1

A

Project Report

On

Improvement of Hardfacing on the Gate Seat

Arrangement from a Quality and Cost

Perspective

BY

Sathvik Divi 2010A4PS317H

Rahul Rochlani 2011A4PS289H

Under the Supervision

Of

Prof. S.P.Regalla,

Mr. Sriram Natarajan,

Mr. Nidhin Thomas

SUBMITTED IN FULLFILLMENT OF THE REQUIREMENTS OF

ME F376: DESIGN ORIENTED PROJECT

BIRLA INSTITUTE OF TECHNOLOGY AND SCIENCE PILANI (RAJASTHAN)

HYDERABAD CAMPUS

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ACKNOWLEDGMENTS

I take this opportunity to express my profound gratitude and deep regard to my

guides Mr. Sriram Natarajan and Mr. Nidhin Thomas for their exemplary guidance,

monitoring, constant encouragement and their unwavering support throughout the

course of this project. I would also like to thank Prof. S.P.Regalla for giving us this

golden opportunity to work with FMC for this project apart from his guidance and

unwavering support throughout this project. I would also like to thank Ms. Aamani

Kanthala for coordinating this project from the Company’s side and ensuring a

cordial and conducive environment.

I would also like to thank the FMC authorities for letting us undertake this project.

Last but not the least; I would like to thank my team mate Sathvik Divi for making

this whole experience a fun-filled one and also for furthering my learning curve in a

big way.

All in all, this project has been an enriching experience and has taught me a lot.

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Birla Institute of Technology and Science-Pilani,

Hyderabad Campus

Certificate

This is to certify that the project report entitled “Improvement of Hardfacing

in a Gate Seat Arrangement from a Quality and Cost Perspective” submitted

by Mr. Rahul Rochlani, ID No. 2011A4PS289H in fulfillment of the

requirements of the course ME F376, Design Oriented Project Course,

embodies the work done by him under my supervision and guidance.

Date: December ,2013 (Prof. S.P.Regalla)

BITS- Pilani, Hyderabad Campus

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Contents

Title Page………………………………………………………………………01

Acknowledgements………………………………………………........02

Certificate…………………………………………………………….........03

Abstract……………………………………………………………………….....05

Introduction………………………………………….............................06

Binding Metals……………….………………………………………………...08

Mechanical Properties……………………………………………………...11

Thermal Properties............................................................16

Cemented Carbide…………………………………………………………….17

Chromium Carbide…………………………………………………………….18

Why Tungsten Carbide………………………………………………………19

Structure of Tungsten Carbide…………………………………………..21

Base Metal…………………………………………………………………………23

Hardfacing Techniques……………………………………………………….27

HVOF Thermal Spray Process...............................................35

Method Applying Tungsten Carbide to Inconel using Sintering

Process……………………………………………………………………………….46

Chromium Carbide: An Alternative......................................56

References………………………………………………………………………….61

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ABSTRACT

Hardfacing is a low cost method of depositing wear resistant surfaces on metal

components to extend service life. It is used in the Slab Gate Valves designed

by FMC Technologies Ltd to be used as the Wellhead Annulus Outlet Valve or

as the Production master and Wing Valve. Although used primarily to restore

worn parts to usable condition, hardfacing is also applied to new components

like the above mentioned valves before being placed into service. In addition

to extending the life of new and worn components, hardfacing provides the

following benefits:

• Fewer replacement parts needed.

• Operating efficiency is increased by reducing downtime.

• Less expensive base metal can be used.

• Overall costs are reduced.

This project aims to understand the process of hardfacing, know why tungsten

carbide is used as the hard facing agent primarily and also the different kinds

of base metals that are used. This project also aims at understanding the

general techniques used in hardfacing. Apart from, it aims to improve the

hardfacing at the gate seat arrangement from a quality as well as a cost

perspective.

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INTRODUCTION Hardfacing is a metalworking process where a harder and tougher metal is applied to a base

metal. Hardfacing is applied to the base metal to make it more wear resistant and also

durable. In addition to extending the life of new and worn components, hardfacing provides

the following benefits:

• Fewer replacement parts needed.

• Operating efficiency is increased by reducing downtime.

• Less expensive base metal can be used.

• Overall costs are reduced.

There are various ways in which hardfacing can be done. These Processes are:

Welding

Brazing

Sintering

Welding and Brazing processes are commonly used in the process of hardfacing equipments

used in drilling and other such processes. Sintering and other similar powder metallurgical

processes are also used occasionally to Hardface components used in processes like oil

drilling, cylinder body in piston-cylinder arrangements used in automobiles, actuators, etc.

There are various welding processes used in Hardfacing. They are:

Shield Metal Arc Welding

Gas Metal Arc Welding

Oxyfuel Welding

Submerged arc welding

Electroslag welding

Plasma Transferred arc welding

Thermal Spraying

Laser Cladding

Hardpaint

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The harder and tougher metal that is applied to the base metal is carbide. Some possible

carbides are:

Tungsten Carbide

Chromium Carbide

Titanium Carbide

Tantalum Carbide ( Common name – RAMET)

Molybdenum Carbide

Vanadium Carbide

Hafnium Carbide

Metals like nickel, chromium, cobalt, iron and silver are added to the above mentioned

carbides to make them tougher. Carbides are one of the hardest compounds found. But, as

hard as they might be, they are also very brittle.

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BINDING METALS

Of all the above mentioned carbides, Tungsten carbide has been extensively exploited in

tool industries because of its high hardness and abrasion resistance. However, processing of

tungsten carbide is difficult as it is a refractory material and cannot be densified by solid

state sintering at high temperatures. Hence, a binder phase is incorporated in the system to

facilitate the densification of the material at lower temperatures.

Binders that are commonly used in Hardfacing are:

Cobalt

Nickel

Chromium

Molybdenum

Iron

Apart from the above mentioned metals, their combinations are also used as binder phases.

The carbide when mixed with the binding phase forms a composite. The consolidation of the

composite is achieved by heating the powders above the melting point of the binder phase.

Pressure-assisted sintering is used to obtain fully-densified cemented carbides with high

volume fractions of tungsten carbide. The liquid formation temperature should be low

enough to avoid the dissolution of tungsten carbide. The use of the binder phase is not

limited to the densification process. The properties of the carbide can be tailored by on the

nature and composition of the binder phase. The binder phase affects the toughness,

ductility, and hardness. The binder phase also affects the abrasion resistance of the

composite. All of these properties shall be seen in the following sections of this report.

The suitability of the binder phase is dictated by the mechanical properties desired for the

final cemented carbide. Conventional cemented carbides often contain cobalt as the binder

phase because it satisfies the desired property requirements. The binder imparts the

necessary toughness and ductility, and facilitates the production of the cemented carbides

at temperatures much lower than the melting point of the carbides. However, cobalt is

strategic material, and is therefore expensive and comparatively rare. Despite early

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attempts to find a substitute for cobalt as the binder phase, production has been greatly

limited to modelling in laboratory scale.

The major restriction for the use of substitute materials arises from inadequate mechanical

property generation. Iron and nickel, in combination with low-melting constituents like

boron and silicon, are the major candidate materials for the binder phase. Comparable

mechanical properties with the cobalt-cemented carbide were achieved for the Fe/Ni alloy

and the alloy in combination with cobalt. The additional advantage realized was that the

binder phase was heat treatable. However, addition of low-melting constituents (e.g.,

boron) is deemed necessary to facilitate liquid phase sintering at low temperatures. Also,

poor distribution of Ni-based binders between the tungsten carbide particles during milling,

resulting in poor densification and high porosity levels has been reported.

Even distribution of the binder is desired to facilitate liquid film formation of binder melting

between individual tungsten carbide grains, which enhances densification. The presence of

agglomerates of binder phase in the initial blend is harmful and has similar effects as poor

distribution. The availability of relatively coarse binder powders will negate this issue.

However, the size has to be adjusted to combine with the carbide powder to provide good

initial packing densities. The above section summarized the binder phases which are

potential substitutes for cobalt. An advantage of these systems is that they are inexpensive

and readily available. The microstructures obtained for the various systems showed

excellent bonding between the binder and the carbide. Ni-Cr-Mo Was investigated as a

replacement for cobalt in tungsten carbide cemented carbides to improve the mechanical

properties and oxidation Resistance. The presence of Mo confers improved corrosion

resistance, in both acid and alkaline media, together with increased toughness, for a given

hardness, in comparison With equivalent tungsten carbide/cobalt cemented carbides. It was

observed that Mo partitions to both the tungsten carbide and the binder phase (nickel

phase).

The properties required for the binders are similar to those required in liquid phase

sintering, i.e., the binder should be liquid and it should wet the carbide at the infiltration

temperature. This will ensure reasonably homogeneous microstructures of tungsten carbide

with various Co and Ni alloys as the binder phase. It was also established that at high

infiltration temperatures, no excessive grain growth of tungsten carbide was seen in

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samples with highly alloyed binder phases. This signifies that the alloying additions inhibit

grain growth, perhaps due to limited solubility of the WC ion in the binder.

Generally, Cr-Ni rich binder phases are often known for good corrosion properties. Nickel-

based alloys are used in powder form to enhance Wear and corrosion resistance and are

applied by various hard-facing techniques .These alloys typically contain Ni, Cr, B, Si, and

often small amounts of C and Fe. Both B and Si form low melting eutectics with nickel or

nickel-solid solution. The lower melting point of these alloys over conventional super alloys

is an advantage as it eases the processing. Boron and silicon act as deoxidizers and improve

the properties of the coating material and in bonding to the substrate, as Well as melting

temperature depressants for Ni and Cr via the formation of eutectic liquids. Both are

expected to diffuse into the substrate during the coating procedures.

The presence of carbon is of prime importance in

the development of cemented carbides. Insufficient carbon levels lead to the formation of

brittle Eta phase. Eta phase is a carbon deficient form of tungsten carbide that results in a

harder, more brittle cemented carbide part. The Eta phase is a double carbide and its

composition depends on the binder phase used (e.g., Co3W3C for Co binders, and Fe3W3C for

iron-based binders). Its presence adversely affects the mechanical properties, and is

attributed to its brittle nature. Eta phase formation occurs due to instability of tungsten

carbide in presence of low carbon austenite. The tungsten carbide breaks down and reacts

with the austenite to form the Eta phase. The hardness is maximum for a Carbon content

which results only in the formation of tungsten carbide. Excess carbon results in presence of

free graphite, which degrades the strength and hardness of the carbide. The desired carbon

level can be ascertained by calculating the total carbon, tungsten and iron content from

chemical analyses. The presence of Ni reduces the carbon requirement needed to prevent

eta formation. It is believed Ni prevents the reaction of the matrix with the carbide. The

addition of TaC and VC increases the hardness and the abrasion resistance of the material.

Their effects are more pronounced for low binder contents.

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Mechanical Properties

Hardness:

It is the most important property for carbide. The more the hardness, the

more is its resistance to abrasion. The Standards used for determining hardness are

ASTM B-294 standards. Hardness values are expressed in terms of Rockwell A,

Rockwell C or Vickers values. Hardness decreases as the grain size increases. As the

temperature increases, the grain size increases and this results in decreasing values

of hardness.

Wear Resistance:

This property or a combination of properties is related to surface phenomena. When

two surfaces slide against each other, material will be removed from both of them. At a

low load, this loss of material will take place through the loss of single grains or parts of

single grains. This process is generally referred to as attrition. At higher load, the loss of

material takes place by clusters of grains becoming detached. This process is known as

abrasion. Both these processes, leading to loss of surface material, contribute to wear. In

practice, the material loss is often also affected by the local environment, particularly if

corrosion or oxidation is encountered. The nature of wear is very complex and the wear

rate depends on many variables. General values for comparative purposes should be

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viewed critically. However, evaluations of wear resistance can be done in the laboratory

under standardised conditions. Such evaluations indicate the ranking between the

tested materials under these specified conditions only.

Transverse Rupture Strength:

Mechanical strength of carbide is determined by using

Transverse Rupture method rather than a tensile test. Carbides are brittle materials

and hence, they are extremely sensitive to any notch or surface defects. The

Transverse Rupture Strength (TRS) is determined by placing a sample (ASTM B-406,

ISO 3327 standard) and loading it at the centre until it fractures. This value is called

the TRS (It is also the bending strength). Generally, several tests are conducted and

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their mean value is taken as the final TRS because, carbides being brittle materials,

exhibit a range of fracture values caused by the existence of micro-voids, inherent in

all brittle materials.

TRS increases as the grain size increases. As the binder content increases, the TRS also

increases.

Toughness:

Toughness is the ability to resist fracture. Toughness varies with the grain

size and also the percentage of binder metal present. It increases as the percentage

of binder metal increases and also increases as the grain size increases.

Compressive Strength:

Brittle materials fail due to shear fracture rather than true

compression unlike ductile materials. Carbides being brittle materials exhibit high

compressive strength.

Compressive strength increases with decreasing binder content and decreases with

increasing grain size.

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Modulus of Elasticity (E):

It gives the stiffness of a material. It is a measure of the amount of

stress a material can handle for a given amount of strain. Young’s modulus decreases

with increasing binder content.

(Young’s modulus and

Poisson’s ratio as a function of binder metal content, the thick yellow line denotes E and

the dotted line denotes Poisson’s ratio)

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Fatigue Strength*:

Fatigue Strength increases with decreasing tungsten carbide grain size

and decreasing binder content. For cemented Carbides, it is usually taken as 65-85 %

of the static compressive strength at 2 X 106 cycles. Fatigue strength can be calculated

by the formulae

Sf=aNb where a= (fSut)2/ Se and b= (-1/3)log( (fSut)/Se)

Sf is the fracture strength, Se is the endurance limit, N is the cycles to failure, Sut is the

ultimate tensile strength f is the Fatigue strength fraction.

(*- for further reference- Shigley’s Mechanical Engineering Design (SI Units), ninth

edition)

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THERMAL PROPERTIES

Coefficient of Thermal Expansion:

Carbides generally have low coefficient of Thermal expansion (CTE). CTE increases with

increasing binder percentage and can be further increased by addition of alloying agents.

Thermal Conductivity:

Carbides like tungsten carbide and silicon carbides are good

conductors of heat. Grain size has no effect of the thermal conductivity. But,

additives like titanium carbide or tantalum carbide will decrease the thermal

conductivity.

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Cemented Carbide (Tungsten Carbide + Co/Ni)

Tungsten carbide is widely used today for the purpose of hardfacing. Binding metals like

Nickel or cobalt are added to it to make it tougher. WC+Ni/Co are also known as cemented

carbides. There are various grades of cemented carbides available based on the percentage

of binder materials and also the percentage of carbon present. Cemented Carbide is

preferred for parts that must withstand all forms of wear (abrasion, erosion, corrosion,

metal to metal galling) and exhibit a high degree of toughness.

Tungsten carbide with nickel/cobalt exhibits high degree of toughness and high compressive

strength, resists deflection ad retains its hardness values at high temperatures. Tungsten

Carbide is extensively used for hardfacing these days because of its unique combination of

toughness and hardness. But, Tungsten carbide has a density of around 14 g/cc. The

particles’ specific gravity is high (twice the value of the metal matrix). This makes them to

sink into the matrix thus exposing the outer layer of the base metal. This leads to a relatively

rapid wear. Also, the cost of hardfacing a base metal with tungsten carbide is costly. Apart

from that, it has been found that a brittle inter-metallic phase frequently forms at the inter-

phase between the particles and the base metal matrix. This too leads to premature wear.

WC is resistant to most corrosive media. Presence of binder material makes it susceptible to

leaching in the presence of a strong acid or alkali. The binder material is attacked by the

strong corrosive media leaving an unsupported carbide skeleton. The carbide particles then

abrade away quite readily exposing a new surface to be attacked.

WC-Co grades are corrosion resistant at neutral pH (7). Same holds true for grades that

contain additives like TiC, Tac, Nbc, etc. Certain Tic-Ni grades possess highest corrosion

resistance down to pH 1. But, these are inferior to WC-Co grades in thermal conductivity

and are more brittle.

When Corrosion resistance is the prime design criteria, specially alloyed WC-Ni grades are

used. These grades are resistant till pH 2 to 3. Also, at high temperatures WC decomposes

to tungsten and carbon. Oxidation starts at a temperature of 500-6000 C. WC reacts with

Fluorine at room temperature and chlorine at higher temperatures. WC dissolves readily in

H2O2 and is unreactive to H2 till its melting point.

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Chromium Carbide + Nickel

According Patent No. 4173457 filed in 1978 by Thomas J Smith, Chromium Carbide with

Nickel as the binding metal can be used to hardface base metals like steels. The Specific

gravity of Chromium carbide is less than that of tungsten carbide. This results in a better

steel matrix. As the Chromium carbide particles have density approximating that of steel

(density of chromium carbide particles is around 6.8-7.0 g/cc), there is a more optimum

distribution of particles throughout the steel matrix when compared to the matrix in case of

cemented carbide, where the carbide particles tend to sink into the matrix due to their high

specific gravity. Also, no brittle inter-metallic phase exits at the interface of the particles and

the matrix.

Also, no additional techniques are required to enhance particle bonding. This kind of

hardfacing is also more economical than WC-Ni/Co.

The hardness of Chromium Carbides bound with Nickel is usually in the range of 87.8-88.8

Rockwell A. A typical Composition is 10% Ni and this gives a hardness of 88.1 on Rockwell A

scale at a density of 6.9 g/cc.

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Why Tungsten Carbide?

The Following table shows the properties of various carbides that could be used as

alternatives.

Carbide

Formula

Vickers

hardness

(HV) @ room

temperature

(200 C)

Rockwell A

hardness

values @

room

temperature

Ultimate

Compressive

Strength

MPa(Ksi)

Transverse

Rupture

Strength (TRS)

MPa(Ksi)

Modulus of

Elasticity (E)

GPa( 106 ksi)

TiC 2930 93 1300-3900

(193-522)

280-400

(40.6-58)

370 (52.9)

HfC 2860 84 - - -

VC 2800 83 620 (89.9) 70 (10.1) 360 (51.4)

NbC 2400 83 1400 (203) - 270 (38.5)

TaC 1570 82 - - 470 (68.2)

Cr3C2 - 81 100 (14.5) 170-380

(24.7-55.1)

280 (40.0)

MoC - 74 2700 (392) 50 (7.3) 375 (53.6)

WC 2400 81 2700-3600

(392-522)

530-560

(76.9-81.2)

665 (95)

From the table above, we see that the carbides of Titanium, Hafnium, Vanadium, Niobium,

Tantalum and Chromium have hardness values comparable to that of Tungsten Carbide. But,

only Titanium Carbide has a Ultimate compressive strength comparable to that of tungsten

Carbide. Also, these carbides have a relatively low modulus of elasticity when compared to

Tungsten Carbide. All these indicate that while there are other carbides (those of Ti, Mo, Cr,

Nb, Ta, V, Hf) that are have hardness values comparable to tungsten carbide, they do not

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offer the kind of toughness that tungsten carbide offers for the same hardness. An optimum

mix of hardness and Toughness is desired for Carbide that is to be used in Hardfacing. Also,

we can see from the young’s modulus values that Tungsten Carbide offers the maximum

resistance to deflection than other carbides.

Hence, Tungsten Carbide with suitable Binding metals such as Chromium, Nickel, Cobalt and

etc is used for hardfacing.

The preferable particle size is 14-30 mesh (US size standard). The average grain sizes

observed are usually in the range of 0.5 to 25 microns. The preferred range of grain size is

0.5 to 15 microns. The Binder percentages are also within the range 3 to 25 %.

For carbides that are produced by sintering, the particles undergo grain growth. For

example, in the case of chromium carbide, particles having an average size of 0.5 to 5.0

microns are used, the grains will normally grow in size to average within the range 5 to 25

microns. As we have seen earlier, that Grain growth increases the toughness and decreases

the hardness of a material, certain compounds are added to prevent or restrict the growth

of the grains. The compounds that are added to restrict the size of the grains are called as

growth inhibitors. Growth inhibitors like titanium carbide, molybdenum carbide, vanadium

carbide, tantalum carbide are added either in combination or alone. Some carbides like

Tantalum Carbide are also used as an anti-galling agent.

The base metal matrix can further be improved by adding certain bonding agents. These

bonding agents are fine powders that are used in minor amounts depending upon the

degree of additional alloying of the matrix which is desired. They enhance the boding of the

particles with the matrix, improve the welding characteristics of hardfacing materials and

assure a uniform distribution of particles in the matrix.

Some of the bonding agents that are used and their preferred ranges are given in the table

below.

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Bonding Agent Preferred Range (%)

Ferrosilicon (50/50) 0.5-15

Ferromanganese (35/65) 0.2-5

Siliconmanganese (40/60) 0.2-5

Ferromolybdennum (40/60) 0.2-5

Ferroboron (80/20) 1.0-10

Carbon 0.01-2

Chromium Carbide 0.5-10

Nichrome (80/20) 0.5-15

Ferrochrome 0.2-5

Structure of Tungsten Carbide

Tungsten Carbide exhibits three structures. They are:

α-WC

β-WC

Gamma WC

The alpha form has a hexagonal structure. It consists of only Tungsten and Carbon. When

Binding metals like Nickel and Cobalt are added, we get a Rock-salt like structure and this is

called the Beta form. When additional compounds are added (like TaC, TiC, nichrome as

growth inhibitors or bonding agents), we get a Gamma form.

A picture of the alpha form of tungsten carbide is given below. The Gray atoms are Carbon

and the blue atoms are tungsten.

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In the Beta form, each of the two atom types forms a separate face-centred cubic lattice,

with the two lattices interpenetrating so as to form a 3D checkerboard pattern. Alternately,

one could view this structure as a face-centred cubic structure with secondary atoms in its

octahedral holes. Generally, this structure is more likely to be formed if the cation is slightly

smaller than the anion (a cation/anion radius ratio of 0.414 to 0.732). Given in the next page

is a picture of rocksalt structure:

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BASE METAL

A base metal is the metal to which hardfacing is done to improve certain mechanical

properties. In most cases, the base metal is Steel or its alloys. Generally, base metals that

can be hardface are:

Stainless Steels

Manganese Steels

Cast-Iron Steels

Nickel based Alloys

Copper base Alloys

However, for this project, Inconel is the base metal. Inconel is primarily an alloy of Nickel

and Chromium. Notable properties of Inconel are its high heat resistance, high creep

rupture strength, low expansion and good electrical resistance. There are various alloys of

Inconel. They are:

Inconel-600

Inconel-617

Inconel-625

Inconel-718

They are many more alloys like 750-X, etc. But, the primarily used ones are Inconel-625 and

Inconel-718.

Inconel-625:

Inconel-625 Chemical composition is as follows:

% Ni Cr Mo Nb+N Fe Al Ti C Mn Si Cu P S

Min. 58 20 8 3.15 - - - - - - - - -

Max. - 23 10 4.15 5 0.4 0.4 0.1 0.5 0.5 0.5 0.015 0.015

Inconel-625 has a density of 8.4 g/cm3 and has a melting point in the range of 1290-13500 C.

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Properties of Inconel-625 are:

Excellent corrosion resistance of different kinds of media in both oxidation and

reduction environments

Excellent resistance of pitting and crevice corrosion, and won't happen stress

corrosion cracking because of chloride

Excellent resistance of the inorganic acid corrosion performance, such as nitric

acid, phosphoric acid, sulfuric acid, hydrochloric acid and the mixture of sulfuric

acid and hydrochloric acid

Excellent corrosion resistance of different kinds of inorganic acid mixture

performance

Good corrosion resistance of a variety of concentrations of hydrochloric acid

when the temperature up to 40℃

Good machining and welding, no weld cracking sensitivity

Have pressure vessel authentication for the wall temperature between -196 to

450℃

Apply for the highest standard lever VII of acidic environment

Inconel-625 has a face-centred cubic lattice structure. Dissolve out the carbon granule and

instability quaternary phase, then change it to stability Ni3 (Nb, Ti) trimetric lattice around

650℃ after a long time heat preservation. Nickel-chromium content will strength the

mechanical performance in the state solution while suppress the plasticity. Inconel-625 is

also known to have very good corrosion resistance in many media, especially with excellent

resistance to pitting, crevice corrosion, inter-crystalline corrosion, and erode in oxide, also

good resistance to inorganic acid corrosion, such as nitric acid, phosphoric acid, sulphuric

acid and hydrochloric acid. Inconel-625 can resist the alkali and organic acid corrosion in the

oxidation and reduction environment. Effect resists the chloride reduction stress corrosion

cracking. It exhibits high corrosion resistance to the sea-water and salting liquid, as well as

at high temperatures. Inconel-625 is also known for its resistance towards chlorine

corrosion.

Inconel-625 is typically used in the following fields:

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The Organic chemical process parts containing chloride, especially in the use of

acid chloride catalyst

The digester and bleacher in the use of paper pulp and paper making industry

Absorption tower, re-heater, gas import board, fan, blender, fair water fin, flue

and so on for using in flue gas desulfurization system

The equipment and parts in the use of acidic gas environments

Inconel-718:

Inconel-718 has the following chemical composition:

% Ni Cr Fe Mo Nb Co C Mn Si S Cu Al Ti

Min. 50 17 2.8 4.75 - - - - - - 0.2 0.7

Max. 55 21 Balance 3.3 5.5 1 0.08 0.35 0.35 0.01 0.3 0.8 1.15

It has a density of 8.2 g/cm3 and its melting point is in the range of 1260-13400 C. Properties

of Inconel-718 are as follows:

High tensile strength, endurance strength, creep strength and rupture strength at

700℃

High resistance to oxidation at high temperatures like 1000℃

Steady mechanical performance in the low temperature

Inconel-718 alloy has an Austenitic structure. It is highly resistant to stress corrosion

cracking and pitting ability at high temperature and also at low temperatures. Also, it shows

high resistance to oxidation even at high temperatures.

The elevated temperature strength, excellent corrosion resistance and workability at 700℃

properties made it use in a wide range of high requirement environments. Some places

where it is extensively used are:

Steam Turbine

Liquid-Fuel rocket

Cryogenic Engineering

Acid Environment

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Nuclear Engineering

To sum it up, Inconel and its alloys are oxidation and corrosion resistant materials well

suited for extreme environments subjected to high pressure and kinetic energy. When

heated, Inconel forms a thick, passive and a stable oxide layer. This protects the surface

from further attack. Inconel retains its strength at high temperatures. This makes it

attractive for high temperature applications where aluminium and steel would succumb to

creep as a result of thermally induced crystal vacancies. Inconel’s high temperature strength

is developed by solid solution strengthening or precipitation hardening (depends on the

alloy). In age hardening, small amounts of Niobium combine to form the inter-metallic

compound Ni3Nb (Gamma Prime). Gamma prime forms small cubic crystals that inhibit

creep and slip effectively at elevated temperatures.

It is difficult to machine Inconel using traditional techniques because of rapid work

hardening. Work Hardening causes the workpiece or the tool to deform plastically. The

majority of machining can be performed with the workpiece in a solution form.

Welding Inconel alloys is difficult due to cracking and microstructural segregation of alloying

elements in the heat affected zone.Inconel is often encountered in extreme environments.

It is commonly used in gas turbine blades, seals, combustors, turbocharger rotors and seals,

high temperature fasteners, chemical processing and pressure vessels, heat exchanger

tubing, natural gas processing with contaminants such as H2S and CO2.

Hence, Inconel alloys are generally subjected to heat treatment techniques. Inconel-718 has

good welding characteristics after it is subjected to age-hardening or annealing.

27

Hardfacing Techniques

Metal Parts often fail their intended use because of wear, which causes them to lose their

dimension and functionality. Hardfacing is the application of wear resistant weld metals to a

part’s surface. It can be one by:

Welding

Brazing

Sintering

Welding techniques to apply hardfacing to a base metal are:

Tungsten Inert gas Welding

Metal Arc Welding

Gas Metal Arc Welding

Submerged Arc Welding

Plasma Transferred Arc Welding

Laser Weld Deposition

Power Weld Deposition

High Velocity Oxy Fuel (HVOF) Spray Deposition

Plasma Spray Deposition

Tungsten Inert Gas Welding

In TIG (Tungsten Inert Gas) welding, an arc is drawn

between a non-consumable tungsten electrode and

the work piece. The electrode, the arc and the weld

pool are protected from the atmosphere with an

inert shielding gas. For manual welding, the

hardfacing material is in the form of a rod.

Advantages of the TIG process include simple

manual operation and good control of the welding

arc. The process can also be mechanized, in which

case a manipulator is used to move the work piece

28

in relation to the welding torch and the hardfacing rod or wire. Rods are also used for

hardfacing with the Oxy-Acetylene welding process. With the correct operation, a very low

level of iron dilution can be achieved in the overlay.

Metal Arc Welding:

In this process an arc is drawn between a coated

consumable electrode and the work piece. The

metallic core-wire is melted by the arc and is

transferred to the weld pool as molten drops. The

electrode coating also melts to form a gas shield

around the arc and the weld pool as well as slag on

the surface of the weld pool, thus protecting the

cooling weld pool from the atmosphere. The slag

must be removed after each layer.

Manual Metal Arc (MMA) welding is still a widely

used hardfacing process. Due to the low cost of

the equipment, the low operating costs of the

process and the ease of transporting the

equipment, this flexible process is ideally suited to

repair work. Benefits of MMA Welding are:

Flexible

Low Cost

Mobile

Ideal for Repairs

29

Gas Metal Arc Welding:

This is an arc welding process where hardfacing wire is

fed continually from a spool through the welding torch

into the arc, where it is melted and transferred to the

work piece.

In the case of MIG welding, also known as Gas Metal

Arc Welding (GMAW) the weld pool is protected from

the atmosphere with a stream of shielding gas. These

MIG processes are very flexible, i.e. they can be

partially or fully mechanized and they are suitable for a

wide range of applications.

Benefits of GMAW are:

Flexible Operation

Can Be Partially Or Fully Mechanized

Suitable For Wide Range Of Applications

Submerged Arc Welding Process (SAW):

In this process a mineral based fluxing powder flows around the consumable wire and is

melted by the arc. It forms a gaseous shield around the arc and also forms a slag on top of

the weld pool, thereby shielding the cooling weld pool from the atmosphere.

30

Plasma Transferred Arc Welding (PTA):

The PTA process recommends itself due to its

ease of automation and thus a high degree of

reproducibility of the welded overlays. In

addition, because of the highly concentrated

heat source, this process benefits from high

powder utilization and can achieve a very low

level of iron dilution in the overlay.

Because the hardfacing materials are in powder

form, it is possible to produce overlays from

many different materials and combinations of

materials with a wide range of hardness and

other properties. Its main Benefits are:

Can Be Highly Automated

High Powder Utilization

Low Dilution

Very Wide Range Of Hardfacing Materials

Laser Weld Deposition:

When overlaying with a laser an optical arrangement

is used to focus the laser beam on the work piece and

heat it. Simultaneously hardfacing material in the form

of powder is introduced into the laser beam and

melted. Due to the narrow heat affected zone and the

fast cooling rate the heat input is low, thereby

producing an almost stress free overlay.

Compared with other welding processes with higher

heat inputs, for a given hardfacing alloy the fast

cooling rate of the laser process produces an overlay

31

with a significantly higher hardness and finer microstructure.Benefits of Laser weld

deposition are:

Low Heat Input

Fast Cooling

Almost Stress Free Overlays

High Hardness

Fine Microstructure

Powder Weld Deposition:

A specially designed Oxy-Acetylene torch is used

for powder welding. The work piece is heated

with the torch. The powder is introduced into the

gas stream from the integral powder hopper and

then transferred to the work piece through a

flame.

This process is similar to the Oxy-Acetylene

process with the exception that the hardfacing

takes place at lower temperatures. This

minimizes oxidation and distortion of the work

piece ad enables easy surfacing of edges.

Benefits of Powder welding are:

Low Dilution

Easy Application

Low Distortion

Ideal for Edge Building

32

HVOF Spray Deposition:

In the High Velocity Oxy-Fuel (HVOF) process, powder is introduced axially into a chamber in

which a gas flame is constantly burning under high pressure. The exhaust gas exits through

an expansion nozzle which produces a high velocity gas stream. The powder particles are

heated in this gas stream and transferred by it with kinetic energy to the surface of the work

piece, forming a dense coating with excellent bonding properties.

Due to the moderate transfer of heat to the powder particles and to the work piece, which

remains relatively cool, there is little metallurgical change to either the sprayed material or

work piece. This is the most used method for hardfacing inconel with tungsten carbide.

Benefits of HVOF are:

Very Dense Coatings

Excellent Bonding

Minimal Metallurgical Changes

Minimal Temperature Effects

33

Plasma Spray Deposition:

In the Plasma Spraying Process powder is softened or melted in the plasma gas stream,

which also transfers the particles to the work piece.The plasma arc is not transferred to the

work piece, it is contained within the plasma torch between an axial electrode and a water

cooled nozzle.

The process is operated in normal atmosphere, in a shielding gas stream (e.g. Argon), in a

vacuum or under water.Due to the high temperature of the plasma gas stream the Plasma

process is especially suitable for spraying high melting point metals as well as their oxides

and carbides.Benefits of Plasma Spraying are:

Operates In Several Environments

Ideal for High Melting Point Materials

From the above mentioned processes, a suitable process chosen after a careful

consideration of the following features:

1. Design :

The design or geometry of the component to be hardfaced will

determine the amount of weld deposit required and the ease of

applying it.

2. Position:

The highest deposition rates are achieved in the flat and

horizontal positions.

34

3. Process:

The welding process and the process parameters chosen will

govern the time required as determined by the deposition rate,

deposition efficiency and operator factor.

4. Costs:

Additional Costs associated with the work such as transportation,

machining, heat treatment and inspection.

Deposition efficiencies for some processes are given below:

Process Deposition Efficiency (%)

Manual Arc Welding 60-70

Tungsten Inert Gas Welding 95

Gas Metal Arc Welding 95

Submerged Arc Welding 90-95 (wire only)

Gas Welding (bare rod) 95

The deposition rates for some processes are given below:

Procedure Deposition Rate (lbs/hr)

Gas Metal Arc Welding 5 to 12

Shield Metal Arc Welding 3 to 5

Submerged Arc Welding 8 to 25

Oxyacetylene Welding ( Oxyfuel Welding) 3 to 5

Based on the cost of operation, deposition rates and their efficiencies and tool costs,

suitable welding techniques are used.

35

HVOF Thermal Spray Process

The HVOF (High Velocity Oxy-Fuel) thermal spray process is a form of the flame spraying

process, but utilizing only powder as the coating material rather than wire or rod. The

Diamond Jet HVOF thermal spraying is a flame deposition technique whereby powder

material is melted by the use of combustion of oxygen and a fuel gas and is propelled at a

high velocity by the use of compressed air, towards a surface as shown in fig. In the

combustion zone, the powder material enters the flame, where it becomes molten or semi-

molten, depending on the melting temperature and the feed rate of the material. The flame

temperature for the HVOF process is between 2300 - 3000°C. The molten or semi-molten

particles are then propelled out of the gun nozzle at supersonic velocities of over 1350m/s

towards the substrate or forming die.

Advantages of the HVOF technique

The HVOF process is designed around producing these high velocities, and this contributes

the advantages that HVOF has over other thermal spray processes in terms of particle

condition including:

More uniform and efficient particle heating, due to the high turbulence experienced

by the particles within the combustion chamber

36

Much shorter exposure time in flight due to the high particle velocities

Lower surface oxidation due to short particle exposure time compared to other

thermal spraying techniques

Reduced mixing with ambient air once jet and particles leave the gun

Lower ultimate particle temperatures compared to other processes such as plasma

or arc guns, as the these processes operate at temperatures of 16000 and 6000°C as

opposed to 3000°C in the HVOF (oxygen/propylene mixture) process

HVOF Thermal Spraying system

The objective of the HVOF process is to transfer kinetic and thermal energy to powder

particles, with high efficiency. This energy is achieved by a combination of gases to combust

and propel these particles. The HVOF thermal spraying system consists of -

A gas supply unit

Flow meter unit

Powder feed unit,

Diamond Jet (DJ) gun.

37

HVOF Spraying Procedure:

The procedure mainly consist of 4 steps:

Surface Preparation

Pre-Heat treatment

Spraying Process

Post heat treatment

Surface Preparation:

Cleaning is generally carried out by eroding the surface by a harder material, which is with

the grit (sand) blasting unit. Surface treatment has proved to provide sufficient bond

strengths between the deposit material and the receiving substrate. It increases residual

stress by very little amount but decreases thermal stress.

Pre-heat Treatment

Moisture build up on the substrate surface may be removed by exposing the surface to a

high temperature, a process known as pre-heat treating. Pre-heating the substrate may

reduce or elevate thermal stresses, dependent on the exposure temperature. The pre-heat

temperature for a steel substrate is usually within the range of 90 to 150°C, but a pre-heat

temperature of 450°C reduces greater amounts of residual stress built up m WC-Co

coatings, compared to temperatures below 150°C. Pre-heating is carried out by igniting the

gun (with a flint lighter), and heating the substrate with the guns’ flame, up to the desired

pre-heat temperature prior to deposition.

Spraying Process

First the WC-Co powder is fed in the hopper of powder feed assembly. The required nozzles

are cleaned and the gas pressures are set to get the required flow rates. Then the gun is

ignited and air flow rate is set to 325LPM. The spraying distance is controlled by a linear

motor, which also controls the deposition rate and thickness.

38

Tungsten Carbide-Cobalt Spray Parameters

Oxygen pressure (Bar) 10.3

Oxygen flow (LPM) 265

Propylene pressure (Bar) 6.9

Propylene flow (LPM) 73.0

Air pressure (Bar) 5.2

Air flow (LPM) 325

Nitrogen pressure (Bar) 12.1

Nitrogen Flow (LPM) 325

Spray distance (mm) 200

Powder feed rate (g/min) 38

Post heat Treatment

The post-heat treatment is carried out in the furnace under a nitrogen atmosphere. Post-

heat treatment of WC-Co components is used for to reduce the residual stress that had built

up during spraying. Post-heat treatment has been proved as an effective stress relief

process, where tungsten carbide-cobalt coatings or spray-formed components are elevated

to temperatures of 650°C.

In order to spray-form thick tungsten carbide cobalt (WC-Co) components, certain problems

have to be overcome More specifically these problems include minimizing residual stresses

(which cause shape distortion in the components), therefore maintaining the integrity of the

deposit on a microstructural scale. So residual stress variation with respect to different

parameters have been studied. For the measurement of residual stress we use Clyne’s

method.

39

Clyne’s Method:

Stress at top of the deposit

(𝜎𝑐)𝑦=ℎ𝑐= −Δ𝜖 (

𝐸′𝑐ℎ𝑠𝐸′𝑠

ℎ𝑐𝐸′𝑐 + ℎ𝑠𝐸′𝑠) + 𝐸𝑐𝜅(ℎ𝑐 − 𝛿)

Stress at bottom of the deposit

(𝜎𝑐)𝑦=0 = −Δ𝜖 (𝐸′𝑐ℎ𝑠𝐸′𝑠

ℎ𝑐𝐸′𝑐 + ℎ𝑠𝐸′𝑠) + 𝐸𝑐𝜅(𝛿)

Stress at top of the substrate

(𝜎𝑠)𝑦=0 = Δ𝜖 (𝐸′𝑐ℎ𝑠𝐸′𝑠

ℎ𝑐𝐸′𝑐 + ℎ𝑠𝐸′𝑠) − 𝐸𝑐𝜅(𝛿)

Stress at top of the substrate

(𝜎𝑠)𝑦=ℎ𝑠= Δ𝜖 (

𝐸′𝑐ℎ𝑠𝐸′𝑠

ℎ𝑐𝐸′𝑐 + ℎ𝑠𝐸′𝑠) − 𝐸𝑐𝜅(ℎ𝑠 + 𝛿)

Where σc and σs are the respective stresses in the coating and substrate

Δ𝜖 = (αs − αc)ΔΤ

Where α c and αs are the respective coefficients of thermal expansions for the substrate

and coating, and ΔT is the difference in temperature

𝐸′𝑐 = 𝐸𝑐

(1 − 𝜈𝑐)

𝐸′𝑠 = 𝐸𝑠

(1 − 𝜈𝑠)

Where Ec and Es are the respective coating and substrate stiffnesses. κ is the curvature of

the beam (given as 1/R, where R is bending radius and δ is the overall deflection of the

beam)

40

41

Effect of various parameters on residual stress:

Residual stress arises during deposition and can be reduced by limiting the rise and

fluctuation of the deposition temperature. This can be achieved by the utilization of a

carbon dioxide cooling system and automated traverse movement of the spray gun, which

together enabled continuous deposition at a steady temperature of 500°C, reducing the

residual stress by 58% compared to manual spraying. A spraying distance of 200mm and a

powder feed rate of 38g/mm exhibits the lowest stress within the deposit using the

automated system. The minimization of the residual stress can increase the maximum

deposit thickness achieved from 0mm to 12mm.

Variation with spraying distance

42

At smaller coating distances the residual stress is higher which gives rise to higher cooling

stresses. Also the variation is higher for low distance.

At higher distances the stresses are further reduced but the coating quality is poor due to

increased flight timings of the particles which results in cooling and poor adhesion of

particles to the substrate.

Stress values are significantly lower at distance 200mm and the deposition temperature

remains constant at 500°C

Variation with Powder Feed Rate

43

The stress change in the deposit sprayed at a distance of 200mm, is on average 252MPa

when a feed rate of 55g/min is used. The stress reduces (at the same distance) to 203MPa

when a feed rate of 45g/min is used. Similarly, the stress reduces to 73 and 36MPa for feed

rates of 38 and 25g/min respectively. A micrograph of the top of sprayed surfaces, formed

using the various powder feed rates, is shown in figure. Lowest stress change results are

observed at feed rates of 25g/min, however the coating is of such poor quality, that the

stress is not relevant. This is because the small amount of material deposits on each pass of

the HVOF gun, results in a sparse distribution of particles. At this rate, inter-particulate

bonding is impossible until several passes are made and this yields poor bond strength

between the substrate and the deposit, and increased porosity. Residual stress values

increase with increasing powder feed rate, at each spray distance. At feed rates of 45gmin‘1

and higher, the number of voids decrease, but the quantity of unmelted particles increases.

A powder feed rate of 38 g/min, yields good quality deposits for the tungsten carbide-cobalt

material, with a minimum pass thickness together with low residual stress.

Variation with Deposit thickness

44

The variation of stress with deposit thickness is non-linear. It can be divided into 3 zones – In

zone-A there is a sharp decrease with deposit thickness upto 1mm. the stress decreases at

lower rate at thickness 1mm to 2 mm. At greater thickness, the stress again decreases at

higher rate. A deposit thickness of 1.2mm produces a stress free deposit so it is used in the

process.

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Brazing

Brazing of a ceramic or a carbide to a substrate using filler metal is often used for different

applications. In the brazing processes, the differential expansion and contraction between the

ceramic and the metal, and the resulting effect on the joint properties, becomes a major concern.

Brazing is the preferred method of joining the carbide and its supporting mass of steel. Further,

silver based alloys popular due to their low melting temperatures, excellent Wetting

characteristics and good mechanical properties. These alloys generally contain Cu, Zn, Cd, Ni and

Mn, and operate at temperatures in the range 640—840° C. Copper-based brazing alloys are also

used, considering the high price of silver. These alloys typically contain Cu, Ni, Mn, Si and Zn, and

operate at temperatures between 870—1100° C. One of the requirements of the braze filler is

that it is of sufficient thickness so that it can absorb the stresses introduced due to the difference

in the expansion coefficient of steel and tungstencarbide. A silver-base alloy, Ag71Cu27Ti2, is used

as the filler metal to brazeSi3N4 to an Inconel base metal. The bonding mechanism between the

two is attributed to the diffusion of Ag and Cu alloy into the grain boundaries of Inconel. This

diffusionresults in mechanical anchoring.

Sintering

Sintering process is used to create the coating composition (Hardfacing Material + Binder Phase)

apart from being used to apply the coating composition onto the substrate surface. Sintering is a

method for creating objects from powders, including metal and ceramic powders.It is based on

atomic diffusion. Diffusion occurs in any material above absolute zero, but it occurs much faster

at higher temperatures. In most sintering processes, the powdered material is held in a mould

and then heated to a temperature below the melting point. The atoms in the powder particles

diffuse across the boundaries of the particles, fusing the particles together and creating one

solid piece. Because the sintering temperature does not have to reach the melting point of the

material, sintering is often chosen as the shaping process for materials with extremely high

melting points such as tungsten and molybdenum.

Sintering is traditionally used for manufacturing ceramic objects but finds applications in

almost all fields of industry. The study of sintering and of powder-related processes is

known as powder metallurgy.

46

Hardfacing Inconel with Tungsten Carbide Using Sintering Process

A method of applying a particulate material to a substrate includes:

Removing impurities from the surface of the substrate

Forming a coating composition having particles of the bonding material and the

particulate material

Applying the coating composition to the substrate surface, thus creating a diffusion

bond between the substrate, bonding material and the particulate material.

This generates a continuous interface between the substrate surface and particulate

material such that the change in mechanical properties between the substrate and the

particulate material occurs in a direction normal to the plane of the substrate surface. This

minimizes the residual strain and coefficient of thermal expansion mismatch between the

substrate and particulate material. The surfaces of the particulate materials are chemically

wetted by the particles of the bonding material.

The impurities are removed from the substrate surface by thermal decomposition, chemical

decomposition, electrolytic decomposition, oblation by ions (using high energy beams),

ultrasonic spray fluxing. The coating composition is applied to the substrate surface by

Sintering Process.

The particulate material can be of a material like tungsten carbide, titanium carbide,

vanadium carbide, titanium diboride, hafnium carbide, molybdenum carbide, diamond

hafnium diboride, zirconium carbide, chrome carbide, tantalum carbide, etc or a

combination of the above. The substrate used is Inconel.

Good interfacial bonding is desired between tungsten carbide and the nickel brazes to avoid

tungsten carbide pull-outs during actual service of the material. As mentioned earlier, good

wettability is the key to the problem. Wettability can be tested by sintering a tungsten

carbide-nickel braze mixture compact above the liquid temperature of the braze alloy

(Where the nickel braze alloy is present as a liquid phase only). Poor Wettability results in

swelling of the compact and liquid exuding from the compact. The exuded liquid appears as

47

small spherical balls on the surface of the compact. This processing step helps in isolating

the suitable braze alloys for further processing.

A tungsten carbide-nickel braze composite can be developed in two different Ways. The first

method involves sintering a mixture of tungsten carbide-nickel base mixture above the

liquids temperature of the nickel braze to allow for densification of the material in presence

of a liquid phase. The limitation with this process is that the effect of relative size and

density of the nickel braze and the tungsten carbide becomes prominent during mixing and

infiltration. The nickel braze powders are generally available in the coarser size range, as

opposed to tungsten carbide which is preferred in finer sizes to enhance the mechanical

properties. This can lead to segregation during mixing. More critical are the sintering

problems associated with the powder size. On melting, nickel braze leaves behind a large

pore (site of its original occupation), and fill up smaller ones. This would lead to poor

densification on completion of the sintering process.

The other method involves fabrication of tungsten carbide-nickel braze composite by

selective infiltration of tungsten carbide porous forms by nickel braze liquid. Nickel brazes

and tungsten carbide, in the form of tape-cast sheets, are stacked one over the other and

sintered above the liquids temperature of the nickel brazes. Densification is achieved

through capillary action and gravity-induced infiltrations of nickel braze liquid into the

tungsten carbide sheet. This method is not limited by nickel braze particle size, as it is liquid

prior to infiltration. Another advantage with this

Design is that homogenous composite can be developed by controlling the pore size of the

tungsten carbide tape-cast sheet.

The bonding process requires the nickel braze alloy as a cementing agent between the

Inconel substrate and the tungsten carbide. The liquid should be minimized at the interfacial

joint between the braze alloy and the substrate to reduce the occurrence of Weak regions

which result in interfacial cracking. Three schematic designs are depicted in FIGS. 1—3. In

FIG. 3, a pre-developed tungsten carbide-nickel braze composite is laid over the Inconel

substrate, and is heated above the nickel braze liquidus temperature. Cementing is achieved

through reaction between the nickel braze in the composite and the Inconel. FIG. 1 depicts

the nickel braze as the interlayer, with tungsten carbide at the top and the Inconel at the

bottom. On heating above the liquids temperature, the Nickel braze melts, wets the Inconel

and infiltrates the tungsten carbide layer. The limitation with such an arrangement is that

48

the amount of liquid at the interface cannot be controlled. FIG.2 is an alternate

arrangement to the earlier design where the tungsten carbide forms the interlayer with the

nickel braze at the top and the Inconel at the bottom. In this lay-up procedure, gravity aids

the infiltration process and the excess liquid remains at the top. Both the above processes

yield the composite and the hard-faced Inconel in a single operation.

The following points need to be noted:

1. The process involves melting of the nickel braze alloy,

2. Bonding is achieved through the formation of eutectic liquids between the

braze alloy and the Inconel

3. Infiltrations of nickel braze liquid into tungsten carbide by capillary action /

gravity.

49

In this process, the constituent powders are tape cast into sheets and these sheets are used

in the process of sintering. Tape-cast fabrication involves three main stages. The first stage

50

involves the selection of a suitable binder system that is compatible with the powders and

yields a flexible homogenous sheet. In the second stage, the composition of the sheet and

fabrication routes should be considered to allow for maximum solids loading (i.e., volume

fraction of tungsten carbide) without sacrificing the flexibility and the uniformity of the

sheets. For generation of a tape-cast sheet, the additive system primarily comprises binder,

plasticizer, solvent, and a surfactant (if required). The binder provides the necessary

bonding between the powder particles which facilitates room temperature handling. The

plasticizer is used to induce flexibility in the sheet which allows it to conform to substrates

with a slight curvature. Surfactant is added to disperse agglomerates adherent due to Weak

van der Waal forces or presence of moisture. One important aspect of the additive system is

that its residual ash content needs to below. This is critical for systems like tungsten carbide

which require close carbon control.

The binder system employed needs to blend in with the tungsten carbide powder to provide

for homogenous tape-cast sheets. In this process, Lupasol PS is used as the binding agent.

The casting substrate is also of crucial importance as it defines the surface finish and the

thickness of the tape-cast sheet. This essentially dictates the surface smoothness of the final

composite. Desirable properties of the casting surface are:

Smooth glassy finish and Poor wettability so as to ensure easy release of the sheet on

drying

Porous surface as this decreases the drying time and higher volume fractions of the

tungsten carbide can be achieved

A Hydrophilic polythene sheet is used as it conforms to the above mentioned factors.

The thickness of the sheet can be controlled by two Plexiglass slides which are mounted on

the two ends along the width of the substrate and extended parallel along the length

direction. This forms a cavity which holds the slurry. The thickness of sheet can be

controlled by the thickness of the glass slides. The pores present in the casting substrate

remove water by capillary action and consequently increases the solids loading through

particle rearrangement.

51

Given below is a schematic arrangement for the Tape-Cast Fabrication.

52

It is critical for Inconel to be free of any oxides or any such contaminants. For this, Inconel is

subjected to thermal treatment to burn all the volatile constituents and reduce the oxides. After

53

this, it is cleaned with acetone. Given below is a flow chart showing the cleaning procedure:

The Nickel braze should be pre-densified prior to the bonding process. Nickel braze alloy is very

sensitive to impurities and the presence of impurities offset their liquids to high temperatures. A

flow chart depicting the pre-densified Nickel brazes production is given below:

54

Using the arrangement as shown in Fig.1 gives the final micro-structure as show below:

Here, even though the sample is fully dense, a crack persists through the interface. Also, excess

liquid is present at the interface and this causes the delamination to occur during the cooling

operation. During cooing, the tungsten carbide-nickel braze composite, the excess liquid and Inconel

undergo shrinkage. The Shrinkage rate is slower for the coating as compared to the excess liquid due

to the presence of tungsten carbide. This results in a restrained shrinkage. This influences the crack

to occur at the composite coating and the excess nickel braze liquid interface and leads to

delamination.

Hence, an arrangement as shown in Fig. 2 is used. Here, any excess liquid which is present

subsequent to infiltration is left on top and can be removed and reused. The Nickel braze acts like a

liquid reservoir on melting. Moreover, the infiltration of nickel braze is aided by capillary forces and

gravity. Once the full infiltration is achieved, the nickel braze liquid reacts with the Inconel to provide

interfacial bonding. This controls the liquid at the interface and thus, minimizes the chances of

interfacial cracking. The resulting product from this arrangement is as follows:

55

56

Chromium Carbide: An Alternative

As discussed earlier even though Tungsten carbide has the best optimum mix of

toughness, hardness and other such mechanical properties, it still has some drawbacks.

Being a high density material, it tends to sink in to the base metal, especially the lower

density metals under the impact of continuous high loads. Apart from that, in a subsea

system, corrosion due to chlorine and fluorine is a major concern. Even though Tungsten

Carbide is corrosion resistant to most extremely corrosive alkaline and acidic media, it is

susceptible to chlorine corrosion at elevated temperatures and susceptible to fluorine

corrosion at room temperatures. Also, in cases where Tungsten Carbide has been used as

the hardfacing agent, formation of brittle intermetallic phases called Eta Carbides was

observed.

Eta phase is a carbon deficient form of tungsten carbide that results in

a harder, more brittle cemented carbide part. Insufficient carbon levels are generally the

result of improper formulation of the carbide powder, long term exposure of unsintered

parts to the atmosphere, or poor control of sintering conditions. Eta phase is generally

considered to be harmful to the performance of cemented carbide parts. Keeping these

factors in mind, a viable alternative to tungsten carbide which offers similar quality, less

costly and more corrosion resistance was explored.

Chromium carbide is one of the best known anti-corrosion materials. Chromium is

used as an additive in various cases where corrosion resistance is an important criterion.

The gate seat arrangement inside the valve is exposed to a corrosive environment where

chlorine, fluorine and hydrogen sulphide are the major players. Using chromium carbide in

these extremely corrosive places would enhance the corrosion resistance of the system.

Also, Chromium carbide has a density of 6.8-7.0 g/cc. As inconel has a density of 8 g/cc

and most base metals have a density less than 8, this ensures that Chromium Carbide does

not sink into the matrix at continuous high loads. Also, no brittle intermetallic phases (Eta

Carbides) were observed in the case of Chromium. From cost point of view, using Chromium

carbide is cheaper. Hence, in suitable conditions if not all, Chromium Carbide can be used as

an alternative to Tungsten Carbide.

57

58

To check for the feasibility of Chromium Carbide, stresses at the surface were evaluated. To

simplify calculations, the entire body was assumed to be made of chromium carbide. The

59

case was assumed to be a rectangular flat plate. A uniform load acts in a circular region of

radius r. The dimensions of the flat plate are 13.08 inches and 6.54 inches.

Case 1b. (as in the above diagram) which applies to a flat plate of rectangular dimensions

was taken from a book “Roark’s Formulas for stress and strain” by Warren Young & Richard

Budynas.

a: length, b: breadth, t: thickness

The stress at the centre was calculated for a load of 10000 Psi= 68.9 MPa

The value of a/b ratio is 2. From the table in the above diagram, the value of β = 0.958.

The values when plugged into the formula for stress gave a value of 158.33 MPa. As the

rupture strength value of Chromium carbide varies from 170-380 MPa, the lower limit of

170 MPa was considered to evaluate the factor of safety. The value of factor of safety was

obtained to be just over 1 (n= 1.07). As, the factor of safety values are not satisfactory, the

lower loads like 5000 or 6000 psi were considered. At 6000 psi, the factor of safety was

obtained as 1.8.

60

These values were obtained for the worst case of the entire slab of the gate valve being

made of chromium carbide. However, chromium carbide will be only at the surface and the

body of the slab is made of inconel.

Hence, it can be suggested that chromium carbide is a viable alternative to tungsten carbide

at lower loads of 6000 psi. FMC has a valve series of 120 plus which are rated to be operated

at 5000-6000 psi. Hence, chromium carbide can be used at much lower loads.

61

References

1. Sandvik Carbide Report- Understanding Cemented Carbide

2. Designer’s Guide to Tungsten carbide- General Carbide

3. Hardfacing Guide by Lincoln electrics

4. US Patent No. 4173457: Hardfacing Composition of Nickel bonded sintered

Chromium Carbide particles and tools Hardfaced thereof

5. US Patent No. 6436470 : Method of Applying a Hardfacing Material to a Substrate

6. Hardfacing by welding- M. Riddihough

7. Roark’s Formulas for Stress and Strain by Warren Young & Richard Budynas

8. General Carbide grade Specifications

9. www.superalloys.com

10. www.generalcarbide.com

11. www.specialmetals.com

12. Economics of Hardfacing- Postle Industries