Heat Treatment1

7/30/2019 Heat Treatment1

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1Prof.SAJeurkar /TE(Mech) /NKOCET Solapur


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Definition of Heat TreatmentThese are controlled heating and cooling operations to change

mechanical properties of Steels and alloy Steels in solid state.

All heat treatments are carried out below solidus temp.

Heat treatment Concept

Steels are heated above upper critical temperature, they

are held at this temp. for sufficient length of time and then

cooled in proper medium to obtained required phase

changes and micro structure.

Austenite phase is converted into Pearlite/ Martensite /Bainite

phases depending upon cooling rates, therefore heat treatments

can be designed by changing method of cooling to obtain

desired mechanical properties.



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Tools used for heat treatment

1) Fe-Fe3C Diagram

2) T-T-T diagram

3) C-C-T diagram

3) Cooling curves

Objectives of Heat Treatment

1. To Improve Ductility

2. To relieve internal stresses

3. To harden and strengthen metals and alloys

4. To refine grain size

5. To improve machinability

6. To improve electrical and magnetic properties



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General Heat Treatment Cycle

Heating/Austenitising



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Fe-Fe3C Diagram and cooling curves



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Austenite

Fine PearliteMartensite

Coarse Pearlite

Bainite

Fast Cooling

Air cooling

Normalizing

Intermediate

Cooling

Slow Cooling

Reheating



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Austenite (Parent Phase)

PearliteBainite

Martensite

Where Fe3C nucleate

First, followed by ferrite

1)Coarse Pearlite where

interlammellar spacing

is large

2)Fine Pearlite where

interlammellarspacing is less.

Where ferrite nucleate

first followed by Fe3C

1)Upper Bainite which is

feathery type.

Where

austenite(FCC)

is converted into BCT

martensite

1)Lath

2)Plate

3)Needle2)Lower Bainite

which is acicular

type

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Part of the equilibrium phase diagram for the Fe-C system



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How to plot T-T-T Diagram

1. Take small sized 25mm 25 mm samples of say 0.8%C eutectoid steels about 10 numbers

2. Heat these samples in furnace at A1+ 50 0C i.e.. 773 0C

3. Hold at this temperature so that surface and core temperature become same.

4. Quickly transfer these samples to a furnace held at 722 0C5. Now hold the first sample at 722 0C for 1 sec and then Quench it in water.

6. Hold second sample at 722 0C for 5 sec and then Quench it in water.

7. Hold third sample at 722 0C for 10 sec and then Quench it in water.

8. Hold fourth sample at 722 0C for 60 sec and then Quench it in water.

9. Hold remaining samples at 0C for various longer times and Quench it in water.

10. Now cut the samples, polish them and observe under metallurgical microscope forpearlite and martensite transformation.

11. Repeat the procedure for 721 0C ,720, 0C - - - - till room temperature.

12. Record the time required to start Pearlite transformation and end of Pearlite

transformation.

13. Plot a graph of % transformation on Y-axis and Time on X-axis for each sample for given

temp (say 722 0C) Collect this information for all temp. 721,720,719,- - - till room

temp.

14. Now plot on Y-axis Temperature and on X-axis time scale. Transfer above information

i.e.. time to start pearlite transformation Ps and time to finish pearlite transformation

Pf for all temperatures. Join all start points to get Start Curve and Join all end points

to get Finish Curve.9Prof.SAJeurkar /TE(Mech) /NKOCET Solapur


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Take small 25mm 25mm

size samples of eutectoid

steels

Heat these samples

at A1+50 0Ci.e. 773 0C

Quickly Transfer the

samples to a salt bath

furnace held at 722 0C(below A1 0C.)

Quench in

water bath.

Polish, etch and

observe microstructure

1 2

1

1

1

1

2

2

2

2

3

3

3

3

3

4

4

4

4

4

5

5

5

5

5

Remove

after 5 sec

Remove

after 30 sec

Remove

after 5 min

Remove

after 20 min

Remove

after 1hr



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Temp.

Time in Log Scale

Coarse pearlite

Fine pearlite

Upper Bainite

Lower Bainite

Stable Austenite

Unstable austenite

Unstable Austenite +Martensite

Martensite

A1

Ms

Mf

Ps Pf

Bs

Critical

Cooling

Rate CCR

T-T-T Diagram For Eutectoid Steel

Nose

Bf



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TTTdiagramquenching cycle



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Normalizing Heat Treatment

The Component is Heated to austenitising

temperature(A3/A1/Acm+50 0C for

Hypoeutectoid/Eutectoid/Hypereutectoid

steels.

It is held at this temp. for sufficient length of

time to equalize surface and core temp.

(usually 3-5 min per mm of cross sectionthickness)

Then it is cooled in Air.



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Temp.

% carbon



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Objectives/Purpose of Normalizing

To produce a harder and stronger steel than fullannealing.

To produce fine Pearlite

To improve the machinability.

To modify and refine the grain structure.

To obtain a relatively good ductility without reducing thehardness and strength.

To homogenize microstructure as in the case of casting.



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Applications of Normalizing H.T.

1. Steel castings

2. Steel Forgings

3. Shafts, Spindles, Round

bars.

4. Pins, Cotters, Sheets,Plates, Rolled Bars



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Annealing Heat Treatment

The Component is Heated to austenitising

temperature(A3/A1/A1+50 0C for

Hypoeutectoid/Eutectoid/Hypereutectoid

steels).

It is held at this temp. for sufficient length of

time to equalize surface and core temp. (usually

3-5 min per mm of cross section thickness)

Then it is cooled in Furnace.



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A1+50 degree



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Objectives/Purpose of Annealing

To relieve internal stresses generated in previous manufacturing likerolling, forging, casting.

To improve machinability.

To produce Coarse Pearlite.

To Soften the steel.

To refine & homogenize the structure.

To remove gases.

To improve ductility/malleability required for further processing.

To make the steel suitable for next heat treatment.



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Types of Annealing Processes

1) Full annealing

2) Process annealing

3) Stress relief annealing

4) Spherodising



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Full annealing

Procedure

1) Heat hypo eutectoid/eutectoid/hypereutectoid steels above

A3/A1/A1+50 0C.

2) Hold/soak according to section thickness

3) Cool in the furnace slowly at a rate 30-200 0C per hour.

Objectives

1) To obtain fine ferrite and pearlite mixture

2) To obtain fine cementite and pearlite mixture.3) To increase machinability of steels.

4) To reduce strength and hardness

Applications

Rolled bars,sheets,forgings and castings



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Process annealing

Procedure

1) Cold worked steels are heated above its recrystallization

temp(600 degree C). That is below A1 temp.


3) Cool in the air slowly at a rate 30-200 0C per hour.

Objective

1) It removes the effect of cold working

2) It soften the steels and make the steel suitable for

further cold working.

ApplicationsSheets, wires ,rods, plates manufactured by various cold

working operations.



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Stress relief annealing

Procedure

1) Cold worked steels are heated to a temp between 500-5502) Hold/soak according to section thickness for 1 to 2 hrs.

3) Cool in the air.

Objectives1) To relieve internal or residual stresses.

2) To remove warpage of steels

3) To eliminate the chances of corrosion.

ApplicationsRolled,extruded,welded and cast component of

ferrous and nonferrous alloys.



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Spherodising

Procedure

1) Only hypereutectoid steels are spherodised.

2) Heat below A1 temp(between 650 and 700 0C)3) Heating and cooling alternately at temps just above and just below A1

temp.

4) Cooling very slowly in the furnace.

Objective

1) To break pearlite and cementite network

of hypereutectoid steels which is hard and brittle.

2) To convert lamellar cementite into globular

cementite to improve machinability of steel.

Application

1) Hypereutectoid steel shafts, bars.

2) All high carbon cutting tools.

3) High Carbon ball bearings.



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Hardening/Quenching

1) Austenitise above A3/A1/A1 +50 0C for

Hypo eutectoid/Eutectoid/Hypereutectoid steels2) Hold/soak according to section thickness

3) Cool with required cooling rate in a suitable quenching medium(e.g.

Water, Oil, Brine solution) exceeding CCR of given steel.



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Objectives/Purpose of Hardening

To increase Hardness of steel.

To improve Wear resistance of Steel.

To increase Strength of steel.

To obtain Martensite phase in steel.

To improve cutting ability of steel required for Tool Steels.



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http://localhost/var/www/apps/conversion/tmp/scratch_2/Martensitic%20transformation%20-%20YouTube%20[freecorder.com].flv


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Types of Hardening

1. Conventional hardening

2. Two media quenching.

3. Stepped Quenching/ Marquenching / Martempering

4. Austempering

5. Hardening with self Tempering



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Conventional Hardening

Procedure

1) Heat hypoeutectoid/eutectoid/hypereutectoid

steels above A3/A1/A1+50 0C


3) Cool in water or Brine solution.

Objective/Purpose

1) Hardening of Medium Carbon steels

2) Commonly used heat treatment.

Applications

Chisels, daggers, plough shave



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Two media Quenching

Procedure

1) Heat hypoeutectoid/eutectoid

/hypereutectoid steels above A3/A1/A1+500C


3) Cool in water initially then cool in oil.

Objective/Purpose

1) Hardening of Hupoeutectoid steels

2) To reduce distortion, cracking

Applications

Taps, dies, milling cutters, Ball and roller bearings



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Martempering

Procedure


/hypereutectoid steels aboveA3/A1/A1+50 0C

2) Hold/soak according to section

thickness

3) Cool in water below the nose in

water, Hold isothermally to

equalize surface & core temp.4) Then cool again so that surface

and core cool at the same time.

Objective/Purpose

1) Hardening of High carbon & Low alloy steels

2) To reduce distortion, cracking3) Chances of quench cracks are eliminated.

Applications

High carbon low alloy steels which are thick in cross sections.



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Austempering

Procedure


/hypereutectoid steels above

A3/A1/A1+50 0C2) Hold/soak according to section thickness

3) Cool in water below the nose in water,

Hold isothermally till Bs and Bfcurves are

Cut i.e.. Bainite transformation is

completed.

4) Then cool in air.

Objective/Purpose

1) The main purpose of this heat treatment is to obtain Bainite and not Martensite.

2) Properties of lower Bainite are similar like tempered martensite.

3) To obtain high hardness and toughness.

Applications

0.3 to0.5% c steels Heavy duty structural parts, springs, lock washers, screws, pins ,

needles, cultivator shovels



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Hardening with self tempering

In this method hardening and tempering are combined. The component is taken outfrom the quenching bath ,when it have some heat in it i.e.at @200-250 0C

This heat come out from the core of the job and start heating Martensite which is

formed during quenching. This heat convert Martensite into Bainite.(Self Tempering)

It is used for chisels, sledge hammers, centre punches, shafts, collars, gears which

require high surface hardness. The disadvantage is, no close control on depth ofhardness



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Various Types ofFurnaces Used in Heat

Treatments

http://localhost/var/www/apps/conversion/tmp/scratch_2/IndustrialFurnaceshttp://localhost/var/www/apps/conversion/tmp/scratch_2/IndustrialFurnaces


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Concept of Hardenability

Hardenability is the capability of an alloy steel to form martensite as a result

of a given heat treatment.

High hardenability in a steel means that the steel forms martensite notonly at surface but to a large degree throughout the interior.

Hardenability more related to depth of hardness of a steel upon heat treat.

The depth of hardening in a plain carbon steel may be 2-3 mm vs. 50

mm in an alloy steelHardenability is not hardness. It is a qualitative measure of the rate at

which hardness decreases with distance from the surface because ofdecreased martensite content.

Hardenability is measured by the Jominy end-quench test, performed for

standard cylindrical specimen, standard austenitization conditions, and

standard quenching conditions (jet of water at specific flow rate and

temperature).


The Jominy End Quench Test measures Hardenability of steels Hardenability is a measure of


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The Jominy End Quench Test measures Hardenability of steels. Hardenability is a measure of

the capacity of a steel to be hardened in depth when quenched from its austenitizing

temperature. Hardenability of a steel should not be confused with the hardness of a steel.

The Hardness of a steel refers to its ability to resist deformation when a load is applied,

whereas hardenability refers to its ability to be hardened to a particular depth under a

particular set of conditions. Information gained from this test is necessary in selecting theproper combination of alloy steel and heat treatment to minimize thermal stresses and

distortion when manufacturing components of various sizes

To perform the Jominy Test: First, a sample specimen cylinder either 100mm in length and

25mm in diameter, or alternatively, 102mm by 25.4mm is obtained. Second, the steel sample

is normalized to eliminate differences in microstructure due to previous forging, and then it

is austenitised. This is usually at a temperature of 800 to 900C. Next, the specimen is rapidlytransferred to the test machine, where it is held vertically and sprayed with a controlled flow

of water onto one end of the sample. This cools the specimen from one end, simulating the

effect of quenching a larger steel component in water. Because the cooling rate decreases as

one moves further from the quenched end, you can measure the effects of a wide range of

cooling rates from vary rapid at the quenched end to air cooled at the far end.

Next, the specimen is ground flat along its length to a depth of .38mm (15 thousandths of

an inch) to remove decarburized material. The hardness is measured at intervals along its

length beginning at the quenched end. For alloyed steels an interval of 1.5mm is commonly

used where as with carbon steels an interval of .75mm is typically employed. And finally the

Rockwell or Vickers hardness values are plotted versus distance from the quenched end.



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JOMINY.flv


/
http://localhost/var/www/apps/conversion/tmp/scratch_2/JOMINY.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_2/JOMINY.flv


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Quenching Baths/Quenching Media

Number Quenching

bath

Characteristics Drawbacks Applications

1 Water Low cost, abundantly available, easy handling,

No pollution/ disposal problem,

High thermal stresses,

Corrosion & scaling, form

steam

PCS,

alloy steels

2 Aqueous

solutions

Water+NaCl or

Water+CaCl2 mixtures

called as brine solutions

Water+NaOH mixtures

called as caustic solutions

high cooling rates, low distortion,

High cost, Corrosion &

Disposal

Problem, affect

Human skin,

High labour

cost

All types of

PCS, alloy

steels

3 Oils Paraffin based mineral oil

Low cooling rates, low distortion and warpage,

Uniform cooling,

High cost,

Chances of

fire

All types of PCS, alloy

Steels with Large cross

sections

4 Air abundantly available, easy handling,

No pollution/ disposal problem, low distortion

and warpage,

Suitable only For air

Hardening steels

alloy Steels with

Large cross sections

5 Salt baths 50%KNO3+50%NaNO3

50%NaNO3+50%KNO2

20%NaOH+80%KOH

Uniform cooling, free from Oxidation ,carburization,

Suitable for selective hardening

High cost, Corrosion &

Disposal

Problem, affect

Human skin,

High labour

Cost.

All types of

PCS, alloy

Steels with

Large and

Thin cross

Sections.


Tempering


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Martensite is very hard and brittle. Tempering is applied to hardened steel to reduce

brittleness, increase ductility, and toughness and relieve stresses in martensite structure.

This process increases ductility and toughness but also reduces hardness, strength and

wear resistance marginally. Increase in tempering temperature lowers the hardness.

In this process, the steel is heated to lower critical temperature keeping it there for

about one hour and then cooled slowly at prescribed rate.

Tempering



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Tempering

Low temp. Tempering (100-250 0C)

High C Martensite -carbide+ Low C martensite

(BCT) (HCP)

-carbide is similar like Cementite (Fe3C ) but its chemical formula is(Fe2.4C)This mixture has high strength, hardness, low ductility and toughness.

Used for low alloy steels, cutting and measuring tools.

Medium temp. Tempering(250-500 0C)

-carbide + Retained austenite Fe3C + Ferrite + Bainite

(HCP) (Orthorhombic) (BCC) (Ferrite+ -carbide)

This mixture has High toughness, ductility, High Yield point

Used for Coil, laminated, Leaf Springs.

High temp. Tempering (500-700 0C)

Low C martensite Ferrite(BCC) (BCC)

This mixture has High toughness.

Used for connecting rods, shafts, spindles, crankshafts



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Selective Heat Treatments(Surface Hardening)

Introduction

Carbon steels that have minimum carbon content of 0.4%, or alloy steels

with a lower carbon content can be selectively hardened in specific regions

by applying heat and quench only to those regions. Parts that benefit by

flame hardening include gear teeth. These techniques are best suited formedium carbon steels with a carbon content ranging from 0.4 to 0.6%.

Common Selective Hardening Processes include:

1. Flame Hardening

2. Induction Hardening



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Flame hardening

Flame HardeningA high intensity oxy-acetylene flame is applied to the selective region.

The temperature is raised high enough to be in the region of Austenitetransformation.

The "right" temperature is determined by the operator based on

experience by watching the color of the steel. The overall heattransfer is limited by the torch and thus the interior never reachesthe high temperature. The heated region is quenched to achievethe desired hardness. Tempering can be done to eliminatebrittleness.

The depth of hardening can be increased by increasing the heatingtime. As much as 6.3 mm of depth can be achieved. In addition,large parts, which will not normally fit in a furnace, can be heat-treated. The image on the left shows a large gear where the teethare being



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Flame hardening



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Induction Hardening

In Induction hardening, the steel part is

placed inside a electrical coil which has

alternating current through it. This energizes

the steel part and heats it up. Depending onthe frequency and amperage, the rate of

heating as well as the depth of heating can be

controlled. Hence, this is well suited forsurface heat treatment. The details of heat

treatment are similar to flame hardening.



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Induction Hardening



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Case Hardening

The Carbon content in the steel determines whether it can be directlyhardened. If the Carbon content is low (less than 0.25% for example) then

an alternate means exists to increase the Carbon content of the surface.

The part then can be heat-treated by either quenching in liquid or cooling in

still air depending on the properties desired. Note that this method will only

allow hardening on the surface, but not in the core, because the high carbon

content is only on the surface. This is sometimes very desirable because it

allows for a hard surface with good wear properties (as on gear teeth or

knife), but has a tough core that will perform well under impact loading.

It is possible to add additional carbon to the outer surface of a component

low in carbon. This carbon casecanthenbe heat treated in the normal waygiving a hard outer skin and a soft core. This process is known as case

hardening. Adding carbon in a process is known as carburizing.



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Case Hardening: Carburizing

Carburizing is a process of adding Carbon to the surface.

This is done by exposing the part to a Carbon rich atmosphere at an

elevated temperature(950-1000 degree Centigrade)and allows diffusion to

transfer the Carbon atoms into steel. This diffusion will work only if the steel

has low carbon content, because diffusion works on the differential of

concentration principle. If, for example the steel had high carbon content

to begin with, and is heated in a carbon free furnace, such as air, the carbon

will tend to diffuse out of the steel resulting in decarburization.

There are a number of different ways to carburize or add carbon to the

surface of a metal. We will briefly discuss three methods:

The pack method

Gas Carburizing

Liquid Carburizing



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The Pack Method

The component is placed in a box surrounded by a carbon rich materialand placed in a furnace at 920 degree C. The carbon is absorbed into

the austenite. The depth of penetration of carbon into the surface

depends on the temperature and the time spent in the furnace. To

carburize to a depth of 1mm the component will need to be left in

the furnace for up to 12 hours. After cooling, the component is immersedin a bath of molten salt. The salt is kept at a temperature of 780 C.

The component remains immersed for a half an hour and it is then

quenched in water. The salt used in this process will not add carbon

or remove carbon from the surface. It is a neutral salt.

The molten salt is used because it provides uniform heating over the entiresurface of a complicated shape.



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Gas Carburizing and Liquid Carburizing

Gas Carburizing:Gas Carburizing is conceptually the same as pack carburizing, except that

Carbon Monoxide (CO) gas is supplied to a heated furnace. The carbon-rich

gas circulates within the furnace and around the component/part which is to

be hardened. The deposition of carbon takes place on the surface of the

part. This is a faster method of carburizing than the pack method and

greater control over the process is possible.Liquid Carburizing:

The steel parts are immersed in a molten carbon rich bath. In the past, such

baths have cyanide (CN) as the main component. This process produces a

thin, hard shell that is harder than the one produced by other carburizing

methods, and can be completed in 20 to 30 minutes compared to several

hours so the parts have less opportunity to become distorted. It is typically

used on small parts such as bolts, nuts, screws and small gears. The major

drawback of cyaniding is that cyanide salts are poisonous. Therefore, safety

concerns have led to non-toxic baths that achieve the same result.



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Nitriding

Nitriding involves the diffusion of nitrogen into the

surface layers of a low carbon steel at elevated

temperature. The formation of nitrides in the

nitrided layer provides the increased hardness.Nitriding is typically carried out in the temperature

range of 500 - 575C, this is in the ferritic state rather

than the austenitic used for carburising. This is

possible since ferrite has a much higher solubility fornitrogen than it does for carbon.



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Carbo-nitriding

Carbo-nitriding is a variation of carburising where both carbon and

nitrogen bearing species are used in the gaseous state, usually theinclusion of ammonia in with the carburising gas mixture. Carbo-nitriding

is carried out in the austenite state, i.e.. temperatures above 850C,

typically 870C. The case depths are typically lower than those achieved

by carburising alone however the surface hardness levels can be higher

Heat Treatment1

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