POSTWELD HEAT TREATMENT

Table of Contents Introduction ............................................................................................................................................ 2

Principles of Heat Treatment ................................................................................................................... 5

Post Weld Heat Treatment ...................................................................................................................... 7

Thermal Stress Relief ............................................................................................................................... 8

Fundamentals of PREHEAT .................................................................................................................... 11

POSTWELD HEAT TREATMENT ............................................................................................................... 15

The post and pre heating of the Residual Stresses .................................................................................. 17

Conclusion ............................................................................................................................................ 18

References ............................................................................................................................................ 19

Introduction

Steel is a very important component in today’s world. When Steel passes through the welding

zone and through the heating process all the heated portions synchronize into the microstructure

that is quite different from the base metal that is often known as HAZ, or in other words we can

also state it as Heat affected Zone. During the process of rapid cooling and heating that takes

place in several areas that produces the cycle of thermal zone near the line region. The thermal

cycle can easily cause the uniform heating and then cools the material that is within the heat

zone. The above process helps in generating harder heat affected zone that includes cracking

susceptibility in the base and the weld metal. These residual contraction of metals move along

with the length of the weld that is prevented from all the large adjacent cold body of the metal.

The residual tensile stresses are prepared along with the weld of the heating zone. All the

properties of the weld causes more problems that are often related with the base metal properties

and in many other cases all the performance of the above mentioned cases are also governed by

them. The post welding heat treatment machine can be more descriptively understood from the

figure given below.

The cubical structure of the Heat affected Zone (HAZ)

Post Weld Heat Treatment

All the problems in relation to the process is quite helpful for the process of production and to

also get rid of the problems that are caused due to some exclusive preheating process that takes

place after Welding and hence with the help of that process the Post Weld Heat Treatment can be

easily employed. The effective pre heating and post heating primary techniques can easily accept

all the heat affected properties that have the least potential for the hydrogen component that

creates the hydrogen induced cracking. These welding process is often used in bridges, ships and

pressure vessels that are used in several industrial machines and automobile stocks.

Principles of Heat Treatment

The heat treatment can be easily defined as the series of sequence process of cooling and heating

in respect to the desired combination that is associated with the properties of steel and the

changes in the properties of steel after the process of heat treatment is because of the structural

change and the and the transformation of phase that occurs during the treatment process and all

the factors that can determine the controlling structures which are known as the principle of heat

treatment process and at the same time is important for the principles of heat treatment procedure

that are explained in the points below :

1. Transformation Phase during the process of heating

2. Effect of controlling rate and the process of structural change that takes place during

cooling process.

3. The effect of carbon content and the process of alloying the elements that are used in Post

Weld Heat Treatment.

4. All the factors that constitute the structural changes that take place in the heating process.

The graph below explains the elementary process of heat treatment that takes place in Post Weld

Heat Treatment

Graph reflecting process of Post Weld Heat Treatment

The elements and their proportionality of heat emission

Post Weld Heat Treatment

The residual stress has high level occurrence level that is due to the process of weldment that

restrains the parent metal during the solidification of weld that stresses highly on the yield of the

material strength that are highly stressful and can yield high strength of material that easily

combines with the normal stress level that may exceed the design stress. The removal of all the

residual stress takes place because of the thermal effect and energy that is received by the metals

that allows the boundary of grain sliding from the defects such as vacancies, dislocation and

slip planes. The most significant prospect of Post Weld Heat Treatment is to prevent the system

from Brittle Fracture. The Post weld heat treatment helps the entire system to harden and soften

the zones that can make all the matches quite easy for the entire system. The removal of the

residual stresses can become quite necessary to achieve dimensional stability. The heat treatment

process consists the annealing solution and the stress relief factor that depends on all the

requirement that are important for the Post Weld Heat Treatment.

The Process of Post Weld Heat Treatment

Thermal Stress Relief

The result of the residual stress that is obtained from different welding process that reduces the

thermal post weld stress that provides a relief for the heat treatment. The residual stress that are

remaining in the material after the stress relief treatment that will be entirely depending on the

rate of cooling . The percentage of internal stress will be completely independent on the type of

steel that is the composition of the yield strength that is the effect of time varying the temper that

can be easily understood from the graph that is stated below.

The Thermal Stress Relief in PWHT

The temperature often reaches the optimum level during the stress relief statement that is quite

far from the greater effect that reveals all the stress that is quite stressful in the length of time and

has the specific specimen for the temperature that is most suitable for the entire process. The

more closer temperature will result in the re crystallization or the critical temperature subsequent

that will be more effective in removing the residual stresses. The thermal stress relief is the

treatment that can be easily employed to reduce the residual stresses that have many important

properties which are taken into account in this process of heating. The microstructure leaves a

tensile impact on the properties of heating that are affected by the stress relief treatment. The A

post weld that has the heat treatment at 1040 degree Celsius to 1090 degree Celsius that contains

the sphereoidizes, the ferrite that are present in different weldment that significantly reduces the

formation of sigma and the treatment also takes some of the ferrite solutions into the process that

has an intermediate and can easily hold at 595 degree Celsius and is often recommended for the

weldment that are used to relieve the residual stresses and at the same time reduces the

susceptibility for cracking all the post during the weld heat treatment. The heat affected zone is

on the verge of vicinity of the welded joints that are aided for consideration and the post welded

joints that are aided for consideration by the process of post weld heat treatments and the

properties that are in their specific zones are later improved with the help of reduction of residual

stresses that is together with all the metallurgical changes that takes place in the process of helps

in increasing the carbon content and the alloy content that are cross sectional in the thickness

part. The temperature is often recommended for relieving the low carbon steels that are between

595 degree Celsius and 675 Celsius. The process takes one hour per inch of the thickness to uses

it in the basis of the compound to determine the length of the desired temperature that are present

in larger periods and are also required at the temperature scale of 595 in order to achieve the

Graph Trending the Thermal stress Relief

same degree level of stress that is relieved at 675 degree Celsius. The thermal stress relief can be

easily conducted for the furnace that is suitable for the heating process in the entire weldment.

The same portions of weldment is heated only if the structure is uniformly cross sectional and are

unheated at most of the ends that are free to move and not to restrict the entire expansion of the

parts that are heated.

The Heat pattern and the Microstructure state of PWHT

The heating and the cooling process must be in a gradual form along with the rate that will make

sure that there is a uniform temperature across the wall full of thickness. In general the greater

difference between the maximum and the minimum thickness that should be a slower process as

the rate of temperature is also slow. The variables factors such as the strength characteristics of

the geometry and the metal of the parts are not possible to recommend the formula that is for

maximum heating and for cooling as well .

Mechanical properties of the components used in the PWHT process.

Fundamentals of PREHEAT

The heating operation of the metals are pre determined temperature before engaging into some

welding process that is primarily known as preheating. The different details and the modes that

are quite different in all the different situations that are basically done for the general purpose

and at the same time it greatly influences all the cooling behavior that takes place after welding

process, so that the level of shrinkage can be easily lowered and the cooling rate can be easily

milder. The pre heating prepares the metal that makes it more receptive in terms of welding and

understand the importance of preheating that increases the thickness of the base metal that is just

because of the rapid quench capability that is along with the rigidity of the welded structure

which is obtained from the derived constraints . Generally the higher amount of preheat

temperature lower the total heat input and satisfies the conditions that are more favorable for

resisting the marten site formulization that increasing the hardness of the compound and

contributes high quality welds. The precautions that hardens the steels must be preheated

properly in order to decrease the cooling rate that takes place after the welding process. For a

specific steel the requirement of the preheating process, requirements, and the critical evaluation

of the preheating temperature can be easily selected according to the percentage of carbon that is

present in steel.

Fundamental Of Preheat

Graville Weldability Diagram

Relationship among carbon equivalent and the critical preheat temperature

From the above tables we can easily recommend the interpass and the preheat temperature that is

completely based on chemistry of the base metals and also the thickness of the elements that are

structured.

The minimum temperature of the preheating process can be easily assured by avoiding all the

cracking that completely depends on the factors that are mentioned below:

The conditions for the base metal in respect to welding.

The different constraint level.

The available Hydrogen risk

Thickness of the base material

Carbon equivalence that expresses the content of carbon and other alloys.

The rapid heating and cooling process is one the most significant characteristics of welding that

produces a hard microstructure within the HAZ. The tough HAZ microstructure is one of the

factors that are responsible for the deterioration of the welds. The heat affected zone is often

cooled with different rates that includes all the different regions of microstructure that are

considered as the source of failure in the weak welded joints.

POSTWELD HEAT TREATMENT

The post weld heat treatment is the procedure that is used to influence the structure of the

properties that are obtained in different weld and in all the heat affected zones. The

implementation of the proper provisions are important for welding the cooling retard in the

cooling rate after the Welding process. The functions of the PWHT is to measure the temper of

the weld metal and the HAZ that are used in order to reduce the hardness of the metal and at the

same time increases the toughness as well to decrease the residual stress that are associated with

the concept of welding.

The Post Weld Heat Treatment Process

The illustration of different single HAZ pass in reference to the Iron carbon equilibrium.

The available literature that is available on the concept of Post Heat Treatment can easily find

out that the recommendations are often dependent on different specific alloys that can filter the

metals that are involved in the above process and also for the restraint and the thickness of the

welded joints. The technique of post heating minimizes the hydrogen potential that is induced by

several cracking. The HIC provides three variables that must be present for a sensitive

microstructure that is quite sufficient for the hydrogen level and the necessity of the PWHT that

depends on the services and the materials that are involved in the process requirement.

The post and pre heating of the Residual Stresses

While going through the post pre welding het treatments are the properties of the mechanical

elements that represent the properties of the electronic beam that are in from of welded alloys

summarizes that the welded state of the existing samples are the ones that are above 80% ductile

and about 95% of the metals impacts the toughness of the base metals that have low temperature

along with stress relieving subsequent in respect to the welding operations that improves the

tensile properties of the residual stress and at the same time decreases the toughness of the fusion

department. The effect of the heat treatment that completely depends on the mechanical

characteristics are concluded to assess all the effects of the PWHT in respect to the mechanical

properties that have several properties such as tensile strength, hardness and the impact tests that

are employed on the welded joints that are under two different conditions which is the heat

treated condition and the welded condition. The results reflect the post weld heat treatment that

improves the toughness by 15% and figures out the tensile differences that has the effect of

reducing all the residual stress by 70%.

The Pre heating and Post Heating residual stress in Charts

Conclusion

The study of the above topic reviews the efficient post heat and pre heat that can be understood

with the help of primary means that are acceptable for the heat affected property zone that

provides the minimum potential for the elements that are hydrogen induced and has the

minimum cracking for all the residual stress that can be created in the heat zone. Many

researchers have concluded that the microstructure that are responsible for HAZ is quite

responsible for all the deterioration of the cold and the weld susceptibility that recommends all

the heat treatment that is necessary for several technical problems that are associated with the

conventional materials of steel and welding process that can be easily eliminated without the

post heating and the pre heating treatments that are applying with several welding process that

are consumable goods. Many of the researchers observes and increases the preheating the

PWHT microstructures that are welded goods that are specialized in all types of ferrous metals

while studying the effects of post and pre weld heating technique on the different mechanical

properties. Many researchers even said the all the residual stress increases the preheat

temperature and recommends that it is almost impossible to extract complete residual stress

among the welded structures by different experimental methods and these disadvantages can be

easily resolved by computational analysis technique.

References

Aloraier, R. Ibrahim, P. Thomson, FCAW process to avoid the use of post weld heat

treatment, International Journal of Pressure Vessels and Piping 83: 394–398 (2006)

A.G. Olabi and M.S.J. Hashmi ,The effect of post-weld heat-treatment on mechanical-

properties and residual-stresses mapping in welded structural steel, Journal of material

Processing Technology, Volume 55, issue 2,: 117-122 (1995)

An Article on Post Weld Heat Treatment (PWHT), May 2004, Issue No. 09 of Practical

Welding Letter., issues -3 :51-54(2007-08)

ASM Handbook: Welding, Brazing & Soldering Olson, David L. 9th Ed. Vol. 6, ASM

International,:1299 (1993)

Elia E.Levi, Welding advisors.com, welding alloy steels.

Enrico Armentani, Renato Esposito, Raffaele Sepe ,The influence of thermal properties

and preheating on residual stresses in welding, International Journal of Computational

Materials Science and Surface Engineering Volume 1, : 146 – 162 (2007)

G. Thomas, V. Ramachandra, R. Gamesman and R. Vasudevan ,Effect of pre- and post-

weld heat treatments on the mechanical properties of electron beam welded Ti-6Al-4V

alloy, Journal of material science, volume 28 :4892-4895(1993)

GHOSH P. K. ; GUPTA P. C. ; POTLURI N. B. ; GUPTA Yogesh ; Influence of pre and

post weld heating on weldability of modified 9Cr-1MoVNb steel plates under SMA and

GTA welding processes,Iron and Steel Institute of Japan, Tokyo, vol. 44, no7: 1201-1210

(2004)

Hisaki OKABAYASI and Ryochi KUME ,Effects of Pre-and Post- Heating on weld

cracking of 9Cr.-1 Mo-Nb-V Steel, Transaction of the Japan Welding

Society,Vol.19,No.2: (1988)

Ivan Hrivňák, ,A review of the metallurgy of heat treatment of welded joints,

International journal of Pressure vessels and piping, Welding Research Institute, 832 59

Bratislava, Czechoslovakia, Vol.20, Issue 3,: 223-237( 2003)

J. Schmidt, D. Pellkofer and E. Wei ,Alternative methods for post weld treatment of

austenitic pipe welds to increase the operational safety of BWR plants, nuclear energy

and science, volume -174,issue- 3, : 301-312 (1997)

Lee, S., Kim, B.C., Kwon, D., Correlation of microstructure and fracture properties in

weld heat affected zones of thermo mechanically controlled processed steels,

Metallurgical Transactions A,:(1992)

LIN Y. C.; LEE K. H., Effect of preheating on the residual stress in type 304 stainless

steel weldment, Journal of materials processing technology, vol. 63, no1-3: 797-801

(1997)

M Eroğlu, M Aksoy and N Orhan ,Effect of coarse initial grain size on microstructure

and mechanical properties of weld metal and HAZ of a low carbon steel, Material

Science and Engineering, Vol.259,Issue 1-2,: 59-66 (1999)

M. Pouranvari ,On the weldability of grey cast iron using nickel based filler metal,

Materials and Metallurgical Engineering Department, School of Engineering, Islamic

Azad University, Dezful Branch, Dezful, Iran. Tel.: +98 91 24075960; fax: +98 21

88522421.: (2010) R. N. S. Fassani; O. V. Trevisan, Analytical modeling of

multipass welding process with distributed heat source, Journal of the Brazilian Society

of Mechanical Sciences and Engineering, J. Braz. Soc. Mech. Sci. & Eng. vol.25 no.3

Rio de Janeiro : ( 2003)

M.S.J. Hashmi and A.G. Olabi ,Effects of post-weld heat-treatment soaking temperature

on the mechanical properties and residual stresses of a marten site stainless-steel welded

component, Journal of material Processing Technology, Volume 38, issue1-2 , : 387-398

(1993)

Mike Roth, Welders turn to induction heating for preheating, stress relieving, : (2001)

N. Yurioka, Comparison of Preheat Predictive Methods,IIW Doc. IX-2135-04, Nippon

Steel Corporation, Technical Development Bureau ,20-1, Shintomi, Futtsu-City, Chiba-

Pref. 293-8511 JAPAN

Nenad Ili, Milan T. Jovanovich, Mi a Todorovi , Milan Trtanj and Petar aponji

,Microstructural and Mechanical Characterization of Post weld Heat-Treated Thermite

Weld in Rails,Materials characterization, Vol.43, Issue 4,: 243-250(1999)

R. Scott Funderburk ,Key Concepts in Welding Engineering- fundamentals of Preheat,

Welding Innovation Vol. XIV, No. 2,: (1997)

R. Scott Funderburk ,Key Concepts in Welding Engineering- Postweld Heat treatment,

Welding Innovation Vol. XV, No. 2, : (1998)

S. Ravi, V. Balasubramanian and S. Nemat Nasser ,Influences of post weld heat

treatment on fatigue life prediction of strength mismatched HSLA steel welds, Centre of

Excellence for Advanced Materials, University of California at San Diego, La Jolla, CA

92093- 0416, USA, (2004)

Tadashi Kasuya, Nobutaka Yurioka,Makoto Okumura ,Methods for predicting maximum

hardness of Heat Affected Zone and selecting necessary Preheat temperature for Steel

Welding,Nippon Steel Technical Report No.65,: (1995)

Takamura, Method for TIG welding 1.25Cr-0.5 MO steel pipe for which preheating and

postheating treatments can be effectively omitted, United State Patent ,No. 5,435,858,:

(1995)

Teng, Tso-Liang, Chang, Peng-Hsiang, , A study of residual stresses in multi-pass girth-

butt welded pipes, International Journal of Pressure Vessels and Piping 74: 59–70 (1997)

The Procedure Hand Book of Arc Welding, fourteenth edition, The James F. Lincoln Arc

Welding.

Wiley VCH ,Tool Steels – A Guide for the Welding of Tools Steels, Advanced Materials

Journal, (2004)

Y.C. Lin, K.H. Lee ,Effect of preheating on the residual stress in type 304 stainless steel

weldment, Journal of Materials Processing Technology, Volume 63, Issues 1-3,: 797-

801(1997) C. P. Chou and Y. C. Lin ,A new technique for reducing the residual stress

induced by welding in type 304 stainless steel, Journal of materials processing

technology, vol. 48, issue 1-4,: 693-698(1995)