Welding Innovation Vol. XV, No. 2, 1998

What is PWHT?Postweld heat treatment (PWHT),defined as any heat treatment afterwelding, is often used to improve theproperties of a weldment. In concept,PWHT can encompass many differentpotential treatments; however, in steelfabrication, the two most common pro-cedures used are post heating andstress relieving .

When is it Required?The need for PWHT is driven by codeand application requirements, as wellas the service environment. In gener-al, when PWHT is required, the goal isto increase the resistance to brittlefracture and relaxing residual stresses.Other desired results from PWHT mayinclude hardness reduction, and mate-rial strength enhancements.

Post HeatingPost heating is used to minimize thepotential for hydrogen induced crack-ing (HIC). For HIC to occur, the follow-ing variables must be present (seeFigure 1): a sensitive microstructure,a sufficient level of hydrogen, or a highlevel of stress (e.g., as a result ofhighly constrained connections). Inferritic steels, hydrogen embrittlementonly occurs at temperatures close tothe ambient temperature. Therefore, itis possible to avoid cracking in a sus-ceptible microstructure by diffusinghydrogen from the welded area before

it cools. After welding has been com-pleted, the steel must not be allowedto cool to room temperature; instead, itshould be immediately heated fromthe interpass temperature to the postheat temperature and held at this tem-perature for some minimum amount oftime. Although various code and ser-

vice requirements can dictate a varietyof temperatures and hold times, 450°F(230°C) is a common post heatingtemperature to be maintained for 1hour per inch (25 mm) of thickness.

Post heating is not necessary for mostapplications. The need for post heat-ing assumes a potential hydrogencracking problem exists due to a sen-sitive base metal microstructure, highlevels of hydrogen, and/or high stress-es. Post heating, however, may be acode requirement. For example,ASME Section III and the NationalBoard Inspection Code (NBIC) bothhave such provisions. The Section IIIrequirement for P-No. 1 materials is450 to 550°F (230 to 290°C) for a min-imum of 2 hours, while the NBICrequirement is 500 to 550°F (260 to290°C) for a minimum of 2 hours.Furthermore, post heating is often

required for critical repairs, such asthose defined under the FractureControl Plan (FCP) for NonredundantMembers of the AASHTO/AWS D1.5Bridge Welding Code. The FCP provi-sion is 450 to 600°F (230 to 315°C) for“not less than one hour for each inch(25 mm) of weld thickness, or twohours, whichever is less.” When it isessential that nothing go wrong, postheating can be used as insuranceagainst hydrogen cracking. However,when the causes of hydrogen crackingare not present, post heating is notnecessary, and unjustifiable costs mayresult if it is done.

Stress RelievingStress relief heat treatment is used toreduce the stresses that remain lockedin a structure as a consequence ofmanufacturing processes. There aremany sources of residual stresses, andthose due to welding are of a magni-tude roughly equal to the yield strengthof the base material. Uniformly heatinga structure to a sufficiently high tem-perature, but below the lower transfor-mation temperature range, and thenuniformly cooling it, can relax these

Postweld Heat Treatment

Key Concepts in Welding Engineering

by R. Scott Funderburk

The need for postheating assumes apotential hydrogencracking problem

exists...

Figure 1. Criteria for hydrogeninduced cracking (HIC).

Welding Innovation Vol. XV, No. 2, 1998

residual stresses. Carbon steels aretypically held at 1,100 to 1,250°F (600to 675°C) for 1 hour per inch (25 mm)of thickness.

Stress relieving offers several benefits.For example, when a component withhigh residual stresses is machined,the material tends to move during themetal removal operation as the stress-es are redistributed. After stressrelieving, however, greater dimensionalstability is maintained during machin-ing, providing for increased dimension-al reliability.

In addition, the potential for stress cor-rosion cracking is reduced, and themetallurgical structure can beimproved through stress relieving. Thesteel becomes softer and more ductilethrough the precipitation of iron car-bide at temperatures associated withstress relieving.

Finally, the chances for hydrogeninduced cracking (HIC) are reduced,although this benefit should not be theonly reason for stress relieving. At theelevated temperatures associated withstress relieving, hydrogen often willmigrate from the weld metal and theheat affected zone. However, as dis-cussed previously, HIC can be mini-mized by heating at temperatureslower than stress relieving tempera-tures, resulting in lower PWHT costs.

Other ConsiderationsWhen determining whether or not topostweld heat treat, the alloying sys-tem and previous heat treatment of thebase metal must be considered. Theproperties of quenched and temperedalloy steels, for instance, can beadversely affected by PWHT if thetemperature exceeds the temperingtemperature of the base metal. Stressrelief cracking, where the componentfractures during the heating process,can also occur. In contrast, there aresome materials that almost alwaysrequire PWHT. For example, chrome-

molybdenum steels usually needstress relieving in the 1,250 to 1,300°F(675 to 700°C) temperature range.Thus, the specific application and steelmust be considered when determiningthe need, the temperature and time oftreatment if applied, and other detailsregarding PWHT.

The filler metal composition is alsoimportant. After heat treatment, theproperties of the deposited weld canbe considerably different than the “aswelded” properties. For example, anE7018 deposit may have a tensilestrength of 75 ksi (500 MPa) in the “aswelded” condition. However, afterstress relieving, it may have a tensilestrength of only 65 ksi (450 MPa).Therefore, the stress relieved proper-ties of the weld metal, as well as thebase metal, should be evaluated.Electrodes containing chromium andmolybdenum, such as E8018-B2 andE9018-B3, are classified according tothe AWS A5.5 filler metal specificationin the stress relieved condition. TheE8018-B2 classification, for example,has a required tensile strength of 80ksi (550 MPa) minimum after stressrelieving at 1,275°F (690°C) for 1 hour.In the “as welded” condition, however,the tensile strength may be as high as120 ksi (825 MPa).

The objective of this article is to intro-duce the fundamentals of postweldheat treatment; it is not meant to beused as a design or fabrication guide.For specific recommendations, consultthe filler metal manufacturer and/or thesteel producer.

For Further ReadingASM Handbook, Volume 6 – Welding, Brazing,

and Soldering. American Society for Metals,1993.

Bailey, N. Weldability of Ferritic Steels. ASMInternational/Abington Publishing, 1994.

Evans, G.M. and Bailey, N. Metallurgy of BasicWeld Metal. Abington Publishing, 1997.

Metals Handbook, Volume 4 – Heat Treating.9th Edition. American Society for Metals,1981.

When determiningwhether or not toPWHT, the alloying

system and previousheat treatment of thebase metal must be

considered

Figure 2. Post heat applied immediately after last pass.