Key Concepts in Welding Engineering by R. Scott Funderburk Postweld Heat Treatment What is PWHT? it cools. After welding has been com- Postweld heat treatment (PWHT), pleted, the steel must not be allowed defined as any heat treatment after to cool to room temperature; instead, it welding, is often used to improve the should be immediately heated from properties of a weldment. In concept, the interpass temperature to the post Figure 1. Criteria for hydrogen PWHT can encompass many different heat temperature and held at this tem- induced cracking (HIC). potential treatments; however, in steel perature for some minimum amount of fabrication, the two most common pro- time. Although various code and ser- cedures used are post heating and required for critical repairs, such as stress relieving. The need for post those defined under the Fracture heating assumes a Control Plan (FCP) for Nonredundant When is it Required? potential hydrogen Members of the AASHTO/AWS D1.5 Bridge Welding Code. The FCP provi- The need for PWHT is driven by code cracking problem sion is 450 to 600°F (230 to 315°C) for and application requirements, as well exists... “not less than one hour for each inch as the service environment. In gener- (25 mm) of weld thickness, or two al, when PWHT is required, the goal is vice requirements can dictate a variety hours, whichever is less.” When it is to increase the resistance to brittle of temperatures and hold times, 450°F essential that nothing go wrong, post fracture and relaxing residual stresses. (230°C) is a common post heating heating can be used as insurance Other desired results from PWHT may temperature to be maintained for 1 against hydrogen cracking. However, include hardness reduction, and mate- hour per inch (25 mm) of thickness. when the causes of hydrogen cracking rial strength enhancements. are not present, post heating is not Post heating is not necessary for most necessary, and unjustifiable costs may Post Heating applications. The need for post heat- result if it is done. ing assumes a potential hydrogen Post heating is used to minimize the cracking problem exists due to a sen- Stress Relieving potential for hydrogen induced crack- sitive base metal microstructure, high ing (HIC). For HIC to occur, the follow- levels of hydrogen, and/or high stress- Stress relief heat treatment is used to ing variables must be present (see es. Post heating, however, may be a reduce the stresses that remain locked Figure 1): a sensitive microstructure, code requirement. For example, in a structure as a consequence of a sufficient level of hydrogen, or a high ASME Section III and the National manufacturing processes. There are level of stress (e.g., as a result of Board Inspection Code (NBIC) both many sources of residual stresses, and highly constrained connections). In have such provisions. The Section III those due to welding are of a magni- ferritic steels, hydrogen embrittlement requirement for P-No. 1 materials is tude roughly equal to the yield strength only occurs at temperatures close to 450 to 550°F (230 to 290°C) for a min- of the base material. Uniformly heating the ambient temperature. Therefore, it imum of 2 hours, while the NBIC a structure to a sufficiently high tem- is possible to avoid cracking in a sus- requirement is 500 to 550°F (260 to perature, but below the lower transfor- ceptible microstructure by diffusing 290°C) for a minimum of 2 hours. mation temperature range, and then hydrogen from the welded area before Furthermore, post heating is often uniformly cooling it, can relax these Welding Innovation Vol. XV, No. 2, 1998 residual stresses. Carbon steels are Other Considerations The filler metal composition is also typically held at 1,100 to 1,250°F (600 When determining whether or not to important. After heat treatment, the to 675°C) for 1 hour per inch (25 mm) postweld heat treat, the alloying sys- properties of the deposited weld can of thickness. tem and previous heat treatment of the be considerably different than the “as base metal must be considered. The welded” properties. For example, an Stress relieving offers several benefits. properties of quenched and tempered E7018 deposit may have a tensile For example, when a component with alloy steels, for instance, can be strength of 75 ksi (500 MPa) in the “as high residual stresses is machined, adversely affected by PWHT if the welded” condition. However, after the material tends to move during the temperature exceeds the tempering stress relieving, it may have a tensile metal removal operation as the stress- temperature of the base metal. Stress strength of only 65 ksi (450 MPa). es are redistributed. After stress relief cracking, where the component Therefore, the stress relieved proper- relieving, however, greater dimensional fractures during the heating process, ties of the weld metal, as well as the stability is maintained during machin- can also occur. In contrast, there are base metal, should be evaluated. ing, providing for increased dimension- some materials that almost always Electrodes containing chromium and al reliability. require PWHT. For example, chrome- molybdenum, such as E8018-B2 and E9018-B3, are classified according to In addition, the potential for stress cor- When determining the AWS A5.5 filler metal specification rosion cracking is reduced, and the in the stress relieved condition. The metallurgical structure can be whether or not to E8018-B2 classification, for example, improved through stress relieving. The PWHT, the alloying has a required tensile strength of 80 steel becomes softer and more ductile system and previous ksi (550 MPa) minimum after stress through the precipitation of iron car- relieving at 1,275°F (690°C) for 1 hour. bide at temperatures associated with heat treatment of the In the “as welded” condition, however, stress relieving. base metal must be the tensile strength may be as high as 120 ksi (825 MPa). Finally, the chances for hydrogen considered induced cracking (HIC) are reduced, The objective of this article is to intro- although this benefit should not be the molybdenum steels usually need duce the fundamentals of postweld only reason for stress relieving. At the stress relieving in the 1,250 to 1,300°F heat treatment; it is not meant to be elevated temperatures associated with (675 to 700°C) temperature range. used as a design or fabrication guide. stress relieving, hydrogen often will Thus, the specific application and steel For specific recommendations, consult migrate from the weld metal and the must be considered when determining the filler metal manufacturer and/or the heat affected zone. However, as dis- the need, the temperature and time of steel producer. cussed previously, HIC can be mini- treatment if applied, and other details mized by heating at temperatures regarding PWHT. For Further Reading lower than stress relieving tempera- tures, resulting in lower PWHT costs. ASM Handbook, Volume 6 – Welding, Brazing, and Soldering. American Society for Metals, 1993. Bailey, N. Weldability of Ferritic Steels. ASM International/Abington Publishing, 1994. Evans, G.M. and Bailey, N. Metallurgy of Basic Weld Metal. Abington Publishing, 1997. Metals Handbook, Volume 4 – Heat Treating. 9th Edition. American Society for Metals, 1981. Figure 2. Post heat applied immediately after last pass. Welding Innovation Vol. XV, No. 2, 1998.