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Welding Innovation Vol. XV, No. 2, 1998

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

Stress relieving offers several benefits.For example, when a component with

high 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 be

adversely 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 1,300F(675 to 700C) 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 as

welded 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 1,275F (690C) 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 Basic Weld Metal. Abington Publishing, 1997.

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

When determiningwhether or not to

PWHT, the alloyingsystem and previousheat treatment of thebase metal must be

considered

Figure 2. Post heat applied immediately after last pass.