Various codes and specifications require that welded structures such as pressure vessels and offshore platforms be post weld heat treated (PWHT), depending on the type and thickness of the welded joint. Post weld heat treatment reduces the effect of any stresses induced by the welding process and tempers the heat-affected zone. The PWHT may be performed several times on a structure during fabrication and after weld repairs, resulting in an accumulation of the total time at soaking temperature. In some instances, the original welds of the structure or vessel may be subjected to cumulative PWRT cycles which exceed the amount of time qualified for by the original welding procedure qualification tests. Because PWHT, in some instances, may result in the loss of both the heat-affected zone and weld metal strength and toughness,1 the mechanical properties of the weld-joint may deteriorate, as the vessel is repaired repeatedly. This may be undesirable, since it is not known whether the mechanical properties of a weldment are still acceptable.
Studies1,2 have been conducted to study the effect of a long PWHT on the properties of constructional and pressure vessel steel weldments. Post weld heat treatment may have a beneficial, detrimental or negligible effect on the properties (especially toughness) of the weidments, depending on the chemical composition of the steel, welding procedure used and PWHT time and temperature. The purpose of the current study is to provide detailed information on the effect of a long PWHT on the microstructure and mechanical properties of a welded joint in ASTM A302 Gr B pressure vessel steel. Weld-joints of the steel in the as-welded and post weld heat treated conditions were studied. The mechanical properties of the weldments were determined by heat-affected zone (HAZ) and weld metal Charpy impact and hardness tests, HAZ fracture toughness tests and transverse weld tensile tests. Tests on the HAZ were supplemented by Charpy impact and hardness tests on HAZs generated from thermal simulation techniques. Metallographical examination included optical and transmission electron microscopy.
2 EXPERIMENTAL PROCEDURE
2.1 Materials and welding procedure
Normalized ASTM A302 Gr B plate material from manufacturers A and B, with dimensions of approximately 500 x 500 x 30 mm was used. The test steels had a ferrite-pearlite/bainite microstructure. Welds were deposited using the submerged-arc welding (SAW) process; the materials used were a commercial SD3-Mo wire (3.2 mm dia.) in combination with an OERLIKON OPl2lTT basic welding flux. The chemical compositions of the base metal and weld metal