Post weld heat treatment of thermo-mechanical controlled process steel plates

Abstract

Thermo-mechanical controlled process (TMCP) steels have been developed to offer several inherent advantages due to its extremely fine-grained microstructure. The higher yield strength of TMCP steel is obtained through grain refinement and the effective reduction of the carbon alloy provides excellent weldability. TMCP Grade HBL 385 is widely used in Japan to create high strength, excellent earthquake resistant and yet easily weldable building frames. However, due to its recent introduction, there is little information regarding TMCP’s material properties after welding and relatively little is known concerning about the effects of PWHT upon both TMCP’s tensile strength and durability.

This paper is organized as follows. The first part of this paper will discuss the mechanical properties of the high strength low alloy (HSLA) steel after undergoing post weld heat treatment (PWHT) and its effectiveness will be evaluated. The second part will investigate the effectiveness of PWHT through the finite element simulation of a plate-to-plate T-joint. A total of seven 16 mm specimens were machined out from 2000 mm x 300 mm x 16 mm TMCP Grade HBL 385 steel and six specimens were welded on both sides of the specimen with a run of 100 mm. Prior to the tensile test, five specimens underwent PWHT in five different holding temperatures and holding times.

Data captured from the seven tensile tests showed that while the yield and ultimate tensile strength were generally quite similar to the control specimen after welding, there was a drastic 63% decrease in the ductility. However, after PWHT of 570°C for 38 mins, the steel regained 16% its ductility, which is still 47% lower than its original ductility before welding.

Overall, it was seen that HBL 385 shows promise for building frame applications, but the deterioration in its ductility after welding should be addressed before using it for building construction in earthquake prone regions.