A Study on Stress Relief Heat Treatment (Report 3)

Stress Relief Test of HT 60 Using H-type Restraint Specimen

Abstract

The maximum residual stress in a welded plate has been measured on the weld bead. The weld bead would behave differently from base metal in stress relief heat treatment. In this report, stress relief treatment effect in a composite specimen which consisted of base metal and deposited metal was studied using HT 60.
A modified H-type specimen in which the weld bead was parallel to tensile residual stress, namely the so-called HL specimen, was proposed. To determine the effect of weld, an HB specimen, in which all parts were composed of base metal and the residual stress was induced by thermal contraction, was also used. Using these specimens, plastic deformation of weld and base metal could be measured by use of Pfender contact strain gauge. Finally, a wide welded plate (500×500 mm) was also used to study stress relief heat treatment effect.
The results obtained from above three test specimens and relaxation tests were compared mutually and the mechanism of stress relief was discussed. The results are summarized as follows:
1 The decrease of residual stress in HL specimen due to stress relief treatment was in relatively good agreement with that in wide welded plate.
2 The reduction of residual stress in HB specimen coincided comparatively well with relaxation test results and deviated from that in wide welded plate only when stress relief temperature was lower than 600°C.
3 The abrupt reduction of residual stress in HL specimen at lower temperature of stress relief treatment might be due to structural changes of deposited metal and heat affected zone.
4 The reduction of residual stress in H-type specimen was mainly due to plastic deformation of tension member and some part of plastic deformation of tension member was absorped into elastic strain of restraint members, so stress reduction per unit plastic deformation became smaller in I-I-type specimen than in relaxation test. But when stress relief temperature became higher than 600°C, a plastic deformation took place in restraint members and the residual stress amounted to a certain value specified by base metal properties independently of type of specimens.