A Study on Stress Relief Heat Treatment (Report 4)

Reduction in Stress on Welded Region of HT 60 during Heating

Abstract

Welded region, usually on which the maximum residual stress has been induced in welded structures, shows a hardened structure and must be subjected to structural changes during stress relief heat treatment. This changes may cause a different relaxation property of welded region from base metal. In order to clarify the influence of welded region upon reduction in stress in the heating stage, various types of H-type restraint specimen were subjected to low-temperature heat treatment. Thermal expansion measurement and X-ray diffraction test on welded region were also carried out to identify the structural changes. The obtained findings are as follows.
1. Reduction in residual stress took place on heating stage over than 250°C and was small in the holding stage. In accordance with this stress reduction, tensile plastic strain occurred.
2. With the existence of pre-stress-relieving at 620°C, reduction in stress as well as plastic strain did not take place on heating stage lower than 400°C. This means that pre-stress-relief retards reduction in stress by its stabilizing effect.
3. Deposited metal and synthetic heat affected zone showed a peculiar expansion at 250°C and thereafter only on heating. This peculiar expansion phenomenon became noticeable with decreasing cooling rate from 1350°C in the heat affected zone.
4. X-ray diffraction tests indicated the existence of retained austenite in deposited metal as well as in synthetic heat affected zone. Volume of retained austenite in synthetic heat affected zone increased with decreasing cooling rate from 1350°C.
5. A small precipitation was found in deposited metal after heat treatment over than 300°C.
6. Enhanced reduction in stress in welded region would be attributable to the lowered resistance for plastic deformation, that is, strain field introduced by both quenching and precipitation as well as by phase change.