Abstract
In his study, experiments were performed to make clear the influence of the contentious cooling transformation in heat affected zone (HAZ) of the 1Cr-1/4Mo (SCM4 and SCM22) steels on notch toughness, tensile properties and HAZ cracking by making the SH-CCT diagram, heat cycle simulating tests and slit type cracking tests. The results of these test are summarised as follows.
(1) The impact value falls strikingly when the single heat cycle of 1, 350°C is given, and those values (vE20) are 0.5-1 kg-m/cm2 for SCM4 and 2-3 kg-m/cm2 for SCM22. However, the influence of the cooling rate in the heat cycle cannot be especially recognized.
(2) The impact value will be remarkably improved when the cooling time of the heat cycle from 800 to 500°C is made less than 20s for SCM4 and 10s for SCM22 (the area percentage of the martensite are in the range 80-100%) and then postheated at 600°C for 2 or 3 hours.
(3) In slit type cracking tests on the tested steels, when the bainite structure of the HAZ was made approximately more than 40-60% by preheating, the HAZ cracking cannot be recognized even though wet coated electrode (12cc H/100g) was used.
