Retained Austenite and Notch Toughness in Synthetic HAZ of 9% Ni Steel: —Studies on Retained Austenite in Cryogenic Steel Welds (Report 2)—

The effect of retained austenite on notch toughness in HAZ of 9% nickel steel welds had been discussed in author's previous paper. In this paper, the above correlation and factors affecting on the formation of retained austenite during weld process were investigated in more detail by using the simulation technique. The obtained result indicated a noticeable effect of a peak temperature, in particular near 620°C being closer to A_CCI temperature (about 630°C), of secondary thermal cycle, after primary thermal cycle 850 to 1350°C, on the amount of retained austenite, and however, there was a lesser effect of another factors, such as heating and cooling rates etc. on it. In the other hand, since the formation of globular austenite occured rapidly and abundantly during heating process in welding, as comparing the ordinary heat treatment, there was a variation in the amount of the retained austenite in very short time if holding at the peak temperature of tempering cycle. It was found that there was a fairly good correlation between the amount of retained austenite and toughness. When the grains, however, were remarkably coarsened by the primary thermal cycle similar to that near weld bond, the toughness was no longer improved regardless of the amount of retained austenite, unless the secondary thermal cycle being above 800°C. This secondary cycle resulted in the grain refinement and the formation of the enriched zone of alloying elements, where the stable austenite precipitated in 3rd thermal cycle, mainly, at prior austenite grain boundaries.