Notch Toughness and Retained Austenite in Ni Steel Welds Matched Ferritic Filler Metals: —Studies on Retained Austenite in Cryogenic Steel Welds (Report 1)—

The retained austenite and V-Charpy toughness of the weld metal, the weld bond and the heat-affected zone (HAZ) of 9 % Ni steel welds were investigated. 20 mm thick test plates were welded using the automatic gas tungsten-arc process and a ferritic filler metal having chemical compositions similar to those of the base metal. The weld specimens were prepared by deposition of single pass and multi-pass weld beads into single bevel groove of the base metal. Some of the single pass weld specimens were heated at 550°C, 600°C, and 650°C for 1 hr in order to evaluate the influence of the post weld heat treatment. The heat treatment at 600°C resulted in peak value of toughness at the weld bond as well as large amounts of retained austenite, 10-17 %, at the weld bond and the HAZ. The austenite produced in this condition, however, was unstable and decreased in less than 10 % by sub-zero treatment at — 196°C. In the multi-pass weld specimens, both the HAZ and the weld metal showed the good toughness comparable with those of the base metal, and then the bond toughness of half size specimen including the primary bead was excellent because of being adequently affected heat histories of the subsequent passes. A sufficient amount of the austenite was present in whole welds of the multi-pass specimen as a consequence of tempering due to the subsequent passes, though the weld heat cycles were in relatively short time comparing with furnace heat cycles. From the results obtained above, it was found that there was the considerable correlation between the thermally stabled austenite and the notch toughness in the weld zone.