ニッケル基超耐熱合金の溶接熱影響部高温割れとミクロ組織との関係

Abstract

Relations between weld heat-affected zone cracking and microstructures of several nickel-base superalloys by the hot ductility test, i.e. the Gleeble test, were determined in comparison with the brittle temperature range between the nil-strength temperature (N.S.T.) and the temperature at which the ductility begins to recover on-cooling from N.S.T. and the recovery velocity of ductility.The conclusions obtained are as follows.(1) In the wrought alloys, constitutional liquation of grain boundaries begins at the nilductility temperature (N.D.T.) and the liquation spreads out to all grain boundaries at N.S.T. and the brittle temperature range is determined by the degree of the solidification temperature drop caused by the constitutional liquation.(2) In the cast alloys, constitutional liquation produces solute-rich regions around and between the preciptates, along which fracture propagates. Consequently the ductility is lost completely and its recovery is slower than the wrought alloys.(3) The recovery velocities of ductility of all the alloys tested are almost the same, but the brittle temperature range is 60°C for Hastelloy X cast alloy, 70°C for Inconel 718 wrought alloy and Inconel 706 wrought alloy, and 170°C for Inconel 718 cast alloy. The results coincide well with our job experiences of hot cracking tendencies of the heat-affected zone in TIG arc welding.(4) Even with the same chemical composition, the cast alloy has a wider brittle temperature range than the wrought alloy.(5) Inconel 706 wrought alloy, obtained by improving the fogeability and machinability of Inconel 718 wrought alloy, exhibits low hot cracking sensitivity nearly equal to Inconel 718 wrought alloy.