METHOD FOR DECIDING INTERGRANULAR STRESS CORROSION CRACKING RESISTANCE OF HEAT TRANSMISSION PIPE MADE OF NI-BASE ALLOY

摘要

PURPOSE:To more exactly decide the stress corrosion cracking resistance of a heat transmission pipe made of an Ni-base alloy by measuring the conformable precipitation ratio of the chromium carbides precipitating at the grain boundary of said heat transmission pipe. CONSTITUTION:The precipitated carbides 3 assume 3 kinds of precipitation forms as shown by A-C when the heat transmission pipe made of the high-chromium Ni-base alloy is subjected to an annealing treatment at the temp. above the carbon solutionization temp. and is then subjected to a grain boundary strengthening heat treatment to precipitate the chromium carbides 3 at the grain boundary I after cooling down to room temp. A is the form in which the unsolutionized chromium carbides 2 are dispersedly formed within the grains and are precipitated non-conformablly with the base material. B is the form in which the carbides 3 are precipitated conformablly only in the base material on one side. Further, C is the form in which the carbides 3 are precipitated alternately matchingly in the base materials sandwiching the grain boundary 1. In the case of the alloy 690, the stress corrosion cracking resistance of said alloy is improved by subjecting the alloy to the annealing treatment at the temp. above the carbon solutionization temp., then cooling the alloy down to room temp. and subjecting the grain boundary strengthening heat treatment to precipitate the carbides 3 at the grain boundary I by holding for 15hr at 700 deg.C.