Ni基超耐熱合金Waspaloyの再熱割れ現象

抄録

In this study, the reheat cracking phenomena in Waspaloy was researched to get beneficial information in order to make clear the mechanism of reheat cracking, that often occurred during the post weld heat treatment (PWHT) of Ni-base superalloys.
Main results obtained in this study are as follows;
(1) Reheat crackings initiated and propagated at grain boundaries adjacent to the fusion lines in HAZ. The initiation and propagation range shifted to lower stress and temperature sides due to slowing down heating rate to PWHT temperature from 4100°C/min to 30°C/min.
The critical initiation stress of the reheat cracking also decreased associated with sluggishing heating rate to PWHT temperature.
(2) Influences of various factors on the local deformability (δ_c) of coarse-grained zone were investigated, which resulted in following conclusions.
(a) δ_c values at 800, 850 and 900°C became constant by using tensile rate less than 2.2 cm./min.
(b) Minimum δ_c value was obtained at 850°C.
(c) The detrimental influence of oxygen-containing environment on δ_c, at 800, 850 and 900°C during simulating weld thermal cycles was found and oxygen embrittlement was particularly observed in the case of local melting occurring at grain boundaries.
(d) δ_c value at 850°C decreased with increase of grain size.
(e) δ_c values at 800, 850 and 900°C depended upon hardenability of matrix. Hardening matrix resulted in decreasing δ_c values at 800, 850 and 900°C.
(f) The beneficial effect of Ca addition was observed. Adding 50 ppm Ca promoted δ_c values at 800, 850 and 900°C due to fixing S atoms as Ca-S inclusions.