Alloy HH is being used more than other heat resistant steels and especially useful as material for trays. This alloy can be classified into Type I in which ferrite is mixed and Type II of perfect austenite. At present, however, the alloy is unable to be selected for each use according to its use condition. From this point of view, the authors have been examining its basic properties of thermal fatigue.
In this report, described are the relation between structural change and thermal fatigue strength and furthermore, studies on crack propagation, the effect of casting defects against thermal fatigue strength and the life estimation.
The results obtained are as follows:
(1) The effect of maximum temperature appears in Type I, The thermal fatigue life greatly lowers on the high strain range side if the temperature rises as much as 100°C from 850°C.
(2) The grain size dependence of thermal fatigue strength of this material is scarcely found when the grain size number changes within the limit of -1.5 to -4.5.
(3) In less than about 1×103
cycle range, thermal fatigue life of Type II is longer than that of Type I. This is due to the distribution of eutectic carbides and high temperature strength of matrix. In more than 1×103
cycle range, thermal fatigue life is governed by secondary carbides which make the life of Type I and II equal.
(4) The characteristics of crack propagation is formulated by dl/dN=C
and the life estimation regarding the slag inclusion size as initial crack length shows favorable agreement with experimental data.