Characteristic features of the embrittlement occurring in the temperature range from 1 000 to 600°C in various kinds of steels were examined by hot stage tensile test as well as fractographic analysis, and the mechanism of this embrittlement is discussed.
The embrittlement is caused by the slower strain rate of the tensile test and fracture occurs along the austenite grain boundary by either grain boundary sliding or by localization of strain in the film-like proeutectoid ferrite produced by the γ-α transformation.
Steels such as ferritic steel and electrolytic iron do not have this embrittlement, while carbon steels containing 0.05 to 0.4% carbon and fully austenitic steels reveal this embrittlement. In the case of carbon steels, sulfur, nitrogen and niobium are detrimental elements while aluminum and phosphorus suppress it depending on their content and state of existence in the steels.
The stress-strain analysis by an Instron type machine shows that the restoration process either by dynamic recovery or by dynamic recrystallization plays an important role in this embrittlement. Another finding is that proeutectoid ferrite deforms preferentially in the austenite plus ferrite region because of very low levels of flow stress and work hardening rate in this ferrite.
Thus, factors governing this embrittlement are the degree of ease of the recrystallization in austenitic steel relating to the grain boundary sliding, and the formation of film-like proeutectoid ferrite produced by the γ-α transformation in the case of carbon steels. Grain boundary precipitates such as sulfides and carbonitrides act as nuclei of voids and thus promote this embrittlement.