Embrittlement Factors in High-Cr Ferritic Heat Resisting Steels

Abstract

Heating brittleness of dual phase δ-ferrite/martensite steel and single phase martensite steel was investigated. In order to clarify the heating embrittlement factor for these steels, Charpy impact testes, transmission electron microscopy and determination of precipitates were carried out using specimens tempered at 700°C and 800°C and reheated at 550 and 650°C for 10²10⁴ h.
In the case of dual phase steel, the embrittlement of the materials and the rise of the ductile-brittle transition temperature were caused by the precipitation of Fe₂Mo, while the embrittlement of the single phase martensite steel was due to the increase in the number of M₂₃C₆ precipitates. In both steels, the increase in the transition temperature was found to be correlated to the total amount of the precipitates. When the precipitates were small enough, the toughness of the materials of heating embittlement could be recovered by means of heating at 800°C. On the other hand, when the precipitates grew over 200nm, the toughness could not be recovered.