The Effect of Carbon on the Properties of 25Cr-20Ni Heat Resisting Steel

Abstract

The effect of carbon on the mechanical properties at room and high-temperature of 25Cr-20Ni steels was investigated for sand mould ordinarily cast, metal mould centrifugally cast and forged specimens. For the ordinarily cast specimens heating-cooling repeat tests and oxidation tests were also carried out to investigate the influence of carbon on thermal fatigue strength and oxidation resistance.
The results obtained were as follows:
1) Ferrite and sigma phases were appreciated in the as-cast microstructure of the ordinarily cast specimen with 0.05% of carbon. The carbides observed in another as-cast specimens were identified as M₂₃C₆ and M₇C₃ by X-ray diffractions of electrolytically extracted residues, of which M₇C₃ increased with increasing carbon content and none of M₇C₃ was identified above 0.75% of carbon. It was reduced that M₂₃C₆ was stable in 25Cr-20Ni cast steel, because of the transformation of M₇C₃ to M₂₃C₆ at high-temperature.
2) At room-temperature, as the carbon content of both cast and forged specimens increased, hardness and yield strength increased and ductility adversely decreased. In ordinarily cast specimens tensile strength was maximum at 0.27% of carbon and above 0.47% of carbon the strength remained nearly constant as well as that in centrifugally cast specimens. The tensile strength offorged specimens increasing with increasing carbon content. Generally forged specimens showed the most excellent properties at room temperature.
3) At 871° and 982°C tensile strength increased with increasing carbon content in all. On the whole the tensile properties of centrifugally cast specimens were better than those of ordinarily cast at high temperature, but such excellence of forged specimens in the properties as appreciated at room temperature had not been already observed at 871°C.
4) The stress rupture strength of ordinarily cast specimens was effectively improved with increasing carbon content from 0.05 to 0.95% up to about 3000hr at 871°C and about 1000hr at982°C, over which the strength decreased more or less above 0.75% of carbon because of the coarsening of secondary carbides and the agglomeration of primary carbides. In the forged specimens such carbon effect on the strength was not appreciated. The strength of centrifugally cast specimens was more excellent than that of ordinarily cast, and forged specimens showed the extremely inferior strength compared with cast specimens.
5) It was deduced that the thermal fatigue strength of 25Cr-20Ni cast steel was improved by addition of carbon to about 0.5%.
6) The increasing of carbon to a proper content in order to improve high-temperature strength would not bring about the decrease of oxidation resistance in 25Cr-20Ni cast steel.