THE EFFECT OF CARBON CONTENT ON THE MICROSTRUCTURE OF AN EXPERIMENTALHEAT-RESISTANT STEEL

The effect of carbon content on the microstructure of a HP-Nb type hea t-resistant steel (35Ni-25Cr-1Nb) is examined for carbon contents of 0 .35, 0.44, 0.56, and 0.62 wt.%. In the cast state, the alloys contain both eutectic Cr7C3 and eutectic NbC. Upon aging, Cr23C6 carbides prec ipitate out from the supersaturated austenite. The distribution of the Cr23C6 carbides was better, that is, more uniform, in the alloys cont aining 0.44 or 0.56 wt.%C than in the alloys containing 0.35 or 0.62 w t.%C. Both the amount of the eutectic Cr7C3 carbides at the grain boun daries, and the Cr23C6 carbides within the grains, increased with incr easing carbon content. The relationship between the microstructures de veloped and the creep and high-temperature tensile properties is brief ly discussed.