Abstract:

The microstructure and strengthening mechanism of ultrahigh strength 23NiCo steel were studied. Upon tempering at 430℃, the martensite was decomposed to form a lot of cementite platelets concomitant with a significant drop in toughness. Tempering at 440～455℃ resulted in peak strength due to the precipitation of coherent zones of fine carbides. The peak in toughness was attained in the absence of cementite and the formation of reverted and stable austenite. Tempering at higher temperatures resulted in loss of both strength and toughness. The drop of the strength tempering at high temperature is due to precipitations coarsening and loss of the coherent with the matrix.

Key words: ultrahigh-strength-steels, secondary hardening, temper