Producing a Tough, High Strength Cast Steel Free of Temper Embrittlement

Temper embrittlement of a cast 3Ni-3Cr steel was produced by small amounts of either P or Mn in samples tempered at 650°C and air cooled. Auger spectroscopy examinations showed P embrittlement was produced by cosegregation of P, Ni, and Cr. Embrittlement by Mn appeared to be caused by Mn and S segregation. In coarse grained materials fractures were intergranular. In fine grained material, both intergranular and brittle transgranular fractures were observed, and Auger mapping showed impurity enrichment on transgranular fracture surfaces. Segregation and fracture on martensite packet boundaries is suggested as the origin of the transgranular fracture. Changing the tempering temperature and adding a half percent Mo solved the embrittlement problem. Water quenching was not required. A 150 mm thick casting of the new composition showed good tolerance for P and Mn, a yield strength of 500 MPa, and good toughness down to −80°C.