Fracture and fatigue in M-50 and 18-4-1 high speed steels

Abstract

The plane strain fracture toughness values,K _Ic , the fatigue crack growth rates and the tensile properties of M-50 and 18-4-1 high speed steels have been measured as a function of tempering temperature. The M-50 was a vacuum arc remelted grade (VIM-VAR) and the 18-4-1 an electroslag grade, and both are used in mainshaft gas turbine bearings. At the usual hardness for bearings, Rockwell C 62, 18-4-1 exhibited a slightly higher fracture toughness (21 MPa·m^1/2) than M-50 (18 MPa·m^1/2). The fatigue crack growth rates were very similar, and in the slow growth region followed the usual power law,dC/dN=(ΔK)^m withm=3 to 4. The crack propagation rates were still significant at values as low as ΔK=5 MPa·m^1/2. SEM studies of the fracture surfaces showed complex transgranular fracture paths for both steels. The tensile strengths and the elongations of M-50 were somewhat higher than the corresponding values for 18-4-1 but the yield strengths of the two steels were similar. The microstructures of these steels were markedly different, with M-50 exhibiting 2.6 vol pct undissolved carbides and the 18-4-1 showing 15.2 vol pct carbides, but the fatigue and fracture behaviors were similar.