Fracture micromechanics of static subcritical growth and coalescence of microcracks in sintered Fe-1.5Cr-0.2Mo-0.7C steel

Nucleation of microcracks, their growth and coalescence are analysed in powder metallurgy (PM). Fe-1.5Cr-0.2Mo-0.7C steel by fractography allied to surface replica microscopy - at several stress levels as the maximum tensile stress in three-point bend specimens was raised to 99-6% of the transverse rupture strength TRS of 1397 MPa. The fatigue limit in this material is ~240 MPa, at which stress level no microcracks were detected in static loading. Numerous microcracks, ranging in size from < 5 to ~20 mum, however, were nucleated above 800 MPa, i.e. beyond the yield strength of 620 MPa. With increasing stress, some microcracks became dormant, whilst others grew subcritically, stress step-wise, to some 400 mum. Of particular importance are observations of the coalescence of two and three of such microcrack systems to produce a critical, propagating crack. The then estimated stress intensity factor Ka, could reach K1c, independently estimated to be -36 MPa m1/2. Microcrack coalescence was associated with easy paths for crack growth, principally prior particle boundaries linking pores. Ways of making subcritical crack growth more difficult and hence improving both static and dynamic mechanical properties, are considered.