Microstructure and mechanical properties of sintered (2-4)Mn-(0 center dot6-0 center dot 8)C steels

Mechanical properties of 2-4% manganese PM steels were determined in tensio n and in bending following laboratory sintering in dry, hydrogen rich atmos pheres. Young's modulus determined by an extensometric technique was about 115 GPa; when measured by an ultrasonic method it was about 153 GPa, in acc ordance with the 'law of mixtures'. The microstructures, significantly devo id of oxide networks, were predominantly pearlitic, but frequently with var iability for specimens similarly processed, resulting in appreciable variat ions in the stresses for macroscopic yielding and fracture. The majority of the experiments were conducted on 3 and 4Mn-0.6C alloys and for these R-0. 1 was in the range 275-500 MPa, tensile strength (TS) 300-600 MPa, and (app arent) transverse rupture strength (TRS) 640-1260 MPa. Statistical techniqu es were employed to analyse the data. When careful control of processing wa s maintained, the Weibull modulus was highest, at about 17, for TS of furna ce cooled specimens, and lowest, about 6, for TRS of the rapidly cooled spe cimens. In order to interpret the significant differences between the TRS a nd the TS values, both apparently measuring the critical stress for crackin g after strains of up to 7%, a two stage normalising technique for TRS was adopted. By taking account of the plastic strains preceding failure, the el astic 'strength of materials' formula was modified to allow true fracture s tresses to be calculated. It was also postulated that failure was initiated from a population of flaws of variable size and then the 'normalised' bend strengths, smaller than TRSs, were shown to correspond well with TSs. It i s suggested that this combined plasticity correction and Weibull analysis a pproach, which has a sound scientific basis, should be employed to interpre t bend test data in preference to empirical correlations between TS and TRS .