Quantitative assessment of strengthening parameters in ferrite-pearlite steels from microstructural measurements

The influence of various ferrite–pearlite microstructures on the hardness and proof stress of a range of C–Mn steels has been studied with respect to a Hall–Petch analysis. In contrast with previous reports, the average pearlite interlamellar spacing was best found to quantify the microstructure when account is also taken of the ferrite volume fraction, austenite grain size, and calculated cementite lamella thickness. The mean uninterrupted distance in the ferrite described by a simple law of mixtures was found to correlate well with measured hardness. Although this parameter gave good agreement with strength figures an improved prediction was suggested by considering an equation of the form σ_ys^C = σ_o + [K_α + (K_α − K_α)V_p]d_c^−1/2, where the composite ferrite grain size d_c can be expressed as d_c^1/2 = V_pd_p^1/2 + V_αd_α^1/2, σ_o is the friction stress, and V_p and V_α are the volume fractions of pearlite and ferrite, respectively. Similarly, K_p and K_α are the Hall–Petch parameters for pearlite and ferrite concerning the ease of slip across a grain boundary. The friction stress was found to be very close to that for pure ferrite, with increasing volume fraction of proeutectoid ferrite having the effect of increasing the Hall–Petch slope while retaining the same friction stress.