The influence of lath boundaries and carbide distribution on the yield strength of 0.4% C tempered martensitic steels

Abstract

There has been continued interest over the last few years in the relationships between the yield stress and the microstructural parameters of lath martensites and bainites. As far as the grain size is concerned, various researchers have concluded that a Hall-Petch relationship exists between the yield stress and the average diameter of the packets of the parallel elongated martensite-bainite grains which are termed laths. Other work has shown a reciprocal relationship between the yield stress and the lath width. If for low carbon steels an explanation can be given for this behaviour — the effective grain size given as a function of lath width and lath length, the latter expressed as the packet diameter — the absence of any influence of packet size on yield stress for higher carbon (about 0.4%) steels requires understanding. Further, since carbides coalesce on lath boundaries, it also needs to be explained why an Orowan relationship applies between the yield stress and the interparticle spacing, since the theory is based on the stress necessary for a dislocation to bypass precipitates. Using the approach developed by Rhines which relates the yield stress to the average resistance of the grain boundary area to shear propagation, in this paper it is attempted to show that the grain boundary resistance to shear propagation increases with the coverage of grain boundaries by carbide precipitates. The formula derived to interpret this type of behaviour has the same form as the Orowan equation, with the exception that the interparticle spacing coefficient depends on the carbide fraction volume. It is shown that the formula is in good agreement with all the experimental data obtained for low and medium carbon steels. Additionally the cementite precipitates remaining in the lath interior may reduce the lath length contribution to the yield stress from its maximum value (which is approximately equal to the packet diameter) for low carbon steels containing few intralath particles to a virtually constant value in medium (0.4%) carbon steels containing more intralath particles.