Importance of k_y (Hall-Petch slope) in determining strength of steels

Abstract

A review of the literature has shown that the k _y value (Hall-Petch slope) for ferrite-pearlite steels is sensitive to small changes in composition and heat treatment, and can vary over a wide range from 14 to 24 MN m⁻² mm^1/2. Generally, however, the lower yield strength is only marginally influenced by this variation in k _y value because of a strong inter-relationship between the k _y value and σ₀ (intercept in the Hall-Petch equation), such that high ky values are accompanied by low (σ₀ values and vice versa. The variability in k _y value value is believed to be caused by differences in the amount of interstitials (mainly carbon) present at the grain boundaries influencing the ease of nucleating dislocations from the boundary regions. Explanations to account for the differing degrees of interstitial segregation to the boundaries and the inter-relationship between k _y value and σ₀ value are presented and discussed.