On Predicting Quenched And Tempered Hardness Of Mixed Microstructures Using Steel Chemistry

A method of predicting tempered hardness of mixed microstructures has been formulated, which uses the quenched hardness and steel chemistry as independent variables.  This calculation is based upon a method first proposed in 1947 by Crafts and Lamont for mixed microstructures and modified using the 1977 chemistry-based tempered martensite hardness calculation of Grange, Hribal, and Porter.  The tempered hardness prediction was examined using Jominy end-quench bars tempered between 204°C (400°F) and 704°C (1300°F). The measured Jominy hardness after tempering was used to make adjustments to the Crafts and Lamont parameters used in the hybrid model. Both plain carbon (SAE 1045) and low alloy (SAE grades 8620, 4130, 5160, 6150, and 4140) were used to validate the chemistry-based hardness prediction.  In combination with the ASTM A255 Jominy hardenability calculation, the proposed method can be used to predict the quenched and tempered hardness profile of a round bar based upon chemistry, quench severity, and tempering temperature.