A simple method for prediction of shelf life of bake hardening steels

Abstract

The shelf life of bake hardening (BH) steel is generally reported to be inferior to that of conventional grades due to presence of solute elements in the steel matrix. Hence the assessment of shelf life of BH steel is essential for automobile manufacturers to produce the components free from defects but a reliable method is not yet available to assess it in an industrial condition. Hence to formulate a simple method for assessing shelf life of BH steel, aging behavior of BH steels has been studied at various temperatures (50-100°C). Analysis of data showed higher activation energy (88-102 kJ/mol) than that of the diffusion of C or N in iron. Using the derived activation energy, k-value in Hundy equation was modified. The prediction of shelf life using earlier correlation was compared to that of modified equation. Based on the present analysis, a simple accelerated test has been presented for prediction of the shelf life applicable in an industrial practice. © 2008 ISIJ.