Early Stages of Precipitate Formation in a Dual Hardening Steel∗

Abstract

Abstract For plastic mold steels, secondary hardening as well as hardenable, corrosion-resistant steels are used. Secondary hardening can be achieved through alloying with Ni, Ti, Al and Cu. The precipitation sequence of combined Cu and intermetallic NiAl particles depends on the precise ratio of the alloying contents. In this study, a dual hardening iron-based alloy with a high ratio of Ni/Cu and Al/Cu and low Carbon content is investigated. These dual hardening steels represent alloys that utilize both carbidic and intermetallic secondary hardening precipitates to achieve secondary hardening. The investigations focus on the initial precipitation of intermetallic particles in order to analyze the sequence of precipitate formation for a specific ratio of alloying elements. DSC measurements were used to identify characteristic precipitation temperatures between 360 °C and 600 °C and the corresponding heat treatment conditions were reproduced using a quenching dilatometer. The initial stages of precipitate formation were examined using high-resolution atom probe tomography. C-enrichments in the form of retained austenite films were evident in the examined material. Heterogeneous nucleation of Cu-NiAl precipitates on these retained austenite films was attributed to exothermal reactions at 460 °C.