Cryogen and heat treatments of boron-lacquered high-strength cast iron

Abstract

This work investigated the effect of the interrupted isothermal quenching and subsequent cryogenic treatment. It studied aspects of direct hardening with the purpose of creating specific hardening structures in the matrix of high-strength cast iron. Here boron microalloying was used and the investigation focused on deeply refined structures with less than 0.002% sulfur content. The influence of thermal and cryogenic treatment on the evolution of the phase transformation and composition was studied. To obtain different structures in the metal matrix, four groups of samples were prepared, and were subjected to different duration of austenitization at 950 °C. The process of formation of the bainite phase was carried out at 400 °C and interrupted at controlled intervals of 7, 15, 30, 45, 60 and 75 min. The samples were then hardened in liquid nitrogen.This paper presents for the first time a novel approach of step-wise interrupted austenisation followed by cryogenic quenching of cast iron, which is usually impossible as cast irons cracks in all attempts of hardening them from high temperatures. The combination of isothermal hardening and cryogenic processing allows to induce in a controlled manner a wide range of structural mixtures of bainite and martensitic phases.