Effect of δ ferrite on the impact property of hot rolled 12Cr – Ni steel

Abstract

12Cr – Ni steel is a low-carbon, corrosion-resistant, low-cost stainless steel. It is developed as an alternative to mild steel in applications requiring good corrosion resistance, high strength, excellent impact resistance and good weldability. Since 12Cr – Ni does not depend upon large amounts of expensive alloying elements like nickel and molybdenum, it maintains a low cost advantage compared to other stainless steels. The composition is designed to pass through (δ +γ) phase field in Fe – Cr binary system with austenite transforming to martensite during air cooling after hot rolling. The final microstructure of the steel is to be martensite and banded delta (δ) ferrite elongated in the rolling direction. The effect of delta ferrite on impact properties of 12Cr – Ni has been studied to some degree but controversial literature still exists. The aim of this investigation was to clarify the effect of δ ferrite on impact property with respect to volume fraction, morphology of δ ferrite and degree of banding. Three grades of 12Cr – Ni with slightly different chemical compositions were investigated, which yielded different volume fractions of δ ferrite. Microstructures were examined by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The final microstructure was lath martensite due to low carbon and banded δ ferrite elongated in the rolling direction, as expected. The volume fraction of each phase was determined by an image analyzer in order to investigate the quantitative relationship between the amount of δ ferrite and impact properties. It was seen that with increasing δ ferrite volume fraction, toughness increases, Brinell hardness of the material decreases, the microhardness of martensite increases. The microhardness of δ ferrite seems to stay the same. Charpy V-notch fracture toughness transition curves of alloys were determined and fractured surfaces were characterized using SEM. With increasing test temperature and δ ferrite volume fraction, cleave fractured area decreases. Degree of banding of the microstructures was assessed per ASTM E 1268-01. The microstructural banding index was found to decrease as the impact energy of as-rolled plate increased.The above results suggest δ ferrite is beneficial to the toughness. With the correct balance of ferrite and austenite stabilizing elements, optimum combinations of toughness and hardness can be achieved. Keywords: ferritic stainless, martensitic stainless, toughness, delta ferrite, degree of banding.