Formation of Crystal Structure in Hot Forging of Powder Carbon Steels

Abstract

The crystal structure acquired by powder steels produced from undoped iron powder and mixtures of iron with 1.0 or 1.5% graphite in hot forging was studied by X-ray diffraction and optical microscopy. The heating temperature of the samples to be forged was varied in the range 950–1150°C. The fundamentally different dependence of the lattice distortion on the heating temperature for forging of the outer and inner layers of the samples was found. The lattice imperfection increases with forging temperature in the outer layers of the hot-forged samples made of the undoped iron powder. The γ→α martensitic transformation occurs when the samples are heated above 1100°C and rapidly cooled down. The lattice imperfection decreases in the inner layers of the samples made of the undoped iron powder. In the case of carbon steel samples, the lattice distortion and hardness increase with heating temperature, reaching the maximum at 1100°C and thus characterizing the γ→α martensitic transformation. When temperature increases to 1150°C, the lattice imperfection reduces as carbon burns out more intensively at elevated temperatures, but the hardness and tensile strength decrease insignificantly. This is attributed to the quenching temperature of steel to form a ferritic cement mixture, whose structure is presented by sorbitol with a microhardness of 2700– 2900 MPa, troostite (3000 MPa), and bainite (4500 MPa).