Formation of Crystal Structure in Hot Forging of Powder Carbon Steels
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STRUCTURAL MATERIALS RESEARCH FORMATION OF CRYSTAL STRUCTURE IN HOT FORGING OF POWDER CARBON STEELS A.A. Mamonova1,2 and G.A. Bagliuk1 UDC 621.762.5 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 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 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). Keywords: powder steel, hot forging, structure, phase analysis, crystal lattice, distortion.
INTRODUCTION The minimum porosity is one of the most important conditions for imparting high strength and service properties to structural powder materials. This predetermined the extensive development of powder metallurgy methods relying on hot shaping of preliminary compacted powder ingots in the last decades [1–4]. Since materials heated to hot shaping temperatures have much lower yield stress than cold-shaped materials of the same composition, hot shaping can result in virtually pore-free state at relatively low pressures. Rapid cooldown of forged pieces subjected to hot plastic deformation enables their thermomechanical treatment and imparts specific structure to the materials determining high mechanical and service properties [5–7].
1Frantsevich
Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, Kyiv,
Ukraine. 2To
whom correspondence should be addressed; e-mail: [email protected].
Translated from Poroshkova Metallurgiya, Vol. 59, Nos. 3–4 (532), pp. 149–159, 2020. Original article submitted May 8, 2018.
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1068-1302/20/0304-0232 2020 Springer Science+Business Media, LLC
The densification of porous ingots in hot forging involves various successive or overlapping processes and transformations in powder materials compared to other deformation methods [8, 9]. The structurization of porous powder ingots in hot forging proceeds under sev
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