Retained Austenite Transformation during Heat Treatment of a 5 Wt Pct Cr Cold Work Tool Steel
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RODUCTION
INDUSTRIAL processes, such as rolling, cutting, forming, and punching, taking place at temperatures below 473 K (200 °C) are usually referred to as ‘‘cold work processes.’’ Steels that are used to make tools for these applications are, hence, called ‘‘cold work tool steels.’’[1,2] These steels must be able of to attain properties, such as high hardness, high compressive strength, and good toughness, to be suitable for cold forming of advanced high-strength steels. The fairly high alloying content of 5 wt pct Cr cold work tool steels results in low martensite start (Ms) and martensite finish (Mf) temperatures. Thus, the
M. ARBAB REHAN is with the Department of Engineering Science, University West, 461 86, Trollha¨ttan, Sweden, and also with Uddeholms AB, 683 85, Hagfors, Sweden. Contact email: [email protected] ANNA MEDVEDEVA is with Uddeholms AB, 683 85, Hagfors, Sweden. LARS-ERIK SVENSSON and LEIF KARLSSON are with the Department of Engineering Science, University West. Manuscript submitted November 21, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS A
transformation of austenite to martensite does not go to completion when quenching to room temperature after austenitization. As a consequence, some amount of austenite is retained in the as-quenched microstructure, which otherwise consists of martensite (and sometimes B) and undissolved primary carbides. Before being put into use as a tool, the steel is then tempered to enhance its ductility and to precipitate alloy carbides. The tempering temperature influences both the stability of retained austenite and the precipitation of carbides in the tempered martensite; therefore, it has a large effect on the properties.[1,3–5] Retained austenite transformation has been a subject of scientific interest for a long time, but characterization and understanding is still a challenge.[4–8] Literature suggests that the retained austenite can transform into ferrite and cementite,[6] martensite,[5] or B.[4] Ferrite and cementite form during isothermal treatment at the tempering temperature, while the latter transformations occur during cooling from the tempering temperature. It has been shown that the isothermal transformation into ferrite and carbide is a two-step process.[6–9] First, precipitation of cementite occurs from retained austenite, which is gradually followed by transformation into ferrite
II.
EXPERIMENTAL
A. Material An electroslag remelted ingot with the chemical composition of Uddeholm Caldie was produced. This was hot rolled into a round billet with a radius of 80 mm and then soft annealed. The chemical composition from the center of the steel billet is presented in Table I. B. Heat Treatment All specimens were heat treated in a push rod dilatometer (Dil 805 A/D, Bahr-Thermoanalyse). Cylindrical specimens (l 9 d = 10 9 4 mm) in soft-annealed condition were taken from the center of the steel billet. The specimens were heated with a high-frequency inductive coil in a vacuum chamber. Specimen temperatures were measured by a thermocouple, spot welded on the
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