Effect of Austempering on Plastic Behavior of Some Austempered Ductile Iron Alloys
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THE industrial application of austempered ductile iron (ADI) has grown in recent years. The material has a number of mechanical properties that make it attractive for structural applications in transport such as automotive, heavy vehicles, and many other industries. The material can be tailored to have properties such as high strength, high wear resistance, high fracture toughness, and high fatigue strength.[1–3] ADI is an alloyed ductile iron that has been subjected to a two-step heat treatment process known as austempering. The first step is austenitizing. The ductile iron is heated to the austenitizing temperature for sufficient time to obtain a fully austenitic matrix saturated with carbon. The next step is austempering, during which the ductile iron is quenched to the austempering temperature and held there for a period of time, the austempering time, before cooling to room temperature. During austempering, the fully austenitic matrix transforms into acicular ferrite and stabilized high carbon austenite, a JAKOB OLOFSSON, PhD Student, and INGVAR L. SVENSSON, Professor, are with the Department of Mechanical Engineering, Materials and Manufacturing - Casting, School of Engineering, Jo¨nko¨ping University, SE-551 11 Jo¨nko¨ping, Sweden. Contact e-mail: [email protected] DAN LARSSON, Quality Engineer, formerly with the Department of Mechanical Engineering, Materials and Manufacturing - Casting, School of Engineering, Jo¨nko¨ping University, is now with Vestascastings Guldsmedshyttan AB, SE-711 78 Guldsmedshyttan, Sweden. Manuscript submitted January 26, 2011. Article published online July 20, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
matrix called ausferrite. This reaction, where austenite (c) decomposes into ferrite (a) and high carbon austenite (cHC), is known as the first stage reaction,[2] Eq. [1]. c ! a þ cHC
½1
The first stage reaction consists of two substeps. In the first substep, ferrite nucleates and the entire matrix transforms into acicular ferrite and austenite with a low carbon content of about 1.2 to 1.6 pct. In the second substep, ferrite grows instead of further nucleation, while the carbon diffuses into austenite and creates high carbon austenite with a carbon level of about 1.8 to 2.2 pct.[4] The low carbon austenite is metastable: metastable austenite may exist at room temperature, but when further cooled or stressed, it may transform to martensite.[4] Martensite is a hard and brittle phase that causes, e.g., machining problems, and strain-induced martensite strongly influences the mechanical properties of ADI materials.[4,5] The austempering time must thus be long enough to ensure that high carbon austenite is obtained and the formation of martensite is avoided. If the austempering time is too long, however, the high carbon content austenite becomes saturated with carbon and decomposes into ferrite (a) and carbide (e).[4] This is known as the second stage reaction,[2] Eq. [2]. cHC ! a þ e
½2
This microstructure contains carbide, which makes the material brittle. The paramete
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