Attributes of Q&T Ductile Iron with Novel Microstructures

• Published in: International journal of metalcasting 2023-04, Vol.17 (2), p.631-647
• Main Authors: Gundlach, Richard B., Tartaglia, John M.
• Format: Article
• Language: English
• Subjects: Characterization and Evaluation of Materials; Chemistry and Materials Science; Materials Science; Metallic Materials; Structural Materials; Technical Paper
• ISSN: 1939-5981; 2163-3193
• DOI: 10.1007/s40962-022-00861-2

A novel approach to austenite grain refinement was applied to quench and temper ductile iron in order to improve the tensile properties. Castings of two ductile iron alloys were intercritically (IC) austenitized near the upper critical temperature to produce a fine-grained lamellar austenite-ferrite structure and were subsequently quenched and tempered. The lamellar austenite + ferrite formed in the dendrite core region without forming a continuous network of austenite in the interdendritic regions. The austenite grain size was substantially finer than the polygonal austenite grain structure produced by conventional hardening above the critical temperature. In contrast to earlier researchers of IC austenitized DI who produced more ferrite by austenitizing at lower IC temperatures, the present study achieved higher austenite fractions and thus greater lamellar martensite contents after quenching. On tempering up to 540 °C (1000 °F), the supercritically heat-treated martensitic structures produced secondary graphite, whereas none was observed in the intercritically heat-treated materials. Differing upper critical temperatures and manganese and silicon contents were measured in the core and interdendritic regions for one of the as-cast irons. The resulting properties of the lamellar martensite and ferrite microstructure were determined and compared to those of conventional quench and tempered ductile iron with a fully martensitic microstructure. When the properties are compared to the minimum specifications for the Q&T grades of ASTM A536 Grade 120-90-02 and ISO 1083 Grade 900-2, the intercritically austenitized alloys exhibited the specified tensile properties with high ductility.