Abstract
Although extensive aging and strain aging (bake hardening, BH) studies have been carried out on dual-phase steels, the aging behavior of the dual matrix structure (DMS) ductile iron (DI), as a potential way to improve its mechanical properties, has not been addressed until now. This research was designed to study the aging behavior of DI with a ferrite-martensite matrix structure. DMS-DI with a martensite volume fraction of 30% was produced by intercritical austenitizing at 785 °C followed by quenching in water to room temperature. Aging treatments were carried out without pre-straining at aging temperatures of 140, 170, and 220 °C for 2–10,000 min. DMS-DI was investigated by light optical microscopy (LOM) for unaged samples and scanning electron microscopy (SEM) for selected samples after aging treatments. The effect of aging conditions on the mechanical properties were investigated. Microhardness measurements for ferrite and martensite were also examined as a function of aging conditions. The increase in yield strength due to aging was determined. The results indicate that the aging conditions have a small effect on the ultimate tensile strength UTS. It is shown that the yield strength increased to a maximum value of 45 MPa (~11% increase) after aging for particular time, which is found to be dependent on the aging temperature. The peak aging response is followed by a decrease in yield strength, which is observed to be attributed to martensite tempering as confirmed by microhardness measurements.
Highlights
This study showed that vanadium additions to ferritic ductile iron increased the yield strength by up to 60% as well as hardness and the modulus of elasticity but reduced the ductility
The aim of the present study is to address the aging behavior of dual matrix structure (DMS)-ductile iron (DI) with a ferrite-martensite matrix structure, by applying varied aging conditions and investigating the microstructure characteristics, tensile behavior, macro, and microhardness of the aged
Mn contents are the most significant elements that influence the range of intercritical region
Summary
Due to the combination of good mechanical properties and low cost of ductile iron (DI), it is widely used in many critical engineering applications [1]. The mechanical properties of DI depend on both nodule characteristics and the constitution of the matrix. The former develop during the solidification stage and the latter results from the transformation of the austenite at high temperature [2]. Since the 1960s, various methods have been carried out to improve the mechanical properties of DI. These methods relate to all known heat-treatments such as austenitizing, normalizing, annealing, quenching, and tempering [4]
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