Elsevier

Materials Characterization

Volume 49, Issue 5, December 2002, Pages 445-454
Materials Characterization

The effect of martensite volume fraction and particle size on the tensile properties of a surface-carburized AISI 8620 steel with a dual-phase core microstructure

https://doi.org/10.1016/S1044-5803(03)00070-6Get rights and content

Abstract

This study is focused on the production of a dual-phase steel structure in the core of a surface-carburized AISI 8620 cementation steel and the effect of martensite volume fraction (MVF) and martensite particle size (MPS) on tensile properties. Experimental results showed that, compared with specimens with a fully martensitic microstructure in the core, those with a dual-phase microstructure in the core exhibited slightly lower tensile and yield strength but superior ductility without sacrificing surface hardness. In specimens with a dual-phase microstructure in the core, the tensile strength increased and ductility decreased with increasing MVF. Both the tensile strength and the ductility increased with decreasing MPS at constant MVF. The best combination of tensile strength and ductility was obtained with a fine MPS at a constant MVF of 25%.

Introduction

Machine components such as shafts, gears and cams often require a very hard surface that can resist wear and a soft, tough core that can withstand the impact stresses that occur during operation. An established method for the production of such a combination of hard case and soft, tough core is case hardening of steels through carburizing and quenching. This procedure involves first the addition of carbon to the surface of a low-carbon steel to produce a composite consisting of a high-carbon steel case and a low-carbon steel core. During subsequent quenching, the high-carbon austenitic surface layer transforms to martensite. The microstructure of the core region is determined by the carbon content and by the base hardenability of the steel. If the steel has low hardenability, the core may transform to ferrite and a small amount of pearlite, depending on the quenching rate. If the steel has high hardenability, the core may transform to martensite.

Low-carbon or low-alloy steels (such as AISI 8620 cementation steel) are often used for carburizing applications. At the same time, their chemical compositions are not unlike those of dual-phase steels, in that the latter contain low percentages of C (<0.3%), Si, Mn, Cr and other alloying elements. Thus, surface-carburized steels have the potential for producing a dual-phase microstructure in the core region. Dual-phase steels typically do not contain enough alloying elements to have good hardenability using the normal quenching process. However, when the steel is subsequently tempered in the ferrite-plus-austenite region, the austenite portion enriches in carbon, which provides needed hardenability. During quenching, the austenite transforms to martensite, the final microstructure consisting of a dispersion of 20–25% of hard martensite particles in a soft, ductile ferrite matrix. In addition to martensite, the microstructure may contain small amounts of other phases such as retained austenite, new ferrite, pearlite and bainite, depending on cooling rate [1]. Dual-phase steels thus processed have relatively high tensile strength, continuous yielding behavior, a low 0.2% offset yield strength and usually a high uniform and total elongation when compared with other high-strength low-alloy steels of similar strengths. This combination of high tensile strength with good ductility is a major asset of dual-phase steels relative to other high-strength steels.

Studies [2], [3], [4], [5] have shown that the martensite volume fraction (MVF) is a dominant factor in controlling strength and ductility. It has been found that optimum properties were obtained with dual-phase steels that contained ∼20% martensite [2], [6], [7], [8]. It has also been shown that, for a constant MVF, a microstructure of finely dispersed martensite yielded better combinations of strength and ductility than a coarse one [9], [10], [11]. Other factors that have been reported to influence the ductility of dual-phase steels include the composition of the martensite, alloy content of the ferrite, retained austenite and amount of new ferrite (also called epitaxial ferrite) [8], [9], [12], [13], [14], [15].

Conventional heat treatment methods for surface-carburized steels involve quenching immediately following carburizing at 850–950 °C or reheating the surface-carburized steel to 850–950 °C and then quenching to room temperature. During this latter heat treatment, both the case and the core regions remain in the austenitic phase before quenching. Therefore, this heat treatment does not change the carbon content and hardenability of either the case or the core region. However, if the material is subsequently tempered at a temperature such that the case remains in the austenite region while the core is in the ferrite-plus-austenite region, then quenching should not significantly change the hardness of the case. However, the core should acquire more ductility due to the formation of a dual-phase microstructure.

The purpose of the present investigation was to study the production of dual-phase steel microstructures with different MVF and martensite particle sizes (MPS) in the core of surface-carburized steels without sacrificing surface hardness. The program was also to study the effect of these parameters on tensile properties.

Section snippets

Experimental procedure

The chemical composition of the as-received AISI 8620 cementation steel used in this investigation is given in Table 1 and its initial microstructure is illustrated in Fig. 1. It was supplied in the form of hot-rolled 22 mm round bar. Before carburizing, the bars were machined down to a diameter of 10 mm for a preliminary study of the microstructural response to dual-phase heat treatment.

The machined bars were gas carburized at 925 °C by a standard method in which the carburizing potential of

Heat treatments and microstructures

Fig. 5 is an illustration showing the locations on the Fe-C phase diagram of the conventional heat treatment and the production of dual-phase steel structures with various MVF in the core of the surface-carburized AISI 8620 cementation steel. During the conventional heat treatment at 900 °C (points a and b), both the carburized surface (∼0.8% C) and the core of the specimen (∼0.2% C) remained in the austenitic single-phase region. Oil quenching from 900 °C to room temperature produced a

Conclusions

  • 1.

    Surface-carburized steel specimens with a dual-phase structure in the core have superior ductility without sacrificing surface hardness and only a slightly lower tensile strength when compared with conventionally heat-treated surface-carburized steel.

  • 2.

    MVF and MPS in the core can be controlled to influence the strength and ductility in surface-carburized steels with a dual-phase structured core.

  • 3.

    The strength increases and ductility decreases with increasing MVF in the core.

  • 4.

    At the same MVF values,

Acknowledgements

We would like to thank Ozer Pamuk and the Hema Gear Industry Company for providing their heat treatment facilities.

References (19)

  • R.G. Davies et al.

    Physical metallurgy of automotive high strength steels

  • R.D. Lawson et al.

    The effect of microstructure on the deformation behaviour and mechanical properties of a dual phase steel

  • J.Y. Koo et al.

    Design of duplex low carbon steels for improved strength: weight applications

  • R.G. Davies

    On the ductility of dual phase steels

  • S.S. Hansen et al.

    Structure/property relationships and continuous yielding behaviour in dual-phase steels

  • R.G. Davies

    Influence of martensite composition and content on the properties of dual phase steels

    Metall. Trans.

    (1978)
  • R.G. Davies

    Early stages of yielding and strain aging of a vanadium-containing dual phase steel

    Metall. Trans.

    (1979)
  • A. Marder

    Factors affecting the ductility of dual phase steels

  • X.L. Cai et al.

    The dependence of some tensile and fatigue properties of a dual-phase steel on its microstructure

    Metall. Trans.

    (1985)
There are more references available in the full text version of this article.

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