Mechanical properties and microstructural features of AISI 4340 high-strength alloy steel under quenched and tempered conditions

Abstract

In this work, the mechanical properties and microstructures of AISI 4340 high strength alloy steel under different tempering conditions are investigated. The specimens are quenched and tempered to a martensite structure and loaded to fracture at a constant strain-rate of 3.3×10⁻⁴ s⁻¹ by means of a dynamic material testing machine (MTS 810). The mechanical properties and strain-hardening exponent are considered as function of the tempering conditions. The morphological features of the as-quenched martensite and their evolution during tempering are described. Fractographs of the specimens are also made in order to analyse their fracture and embrittlement mechanisms. The results indicate that the mechanical properties and microstructural features are affected significantly by tempering temperature and holding time. The strength and hardness of tempered martensite drop as the tempering temperature and holding time are increased. However, the ductility increases with increasing tempering temperature and holding time, except when tempered martensite embrittlement occurs. Microstructural observations reveal that the carbide precipitates have a plate-like structure at low temperatures, but are spheroid-like at high temperatures. Under the tested conditions, the fracture appearances show that the material failed in a ductile manner except for the case of tempering at 300°C, where tempering martensite embrittlement occurs due to the existence of retained interlath austenite.

Introduction

An understanding of the mechanical properties of metals during deformation over a wide range of loading conditions is of considerable importance for a number of engineering applications. When discussing high strength steel, it is crucial to realise that the definition of so-called high strength depends entirely upon how the steel is to be used. These usages tend to fall into a number of different categories where different combinations of properties are required. In each of these categories, works being carried out to develop higher strength steels have to take the manufacturing processes, the heat treatment and the alloying technology into consideration [1].

There are several well-known structures in steel, such as ferrite/pearlite, bainite, martensite and austenite. Each of them has very different mechanical properties 2, 3, 4. Therefore, it is possible to obtain the highest strength from any one of these structures and it is likely that the highest strength steel in each of these categories will be of wide application. Generally, quenching and tempering are well-established means to produce strengthening in steel which can be achieved mainly due to the precipitation of a fine dispersion of alloy carbides during tempering [5]. Known for forming the highest level of strength in a steel, the martensite structure is rarely used in an untempered condition because a large number of internal stresses associated with the transformation cause the material to be lacking in ductility 6, 7, 8, however, low-temperature tempering is sufficient to reduce these stresses considerably without essentially changing the basic features of the martensitic structure. Therefore, from the commercial point of view, the study of martensitic steels have to include that of steels tempered in the range of 200–250°C.

However, apart from the effect of tempering temperature, the strength of the martensitic structure is dominated by the carbon content and the (M_s–M_f) temperature range 9, 10. In the case of low-carbon martensite, the martensite units form in the shape of lath, grouped into larger sheaves or packets. Its substructure consists of high densities of dislocations arranged in cells, and is superficially similar to that developed in iron by a heavy cold-working process. In the case of high-carbon steels and iron alloys with M_s below the ambient, their structure is plate martensite, which consists of very fine twins with a spacing of about 50 Å. Their crystal structure may be either (bct) or (bcc). However, in the case of medium-carbon steels, since they may contain a mixture of lath and plate martensite, their structure is more complicated. These results also indicate that the mechanical behaviour of a quenched-and-tempered steel depends strongly on its microstructure. Thus, the study of effects of the microstructure and dislocation structure of a steel on its strength, ductility and fracture characteristics is of great importance from the viewpoint of both theory and practice.

Although AISI 4340 steel is a widely-used low-alloy martensitic steel that provides an advantageous combination of strength, ductility and toughness for the applications of machine part-members, it is susceptible to embrittlement during the tempering procedure within a specified temperature range. In order to prevent this fault, a study on the microstructure and mechanical properties of AISI 4340 steel under different tempering conditions becomes necessary, these questions being focused on in this study. Further, the behaviour of tempered embrittlement as well as its formation mechanisms are also described.