Mechanical behavior of CrMo steel with tempered martensite and ferrite–bainite–martensite microstructure
Introduction
The influence of microstructure on mechanical properties in steels has been a subject of considerable research interest for many years [1], [2], [3]. The microstructure of conventional steels often makes it impossible to obtain concurrently good ductility, toughness and high strength. Evolution of newer steel with improved combinations of strength, ductility and toughness has led to the emergence of a series of mixed or multi-phase structures in which Advanced High Strength (AHS), represents a distinguished class [3], [4], [5]. Some applications, especially transportations require economical high strength steel with good ductility and formability. The AHS steels were developed to satisfy an increasing need, primarily in the automobile industry, for new high strength steels that permit weight reduction without dramatically increasing costs [6], [7], [8], [9]. The mechanical properties of mixed microstructure in high strength steels have been widely investigated. Many researchers reported a good combination of strength, toughness and ductility for mixed microstructure [6], [7], [8], [10], [11].
Sanctis and Lovicu [10] developed a model to examine the effect of the soft and hard phases on the tensile property for mixed microstructure in high strength steels. They found that a simple model based on the law of mixtures may result inaccuracy in predicting mechanical strength, when applied to product having a large difference in the amount of a hard constituent.
Matlock and Krauss [11] also investigated the effect of microstructure on mechanical properties in micro-alloyed, multiphase steels. They found that tensile properties and fracture toughness of ferrite–bainite–martensite (FBM) structure of micro-alloyed steels are inferior to those of conventional steels.
Sankaran et al. [6], [7], [8] developed a multiphase microstructure by thermomechanical processing in micro-alloyed steels in order to produce a mixed structure. They reported that the proof and tensile strengths of FMB microstructure are increase by 17% and 20%, respectively, compared with the values corresponding to the conventional microstructure.
In all the previous studies made on multiphase steels, both thermomechanical and heat treatments have been used to produce FBM microstructures. However, it is of interest to see, if mechanical behavior could be improved upon by only step quenching treatment. As of now, no literature is available on the improved mechanical properties by the step quenching heat treatment processes. The objective of this investigation is to study the correlation between the microstructure and mechanical behavior of FBM and tempered martensite microstructure in 42CrMo4 steel.
Section snippets
Experimental methods
The material used in this investigation was 42CrMo steel with the composition (wt.%): Fe–0.35C–1.10Cr–0.23Mo–0.52Mn–0.36Si–0.014P–0.006S. The heat treatment cycles were designed so as to produce significantly different microstructures. These included one tempered martensite microstructure and six FBM microstructures. Quench tempering and step quenching heat treatments produced a tempered martensite and an equiaxed ferrite–bainite–martensite microstructure, respectively. The detailed heat
Results and discussion
The microstructures of the quench tempered and step quenched specimens are shown in Fig. 1a through d, respectively. As can be seen in these figures, the microstructure of the quench tempered specimen was tempered martensite, whereas the microstructure of the steel in the step quenching condition was ferrite (white areas) and mixture of bainite and martensite (black areas). In the color tint etching, ferrite, bainite and martensite were observed in azure blue, brown and white colors,
Conclusions
The mechanical properties of tempered martensite, is more favorable than those of FBM microstructure. Decreasing bainite transformation temperature from 430 to 400 °C increases, the yield and tensile strength but decreases impact energy. Increase in the yield and tensile strength and the related reduction of impact energy are results of increasing bainite dislocation density.
Yield-drop effect was observed in FBM microstructure with ferritic matrix. This effect can also be attributed to
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