Abstract
45Mn2 steel, a low-alloyed steel, has been heat treated at 1100⁰C for 20 minutes followed by a quench in water at 470⁰C for 3 seconds. Metallographic examination of the material, using optical microscopy and SEM/EDS, has revealed the different phases that are present in the steel matrix. The presence of pearlite, ferrite, and small proportions of martensite and bainite was observed. The distribution of the phases may be associated with the cooling rate, where slow cooling rates would favor the formation of large proportions of ferrite and pearlite, while faster cooling rates would favor the formation of martensite. Additionally, the presence of retained austenite was observed, and attributed to the slow cooling rate, lack of deformation during cooling, and protective MnS inclusions, leading to increased retention of austenite after quenching.
Introduction
45Mn2 steel is a low alloyed steels, used for applications that do not require great strength or resistance to wear and corrosion. It is a toughness steel, with low carbon content and its main alloying elements are Mn and Si. It is mainly composed of ferrite and pearlite, with small proportions of bainite and martensite. The heat treatment of 45Mn2 steel involves quenching from a single austenitizing temperature, which results in a tempered martensite and a hardness that decreased with increasing cooling rate.
The aim of this work was to assess the microstructures obtained in 45Mn2 steel after a heat treatment consisting of austenitization at a temperature of 1200°C for 20 minutes followed by a quench in water at 470°C for 3 seconds. The results obtained were then compared with literature values to assess the different phases generated and their potential effects on the physical and mechanical behavior of the material.
Experimental
The samples were austenitized at a temperature of 1200°C for 20 minutes and then quenched in water at 470°C at a rate of 3 seconds. The samples were then mounted and ground using standard metallographic techniques. The specimens were then subjected to optical microscopy (OM) using a x100 objective and secondary electron microscopy (SEM) using an energy dispersive spectroscopy (EDS) attachment to perform the analysis of phase distributions and to observe the presence of MnS inclusions.
Results and Discussions
The results of the metallographic examination revealed the presence of anthracite colored ferrite, silver colored pearlite, and small proportions of martensite (light gray regions) and bainite (greenish gray regions). The ferrite, which occupies a major fraction of the microstructure, is delimited by small MnS inclusions, while the pearlite and bainite constitute a minor fraction of the microstructure.
The presence of a large amount of ferrite may be attributed to the cooling rate. Slow cooling rates, probably due to the thickness of the sample, lower the rate of martensite and bainite transformation, resulting in increased proportions of ferrite.
The presence of retained austenite was also observed, particularly around MnS inclusions which lead to formation ofprotected islands of austenite. Additionally, oxidation may take place due to the hot quench, leading to building of surface layers of oxide scale, whereas the core of the sample, below the oxide layer, may remain unaffected.
Conclusions
The microstructural analysis of 45Mn2 steel after a heat treatment consisting of austenitization at a temperature of 1200°C for 20 minutes followed by a water quench at 470°C for 3 seconds revealed the presence of ferrite, pearlite and a small fraction of martensite and bainite. Ferrite was the majority phase and is attributed to the relatively slow cooling rate, while the presence of retained austenite was attributed to the protective MnS inclusions that lead to formation of islands of austenite. The presence of austenite should be further investigated as it may affect the physical and mechanical characteristics of the steel.
