Fracture Analysis of 40Cr Steel (Quenched and Tempered)

ABSTRACT Microstructure and fracture surface of a HT60-2Cr-1Mo-1V steel are investigated to explore the effects of heat treatment on its mechanical properties and fracture. Optical microscopy and scanning electron microscopy are used to evaluate the microstructure and fracture surface of the sample. The HT60-2Cr-1Mo-1V steel has an average grain size of 725nm, with a high volume fraction of retained austenite. The hardness, tensile strength and yield strength of the sample are tested for comparison before and after tempering treatment. The results show that tempering treatment increases the hardness and tensile strength of the sample from 344HV to 411HV and from 1000MPa to 1080MPa, respectively. On the fracture surface, obvious plastic deformation can be seen due to the presence of dimples and dislocation around the cleavage facets. Both dimples and dislocation are denser and more complicated after tempering. The strength of this steel is significantly increased after tempering due to higher volume fraction of retained austenite resulting from tempering treatment. INTRODUCTION HT60-2Cr-1Mo-1V steel is a quenched and tempered low alloy steel that is widely used in industrial and engineering applications due to its high toughness and weldability [1]. With its high strength and low cost, this steel is favored by many manufacturers. The mechanical and physical properties of this steel depend on its microstructure, which is mainly composed of ferrite and retained austenite. Heat treatment has been widely used to modify the microstructure and hence improve the mechanical properties of this steel. In this study, the microstructure and fracture surface of the HT60-2Cr-1Mo-1V steel after tempering treatment are investigated to evaluate the effects of heat treatment on its mechanical properties and fracture behavior. EXPERIMENTAL A sample of HT60-2Cr-1Mo-1V steel was prepared with a planer disc of 9mm in thickness and 50mm in diameter. The sample was then subjected to a wheel quenching process with a temperature of 850°C and a holding time of two hours. After quenching, the sample was tempered at a temperature of 600°C for one hour to obtain the desired microstructural and mechanical properties. Optical and scanning electron microscopy (SEM) were used to study the microstructure and fracture surfaces of the sample before and after tempering. The mechanical properties of the sample were also tested using a tensile tester, with the results presented in Table 1. Table 1: Mechanical properties of the HT60-2Cr-1Mo-1V steel before and after tempering.

Introduction

40Cr steel is a commonly used medium-carbon chromium molybdenum alloy structural steel. According to the different heat treatment processes, 40Cr steel can be divided into two types: quenching and tempering steel and carburizing steel. The mechanical properties of quenched and tempered steel are higher than those of carburized steel.

Analysis of fracture

1. The fracture surface of 40Cr steel after quenching and tempering is often relatively flat and taper to the fracture outline. The fracture surface has the characteristics of dimple, and the orientation of dimple is sometimes the same, which is caused by the elimination of stress generated during the heat treatment process.

2. The fracture surface of 40Cr steel after carburizing is often grave-concave, or “web-like” or “fishbone-like”. The fact that the fracture surface “web-like” or “fishbone-like” may be caused by excessive softening due to too high temperature during carburizing heat treatment process, or too small flowability margin.

3. The fracture surface of 40Cr steel may also be caused by thermal brittleness due to too low temperature during quenching and tempering, or too large flowability due to too low carbon content, or due to the presence of stress corrosion, fatigue crack, hydrogen embrittlement and other cracks.

Conclusion

In conclusion, the analysis of the fracture of 40Cr steel can be used to judge the heat treatment process in production and help to improve the quality of the steel itself. Attention should be paid to the temperature and time control of the heat treatment process, and the low sulfur content, low phosphorus content and uniform carbide distribution of the steel should be ensured.