Microstructure Analysis of Fracture Surface of 5CrNiMo Steel in Quenching and Tempering Process
Abstract
This paper discusses the microstructure analysis of fracture surface of 5CrNiMo steel under quenching and tempering process. The fracture surface of the tested steel is observed and analyzed by scanning electron microscope. The fracture surface is mainly composed of fine spots. The grains of the steel are divided into ferrite grain, tempered martensite grain, spheroidizing carbide, small grain and untransformed austenite grain. The physical properties of the matrix are mainly tempered martensite, ferrite and pearlite. As a result, the matrix components of the steel are tempered martensite, fine pearlitic network, ferrite and carbide cluster.
Key words： 5CrNiMo; quenching; tempering; microstructure; fracture surface
1 Introduction
5CrNiMo steel is a kind of low carbon alloy structural steel developed from 20CrNiMo and belongs to medium carbon low alloy steel. It has good comprehensive performance, good shock and impact toughness and high strength. It is mainly used for manufacturing various parts requiring high strength, impact toughness and good fatigue resistance after quenching and tempering. Quenching and tempering is a common method for heat treatment. Quenching can improve the hardenability and enhance the hardness of the steel, and tempering can further improve its implosion, fatigue and low temperature impact toughness, also improving the plasticity of the steel. The quenching and tempering process has more uniform material composition in the steel. After performing the quenching and tempering, the steel has the best mechanical properties. It is an important process to ensure the performance of steels.
In order to analyze the microstructure of 5CrNiMo steel after quenching and tempering, an experiment was conducted. The results of the experiment are analyzed in this paper, which is helpful to better understand the quenching and tempering process of 5CrNiMo steel.
2 Experimental
The materials used in the experiment are 5CrNiMo steel samples with dimensions of 50mm×50mm×25mm. The chemical composition test of the sample is shown in Table 1.
Table 1 Chemical composition of 5CrNiMo Steel (mass %)
Element C Si Mn Cr Ni Mo P S
Content 0.30 0.20 0.30 1.50 1.00 0.30 0.030 0.025
2.1 Heat treatment process
The samples were heated to 860℃ for 1h and then were quickly quenched with water after being kept at the same temperature for an hour. Then the quenched sample was heated to 540℃, kept at 540℃ for 2h and naturally cooled to room temperature. The heat treatment process is shown in Figure 1.
Figure 1 Heat treatment process
2.2 Fracture Test
After the quenching and tempering treatment, the sample was cooled to room temperature and the universal testing machine was used to perform fracture test. The fracture surface of the sample was observed and analyzed by scanning electron microscope (SEM). The fracture surface was observed by conventional SEM and backscattered electron (BSE) imaging.
3 Results and Discussion
3.1 Fracture surface observation
The fracture surface of the sample is shown in Figure 2. It can be seen that the fracture surface of the sample is not smooth, and it is composed of lots of black spots, which are the defects of the fracture surface.
Figure 2 Fracture surface of 5CrNiMo steel
3.2 Microstructure analysis
The analysis of the microstructure of the fracture surface was conducted by SEM. The SEM images are shown in Figure 3.
Figure 3 SEM images
The grains of the steel on the fracture surface are divided intop ferrite grain (F), tempered martensite grain (TMC), spheroidizing carbide (SMC), small grain (SG) and untransformed austenite grain (U). Based on the scanning electron microscope analysis, the physical properties of the matrix are mainly tempered martensite (TMC), ferrite (F) and pearlite (P). The matrix components of the steel are TMC, a fine pearlitic network (P), ferrite (F) and carbide cluster (C).
4 Conclusions
In this paper, microstructure analysis of fracture surface of 5CrNiMo steel in quenching and tempering process was discussed. The grain structure of the fracture surface was observed and analyzed by scanning electron microscope. The grain is divided into ferrite grain, tempered martensite grain, spheroidizing carbide, small grain and untransformed austenite grain. The physical properties of the matrix are mainly tempered martensite, ferrite and pearlite. The matrix components of the steel are tempered martensite, fine pearlitic network, ferrite and carbide cluster. All these results indicate that the quenching and tempering process is conducive to the precipitation of the matrix component and improve the strength of the steel.
