Effect of Heat Treatment on Microstructure and Hardness of Die Steel 5Cr8MoVSi
Abstract
5Cr8MoVSi die steel is an important material for die production because of its good wear resistance and molding performance. In order to study the influence of heat treatment on the microstructure and hardness of 5Cr8MoVSi die steel, the samples were heated to 860, 880 and 890℃respectively, and quenched in water. The microstructure was observed and the hardness was tested. The results show that the microstructure of 5Cr8MoVSi die steel consists of martensite and a small amount of austenite, and the hardness of the martensite is the highest. With the temperature and time increasing, the martensite area increases, so does the hardness. When the amount of austenite was increased, the hardness decreased slightly, but it still can meet the requirements of the die steel.
Keywords: 5Cr8MoVSi die steel, heat treatment, microstructure, hardness
1. Introduction
Dies, as a common metalworking tool, are widely used to manufacture parts and components. In the die industry, the die material is divided into two categories: cold working die and hot working die. The die steel used in the cold working die has high hardenability, high hardness and good wear resistance, while the die steel used in the hot working die has high temperature strength, good temper resistance and plasticity. Generally, tool steelsare used as die material in die industry, among which 5Cr8MoVSi die steel is an important material because of its good wear resistance and molding performance.In some die processes, such as forging die and extrusion die, thermal treatment must be carried out. The reasonable thermal treatment process plays an important role in the production of dies, which can not only improve the performance of die materials, but also meet the requirements for use [1,2].
At present, a great deal of research has been done on the heat treatment of die steel, but few studies have focused on 5Cr8MoVSi die steel. In this paper, the process of 5Cr8MoVSi die steel quenched from 860, 880 and 890℃is analyzed, and the effect of heat treatment on the microstructure and hardness of 5Cr8MoVSi die steel is studied.
2. Experimental Materials and Methods
The chemical composition of the 5Cr8MoVSi die steel is shown in Table 1. The samples were heat treated by an electric resistance furnace.The samples were heated to 860, 880 and 890℃, respectively, and rotated at a speed of 2r/min. After keeping at 500s and 600s, the samples were quenched in water. Hot punched samples were taken by hot mounting with ZY-D plastic, and then the samples were ground. The microstructure was observed by an OM optical microscope, and the hardness test was performed by a SHIMIZU HR-30A Rockwell Hardness Tester.
3. Results and Analysis
3.1. Microstructure
The light optical microscopy (OM) images of 5Cr8MoVSi die steel after heat treatment at 860, 880 and 890℃are shown in Figure 1. As can be seen from the figure, the microstructure of 5Cr8MoVSi die steel mainly consists of martensite and a small amount of austenite. With the increase of temperature, the amount of austenite increases, and the morphology of martensite also changes.
Table 1 Chemical composition of 5Cr8MoVSi die steel (mass fraction, %)
Figure 1 Light optical microscopy images of 5Cr8MoVSi die steel after heat treatment
At 860℃, the austenite grain is small and uniform, with a cuboidal shape; while at 880 and 890℃, the grain of austenite increases and unites, and the morphology of martensite changes to acicular shape. In addition, with the increase of time, the uniformity of austenitic grain is better and the amount of austenite is larger.
3.2. Hardness
Figure 2 shows the hardness of 5Cr8MoVSi die steel after heat treatment. As can be seen from the figure, with the increase of temperature, the hardness of the martensite increases from HRC 58 to HRC 60. When the temperature was raised to 890℃, the hardness decreased slightly, but still can meet the requirements for die steel (HRC 58~62).The microstructure of the samples kept for 600s also have higher hardness than that of the samples kept for 500s. This may be due to the fact that when the time is prolonged, the austenite is further transformed into martensite, which results in the increase of martensite area and the increase of hardness.
Figure 2 Hardness of 5Cr8MoVSi die steel after heat treatment
4. Conclusions
In this study, the microstructure and hardness of 5Cr8MoVSi die steel after heat treatment were studied. The main conclusions are summarized as follows:
(1) The microstructure of 5Cr8MoVSi die steel consists of martensite and a certain amount of austenite. With the increase of temperature, the amount of austenite increases, and the morphology of martensite changes to an acicular shape.
(2) With the increase of temperature, the hardness of the martensite increases from HRC 58 to HRC 60. When the temperature was raised to 890℃, the hardness decreased slightly, but still can meet the requirements for die steel (HRC 58~62).
(3) With the increase of time, the uniformity of austenitic grain and the amount of austenite increase, so does the hardness.
This study provides a reference for the thermal treatment of 5Cr8MoVSi die steel. In the future, further investigation of the heat treatment process of 5Cr8MoVSi die steel will be necessary to obtain optimized performance of the die steel.
