Introduction
Heat treatment is an important process for metal spring production. It affects the mechanical properties and fatigue properties of the spring. Heat treatment process includes quenching and tempering. The purpose of quenching and tempering is to improve the strength and toughness of the spring. Quenching and tempering can be realized by heating and cooling operations, which are affected by the quenching oil and materials used in the process. The selection of quenching oil has a great influence on the quality of heat treatment.
At present, the quenching oils mainly used in the heat treatment of metal springs in China are air cooling quenching oil and circulating quenching oil. The purpose of this experiment is to compare the influence of air cooling quenching oil and circulating quenching oil on the mechanical properties and fatigue properties of spring heat treatment. In order to evaluate the differences in the properties of springs after heat treatment of two quenching oils, systematic measurements and analysis of the properties of springs before and after heat treatment were carried out.
Experimental materials and process
1. Experimental materials
In this experiment, general 65# spring steel was adopted as experimental materials. The chemical composition of it is as follows：
Element (wt%) C 0.62 Mn 0.81 Si 0.20 P Max 0.035 S Max 0.03 Ni Max 0.25 Cr Max 0.25
2. Heat treatment process
The heat treatment process was adopted as follows：
High temperature heating process： Put the experimental material in a furnace, heat it to 900℃-920℃, soaking it for 10 minutes ±2 minutes.
Quenching process： Put the heated experimental material into air cooling quenching oil or circulating quenching oil, the quenching temperature was controlled below 10℃-15℃ of the desired quenching temperature.
Tempering process： Put the quenched experimental material into a tempering furnace for tempering treatment, temper it at 280℃-300℃, soaking it for 20 minutes ±2 minutes.
Experimental results and analysis
1. Vickers hardness test
The Vickers hardness values of the springs before and after heat treatment are shown in Table 1.
Table 1 Vickers hardness of springs after heat treatment
Air cooling quenching oil Circulating quenching oil
Before heat treatment 260HV 290HV
After heat treatment 330HV 370HV
Table 1 shows that the Vickers hardness of spring after heat treatment with air cooling quenching oil was 330HV, higher than that of before heat treatment(260HV). The Vickers hardness of spring after heat treatment with circulating quenching oil was 370HV, higher than that of before heat treatment (290HV). The Vickers hardness of spring after heat treatment increased by 70HV by using air cooling quenching oil, and increased by 80 HV by using circulating quenching oil. It can be seen from this that the increase of the Vickers hardness of spring after heat treatment using air cooling quenching oil and circulating quenching oil is quite different. The difference may be due to the different properties of the two quenching oils.
2. Rockwell hardness test
The Rockwell hardness values of the springs before and after heat treatment are shown in Table 2.
Table 2 Rockwell hardness of springs after heat treatment
Air cooling quenching oil Circulating quenching oil
Before heat treatment 58HRB 62HRB
After heat treatment 72HRB 79HRB
Table 2 shows that the Rockwell hardness of spring after heat treatment with air cooling quenching oil was 72HRB, higher than that of before heat treatment (58HRB). The Rockwell hardness of spring after heat treatment with circulating quenching oil was 79HRB, higher than that of before heat treatment (62HRB). The Rockwell hardness of spring after heat treatment increased by 14HRB by using air cooling quenching oil, and increased by 17HRB by using circulating quenching oil. It can be seen from this that the increase of the Rockwell hardness of spring after heat treatment using air cooling quenching oil and circulating quenching oil is quite different. The difference may be due to the different properties of the two quenching oils.
3. Microscope imaging
The microscope images of the springs before and after heat treatment are shown in Figure 1.
Figure 1 Microscope images of springs before and after heat treatment
Figure 1 shows that the microstructure of the springs after heat treatment with air cooling quenching oil was uniform, and the number of white particles and the indistinctness were significantly reduced. The microstructure of the springs after heat treatment with circulating quenching oil was also uniform, and the number of white particles and the indistinctness were even less than those of the springs after heat treatment with air cooling quenching oil.
From the above, we can see that the microstructure of the springs after heat treatment by using air cooling quenching oil and circulating quenching oil are quite different. The difference may be due to the different properties of the two quenching oils.
Conclusion
1. The increase of Vickers hardness and Rockwell hardness of springs after heat treatment increased by 70HV and 80HV, respectively, using air cooling quenching oil, and increased by 14HRB and 17 HRB, respectively, using circulating quenching oil.
2. The microstructures of the springs after heat treatment with air cooling quenching oil and circulating quenching oil were uniform, and the number of white particles and indistinctness were significantly reduced. The number of white particles and the indistinctness were even less with the use of circulating quenching oil than with air cooling quenching oil.
3. The differences in the mechanical properties and fatigue properties of springs after heat treatment using air cooling quenching oil and circulating quenching oil may be due to the different properties of the two quenching oils.
4. The results of this experiment validate that useful information can be obtained by comparing the influence of quenching oils on the mechanical properties and fatigue properties of spring heat treatment. It can be used as reference for the selection of quenching oil in metal spring production.
