Influence of heat treatment process on the organization and properties of 45# steel

The effect of changing the heating temperature and cooling rate on the microstructure and mechanical properties of 45# steel was studied. The results show that different heat treatment heating temperatures will change the microstructure of 45# steel and its hardness properties; simultaneously, the heat treatment temperature of 45# steel cannot be very high nor too low; the more appropriate heat treatment temperature is 840 ℃. Different cooling rate on the organization of 45# steel properties of the impact is more pronounced; the cooling rate of the change directly to the organization of 45# steel changes, so obviously change its performance.

45# steel is cost-effective and high-quality carbon structural steel, so the scope of application is very wide. 45# steel physical and chemical properties of modern industry are important, so studying 45# steel is very important. In recent years, to change the heat treatment process to enhance the performance of 45# steel, the research in this area has made great progress, but also there will be more new technology and results. In this paper, we mainly change the heating temperature and cooling rate to study the effect of the heat treatment process on the organization and properties of 45# steel.

1. Test materials and methods

1.1 Test material

The original state of 45# steel selected for this test is a hot rolled state, 45# steel specimens 9, 4% nitric acid alcohol solution, anhydrous ethanol, test the chemical composition of 45# steel for C: 0.43, Si: 0.24, Mn: 0.66, P: 0.03, S: 0.03, Ni: 0.22, Cu: 0.23.

1.2 Test equipment

Heating equipment: crucible resistance furnace; hardness performance testing equipment: Rockwell hardness tester; metallographic organization observation equipment: metallographic pre-mill, polishing machine, hair dryer, cotton balls, MR-5000 metallographic microscope.

1.3 Test method

First of all, the 45# steel processing into nine metallographic specimens; the above specimens were divided into three groups, each group of three specimens were placed at temperature of 760 ℃, 840 ℃, 940 ℃ temperature insulation for 30 minutes, and the specimens of the various groups of cooling treatment in oil, cooling treatment in the water and cooling treatment in the air; and then corrode the specimens, the organization of the 45# steel to observe and analyze, and then measured the hardness value of each specimen. Then the hardness value of each specimen is measured.

2. Test results and analysis

2.1 Mechanical properties analysis

As shown in Table 1, the heat treatment process is different; the same heat treatment heating temperature, the hardness is not the same. Take three 45# steel specimens for normalizing, respectively, in 760 ℃, 840 ℃, and 940 ℃ in the heating measured. The hardness value of the specimen was 8.99, 16.1, and 16.3 HRC. 45# steel oil quenching of the specimen, there are three, respectively, in 760 ℃, 840 ℃, and 940 ℃ in the heating, measured the hardness value of the specimen were 24.1, 24. 32.5 HRC. The remaining three specimens were quenched in oil, respectively, at 760 ℃, 840 ℃, and 940 ℃ in the heating; the hardness value of the specimen was 24.1, 24. 32.5 HRC, 32.5 HRC. The remaining three specimens for water quenching, respectively, in 760 ℃, 840 ℃, and 940 ℃ in the heating, the measured hardness of the specimens were 34.1, 58.1, 56.3 HRC. from the above data can be seen, quenched specimens than normalized hardness. At the same time, water-quenched specimens’ hardness than oil-quenched is also much larger. This is because of the difference in cooling rate caused by the different organizations. After quenching in oil, the organization of the main bainite, pearlite residual ferrite, etc., water quenching and cooling treatment of the organization of the main martensite, and martensite to be higher than the hardness of other organizations.
Table.1 Rockwell hardness value of 45# steel after heat treatment

Heating temperature/℃	Cooling method	Average hardness value/HRC
760℃	Air cooling	8.99
	Oil cooling	24.1
	Water-cooling	34.1
840℃	Air cooling	16.1
	Oil cooling	24
	Water-cooling	58.1
940℃	Air cooling	16.3
	Oil cooling	32.5
	Water-cooling	56.3

2.2 Metallographic microstructure analysis

45# steel by 760 ℃ normalizing treatment, the organization of the grain is very large; 45 # steel heating temperature 840 ℃, the organization of the grain is very small. Due to the lower 760 ℃ heat treatment heating temperature, austenite distribution is not uniform, and the 940 ℃ temperature is higher, the organization of the grain becomes very coarse.
45# steel for water quenching treatment, the different heating temperatures will get a different organization, and thus the hardness will be different. With the heating temperature of 760 ℃, the resulting organization has more ferrite. At the same time, the heating temperature of 840 ℃ and 940 ℃, the organization obtained is martensite. And when the heating temperature is 840 ℃, the martensite obtained is finer, mainly because the austenite grains obtained are smaller. And add a temperature of 940 ℃, the austenite grain becomes coarse to get coarse martensite organization.
The 45# steel for oil quenching treatment, not all martensite organization. A heating temperature of 760 ℃ in the oil quenched after the organization is mainly composed of bainite, pearlite residual ferrite, etc. While the heating temperature is 840 ℃ and 940 ℃, the organization after quenching in the oil is mainly composed of bainite and pearlite, but 940 ℃ oil quenching after pearlite organization content is more.

3. Conclusion

• 1) Different heat treatment heating temperatures will change the microstructure of 45# steel and its hardness properties; at the same time, the heat treatment temperature of 45# steel cannot be very high, nor can it be too low; the more appropriate heat treatment temperature is 840 ℃.
• 2) Different cooling rate on the organization of 45# steel properties of the impact is more obvious; the cooling rate of the change directly to the organization of 45# steel changes, so it changes its performance.

Author: Wang Guoxin