Effect of Heat Treatment Process on Microstructure and Mechanical Properties of 15CrMo Forgings

The effect of the heat treatment process on the microstructure and mechanical properties of 15CrMo forgings was analyzed by means of a heat treatment test, tensile test, impact test, and metallographic inspection. The results show that at 910 ℃ × 100min normalizing, water cooling, and 640 ℃ × Under the tempering process of 300 min, 15CrMo forgings obtain uniform bainite tempering structure and have the best comprehensive mechanical properties.

15CrMo steel forgings are low-alloy heat-resistant steel based on chromium and molybdenum. The maximum working temperature can reach 600 ℃. They have good high-temperature oxidation resistance, thermal strength, and good sulfur and hydrogen corrosion resistance. It is widely used in synthetic chemical vessels, conventional thermal power plants, petroleum refining equipment, hydrocracking units, and other high-temperature processing equipment. Abnormal mechanical property data of 15CrMo steel often occur in production, which will greatly impact enterprises. Through studying the metallographic structure and mechanical properties of 15CrMo steel forgings under different heat treatment processes, this paper finds the corresponding relationship between their mechanical properties and metallographic structure in order to provide a reference for the theoretical research and practical production of this metal material.

1. Test materials and methods

The test material is 15CrMo steel forgings (normalized as supplied), and its chemical composition is shown in Table 1. High-temperature box-type resistance furnace is used for heat treatment. The sample is placed in the furnace at room temperature and then heated to different temperatures (910, 920, 930, and 940 ℃, respectively) according to the set heating rate and kept for 100min. Different cooling methods are used: water cooling, spray, and air cooling, and then 640 ℃ × See Table 2 for the specific heat treatment process.

CMT5305 microcomputer-controlled electronic universal testing machine is used for tensile tests. The sample specification is ϕ 10mm, test 3 samples in each group, and take the average value; The impact test is conducted with ZBC2602N-3 Charpy pendulum impact tester, and the sample size is 10mm × 10mm × 55mm, the test temperature is – 30 ℃, and the average value of 6 samples in each group is taken; The THBS3000E Brinell hardness tester is used for hardness test, and each group tests 3 data. The load is 7.355kN, and the load is maintained for 15s; Metallographic examination was carried out with GX51 optical microscope; The impact fracture morphology was observed by JSM-IT300 scanning electron microscope (SEM).

Table.1 Chemical composition of 15CrMo steel forgings

Element	C	Si	Mn	P	S	Cr	Ni	Cu
Detection value	0.15	0.22	0.51	0.007	0.003	1.01	0.13	0.47
Standard value	0.12–0.18	0.10–0.60	0.30–0.80	≤0.025	≤0.010	0.80–1.25	≤0.30	0.45–0.65

Table.2 Heat treatment process of 15CrMo steel forgings

Sample No	Heat treatment process
	Normalizing	Normalizing Cooling Method	Tempering
1-1	910 ℃×100 min	Water-cooling	640 ℃×300 min
1-2		Spray
1-3		Air Cooling
2-1	920 ℃×100 min	Water-cooling	640 ℃×300 min
2-2		Spray
2-3		Air Cooling
3-1	930 ℃×100 min	Water-cooling	640 ℃×300 min
3-2		Spray
3-3		Air Cooling
4-1	940 ℃×100 min	Water-cooling	640 ℃×300 min
4-2		Spray
4-3		Air Cooling

2. Test results and discussion

2.1 Mechanical property test

See Table 3 for mechanical property data of 15CrMo steel forgings. It can be seen that the tensile strength of sample 3-1 and sample 3-3 does not meet the standard requirements; The hardness values of sample 2-1, sample 3-1, sample 3-2, sample 3-3 and sample 4-1 do not meet the requirements of NB/T47008-2017 Carbon and Alloy Steel Forgings for Pressure Equipment; The impact absorption energy of samples 1-3, 2-3, 3-2, 4-1 and 4-2 at – 30 ℃ is lower than the standard value (≥ 41J). At the same normalizing temperature, with the increase of normalizing cooling rate, the yield strength, tensile strength, and hardness all show an upward trend. At the same time, the elongation and area reduction change is not obvious, as shown in Figure 1.

2.2 Microstructure observation

The microstructure of 15CrMo steel forgings with different cooling methods at 910 ℃ normalizing temperature is shown in Figure 2. The material’s mechanical properties with water cooling are better than spray and air cooling. The microstructure of samples 1-1 and 1-2 is uniform bainite tempered structure; The microstructure of samples 1-3 is massive ferrite+pearlite.

2.3 Fracture analysis

SEM morphology of impact fracture of 15CrMo steel forgings with different cooling methods at 910 ℃ normalizing temperature is shown in Fig. 3. The fracture morphology of samples 1-1 and samples 1-2 is dimple fracture, and samples 2-3 is cleavage fracture.

Table.3 Mechanical property data of 15CrMo steel forgings

Sample No	ReL/MPa	Rm/MPa	A/%	Z/%	-30 ℃ KV2/J	Hardness/HBW5/750
1-1	463.7	585.7	30	82	300/300/305/300/302/316	177/174/174
1-2	426	578	29.5	78.3	267/82.9/245/260/257/287	170/170/169
1-3	333	503.7	34.2	75.3	49.7/19.2/14.7/219/17.719.3	140/140/139
2-1	503.3	604.7	28.8	81.3	328/338/326/313/332/324	175/183/180
2-2	404	563.3	29.5	74.3	310/334/329/314/269/326	148/160/153
2-3	425	550.3	29.3	78	35.2/78.5/53.6/42.4/36.8/59.0	160/157/152
3-1	563	681.3	24.2	78.3	301/301/300/311/308/305	193/207/217
3-2	486.3	611	25.5	78	12.8/16.5/233/112/195/247	177/195/187
3-3	508.3	641.7	23.8	71.7	226/214/275/289/276/193	197/197/179
4-1	450.3	571.3	29.3	82.3	>300	170/171/181
4-2	402.7	533.3	29.5	80.7	10.8/14.5/11.5/ 13.3/12.2/11.5	161/161/164
4-3	409	535	28.2	78	10.2/8.84/9.33/9.37/9.57/7.75	161/164/161
Standard value	≥280	480–640	≥20	/	≥47	118–180

Fig.1 Comparative analysis of mechanical properties of 15CrMo steel forgings
(a) Strength; (b) Fractional shrinkage and elongation; (c) Hardness; (d) Impact absorbed work

Figure.2 Microstructure of specimens at 910°C normalizing temperature
(a) water cooling; (b) spraying; (c) air cooling

Figure.3 Fracture morphology of specimens under normalizing temperature of 910℃
(a) water cooling; (b) spraying; (c) air cooling

3. Discussion

Through the test of mechanical properties, it is found that 15CrMo steel forgings are cooled at different cooling rates at the same normalizing temperature, and the comprehensive mechanical properties are: water cooling>spray>air cooling. When air cooling is adopted for cooling, the undercooling of austenite is small, the transformation temperature is high, the diffusion ability of various elements is strong, and the austenite has the conditions for diffusive transformation, so the microstructure is ferrite+pearlite. When water cooling is used for cooling, the cooling rate increases. The long-range diffusion ability of carbon, iron and other atoms in austenite decreases and becomes a semi-diffusion and semi-sheer type of intermediate-temperature transformation product, namely the bainite structure. There are many dislocations in the bainite structure, and the strength is relatively high. The carbide in the bainite is fine and dispersed in the matrix, which improves the toughness.

The effect of austenitizing temperature on the properties of 15CrMo steel essentially affects the microstructure and structure of steel through the normalizing process. With the increase of normalizing temperature, 15CrMo steel can dissolve more alloy elements within the range of austenitizing temperature; that is, the effect of solid solution strengthening is enhanced, increasing material strength and hardness, even exceeding the upper limit of the standard requirements. Therefore, 15CrMo steel forgings have the best mechanical properties under 910 ℃ normalizing and water cooling conditions.

4. Conclusion

• 1) At the same normalizing temperature, the mechanical properties of 15CrMo steel forgings by water cooling are better than those by spray and air cooling because the bainite structure is obtained by water cooling. There are many dislocations in the bainite structure, and the carbide is dispersed in the matrix, making its comprehensive mechanical properties the best;
• 2) At the normalizing temperature of 910 ℃, the dispersion strengthening, solution strengthening, and grain refinement strengthening reach the optimal equilibrium state, and the comprehensive mechanical properties of 15CrMo steel forgings are the best.

Author: Li Gang, Li Qingsong, Fu Ying, Ruxin, Liu Haijin, Wang Jimin

Source: China 15CrMo Forgings Manufacturer – Yaang Pipe Industry (www.epowermetals.com)