35CrMo Steel Water Quenching vs. Oil Quenching: The Difference Explained

35CrMo steel, as the common material of our company’s parts, is often quenched and tempered by oil cooling or water quenching oil cooling.

After the multi-purpose furnace production line is put into production, the equipment cannot complete the water quenching oil cooling process.

The oil-quenched workpiece (especially the rolled bar without surface processing) has the problem of low hardness and can not meet the drawing requirements, so it needs to be reworked for secondary quenching.

This not only affects the use performance of parts, but also increases the production cost and causes unnecessary losses.

Through analysis, the low hardness is mainly caused by insufficient cooling speed.

According to relevant data and production experience of other heat treatment plants, 35CrMo steel bars can be cooled by brine to increase the cooling rate.

Since our company has not conducted water quenching in the production of 35CrMo steel bars before, we need to test the water quenching and oil quenching methods of 35CrMo steel bars, compare the mechanical properties, and then decide whether to implement water quenching of 35CrMo steel bars.

1. Determination of test plan

(1) Test materials

The test material is 35CrMo steel bar produced by our company( φ 60mm), see Table 1 for chemical composition.

Table 1 Chemical Composition of 35CrMo Steel Bar (Mass Analysis) (%)

ProjectCSiMnCrMoSPCuNi
National standard0.32~0.400.17~0.370.40~0.700.80~1.100.15~0.25≤0.035≤0.035≤0.30≤0.30
Test materials0.360.260.560.990.170.030.020.10.1

(2) Test instruments and equipment

Multi-purpose furnace production line for test equipment is shown in Figure 1.

The Rockwell hardness tester and CMT8202 microcomputer are used to control the electronic universal testing machine.

Fig. 1 Multi purpose furnace production line

(3) Test method

The test is divided into two parts: comparing the depth of the hardened layer of two quenching methods;

The tensile test is carried out for the samples of two quenching methods, and the tensile strength and elongation are compared.

2. Test process and result analysis

2.1 Hardening depth

Take φ60mm bar as test sample, and conduct oil quenching and water quenching.

(1) Oil quenching process

Quenching 860 ℃, holding for 45min, oil cooling to 150 ℃;

Temper at 200 ℃, hold for 180min, and air cool to room temperature.

(2) Water quenching process

Quenching 860 ℃, holding for 45min, water cooling to 150 ℃;

Temper at 200 ℃, hold for 180min, and air cool to room temperature.

There is no quenching crack on the surface of the treated sample.

The hardness of the machined surface shall be measured every 2.5mm from the surface to the center, as shown in Fig. 2.

The hardness results are shown in Table 2.

It can be seen from Table 2 that the hardness after water quenching is high and the hardened layer is deep.

Fig. 2 Hardness Test of Hardened Layer

Table 2 Hardness of Hardened Layer

Distance to surface/mm02.557.51012.51517.52022.52530
Water quenching hardness HRC535250464037.5353332302825.5
Oil quenching hardness HRC33323130.5302827.52624232322.5

2.2 Comparison of mechanical properties

35CrMo steel bars are subject to oil quenching and water quenching, and the hardness can reach 28~30HRC by adjusting the tempering temperature.

(1) Oil quenching process

Quenching 860 ℃, holding for 45min, oil cooling to 150 ℃;

Temper at 520 ℃, hold for 90min, and air cool to room temperature. The process curve is shown in Fig. 3.

Fig. 3 Oil quenching process curve

(2) Water quenching process

Quenching 860 ℃, holding for 45min, water cooling to 150 ℃;

Temper at 580 ℃, hold for 90min, and air cool to room temperature. The process curve is shown in Fig. 4.

Fig. 4 Water quenching process curve

The test bar is processed into a tensile sample for tensile test, and the results are shown in Table 3.

It can be seen from Table 3 that the strength after water quenching is relatively high and the toughness is relatively low, with little difference overall.

Table 3 Mechanical Properties

Quenching cooling mediumTensile strength/MPaElongation (%)
brine88014
oil81419

2.3 Result analysis

From the above test results, it can be concluded that the tensile strength is significantly improved, but the elongation is reduced under the same surface hardness, because the water quenching cooling speed is fast, the hardened layer is deep, and the hardness is high.

This can solve the problem of the insufficient hardness of 35CrMo steel bars after oil quenching.

3. Benefit analysis

Changing some 35CrMo steel parts to water quenching can not only meet the production requirements, and reduce the rework rate, but also save costs and production costs.

Electricity saving=electricity cost for rework (equipment power × annual production furnace × rework rate × electricity price)+electricity cost saved by existing process (equipment power × save time × annual production furnace × electricity price) = (209 × 1.5+78 × 3) × 120 (about 120 heats of 35CrMo steel production throughout the year) × 10% × 1.2+91 (washer power) ×0.5×120×1.2yuan = 14436 yuan.

Labor cost saved = (time saved+rework time) × number of operators × annual production furnace × unit price of man hour=(0.5+4.5×10%)×2×120×8.571yuan = 1954yuan.

Material cost saved = (quenching oil loss per furnace+cleaning fluid loss) × annual production heats=(50+20) × 120 yuan = 8400 yuan.

The annual total is about 24790 yuan.

4. Conclusion

35CrMo steel bars can be quenched by water instead of oil.

Not only can the hardness meet the requirements, but also have little impact on the comprehensive mechanical properties.

Related reading: Metal Mechanical Properties Chart

In addition, the rework rate is greatly reduced, the labor efficiency is improved, and the production cost is saved.

At the same time, the lower quenching temperature and the elimination of oil smoke generated in the oil cooling quenching process of the material bring better economic benefits and have the advantage of energy saving and emission reduction.

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