The surface carbon content and its distribution gradient of carburized parts have an important influence on the properties of carburized layer.
Too high or too low surface carbon content or surface decarburization will have a negative impact on the surface strengthening effect of carburized parts.
The process method of certain carburized parts produced by our company is carburizing → air cooling in the furnace → machining → heating and quenching → cleaning → tempering → grinding.
The workpiece surface is slightly decarburized during reheating and quenching after carburizing, and then the decarburized layer is removed by grinding.
At the same time, the distortion during quenching is corrected, which can effectively improve the dimensional accuracy of the product.
Based on the production practice of our company, this paper analyzes the distribution of carbon content on the surface of carburized parts, determines the depth of decarburized layer and verifies the rationality of grinding allowance.
1. Test plan
The carburized parts are made of 20CrMnMo, the surface carbon concentration is required to be 0.75% ～ 0.95%, the carburized effective hardened layer depth is 1.8 ～ 2.4mm, and the surface hardness is 56 ～ 62HRC.
Since the shape and structure of carburized parts are not convenient for surface carbon detection, furnace samples are used for representative detection.
Specification of furnace sample：φ 25mm × 50mm；material: 20CrMnMo；quantity: 6.
Carburizing equipment is TQF-27-ERM of EPSON multipurpose furnace production line.
(1) Carburizing.
The furnace test bar is marked with No. 1-6. The furnace is air cooled after carburizing with the workpiece.
See Fig. 1 for the carburizing process curve.
Fig. 1 Carburizing air cooling process curve
(2) Secondary quenching.
After carburizing and air cooling, No. 4~6 test bars are reheated and quenched along with the workpiece.
The atmosphere carbon potential is set to 0.18%, heated to 840 ℃± 10 ℃ for 1h, and quenched with Houghton K oil.
(3) Sample preparation.
The φ25mm×10mm surface carbon sample shall be prepared from the end of the test bar by wire cutting, and the detection surface and mark 1~6.
(4) Surface carbon detection.
① Clean the surface carbon sample.
② Measure the original length of the sample with a micrometer and record it.
③ The surface grinding amount is 0.1mm, the length is measured and recorded.
④ The direct reading spectrometer detects the surface carbon content and records it.
⑤ The length and carbon content shall be measured once every 0.1mm of grinding.
(5) Effective hardened case depth detection.
For test bars 4~6, Vickers hardness tester is used to measure the depth of hardened layer.
(6) Sectional inspection of carburized parts.
The depth of hardened layer of carburized parts was dissected and metallographic analysis was carried out.
2. Test results and analysis
Six samples were ground for 8 times and the surface carbon content was measured.
See Table 1 and Table 2 for records of single grinding amount, cumulative grinding amount and corresponding surface carbon content of each test bar.
Table 1 Test Results of Surface Carbon of Sample 1-3
| Grinding times | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | |
| 1# | Single grinding amount/mm | 0.11 | 0.12 | 0.1 | 0.09 | 0.09 | 0.09 | 0.11 | 0.11 |
| Cumulative grinding amount/mm | 0.11 | 0.23 | 0.33 | 0.42 | 0.51 | 0.6 | 0.71 | 0.82 | |
| Carbon content (%) | 0.83 | 0.8 | 0.8 | 0.78 | 0.77 | 0.75 | 0.73 | 0.7 | |
| 2# | Single grinding amount/mm | 0.14 | 0.11 | 0.13 | 0.1 | 0.1 | 0.08 | 0.11 | 0.1 |
| Cumulative grinding amount/mm | 0.14 | 0.25 | 0.38 | 0.48 | 0.58 | 0.66 | 0.77 | 0.87 | |
| Carbon content (%) | 0.82 | 0.79 | 0.79 | 0.77 | 0.75 | 0.73 | 0.71 | 0.68 | |
| 3# | Single grinding amount/mm | 0.13 | 0.1 | 0.14 | 0.1 | 0.1 | 0.08 | 0.11 | 0.11 |
| Cumulative grinding amount/mm | 0.13 | 0.23 | 0.37 | 0.47 | 0.57 | 0.65 | 0.76 | 0.87 | |
| Carbon content (%) | 0.84 | 0.8 | 0.81 | 0.78 | 0.77 | 0.76 | 0.73 | 0.71 | |
Table 2 Surface Carbon Test Results of Sample 4-6
| Grinding times | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | |
| 4# | Single grinding amount/mm | 0.15 | 0.11 | 0.1 | 0.12 | 0.08 | 0.08 | 0.11 | 0.1 |
| 0.15 | 0.26 | 0.36 | 0.48 | 0.56 | 0.64 | 0.75 | 0.85 | ||
| Cumulative grinding amount/mm | |||||||||
| 0.49 | 0.62 | 0.75 | 0.77 | 0.79 | 0.76 | 0.74 | 0.71 | ||
| Carbon content (%) | |||||||||
| 5# | Single grinding amount/mm | 0.13 | 0.11 | 0.13 | 0.1 | 0.09 | 0.09 | 11 | 0.1 |
| 0.13 | 0.24 | 0.37 | 0.47 | 0.56 | 0.65 | 0.76 | 0.86 | ||
| Cumulative grinding amount/mm | |||||||||
| 0.58 | 0.71 | 0.79 | 0.78 | 0.76 | 0.74 | 0.72 | 0.69 | ||
| Carbon content (%) | |||||||||
| 6# | Single grinding amount/mm | 0.11 | 0.1 | 0.13 | 0.12 | 0.08 | 0.09 | 0.11 | 0.1 |
| 0.11 | 0.21 | 0.34 | 0.46 | 0.54 | 0.63 | 0.74 | 0.84 | ||
| Cumulative grinding amount/mm | |||||||||
| 0.53 | 0.68 | 0.79 | 0.78 | 0.76 | 0.75 | 0.72 | 0.69 | ||
| Carbon content (%) | |||||||||
The distribution gradient curve of surface carbon content is drawn according to the data in the table.
As shown in Fig. 2, No. 1~3 curves are the curves obtained after carburizing air cooling, and No. 4~6 curves are the curves obtained after carburizing → air cooling → secondary quenching.
Fig. 2 Distribution Gradient Curve of Carbon Content on the Surface of Carburizing Test Bar
The test result of hardened layer of 4~6 test bars after quenching is 2.3~2.5mm, slightly higher than the upper limit of process requirements.
According to the test data, the surface carbon content of test bars 1~3 after carburizing and air cooling is between 0.8%~0.85%, the surface carbon content distribution curve drops gently near the surface layer, and the sealing performance and atmosphere protection effect of multi-purpose furnaces are good;
However, the carbon potential of the atmosphere of test bars 4~6 is much lower than that of carburizing during secondary heating, so the surface carbon content is obviously “low head” on the surface, and the carbon content at the depth of 0.1~0.15 mm is 0.49%~0.58%.
When the surface carbon content is 0.75%, the depth of the decarburized layer of the test bar is about 0.29~0.36mm, and the process requires a grinding allowance of 0.3~0.4mm.
Therefore, the carburized parts can guarantee a certain surface hardness and wear resistance after grinding, and the depth of the hardened layer after grinding can also meet the final requirements of the product.
The section inspection results of carburized parts after grinding are shown in Table 3.
Table 3 Sectional Inspection Results of Carburized Parts
| Material quality | Surface hardness HV | Hardened case depth/mm | Carbide grade/grade | Amount of retained austenite | Surface decarburization |
| 20CrMnMo | 699 | 2.1 | 1 | 20% | None |
3. Conclusion
According to the test results of carburized furnace samples and carburized parts, the distribution control of surface carbon content is reasonable, the grinding reserve is reasonable after carburizing and quenching, and there is no decarburized layer on the workpiece surface after grinding, which not only meets the surface hardness and wear resistance, but also facilitates the improvement of dimensional accuracy.
