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Analysis of carbon content on the surface of carburized parts

The carbon content and its distribution gradient on the surface of carburized parts have an important influence on the properties of the carburized layer. Too high or too low surface carbon content or surface decarburization will have adverse effects on the surface strengthening effect of carburized parts. The process flow of a certain carburized part is carburizing→air cooling in the furnace→machining→heating and quenching→cleaning→tempering→grinding. The decarburized layer is removed by the cutting method, and the distortion occurred in the quenching process is corrected, which can effectively improve the dimensional accuracy of the product. In this paper, combined with the actual production, the surface carbon content distribution of the carburized parts is detected and analyzed, the depth of the decarburized layer is determined, and the rationality of the grinding allowance is verified.

1.Experimental protocol

The material of carburizing parts is 20CrMnMo, the surface carbon concentration is required to be 0.75%~0.95%, the depth of the effective hardened layer of carburizing is 1.8~2.4mm, and the surface hardness is 56~62HRC. Because the shape and structure of the carburized parts are not convenient for the detection of surface carbon, the samples with the furnace are used for representative detection.

(1)Vacuum carburizing. The test rods with the furnace are marked with Nos. 1 to 6, and the furnace is air-cooled after carburizing with the workpiece. The carburizing process curve is shown in Figure 1.

(2) Secondary lighting. After carburizing and air-cooling, the No. 4~6 test bars are reheated and quenched with the workpiece, the carbon potential of the atmosphere is set to 0.18%, and the temperature is heated to 840℃±10℃ for 1h.

(3) Sample preparation. The test bar is cut from the end of the coin to obtain a 25mm × 10mm surface carbon content sample, and the detection surface and the number 1~6 are marked.

(4) Surface carbon detection. ①Clean the surface carbon content sample. ②Use a micrometer to measure the original length of the sample and record it. ③ The surface grinding amount is 0.1mm, and the length is measured and recorded. ④ Direct reading spectrometer to detect and record the carbon content on the surface.

(3) Measure the length and carbon content once every 0.1 mm of grinding.

(5) Depth detection of effective hardened layer. The depth of the hardened layer was detected by a Vickers hardness tester for No. 4-6 test bars.

(6) Physical autopsy of carburized parts. The hardened layer depth and metallographic analysis were carried out on the carburized parts.

2 Test results and analysis

A total of 6 samples were ground for 8 times and the surface carbon content was detected. The single grinding amount, cumulative grinding amount and the corresponding surface carbon content records of each test bar are shown in Table 1 and Table 2.

Draw the distribution gradient curve of carbon content on the surface according to the table data, as shown in Figure 2, the curves 1~3 are the curves obtained after carburizing-air cooling, and the curves 4~6 are obtained after carburizing → air cooling → secondary quenching the curve.

The test results of the hardened layer of No. 4~6 test bars after quenching are 2.3~2.5mm, which is slightly higher than the upper limit of the process requirements.

According to the test data, the surface carbon content of No. 1~3 test bars is between 0.8% and 0.85% after carburizing and air cooling, and the surface carbon content distribution curve decreases gently in the near-surface layer. Good; and the carbon potential of No. 4-6 test bars is much lower than that of carburizing during the secondary heating, so the surface carbon content has an obvious “head down” phenomenon in the surface layer, and the carbon content at the depth of 0.1~0.15mm is 0.49 %~0.58%. With the surface carbon content of 0.75% as the limit, the depth of the decarburized layer of the test bar is 0.29~0.36mm, and the process requires a grinding allowance of 0.3~0.4mm, so the carburized parts can ensure a certain surface hardness and wear resistance after grinding. , The depth of the hardened layer after grinding can also meet the final requirements of the product.

3.Conclusion

According to the test results of carburized samples and carburized parts, the distribution of surface carbon content is reasonably controlled, the grinding reserve after carburizing and quenching is reasonable, and there is no decarburization layer on the surface of the workpiece after grinding, which not only satisfies the surface hardness And wear-resistant requirements, but also conducive to the improvement of dimensional accuracy.

Selection of carburizing equipment: The RVC series vacuum carburizing furnace produced by SIMUWU is a high-quality product for the vacuum carburizing process of tools and molds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum carburizing process.