How to control the surface carbon content of carburized steel?

The surface carbon content and its distribution gradient of carburized steel parts have an important effect on its properties. High or low surface carbon content or decarburization will affect the surface strengthening of carburizing steel parts. In general, the carburizing process including carburizing → furnace air cooling → mechanical processing → heating quenching → cleaning → tempering → grinding, carburizing reheating quenching process to make the workpiece surface slightly decarburizing, grinding treatments can remove the decarburizing layer, and correct the distortion in the quenching process, can effectively improve the dimensional accuracy of products. Several experiments have proved that the surface carbon content of carburized parts can determine the depth of the decarburization layer and verify the necessity of grinding allowance.

 

Test design scheme

Carburizing material 20CrMnMoH, surface carbon concentration requirements 0.75% ~ 0.95%, carburizing effective hardening layer depth 1.8 ~ 2.4mm, surface hardness 56 ~ 62HRC. Because the shape and structure of the carburizing steel part are not convenient for the detection of surface carbon, the sample with the furnace should be used with size 25mm ×50mm, and the quantity 6. Carburizing equipment for Ipsen multi-purpose furnace production line QTF-27-ERM.

(1) Carburizing.

With the furnace test steel bar marked no. 1-6, carburizing after air cooling in the furnace.

(2) Secondary quenching.

After carburizing and air cooling, the 4 ~ 6 test steel bars are re-heated and quenched with the workpiece. The carbon potential of the atmosphere is set at 0.18%, heated to 840℃±10℃ for 1h, and the Hofton K oil is quenched.

(3) Sample preparation.

The 25mm×10mm surface carbon sample was made from the end of the test rod by line cutting, and the test surface and label 1 ~ 6 were marked.

(4) Surface carbon detection. Clean the surface carbon sample; Measure the original length of the sample with a micrometer and record it; Surface grinding amount 0.1mm, measured and recorded the length; The surface carbon content was detected by direct reading spectrometer; Measure the length and carbon content once per grinding 0.1mm.

(5) Effective hardening layer depth detection. The depth of the hardened layer was measured by Vickers hardness tester for No.4 ~ No.6 test bar.

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

 

Test results and analysis

Three samples were grinded 8 times and their surface carbon content was measured. The single grinding amount, cumulative grinding amount, and corresponding surface carbon content of each test rod were recorded in detail. According to the test data, the surface carbon content of No. 1 ~ 3 tested bars is between 0.8% ~ 0.85% after carburizing and air cooling and the surface carbon content distribution curve decreases gently near the surface layer. The multi-purpose furnace sealing performance and atmosphere protection effect is better. However, due to the carbon potential setting of the atmosphere in the secondary heating of No.4-6 test bars was much lower than that in the carburizing, the surface carbon content was significantly “reduced” in the surface, and the carbon content at the depth of 0.1 ~ 0.15mm was 0.49% ~ 0.58%. With the surface carbon content of 0.75% as the limit, the depth of the decarburization layer of the test rod is about 0.29 ~ 0.36mm, and the grinding allowance is 0.3 ~ 0.4mm according to the process requirements. Therefore, the surface hardness and wear resistance of carburized parts can be guaranteed after grinding, and the depth of the hardened layer after grinding can also meet the final requirements of the product. Inspection results of carburized parts after grinding are shown in the following table.

Materials	Surface hardness HV	Hardened layer depth /mm	Carbide grade	Residual austenite volume	Surface decarburization
20CrMnMoH	699	2.1	1	20%	None

 

According to the test results of carburizing steel bar and carburizing parts, we can know that the uniform surface carbon content distribution and the appropriate grinding after carburizing quenching can improve the surface hardness and wear-resisting performance, as well as the dimensional accuracy.