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Inspection requirements for hardened zone of induction hardening parts

At present, induction hardening has been widely used in the production of automobile parts to improve the strength and increase the surface hardness. In order to confirm that the quenching quality of induction hardening parts meets the technical requirements specified in the drawing, the appearance, hardness, hardening layer depth, hardening area, metallographic structure, deformation and crack should be inspected. However, in practical application, some enterprises only invest in machine tools or outsourcing induction hardening treatment, and do not seriously inspect and control the hardening area, which leads to the occurrence of quality problems. The following will introduce some cases to illustrate the importance of hardening area and inspection requirements of induction hardening parts.
In order to detect the hardened area and hardened layer depth of induction hardening parts, it is necessary to take samples and grind samples. Because it is time-consuming and laborious, as the hardened area of working face, some enterprises often use hardness tester to test the hardness in the hardened area at random, but do not judge whether the hardened area meets the requirements. If the hardened area is just in the transition or stress area, even if other inspection items meet the requirements, it will also cause stress concentration and fracture after stress.

Hardened area of stepped shaft

Many types of stepped shafts are involved in automobile parts. For a 40Cr front shell product (see Figure 1), the hardened layer is required to be 5-7mm deep. Due to the imperfect quenching process, in order to ensure that the hardened layer area is too large, the coil stays at the fillet for a long time, resulting in too deep hardened layer at the fillet, too large stress and easy to crack.

20210412214603 25595 - Inspection requirements for hardened zone of induction hardening parts

Figure 1 required hardened area

In order to avoid this situation, the manufacturer adjusted the quenching area as shown in Fig. 2, and the quenching layer was interrupted, which greatly reduced the torsional strength of the front shell and caused early fracture. For the parts with front shell bearing torque, the hardened area must be too large in diameter and chamfered. Therefore, for similar stepped shaft products, to check whether continuous and uninterrupted hardened area is obtained, it is necessary to consider sampling inspection at the step part (dotted box in Figure 1).

20210412214650 16640 - Inspection requirements for hardened zone of induction hardening parts

Figure 2 actual hardened area

Hardened area of spline shaft

For a brake camshaft product, the drawing requires that the hardened layer in the local polished rod area is 1 ~ 3mm deep, and there is a 15mm interval between the hardened area boundary and the spline undercutting. Through the analysis of the real object fracture in the spline transition area, it is found that the polished rod fracture has 1.3mm deep tempered martensite structure, the spline undercutting fracture is pearlite + ferrite metallographic structure, and the fracture is just the transition area between the hardened area and the original state, The surface hardness decreases from 60HRC to 17hrc, and changes from the surface residual compressive stress state to the tensile stress state.

Hardening area of polished rod shaft

Through the analysis of the failure parts of a balance suspension shaft, it is found that the fracture position is in the boundary area between the quenched area and the non quenched area. The CAE analysis shows that the fracture position is just the stress area. After checking with the technical requirements, it is found that the length of hardened zone is only 270mm, which is lower than 350mm. In the daily hardness and metallographic testing, the hardened area was not confirmed, and this problem was not found. After the intermediate frequency quenching process of the balance suspension shaft was improved by the manufacturer, the length of the hardening zone increased to the technical requirements. After market verification, the fracture failure rate of the balance suspension shaft was reduced by more than 95%.

Integral quenching shaft products

Take the automobile half shaft as an example, the hardened area is from the inner R angle of the half shaft flange to the end of the spline. When using the scanning quenching method, when the transition section between the rod and spline is scanned, the magnetic line of force on the effective ring often deviates, that is, the rod is not heated to the transition area, but the spline is heated, so the step part is difficult to guarantee in the process control. In order to shorten the testing time and grasp the quality of induction hardening of half shaft as soon as possible, the sampling position at the dotted box in Figure 3 is recommended.

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Fig. 3 sampling position of half axis scanning quenching

Epilogue

In the hardening area of induction hardening parts, one or several sections should be selected for anatomical detection and analysis. In addition to detecting the depth and microstructure of hardening layer, the detection of hardening area should also be considered.

Local hardening parts

For parts with short hardened zone length, the whole zone can be taken for inspection in whole section or section. For the parts with long hardened area, the representative area can be selected for inspection, and the representative area is usually the section, stress surface or transition area with variable size.

Integral induction hardening parts

It is necessary to take samples from several places for quality inspection. The selected representative area is the same as the selection principle of local quenched parts, so as to effectively judge the overall induction quality with the least section as far as possible.

Authors: Miao Guoliang, Miao Guoliang, Xu Longli

Source: Network Arrangement – China Pipe Sleeve Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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