Analysis of quenching defects and process improvement of intermediate frequency heating power supply for wheel support shaft

The wheel support shaft is a heavy-duty cylindrical variable cross-section part with a relatively complex structure. Its material is equivalent to domestic 35G steel. The heat treatment of the support shaft adopts medium frequency heating power quenching treatment, and the heat treatment technology requirements are strict. The main requirements are: ① The minimum hardened width of the annular plane area between diameter 140 and 190 is ≥ 5mm, and the depth of the hardened layer in the cylindrical area of 140 diameter is ≥ 1mm. No tempering is required after quenching. ② The first inspection is carried out during the heat treatment of the workpiece. In addition to inspecting the hardness and quenching defects, special attention is paid to inspecting the micro-cracks and distortion defects of the workpiece, and the product quality is strictly controlled. During production, it was found that when the initial test process was used, the workpiece had insufficient hardening depth, low hardness, and fine crack defects, making the workpiece unqualified. For this purpose, follow-up inspection analysis and process improvement were carried out.

The depth of the hardened layer of the support shaft reaches 4-8.5mm, and the surface is strengthened by a medium-frequency heating power supply. The frequency is selected in the range of 2-4KHZ. A special inductor is designed according to the requirements of workpiece quenching. Its cross-section adopts a bevel shape to adapt to the heating of annular stepped workpieces; the inner diameter of the inductor is larger, making it slightly farther away from the cylindrical area to reduce the heating speed of the 140mm diameter cylindrical area. The regional heating rate tends to be uniform. During heating, the inductor should be appropriately held to heat the transition zone between the annular and cylindrical sections under the action of heat conduction and weak induction; then move the inductor to the spline area, and adjust and control its moving speed and heating power according to process requirements to obtain the requirements. hardened layer. During quenching and cooling, due to the large size of the workpiece, the depth of the quenching layer, the large range of structural changes and the strict requirements for distortion, after calculation, the local deformation of the support shaft must be controlled within ≤0.03mm to meet the requirements. The quenching medium is a water-based quenching liquid with a mass fraction of 5%-8% additives. It is spray-cooled and quenched. The spray pressure is maintained at 0.2MPA to form a cooling zone with a height of 80mm on the surface of the workpiece to ensure uniform and good quenching cooling.

The frequency of the manufactured shaft medium frequency heating power supply is 3.5KHZ. The annular gap between the sensor and the workpiece is 2mm. Experimental tests show that the hardened layer of the workpiece in the experimental process is shallow, the depth of the hardened layer does not meet the technical requirements, and the hardness is low. At the same time, it is found that if the quenching liquid additive is insufficient during production, or too much water is added, the coolant is When the mass fraction is ≤2%, fine cracks will appear in sensitive parts of the workpiece, causing the product to fail. After inspection and analysis, the following process improvement measures are proposed:

(1) The residence time of the inductor at the starting heating position is lengthened so that the workpiece can be fully heated and austenitized completely. The hardening effect is good.

(2) Increase the residence time in transition zone 2 so that transition zone 2 is fully heated and the hardening effect is improved.

(3) Appropriately increase the output power to increase the heating depth, thereby increasing the quenching and hardening depth.

(4) The rising speed of the sensor slows down. It also makes the surface of the workpiece heated more evenly and fully, so that the workpiece transforms more fully during cooling. The depth of the hardened layer increases and the hardness increases, which has reached the technical requirements.