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How to control distortion in vacuum heat treatment of sheet parts?

Thin metal parts are a kind of parts with high requirements for distortion. Taking 35CrMnSiA steel parts as an example, it is a common quenched and tempered steel with good hardenability and is widely used in the production of various thin sheet parts. When processing the quenching in this vacuum high-pressure gas quenching furnace, the high-purity high-pressure (≥ 0.4MPa) nitrogen (99.995% or more) used basically meets the requirements for quenching hardness. Although the distortion of workpieces can be solved by heat treatment in vacuum high-pressure gas quenching furnace, the flow mode of nitrogen during high-pressure nitrogen quenching and cooling puts forward new requirements for the treatment of thin pieces. At this time, it is necessary to improve the furnace charging mode of the sheet parts according to the characteristics of vacuum gas quenching to achieve the purpose of controlling distortion.

Traditional charging method:

Workpiece is φ 100mm × The hardness of 3mm thin piece (see figure) shall be controlled within 45~48HRC, and the flatness after heat treatment shall be within 0.30mm. 35CrMnSiA steel has good hardenability, and the cooling rate of high-purity high-pressure nitrogen can meet its cooling requirements. The vacuum heat treatment process is shown in the figure below. The furnace charging method is to vertically hoist the steel wire under the material rack.

After heat treatment of the sheet parts according to the above process, the surface of the sheet parts shall be bright and clean (metallic color) without oxidation and decarburization; The quenching hardness is above 49 HRC, and the tempering hardness is basically controlled within the technical requirements. However, the flatness is out of tolerance, and the yield of heat treatment is only 75%, which does not achieve the expected effect.

Causes of distortion:

All the factors that affect the quenching distortion of the workpiece affect the quenching distortion of the workpiece through the effect of the two characteristics of the workpiece. The first characteristic is the ability of the workpiece to bear the quenching cooling speed; The second characteristic is the cooling speed obtained by the workpiece during quenching and cooling. Only when the speed of the cooling speed obtained by the workpiece is completely controlled within the speed of the cooling speed it can withstand, can the quenching results of the required hardening hardness, depth of the hardened layer, no quenching cracks, and no out of tolerance quenching distortion of the workpiece be obtained.

Influence of charging method:

During the quenching and cooling of workpieces, the hanging method has a great influence on the cooling speed. If improper hanging tools are used, or the distance between workpieces is too small, the average cooling speed will be reduced, which may cause excessive quenching distortion of some workpieces. In actual production, when the sheet workpiece is quenched and discharged from the furnace, individual workpieces fall off from the iron wire and fall onto the basket, and the workpieces are in disorder as a whole, which indicates that the sheet workpiece is subjected to the impact of high-pressure nitrogen in different directions during quenching and cooling, and it swings irregularly, which makes the flow direction and pressure of high-pressure nitrogen between workpieces irregular, and this irregularity makes the flow of quenching medium uneven, It causes uneven cooling of the sheet parts and causes distortion out of tolerance. It can be seen that the distortion of a thin piece is affected by a combination of thermal stress, organizational stress and external impact force. Therefore, reasonable control of charging mode, reduction of impact force and proper reduction of undercooling are the keys to control the distortion of the sheet.

For thin pieces, the uniform and fixed charging method of longitudinal parallel and downward vertical reduces the external impact of high-pressure nitrogen on the workpiece. At the same time, the quenching temperature is adjusted to reduce the undercooling properly, which can effectively control the quenching distortion of the workpiece. The specific charging method is as follows:

The improved process and furnace charging method were used to carry out vacuum heat treatment on 30 thin pieces of the same batch. The hardness of the pieces after heat treatment was tested to be 45~47HRC, the flatness was basically controlled to be 0.03~0.25mm, and the yield of heat treatment products reached 99.7%. The adjusted process has solved the distortion problem of the sheet parts very well, and all technical indicators meet the requirements, which can be used for mass production.

Heat treatment equipment selection: The VOGQ series vacuum heat treatment furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment process of tools and dies. Good temperature control accuracy and uniformity ensure the effective implementation of the vacuum heat treatment process. SIMUWU focuses on the manufacturing of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum gas quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.