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Analysis of vacuum heat treatment process for wind power products

In recent years, the wind power industry has developed rapidly, and the wind power gearbox is the core component of the wind power unit. The service conditions of wind power gearboxes are harsh, with high power, large speed difference, high precision, long service life and high reliability requirements. In order to achieve a long service life and high reliability of the transmission system in a complex working environment, strict requirements are put forward for the structural design and manufacturing process (including material processing technology and surface treatment technology, etc.) This type of wind power product has high requirements on its surface hardness, wear resistance and other mechanical performance indicators. In actual production, gears, gear shafts, and bearings for wind power mainly use vacuum carburizing and quenching processes. By adopting a reasonable vacuum heat treatment process design, it can finally meet the specified technical requirements. Therefore, vacuum heat treatment has brought good mechanical performance improvements to wind power products.

The performance of materials is the basis of the product’s carrying capacity. Reasonable selection of materials and vacuum heat treatment process can improve the carrying capacity of wind power products. At present, 18CrNiMo7-6 has become the mainstream standard steel for wind power products. Its characteristics are as follows: high tensile strength and toughness of the core; high fatigue strength of the core and carburized layer; high hardenability; micro distortion after vacuum quenching; good high temperature performance.

Product technical indicators are as follows:

1) The depth of the effective hardened layer is 2.0-6.0mm (550HV), most of which are deep penetration layers;

2) After vacuum quenching, the surface hardness is 58-62HRC; the core hardness is 30-47HRC;

3) The depth of internal oxidation varies according to the penetration layer requirements of different products;

4) The surface texture and core texture meet the relevant standards.

At present, by adopting the slow cooling and reheating quenching process after vacuum carburizing, the performance after vacuum heat treatment meets the material requirements. During the processing, the vacuum heat treatment deformation should be minimized to ensure that the grinding allowance of each product surface is consistent during the machining process, so that each product surface can finally obtain consistent carbon content, metallographic structure and hardness, so as to obtain uniform working performance.

Vacuum heat treatment process: production preparation → loading → pre-cleaning → gas carburizing slow cooling + vacuum quenching again → vacuum tempering → inspection → packaging → delivery.

1.Control of product deformation: Under the premise that the reliability, stability and accuracy of vacuum heat treatment equipment are guaranteed, the furnace loading method, carburizing temperature, quenching heating rate, vacuum quenching temperature, quenching cooling temperature and quenching medium are the key factors affecting product deformation in the vacuum carburizing and quenching process.

During the heating process, the heating method of preheating and step heating is adopted. The specific preheating temperature and holding time are determined according to the product structure and the furnace loading of the product; for carburizing and quenching products with complex structures and shapes, compensation pads, washers, mandrels, suitable fixtures, and reasonable clamping arrangements are used; hot oil is used for quenching during quenching and cooling. The temperature distribution difference in the process improves its uniformity, thereby reducing distortion.

2.Control of the internal organization of the product during processing: Reasonably select the vacuum heat treatment process temperature, holding time and carbon potential, and strictly control the carbon concentration on the surface of the workpiece. From the perspective of vacuum heat treatment process, the size of austenite grains before quenching is related to the austenitization temperature and time, surface carbon concentration, and original structure before quenching and heating. The higher the austenitization temperature, the longer the time, and the larger the austenite grains; the increase of the surface carbon concentration to a certain extent will lead to the incorporation of excess carbides into the austenite, which will promote the growth of austenite grains, making the martensite coarse after quenching, and will also increase the amount of retained austenite. The effect of quenching temperature on product deformation is also critical. Therefore, on the premise of ensuring that the structure after quenching is qualified, try to choose a lower quenching temperature.

Using this process to process several different wind power products, all of them meet the technical requirements of the products.