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Vacuum gas Quenching of 45CrNiMo1VA Steel Spinning Mandrel

In order to improve the material utilization rate of cylindrical parts, factories often use strong spinning forming technology. Spinning mandrel (also known as mandrel) is an important process equipment that is located in the inner hole (or inner cavity) of the workpiece to guide and bear pressure during spinning.

1 Analysis of the reason for the cracking of the first mandrel

The outer diameter of the first mandrel is φ160～180 mm. The mandrel has a longitudinal crack that runs through the entire length of the mandrel, which is a typical quenching crack. From the end face, the crack almost runs through the entire cross section, that is, it is almost cracked. This also shows that the hardenability of the mandrel material is very good, the mandrel is completely hardened, and the quenching stress is too large.

The hardness of the test mandrel after heat treatment is: 55 ~ 58 HRC.

The heat treatment process:

1) Preheating: pit type tempering furnace, 400 ℃ for 3 h 30 min.

2) Quenching: controlled atmosphere multi-purpose furnace, carbon potential 0.4%, 880 ℃ for 3 h 30 min, oil cooling.

3) Tempering: vacuum tempering furnace, 180 ℃ for 5 h, air cooling.

After investigation, 45CrNiMo1VA steel is a high-grade high-quality steel smelted by vacuum smelting and purified by electroslag remelting, so it is unlikely that the cracking is caused by the inclusion of raw materials. According to the macroscopic appearance of the mandrel crack and the investigation, it is preliminarily believed that the main reason for the crack is that the hardenability of the material is very good, the quenching method is not suitable for the workpiece and the material, and the quenching stress is too large.

2 Vacuum gas quenching test of the second mandrel

2.1 Test plan and implementation

The material of the second mandrel is still 45CrNiMo1VA steel. In order to reduce the effective thickness of the mandrel rod and further reduce the quenching stress, before quenching, on the premise of not affecting the use of the mandrel rod, a through hole with an inner diameter of about 60 mm was drilled in the core of the mandrel rod. From round bar-shaped parts to thick-walled tubular parts with a wall thickness of 50-60 mm. Then carry out the vacuum gas quenching test according to the following process plan.

Quenching process: enter the furnace at room temperature, preheat at 180 °C for 1 h, preheat at 650 °C for 2 h, hold at 880 °C for 1 h for 30 min, and charge high-purity nitrogen with 30 kPa for forced circulation cooling and gas quenching.

After quenching, low temperature tempering was performed twice.

2.2 The effect of vacuum gas quenching

After the above-mentioned heat treatment of the mandrel, there is no crack or obvious deformation in the appearance inspection, and the test hardness is 55~55.5 HRC, which meets the design requirements of the mandrel.

The mandrel is ground and processed for flaw detection, and there is no crack. This mandrel is used for spinning process trials and product production applications.

3 Summary

3.1 45CrNiMo1VA steel has good hardenability

According to the above analysis of the crack morphology of the first mandrel and the high pressure gas quenching test of the second one, the 45CrNiMo1VA steel has good hardenability.

3.2 45CrNiMo1VA steel mandrel should adopt qacuum gas quenching

Due to the good hardenability of 45CrNiMo1VA steel, oil quenching is easy to crack the mandrel, and gas quenching is used instead. The cooling rate of mandrel quenching is appropriately reduced, and the quenching stress is significantly reduced, so that the mandrel can not only meet its hardness, Wear resistance requirements, and can prevent quenching cracks.

3.3 Appropriate pressure should be selected for vacuum gas quenching

For vacuum gas quenching, an appropriate gas quenching pressure should be selected according to the quenching needs of the workpiece and if the equipment conditions allow. When the hardenability of the workpiece material is very good, a lower gas quenching pressure should be selected under the premise of ensuring the quenching quality, which is conducive to reducing the consumption of high-purity nitrogen or inert gas, reducing production costs and prolonging equipment life. service life.

45CrNiMo1VA steel has good hardenability. The 45CrNiMolVA steel spinning mandrel adopts vacuum high pressure gas quenching furnace and vacuum gas quenching process instead of controlled atmosphere furnace and oil quenching process, which can not only meet the requirements of hardening and wear resistance, but also can Prevent quenching cracks from occurring. When vacuum gas quenching is used, a lower gas quenching pressure should be selected on the premise of ensuring the quenching quality.

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