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Deformation and Improvement of Slicing Knife Vacuum Quenching

Slicing knives are a type of cutting utility knife and therefore require good hardness and a straight profile. For many years, the conventional heat treatment and quenching process specification has been adopted. The warpage and deformation of the hot rear end face are large, and the defective product rate is particularly high. After grinding, it still fails to meet the technical requirements of the drawing. Therefore, it is very important to find out the cause of deformation and improve the vacuum quenching process.

1 Analysis of the causes of quenching deformation of the slicing knife

1.1 Deformation due to thermal stress

After analyzing the deformation, it is believed that the yield strength of the workpiece decreases with the increase of the heating temperature during heating. The residual stress (cold deformation stress, machining stress, etc.) already exists in the workpiece. When the yield strength at high temperature is reached, it will cause uneven plastic deformation of the workpiece, resulting in shape distortion and residual stress relaxation. During quenching and cooling, the thermal stress and structural stress formed by the asynchrony of the cooling speed of the workpiece cause the workpiece to warp locally.

During the heating and cooling process of the long thin slice knife, the volume will change due to thermal expansion and contraction and the volume change due to the tolerance of the old and new during tissue transformation. When the workpiece is heated to the quenching temperature, the yield strength is significantly reduced and the plasticity is greatly improved. The yield strength of T12 steel is lower. The internal stress of the workpiece has exceeded its own yield strength, so the workpiece will warp and deform at both ends.

The material of the slicing knife is T12 steel, which is a high carbon steel with poor thermal conductivity. The heating temperature of vacuum quenching is higher than the Ac1 line and lower than the Acm line, and the temperature is relatively high, so multiple preheating and slow heating should be used. , so as to avoid the workpiece deformation, cracking and scrapping.

1.2 Deformation caused by tissue stress

The T12 steel used for the workpiece has a large carbon content, so the specific volume of martensite is large, so the structural stress is large. The change in volume is often associated with heating and cooling, as it is related to the coefficient of expansion of the steel. The generation of tissue stress originates from the contraction and expansion of volume due to the anisochronous nature of tissue transformation due to volume expansion and contraction. The internal stress that causes heat treatment deformation is the composite stress formed by the combined action of thermal stress and tissue stress. The result of the combined effect of thermal stress and tissue stress is uncertain, and may be different due to different cooling conditions and quenching temperatures. The quenching stress is caused by The combined effect of rapid cooling and rapid heating stress and the stress caused by different tissue transformations.

1.3 Effect of carbon content on deformation

The carbon content of T12 steel is 1.2%, which belongs to high carbon steel. The carbon content of the slicing knife is very large, so it will cause deformation because of the large volume change due to phase transformation during quenching; and the workpiece is long and thin. Volume changes have a big impact on shape.

Relationship between volume change and carbon content during martensite formation：

1.4 Influence of quenching medium on deformation of parts during quenching heat treatment

According to the isothermal transformation diagram of carbon steel, in order to suppress the occurrence of non-martensitic transformation, rapid cooling is required near the “nose temperature” of the c-curve (about 550 °C), and in the temperature range above 650 °C or below 400 °C, and Rapid cooling is not required, especially when martensite transformation is found near the Ms line, slow cooling is required to minimize thermal stress and structural stress during martensitic transformation to prevent quenching deformation. Therefore, the ideal quench cooling rate for steel is shown in the figure:

Generally considered. The influence of the cooling rate of the quenching medium at 300 °C on the deformation is the key. The quenching medium should be selected reasonably according to the hardenability of the steel, the cross-sectional size of the part and the surface roughness.

Commonly used quenching media are water, oil, and salt water solution, molten salt, air, etc. The cooling characteristics of water are not ideal. When the fast cooling range is 400-650 °C, the cooling speed of the water is very small, about 200 °C/s, and in the slow cooling range below 400%, the cooling speed of the water is greatly increased. About 300~C reaches the maximum value of 800℃/s, which maximizes the quenching deformation and cracking tendency of the parts. In general, carbon steel often uses water or aqueous solution with high quenching intensity as the quenching medium; and alloy steel generally uses oil as the quenching medium. Therefore, the correct principle for selecting a quenching medium is to select a quenching medium with a small quenching intensity as much as possible on the premise of ensuring hardening to reduce quenching deformation and cracking.

When analyzing the quenching process of the slicing knife, it was found that the cooling rate of the workpiece and the selection of the quenching medium were not suitable during cooling, which resulted in the warping deformation of the workpiece at the end.

2 Improve the vacuum heat treatment process

In the process of vacuum heat treatment of the slicing knife, it is very difficult to meet the deformation amount required by the process. In the test, we use the following methods to control the warping deformation of the circumferential plane of the slicing knife.

① Preparation before vacuum quenching: remove rust marks, dirt, cracks, bumps, deformation and other defects on the surface of the slicing knife.

② Choose a suitable pallet rack: Use a pallet rack with a diameter greater than 500mm, and its surface is required to be flat. Stack the slicing knives on the pallet rack one by one, and divide the slicing knives into four equal parts. Use iron wire to slice between each equal part. The knives and slicing knives are connected to each other, so that the slicing knives overlap and have a certain gap. Each pallet rack can place 6-7 slicing knives.

③ Stress relief annealing: Stress relief annealing at 450°C eliminates cutting stress and has a good stabilization effect on the size of the slicing knife.

④ Quenching and heating: Put the slicing knife in a vacuum furnace that has been raised to 850℃ to heat and keep it warm.

⑤ Cooling method: put the slicing knives kept at 850℃ for 1 hour in oil with compressed air for cooling. Since the slicing knives are fixed on the pallet rack, it is relatively stable, and there is a gap of 2-3 mm between the slicing knives. It ensures that the slicing knife can be cooled evenly and prevents the occurrence of soft spots. When the slicing knife is cooled to about 120°C, the oil is cooled to room temperature.

⑥ Tempering: After cooling to room temperature, the slicing knife should be tempered at low temperature immediately. Tempering equipment: use a low temperature tempering oil tank, and the heating temperature is 150℃.

3 effects

After the slicing knife is post-treated by the above methods, after inspection, the warpage deformation and hardness of the two ends of the slicing knife meet the process requirements. Improve production efficiency and meet the needs of mass production.