The function of heat treatment for Cutting Tools

Heat treatment for cutting tools refers to a metal hot working process in which materials are heated, insulated and cooled in the solid state to obtain the desired structure and properties.  Generally, refers to the treatment of metal materials, especially steel.  It can also be said that the characteristics of the material are changed through temperature control and cooling rate adjustment.

Heat treatment of cutting tools can be classified into vacuum heat treatment, salt bath heat treatment, chemical heat treatment, and coating treatment.  This article mainly explains salt bath heat treatment in details.

What is salt bath heat treatment?  The advantages and disadvantages of its processing are What?

The heating principle of the salt-bath furnace is to melt industrial salt into liquid as a heating medium, then put the cutting tools to be heat treated into melted liquid salt inside for heating, and finally carry out heat treatment.  As a layer of salt film coating is formed on the heating surface of the workpiece under the melted liquid salt coating, air can be isolated, oxidation can be better reduced, decarburization can be prevented, heating is more uniform due to the fact that the heating medium is liquid, and the heating medium of the liquid salt can be heated faster, thereby effectively saving time.  Summarize the advantages of the salt-bath furnace: air isolation reduces oxidation and decarburization, the liquid heating temperature is more uniform, heating speed is faster, and the workpiece can be locally heat treated.

Advantages of salt bath heat treatment:

1. Quenching heating time is short; crystal grains will not coarsen and toughness is strong.

2. The furnace temperature distribution is good, the heating is uniform, and the overall stability of the product is good.

3. The high-temperature area cools quickly, the low-temperature area cools slowly, quenching is easy, and deformation decreases

4. Able to respond to a wide range of heat treatment process changes (intermittent quenching, isothermal quenching, partial heat treatment, short-time heating, cooling, complex temperature changes, etc.)

5. Less oxidation and decarburization, high surface hardness and strong wear resistance.

6. The heat treatment temperature is stable, the time adjustment can be completed in a short time and can correspond to multiple varieties and batch production.

7. Equipment costs are relatively low.

The reason why heating time can be shortened is closely related to salt bath heat capacity and thermal conductivity.  The temperature difference between the surface and the central part of the 25mm High-speed tool steel during quenching takes 3min at 900℃ and only 2min at 1180℃, and the temperature difference between the inside and outside disappears.  Generally, heating with a vacuum furnace or the like requires about 20-30min, and the difference is obviously increased.  The particle size of steel crystallization depends on the size of austenite crystal directly above the transformation point, so in order to obtain fine austenite crystal size, it is very important to increase the heating speed of steel.  Salt bath heat treatment, due to the high uniformity of bath temperature, can heat evenly in a short time.

During heating in a vacuum furnace or the like, uneven heating of the treated parts is easily caused, so the time is increased or the quenching temperature is increased, which will cause coarsening of austenite crystal grain size and the decrease of toughness.  Comparing the austenitizing time of vacuum furnace and salt-bath furnace shows that the austenitizing time of vacuum furnace is 4-5 times as long as that of the salt-bath furnace at 1000℃ and about 9 times under 1200℃. In addition, with the recent progress in cooling vacuum furnace by high-pressure gas, quite a lot of materials can be quenched.  Even so, compared with oil cooling and salt bath cooling, gas cooling has poor heat conduction and cooling is not as good as salt bath cooling.

The outstanding advantage of the salt bath is its strong cooling capacity in high-temperature areas. High-speed tool steel is an extremely sensitive material for heat treatment because the precipitation temperature of carbide is 1000-600℃.  If carbide is precipitated, the toughness and hardness of the material will decrease.  Therefore, the key to High-speed tool steel's heat treatment is how to rapidly cool in this temperature range.  When salt bath cooling is used, the cooling speed above 700℃ is high, so carbide precipitation can be avoided.  In addition, if cooling is performed rapidly in the low-temperature region where martensite is formed, the strain increases, so it is also desirable to avoid it.  Compared with oil quenching, the cooling capacity of the salt bath below 450℃ is reduced, which is also desirable.  Judging from these properties, salt bath heat treatment is the most suitable method for quenching in High-speed tool steel.

The purpose of the high-speed steel heat treatment is to improve the strength and hardness of cutting tools, enhance wear resistance, make cutting tools sharper and improve the properties of high-speed steel props:

1. Carbide Transformation and Transformation.

The core of high-speed steel heat treatment is the transformation and transformation of carbides.  Well handled, it guarantees high hardness, wears resistance and red hardness of high-speed steel.  Poor handling leads to overheating and overburning, thus affecting tool life.  In order to improve the service life of high-speed steel cutting tools, it is necessary to have a deep understanding of carbides in steel and the formation and change rules of carbides. The most intuitive thing to do is to check the non-uniformity of carbides, improve the morphology and distribution of carbides through pressure processing, pay attention to the solubility of carbides during quenching and heating, and check the change of tempered carbides.

2. check the grain size must see carbide solubility

Grain size inspection of high-speed steel is an important means to inspect quenching quality of high-speed steel.  Some are even used as the only means except hardness measurement.  What is the effect of grain size on the life of high-speed steel cutting tools?  Domestic and foreign counterparts have different views and even great differences.

The former Soviet Union carried out tests on straight twist drilling of ￠8mmP6M5(M2) steel. The results show that the tool has the best service life (460 boreholes) when the grain size is 9.5. If the grain size is too fine or too coarse, the life of the drill bit will be reduced.  There should add two points: carbide dissolution should be sufficient;  Hardness > 65HRC after heat treatment.

Different cutting tools have different requirements for quenching austenite grain size and carbide dissolution degree, such as gear hob grain size grade 9, turning tool grade 8, carbide dissolution should be sufficient, machine tap should be fine grain size 10.5, carbide dissolution is generally enough.

Grain size is at best a reflection of quenching heating temperature. For cutting tools requiring high red hardness, carbide dissolution degree is more important than grain size. Therefore, when looking at metallography in front of the furnace, both must be taken into account and cannot be neglected.

3. Hardness

For high-speed steel tools, it is easy to obtain high hardness > 64HRC, but it is not easy to obtain the ideal metallographic structure.  For example, super-hard high-speed steel such as M42 has a suitable range instead of the higher the hardness, the better.  The hardness is too high, not only will it not last long, but be shortened.  For a specific tool made of a specific steel grade, there is a practical hardness value under specific working conditions. People have found in practice that high-life cutting tools are generally of high hardness, but those with high hardness do not necessarily have high life.  We should strive for high hardness on the premise of sufficient toughness.

4. Stabilize tool life

The high-quality tool product not only has a high average life but also has stable quality and good repeatability of tool life. The problem of China cutting tools factory is not every tool has a low tool life, but the average tool life is greatly reduced due to the wide difference in tool life and too large dispersion.  Many domestic tools have done such tests: compared with single pieces, domestic tools are no worse than foreign ones, but if you take 10 PK, there will always be 1~2 pieces of domestic tools that are not satisfactory.

The stability of product quality and tool life is a comprehensive reflection of a factory's technical level and management level.  It is not easy to make the tool life high. It is a systematic project. From the control of raw materials, mechanical processing, heat treatment, to the final packaging and delivery of products, there must be a set of strict rules and regulations, correct process documents, complete testing methods and good quality of staff.

To sum up, improving cutting tool life requires not only high-quality raw materials, machining level, and strict and fine heat treatment, but also an important determinant of tool life.

When we see the problems, it’s the beginning of changing.  To be a world-class cutting tool manufacturer is the direction and mission that Heygo Tools strive all the time.