Heat Treatment of High-Speed Steel: 5 Tips You Should Know

High-speed steel is the abbreviation of high-speed tool steel, commonly known as “sharp steel” (also known as “wind steel” because it can be hardened in the air).

It has been many years since American Fred W. Taylor and his assistant M. Whiet invented high-speed steel in 1898.

Looking at all the steel grades in the world at present, no matter the complex relationship between chemical composition, structure and performance, or the difficulty of the whole manufacturing process such as smelting, casting, forging, roll rolling, drawing, plastic forming and heat treatment, high-speed steel is undoubtedly one of the most difficult steel grades.

Some people say that “high-speed steel is mysterious”, while others say that “high-speed steel is unpredictable”.

After graduation from university in 1968, the author has been engaged in high-speed steel heat treatment for 50 years, accumulated rich practical experience, and recorded many lessons from failure.

Now, the author has summarized five high-speed steel heat treatment scriptures to share with colleagues.

Heat Treatment of High-Speed Steel: 5 Tips You Should Know 1

1. Salt bath for graded quenching of high-speed steel

Since China has salt bath heating and quenching of high-speed steel, the 5-3-2 formula (mass fraction,%) of the former Soviet Union has been used, namely 50BaCl2+30KCl+20NaCl, the melting point temperature is 560 ℃, and the service temperature is 580 ~ 620 ℃.

For tools or HSS steel parts with effective size less than 20mm, they can reach high hardness above 65HRC;

HSS-E steel parts can reach ≥ 66HRC.

The tool industry in China uses this graded quenching process to create provincial, ministerial and national excellence, which shows that it has great vitality.

With the development of the times and the progress of technology, as people deepen their understanding of the importance of cooling speed, it is found that the average cooling speed of the workpiece at 800 〜 1000 ℃ is less than 7 ℃ ⁄ s, and there will be carbide precipitation which will affect the hardness and other properties.

Therefore, calcium based grading salt was introduced from Europe and the United States at great expense.

Its formula (mass fraction,%) is 48CaCl2+31BaCl2+21NaCl, the melting point is 435 ℃, and the service temperature is 480 〜 560 ℃.

In order to simplify the formula, some factories in China have changed to 50CaCl2+30BaCl2+20NaCl.

The melting point is slightly higher than that of traditional calcium based salts, but the classification temperature remains at the level of 480~560 ℃.

As for the Ca based salt bath technology, the former Soviet Union made it public in the 1940s, spread it to China in the 1950s, and many factories tried it in the 1960s.

When the author worked in Guilin from 1974 to 1978, he also used Ca based salt bath.

Because the furnace was opened once a week, the furnace was shut down for a long time, and the salt bath was too hygroscopic, he was forced to eliminate it.

Some factories have conducted field tests on the cooling rate of the graded salt bath, and the cooling rate of φ40mm workpieces at 800 〜 1000 ℃ and 550 ℃ is exactly 7 ℃ ⁄ s, which means that the effective size can be hardened completely below 40mm, a string of φ25mm workpieces are cooled at 500 ℃ calcium base, and the cooling rate at 800 〜 1000 ℃ is 9 ℃ ⁄ s.

There is no doubt that the cooling rate of 580-620 ℃ barium based salt bath workpiece at 1000-800 ℃ is certainly slower than that of 480-560 ℃ calcium based salt bath.

When the effective diameter of the workpiece is 20~40mm, calcium based salt is superior, but the size is below 20mm.

It is unnecessary to change to calcium-based salt.

The key is how to control the temperature of the salt bath below 600 ℃.

For workpieces with a diameter of more than 40mm, oil cooling can be used first, then graded salt cooling, and then graded in nitrate, so as to ensure that the hardness after heat treatment is ≥ 65HRC.

2. Tempering degree and times

After quenching, high speed steel must be tempered in time for four purposes:

① Completely eliminate quenching stress.

② The residual austenite shall be fully decomposed.

③ It produces the best secondary hardening effect.

④ To achieve the required comprehensive mechanical properties and good use performance.

Tempering temperature is 540 〜 560 ℃.

Whether it is salt bath quenching or vacuum quenching, it is proposed to use 100% KNO3 or 100% NaNO3 salt bath for 1h.

After each tempering, it must be cooled to room temperature before the next tempering.

The number of times of tempering is generally 3.

If the tempering is not sufficient, or those who have been isothermal quenched, as well as high-performance high-speed steel parts, 4 times of tempering shall be carried out.

The tempering degree is generally divided into three levels, not based on the tempering times, but on the metallographic the final say.

Level I (sufficient): It is characterized by black tempered martensite+speckled carbide metallographically.

Class II (general): white areas exist in individual areas or carbide deposits.

Third level (insufficient): most of the field of view is white area, and quenched grains can be seen faintly.

If tools such as steam treatment and oxygen nitrogen treatment need to be surface strengthened in the tempering temperature area, the tempering degree can reach Grade II, which can save energy.

The tempering degree shall be checked by etching with 4% nitric acid alcohol solution at a temperature of 18 〜 25 ℃ and an etching time of 2 〜 4 min, and observed under a 500 fold microscope based on the worst field of view.

3. Secondary bainite treatment

In order to improve the toughness, strength and cutting performance of the tool, tool factories often adopt a bainite treatment, that is, after the neutral salt bath is graded at 480 ℃ to 560 ℃, it is immediately transferred to a 240 ℃ to 280 ℃ nitrate bath for isothermal 1 to 2h;

The secondary bainite treatment is applicable to oversized cutters with extremely complex shapes (such as milling cutters and hobs with modulus>15 and perforated cutters with effective thickness>100mm).

40% ~ 50% lower bainite is produced during the first bainite treatment, and the rest is residual austenite and a small amount of carbide.

During the first tempering, a large amount of residual austenite is transformed into martensite.

After the first tempering, do not cool it in the air.

Instead, it is directly transferred to the salt bath at 240 〜 280 ℃ for isothermal treatment for a certain period of time to prevent the transformation of retained austenite into martensite and into bainite, which is the so-called secondary bainite treatment.

This can reduce and prevent the cracking tendency of large and complex tools.

Although the secondary bainite treatment process is more complex, it is very beneficial to prevent large tools from cracking during heat treatment.

Tempering process shall be heated slowly. Each tempering shall be carried out at a temperature lower than 500 ℃.

It is not allowed to blow after tempering. It is better to cool it statically.

Due to the secondary bainite treatment, four times of tempering may not be sufficient, and one time of tempering shall be added.

4. Heat treatment of friction welding tool

In order to save expensive high-speed steel, friction welding is widely used at home and abroad to produce rod cutters with a diameter of more than φ10mm.

A high temperature of more than 1000 ℃ is generated during friction welding, and a large temperature difference is generated in a small area on both sides of the weld.

If air cooling is conducted directly after welding, martensite transformation occurs on the high-speed steel side of the weld, while pearlite transformation only occurs on the air cooling side of the structural steel.

Due to the difference in specific volume, large organizational stress will be induced, resulting in cracking.

For this reason, the tool after welding shall be put into a 650~750 ℃ furnace for thermal insulation immediately.

After the charging tank is filled, the tool shall be kept for 1~2h for annealing.

The tool shall be discharged from the furnace for air cooling when the furnace is cooled to below 500 ℃.

If the production volume is too large to implement the above process, the heat preservation temperature of the weldment should be 740 〜 760 ℃, and the heat preservation time should be extended to 2 〜 3h, so that both sides of the weld can be fully transformed into pearlite+sorbite, and then air cooled and re annealed.

The focus of the debate on the quenching of friction welding tools is whether to overheat the weld.

Arguments for super weld heating:

The original structure after welding is improved, the welding quality is tested, the weld strength is improved, and the high-speed steel is fully used;

Argument lower than weld heating: in order to prevent quenching cracks and avoid quality disputes.

Since the successful vacuum quenching of welding tools, there has been less doubt about the crack caused by super weld heating after salt bath quenching.

The author insists on the view of super weld heating, and practice has proved that super weld heating has no direct relationship with quenching cracks.

At present, most tool factories use heating 15 〜 20mm lower than the weld seam, which actually shortens the cutting length of high-speed steel, causing waste, and is very uneconomical.

It is strictly forbidden to pickle the tools heated by super weld.

If it is necessary to pickle, the acid concentration, pickling time and acid temperature must be controlled to prevent hydrogen embrittlement.

5. Cryogenic treatment

The microstructure of high speed steel tool after normal quenching and tempering is tempered martensite+trace retained austenite+carbide.

The author believes that it is unnecessary to eliminate the remaining trace (<5%) retained austenite.

After normal quenching and tempering for 550∼570℃×1h×3 times, the heat treatment of high speed steel tools has reached the acme, and then deep cooling treatment may add insult to injury.

Austenite is a very soft phase in the steel structure, and its hardness is only about 200HBW.

Compared with the hardness of 65 〜 66HRC used for high-speed steel tools, it can be seen that too much retained austenite will obviously not make the tools hard.

Japanese scholar Ichiro Iijima et al. believed through experiments that the residual austenite below 15% will not reduce the tool hardness, but can improve the plasticity and toughness of the steel.

Therefore, it is definitely harmful to toughness to reduce residual austenite by cryogenic treatment.

From the 1970s to the beginning of the 21st century, many domestic tool factories have done cold treatment and cryogenic treatment for high-speed steel cutters.

There have been many failures and few successes.

Our Company has also done cryogenic treatment for several years, but no results have been achieved.

Therefore, the equipment has been put on hold.

Compared with other superhard materials, the biggest advantage of high-speed steel tools is that their toughness is slightly higher.

Cryogenic treatment can further reduce the residual austenite, and their toughness is even worse.

Isn’t it not to sprinkle salt on the wound?

Practice has proved that less than 5% retained austenite is harmless to tool use.

The hardness of HSS steel is 65 〜 66HRC, and the hardness of HSS-E steel is 66 〜 67HRC.

Under the same conditions, the higher the hardness, the less wear, and the higher the tool durability.

From this, it can be judged that the retained austenite that will reduce the hardness is obviously not welcome.

However, the tool life has never been determined by the hardness.

Too high hardness leads to increased brittleness, which will not increase the service life, but will reduce it.

There are many factors that affect the life of high-speed steel tools.

We should not blindly pursue high hardness.

Our principle is to strive for high hardness on the premise of meeting the toughness.

Experience shows that cryogenic treatment of fully tempered tools will not increase the hardness, let alone the thermal hardness, but will reduce the toughness.

Some domestic tool factories have added cryogenic treatment to some cutters, such as shaving cutters and small module hobs, in order to eliminate stress and stabilize the size, because both of them are centered on the inner diameter.

It is hoped that the inner diameter of the cutter will not change during use.

There are also some high-end measuring tools and molds made of high-speed steel for cryogenic treatment to stabilize the size.

From the above analysis, it is not difficult to find that after normal quenching and tempering, there is a trace of retained austenite in the structure of high-speed steel, which does not greatly affect the use of tools and comprehensive mechanical properties.

Is it necessary to conduct cryogenic treatment?

This controversial issue needs to be supported by a large number of experimental data and application examples, but the author holds an opposing view after experiments.

It is a powerful argument that hundreds of tool manufacturers in China have not been visited.

It is widely reported that it is basically the scientific research achievements of universities and research institutes or the products of laboratories, but the promotion fails.

The so-called new tempering process is a flash in the pan.

It is still a mature process that has been widely used in mass production for three times.

Practice is the only criterion to test the truth, and any new process must withstand the test of practical production.

Conclusion

High speed steel heat treatment is too profound, but as long as we take it seriously, dare to explore, practice repeatedly, and innovate boldly, we will certainly be able to make high-quality and long-lived cutting tools and make due contributions to the revitalization of the mechanical industry.

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