1. Salt bath for graduated hardening of high speed steel
China has used salt bath heating and quenching of high-speed steel using the former Soviet Union's 5-3-2 (mass fraction, %) formula, which includes 50BaCl2+30KCl+20NaCl. This formula has a melting point temperature of 560℃ and a serving temperature of 580 to 620℃.
For HSS steel tools or parts with effective sizes less than 20 mm, high hardness levels above 65HRC can be achieved. HSS-E steel parts, on the other hand, can reach a hardness ≥ 66HRC.
The tool industry in China has used this gradual tempering process to achieve provincial, ministerial and national excellence, demonstrating its great vitality.
As time passes and technology advances, people have realized the importance of cooling speed, and it has been discovered that the average cooling speed of a workpiece from 800 to 1000 ℃ is less than 7 ℃ ⁄ s. This slow cooling rate results in carbide precipitation, which affects the hardness and other properties of the steel.
As a result, calcium-based grading salt was introduced to China from Europe and the United States at significant cost. Its formula (mass fraction, %) is 48CaCl2+31BaCl2+21NaCl, with a melting point of 435℃ and service temperature ranging from 480 to 560℃.
To simplify the formula, some Chinese factories have changed to 50CaCl2+30BaCl2+20NaCl. This alternative has a slightly higher melting point than traditional calcium-based salts, but the classification temperature remains between 480-560°C.
The former Soviet Union first introduced Ca-based salt bath technology in the 1940s and later shared it with China in the 1950s. Many factories experimented with it in the 1960s.
During his tenure in Guilin from 1974 to 1978, the author used a Ca-based salt bath. However, due to the infrequent operation of the furnace and the high hygroscopicity of the salt bath, he had to discontinue its use.
Some factories have conducted field tests on the cooling rate of graduated salt baths. Specifically, the cooling rate of φ40mm parts at 800-1000°C and 550°C is 7°C/s, indicating that the effective size can be fully hardened below 40mm. Meanwhile, a series of φ25mm workpieces are cooled at a calcium base of 500°C, and the cooling rate at 800-1000°C is 9°C/s.
Undoubtedly, the cooling rate of barium-based salt bath parts at 580-620℃ from 1000 to 800℃ is slower than that of calcium-based salt bath parts at 480-560℃.
For parts with an effective diameter of 20-40 mm, calcium-based salt is preferable, but unnecessary for sizes below 20 mm. The crucial factor is how to control the temperature of the salt bath below 600°C.
For parts with a diameter greater than 40mm, oil quenching can be applied first, followed by graded salt quench and then graded nitrate to ensure a heat treatment hardness of ≥65HRC.
2. Quenching degree and times
After quenching, high-speed steel must be tempered for four purposes:
① To completely eliminate quenching stress.
② To fully decompose residual austenite.
③ To produce the best secondary hardening effect.
④ To achieve the required comprehensive mechanical properties and optimal performance.
The recommended tempering temperature is between 540 and 560 ℃.
Whether salt bath quenching or vacuum quenching is used, it is recommended to use 100% KNO3 or 100% NaNO3 salt bath for 1 hour.
After each tempering process, the steel must be cooled to room temperature before starting the next tempering process.
Typically, the number of times tempering is performed is three. However, in cases where tempering is insufficient, or for high-performance high-speed steel parts that have been isothermally quenched, four tempering processes must be carried out.
The degree of tempering is generally classified into three levels, based not on the number of tempering cycles, but on the metallographic appearance.
Level I (Adequate): Characterized by the presence of tempered black martensite and speckled carbide in the metallograph.
Level II (Intermediate): White areas or carbide deposits are present in isolated regions.
Level III (Inadequate): The majority of the field of view comprises white areas and tempered grains are faintly visible.
If surface strengthening treatments such as steam treatment and oxygen and nitrogen treatment are required, in the tempering temperature range, a Grade II tempering degree can be achieved, resulting in energy savings.
The degree of tempering must be assessed by acid etching with a 4% nitric acid alcohol solution at a temperature of 18 to 25°C for 2 to 4 minutes and observed under a 500x microscope based on the worst field of view.
3. Secondary treatment of bainite
Tool factories often use a bainite treatment to improve the toughness, strength and cutting performance of tools. This involves classifying the neutral salt bath at 480°C to 560°C and immediately transferring it to a 240°C to 280°C nitrate bath for isothermal treatment for 1 to 2 hours.
Bainite secondary treatment is specifically suitable for oversized cutters with extremely complex shapes, such as cutters and milling cutters with modulus > 15, and drilled cutters with effective thickness > 100mm.
During the first bainite treatment, 40% to 50% less bainite is produced, along with residual austenite and a small amount of carbide.
During the first tempering, residual austenite is transformed into martensite in large quantities.
After the first tempering, the tool must not be cooled in air. Instead, it must be transferred directly to the salt bath between 240°C and 280°C for isothermal treatment for a specific period. This will prevent the transformation of retained austenite into martensite and bainite, which is known as secondary bainite treatment.
This method can help reduce and prevent the tendency of large, complex tools to crack.
The bainite secondary treatment process is more complex but highly beneficial in preventing cracking of large tools during heat treatment.
The tempering process must be heated slowly, and each temper must be carried out at a temperature below 500 ℃.
Blowing after tempering is not permitted; it is better to cool the tool statically.
As a result of secondary bainite treatment, four rounds of tempering may not be adequate and additional tempering must be performed.
4. Heat treatment of friction welding tool
Friction welding is widely used both domestically and abroad to produce rod cutters with a diameter greater than φ10mm, as it helps to save expensive high-speed steel.
During friction welding, a temperature exceeding 1000℃ is generated, resulting in a large temperature difference within a small area on both sides of the weld.
Direct air cooling after welding leads to the transformation of martensite on the high-speed steel side of the weld, while pearlite transformation occurs only on the air-cooled side of the structural steel.
The difference in specific volume induces significant organizational stress, leading to cracks.
To avoid this, the brazed tool should be immediately placed in a 650~750℃ oven for heat insulation after welding.
Once the charging tank is full, the tool should be held for 1 to 2 hours for annealing.
The tool should be removed from the furnace for air cooling when the furnace temperature drops below 500℃.
If the production volume is too high to follow the above process, welding must be maintained at a heat preservation temperature of 740 hours. This process will allow both sides of the weld to be completely transformed into pearlite+sorbite, and the tool can then be air cooled and annealed again.
The debate over quenching friction stir welding tools is focused on overheating the weld. Arguments in favor of weld overheating include improving the original structure, improving welding quality and strength, and making full use of high-speed steel. On the other hand, arguments against overheating the weld include the risk of crack extinction and possible quality disputes.
Since the success of vacuum quenching of welding tools, doubts about cracks caused by overheating of the weld after quenching in a salt bath have diminished. However, the author insists that weld overheating does not directly lead to crack extinction, based on practice and experience.
Currently, most tool factories use heating 15 to 20 mm below the weld seam, which results in reduced cutting length of high-speed steel, waste, and uneconomical practices.
Pickling heated tools by super soldering is strictly prohibited. In cases where pickling is required, the acid concentration, pickling time and acid temperature must be carefully controlled to avoid hydrogen embrittlement.
5. Cryogenic treatment
The microstructure of high-speed steel tools after normal quenching and tempering consists of tempered martensite, traces of retained austenite and carbide.
The author believes that it is unnecessary to eliminate the remaining traces (<5%) of retained austenite.
After normal quenching and tempering at 550-570℃ for 1 hour 3 times, the heat treatment of high-speed steel tools has reached its optimum level, and additional deep cooling treatment may do more harm than good.
Austenite is a very soft phase in structural steel, with a hardness of only about 200HBW. Compared to the desired hardness of 65-66HRC for high speed steel tools, it is clear that an excess of retained austenite will not make the tools harder.
Through experiments, Japanese scholar Ichiro Iijima and his team discovered that residual austenite below 15% would not reduce the hardness of the tool, but could increase the plasticity and toughness of the steel.
Therefore, reducing residual austenite through cryogenic treatment would be detrimental to the toughness of the tool.
From the 1970s to the beginning of the 21st century, many domestic tool factories carried out cold treatment and cryogenic treatment on high-speed steel cutters.
There were numerous failures and only a few successes.
Although our company has been carrying out cryogenic treatment for several years, we have not achieved significant results. Therefore, the equipment was placed on hold.
When compared with other super-hard materials, the most significant advantage of high-speed steel tools is their slightly higher toughness.
Cryogenic treatment can decrease residual austenite, but it also reduces the toughness of tools.
Isn't it like rubbing salt into the wound?
It has been proven that retention of less than 5% austenite is harmless for tool use.
The hardness of HSS steel is 65-66HRC, while that of HSS-E steel is 66-67HRC.
Under similar conditions, the higher the hardness, the lower the tool wear and the longer the tool durability.
From this we can conclude that retained austenite, which reduces hardness, is not welcome.
However, the useful life of a tool is not only determined by its hardness.
Excessive hardness increases brittleness, which does not extend the tool life, but reduces it.
Several factors affect the service life of high-speed steel tools.
Therefore, it is not advisable to blindly pursue high hardness.
Our principle is to pursue high hardness while ensuring adequate strength.
Based on experience, cryogenic treatment does not increase the hardness of fully hardened tools, nor does it improve their thermal hardness. On the contrary, it can decrease your resistance.
However, some domestic tool factories have added cryogenic treatment to certain cutters such as razor cutters and small module milling cutters in order to eliminate stress and stabilize their size. This is particularly important as both tools are centered on their internal diameter, and it is crucial that this does not change during use. Additionally, some high-quality measuring tools and molds made from high-speed steel can benefit from cryogenic treatment to stabilize their size.
After normal quenching and tempering, high-speed steel structures typically retain traces of austenite. Although this does not significantly affect the mechanical properties or use of the tools, there is some debate as to whether cryogenic treatment is necessary.
To determine whether cryogenic treatment is beneficial, a large amount of experimental data and application examples are needed. However, the author's experiences led us to take an opposite view. It is important to note that there are hundreds of tool manufacturers in China that have not implemented cryogenic treatment.
Although cryogenic treatment is often presented as an achievement of scientific research or a laboratory product, its promotion has not been widely successful. The so-called “new tempering process” may be a short-lived trend.
The process in question remains mature and has been widely used in mass production on several occasions.
“Practice is the only criterion for testing truth,” as the saying goes, and any new process must prove its worth through practical production.
Conclusion
The heat treatment of high-speed steel may seem complicated, but with a serious and bold approach, coupled with repeated practice and bold innovation, we can certainly produce high-quality, long-lasting cutting tools and make significant contributions to the revitalization of the mechanical industry. .
