1. Metallographic induction hardening problems
(1) Rapid heating critical point increase
Induction heating has a heating rate ranging from tens to hundreds of degrees per second, and pulse quenching can reach thousands of degrees per second (2000-3000 ℃/s). Due to the fast heating speed and short duration, the quenching temperature is higher than the general salt bath quenching temperature. This results in the transformation of the structure into austenite and homogenization.
Table 1 presents relevant data indicating that the AC1 point of T10 steel and GCr15 steel increases with the heating rate during rapid heating.
Table 1 Relationship between induction heating speed and critical point AC1
| Steel Grade | Heating speed / (℃ / s) | ||||||
| Original state | 10 | 50 | 100 | 150 | 200 | 300 | |
| T10 | girdling | 745 | 760 | 765 | 760 | 765 | 765 |
| extinguish | 735 | 745 | 755 | 755 | 760 | 765 | |
| GCr15 | girdling | 770 | 810 | 825 | 830 | 835 | 830 |
| extinguish | 740 | 750 | 785 | 800 | 815 | 810 | |
From practice, we know that the quenching temperature of induction heating is 80~150℃ higher than that of conventional quenching.
Table 2 shows the recommended heating temperature for high-frequency quenching of common steel.
Table 2 Heating temperature of common steel for high frequency quenching
| Steel Grade | Heating temperature/℃ |
| 45 | 860~920 |
| 50 | 860~900 |
| 40cr | 940~980 |
| T7, T7A | 880~960 |
| T8, T8A | 860~960 |
| Steel Grade | Heating temperature/℃ |
| T10, T10A | 850~960 |
| GCr15 | 920~1020 |
| GCr9 | 900~1000 |
| CrWMn | 850~960 |
| 9SiCr | 880~1000 |
Induction heating has significantly greater power than furnace heating, resulting in faster heating speeds and shorter times required to promote the transformation of pearlite to austenite.
The original structure of the steel has a considerable influence on the nucleation, growth and homogenization of austenite during rapid heating, which in turn affects the induction quenching temperature as well as the resulting microstructure and properties.
Figure 1 illustrates the relationship between the critical point of T8 steel, the heating rate and various original structures.
Flaked perlite is more susceptible to structural transformation during heating than spherical perlite.
Therefore, the induction quenching temperature of steel with different original structures should be as follows: t quenching (annealing state) > t quenching (normalizing state) > t quenching (quenching and tempering (annealing + high temperature quenching)) .
The physical meaning of α0 in the figure is:
Pearlite represents half the distance between two adjacent cementites, while free ferrite represents half the distance between nodes in the dislocation network (see Fig. 1).
As the heating temperature increases, the AC3 point also increases rapidly (see Fig. 1).
Figure 2 shows the temperature required for complete quenching of hypoeutectoid steel at different heating rates.
Figure 2
(2) Rapid heating can cause the steel to obtain fine or ultrafine grains
At low heating rates, the initial austenite grain formed immediately after austenitization decreases significantly with increasing heating rate. However, at high heating rates, the initial austenite grain hardly decreases with increasing heating rate.
Studies have shown that under real induction heating conditions, the heating rate is very high and the initial grain obtained is very small. It was determined that the initial grain size has nothing to do with the heating rate.
However, the growth of the austenite grains formed is related to the heating rate. As heating continues to a certain temperature, the actual size of the austenite grain formed increases with decreasing heating rate, as shown in Fig.
Therefore, as long as the temperature and heating time are properly controlled, induction heating will not result in overheating.
Figure 3
2. Various rapid heating phenomena of high-speed steel
(1) Fast heating of high speed steel blade
As early as 1923-1924, Vologgin of the former Soviet Union began to study high-frequency quenching of high-speed steel tools, but was unsuccessful.
The reason for this was that high-speed steel tools need to be thoroughly quenched to obtain a hardened layer with high hardness and thermal resistance, which can be relatively thick.
There was also concern that poor dissolution of high frequency quenching carbides could affect other properties.
However, this was a superficial insight and induction extinction has not been fully studied.
It was not until 1952 that a breakthrough was achieved.
Gedeberge and others finally managed to temper a 3-10mm W18Cr4V (P18) blade.
Unfortunately, it was not put into industrial production, but it demonstrated that high-speed steel tools could be induction hardened.
(2) Fast heating of fast steel welding
Rod-shaped tools, such as high-speed steel tapered shank drills and end mills, can be heated quickly using techniques such as flash welding or friction stir welding. These methods are capable of heating steel parts to temperatures exceeding 1,000℃ in just a few seconds.
(3) Rapid heating of high-speed steel forgings
The author recommends directly heating the φ60mm high-speed steel billet in the high temperature zone. This means that the cold material must be heated directly in the 1150-1200°C zone without preheating.
This method has been used in production for many years and the forging quality has remained stable.
(4) Application of quenching parameter formula of high speed steel tools
There is a quenching parameter formula in heat treatment of high speed steel tools
That is,
P = t (37 + lg τ）
Where
- P – extinction parameter;
- t — extinguishing heating temperature;
- τ—— Extinguishing heating time.
The symbol P in the formula represents the combined impact of extinction heating temperature and heating time.
During the quenching process, regardless of variations in the heating temperature and quenching heating time, the degree of austenitization remains the same as long as the quenching parameters are identical.
This implies that the quench quality of the tool will remain consistent if P remains constant, whether achieved through rapid heating at a high temperature over a short period or slow heating at a low temperature over a longer period.
(5) Rapid heating and semi-rapid heating in furnaces for high-speed steel tools
In the late 1950s, new technologies for rapid heating and energy-efficient heat treatment were introduced in Beijing, Tianjin, Shanghai, and elsewhere with the assistance of heat treatment experts from the Soviet Union. The implementation of this technology has resulted in numerous successful experiments, but unfortunately, only a limited amount of data remains.
The author has data only on the rapid heating of a drill with a φ14mm conical shank and a slot milling cutter made of W18Cr4V steel from the Shanghai tool factory. According to reports, the quenching heating temperature of W18Cr4V steel has increased from 1270 to 1310 ℃, while the heating coefficient has been reduced from 10 to 6s/mm. Surprisingly, tool life increased slightly instead of decreasing.
(6) Surface modification of high-energy density high-speed steel cutting tools such as laser and electron beam
In recent years, there have been continuous reports on the surface modification of high-speed steel using lasers. These reports point out that high-speed steel can be heated quickly using this method.
The technical method involves applying high energy density plasma to the surface of M42 steel at high speed. This results in a rapid local increase in temperature and rapid cooling at the surface of the material. The temperature rise and cooling speed can reach 104-108k°/s.
As a result, a crystal structure modification layer can be formed on the surface of the part, improving the performance of the material.
(7) Rapid heating of high speed steel has a long history
In the past century, since the introduction of high-speed steel, innovation and reform of its heat treatment process have been ongoing.
Some individuals in the former Soviet Union claimed that steel could be heated at any speed. However, due to the limited conditions at the time, this was only possible through salt bath furnaces and high frequency heating. Furthermore, tempered parts were no longer limited to simple rods or pieces, but still lacked universality.
Relatively successful results have been achieved in the rapid heating application of high-speed steel forged billets. Most people believe that the heating speed of high-speed steel material after pressure processing and annealing can be unrestricted before forging.
Furthermore, with the emergence of new technologies and processes such as laser and electron beam, there are many reports about the surface modification of high-speed steel through rapid heating. This suggests that the rapid heating of high-speed steel has now entered a substantial application stage.
3. Application of induction heating quenching on high-speed steel mechanical blades
High-speed steel is known as “wind steel” due to its good hardenability, which allows it to be tempered in air.
It can be hardened to a hardness greater than 64HRC in air, making it ideal for producing sharp edges. For this reason, it is also known as “steel blade”.
Induction heating quenching of high speed steel is a form of self-cooling quenching that is energy efficient and environmentally friendly. It also offers high production efficiency.
Regardless of the type of steel to be tempered, there are two basic conditions that must be met. First, it must be austenitized and second, it must be cooled immediately.
The cooling rate must be greater than the critical cooling rate of the steel (V).
Induction heating has the unique characteristic of heating the surface of the part. If heating is stopped immediately after austenitizing the surface layer, and the adjacent unheated metal can quickly conduct heat from the heating layer, the surface will be hardened if its cooling rate is greater than V.
The quenching process does not involve spraying a quenching liquid onto the surface, but relies on cooling the internal metal. This unique cooling process can only be achieved with high energy density heating. Induction heating is one of the methods that provides high energy density heating.
Due to the extremely high power density and short heating time, induction heating is also known as pulse heating. The temperature of the part during induction heating can be measured using an infrared photoelectric pyrometer or an optical pyrometer. Alternatively, the quenching temperature can be determined through visual inspection by examining the color of the heated part.
During the induction heating process, the heat generated by the eddy current is mainly used to heat the surface layer of the workpiece. However, there are two types of heat released by the part during this process.
There are two types of heat loss during the heating process: radiant heat, which is emitted from the heating surface to the air, and conduction heat, which is conducted from the heating layer of the part to the center.
The impact of internal heat conduction, mainly in the heating layer, deepens the theoretical understanding of the process. The depth of the heating layer is indicated by d and is equal to 0.2 mm, where t represents the heating time in seconds. Heat loss increases as power density decreases and heating time is prolonged.
If the part is relatively thin, heat conduction will be quickly transmitted from the surface to the core, resulting in uniform heating of the entire section.
High speed steel is a self-hardening material, which means it will harden immediately after stopping the heating process.
