Realize High-Speed Steel Induction Heating And Quenching

1. Metallic problems of induction hardening

(1) The critical point of rapid heating rises

The heating speed of induction heating ranges from tens of degrees per second to hundreds of degrees per second, and pulse quenching reaches thousands of degrees per second (2000～3000℃/s). Because of the fast heating speed and short duration, the quenching temperature is higher than that of ordinary salt bath quenching. The temperature must be high in order to transform the structure into austenite and homogenize. Table 1 shows the relevant data that the Ac1 point increases with the acceleration of the heating rate when the T10 steel and GCr15 steel are heated rapidly.

The specific power of induction heating is much larger than that of heating in the furnace, so the heating speed is fast, so that the time required for the start and completion of the temperature rise of the pearlite to austenite transformation is short.

The original structure of steel has a great influence on the nucleation, growth and homogenization process of austenite during rapid heating, and therefore also significantly affects the temperature of induction quenching and the structure and performance of quenching. Lamellar pearlite is easier to complete the microstructure transformation process during heating than spherical pearlite. Therefore, the induction quenching temperature of the same steel with different original microstructures must be: t quenching (annealed state)> t quenching (normalized state)> t quenching [ Tempering (annealing + high temperature tempering)]. For pearlite, it means half of the distance between two adjacent cementites; for free ferrite, it means half of the distance between the nodes of the dislocation network.

(2) Rapid heating can make steel obtain fine grains or ultra-fine grains

In the lower heating rate range, as the heating rate increases, the initial austenite grains formed just after austenitization are significantly reduced, but when the heating rate is high, the austenite starts The crystal grains almost no longer decrease with the increase of heating rate. Practice has proved that under the actual conditions of induction heating, the heating speed is extremely high, and the initial grains obtained are extremely small and have nothing to do with the heating speed. However, the growth of the formed austenite grains is related to the heating rate. When heating to a certain temperature, the smaller the heating rate, the larger the actual austenite grains formed, as shown in Figure 3 [2], so as long as the heating temperature and heating time are properly controlled, induction heating will not Produce overheating.

2. Various phenomena of rapid heating of high-speed steel

(1) Fast heating of high-speed steel blades

As early as 1923 to 1924, Vologkin in the former Soviet Union began to study high-frequency hardening of high-speed steel tools, but it was not successful [3]. The reason for the failure is that high-speed steel tools must be completely hardened, or the resulting high-temperature hardening and high-strength hardened layer is relatively thick, and there is also concern that the high-frequency quenching carbide dissolution will affect other properties. But this is a relatively superficial perception, and induction hardening has not been studied in depth. It was not until 1952 that there was a breakthrough. Gedeberg and others finally succeeded in quenching W18Cr4V (P18) blades with a size of 3-10mm. Unfortunately, it did not move towards industrial production, but it is sufficient to show that high-speed steel tools can be induction-hardened .

(2) Rapid heating of high-speed steel weldments

High-speed steel taper shank drills, end mills and other rod-shaped tools, whether flash welding or friction welding, are examples of rapid heating, and steel parts can be heated to above 1000°C in a few seconds.

(3) Rapid heating of high-speed steel forgings

The author advocates that the φ60mm high-speed steel billet is directly heated in the high temperature zone [4], that is, the cold material is directly fed into the 1150～1200℃ zone without preheating. It has been in production for many years and the forging quality is stable.

(4) Application of quenching parameter formulas for high-speed steel cutters

There is a quenching parameter formula in the heat treatment of high-speed steel tools

• That is, P=t(37+lgτ)[5]
• In the formula, P—— quenching parameter;
• t——Quenching heating temperature;
• τ——Quenching heating time.

The P in the formula represents the combined effect of quenching heating temperature and heating time. In the quenching process, no matter how the quenching heating temperature and heating time change, as long as the effect of the two is the same as the quenching parameter, the degree of austenitization should be equivalent. It means that high temperature short time (rapid heating) and low temperature long time, as long as P is the same, the quality of tool quenching is the same.

(5) Rapid heating and semi-rapid heating in the high-speed steel tool furnace

At the end of the 1950s, with the help of Soviet heat treatment experts, Beijing, Tianjin, Shanghai and other places promoted rapid heating and energy-saving heat treatment technology, and gained a lot of successful experience. Unfortunately, there are not many materials left. The author only has Shanghai Tool Factory W18Cr4V steel φ14mm taper shank drill and slot milling cutter rapid heating data [6]. It is reported that the quenching heating temperature of W18Cr4V steel is increased from the conventional 1270～1280℃ to 1300～1310℃, the heating coefficient is shortened from the original 10～12s/mm to 5～6s/mm, and the tool life is not reduced but slightly increased.

(6) Surface modification of high-speed steel cutting tools with high energy density such as laser and electron beam

In recent years, there have been reports on the surface modification of high-speed steel such as laser [7], which pointed out that high-speed steel can be heated quickly. The technical method is to apply high-energy density plasma on the surface of M42 steel at high speed, so that the surface of the material will have local rapid heating and cooling. The heating and cooling speed can reach 104～108K°/s, so it can be formed on the surface of the workpiece. The modified layer of crystal structure achieves the purpose of improving the performance of the material.

(7) The rapid heating of high-speed steel has a long history

For more than 100 years since the advent of high-speed steel, people have never stopped the innovation and reform of its heat treatment process. There were people in the former Soviet Union that it was possible to heat steel at any speed. Limited to the conditions at the time, only limited to salt bath furnaces and high-frequency heating, and the quenched parts are all rods or sheets that can no longer be simple, and they are not universal. The application of rapid heating of high-speed steel forging blanks is relatively successful. Most people believe that the heating speed of high-speed steel materials after pressure processing and annealing before forging can be unlimited [8]. When new technologies and processes such as lasers and electron beams appear, there are frequent reports on the surface modification results of high-speed steel rapid heating, indicating that rapid heating of high-speed steel has entered a substantial application stage.

3. Application of induction heating and quenching in high-speed steel mechanical blades

High-speed steel has good hardenability and can be quenched in the air, so it is called "wind steel". Its hardenability is also good, and it can be quenched to more than 64HRC in the air, resulting in a very sharp cutting edge. Also called "Fenggang". High-speed steel induction heating quenching is self-cooling quenching, which is energy-saving and environmentally friendly, and has high production efficiency.

No matter what kind of steel quenching requires two basic conditions: the first is to be austenized, the second is to cool immediately, the cooling rate should be greater than the critical cooling rate of steel (vPro). The characteristic of induction heating is that the surface of the workpiece is heated. If the surface layer is austenitized and the heating is stopped immediately, and the adjacent unheated metal can quickly conduct the heat of the heating layer, and its cooling rate>v, the surface will be It is hardened. It does not rely on the surface spray quenching liquid for rapid cooling, but is cooled by the internal cold metal. This special quenching process can only be achieved under high-energy density heating. Induction heating is one of the high-energy density heating methods. Because of the extremely high power density and extremely short heating time, it is also called pulse heating.

The temperature of induction heating can be measured by infrared photoelectric pyrometer or optical pyrometer, or visual inspection (according to the color of the heated workpiece) to judge the quenching heating temperature.

The heat generated by the eddy current on the workpiece during induction heating is mainly used for the surface layer required for heating, but there are two kinds of heat emitted from the workpiece during this process. The first is from the heating surface to the air, which is called radiation. Heat; The second type is conducted from the heating layer of the workpiece to the core, which is called conduction heat. These two kinds of heat loss, especially the effect of inward heat conduction, deepen the theoretical depth of the heating layer, you can use d depth = 0.2 (mm), where t is the heating time (s). As the power density decreases and the heating time increases, the loss increases. If the workpiece is relatively thin, heat conduction will quickly transfer from the surface to the core, and the entire section will be hot through. High-speed steel is a self-hardening material, and it will be hardened as soon as the heating stops.

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