HSS Heat Treatment

---

Heat Treatment of HSS

The heat treatment of high-speed steel usually includes spheroidizing annealing before machining and quenching and tempering after forming. After forging, high-speed steel must undergo spheroidizing annealing (pre-heat treatment). The purpose is not only to reduce the hardness of the steel to facilitate cutting, but also to prepare the structure for the final heat treatment.

1.HSS Spheroidizing Annealing

The spheroidizing annealing process of high-speed steel has two types: ordinary spheroidizing annealing and isothermal spheroidizing annealing. The annealing temperature of W18Cr4V steel is 860~80 °C, which is slightly higher than the A1 temperature, and the holding time is (2-3) h. In this way, there are not many alloying elements dissolved in the austenite, and the austenite is less stable and easy to transform into Soft tissue. If the heating temperature is too high, a large amount of carbon and alloying elements will be dissolved in the austenite, and its stability will increase, which is not good for annealing. The annealing temperature of W6Mo5Cr4V2 steel adopts the lower limit of the above temperature. In order to shorten the annealing time, isothermal spheroidizing annealing can be used, that is, keeping the temperature at 860~880°C, quickly cooling to 720~750°C and holding it down to 500°C. The annealed structure of W18Cr4V steel is that uniform and fine carbide particles are distributed on the sorbite matrix (as shown in Figure 1). The hardness after annealing is 207~255HBS.

Fig 1 Microstructure of high-speed steel after annealing

2.  HSS Quenching

The superiority of high-speed steel can only be exerted after correct quenching and tempering. The quenching temperature is generally much higher in lower alloy cutting tool steel. For high-speed steel, the higher the quenching heating temperature, the more the amount of alloying elements dissolved into austenite, and the higher the alloying elements of martensite after quenching. Only martensite with a high content of alloying elements has high red hardness. The alloying elements (W, Mo, V) that have the greatest effect on the red hardness of high-speed steel only increase sharply when the temperature is above 1000°C. When the temperature exceeds 1300°C, although the content of these alloying elements can continue to be increased, this At this time, the austenite grains grow up sharply, and even local melting occurs at the grain boundaries. Therefore, the toughness of quenched steel is greatly reduced. Therefore, for the quenching heating temperature of high-speed steel, as long as it does not overheat, the higher the quenching temperature of high-speed steel, the better its red hardness. The quenching heating temperature of commonly used high-speed steel is shown in Table 1.

Grade	W18Cr4V	W6Mo5Cr4V2	W6Mo5Cr4V3	W6Mo5Cr4V2Co5
Oil quenching temperature range	1260~1310	1200~ 1240	1200~ 1230	1210-1230
Commonly used oil quenching temperature	1280	1220	1220	1220

Table 1 Commonly used high-speed steel quenching heating temperature

Because of the poor thermal conductivity of high-speed steel and the extremely high quenching temperature, it is often heated in two or three stages, that is, preheated at 800~850C, and then heated to the quenching temperature. Large blades (tools) and complex blades (tools) should be preheated twice (three-stage heating), the first time at 500~600°C, the second time at 800~850°C. In addition, high-speed steel is used Preheating first can also shorten the residence time in high temperature treatment, which can reduce the risk of oxidative decarburization and overheating.

The supercooled austenite transformation curve of W18Cr4V steel is shown in Figure 2. Due to the high degree of austenitic alloy, the decomposition rate is relatively slow. The pearlite transformation interval is between A1 and 600°C, and the fastest time from the beginning to the end of the transformation is 1.0-10h. From 600°C to BS (360°C) is the supercooled austenite medium temperature stable zone, and from BS to 175°C is the bainite transformation zone, but the transformation does not proceed to the end. Below MS (220°C) is the martensite transformation range. After quenching, it contains 70% cryptocrystalline martensite and 20% to 25% retained austenite. During the cooling process, the medium temperature stays or the slow cooling will cause thermal stabilization of austenite, which will reduce the MS point and increase the amount of retained austenite.

Fig 2 CCT diagram of W18Cr4V high-speed steel heated to 1300°C

The quenching and cooling of high-speed steel is usually carried out in oil, but for complicated shapes, slender rods or thin parts, hierarchical quenching and austempering can be used. When the cooling rate is too slow, carbides will precipitate from the austenite in the temperature range of 80~1000°C, which will have an adverse effect on the red hardness of the steel: staged quenching can increase the amount of retained austenite by 20%~30% , So that the deformation and cracking tendency of the workpiece are reduced, and the strength and toughness of the steel are improved. The normal quenched structure of Gaolian Steel is martensite (60% ~ 65%) + carbide (10%) + retained austenite (25% ~ 30%) (as shown in Figure 3). It must be emphasized that for woods, the oil quenching grading temperature retention time should generally not be too long, otherwise a large amount of secondary carbides may be precipitated, which is detrimental to the performance of the steel.

Fig 3 The structure of high-speed steel after quenching at 1280°C

In addition to martensite, carbides and retained austenite, the austempered structure of high-speed steel also contains lower bainite. Austempering can further reduce the deformation of the workpiece and improve the toughness.

3. HSS Tempering

In order to eliminate the quenching stress, stabilize the structure, reduce the amount of retained austenite, and achieve the required performance, high-speed steel generally undergoes three tempering treatments at 560°C for 1h. The tempering transformation of high-speed steel is more complicated. The most significant change in the structure during the tempering process is the precipitation of alloy carbides from martensite and residual steel, which causes changes in the composition and properties of the steel’s matrix.

During the tempering process, from room temperature to 270°C, ε carbides are first precipitated from martensite, and then gradually transformed into Fe3C and grow up. The hardness decreases accordingly: in the tempering temperature range of 400~500°C Inside, the Cr in the martensite transfers to the carbides. At the same time, the cementite-type carbides gradually transform into dispersed Cr-rich alloy carbides (M6C), which gradually increases the hardness of the steel. Between 500~600°C, the hardness, strength and plasticity of steel are improved, and dispersed tungsten (aluminum) and vanadium carbides (W2C, Mo2C, VC) are precipitated from martensite at 50570. The hardness of the steel is greatly improved, and the maximum hardness and strength can be reached. Therefore, high-speed steel is mostly tempered at 550~570°C.

It must be pointed out that due to the large amount of retained austenite in high-speed steel, 10% of the retained austenite remains untransformed after one tempering, and only after two tempering can it be lower than 5%. The structure after normal tempering is tempered martensite + carbide, as shown in Figure 4, the hardness is 63~66HRC.

Figure 4 The structure of high-speed steel after three times of normal quenching and tempering

In order to reduce the number of tempering, cold treatment (-70°C~-80°C) can be carried out. Because the retained austenite will be quickly stabilized after the high-speed steel is refrigerated at room temperature (30~60) min, the cold treatment is the best It should be carried out immediately, and then tempered again to eliminate the stress caused by the cold treatment. High-speed steel should be noted in the tempering process, after each tempering, it must be cooled to room temperature before the next tempering, otherwise it is easy to cause insufficient tempering. Therefore, the metallographic method is often used in production to measure the amount of retained austenite to check whether the tempering transformation of high-speed steel is adequate.To sum up, in the heat treatment operation of high-speed steel, the quenching heating and tempering temperature, the quenching and tempering holding time, and the quenching and tempering cooling methods must be strictly controlled. If the heat treatment process parameters are not properly controlled, defects such as overheating, overburning, tea-like fractures, insufficient hardness, and deformation and cracking are likely to occur.

Shanghai Geheng Vacuum Technology Co., Ltd. is a vacuum heat treatment furnace manufacturer with many years of experience in design, manufacturing and production. Mainly produce vacuum heat treatment furnace, vacuum sintering furnace, vacuum brazing furnace, New Energy And Environmental Protection Equipment and other vacuum equipments.Provide customized services, equipment work area can be based on customer output.

Learn more:

Vacuum Oil Quenching Furnace

Vacuum Annealing Furnace

Horizontal Type Single Chamber Vacuum Annealing Furnace

Copyright: SIMUWU Vacuum Furnace