In fact, W18Cr4V steel is a W series high-speed tool steel. W18Cr4V has high strength, high compression resistance, high thermal stability and high hardness and high temperature hardness, the steel has high thermal hardness, good wear resistance, tempering resistance, large hardening depth, and the bearing capacity ranks first in all kinds of high-speed steel. However, the toughness, machinability and thermal conductivity are poor, and the quenching deformation is medium. The cost of W18Cr4V is high, the manufacturing process is poor, the heat treatment process is complicated, and the deformation of the parts after quenching and tempering is difficult to control. However, it is found that the steel is brittle and easy to produce the phenomenon of falling edge, the main reason is that the uniformity of carbide bar is large.
W18Cr4V high speed steel must go through spheroidizing annealing after forging, which is conducive to cutting. The reworked workpiece is also subjected to spheroidizing annealing before the second quenching. Otherwise, the second quenching is heated. The grain size will overgrow and make the workpiece brittle.
W18Cr4V cold pressing blank softening process, using upper limit temperature heating, stage isothermal, and then additional isothermal tempering process.
W18Cr4V high-speed steel should be preheated twice during quenching, because the high-speed steel contains a large number of alloying elements, poor thermal conductivity, so as not to cause deformation or cracking of the workpiece, especially the large complex workpiece is more prominent. By preheating in advance, the residence time at high temperature treatment can be shortened, and the risk of oxidation and decarburization and overheating can be reduced.
The quenching process of W18Cr4V high-speed steel is relatively special, that is, after two preheating, high temperature quenching, and then three high temperature tempering. Production must strictly control the quenching heating and tempering temperature, quenching, tempering holding time, quenching, tempering cooling method. If not properly controlled, it is easy to produce defects such as overheating, overburning, naphthalene fracture, insufficient hardness and deformation and cracking. Oil toughness treatment can improve the plasticity of the steel.
The first preheating of W18Cr4V can dry the moisture on the workpiece, and the second preheating can make the transition from sorstenite to austenite occur at a lower temperature.
W18Cr4V high speed steel contains a large number of insoluble alloy carbide, quenching heating, the temperature must be high enough to make the alloy carbide dissolved into austenite, after quenching martensite alloy element content is high enough, steel will have high thermal hardness. The alloy elements that have the greatest influence on the thermal hardness of high-speed steel are W, Mo and V, and the dissolution amount increases sharply only when the temperature is above 1000℃. When the temperature is ≥1300℃, although the amount of dissolved elements increases, the austenite grains grow rapidly, and even melt at the grain boundaries, resulting in a decline in the strength and toughness of the steel. For high-speed steel, the appropriate grain size is 9.5~10.5.
The quenching temperature of W18Cr4V has a great influence on the performance of the steel. When the quenching temperature rises, the wear resistance, compressive resistance and thermal stability improve, and the toughness of the steel increases with the decrease of the temperature. The peak bending strength of 1230~1250℃ quenching appeared, and the best comprehensive mechanical properties were obtained after tempering at 550~570℃. The surface decarburization layer also significantly increases the susceptibility of the steel to quenching crack and wear crack.
W18Cr4V quenching cooling is usually carried out in oil, but for complex, slender rod or sheet parts, fractional quenching and isothermal quenching can be used. After grading quenching, the volume fraction of residual austenite is increased by 20%~30%, the deformation and cracking tendency of the workpiece are reduced, and the strength and toughness are improved. The microstructure after oil quenching and fractional quenching is martensite + carbide + residual austenite. After isothermal quenching, in addition to martensite, carbide and residual austenite, the main quenched structure also contains lower bainite compared with fractional quenching. Isothermal quenching can further reduce the deformation of the workpiece and improve the toughness.
When W18Cr4V is graded quenching, if the residence time at the graded temperature is too long, a large number of secondary carbides may be precipitated. Isothermal quenching generally takes a long time, with different isothermal time, the amount of bainite obtained is different, in the production usually can only obtain the volume fraction of 40% bainite, and isothermal time is too long can significantly increase the residual austenite volume. This requires cold treatment after isothermal quenching or multiple tempering to eliminate residual austenite, otherwise it will affect the hardness and heat treatment quality of the tempered steel.
W18Cr4V In order to eliminate quenching stress, stabilize the structure, reduce the residual austenite volume, and achieve the required performance, high-speed steel is generally tempered at 560℃ three times. The tempering transformation of high speed steel is more complicated. In the tempering process, martensite and residual austenite change, and excess carbide does not change in the tempering process.
