How to heat treat W18Cr4V high speed steel?
In industrial knife, W18Cr4V is a most popular steel material, it is used for paper cutting knife, veneer cutting knife, wood planer knife, circular knife to cut paper...Then do you know heat treatment of W18Cr4V material? Please review the information below.
High-speed steel has high hardness, proper toughness, good wear resistance and thermal hardness above HRC60 below 600 °C. W18Cr4V is one of the common high-speed steels. It has become a variety of cutting tools in machinery manufacturing. The main material, after heat treatment, can be found in the characteristics of its materials. The high-speed steel is hypereutectoid steel, and the structure after isothermal spheroidizing annealing is a fine carbide alloy of Soxite. The hardness is below 255HB, which is good for cutting and preparation for quenching.
(1) Preheating and quenching of high speed steel
Because high-speed steel has many alloying elements, it contains more alloy carbides, generally in the form of M6C, M23C6, MC, etc., and Fe3W3 C~Fe4 W2C (tungsten high-speed steel), Fe3Mo3C~Fe4Mo2C (molybdenum) The high-speed steel is the main carbide, and its thermal conductivity is poor. In order to prevent cracking of the parts caused by uneven heating of the inner and outer temperatures, it is necessary to perform a preheat treatment at 800 to 850 °C. During the quenching process, the microstructure changes during the quenching process: austenite formation, carbide dissolution and transformation, and austenite grain growth. The temperature at which pearlite transforms to austenite is from 800 to 860. °C began to change.
When the heating temperature is above Ac1, the formation of austenite, the dissolution of carbides, M23C6 can be completely dissolved in austenite, and the M6C and MC type carbides are only partially dissolved. As the austenitizing temperature increases, the amount of alloy carbide dissolved in austenite increases, increasing the carbon content and alloying degree in austenite, so they directly affect the grain size of steel and Hardness after quenching. In general, the grain size of high-speed steel is usually controlled at 8 to 11. If the grain is fine, the carbide is too much, indicating that the quenching temperature is too low or the holding time is too short to function as a high-speed steel; When the heating temperature exceeds 1300 ° C, the amount of carbides in the steel decreases, the hindrance of carbide growth to grain size is weakened, and the crystal grains will be abnormally coarsened, resulting in an increase in brittleness and a decrease in mechanical properties, and thus in specific parts. During the actual heat treatment, a reasonable quenching process temperature should be selected according to the performance and service conditions of the tool.
W18Cr4V steel has a wide range of quenching temperature, usually between 1180 and 1280 °C, the thermal insulation coefficient is 8-15 min/mm, and the minimum holding time is not less than 1.5 min. It should be noted that in order to ensure that the surface is free from oxidative decarburization defects, and that there is overheating or overheating, it should be deoxidized during preheating and heating in the salt bath furnace, and the tool is not allowed to contact or approach the electrode.
The quenching and cooling methods of high-speed steel are as follows: 1 direct oil cooling, air cooling at 300-400 °C, mostly used for simple tools, such as turning tools; 2 primary quenching treatment, cooling in a salt bath at 400-600 °C ; 3 multiple stages of quenching treatment, respectively, in a 300 ~ 500 ° C salt bath for 30 ~ 60min; 4 for austenitic austempering treatment. The oil-cooled or staged quenched structure is quenched martensite + granular carbide + retained austenite (30%), while the bainite austempered structure is bainite + martensite + retained austenite.
(2) Tempering of high speed steel
For quenched tools, multiple high temperature tempering must be performed to achieve the required organization and performance. Martensite, retained austenite and carbides in the quenched structure change during tempering. When tempering below 400 °C, martensite precipitates alloy cementite and uneven aggregation, which causes the hardness, strength and plasticity of steel to decrease. When tempered at 400-600 °C, fine dispersion occurs in martensite. Carbide of chromium, vanadium and tungsten, characterized by dispersion distribution, not easy to aggregate, hardness is improved, in the tempering cooling process, the retained austenite transforms into secondary martensite, at 550-570 ° C, hardness is 63 Between ~66HRC (so called secondary hardening), due to the appearance of this effect, the carbides of vanadium and tungsten are stable and not easy to aggregate, and the martensite after carbide precipitation is difficult to decompose, so that the steel can still be at 600 ° C. Keep the hardness above HRC60.
The tempering of high-speed steel is generally three times, the purpose is to transform the majority of the quenched retained austenite into martensite for the first time; the second transformation of the transformed martensite into tempered martensite, to eliminate the tissue stress, A part of the retained austenite is transformed into martensite structure; the third tempering transforms the secondary quenched martensite into tempered martensite + carbide to facilitate the transformation of retained austenite into martensite. The amount of retained austenite after three tempering is greatly reduced, and the hardness is not reduced and the strength and plasticity are improved.
The structure of W18Cr4V steel after normal tempering is tempered martensite matrix, evenly distributed white bright granular carbide, if the base part appears white, and a small amount of grain boundaries are not completely disappeared, resulting in hardness reduction, which is insufficient tempering, should be needed Perform supplementary tempering treatment.
