The carbon content also has an effect on the strength and plasticity of the steel after quenching and low temperature tempering. For hypoeutectoid steels, as the carbon content increases, the strength of the quenched steel increases to a carbon content of 0.6% to 0 At 7%, it reaches the maximum value; then it decreases, and it is the lowest when it is close to the eutectoid composition. When the carbon content exceeds 1.15%, the strength decreases due to uneven distribution of cementite. In general, as the carbon content increases, the toughness of steel gradually decreases.
Carbon tool steel is usually smelted in electric arc furnace or open hearth furnace. Due to the high carbon content in steel and poor thermal conductivity, during hot working, the loading temperature of steel ingots or large billets during heating should not be too high, and the heating rate (especially at low temperatures) should not be too fast, so as to avoid too large The thermal stress caused cracks. When heating, the steel must be burnt through; however, the residence time at high temperature should not be too long to avoid serious decarburization. When hot working (forging, rolling), it is necessary to ensure that most of the network carbides in the steel can be broken after hot working. Because there are uneven or coarse carbides in the steel, the quality of the steel will deteriorate, the cutting process will become difficult, the mold is prone to cracking during heat treatment, the hardness after heat treatment is uneven, and the edge is easy to chip during use. Therefore, when forging, rolling and hot working carbon tool steel, it must have an appropriate compression ratio (generally greater than 4); for T12 and T13 steels with high carbon content, sometimes the upsetting method must be used for forging. In order to make the carbides in the steel uniformly refined. The final forging and final rolling temperature of carbon tool steel is generally around 800. After forging and rolling, it should be quickly cooled to 650°C, and then slowly cooled to avoid precipitation of coarse or networked carbides.
The carbon tool steel after hot processing has a pearlite structure with high hardness, and its structure does not meet the requirements of the final heat treatment. In order to improve the cutting performance of the steel and prepare the structure for the final heat treatment, spheroidizing annealing is required. The structure and hardness after annealing should meet the requirements of (3-B1298-86). There are continuous networked carbides, and the broken networked carbides are evaluated according to the second level diagram attached to the GBl298-86 standard.
The martensite structure is obtained after quenching, so that the die steel has high hardness and wear resistance. After quenching, there is inevitably a certain amount of retained austenite and coarse martensite, which reduces the mechanical strength of the steel and increases the brittleness. Therefore, there are certain restrictions on the level of quenched martensite for molds made of high-carbon steel. Otherwise, brittle damage may occur when the mold is used. The quenching heating temperature of carbon tool steel is generally selected according to the critical point of the steel, taking A. The quenching temperature can also be higher for steel with a higher Acm point than 30～50℃. In order to improve the surface hardness of larger-sized molds, a higher quenching temperature can be considered. For small-sized molds, a lower quenching temperature can be selected to obtain good mechanical properties. In order to reduce the quenching deformation and cracking of the mold, when the size or use conditions permit, a cooling medium with a slower cooling capacity should be selected. At this time, a higher quenching temperature can be used. For example, the heating temperature of the mold quenched in oil or nitrate salt is about 20°C higher than that of water quenching, so that a deeper hardened layer and higher hardness can still be obtained. The decrease in mechanical properties caused by increasing the quenching temperature can be offset to a certain extent by the slow cooling to reduce the quenching internal stress. If the original structure is fine flake and punctate pearlite structure, cementite is easy to dissolve when heated, and a lower heating temperature should be selected; for steel with a coarse spheroidized pearlite structure, a higher quenching heating temperature can be selected. The time required for quenching and holding must ensure that the inside of the mold reaches the quenching temperature and forms austenite with uniform carbon concentration, otherwise good performance will not be obtained after quenching. Of course, too long holding time will also cause mold overheating, surface decarbonization, waste of energy and lower productivity. Quenching
When heating, in order to prevent oxidation and decarburization of the mold surface, it is generally heated in a salt bath. Because of the low hardenability of carbon tool steel, oil quenching is generally used for molds with an effective thickness of 5mm; molds with an effective thickness of 5-10mm can be quenched in stages in a nitrate bath at 150-160℃; and the effective thickness is 10～ 15mm molds can be quenched in stages in an alkaline bath at 140-160℃; molds with an effective thickness of 15-18r) qlYl can be quenched in water, but are prone to large internal stress and deformation. Therefore, carbon tools Steel is only suitable for making small cross-section molds.
Carbon tool steel has high hardness after quenching, but there is internal stress in quenching. The plasticity 1.3 carbon tool steel is low and the strength is not high. It must be tempered to improve its mechanical properties. When tempering at low temperature, e-carbide (Fe2.4C) in the steel is precipitated from martensite, which has a high degree of dispersion. The carbon content in the martensite decreases, and the hardness of the steel decreases a little, but the strength and plasticity increase , Thereby reducing the phenomenon of die chipping. As the tempering temperature increases, the amount of retained austenite in the steel decreases, and the decomposition is basically completed at 250°C. When tempering above 200℃, the hardness and strength performance of steel will decrease rapidly. Therefore, molds made of carbon tool steel generally adopt low-temperature tempering (≤200°C). For the mold steel used to manufacture forging dies, in order to obtain high toughness, the tempering temperature can be increased to 350-450°C.
Carbon tool steels with hypoeutectoid components, such as 17 steel, have good plasticity and strength, and are suitable for making tools and dies (such as forging dies, chisels, hammers, etc.) that can withstand impact loads and tools for cutting soft materials (such as woodworking). tool). T8 and T9 steel are prone to overheating during quenching and heating, but they have high hardness and wear resistance. They are generally used to make simple-shaped molds and soft metal cutting tools and woodworking tools. Carbon tool steels with hypereutectoid components, such as T10 and T11 steels with a carbon content between 0.95% and 1.15%, can be heated at 780-800°C to maintain a fine grain structure, and the steel will remain in the quenched steel. There are undissolved excess carbides, which is beneficial to wear resistance. Therefore, this steel is widely used and suitable for manufacturing molds with high wear resistance requirements, such as cold punching dies, wire drawing dies, trimming dies, etc. The T12 and T13 steels with carbon content between 1.15% and 1.35% have more excess carbides after quenching, so they have high wear resistance and hardness, and low toughness. They are not suitable for manufacturing tools and molds that can withstand impact loads. , And suitable for manufacturing wire drawing dies, taps, dies, etc.
