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Process Performance of Steel-bonded Cemented Carbide

1. Hot Working of Steel Bonded Cemented Carbide

Steel-bonded cemented carbide has malleability. After forging, it can increase the density of the alloy, improve the distribution of hard phases, reduce segregation of carbides, and shape the material.

The forgeability of steel-bonded cemented carbide depends on the ratio of the hard phase to the steel matrix and the plasticity of the steel matrix. Low- and medium-alloy steel and steel-bonded cemented carbide have good forgeability. The raw materials must be inspected before forging to remove holes, blisters, cracks, delamination, underburning, severe carburization, alloy dispersion and other powder metallurgical production. Blanks with common defects. Because the thermal conductivity of steel-bonded cemented carbide is relatively poor, heating should be slow and uniform, and preheating should be sufficient to prevent oxidation and decarburization. The initial forging temperature can be appropriately increased to increase the plasticity of the die blank. The initial forging temperature is generally 1150～1200℃. In the first and second forgings, try to tap quick hits, and control the deformation of each forging at about 5%. The amount of deformation can be appropriately increased to 10% to 15% during reforming and forging. The final forging temperature should be controlled at 900~950℃. When the die blank is cold to the final forging temperature during forging, the forging should be stopped in time, and the forging should be continued after returning to the furnace for heating, and it must be cooled slowly after forging. Too high initial forging temperature, too low final forging temperature, excessive deformation, too fast heating speed and other reasons can cause defects such as surface cracks, center cracks, axial cracks and corner cracks in cemented carbide forgings.

2. Preliminary Heat Treatment

In order to reduce the hardness, improve the machining performance, and provide favorable conditions for the subsequent processes and quenching, tempering strengthening and other subsequent processes, the steel-bonded cemented carbide needs to be spheroidized before leaving the factory and after forging the blank. , Can be machined. Steel-bonded cemented carbide annealing should generally be carried out in a vacuum furnace or a heat treatment furnace with a protective atmosphere to prevent oxidation and decarburization. Packing protection should be used when it is carried out in a box furnace to ensure that it is filled with a good protective agent. Annealing process is: slowly heating to 850~890℃, holding for about 4h, furnace cooling to about 730℃, holding for 6h, and then furnace cooling to below 500℃ for air cooling. The annealed structure is hard phase+point pearlite+alloy carbide, and the hardness after annealing is about 40HRC.

3. Quenching and Tempering

The quenching process of steel-bonded cemented carbide mainly depends on the composition characteristics of the binder phase and the hard phase and the mold requirements. Steel-bonded carbide quenching and heating must be preheated. Small molds use one-stage preheating method, and larger workpieces must use two-stage preheating method. The overheating tendency of steel-bonded cemented carbide is smaller than that of alloy tool steel, and the quenching heating temperature range is wide. Especially when manufacturing cold-worked mold parts, alloying elements and carbides can be fully dissolved in austenite through high temperature and sufficient heat preservation to improve the alloying degree of quenched martensite and increase tempering resistance. The quenching temperature of WC type steel-bonded carbide is 1020~1050℃, the quenching temperature of TiC type is 950~1000℃, and the quenching temperature of G type is 1200~1280℃.

The heating and holding time of steel-bonded cemented carbide in the salt bath furnace can be selected at 0.5~1min/mm, and the box furnace is doubled to prevent oxidation and decarburization. Generally, hot oil or graded quenching is used for quenching and cooling. Austempering should be used for the quenching of molds with complex shapes or large changes in cross-sectional dimensions to avoid cracking and reduce deformation as much as possible.

After quenching, it should be tempered as soon as possible. The tempering temperature is 180~200℃, and the holding time is 1.0~1.5h; when higher toughness is required, high temperature tempering at 500~550℃ can be used, and 250~350℃ tempering brittle zone should be avoided. . WC steel-bonded cemented carbide will produce secondary hardening when tempered at 450~550℃. During high-temperature tempering, a large amount of carbides are precipitated and the connection phenomenon occurs, which reduces the impact toughness, so avoid using too high tempering temperature.

After annealing and softening the steel-bonded cemented carbide, it can be machined for machining such as turning, milling, planing, drilling, reaming, and tapping. It is advisable to use a low speed, a large amount of back-grabbing and a medium feed rate during processing. Cooling lubricating fluid should not be used to avoid hardening or even cracking due to excessive cooling. Before quenching, the mold should be processed to the final size or leave a very small margin. Try to stop grinding or slightly grind after quenching.