Carbide vacuum sintering
Liquid phase sintering of hard alloy, the binder phase in the liquid phase conditions. The compact in the vacuum furnace heated to 1350 ℃, 1600 ℃. Sintered compact line shrinkage rate of about 18%, volume shrinkage at about 50%, the accurate value of shrinkage depends on the granularity of powder and alloy composition.
Carbide sintering is a complex physical and chemical process. The process involves plasticizer removal, degassing, solid phase sintering, the liquid phase sintering, alloying, densification and dissolving precipitation process. Compact formed in the specific sintering conditions has a certain chemical composition, structure, performance, size and shape of products. The process conditions in accordance with the different sintering plant has great differences.
Carbide vacuum sintering is in less than 1 ATM ATM (1 = 101325 pa) under the sintering process. Under the condition of vacuum sintering, greatly reduce the powder surface adsorption of gases and the densification of gas inside the closed pore blocking effect, is advantageous to the diffusion process and densification, avoiding the sintering process of the metal reaction with certain elements in the atmosphere, can significantly improve wettability liquid binder and hard facies, but the vacuum sintering cobalt attention should be paid to prevent evaporation losses.
Vacuum sintering generally can be divided into four stages, namely plasticizer removal, presintering, high temperature sintering stage, cooling stage.
Plasticizer removal stage began warming at room temperature to 200 ℃ or so, compact powder particles in the surface adsorption of gas under the action of heat from the particle surface, constantly escape from the compact. Compact plasticizer by thermalization transgression in the compact. Maintaining high vacuum degree for gas lift and escape. Different kinds of plasticizer performance of the heat change is not the same, make plasticizer removal process to make a test to determine according to the specific situation. General plasticizer of gasification temperature below 550 ℃.
Before presintering phase refers to the high temperature sintering, cause fixed oxygen and carbon in the powder particle reduction reaction, generate carbon monoxide gas away from the compact, if the gas cannot be ruled out in the presence of liquid phase, will be closed pores residue in the alloy, even pressure sintering, is difficult to eliminate. On the other hand, the serious influence the liquid phase oxidation exist on wettability of hard phase is eventually affect the densification process of cemented carbide. In the liquid phase before, should fully degassing, and USES the vacuum degree as high as possible.
High temperature sintering stage is carbide compact densification critical stage, and the sintering temperature and sintering time is compact realize densification, forming uniform structure, important process parameters to obtain the required performance. Sintering temperature and sintering time depends on the alloy composition, powder particle size, the mixture of grinding factors such as intensity, also is conditioned by the overall design of the material. The cooling stage is cooling speed affect the binder phase composition and structure of the alloy, internal stress. Cooling rate should be in a state of control. Sintering hot isostatic pressing sintering technology is a new, also known as low pressure sintering, in complete degassing, compact surface pore has been closed, binder phase is still under the condition of the liquid phase, with a certain pressure of gas pressure, densification led products. Cemented carbide production process every detail is important and must be rigorous, to control the quality of hard alloy products!