III WC - CO alloy sintered grain growth

1, WC grain grew up mechanism: after sintered alloy average grain size is greater than average size carbide mixture called grain growth.

(1) gather recrystallization: mainly in liquid phase, this process is a solid phase sintering stage. Interface between WC particles increases, due to the diffusion bonding of polycrystalline WC particles into a larger particles, with the diffusion and grain boundary line of large particles gradually eliminate, the formation of large size of WC grain; The higher the temperature, sintering time and the greater the coalescence of recrystallization; Mixture, the smaller the grain size, the longer the wet milling, gathered at a speed of recrystallization, the greater the grew up and gather the recrystallization phenomenon, the more obvious.

(2) liquid recrystallization: in liquid phase sintered body, after some WC grain to dissolve in the liquid phase, and then in other precipitation on the grain, make the latter grew up, this is dissolved precipitation process, this process is very fierce, there are two reasons:

A, WC have larger solubility in liquid cobalt; B, the size of the WC grain uneven, uneven, gathered after recrystallization, produce a small amount of large grain WC; Uneven energy, because of the mixture was strong during the grain in wet grinding in deformation; Irregular shape, such as Angle and protruding parts, larger than the flat parts of energy; When there is liquid phase sintering, the tiny, high-energy first dissolving, has been saturated, the heat preservation time, dissolution and precipitation of WC, the cause of dynamic balance, the size of grain disappearing as a result, the coarse WC particle size increasing. Liquid recrystallization than gathered recrystallization process strongly.

2, the influence of WC grain growth factors:

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(1) the number of liquid: improve the sintering body liquid quantity of various factors, can promote the WC grain growth;

(2) sintering temperature: raise the temperature, make the WC increases the solubility and dissolution rate in the liquid phase, the WC particles increased obviously.

(3) the cobalt content: sintered body fluid phase quantity increases with increase of cobalt content;

(4) carbon content: sintering liquid phase increase with the increase of carbon content in the body, keep the liquid phase time extended, with the increase of carbon content of b improve the carbon content can promote the WC grain growth.

(5) mixture state: A, the smaller the original powder particles, grow up the more serious when sintering; B, the larger the size inhomogeneity, grow up the more serious (still uneven grew up); Grew up, the greater the degree C, grinding and the greater the tendency (still grew up uneven energy);

(6) additive: supplying the alloy with refractory metal carbides, can prevent the WC grain grew up uneven, in order to get the granularity of the fine grain of alloy; Because of adding carbide was adsorbed on the surface of WC particles, can reduce the surface of the WC, bring down the WC solubility in the liquid phase, hinder the WC dissolved in the liquid phase. Hinder recrystallization dissolution mechanism.

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