I - The cemented carbide products sintering mechanism

An overview: the so-called sintering, the powder is heated to a certain temperature, and maintain a certain amount of time, and then cooling, to get the required materials or products, this special heat treatment method called sintering.

Carbide sintered belongs to the liquid phase sintering of multicomponent system, sintering on alloy products play a decisive role.

1, the sintering process of the change of the sintered body: smaller pressure fast volume shrinkage, and strength increase, alloy surface is metal luster, usually, pressure fast porosity is 50%, the products should be less than 0.2%. Sintering process, the qualitative changes between powder particles, the combination of alloy intergranular instead of the contact between the powder particles to form alloy strong overall, which has excellent physical properties.

1, the sintering process of several stages:

(1) removal agent and presintering stage (800 ℃) or less:

The early stage of sintering, with the temperature increases, cracking, gradually forming agent out of the sintered body side by side, at the same time, forming agent increased carbon sintered body more or less. When hydrogen sintering, under 800 ℃ can restore oxide on the surface of the powder, at the same time, the contact stress between powder gradually eliminate, bonding metal powder to produce responses and recrystallization, particle surface diffusion, increase strength.

(2) solid phase sintering stage eutectic temperature (800 ℃ -- 800 ℃) : eutectic temperature refers to the gradual thawing to started to the temperature of the eutectic liquid phase sintered body.

(3) the liquid phase sintering stage (eutectic temperature sintering temperature) : when there is liquid phase sintered body after sintered body contraction finished quickly, carbide grain growth and formation of the skeleton, which laid the basic organization structure of alloy.

(4) cooling stage (sintering temperature - room temperature), the organization of the alloy and binding phase composition varies with the different cooling condition and produce some. After cooling, to get final alloy.

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SIX - Metallographic examination

1, sample preparation, coarse grinding with coarser silicon carbide grinding wheel grinding, grinding surface to 2-3 mm. The internal organization of the proof alloy; Set sample is coarse grinding sample with sulfur circle insert samples, sample set samples should make all polished in a plane. Circle will be set with sample set samples, on special sample device of fine grinding, to eliminate coarse grinding water grinding crack. When the fine grinding of abrasive has two kinds: one is containing silicon carbide (120-180 mesh) and potassium permanganate solution; Another kind is boron carbide (180-220 mesh) with kerosene or a mixture of transformer oil. Polishing can be conducted on similar to the fine grinding device. Mill pad on the canvas, and drop into the zirconia powder (150 mesh) and potassium permanganate solution, no streaks of light should be after polishing grinding surface, further on the iron plate in woolen cloth fine grinding, to eliminate grinding polishing is the formation of the turbidity, drip continuously in the process of fine grinding abrasive (less than 3 micron boron carbide) and lubrication oil, transformer oil or edible vegetable oil), after fine grinding of grinding should be smooth as a mirror.

2, the corrosion of grinding

Porosity is A field of pore area percentage, namely X = A/B * 100%; X for porosity, A is the area of the field of pore, B is the area of the field. Generally choose a field of most pore and the known standard photos to compare and determine porosity. Dirt degree is to put the above 40 microns, irregular shape but the clarity of the edge of black holes. Graphite inclusions generally or tiny dot gathered in nests, its edge is clear, in the process of grinding, flake graphite inclusions are usually the pore, some national standards will even graphite inclusions as the secondary pore. Other defects, layered like dirt under a microscope, but longer than dirt, dirt than straight edges. Crack is thin and long black stripes. Not good pressure caused by the pore usually triangular or water chestnut, if pore focused particularly on the sharp edge of place, usually is not good. Owe to burn the pore generally smaller, about 5 microns, concentrated in the middle of specimen. Burnt generally does not affect the porosity, but serious when burnt, average porosity is larger, in 10-20 microns, and most of the surface.

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