What is the synthetic diamond cubic press machine?
The six-anvil high-pressure synthetic diamond hydraulic press (cubic press) is a device that generates high pressure through oil pressure, and generates high temperature through low voltage and high current to provide the required pressure and temperature (HPHT) for the synthesis of superhard materials. It is currently the main large-scale equipment for the production of superhard materials, which has been used in the research and production of superhard materials such as monocrystalline synthetic diamonds, lab grown diamonds, polycrystalline diamond PCD tool blanks and cubic boron nitride(CBN).
What is the anvil of synthetic diamond cubic press machine?
Carbide anvil is a key component of high pressure and high temperature(HPHT) machine, and is one of the main materials consumed in the production of synthetic diamonds, which can account for more than 40% of the entire synthetic diamond production cost. Therefore, the consumption of anvils in the production of synthetic diamond is a problem that factory is very concerned about. Their service life determines the technological level, synthesis quality, product cost, economic benefit and labor productivity of scientific research and production. Therefore, every synthetic diamond manufacturer hopes to use high-quality anvil and pressure cylinder for synthesis to reduce anvil consumption. Moreover, it can be seen from the current development status of superhard materials that synthetic diamond manufacturers are considering expanding the cavity one after another, which puts forward higher requirements for the specifications and quality of the anvil and pressure cylinder.
According to the structure of the cubic press, the working surface of the anvil is much smaller than the piston area of the oil cylinder. When a group of anvil are driven by the oil cylinder to close to form a high-pressure cavity, the pressure in the cavity can reach more than 10GPa and the temperature of more than 1,000 degrees Celsius. There will be compressive stress, tensile stress, shear stress and others. Therefore, the quality of the cemented carbide anvil is particularly important.
How are the anvils damaged?
1) Due to various defects caused by materials and manufacturing processes
such as uneven grain size and structure, over-burning, under-burning, decarburization, carburization, holes, cracks, delamination, thermal stress, etc., these defects will cause local stress concentration during the synthesis process, resulting in premature failure of the anvil. Small pieces of alloy may come off the top, bottom, shuttle corners or slopes of the anvil.
2) In the absence of local stress concentration caused by defects
most anvil failures are caused by finding one or several cracks on the 46° inclined plane, extending to the top surface and failing. Generally, cracks develop from the inside to the outside. The analysis shows that the maximum shear stress of the force on the anvil occurs on the central axis, and the maximum tensile stress occurs on the 46° inclined plane symmetry line. Then, in the high temperature and high pressure (HPHT) state during synthesis, if the local stress is too concentrated, the small internal cracks will gradually spread to the surface, which is why the cracks are generally generated from the 46° large slope until they spread to the anvil where most of the cracks occur.
3) Uneven temperature can also lead to the phenomenon of crack hammer
the change of temperature can cause the expansion or contraction of the object, which will produce thermal stress. Alternating thermal stress is one of the factors that cause anvil fatigue fracture. The highest temperature of the hammer is the contact point between the hammer and the conductive steel, the lowest temperature is the center of the bottom surface of the hammer, the maximum temperature difference between the hammer and the bottom surface is about 500 ℃, and the temperature of the anvil surface is uneven, which is caused by the stress concentration of the anvil. The reason for cracking and energized anvil life is significantly reduced. These are normal failures, and the mechanism is brittle fracture fatigue failure.
4) The poor stability of the ultra-high pressure equipment of the press
for example, the pressures of the six working cylinders are inconsistent, the strokes are inconsistent, and the pressure of a certain cylinder suddenly drops.
5) Large-size presses
at present, various superhard material manufacturers are pursuing large-size presses for the sake of efficiency, so that the cemented carbide size of the anvil is gradually expanded. Generally speaking, the larger the cemented carbide anvil, the larger the cavity, and its life will be correspondingly reduced. From this aspect, while designing the anvil geometry according to the principle of “large mass support”, we should find the best parameters between expanding the size of the anvil, the cylinder and their lifespan, so as to make the technology and economy reasonable.
What other possible situations may lead to the damage of the anvil and the pressure cylinder?
1) Water leaks
in the synthesis process, if the steel ring suddenly leaks water, the anvil will rapidly expand and contract under high temperature, so that the local stress of the anvil will be concentrated, resulting in cracks and damage.
2) The hardness of the pad is not enough
it is easy to deform after being stressed, which makes the plane appear uneven, which causes the local stress concentration of the anvil, thereby causing the anvil to break.
3) Poor quality of pyrophyllite blocks
such as too large or too small and uneven hardness, will seriously affect the sealing performance of pyrophyllite. When the sealing force cannot withstand the pressure in the high-pressure chamber, it is easy to cause explosion and the anvil squeeze each other and break.
4) The equipment is not running normally
such as high-pressure oil leakage, sudden rupture of oil pipes, failure of high-pressure gauge and travel switch, poor alignment and synchronization, sudden rupture of pressure cylinder and supercharger, and excessive quenching hardness of steel rings and pads. The sudden rupture and the loosening of the screw cause the displacement of the top hammer, which destroys the synchronization of the centering, etc., which will cause high pressure and high temperature (HPHT) explosion and cause excessive damage to the anvil.
5) Insufficient supporting force
the anvil may automatically turn or come out, causing a sudden explosion of high pressure and high temperature (HPHT).
6) High synthesis temperature or the presence of impurities
such as pyrophyllite powder at the conductive parts during assembly, the steel ring will be melted, resulting in local thermal stress concentration of the anvil, and the hammer will be damaged.
7) Improperly installed or debugged anvils
the force will be uneven during the combined pressure-bearing process, which will cause local stress concentration at the end and edge of the anvil, and then cause the hammer to fail.
8) Improper manual operation
such as uneven pads, the anvil is not adjusted in place, the anvil and the steel ring are not well matched, the temperature setting is too high, etc. which are not discovered by the staff in time, may cause unnecessary damage to the anvil.
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