Problems and causes of using graphite sintering dies
At present, the graphite molds for alloys for sintered saw blades can be roughly divided into single-slot, double-slot, and combination molds in addition to the size change. The graphite molds used for sintered alloy drills and integral small round alloy saw blades are also divided into single and combined graphite molds. According to the observation and analysis of practical use problems, it can be divided into the following categories:
1. The mold adheres to the sintered alloy body.
2. Heating in the sintering process does not reach the predetermined temperature for a long time.
3. The life of mold is short, even be cracked on the first pressure (average number of use should be up to 5 times)
The reasons for the above problems, in addition to the analysis of the operating conditions of pressure, current, voltage, temperature, alloy body formulations, etc., the reasons for the mold and graphite itself can be divided into the following categories:
1 The geometrical dimensions of the die being machined are not satisfactory. Under normal circumstances, the parallelism and verticality tolerances of the die and indenter should be controlled within 0.03mm, and the tolerance of the indenter with the cavity die should be controlled within 0.02mm. After the cavity die is assembled with the indenter, it is free to be placed. Can not be separated from each other, when the head is pushed by the hand, it can slide freely, that is similar to the transition fit in mechanical , the first phenomenon will occur when the fit too loose, and the second phenomenon will occur when the molds are not close to each other. A third phenomenon may easily occur when the cavity die is fitted too tightly.
2 The resistance of graphite itself is too large or too small. Sintering of the alloy body requires graphite as a mold and the mold itself must be energized and heated. Under normal circumstances, the voltage and current of a DC power supply equipped on the equipment has its rated value, and the selected graphite has a significantly different resistivity with different degree of graphitization, thesmall hourly resistivity of 4 μΩm and a large time of more than 30 μΩm. If the resistivity is too small, when the current passed has reached the rated value or the resistivity is too large, the voltage passed has reached the rated value. Although the heating exceeds the specified time (usually less than 8min) but the sintering temperature does not meet the requirements, the second phenomenon will occur.
3 The metal content of the mould itself is large, graphite itself and the molten alloy is not physical and chemical reactions. However, if the raw material for producing graphite is large in ash content or the graphitization temperature is low, the residual metal ash (mainly Fe, Cu, Ni, Co, and W) or specially processed dies are brought into contact with the metal in the presence of moisture. The first phenomenon is likely to occur when a metal or metal oxide is present.
4 Graphite itself has large visible pores. Small Asphalt Ball appears if the temperature is low during the kneading process, or compacted powder mixed with sand, metal particles, there will appear visible hole in the interior of the graphitized, just appear in the forming area of the cavity die frame will produce the first phenomenon.
5 Graphite has low strength. There are many reasons that affect the strength of graphite, especially the flexural strength, which can be roughly classified as a. The raw material of the formula is coarse, and the amount of binder added is too large or too small. c. The content of asphaltene (B component) in the binder is low d, the temperature rise during the heating process is irrational or the high temperature maintenance time is short, e, tDuring graphitization, the temperature is up too fast between 800 ℃ and 1800 ℃. The graphite strength too low is easy to produce the third phenomenon.
Sintered mould’s graphite material technical parameter reference standard
Sintered mold’s graphite material does not require as much purity as high-purity graphite. In simple terms, its material should not only have the nature of graphite, but it must have a certain resistance at the same time as its own heating element. Under pressure, it must have a certain mechanical strength.
Mold’s graphite material should be developed in series and standardization. In addition to the size of the specifications, its internal quality should have the following
1 has a certain resistivity, the degree of graphitization should not be too high, but it should not be too low, the resistivity is 9.5~12.8μΩm
2 has a certain mechanical strength, compressive strength of not less than 35MPa, bending strength of not less than 18Mpa
3 does not allow internal and micro cracks
4 Ash does not make specific requirements (but should not exceed 2000PPM).
