The heat resistance of materials
The heat resistance of the alloy
It depends on the grain size and the preferential distribution of impurities around the crystals, to reduce Sharply the level of heat resistance can accumulation of pollutants in the border volume, which weakens the strong bond between the crystals.
Comparative properties of steel
Coarse-grained steel has a high degree of creep resistance than hot-rolled steel type, with the basis of fine grain. In conducting the test was slightly exceeded temperature level required for recrystallization. As a result, the steel with large grains showed less steep slopes of the lines in the diagram. Given the test results, we can make such conclusions. Fine-grained alloys when tested under room and low temperatures, have greater durability. At elevated temperatures the coarse-grained alloy has a high heat resistance, but deteriorate the plastic properties. Such characteristics are suitable for the austenitic structure of alloys, and alloys with ferritic structure.
The influence of impurities
If the alloy has impurities, grain size has a decisive influence. Contaminants that fall in the boundary region, the lower the resistance. When the alloy begins to solidify, crystallize, impurities tend to accumulate in the border zone. In the cast material this trend more pronounced than in strain. However, the deformed material also has a weakened strength at the grain boundary at high temperature. Exception — the impurities of molybdenum, tungsten, niobium, etc. of high-temperature elements, additives which in small amounts has a positive influence on the strength of the alloy at high temperature.
The influence of raznozernistoy
Also the heat resistance decreases in the case of raznozernistoy material. Only the sample with a uniform grain structure showed high heat resistance. If a sample of various-grained composition is exposed to stress at high temperature, it can lead to cracking of the material in the grain boundary. Alloy with a large grain more heat resistant and practical. When tests were conducted of materials of different samples on the duration of existence, he observed tears in the grain boundary, which have nothing to do with the destruction of the samples. Plastic properties of the material much reduced during the second phase of the test.
The influence of the surface quality
Samples of Nickel and those that do not have such protection did not differ in tests of strength. Samples of the material with a polished surface screamed much later than the samples that had a rough raw surface.
Influence of heat treatment
Overheated steel forgings leads to a reduction of heat resistance. Overheating causes enlargement of the structure and intercrystalline oxidation. Must accurately track the temperature. Hot-rolled and hot-stamped materials during aging give the lowering of the heat resistance, if the processing temperature is too high.
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