Influence on the heat resistance of alloys
«Food» stainless steel 20x23н18
It is made from heat-resistant alloy. Its properties, like other heat resistant steels are closely linked to the grain size. From the grain size dependent electrochemical processes occurring in the border zones and the distribution of impurities around the crystal. The accumulation of impurities in the border volumes weaken the strength of the link between the crystals at high temperatures and can cause a sharp decline of heat resistance.
The influence of grain size on creep resistance
Was studied on steel 12x18н10т. Coarse steel had higher creep resistance than hot-rolled with a fine grain. At high temperatures the alloy begins recrystallization. In the case of coarse-grained steel the slope of the lines on the dual graph is not very steep, reflecting the best creep resistance. The same results were obtained when testing stainless steel 20x23н18 with a large grain, which has higher heat resistance, but low ductility.
The influence of grain size on strength
At low and room temperatures the strength characteristics of the alloys with fine grain very high. At higher temperatures the coarse-grained alloys show better heat resistance, but do not have sufficient plasticity. This applies to alloys of austenitic and ferritic structure.
The influence of foreign impurities in the border areas
The mechanism of interaction of the impurities is not sufficiently studied, it was found that a small proportion S, Pb, Bi, Sn, Sb greatly reduce high-temperature characteristics. The presence of a ten-thousandth of a share of lead in Nickel-alloy promotionscom 75−20−2,5 Ti with 0,7% Al, significantly reduces the resistance of the alloy. In the first place during the solidification of alloy kristallizuetsya grain refractory substances and low-melting impurities, which do not dissolve, accumulate in the border areas. They have a significant impact on the quality of cast alloys. From the deformed material weakening of the strength at elevated temperatures may be even greater in the presence of fusible impurities. Not all of the impurities have a detrimental effect on the heat resistance. There is a group of elements (tungsten, molybdenum, niobium, boron), which are additive in small doses, increases the strength of boundary layers. It is also necessary to take into account possible changes in the concentration of alloying elements in the boundary layer after the diffusion or the formation of new phases, which lead to a loss of heat resistance and reduction of plasticity. The difference in the grain size of steel 12x18н10т affects the processes of extraction of chromium carbides at the grain boundaries and the tendency of steel to intergranular corrosion.
Similar changes in the concentration of solid solution at the grain boundaries in other materials. It is different travismathew grains after the homogenization of the alloy at high temperature followed by heating in the range of operating temperatures.
This process is directly linked with the formation of carbide and intermetallic phases, and depends on the grain size. Clearly this process takes place in austenitic steels, hardened by high temperatures, with a coarse-grained structure. Dispersion hardening is very intensive the simultaneous action of stress and temperature, much better than when only one temperature. The critical amount of impurities that lower the melting point, leads to microcarcinoma destruction.
All high-temperature characteristics of high-alloyed alloys and steels in large extent reduced by raznozernistoy of material when the sample is present at the same time the crystals with small and large grain. Such a mixture may occur in products that are subjected to hot pressure treatment, when metal gets in under the critical deformation degree. Coarse-grained structure is formed where the plastic deformation is difficult — for stamping of metal due to uneven cooling of the metal during its deformation. Samples of uniform structure will have a higher heat resistance than those samples that have various-grained structure. The alloy Z 437 at t° 700 °C with a homogeneous structure and a=36 kg/mm2 load duration to failure = 72 hours. Most of the specimens will be destroyed only after 150−200 hours. If the material is various-grained structure — the samples be destroyed within 6−30 hours. Respecting the exact mode of punching, to prevent the appearance of raznozernistoy in detail. Raznozernistoy leads to a lack of constancy of properties and to a decrease in heat resistance.
A large part of the samples will be small tears within grain boundaries. In the area of large grains tears occur more often. The research allowed to establish that the tears appear long before the destruction of the sample. After the initial tears, the viability of the material when the temperature reaches 700−800°C and a voltage of 36/15 kg/mm2 to a considerable extent is lost. First, there is a shallow tear at the metal surface, which further prolonged the test, the number and depth of the tears will gradually increase. On the eve of destruction there tears inside the material, they are not visible on the surface. But a large number will be concentrated closer to the point of destruction. As a rule, the place of destruction is not the same as the order of the first tears.
If samples with various-grained structure under tension are destroyed at high temperatures, fine-grained material is easily extended with such influence. As a consequence, coarse-grained and low-plasticity material will crack along the grain boundaries. Therefore, products with a homogeneous structure are considered more durable. .
It has been speculated that the formation of cracks in the alloy were the result of exposure to a gas environment. To verify the surface is protected by a layer of Nickel with a thickness of 10 µm. Samples Nickel plating was performed by electroplating. During the tests it became clear that the tears are not different from the tears on those samples that were not protected with Nickel.
Greatly influenced by the cleanliness of the treatment, which is confirmed by tests. Due to the local stress concentration tears formed earlier. Macro — and microstructure are formed under the action of deforming forces during hot pressure treatment. Due to overheating of forgings turbine disks above 1160 °C, made of steel ЭИ481, and more than 1170 °C and steel ЭИ4376 before stamping the characteristics of heat resistance decreased. In fact, and in another case, the overheating will cause the consolidation of the structure and intercrystalline oxidation, which is difficult to distinguish under the microscope. The same negative effect will have overheating during heat treatments complexly alloyed heat-resistant alloys. Therefore, you should strictly follow the temperature regime of production.
During hot processing under pressure is crushed structure. Hot rolled and gorjacheotsinkovannyj material has a fine-grained structure and strained state. If the material is subjected to aging, it acquires high mechanical properties at different temperatures, however, at very high temperatures has a low heat resistance. This effect is used with the aim of obtaining products with good mechanical properties at moderate temperatures. This can be called thermomechanical treatment.
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