Influence on the heat resistance of alloys
«Food» stainless steel 20x23н18
It is made of heat-resistant alloy. Its properties, like other heat resistant steels are closely related with 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 of high-temperature weakens the connection between the crystals at high temperatures, and can lead to loss of strength.
The influence of grain size on creep resistance
For example, 12x18н10т it was found that the coarse-grained alloy has higher creep resistance than hot-rolled alloy with a fine grain. At high temperatures the alloys begin recrystallization. If it is coarse-grained alloys, 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 strength but low ductility.
The influence of grain size on strength
At low and room temperatures the alloys with fine grains have very high strength characteristics. At higher temperatures the coarse-grained alloys show better strength, 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 high-temperature impurities are not well understood, but it is established that alloys with a minimum percentage S, Pb, Bi, Sn, Sb, inherent low-temperature characteristics. The presence of a ten-thousandth of a share of lead in Nickel-chromium-titanium alloy 75−20−2,5 Ti with 0,7% Al, significantly reduces heat resisting quality of the alloy. In the first place during the solidification of alloys 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 alloys, the 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 is additive to the alloys 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 reduce the ductility of the alloys. 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 are and other alloys. 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 in heat-resistant alloys and depends on the grain size. Clearly this process demonstrate austenitic alloys, 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, which lower the melting temperature, accelerate the destruction of high-temperature materials.
High-temperature characteristics, which are highly alloyed superalloys, strongly 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 high-temperature alloys fall under the critical deformation degree. Coarse-grained structure is formed where the plastic deformation is difficult — for stamping heat-resistant alloys and alloys uneven when cooled during deformation. Alloys of uniform structure will have a higher heat resistance than the alloys that have various-grained structure. The brand Z 437 at t° 700 °C with a homogeneous structure and a=36 kg/mm2 load duration to failure = 72 hours. A large part of the alloys will be destroyed only after 150−200 hours. If the material is various-grained structure of the fracture of alloys occurs 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 alloys will be small tears within grain boundaries. In the area of large grains tears occur more often. Research alloys has allowed to establish that the tears appear long before the destruction of the alloys. 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 on the surface, then long 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. The greatest number is 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 alloys with various-grained structure under tension are destroyed at high temperatures, fine-grained alloys readily elongate under the 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.
A great influence on the alloys having the surface, which is confirmed by tests. Due to local concentration of stresses at the alloy, tears formed earlier. Macro — and microstructure is formed under the action of deforming forces the alloy during hot working pressure. Due to overheating of forgings turbine disks above 1160 °C, made of steel ЭИ481, and more than 1170 °C steel ЭИ4376 characteristics of heat resistance decreased. In both cases the overheating causes the coarsening 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 pulverizes the alloy structure. Hot-rolled and hot-stamped alloys have fine-grained structure and strained state. If the alloys are subjected to aging, they acquire high mechanical properties at different temperatures, however, at very high temperatures, these alloys have low strength. This effect is used in order to obtain alloys with higher mechanical properties at moderate temperatures. This can be called thermomechanical treatment.
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