Influence on the heat resistance of alloys
Effect of grain size on creep resistance
Coarse-grained steel had higher creep resistance than fine-grained hot-rolled steel. High temperature leads to recrystallization of the alloy. Steel of coarse-grained structure had better creep resistance. The same results were shown by tests of chromium-nickel steel 20X23n18 with coarse grain, which has higher heat resistance, but low ductility. In the segment of heat-resistant steels and alloys. Auremo is a profitable supplier.
Influence of grain size on heat resistance
At reduced and room temperatures, the strength characteristics of alloys with fine grains are very high. With increasing temperature, coarse-grained alloys show better heat resistance, but do not have sufficient ductility. This applies to austenitic and ferritic alloys.
Influence of foreign alloys on the refractory strength
It was found that small portions of S, Pb, Bi, Sn, Sb greatly reduce the heat resistance characteristics. The presence of ten-thousandths of lead in nickel-chromium-titanium alloy 75−20−2.5 Ti with 0.7% Al, significantly reduces the heat resistance of the alloy. During the solidification of the alloy, the refractory grains are the first to crystallize, and the fusible impurities that do not dissolve accumulate in the boundary zones. They have a significant impact on the quality of cast alloys. In deformable materials, the strength decreases with increasing temperature… But there is a group of elements (tungsten, molybdenum, niobium, boron), the addition of which, in small doses, increases the strength of the boundary layers. It is also necessary to consider possible changes in the concentration of the alloying elements in the boundary layer after diffusion or the formation of new phases, which leads to the loss of heat resistance and reduction of ductility. The difference in grain size affects the processes of chromium carbide precipitation at grain boundaries and the tendency of steel to intergranular corrosion. Similar changes in the concentration of solid solution at grain boundaries are found in other elements.
Dispersion hardening
This process is directly related to the formation of carbide and intermetallide phases and depends on the grain size. This process develops clearly in austenitic steels that are quenched at high temperatures and have a coarse-grained structure. The simultaneous action of plastic deformation and high temperature accelerates dispersion hardening.
Material grain size distribution
All heat resistance properties of high-alloyed alloys and steels are greatly reduced by multigrain material, when crystals with fine and coarse grains are present simultaneously in a sample. Such a mixture can occur in products that are hot pressurized when the metal is subjected to critical degrees of deformation. A coarse-grained structure is formed due to uneven cooling of the metal during deformation. Samples with a single structure will have a higher heat resistance than those with a multigrain structure. For ZI437 alloy at t° 700 °C with a uniform structure and a=36 kG/mm2 the load duration to failure = 72 hours. Most of the samples will not fracture before 150−200 hours. If the material has a heterogeneous structure, the samples will fracture within 6−30 hours. By following the exact stamping regime, it is possible to prevent the occurrence of heterogeneity in the parts. Multigranularity results in unstable properties and lower heat resistance.
Ruptures
Most samples will have small nicks within the grain boundaries. In the area of coarse grains, tearing is more common. The study showed that tears appear long before the specimen breaks. After the first fractures occurrence the viability of the material loses considerably after reaching 700−800°С temperature and 36/15 kG/mm2 stress. First there is a shallow tear on the surface of the metal. As a rule, the failure site does not coincide with the first tears.
Gas environment.
It has been suggested that the formation of cracks in the alloy was the result of exposure to the gas environment. In order to verify this, the surface was protected with a layer of nickel, 10 microns thick. Nickel plating of samples was conducted by electroplating. In the process of testing, it turned out that the tears did not differ from the tears on those samples which were not protected with nickel.
Peculiarities of alloy processing
The cleanliness of machining is of great importance, as confirmed by tests. Because of the local stress concentration, tears form earlier. The macrostructure and microstructure are formed by deforming forces during hot work. Overheating will cause structure enlargement as well as intercrystalline oxidation and ultimately loss of strength of complex alloyed heat-resistant alloys. Therefore, the production temperature regime should be strictly adhered to. During hot working under pressure, the structure is pulverized. Hot-rolled material has a fine-grained structure and a stressed state. If the material is subjected to aging, it becomes resistant to thermal shock, however, at very high temperatures it loses its heat resistance. Thanks to an optimised production process, refractory alloys can now be purchased at the best possible price.
Furnizor
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