"Food" stainless steel 20x23n18

It is made of heat-resistant alloy. Its properties, as well as those of other heat-resistant steels are closely related to the grain size. The size of the grain depends on the electrochemical processes occurring in the boundary zones and the distribution of impurities around the crystal. The accumulation of impurities in the boundary volumes weakens the refractory bonds between the crystals at high temperatures and can lead to loss of strength.

Influence of grain size on creep resistance

Using 12x18n10t steel as an example, it was found that a coarse-grained alloy had a higher creep resistance than a hot-rolled alloy with fine grains. At high temperatures, alloys begin recrystallization. If they are coarse-grained alloys, the slope of the lines in the double diagram is not very steep, reflecting better creep resistance. The same results were obtained when testing 20x23n18 chromium-nickel steel with coarse grains, which has higher strength but little ductility.

Influence of grain size on strength

At reduced and room temperatures, alloys with fine grains have very high strength characteristics. At elevated temperatures, coarse-grained alloys show better strength, but do not have sufficient ductility. This applies to alloys with an austenitic and ferritic structure.

Influence of foreign impurities in the boundary regions

The mechanism of interaction of heat-resistant impurities is not well understood, but it has been established that alloys with a minimum percentage of S, Pb, Bi, Sn, Sb are characterized by reduced heat resistance characteristics. The presence of ten-thousandths of lead in the nickel-chromium-titanium alloy 75-20-2.5 Ti with 0.7% Al, significantly reduces the heat resistance properties of the alloy. First of all, crystallization of refractory grains during solidification of the alloys, and the fusible impurities, which are not dissolved, accumulate in the boundary zones. They have a significant impact on the quality of cast alloys. In deformed alloys, the weakening of strength at elevated temperatures can be even greater in the presence of fusible impurities. Not all impurities have a detrimental effect on heat resistance. There is a group of elements (tungsten, molybdenum, niobium, boron), whose additions to alloys increase the strength of the boundary layers. It is also necessary to consider possible changes in the concentration of alloying elements in the boundary layer after diffusion or the formation of new phases, which lead to a loss of heat resistance and a decrease in ductility of the alloys. The difference in grain size of steel 12x18n10t affects the processes of chromium carbides separation on the grain boundaries and the propensity of steel to intergranular corrosion.

Other alloys have similar changes in the concentration of solid solution at the grain boundaries. This is revealed by the different etchability of grains after homogenization of alloys at high temperature followed by heating in the operating temperature range.

Dispersion Hardening

This process is directly related to the formation of carbide and intermetallide phases in heat-resistant alloys and depends on the grain size. High-temperature hardened austenitic alloys with a coarse-grained structure clearly demonstrate this process. The dispersion hardening is very intensive under the action of stress and temperature simultaneously, much better than under the action of temperature alone. The critical amount of impurities that lower the melting point accelerates the degradation of heat-resistant materials.

Material grain size.

The heat resistance properties of high-alloyed refractory alloys are greatly reduced when the material is multigrain, in which crystals with fine and coarse grains are present simultaneously in the sample. Such a mixture can occur in products that are subjected to hot pressure treatment when heat resistant alloys are subjected to critical degrees of deformation. A coarse-grained structure forms where plastic deformation is difficult - when heat-resistant alloys are stamped and when the alloys are unevenly cooled during deformation. Alloys with a single structure will have a higher heat resistance than those alloys that have a different grain structure. In the ZI 437 grade at t° 700 °C with a uniform structure and a=36 ^kG/mm2 the load duration to fracture = 72 hours. Most alloys will not fracture before 150-200 hours. If the material has a heterogeneous structure then the alloys will fracture within 6-30 hours. By following the exact stamping regime, it is possible to prevent the appearance of heterogeneity in parts. Multigranularity results in unstable properties and lower heat resistance.

Ruptures

Most alloys will have small pitting within the grain boundaries. In the area of large grains, tears appear most frequently. A study of the alloys found that, in fact, pitting occurs long before the alloys fail. After the first fractures occur, the viability of the material is largely lost when the temperature reaches 700-800°C and the stress is 36/15 ^kG/mm2. First there is a shallow tear at the surface, and then with prolonged testing, the number and depth of tears will gradually increase. On the eve of failure, there are tears within the material and not visible on the surface. The greatest number will be concentrated closer to the point of failure. As a rule, the place of destruction does not coincide with the places of the first breaks.

Fine-grained metal

Whereas multi-grain alloys fracture under stress at high temperature, fine-grain alloys easily elongate under such stress. As a consequence, a coarse-grained and poorly plastic material will crack at the grain boundaries. Therefore, products with a homogeneous structure are considered more durable.

Gas medium

It has been suggested that the formation of cracks in the alloy was the result of exposure to the gas environment. To test this, a layer of nickel 10 µm thick was applied to the surface. Nickel plating of samples was conducted by electroplating. In the process of testing, it was found that the cracks did not differ from the cracks on those samples which were not protected with nickel.

Processing Features

The alloys are greatly influenced by surface finish, which was confirmed by tests. Because of the local concentration of stresses acting on the alloys, notches form earlier. Macro- and microstructure is formed under the action of deforming forces on the alloy during hot pressure machining. Because of overheating of turbine disc forgings above 1160°C made of EI481 steel, and above 1170°C made of EI4376 steel, the characteristics of heat resistance decreased. In both cases, overheating causes structural enlargement as well as intergranular oxidation, which is difficult to distinguish under a microscope. The same negative effect will be caused by overheating during heat treatments of complex alloyed heat-resistant alloys. Therefore, production temperatures should be strictly adhered to.

During heat treatment under pressure, the alloy refines its structure. Hot-rolled and hot-formed alloys have a fine-grained structure and stress state. If alloys are subjected to aging, they acquire high mechanical properties at different temperatures, but at very high temperatures such alloys have low strength. This effect is used to produce alloys with higher mechanical properties at moderate temperatures. This can be called a thermomechanical treatment.

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