Heat resistance of materials
Heat resistance of an alloy
The heat resistance depends on the grain size and on the preferential distribution of impurities around the crystals. The concentration of impurities in the boundary volume, which weakens the strong bond between the crystals, can sharply reduce the level of heat resistance.
Comparative properties of steel
Coarse-grained steel materials have a higher degree of resistance to creep than hot-rolled steel, which has a fine grain size at its core. Tests have shown an excess of temperature, recrystallization. As a result, steel with coarse grain showed less steep slopes of the line in the diagram. Considering the test results, we can draw the following conclusions. Fine-grained alloys at room temperature and at reduced temperature, have greater strength. At elevated temperatures, coarse-grained alloys have high heat resistance, but their plastic properties deteriorate. Such characteristics are also suitable for alloys with an austenitic structure and for alloys with a ferrite structure.
Effect of impurities
If there are impurities in the alloy, the grain size has a decisive influence. Harmful impurities that enter the boundary regions lower the heat resistance. When the alloy begins to solidify and crystallize, the impurities tend to accumulate in the boundary zone. This tendency is more pronounced in the cast material than in the deformed material. However, the deformed material also has weakened grain boundary strength at high temperature. The exception is the impurities of molybdenum, tungsten, niobium
Influence of the differentiated grain sizes
The heat resistance also decreases if the material is very unequal in grain size. Only a sample with a homogeneous grain structure showed high heat resistance. If a specimen of heterograin composition is subjected to stress at high temperature, it can lead to cracking of the material at the grain boundary. Alloy with coarse grains is more heat resistant and practical. When materials of different specimens were tested for longevity, tears along the grain boundary were observed that had nothing to do with specimen fracture. The plastic properties of the material are greatly reduced during the second phase of testing.
Influence of surface quality
The nickel plated specimens and those without such protection did not differ in strength tests. The material samples with polished surfaces broke much later than the samples that had a rough, untreated surface.
Effect of heat treatment
Overheating of steel forgings produces a reduction in heat resistance. Overheating causes structure coarsening and intergranular oxidation. The temperature conditions must be closely monitored. Hot-rolled and hot-formed materials, when aging, give a decrease in heat resistance if the processing temperature is too high.
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