Properties and defects of high-temperature steel
Heat resistance (resistance to the gaseous medium or steam, high temperature) is the most important property of high-temperature steels. Heat-resistant steel, usually needs to be heat resistant, i.e. at high temperature for a predetermined time to resist creep — the gradual and increasing over time, the deformation under a constant load, leading to destruction.
Hardening heat-resistant steels
Hardening phase along with the chromium carbides are carbides of vanadium, molybdenum, tungsten and other elements as well as intermetallic compounds of type AGV (which as an element, And contains iron and chromium, molybdenum, tungsten, niobium, titanium as the element b) or compounds (Ti, Al). Additives in high-temperature steel refractory elements — molybdenum, tungsten, niobium, tantalum have on hardening phase stabilizing effect, since these elements increase the recrystallization temperature and weaken the diffusion processes. Their action is enhanced if introduced not one, but several elements that weaken the diffusion For this reason, heat-resistant steels alloyed, usually a set of different elements.
The types of high-temperature steel
The processes of diffusion and exchange are inhibited when the steel is not subject to polymorphic transformations. Therefore, as a heat-resistant hard alloy is often used purely ferritic or austenitic steel. Steel ferritic until recently only used as heat-resistant. Recently, however, has developed and is successfully implementing and heat resisting ferritic steels, such as steel 12Х2МВ8ФБ (ЭП503), hardened particles of intermetallic phases FeW. Austenitic steels contain 12−20% CR, they are characterized by much higher heat resistance. Particularly the widely used austenitic steel using as austentatious element 7−30% Ni. Nickel itself relates to corrosion-resistant metals and improves the corrosion resistance of steels in solutions of salts and alkalis, as well as in slightly acidic environments. The content to 20−30%, it increases the heat resistance of iron-chromium alloys. Due to the high cost of Nickel in some heat-resistant steels it partially or completely replace other austenitisation element — manganese. Action as austentatious element is much weaker, especially at high chromium content, along with Mn so it is advisable to introduce a small amount of Nickel 2−4% nitrogen. To obtain high heat resistance recommended additives carbon with vanadium, molybdenum, tungsten, niobium and nitrogen.
Chromium-Nickel, chromenickel-manganese and bromomalonate heat resisting steels have good resistance to General corrosion, but are sensitive to intergranular corrosion, especially after slow cooling in the temperature range 500−850°C. this is Due to selection at these temperatures, chromium carbides at the grain boundaries. In electrolyte solutions to form carbides with the carbon depleted areas of the grain galvanic couples. As a result of structural inhomogeneity of the grain boundaries are subjected to more severe corrosion corrosion. Austenitic steel become insensitive to intercrystalline corrosion, if the carbon content in steel is less than its solubility limit in austenite at room temperature, i.e. less than 0,02−0,03%.
Heat-resistant steel with this low carbon content in electrical arc furnaces to smelt difficult. Therefore, in the smelting of corrosion resistant austenitic steels, the upper limit of the carbon content is usually set at the level of 0,08−0,12%, and a further decrease of carbon concentration in the solution is additive strong carbidopa elements — titanium or niobium. The amount of titanium is determined by the carbon content, and sufficient carbon sequestration the amount of titanium should be at least five times larger than the amount of carbon. The high content of chromium and titanium in steels of this type causes intense oxidation of the metal during casting to form in the mold on the metal surface of the crust, rich in oxides and nitrides of titanium. Inversions crust during the filling of the mold leads to numerous surface defects in the ingot heat-resistant steel, which are forced to expose a solid ingot Stripping to a depth of 10−20 mm.
Remaining in the body of the ingot accumulation of nitrides and oxides form a boundary or a General inhomogeneity of the macrostructure — the so-called titanium porosity. The development of this defect of heat-resistant steels increases with increasing content of titanium in the metal.
To buy, price
In stock, LLC «Electrocentury-steel» a wide range of products from high-temperature stainless steels of various brands at the best prices. Offer advantageous conditions of supply for wholesale and retail buyers. On our website you will find all the necessary information, offers experienced managers-consultants, who are always ready to help with the selection. Product quality is guaranteed by strict observance of the norms of production at all stages. Delivery times are minimal. For wholesale orders there is a reduced discount.