发布时间: 2020-07-13 10:57    


Heat-resistant steel grades - chemical composition and characteristics

For elements and components made working at higher temperatures, exceeding 600 ℃, heat-resistant and creep-resistant steels are used.

In contrast to boiler steels, heat-resistant and creep-resistant products are used in the working temperature range of 800-1150 ℃. These are generally accepted ranges, and each grade in a particular subgroup has a distinct purpose, creep limit, creep strength, and the environment in which it can operate for a specified period of time. These steels are highly resistant to the corrosive effects of the oxidizing atmosphere at high temperatures.

For the property of heat resistance is responsible, among others, Chromium, for example, in H6S2 - X10CrAl7 steels is in the range of about 5-8%. In the presence of high temperature and difficult environment, oxides appear on the surface of the product due to oxidation, increasing their thickness as the temperature increases. The entire product coverage process only lasts until a sufficient layer is formed to withstand a specific environment that adheres to the intact surface and hardly removes, protecting the other alloying additives inside the product.

In many situations, steel savings contribute to the destruction of the product, as too little alloying material causes the oxides to crack and re-creates the subsequent layers, destroying the product in a relatively short amount of time.

Heat resistance

Resistance to gas, chemical vapors, steam, and reagents is determined by the heat-resistant or gas-corrosion resistance and the temperature, at the same time, exceeding 550 ℃ without taking into account the load that the product is exposed to. Heat-resistant steels contain chromium in the range of 5-30% in chemical composition. In addition to chromium, the most common alloying elements are aluminum – Al, and Silicon – Si, and Titan – Ti, and residual Niobium – Nb, Cerium – Ce, and Nitrogen – N.

Some of the heat-resistant steels may, at the same time, serve as creep resistant steels - these alloys with nickel addition with austenitic and austenitic-ferritic structure. The carbon content of the heat resistant steels is as high as 0.30%, thanks to with they have appropriate hardness and abrasion resistance.

It should also be added that silicon and aluminum added to a small extent improve the heat treatment of the material. Due to the structure, we distinguish ferritic steels, heat-resistant austenitic steels, and heat-resistant austenitic-ferritic steels. They are used where, besides high temperature, the products are required as bars, pipes, sheets - resistant to sulfur compounds, engine exhausts, nitriding and carburizing. They are used as steels for carburizing boxes, steels for thermocouple sheaths, rails in ovens, for vacuum chamber parts, for heat-resistant pipes of industrial furnaces.

Creep resistance

In turn, the creep resistance determines the resistance of alloy or high alloy steel to numerous deformations caused by stresses due to operation of the element at high temperatures. Creep-resistant steel grades have an austenitic structure and their chemical composition includes 13 to 28% chromium, and nickel in the range of 8-27%. Depending on the alloy grade, the proportions of these elements are regulated alternately on the basis the intended environment of the particular product (especially in chromium-nickel alloys).

Alloys that increase the temperature range of recrystallization and melting, and at the same time increase the level of atomic bonding of the solids network, are Chromium, Titanium, and Silicon, and also Vanadium, Molybdenum, Tungsten, and Cobalt. Carbon content in typically creep-resistant steels in relation to heat-resistant steels is maintained at a negligible level - up to 0.16%, which is relatively common in austenitic structures.

Properties of creep-resistant steels are increased by curing, and as a result of crushing, but by cold plastic deformation and coagulation of the separation of phases they decreases.

Silchrom valve steels - Application and Specification

Chrome and Silicon - CrSi, co-create a duo known as SilChrom Steel, the official name of which is Valve Steels. They are a small group of creep-resistant steels intended for use in aircraft engines, combustion engines of responsible machinery and in automotive parts and valves.

Mostly manufactured in the form of forgings for the valves, they are resistant to exhaust gases of aircraft engines, are not erodible by dust from exhaust gases, exhibit a high hardness-to-temperature ratio, do not deform during operation, are easy to form during plastic and mechanical processing, and as few of creep-resistant steels, have a martensitic structure.

Valve steels - chemical composition

The carbon content of this steel group is about 0.4-0.6%, the operating temperature is up to 750 ℃, and as a working product is delivered in a softened state. The addition of tungsten and molybdenum in martensitic valve steels counteracts brittleness and increases safety during the tempering process. Valve steel processing involves hardening at temperatures of 1000-1200 ℃ and tempering at 720-850 ℃ with cooling in water or air.

In the group of valve steel there are also less popular steels with austenitic and austenitic-ferritic structure. In austenitic steels the main additives are Nitrogen, Nickel and Manganese, which together with Chromium create austenitogenic elements. With respect to chromium silicon steels, austenitic steels are characterized by even higher strength at high temperatures.