What are stainless steels?
Carbon and low alloy steels are virtually unprotected against atmospheric corrosion, in water and most other environments. They are covered with a film of oxides, which does not have sufficient density and tightness to protect steel from the chemical attack of the environment. At the same time, known, that some alloying elements increase the corrosion resistance of steel. These elements primarily include chromium and nickel..
When chromium is added to steel, less 12 % its corrosion resistance does not increase: it remains at the level of ordinary carbon steels. However, the introduction of chromium into steel in an amount of more 12 % makes it resistant to corrosion in the atmosphere and in most other industrial environments. Steel with a chromium content of more than 12 % called corrosion-resistant or, how often are they called, stainless steels.
Three types of chromium stainless steels
Three types of chromium steels are used: with nominal chromium content 13, 17 and 25-28 %. The composition of the main chemical elements of chromium steels according to GOST 5632-72 presented in the table 1.
Chromium steels, depending on the carbon content, can belong to different structural classes: ferritic, martensitic and mixed - ferrite-martensite. Belonging to one class or another is determined by the diagram of the iron-carbon-chromium ternary system.
Steel with a nominal chromium content 17, 25 and 28 % – 12H17, 08Х17Т, 15X25T and 15X28 – are ferritic steels. Their structure is ferrite and they have no phase transformations.
For steels containing chromium 12-14 % everything is a little more complicated. They are unstable in properties and small deviations in the chemical composition transfer steel from one class to another. So, steel 08X13 with a minimum carbon content and a maximum chromium content is ferritic, and at the minimum chromium content it has a gamma-alpha transformation.
Cooling of steels 20X13, 30X13 and 40X13 in air leads to the formation of martensite in them. The hardness of martensite increases with increasing carbon content, as well as heating temperatures for hardening, which determines the degree of dissolution of carbides in austenite.
Heat treatment of chromium stainless steels
Heat treatment of chromium steels can be different depending on the goal pursued, grade of steel and its chemical composition. The commonly used modes of heat treatment of chromium stainless steels are presented in the table 2
Steel type X13
Steel with 13 % chrome – 08Х13, 12Х13, 20Х13, 30X13 and 40X13 – are the most common and cheapest stainless steels. They are also used for kitchen utensils., and in technology. Steel with low carbon content 08X13 and 12X13 have high ductility and various parts are stamped from them. Steel 20X13, 30X13 and 40X13 have high hardness and increased strength - they are used to make parts of increased strength and wear resistance with high corrosion resistance. Various tools are made from them., including, surgical, and also bearings, springs and other parts for working in an active corrosive environment.
Steel type X17
Steel with 17 % – 12H17, 08X17T and 14X17H2 – chromium has a higher corrosion resistance. Due to the higher chromium content, these steels are also used as heat-resistant (scale-resistant) at operating temperatures up to 900 °FROM.
Steel type Х25-Х28
Steel 15X25T and 15X28 are used for furnace parts, eg, muffles and thermocouple sheaths, for operation at temperatures from 1050 to 1150 °FROM.
Ferritic stainless steel problems
The big disadvantage of ferritic stainless steels is their tendency to coarse grain when overheated., which is not eliminated by heat treatment - these steels do not have phase transformations. Coarse grain creates increased brittleness of steel with the transition of the cold brittleness threshold to the region of positive temperatures.
Source: Gulyaev A.P.. Metallurgy,