Stainless steels: ferrite, martensite, austenite

Stainless steels prized for their high corrosion resistance. All truly stainless steels contain at least 11 % chrome. This chromium content ensures the formation of a thin protective surface layer of chromium carbide when steel reacts with oxygen..

Effect of chromium on the corrosion resistance of steel

It is chrome that makes steel stainless. Besides, chrome is an element, increasing the stability of ferrite. Picture 1 illustrates the effect of chromium on the iron-carbon phase diagram. Chromium causes the austenitic region to shrink then, how the ferritic region increases in size. With a high chromium content and a low carbon content, ferrite is the only phase up to the liquidus temperature.

xromistye-staliPicture 1 – Influence 17 % chromium on the iron-carbon phase diagram. With a low carbon content, ferrite is stable at all temperatures. The letter "M" stands for "metal", eg, chrome or iron, as well as other alloying elements.

There are several types of stainless steels, based on differences in crystal structure and strengthening mechanisms.

Ferritic stainless steels

Ferritic stainless steels contain up to 30 % chrome and no more 0,12 % carbon. Due to its body-centered crystal structure (OCK) ferritic steels have good strength and decent ductility , which are achieved due to solid solution hardening and strain hardening. Ferritic steels are ferromagnetic or, in simple terms, "Magnet". They do not lend themselves to heat treatment. Ferritic steels have excellent corrosion resistance, are moderately pressure-treating and relatively cheap.

Ferritic stainless steels include steels 08Х13, 12H17, 08Х17Т, 15Х25Т, 15X28 according to GOST 5632-72.

Martensitic stainless steels

From picture 1 it is seen, what steel with 17 % chrome and 0,5 % carbon when heated to 1200 ºС forms 100 %-ny austenite, which turns into martensite when quenched in oil. Martensite is then tempered to obtain high strength and hardness of steel (picture 2).

martensitaya-nerzhaveyushchaya-stalPicture 2 - Martensitic stainless steel. Contains coarse primary carbides and fine carbides, which were formed on vacation.

The chromium content in martensitic steels is usually not more than 17 %, otherwise the austenitic region in the phase diagram becomes too small. This leads to, that it becomes technologically difficult to get into it: requires tight control of carbon content and austenitization temperature. The lower chromium content allows the carbon content to be extended from 0,1 to 1,0 %, which makes it possible to obtain martensite of various hardness. Combination of high hardness, strength and corrosion resistance makes these steels suitable for making products such as high quality knives and ball bearings.

Martensitic stainless steels include steels 20Х13, 30Х13, 40Х13, 14Х17Н2 according to GOST 5632-72.

Austenitic stainless steels

Nickel is an element, which increases the stability of austenite. The presence of nickel in steel increases the size of the austenitic region, whereas ferrite almost completely disappears from iron-chromium-carbon alloys (picture 3).

stal-Х17Н8Picture 3 – Cross section of the iron-chromium-nickel-carbon phase diagram at 18 % chrome and 8 % nickel. With low carbon content, austenite is stable at room temperature.

If the carbon content gets lower 0,03 %, then carbides are not formed in the steel at all and the steel is completely austenitic at room temperature (picture 4).

austenitnye-nerzhaveyushchie-staliPicture 4 - Austenitic stainless steel

Austenitic stainless steels are highly ductile, pressure processing and corrosion resistance.

Heat treatment of stainless steels of the austenitic class consists in quenching in water from a temperature 1050-1100 ° C. This heating causes the chromium carbides to dissolve, and rapid cooling fixes the state of a saturated solid solution. Very important to note, that as a result of hardening the hardness of these steels does not increase, but decreasing. Therefore, for austenitic stainless steels, hardening is a softening thermal operation..

Austenitic stainless steel obtains its strength due to cold work hardening - cold working. Austenitic steels can receive work hardening to much higher values, than ferritic stainless steels. With deformations of the order 80-90 % yield point reaches 980-1170 MPa, and the tensile strength is 1170-1370 MPa. Clear, that such work-hardening can be achieved only in the manufacture of these types of products, like a thin sheet, tape, wire.

Austenitic stainless steels are non-magnetic, which gives them an advantage in many applications.

Representatives of austenitic stainless steels are 12X18H9 and 17X18H9, 12Х18Н10Т and 12Х18Н9Т, 08Х18Н10Т, 08Х18Н12Б, 03Х18Н11 according to GOST 5632-72.

Dispersion hardened stainless steels

These steels are also called high-strength stainless steels.. Dispersion hardened stainless steels contain aluminum, niobium or tantalum and get their properties through hardening, strain hardening, aging hardening and martensitic transformation. Steel is first heated and quenched to convert austenite to martensite. Reheating causes hardening particles to precipitate from the martensite, such as NiAl3. The high strength of these steels is achieved even with low carbon content.

Precipitation hardening steels include 07X16H6 steels, 09Х15Н8Ю, 08Х17Н5М3, 04Kh25N5M2, HN40MDTYU according to GOST 5632-72.

Duplex stainless steels

In some cases, a mixture of different phases is deliberately produced in the structure of stainless steels. With appropriate control of the chemical composition and heat treatment modes, steel is obtained with a content, eg, 50 % ferrite and 50 % austenite. This combination of phases in the structure of steel provides it with such a unique combination of mechanical properties, corrosion resistance, pressure and weldability, which cannot be achieved in any other stainless steels. Sometimes they are called in foreign terms - duplex steels.

Two-phase stainless steels include 08X22H6T steels, 03Х23Н6, 08Х21Н6М2Т, 03Х22Н6М2, 08Х18Г8Н2Т, 03Х24Н6М3 according to GOST 5632-72.

Source: D. Askeland, P. Fulay, W. Wright – The Science and Engineering of Materials, 2011