Retained austenite

Lattice parameter Martensite 2-856-2-862A at room temperature austenite (retained) 3-58A... 

Austenitic steels retain the ccp structure right down to room temperature. For this reason these steels cannot be hardened by quenching. 

The highly aHoyed austenitic stainless steels are proprietary modifications of the standard AISI 316 stainless steel. These have higher creep—mpture strengths than the standard steels, yet retain the good corrosion resistance and forming characteristics of the standard austenitic stainless steels. Nickel-Base Superalloys. 

In some cases, the carbon profile may not provide the necessary hardness or other properties. For example, if the carbon content is too high, quenching to room temperature may not produce all martensite at the surface because the high carbon content places the martensite finish temperature, Mj below room temperature. This results in the presence of soft retained austenite, and a low surface hardness. Conversion to martensite by subzero cooling to below the temperature can increase the hardness (Fig. 6) (12). 

Fig. 6. (a) The effect of sub2ero cooling on the hardness gradient in a carburized and quenched 3312 steel where (e) is oil quenched from 925 to 20°C and ( ) is cooled to -195°C. The initial quench to 20°C does not convert all of the austenite to martensite because the high carbon content in the surface region lowers the temperature below 20°C. Subsequent cooling to -195°C converts most of the retained austenite to martensite, raising the hardness, (b) The... 

Nickel normally crystallises in the f.c.c. structure it undergoes a magnetic transformation at 357°C and is ferromagnetic below that temperature. In all the alloys shown in Table 4.21 the f.c.c. (austenitic) structure is substantially retained, and in consequence most of the alloys possess the combination of properties required of materials for widespread industrial acceptability, i.e. tensile strength, ductility, impact strength, hardness, hot and cold workability, machinability and fabrication. 

Similarly it seems that retained austenite may be beneficial in certain circumstances , probably because the austenite acts as a barrier to the diffusion of hydrogen, although in high concentrations (such as those obtained in duplex stainless steels) the austenite can also act as a crack stopper (i.e. a ductile region in the microstructure which blunts and stops the brittle crack). 

The peritectic transformation generally has little effect on the structure, properties or corrosion resistance of steels at room temperature an exception to this occurs in the welding of certain steels, when 6-ferrite can be retained at room temperature and can affect corrosion resistance. Furthermore, since most steels contain less than about 1 -0 oC (and by far the greatest tonnage contains less than about 0-3%C) the eutectic reaction is of relevance only in relation to the structure and properties of cast irons, which generally contain 2-4%C. This discussion, therefore, will be limited to the eutectoid reaction that occurs when homogeneous austenite is cooled. 

Figure 3. Mossbauer spectra for ordered and disordered samples of an alloy of 25% rhodium and 75% iron. The disordered sample was quenched and contains retained austenite (17)...

Figure 6 illustrates a more complicated situation. The sample was a plain iron-carbon steel—an iron foil carburized to about 5 atomic % carbon and then quenched. One sees a rather complex pattern. There is a large central peak from some untransformed high temperature face-centered phase of iron containing carbon in solid solution, retained austenite. There is a strong six-line pattern coming from martensite, a distorted body-centered solid solution of carbon in iron. We also see a... 

Low-temperature p-hydrogen requires the use of materials that retain good ductility at low temperatures. Austenitic stainless steel (e.g. AISI 316L and 304L) or aluminum and aluminum alloys (Series 5000) are recommended. Polytetrafluor-oethylene (PTFE, Teflon) and 2-chloro-l,l,2-trifluoroethylene (Kel-F) can also be used. 

As in the case of corrosion failures, the sequence of steps involved in analyzing wear failures are initial examination of the failed component including service conditions to establish the mode or combination of modes of wear failure, metallographic examination to check if the microstructure of the worn part met the specification, both in the base material and in the hardened case or applied surface coatings, existence of localized phase transformations, shear or cold worked surfaces, macroscopic and microscopic hardness testing to determine the proper heat treatment, X-ray and electron diffraction analysis to determine the composition of abrasives, wear debris, surface elements and microstructural features such as retained austenite, chemical analysis of wear debris surface films and physical properties such as viscosity and infrared spectral determination of the integrity of lubricants and abrasive characteristics of soils or minerals in the cases of wear failures of tillage tools. 

The chloride stress-corrosion cracking of austenitic stainless steels in chloride solutions with samples under tensile stresses has been known since 1940. There are some reports that claim the retained austenitic structure to be responsible for chloride stress-corrosion cracking. 

Microstructure after CVD Ferrite + Cementite Martensite +10% Retained Austenite + Carbide... 

Microstructure after Hardening Martensite + 20% Retained Austenite Tempered Microstructure + Carbide... 

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