# Retained austenite

Retained austenite is a phase in steel or cast iron that is usually undesirable in conventional steel tempering . It is relatively unstable and is transformed by temperature increase in ferrite and cementite and lowering the temperature and mechanical stress ( "Sitram" = stress induced transformation of austenite - martensite) in martensite to.

During the conversion, a flip occurs, from the face-centered cubic space lattice ( austenite ) to a tetragonal body-centered space lattice ( martensite ). In the face-centered cubic lattice , the packing density is greater than in the body-centered tetragonal lattice; the conversion therefore leads to an increase in volume , which can lead to stresses in a workpiece. For example, the conversion of retained austenite into martensite (volume increase) can lead to microcracks within the already existing martensite plates (“barrier structure”) and thus reduce the fatigue strength.

In terms of quality, the transformation of retained austenite (folding the grid) into martensite can be approximately determined by measuring the hardness before and after the low-temperature cooling. The condition can also be quantified by means of a structural examination or X-ray determination of retained austenite.

## Mechanism of origin

 element M S per percent of the element in ° C RAG per percent of the element in% 1) C, N -300 50 Mn -33 20th Cr -22 11 Ni -17 10 Mon –11 9 W. –11 8th Si –11 6th Co +6 -3 Al +17 -4 1) Base alloy = C100

In the case of steels with more than 0.5% by mass of carbon and sufficient proportions of alloying elements, the martensite finish temperature M f is less than 20 ° C. When these materials are quenched from the austenitic state to room temperature, part of the austenitic initial phase is retained in the structure. This austenite is called retained austenite. It is a relatively soft, metastable structural component that can transform into martensite through further cooling or mechanical stress.

The amount of retained austenite can be calculated according to the equation

${\ displaystyle RA = e ^ {- B (M _ {\ text {S}} - T _ {\ text {u}})}}$

estimate depending on the martensite start temperature and the temperature of the quenching medium. The alloy composition is included in the above equation via the temperature. is a temperature-dependent constant. ${\ displaystyle M _ {\ text {S}}}$${\ displaystyle M _ {\ text {S}}}$${\ displaystyle B}$

${\ displaystyle B (T = 20 \, ^ {\ circ} {\ text {C}}) = 1 {,} 1 \ cdot 10 ^ {- 2} \, {\ tfrac {1} {^ {\ circ } {\ text {C}}}}}$

and

${\ displaystyle B (T = -196 \, ^ {\ circ} {\ text {C}}) = 7 {,} 5 \ cdot 10 ^ {- 3} \, {\ tfrac {1} {^ {\ circ} {\ text {C}}}}}$

The table summarizes the effects of the various alloy elements on M S and the retained austenite content (RAG).

Larger amounts of retained austenite can also be present after bainitic transformations, e.g. B. in silicon steels or cast iron. Since silicon hinders the formation of carbides, the carbon that is not soluble in the bainite cannot precipitate in the form of carbides and diffuses into the austenite that is still present. This has the consequence that the carbon content of the austenite increases and at the same time the martensite start temperature decreases. When M S reaches room temperature, the austenite remains completely retained as retained austenite after cooling.

## Retained austenite stabilization

Figure 1: Parameters influencing the stability of austenite and retained austenite

One speaks in steels of "austenite" at> A 3 and "Restaustenitstabilität" in assessing the after quenching to room temperature still present austenite. As shown in Figure 1, the austenite stability depends on various factors. Influenced by the transformation kinetics and the quenching parameters, it forms the basis of the residual austenite stability. B. is influenced by the amount of retained austenite as well as mechanical and thermal loads. The stability of the retained austenite is of great technical importance for the mechanical properties and dimensional accuracy of retained austenite steels. It is useful to differentiate between mechanical and chemical residual austenite stabilization.

### Mechanical stabilization

According to Tammann and Scheil, the compressive stresses caused by the volume increases in the martensite formation in the austenite should prevent further transformation if they exceed a certain amount. The transformation can only be continued when the compressive stress is reduced by further cooling. This is possible because austenite has a coefficient of thermal expansion that is a factor of two greater than martensite and therefore shrinks more when it cools down. According to Rose, on the other hand, the growth of the martensite nuclei is stopped by disturbing the coherence at the austenite-martensite interfaces. These disturbances are said to be caused by dislocations and other lattice defects that arise during the plastic deformation of the austenite by the martensite that forms. In fact, dislocation densities of 10 11 to 10 12 cm −2 are found in austenite areas which are immediately adjacent to the martensite crystals . The mechanical austenite stabilization in steels with a higher carbon content can be so strong that even when cooled to the temperature of liquid helium (4 K), complete martensite formation does not occur.

### Chemical stabilization

After the formation of the first martensite crystals, carbon diffusion takes place during the further martensite formation. These self-tempering effects increase the carbon content in the surrounding austenite and stabilize it. The increased carbon concentration in the austenite locally lowers the martensite start temperature. As a result, the existing undercooling decreases, so that the driving force ΔG (A → M) necessary for the martensitic transformation can no longer be provided. The conversion can only be continued with further cooling and thus falling below the new martensite start temperature M S '.

During the bainitic transformation, too, there is strong carbon diffusion into the austenite that has not yet been converted. If the silicon content is sufficiently high, the austenite can be chemically stabilized to such an extent that it does not convert to martensite when it is subsequently cooled to room temperature.

### Other stabilizations

Further stabilization mechanisms of austenite such as thermal, dynamic and isothermal stabilization can be traced back to mechanical or chemical stabilization or a combination of the two.

## Thermally induced retained austenite transformation

When tempering structural states containing residual austenite, a disintegration of the residual austenite is observed from around 300 ° C. The retained austenite between the martensite needles is converted into ferrite and cementite in a diffusion-controlled manner. Since the cementite is formed between the martensite and prefigures therefore possible crack paths, it is for the observed at about 300 ° C embrittlement blamed. The addition of silicon shifts the cementite precipitation to higher temperatures, so that the austenite initially incompletely converts into carbide-free bainite above 300 ° C and cementite only forms above 380 ° C.

On the other hand, the existing retained austenite content can be reduced athermally by freezing below M S '. The deep-freeze martensite that forms in this process has significantly poorer mechanical properties than “hardness martensite”, as it is not subjected to any tempering or self-tempering processes. M S 'is well below the quenching temperature. The temperature difference to the quenching temperature (T u - M S ') depends on the amount of martensite formed, the waiting time between quenching and deep-freezing and on previous aging at elevated temperatures. Obviously, carbon diffusion from martensite into the austenite takes place during the waiting time and stabilizes it. When the value falls below M S ', such a large driving force ΔG therm is available that the stabilized retained austenite also converts.

## Mechanically induced residual austenite transformation

In the mechanically induced residual austenite transformation, a distinction must be made between stress-induced and deformation-induced transformation, depending on whether the martensite formation takes place below or above the yield point of the austenite.

Figure 2: Temperature dependence of the stress and deformation-induced martensite formation

Figure 2 illustrates the dependence of both processes on the transition temperature. If one cools below M S (M), martensite develops spontaneously on the preformed nuclei (A). At temperatures above M S (M S '), martensite is only formed after an external voltage has been applied, as a result of which the preformed nuclei are capable of growth. Part of the driving force necessary for martensite formation is now applied mechanically, so that applies

${\ displaystyle \ Delta G ^ {(AM)} = (\ Delta G_ {mech} + \ Delta G_ {therm})}$

Since the thermally supplied free enthalpy contribution decreases with increasing temperature, the mechanical contribution must be increased by increasing the voltage. At the temperature M S σ , the stress reaches the yield point of austenite (C). The plastic deformations of the austenite generate new preformed nuclei, so that martensite formation is facilitated. Therefore, the curve of the onset of martensite formation deviates from the extension of the straight lines A – C and runs from C to E. At E, the tension required for martensite formation increases so much that it can no longer be reached. M d is therefore the temperature above which deformation-induced martensite formation is no longer possible.

The martensitic transformation of the retained austenite causes irreversible expansion components due to the volume difference. As a result, the yield point of the material, viewed macroscopically, coincides with the threshold stress of the stress-induced transformation below M S σ . In the case of M S (M S '), the yield point has very low values, since even the smallest stresses lead to a stress-induced transformation. The yield point of the material is identical to the yield point of austenite over M S σ .

## Conversion-induced plasticity

In the case of steels with high residual austenite and metastable austenitic steels, the so-called TRIP steels (transformation induced plasticity), an astonishing strength and ductility is often observed. The increased ductility is due to the deformation-induced martensite formation, which provides an additional hardening mechanism. This deformation-induced martensite formation also takes place preferably in the area of ​​stress peaks and reduces them. This z. B. in the case of rapid stress, the instability of the constriction, which occurs at the stress peaks, is delayed and the solidification capacity of the material is better utilized. To achieve the TRIP effect, complex alloy compositions and complex thermomechanical treatments are usually necessary.

Transformation-induced plasticity phenomena also have a positive effect on the material resistance to crack propagation, since additional energy is required for crack propagation due to the residual austenite transformation in the plastic zone. In addition, in the material area close to the crack tip, the increase in volume associated with the transformation generates residual compressive stresses that close the crack and thus slow down crack propagation.

## literature

• Hans-Jürgen Bargel (Ed.): Material science. 7th edition, Springer, Berlin 2000, ISBN 3-540-66855-1 , p. 181.
• Jürgen Ruge, Helmut Wohlfahrt: Technology of materials. Manufacturing, processing, use. 8th edition, Vieweg, Wiesbaden 2007, ISBN 3-8348-0286-7 , p. 79 ff. (Media combination; with DVD-ROM)
• Dieter Liedtke: Heat treatment of ferrous materials. 1. Basics and applications. 7th edition, expert-Verl., Renningen 2007, ISBN 3-8169-2735-1 , pp. 22, 42.
• Axel Lünenbürger: On the transformation and deformation behavior of bainitic-austenitic silicon steels. Karlsruhe 1991. (Dissertation, University of Karlsruhe (TH), PDF )

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