Measuring retained austenite

Large amounts can be estimated by metallography, but for smaller amounts of retained austenite, another method, such as X-ray diffraction, is required.

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In this column, we will discuss the different methods of measuring the amount of retained austenite in a heat-treated steel sample.

Retained Austenite

Previously [1] we have discussed the transformation of retained austenite. Retained austenite forms in steel when the steel is heated to the austenite region and then quenched. If the quenching temperature is above the martensite finish temperature, Mf, then the austenite-to-martensite transformation will not be completed. In other words, not all the austenite will transform to martensite, and an amount of austenite will remain or be retained.

In many alloys containing more than approximately 0.3 percent carbon, the Mf temperature is below room temperature. Remember from previous columns that the martensite start temperature can be estimated from the equation [3]:

The martensite finish temperature can be estimated from the equation [2]:

Koistinen and Marburger [3] developed an empirical relationship for the presence of retained austenite as a function of the martensite start temperature (Ms in °C) and the quench temperature (°C):

where the martensite start temperature, Ms and quench temperature, TQ, are in °C.

Since, at room temperature, a portion of the austenite is not transformed to martensite, it remains intermingled with the martensite at room temperature. This is particularly true of carburized alloys, since the case carbon content is roughly 0.9%C, and the martensite finish temperature is approximately -100°C. The amount of retained austenite is dependent of the carbon content, alloy content (nickel and manganese are strong austenite stabilizers), quenchant temperature, and subsequent thermal processing [4]. The amount of retained austenite present influences the tensile strength, impact toughness, and fatigue resistance [5].

Measurement of Retained Austenite

There are three main methods for determining the presence of retained austenite in steel.

Metallography

Metallography is a very simple method of examining a steel for the presence of retained austenite.

In this method, the sample micrograph is compared to a series of reference micrographs. Alternatively, the use of image analysis can be used.

This method is accurate when higher amounts of retained austenite are present. It is very difficult to determine very low quantities of retained austenite because of the difficulty distinguishing the retained austenite. Surface preparation is critical to remove plastic deformation due to metallographic preparation [6].

Magnetic measurement of retained austenite. (Courtesy: Stresstech)

Magnetic Induction

The magnetic measurement of retained austenite is a relatively fast and simple method of quantitative measurement of retained austenite [5]. This technique measures the fraction of magnetic phases present in the sample. The non-magnetic phases present are retained austenite. This method was developed to measure the amount of ferrite in stainless steel welds but has been expanded to measure retained austenite [7].

In this method, the mass fraction of retained austenite is determined by [8]:

where JS is the magnetic saturation of the sample, and J0 is the reference magnetic saturation of the austenite-free reference. Since the calculation is based on a reference, the quality of the reference is very important. Historically, it can be reference to plain iron or ferrite, but this will overestimate the amount of retained austenite. The heat-treatment condition must be considered [9].

X-ray diffraction of retained austenite. (Courtesy: NIST)

X-ray Diffraction

Since austenite (face centered cubic, FCC) has a different structure than ferrite (body centered cubic, BCC) or martensite (body centered tetragonal, BCT), the X-ray diffraction pattern will be different. There will be peaks in the diffraction pattern that correspond to FCC peaks. The amount of retained austenite present is determined by examining the peak intensity from each phase.

There are two standards for measuring retained austenite — ASTM E975 [10] and SAE SP-453 [11]. In each of the methods it is assumed that the microstructure has a random orientation and few carbides are present.

These methods compare the hkl 200 peak of martensite and ferrite to the austenite hkl 200 peak and the hkl 220 peak. This method allows for the direct calculation of retained austenite by the intensity of the austenite phase and the ferrite phase [10]:

where fγγ is the volume fraction of austenite, I1 and I2 are the intensity of the hkl peak of austenite and ferrite, and R1 and R2 are the correction factors for austenite and ferrite, respectively.

In the event there is a preferred orientation for the retained austenite or there are significant amounts of undissolved carbides present, the above method provides an inaccurate result. To compensate for this, the preferred method is to use the Rietveld whole pattern method [12]. The method requires a non-linear least squares approach which is difficult to perform. However, the results are more accurate than using the basic comparison method.

Figure 1: Optical micrographs of quenched and tempered 100CrMnSi6-6 steel with approximately (a) 15 percent and (b) 30 percent retained austenite. Images taken at 1000X and etched with 4 percent nital [5].

Conclusion

In this article, the different methods of determining the amount of retained austenite were shown. Large amounts of retained austenite can be estimated by metallography [13], but for smaller amounts of retained austenite (less than 15 percent) other methods such as X-ray diffraction are required [14]. Magnetic Induction can also be used.

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References

  1. D. S. MacKenzie, “Transformation of retained austenite,” Gear Solutions, pp. 32-33, September 2021.
  2. W. Steven and A. G. Haynes, J. of Iron and Steel Institute, vol. 183, p. 349, 1956.
  3. D. P. Koistinen and R. E. Marburger, “General Equation Prescribing the Extent of the Austenite-Martensite Transformation in Pure Iron-Carbon Alloys and Plain Carbon Steels,” Acta Metall., vol. 7, pp. 59-60, 1959.
  4. D. H. Herring, “A discussion of Retained Austenite,” Industrial Heating, no. March, pp. 14-16, 2005.
  5. L. Rothleutner, “Retained Austenite Significant for Strength, Toughness,” Thermal Processing, pp. 14-16, March 2019.
  6. U. Başkaya, G. Karaçalı, M. Özyiğit and E. Kılıç, “The Effect of Sample Preparation Method on Volume Fraction of Retained Austenite,” in 18th International Metallurgy & Materials Congress, 29 Sept. – 01 October, Istanbul, Turkey, 2016.
  7. V. Miguel-Eguía, F. J. Avellaneda, J. Coello, A. Martinez and A. Calatayud, “A Procedure Based on Magnetic Induction to Evaluate the Effect of Plastic Deformation by Multiaxial Stresses on TRIP Steels” Mat. Sci. Forum, vol. 713, pp. 1-6, 2012.
  8. K. Ouda, H. Danninger and C. Gierl-Mayer, “Magnetic Measurement Of Retained Austenite In Sintered Steels – Benefits And Limitations, Powder Metallurgy, vol. 5, pp. 358-368, 2018.
  9. H. E. Exner, “Magnetische Bestimmung von Gefügebestandteilen (Masters Thesis),” University Leoben, 1960.
  10. ASTM, “ASTM E975 Standard Practice for X-ray Determination of Retained Austenite in Steel with near Random Crystallographic Orientation,” ASTM, Conshohocken, PA, 2013.
  11. C. Jatczak, J. A. Larson and S. W. Shin, “SP-453 Retained Austenite and Its Measurement by X-Ray Diffraction,” SAE, Warrendale, PA, 1980.
  12. H. M. Rietveld, “A Profile Refinement Method For Nuclear And Magnetic Structures,” Journal Applied Crystallography, vol. 2, no. 2, pp. 65-71, 1969.
  13. A. Stormvinter, Low Temperature Austenite Decomposition in Carbon Steels, Stockholm: KTH Royal Institute of Technology, 2012.
  14. G. Vandervoort and E. P. Manilova, “Hint For Imaging Phases In Steels”” Adv. Mater. Process., vol. 163, pp. 32-37, 2005.