Retained Austenite

Retained Austenite Measurement

X-ray diffraction is considered to be the most accurate method of determining the amount of retained austenite in steels.

Hardening of steels requires heating to an austenitic phase and quenching to room temperature to produce a hard martensitic phase. Due to incomplete transformation some austenite is retained at room temperature. Retained austenite can have a detrimental affect on the mechanical properties of the steel. Properties such as fatigue strength, toughness, hardness, yield strength and machinability can be influenced by retained austenite. Austenite can transform in service as a result of thermal cycles, plastic deformation, or shock. Shot peening, for example, will transform the austenite on the surface of gear teeth. Exposure to extreme cold renders the austenite increasingly unstable as the temperature diminishes. The transformation of austenite to ferrite involves a nominal 4% volume increase that can lead to seizure and excessive interference in precision gearing and bearings.

X-ray diffraction is considered to be the most accurate method of determining the amount of retained austenite in steels. Lambda Research is the only accredited independent laboratory that uses Bragg-Brentano diffractometers for measurement of retained austenite, as required by ASTM E975 (1) and SAE SP-453 (2). Most other labs use position sensitive detector (PSD) based systems for speed and portability that cannot be calibrated by direct calculation of the relative phase intensities. These systems must use reference samples that usually are not of the correct structure. As a result, the retained austenite measurements on PSD systems can have a proportional error as much as 20% of the value reported.

Lambda Research provides the measurement and certification of retained austenite standards. Lambda is the only accredited independent laboratory that uses Bragg-Brentano diffractometers for measurement of retained austenite, as required by ASTM(1) and SAE(2), providing the only accurate and reliable austenite standards available.

The austenite fraction is determined from the ratio of the austenite and ferrite diffraction peak intensities and the values of “R” for each phase. This method is more commonly known as the “Direct Comparison” method. The term R is derived from the Powder Pattern Power Theorem and depends upon the volume of the unit cell, Bragg angle, crystal structure and composition of the sample. R is inversely proportional to the square of the unit cell volume (v2) or the unit cell dimension to the sixth power (a6). Given the sensitivity of the austenite measurement to the unit cell dimension, Lambda calculates the term R from first principals for each sample alloy and structure per SAE(2).

Most of the diffractometers on the market today are equipped with position sensitive detector (PSD) based systems for speed and portability. These systems cannot be calibrated by direct calculation of the relative phase intensities and must use reference standards that usually are not of the same structure as the test articles.  For the reasons described above, any difference in unit cell dimension between the calibrating austenite standard and the test article will result in a significant error in calibration of the measurement system. It is therefore critical that the reference standards are of the same alloy and structure as the samples being tested on PSD based systems.

Lambda can provide rigorous certification of retained austenite standards per ASTM and SAE.  Standards can be manufactured from the same structure and alloy as the test article to eliminate the associated errors discussed above.

For more information on Retained Austenite Measurement, see Lambda Research Diffraction Notes, No. 3 and Lambda Research Diffraction Notes, No. 4.

Features

Austenite measurement method of Averbach and Cohen and Bragg-Brentano diffractometers in strict adherence to SAE SP-453 and ASTM E975.

R-values are calculated from first principals for each steel alloy tested per SAE SP-453. -The volume percent austenite can be determined in layers as thin as 10 microns.  Measurements can be made nondestructively on the surface of critical components.

Carbides present are routinely identified, and specific data collection schemes are developed for each alloy to avoid and correct for carbide interference.

Carbide content may be obtained by chemical extraction and gravimetric analysis.  Correction for carbide content is available.

Either constant angle or constant area techniques allow measurement of arbitrary sizes and shapes.  Fixtures are available for large samples.

Miller apparatus is available to spin and rock the sample to mitigate texture and grain size effects.

Subsurface austenite distributions may be obtained through the hardened case using electropolishing.

Complete machine shop for sample sectioning and preparation, including wet cut-off saw, wet diamond saw, and wet mechanical polishing or sanding.

References

1. ASTM, “Standard Practice for X-Ray Determination of Retained Austenite in Steel with Near  Random Crystallographic Orientation,” Standard E975-03, American Society for Testing and Materials, Nov. 1, 2003.

2. Retained Austenite and Its Measurement by X-Ray Diffraction,” SAE Special Publication 453, SAE, Warrendale, PA 15096.