When a carbon steel is quenched from the Austenitic, high temperature region, most of the material transforms to fresh Martensite. However, some Austenite is retained in the quenched structure, which may lead to inferior mechanical properties such as lower fracture toughness or fatigue life. Therefore, the use of high carbon steel in critical applications often mandates that the maximum level of Retained Austenite be controlled.
When the Retained Austenite level is at or below 15%, X-ray diffraction methods are the desired analytical tool. Protocols such as ASTM E-975 and SAE SP-453 have been developed to put standardized methods in place that produce accurate results down to 0.5% or less. However, these methods are less useful when carbides such as M23C6, M7C3 or M3C are present. Therefore, more robust methods such as Rietveld refinement are used in such cases to account for carbides, various matrix phases, and preferred orientation. Typical accuracies for this method are in the 1% range.
- Quality control
- Optimization of heat treating process
- Avoiding grinding burn and shot-peening damage
- Control premature failure
- Dimensional control
- Suppression of crack growth during spalling