Determination of Stresses and Retained Austenite in Carbon Steels by X-rays - A Round Robin Study

Residual stresses and retained austenite are two important process-related parameters which need to be controlled and monitored carefully during production and heat treatment of products. X-ray diffraction techniques are normally used in this context, and the purpose of the present study was to investigate the reproducibility and accuracy of these methods for medium and high carbon steels. The work was carried out as a round robin study including nine different laboratories in Sweden and Finland. Stress measurements were carried out on three specimens etched to three different depths, 0 μm, 230 μm and 515 μm. Retained austenite measurements were carried out on three specimens containing approximately 11, 17 and 30 vol.-% of this phase. The stress measurements showed good reproducibility with standard deviations of typically 4% on flat and smooth surfaces and not more than about 8% on etched surfaces. Estimations revealed that specimen misalignment and improper X-ray spot location were the main sources behind the variation in the stress recordings. The determination of retained austenite showed a standard deviation of typically 15% between the different contributors. However, by using identical evaluation methods for all raw data, the data spread could be narrowed by a factor of 3 to 4 despite the fact that different experimental settings were used in the individual laboratories.