Comparison of residual stress measurements on a Jominy test bar

The study: System performance with varying diffraction peak width

The Xstress DR45 utilizes 2D detectors while the Xstress G2R systems use a more traditional line detectors (Figure 1).

2D-vs-1D-detector — Figure 1: Dimensions of the active areas of 1D and 2D detectors.

The two dimensional area detectors designed for X-ray detection give numerous advantages over the line detectors, one being the measurement speed. But the smaller detection range in 2θ angle direction could be seen as a disadvantage in some applications, especially when the diffracted peaks are wide. The width of a detected diffraction peak is affected by the sample material, its microstructure and stress state as well as the used equipment. Hardness of the material correlates with the width of the diffraction peak – Harder the material the wider the full width at half maximum (FWHM) values are. This application study was done to confirm that the residual stress measurements are valid for varying peak widths with both systems and to compare the measurement time on the Xstress DR45 and G2R systems.

The testing method: X-ray diffraction

When a steel is hardened by quenching the diffracted X-ray peaks become broad because of the microstrain due to the formation of martensite. A Jominy test bar is heat treated and then quenched from one end. The cooling rate is fastest at the end being quenched and slower inside the sample. This difference in cooling rate causes the hardness to decrease when moving away from the quenched end. Because the sample is hardest closest to the quenched end the diffraction peaks are the widest there. The varying peak width makes the Jominy bar an excellent sample for this application. The sample used in this study was prepared by Tampere University.

Residual stress measurements were done along 50 mm long line on the Jominy test bars surface using X-ray diffraction (XRD). The measurements were done near the hardness test spots marked on the sample. The measurements setups and measurement locations are shown in Figure 2 and Figure 3.

Figure 2: Measurement setup on Xstress G2R.

Figure 3: Measurement setup on Xstress DR45 and measurement locations along the arrow.

The results

The FWHM-values were similar for measurements performed with both the Xstress DR45 and G2R. The wide diffraction peak from the edge of the sample fit on to the shorter detectors of the DR45. The measurements with both systems showed a similar decrease in the FWHM-value when moving away from the edge of the sample bar. The FWHM results are shown in Figure 4.

Figure 4: On the left the FWHM-values measured on G2R and on the right the FWHM-values measured on DR45.

The residual stress also showed similar behavior for both the DR45 and G2R. At the edge of the sample there was a transition from compression to almost zero stress followed by a steep transition back to compression when moving towards the center of the sample. After that the residual stress increased in a linear fashion. The results are shown in Figure 5.

Figure 5: On the left the surface stress values measured on Xstress G2R and on the right the surface stress values measured on Xstress DR45.

Measurement time was 1 hour and 50 minutes for Xstress G2R and 13 minutes for Xstress DR45. Based on these results the DR45 is a well-suited for fast residual stress measurements and FWHM-value determination even with sample materials with wide diffraction peaks.