Residual Stress & Retained Austenite

Application Note

Shot Peening

Peening effectiveness is normally characterized via the Almen intensity and the % coverage. It should be noted that many potentially different residual stress gradients can result from what may appear to be the same deflection of the Almen strip and observed % coverage. The % coverage is an optical assessment of the as-peened surface and is generally thought of as a measure of the uniformity of peening on the component surface.

However, uniform coverage does not always result in a uniform stress state. Consider the following example of a weldment shot peened with CW-28 shot at 16-18A intensity with 125% coverage. To verify the effectiveness of the peening process, a section of the weld was masked off and was not peened so as to compare the peened and as-welded conditions. The residual stress was mapped over an area encompassing both the peened and unpeened portions of the weld and parent material. It can be seen in the figure on the left that the shot peening technique used had a significant effect on the stress state of the weld and parent material, as seen by the “step” or drop in residual stress near the center of the map.

On the left side of the map, the as-welded stress state has tensile residual stresses in the weld and heat-affected zone (HAZ) and neutral or slightly compressive stresses on average in the parent material. The right side of this map shows the peened area where the characteristic profile is much more compressive (less tensile) and uniform in the parent material, but a reduced tensile residual stress state remains in the weld. This indicates that the peening process had the effect of reducing the tensile residual stresses in the weld and HAZ while introducing compressive residual stresses in the parent material.

The peening was not sufficient to force the weld and HAZ entirely into compression in the sampled volume. This would suggest that the peening parameters could be changed to increase the compressive residual stress imparted upon the weld and HAZ or that a stress relief heat treat cycle could be applied prior to peening. This kind of information cannot be obtained using the standard Almen strip test and the assessment of the % coverage.

Can one use XRD in the lab?

Yes. XRD has been used extensively to measure the residual stress in peened components in a laboratory environment. Since varying peening parameters can result in different subsurface residual stress gradients, the residual stress in peened components must initially be evaluated as a function of depth. In some cases, the effect of varying the peening intensity on the same component results in quite different residual stress gradients as a function of depth. This information is used to select the best peening parameters for the intended application.

Can one use XRD in the field?

Yes. XRD has been used on large components on the shop floor and in the field. Residual stress measurements were performed on a large pinion gear at different locations where various shot peening parameters were applied. The resultant residual stress vs. depth plot (below, left) demonstrates how the effect of the peening process varies with depth as a function of peening intensity, shot size, and shot hardness.

For an accurate assessment of the peening process applied, the actual residual stresses present in the peened component(s) must be measured. This is because the apparent Almen intensity is a result of the total through-thickness effect on the Almen strip. Hence, steeper, more intense compressive stress gradients can potentially result in the same apparent Almen strip curvature as deeper reaching but less intense compressive stress gradients. Additionally, the width/breadth of the x-ray diffraction peak is related to the dislocation density in the sampled volume; thus, an empirical measure of the net work hardening present in the sampled volume due to peening as a function of depth can be obtained (below, right).

Can one use XRD inline and in audit stations?

Yes. Recent advances in detector technology and computing power have made XRD-based residual stress measurement data acquisition and analysis possible in near real time. Once the residual stress profile as a function of depth has been established for a given component and process, surface measurements can be performed completely nondestructively for 100% inline inspection and quality assurance.

Since XRD can be used to measure surface residual stresses nondestructively, it can be used to a) track changes in the residual stress on the exact same part at the exact same location through various production stages, b) quantitatively monitor the resultant residual stresses, and c) subsequently provide verification of quality. Thus, XRD can be used to determine when and how potentially harmful stresses are introduced into a component. The effectiveness of potential corrective actions can then be evaluated, implemented, and monitored.

How can one apply XRD to their parts?

XRD has been used to help identify stress fields created by over-peening that may actually decrease the service life of a component. It has also been used to help reduce the number of parts needed for fatigue testing, which decreases expenses and time requirements.    

Data and Results from Peened Parts

Comparison of residual stress with various peening parameters

Comparison of FWHM with various peening parameters

Additional Applications:

Residual Stress & Retained Austenite