Residual Stress Determination in Surface Layers of Martensitic Stainless Steel Freeflex® Versa using Grazing-Incidence X-ray Diffraction

The presence of high levels of compressive residual stresses at the surface of compressor valves is a determining factor for good fatigue properties and long-life operation. There is a significant focus on continuous improvement within the compressor industry regarding energy efficiency, due to reoccurring sharpening of legislations on compressor performance. Recently, the new compressor steel grade Freeflex^® has been launched, with Freeflex^® Core and Freeflex^® Versa products to enable further development towards improved energy efficiency and compressor miniaturization.

In this work, residual stresses of Freeflex^® Versa have been investigated using X-ray diffraction (XRD). For obtaining an average of macroscopic stresses, the conventional single-reflection XRD was used. To study the macroscopic stress gradient profile of the outermost surface layers, multi-reflection Grazing Incidence XRD (GIXRD) technique has been employed. The investigated material samples were in as produced and tumbled conditions. The results indicate a significant level of compressive stresses in the vicinity of the surface (~1- 2 µm) due to the tumbling process. The residual stresses measured by single reflection were found to be lower compared to the GIXRD residual stresses, indicating that conventional residual stress measurements provide data from a material depth not fully affected by the tumbling process.