Which treatment process produces a residual compressive stress at the surface?

The treatment process that produces a residual compressive stress at the surface of the material is shallow quenching. In shallow quenching, the material is cooled rapidly, but not to the same extent as in deeper quenching processes. This moderate cooling leads to a differential in temperature across the material's cross-section, creating compressive stresses on the surface.

As the outer layer of the material cools and contracts more rapidly than the inner core, the surface tends to become more dense and experiences compressive stress. This residual compressive stress is beneficial as it can enhance fatigue strength, improve resistance to surface cracking, and increase overall durability under cyclic loading conditions.

Other processes listed may not create the same beneficial residual stresses or might lead to different stress states within the material. It’s also important to highlight that the effectiveness of shallow quenching in generating these beneficial stresses is significantly enhanced by the controlled cooling process tailored to achieve the desired microstructural characteristics.