During process development and on a regular basis throughout production, the quality and compliance of surface hardening process should be checked. Besides crack inspection, surface hardening quality can be verified by measuring surface hardness and hardening case depth, which are fundamental to understand the mechanical behavior of the component under working conditions.
When processing large rings and other large parts, it is convenient to verify the hardening process by non-destructive testing (NDT) methods. NDT helps save time while it is not necessary to cut the part to obtain samples for testing. Furthermore, after NDT quality checks, the part is still intact and suitable for use. This is most important for large parts which have usually a high economic value. In addition to that, NDT methods allow for a quality control on 100% of production.
For NDT quality control, SAET is equipped with:
– a portable hardness tester with Leeb probe and HV UCI probe
Leeb method employs a sphere rebounding on the part surface and is recommended for large parts with a high case depth. UCI probe is a Vickers indenter, the testing load is applied by hand and the indentation is measured by an ultrasonic system. UCI method is also suitable for smaller part with thinner hardened layer. Both testing methods leave on the part surface a very small indentation.
– ultrasonic equipment for hardening case depth NDT
The instrument operating principle is based on the scattering phenomenon of the ultrasonic waves propagating through a polycrystalline material. The particles responsible for the scattering phenomenon are the crystalline grains, which are small compared to the ultrasonic wavelength. The backscattered waves have an intensity which depends on the ratio of grain size to the ultrasonic wavelength. For a case-hardened part, the hardened layer has small martensitic grains, while the core material has a coarser structure. The backscattering wave intensity increases with increasing grain size, thus the position of interface between the hardened layer and the core material is measured.
From our experience with large rings, ND testing of surface hardness and case depth is convenient, affordable, and adaptable to numerous track geometries