## 4 Applications
### Applications in the Automotive Industry
#### 4.3.2 Surface-Hardened Parts and Machined Parts  
Surface heat treatments and machining processes are necessary to provide materials with the desired strength, shape, and surface quality. Hardening the material's surface by case-hardening, induction hardening, or nitriding will increase the resistance to surface wear without impairing the material's toughness in the bulk. Usually, the workpiece is already formed or milled to its final shape when surface hardening is applied. Therefore, unavoidable quenching distortions that are due to hardening usually have to be removed afterwards. In order to ensure a high-dimensional workpiece of accurate form, a subsequent fine machining of the workpiece's surface by means of turning, grinding, honing, or polishing is often required after hardening. However, this surface machining process can, again, result in unwanted changes in the material's microstructure. Machining can lead to a local temperature increase on the surface of the workpiece and thus thermally induced damage to the near-surface edge zones--the so-called \"grinding burns\". Depending on the degree of damage, this leads to an unfavorable change in the depth profiles of the hardness and residual stress, even resulting in hardening cracks. The most important quality characteristics of surface-hardened workpieces are the hardening depths, i.e., the surface-hardening depth (SHD) after induction hardening, the case-hardening depth (CHD) after carburizing, the nitriding hardness depth (NHD) after nitriding, and the fusion hardness depth (FHD), e.g., after laser hardening. In addition, the hardness and residual stress values at the surface and in the bulk or even the depth-profiles of hardness and residual stress are relevant in order to assure the quality of such workpieces. All of these material parameters can be determined with 3 MA after appropriate calibration. Figure 23 shows the CHD and SHD calibration results. For all surface-hardening mechanisms, 3 MA not only allows the hardening depth to be determined, but also the hardness at the surface, the core hardness, and the hardness values between. With verification measurements on parts not used in these calibrations, the RMSE values, i.e., the average deviations between the nondestructive and destructive measuring data of 0.07 mm for CHD and 0.13 mm for SHD could be determined. Applications for the determination of FHD with 3 MA have also been previously described [71; 72]. Especially impressive are the results of 3 MA for  
Figure 22: Results of the application of a calibrated 3 MA system: (**a**) screenshot of the results displayed after a single 3 MA measurement and (**b**) profiles of mechanical properties in a sharp transition zone. determining the NHD, e.g., in piston rings [44]. The typical values of NHD were between 60 and 100 \(\upmu\)m. It was found that, for the NHD values determined with 3 MA, the reproducibility is better than for the values determined by metallographic investigations (Nht700 test). In the case of machined workpieces, not only the hardness, but also the residual stress in its depth distribution will be of interest. In order to provide quality information regarding the microstructure of a grinded component, the nital etching method is often used. This method does not work completely nondestructively. Grinding defects, e.g., tempering zones and re-hardening zones, will be indicated by the discoloration of the surface after the etching process. This method is useful so long as the information from the material's surface is sufficient to describe the overall quality of the grinded workpiece. Nevertheless, thermally induced changes in the microstructure and residual stress due to grinding could reach up to 100 \(\upmu\)m material depth or more. Moreover, such deep grinding defects could be \"hidden\", e.g., covered by material with appropriate microstructure and residual stress. In such cases, it will not be possible to detect all grinding burns with nital etching.