Volume 76, Issue 1

The manufacturing process of grinding generally leads to a thermo-mechanical influence on the surface integrity. In addition to the intended development of residual compressive stresses due to the finishing process, disturbances in the grinding process can lead to negative effects such as tensile residual stresses, tempering and even rehardening zones and significantly reduce the component lifetime. In industrial applications, the analysis of Barkhausen noise is becoming increasingly important for the detection of this unwanted thermo-mechanically influenced surface integrity. The non-destructive method reacts sensitively to changes in, for example, the residual stress state as well as the hardness. In addition, other material-, process- and metrology-related influences are described in literature. The investigations presented in this paper deal with the influence of different material states (case-hardening depth, surface carbon content and alloy composition) on the signals of the Barkhausen noise as a function of the surface integrity. It is shown that the signal level is significantly influenced by the material condition and thus individual limit values must be used for evaluation of the surface integrity. ◼

Globalization and international supply chains constantly challenge gear- and steel manufacturers. In the case of large gear units in particular, expensive alloy contents have a direct impact on the material price. Steels with lower alloy contents and therefore cheaper steels are therefore becoming the focus of attention in order to further improve competitiveness. This article therefore compares two materials with different alloying elements and contents and thus different hardenability behaviour. For this purpose, extensive material characterizations as well as pulsator tests were carried out on case-hardened large gears. The aim of these tests was to determine the tooth root load-bearing capacity of the two material variants. Finally, the results are compared, discussed and recommendations for industrial application are derived, taking into account the hardenability of large gears. ◼

Case hardening processes such as carbonitriding can be used to improve the performance of powder metallurgical structural components. Due to the amount of carbon and nitrogen introduced, it is possible to adjust the hardness and residual stress of the surface layer. Due to their porosity, powder metallurgical components show a significantly increased diffusivity and therefore increased demands on the process control. In order to be able to make a quantitative statement about the effects of diffusivity as a function of porosity, common densities of 6.9 g/cm 3 , 7.2 g/cm 3 and 7.35 g/cm 3 for PM-steel are examined in a total of ten process sequences. By means of simulative approaches, the resulting element depth profiles can be calculated and the surface layer condition can be predicted by a subsequent heat treatment simulation. In a two-part work, the mass transport during carbonitriding is investigated in the first part and the resulting surface layer conditions after heat treatment in the second part. By considering different process combinations and porosities, model approaches of volume and pore diffusion can be formulated and quantitative element depth profiles can be predicted and validated depending on the process parameters. ◼

The mechanism of plasma nitriding include the formation of various active species generating nitrogen atoms reacting with the metal. Which species prevail in supplying nitrogen depends on nitriding conditions as well as the nature of the treated metal. Plasma nitriding of low-density powder metal (PM) products results in a formation of the layers whose thicknesses may depend on the gas pressure used for the process. Higher pressure can cause locally deeper penetration of the surface by active nitrogen species formed from ammonia compounds generated by the plasma. While a low processing pressure reduces this effect significantly. The formation mechanism of a locally thicker layer relies on the presence of open porosities in the surface as they can be penetrated by the ammonia species generated by the plasma. The same porosities cannot be penetrated by the ions of nitrogen formed at the same time since their mean free life is much shorter than that of ammonia species. ◼

For the determination of heat transfer coefficients in air-water spray cooling, two methods are presented that are capable of characterizing multi-nozzle cooling set-ups. The methods are based on the quenching of thin-walled tubes or massive cylinders on which cooling curves are recorded at given positions with thermocouples. The temperature dependent heat transfer coefficients were calculated by an inverse calculation and the measured temperature-time-curves could be reproduced with these data in numerical cooling simulations. Next, the determined heat transfer coefficients were used for the calculation of an air-water-spray quenching process of a forging part with more challenging geometry. The calculated results were compared with thermocouple measurements. Different calculation variants for the heat transfer on component surfaces not directly exposed to the air-water spray are shown and discussed. ◼