Volume 78, Issue 1

In order to achieve the goal of a massive reduction of CO 2 -emissions, fossil fuels have to be substituted. In Germany, continuous carburizing furnaces for high capacities are almost exclusively fired by natural gas due to the lower energy costs. Electrical heating and hydrogen combustion are obvious alternatives. While electrical heating elements are state-of-the-art for these types of furnaces, hydrogen combustion has not been investigated. Furthermore, these two alternatives strongly depend on the specific energy mix, which determines the CO 2 -emissions. This case study compares different process heat generation options for continuous ring hearth furnaces for carburizing automotive steel parts by a quantitative approach. The investigated alternatives are natural gas/air heating as the reference, electrical heating and hydrogen/air heating. Besides the energy balances, primary energy consumption and resulting CO 2 -emissions are calculated. Furthermore, possible developments until 2050 are analysed. The results show that both alternative cases have a high potential to decrease CO 2 -emissions which strongly depend on the development of the energy mix and, therefore, the future expansion of renewable energy sources.

Highly stressed gears are usually manufactured from case hardening steels. The case hardening depth (CHD) has a significant influence on the tooth root load carrying capacity. For the materials 16MnCr5 and 20MnCr5, comprehensive investigations have been carried out in the past to determine the optimum case hardening depth up to size module m n = 10 mm on cylindrical spur gears. In certain applications, however, it may be necessary to select an increased CHD, e. g. to reduce the risk of tooth flank breakage. The question therefore arises as to what extent the available findings on the influence of the CHD on the tooth root load carrying capacity can also be transferred to materials of higher hardenability and larger sizes. For this purpose, several materials with different alloy systems and hardenabilities were examined under variation of the case hardening depth. In addition to extensive material characterization, pulsator tests were carried out to determine the tooth root load-carrying capacity on test gears of size m n = 12 mm. It is shown that the results obtained on MnCr steels up to size m n = 10 mm can in principle also be transferred to the variants investigated here.

Magnetic Barkhausen noise analysis is an already industrially used method for the detection of grinding burn and offers several advantages over the established nital etching test with respect to objectivity, cost savings as well as occupational safety and environmental protection. Studies on the detectability of damages and influencing factors such as the condition of the surface zone prior to grinding have so far only existed in isolated cases and mostly limited to a few widely used case-hardened steels. For this reason, various steel grades used in particular in the aerospace sector were considered in this study. For one quenched and tempered, one nitrided and two case-hardened steels, the detectability of damages by means of Barkhausen noise and the influence of different heat treatment parameters were investigated. It was shown that the use of Barkhausen noise analysis is also possible on these steel grades and enables more reliable damage detection than nital etching.

Tribocorrosion is the simultaneous occurrence of wear and corrosion in a tribosystem and their interaction. In many applications, such as media-lubricated rolling bearings and (cutting-)tools in the food industry or medicine, tribocorrosion occurs and leads to a high material loss and damage to materials. The tribocorrosion resistance of mechanically and chemically stressed steel surfaces can be significantly increased by low-temperature plasma nitriding at T < 400 °C. In this process, nitrogen is forcibly dissolved in the surface area (up to approx. 20 μm) in high contents of 15 wt.-% without precipitation. This results in an extreme expansion and distortion of the metal lattice (“expanded martensite”, “expanded austenite”), which leads to an increase in hardness of up to 1000 HV with the same or even increased pitting corrosion resistance. Due to the formation of expanded martensite/austenite, the tribocorrosion resistance of the martensitic steels X40Cr14 and X54CrMnN13-2 and that of an austenitic CrMn steel can be significantly improved compared to the initial state, which is expressed in a 40–70 % lower material loss under tribocorrosive attack. It was found that the tribocorrosion resistance depends on the process parameters of the surface treatment and on the chemical composition of the steels and their crystal lattice.