In contrast to a cold‐forming process, a tempered forming process is able to deform high‐strength steel used for manufacturing automotive bodyworks in a more economic manner. Cold‐formed steel sheets are commonly coated with a Zn or ZnAlMg layer for cathodic corrosion protection. The tempering process would lead to diffusion processes at the steel/coating interface, which is accompanied by the formation of new phases in the coatings. This publication focuses on phase formation in Zn and ZnAlMg coatings on steel sheets, which are heat‐treated at 400 and 750 °C. the authors find that the pure Zn coating remains in the solid state and transforms into the intermetallic δ phase (FeZn10) during heat treatment at 400 °C. The coating melts during heating to 750 °C, but remains in the solid state after transformation into the Γ phase (Fe4Zn9) and α‐Fe. In the ZnAlMg coating, minor iron diffusion occurs at a temperature of 400 °C. Within a dwell time of 600 s, intermetallic Fe–Zn phases are not formed. During heat treatment at 750 °C, phase formation in the ZnAlMg coating is very similar to that in the pure Zn coating, during which Γ (Fe4Zn9) and α‐Fe are formed.
Sealless rolling bearings for use under media lubricated conditions are exposed to high tribocorrosive loads. To achieve the necessary wear resistance while keeping up the corrosion properties of the corrosion resistant steels low-temperature-plasma-diffusion-treatments were conducted. When scaling-up the developed low-temperature-plasma-nitriding-processes to industrial dimensions, it is necessary to reach the required surface conditions independently of the parts placement within the batch. Upscaling to industrial batch processes was analyzed by using the example of rolling bearings. To achieve a homogeneous treatment result, relevant factors like temperature distribution and component placement were taken into account. This allows to draw conclusions about the surface layer properties in dependence of temperature, batch arrangement and position within the batch to verify the treatment results homogeneity.
The development of NbC‐containing martensitic stainless steels has made it possible to unite the properties of high corrosion resistance and wear resistance. If NbC is used as a hard phase instead of chromium carbide, the abrasive wear resistance of the steels is increased due to the greater hardness of NbC. The solubility of chromium in NbC is low. For this reason, chromium is fully available to form a passive surface layer to increase the corrosion resistance. In the steel melt, niobium leads to the formation of primary NbC, which grows very rapidly. Atomization of PM steels leads to the formation of coarse primary hard phases that clog the nozzle. Therefore, until now, steels with NbC as a hard phase are produced using the PM route with so‐called diffusion alloying; however, this production route is very complex and expensive. Herein, a novel Nb‐rich MC‐containing wear‐ and corrosion‐resistant steel that is produced using the usual PM route is presented. This steel consists of a martensitic matrix with evenly dispersed Nb‐rich MC having a volume of 2.5%. Due to the high hardness (>750 HV30) in combination with high resistance to pitting corrosion, the steel exhibits outstanding tribocorrosion resistance in 0.9% NaCl solution.
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.
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