Volume 75, Issue 2

This contribution provides an overview of the concepts of high temperature solution nitriding (HTSN) and the state of the art of the technology. It shows how HTSN can be applied to different classes of stainless steels. New advances of HTSN are presented, in particular the combination of HTSN with low temperature surface hardening, LTSH, is addressed: HTSN is an excellent “pre-treatment” for LTSH for austenitic, duplex, ferritic and martensitic stainless steels. Examples of combined HTSN + LTSH treatments of duplex and martensitic stainless steels are shown.

Additive manufacturing in the powder bed by means of laser powder bed fusion (L-PBF) combines the fast production of metallic components with complex geometrical structures with a high degree of customization and low raw material consumption. However, there is a high demand for the development of new alloys to achieve the desired properties of additively manufactured parts. Due to a lack of knowledge, only a small group of steel powders are currently available for L-PBF. As an example, Cr and CrMn steel powders with additional alloyed nitrogen could be used for L-PBF products for Ni-free medical devices. The present work investigates the possibility of alloying nitrogen to austenitic steel alloy powder and high alloyed martensitic steel powder via gas nitriding. For this purpose, an X30CrMo7-2 powder and an AISI 316L powder for additive manufacturing were arranged to form powder beds of different heights and nitrided at different temperatures, times and nitriding potentials. The nitrogen content was determined from the samples taken at different heights of the powder beds. Individual powder particles were examined by means of metallography, scanning electron microscopy and electron probe micro analysis. It is shown that the nitrogen concentration in the powder is essentially determined by the nitriding temperature. In the X30CrMo7-2 powder, nitride precipitation also occurs at high temperature and N and C gradients are present in the particles with an average nitrogen content of up to 4 mass%. In the 316L powder, a homogeneously distributed nitrogen content of 0.35 mass% can be achieved after gas nitriding at 420 °C for 24 h. The flow properties of both powders are hardly affected by gas nitriding.

The purpose of this study is to clarify the effect of the combined treatment of nitriding and multi-stage peening on the friction and wear characteristics of tool steel AISI D2. In the measurement of the surface roughness, it was clarified that the size of the dimple changes by the multiple peening treatment when the nitriding treatment is not applied. It was indicated that the surface roughness increased by forming the Sn layer on the surface when the nitriding treatment was performed. It was revealed that the hardness increased significantly by applying multiple peening treatment after nitriding treatment in hardness measurement. In residual stress measurement, it was revealed that residual stress of about −1500 MPa is imparted by multiple peening treatment. In addition, it could be shown that higher residual stress is given after the nitriding treatment. It became obvious the friction coefficient of AISI D2 can be greatly reduced by nitriding multiple peening treatment. In addition, it was clarified that the friction coefficient can be reduced even with only the multiple peening treatment.

Carbonitriding serves to improve the properties of steel components. The carbon and nitrogen content and the microstructure comprising martensite, retained austenite, and finely distributed nitrides and carbides are significant parameters determining the properties of a component. Our present knowledge posits that these multi-phase microstructures represent optimal material conditions. The reliability of a process is strongly dependent on the process control. Previous studies examined the use of ammonia sensors in the exhaust gas to determine atmospheric nitrogen potemtial. Such measurements were used to establish the equilibrium ratios in the austenite. Phase diagrams were also found for corresponding alloy systems in order to determine maximum solubilities and precipitation in the presence of interacting carbon and nitrogen content. To further improve the reliability of carbonitriding, this final project discusses the simulation of carbon and nitrogen profiles and the precipitation state of carbides and nitrides in order to develop further controlled carbonitriding towards ensuring the reliable results of heat treatment processes.

Tool surfaces can be protected against corrosion and wear by thin film coatings. In addition to the protective properties, ceramic materials with low thermal conductivity, such as yttrium-stabilized zirconium oxide, can thermally insulate tool surfaces. In an injection molding process, the thermal insulation enables a more precise partial tempering and can thus be used to reduce surface defects on the plastic parts produced. Furthermore, the coating can have a positive effect on the demolding of the plastic parts. The coatings are applied in a hot wall reactor by metal organic chemical vapor deposition (MOCVD). This enables a homogeneous application of zirconium oxide based thin films on steel tools with complex 3D surfaces. The performance of the coatings was evaluated in various injection molding tests and assessed by the quality of the manufactured parts.