During hot rolling of steel, various defects are introduced in the surface and near surface region of the steel strip. These defects will propagate and evolve through the different stages of the hot rolling process and, finally, affect the bendability during subsequent manufacturing processes. Understanding the connection between surface defects and oxide layers and the bendability of high-performance steels is critical for forming process optimisation. This study is to investigate the surface/near surface defects and oxide layers of a high-performance martensitic wear-resistant steel at different stages of the hot rolling process (roughing mill, finishing mill, and after the final processing) and correlate these to the bendability. Pre- and post-bend test analyses were performed using scanning electron microscopy with energy-dispersive spectroscopy, 3D white-light interferometry, X-ray diffraction, and microhardness measurements. Friction free bending tests were performed using an equipment similar as the original VDA 238-100 tool, but upscaled in size managing higher loads. The bendability of samples in levelled condition after finishing process is reduced compared to as-rolled condition. The results show that the formation of red scale between the steel substrate and primary oxides significantly affects the bendability of the martensitic wear-resistant steel in both as-rolled and levelled conditions. Detailed characterisation indicated that crack initiation during bending was primarily concentrated at the red scale region as a result of topographical changes and local deformation of the near surface microstructure. Generation of secondary cracks caused by abrasive wear from mechanical brushing also impact bendability.

The surface and near surface damage features in a martensitic wear resistant steel extracted from three stages (after roughing mill, finishing mill, and levelling) in a hot rolling process has been characterized. Friction free bend tests VDA-238-100 has been performed to assess the deformation behaviour and its correlation to surface properties. The salient conclusions from this study are as follow:⁃After roughing mill, oxide layers of varying coverage and thickness up to 50 μm were observed with a silicon and chromium enriched spinel oxide layer located between the bulk oxide and steel substrate where presence of fayalite (Fe2SiO4) was shown. Localized fracture and removal of the oxides was seen as well as indentation of the oxides into the steel substrate causing local microstructure deformation.⁃After finishing mill the oxide layer thickness was reduced (around 10 μm) with the same silicon and chromium enriched oxide layer closest to the steel substrate. Presence of material transfer from work rolls to steel strip was observed.⁃The levelling introduced abrasive grooves caused by microploughing resulting from interaction with hard particles during mechanical brushing. Deeper valleys were seen in the surface topography compared to as-rolled condition resulting from removal of indented oxides during mechanical brushing. A deformed subsurface microstructure was also observed.⁃The silicon and chromium rich oxide layer improves adhesion of the thicker oxide layer making it difficult to remove during descaling and mechanical brushing.⁃Bend test results showed that the samples in finished condition experience increased plasticity before failure compared to as-rolled condition.⁃The crack initiation was primarily located at regions with presence of silicon rich oxides or defects such as abrasive grooves, surface indents, or local plastically deformed sub-surface areas with reduced ductility.

Polycrystalline Diamond (PCD) is used as a cutting tool for machining non-ferrous metals and composites, especially aluminium alloys. The use of aluminium alloys in the automotive industry is forecasted to increase by 50 % from 2020 to 2030. This implies that machining of aluminium alloys will increase significantly, and the detailed investigation of PCD degradation mechanisms and the development of PCD material and tooling solutions is a necessity for efficient and robust manufacturing processes. This research aims to study the attrition wear mechanisms in Polycrystalline Diamond tools during interaction with aluminium using both a machining test as well as simplified laboratory scale tribological test. Two grades of PCD with similar chemical composition and different diamond grain sizes (10 μm and 25 μm) were used in the experiments and the counter material was a metal matrix composite (Al + 15 vol% of SiC) in machining test and AA6082 in the tribological tests. The wear rate of the two PCD grades were similar in machining tests. The PCD grade with the larger grain size showed a rougher worn surface topography, which can be attributed to the attrition wear by larger removed grains. During the cutting process, the binder phase (Co) is more easily removed compared to the PCD grains. This weakens the mechanical anchoring of the diamond grains in the PCD microstructure. When the diamond grain loses its anchoring, it is detached from the PCD microstructure damaging the cutting tool by attrition wear. Analysis of cutting tool inserts from turning tests and tribological tests revealed two types of wear phenomena, micro-wear and macro-wear both promoted by detachment of diamond grains from the PCD tools. This confirms that attrition wear takes place during machining of aluminium alloys using PCD, and it can be replicated in a simplified laboratory test setup.

Over the years, fretting has been a leading cause of fuel failures for water cooled nuclear reactors. Implementation of liquid lead coolant increases the need for fretting and corrosion-resistant materials. Fe–10Cr–4Al alloy has shown good resistance to thermal aging, oxidation, liquid metal corrosion, and embrittlement in liquid lead environment. The performance of Fe-10Cr-4Al under fretting wear conditions has not been studied previously. This work aims to investigate the fretting wear and friction behaviour of self-mated Fe-10Cr-4Al alloy at varying temperatures, in the presence of a reducing gas environment and liquid lead. The results show that an increase in test temperature from RT to 550 °C decreases the coefficient of friction and promotes the formation of Cr and Fe oxide third body layer. At high temperature in reducing gas atmosphere, the initial plastic deformation and severe adhesion leads to early seizure. Presence of liquid lead has a lubricating effect and allows a third body layer to form on worn surfaces. A detailed description of the initial wear mechanisms of Fe-10Cr-4Al alloy and friction levels under various conditions provides a base for further investigation of this alloy for long-duration fretting wear.

There is a great potential in using different techniques of additive manufacturing (AM) for the production and refurbishment of hot forming dies. In hot stamping of AlSi-coated boron steel, the dies are exposed to a complex combination of abrasive and adhesive wear. This results in tool damage and eventually tool refurbishment is necessary. Laser-metal-deposition (LMD) is an AM technique suitable for tool repairing due to its easier implementation, compared to selective-laser-melting (SLM). However, tribological studies on AM produced tools are still limited, particularly in the context of hot forming. Thus, the aim of this work is to increase the knowledge on the friction and wear behavior of LMD tool materials in hot stamping conditions. Two LMD materials were investigated: a hot-work tool steel (LMD-TS) and a conceptual high-hardness tool material (LMD-HH). Porosity was observed for both materials in the form of lack-of-fusion defects. LMD-TS resulted in a Vickers hardness value of approximately 650 HV, while LMD-HH showed a value of approximately 1100 HV. A high temperature strip drawing tribometer was used to perform friction and wear tests against AlSi-coated boron steel under unidirectional (linear) sliding conditions. Workpiece temperatures during sliding were 600 °C and 700 °C. All tribotests at 700 °C resulted in severe adhesion and AlSi-coating rupture, resulting in higher and more unstable friction. The wear mechanisms observed for LMD-TS were a combination of abrasive and adhesive wear, where ploughing and grooving in the tool material resulted in larger surface defects to accumulate material transfer. At 700 °C, these mechanisms were more severe. LMD-HH did not show significant abrasive damage in any test; the resulting material transfer was thinner and sparser compared to the other LMD material. The change in FeAlSi material transfer for LMD-HH was associated with the reduction in abrasive damage, i.e., ploughing and grooving in the tool material did not take place and thus did not enhance material transfer. The LMD process itself did not seem to have either a positive or a negative effect on the tribological behavior of the tool materials in this tribosystem, and the differences are seen mostly from the hardness perspective. LMD-HH is a promising tool material to reduce wear in hot stamping dies.

This work investigates the quantitative effect of the hydrogen concentration of 100Cr6 bearing steel on the surface-initiated damage induced during lubricated rolling/sliding tribotesting. Hydrogen was introduced to the samples prior to tribotesting by electrochemical pre-charging, and hydrogen concentration was measured using thermal desorption analysis. The surface-initiated damage was quantified by optical profilometry and scanning electron microscopy. An upper limit for the critical hydrogen concentration was determined to be 1.4–2 wppm diffusible hydrogen. At this concentration, the area fraction covered by damage features was found to double compared to uncharged samples. As both charged and uncharged samples exhibited the same type of surface damage (early-stage micropitting), it was concluded that hydrogen did not change the wear mechanism but decreased the number of contact cycles necessary for the initiation of surface defects.

This study investigate the effects of temperature on the tribological properties of Titanium-zirconium-molybdenum (TZM). TZM alloy was subjected to sliding contact against AISI 430 steel counterparts at varying temperatures. The results shows, the coefficients of friction (COF) at interface were 0.77, 0.75, 0.71, and 0.69, in the case of 25 °C, 300 °C, 600 °C, and 900 °C test temperatures. The presence of MoO3 and Mo9O26 phases suggested the wear resistance and decreasing COF at high temperature. Temperature is a critical factor influencing material transfer between TZM and AISI 430 steel. The primary TZM wear mechanism at 25 °C involved scratching, surface deformation, and local cracks, while elevated temperatures near 900 °C intensified material adhesion phenomena with a surface shearing effect on the TZM surface after the trotest.

Additive manufacturing (AM) of ferrous alloys has achieved a technological maturity level that allows for the production of high-performance steel components. Among these alloys, tool steels and maraging steels can be used in die manufacturing for different hot forming processes as both materials have good mechanical properties and thermal stability. In recent years, several works have successfully produced these alloys through selective laser melting, focusing on the optimization and characterization of their microstructure and mechanical properties. However, few studies look into the tribological aspects of these newly produced materials and even fewer consider high temperature friction and wear performance. Therefore, there is a need to understand the high temperature tribological response of tool materials produced by additive manufacturing. In this context, the aim of this work is to investigate the tribological behavior of a tool steel and a maraging steel, both produced by selective laser melting, at different elevated temperatures. A conventionally produced tool steel was used as reference. A reciprocating sliding tribometer was used to perform tests at 40 °C, 200 °C and 400 °C. The counter-body was a WC-Co cemented carbide. Friction and wear of the AM tool steel and reference tool steel were very similar, thus no signs of the AM route having an impact on the tribological behavior were observed. Both tool steels showed reduced wear volume with increasing temperature, while the opposite was observed for their respective WC-Co counter-bodies. Higher temperature also resulted in increased amount of W transferred onto the tool steel wear scars. AM maraging steel showed higher and more unstable friction level, as well as a much larger wear volume at all temperatures; material transfer onto WC-Co was observed at certain temperatures. Overall, tribolayer formation (or lack thereof) was the dominant aspect in the tribological responses.

As hydrogen reduces the fatigue life of 100Cr6 bearing steel significantly, extensive research on the interaction of hydrogen with 100Cr6 is necessary. This study investigated the influence of rolling/sliding tribotesting performed on a micro-pitting-rig on the hydrogen absorption and trapping behaviour of 100Cr6 bearing steel. Thermal desorption mass spectrometry was used to compare the hydrogen desorption spectra of 100Cr6 samples after tribological tests and static heated oil-immersion tests to untested reference samples. The approach was chosen to further understand the influence of both microstructural deformation as well as steel-oil contact on the hydrogen absorption and trapping behaviour of 100Cr6. The tribological test showed a stable friction behaviour and mild wear which was dominated by local plastic deformation of surface asperities. Despite the mild wear, a change in de-trapping temperatures was found for tribotested samples compared to oil-immersed and untested reference samples. This finding indicates that even mild tribotesting conditions alter the hydrogen trapping behaviour of 100Cr6 bearing steel.