Leonardo Pelcastre

Investigations into the occurrence of galling during hot forming of Al–Si-coated high-strength steel:

Galling is a severe form of adhesive wear associated with both cold and hot metal forming operations. In hot sheet metal forming of Al–Si-coated ultrahigh-strength steel (UHSS), transfer occurs from the coated UHSS to the tool surface. This leads to poor quality of produced parts, damage of expensive tooling, and increased downtime for maintenance of the tools. This study thus aims at identifying the salient mechanism(s), which give rise to initiation/occurrence of galling at elevated temperatures. This has been accomplished by analysing actual hot forming tools and through systematic parametric tribological investigations in the laboratory. The analysis of the actual tools has shown that the transferred layer consists of Al, Si, and Fe. The structure of the transferred materials is composed of sintered/compacted wear particles. The parametric study has shown that galling is dependent on the operating conditions. A strong relationship between the contact pressure and material transfer has been observed. The severity of galling is lower for smoother surfaces at low contact pressure. However, at high contact pressure, the influence of roughness under these conditions is insignificant. It has also been observed that hard-tool steel substrates reduce the severity of galling, particularly, at high contact pressure.

High-temperature friction and wear characteristics of hardened ultra-high-strength boron steel:

Abstract The usage of ultra-high-strength boron steel (UHSS) in automotive industry has increased rapidly in the recent past. Forming of UHSS components is performed at elevated temperatures, which also offers the possibility of hardening through quenching directly after forming. However, the influence of hardening on friction and wear during relative sliding between the tool and the workpiece is unclear. Therefore, the friction and wear characteristics at elevated temperatures of hardened and unhardened UHSS and tool steel pairs are investigated in this study. The results show that both friction and wear at all the investigated temperatures are affected by hardening of the UHSS. For uncoated UHSS, the hardening resulted in lower friction and the tool wear increased at low temperatures, but was not affected at elevated temperatures. This was attributed to the higher hardness after hardening combined with the presence of an oxide scale on the UHSS after heating and quenching. For Al—Si-coated UHSS, the hardening reduced friction and tool steel wear at elevated temperatures, and also reduced the wear of the Al—Si-coated high-strength steel at low temperature mainly owing to the formation of an intermetallic layer on the Al—Si-coated UHSS surface after exposure to elevated temperatures.

Abrasive wear behaviour of hardened high strength boron steel

Abstract Abrasive wear in industrial applications such as mining, materials handling and agricultural machinery constitutes a large part of the total wear. Hardened high strength boron steels are known for their good wear resistance and mechanical properties, but available results in the open literature are scarce. This work aims at investigating how different quenching techniques affect the two-body abrasive wear resistance of hardened high strength boron steels. Furthermore, the wear as a function of depth in thicker hardened high strength boron steel plates has also been studied. The material characterisation has been carried out using microhardness, SEM/energy dispersive spectroscopy and three-dimensional optical surface profilometry. The results have shown that water quenched and tool quenched high strength boron steel had similar wear resistance. The main wear mechanisms appear to be microcutting combined with microfatigue. Workhardening during the abrasion process has been found to affect the abrasive wear.

A strategy for wear analysis using numerical and experimental tools, applied to orbital type hydraulic motors

An accurate and reliable wear analysis requires detailed knowledge of the tribological conditions of the studied system. In this work, a numerical model which can quantify wear and is applicable to hydraulic motors is developed. Detailed tribological knowledge can be acquired through strategic experimental testing and numerical simulations. The model is constructed to include the effect on wear from varying lubricant film thickness. The development of the wear model includes consideration of wear observed in the Scanning Electron Microscopy (SEM) analysis of tested motors. The model is of the Archard type, in which the k-value is estimated from experiments, after considering the effect of lubrication. The contact pressure is the solution to a lubrication model that governs both the hydrodynamics of the lubricant film and the direct contact between the rough surfaces. To validate the model, a hydraulic motor is run at different operating conditions and the apparent wear depth is analysed after the tests. Numerical simulations mimicking the same configuration are performed and the predicted wear depths are compared to the experimental results. Similarities and differences are discussed and it is evident that a clear correlation exists between the wear predicted with the model and the measurement data of the apparent wear in the hydraulic motor. There are also discrepancies because of the model simplicity and the uncertainty in the specifications of the tested system. The results imply that wear analysis using numerical simulations aid the understanding of wear in machinery. The combined knowledge of physical conditions on different important scales enables in-depth analysis with numerical tools which cannot be achieved through experimental investigations alone. Furthermore, the numerical model can be refined leading to better wear predictions.

Experimental Evaluation of Galling Under Press Hardening Conditions

Severe adhesion, also referred to as galling, is a critical problem in press hardening, especially in stamping tools used for hot forming of Al–Si-coated ultra-high strength steel. Galling is known to develop rapidly on the tool surface and it negatively affects the quality of the formed products. Earlier research on this topic has focused on the galling initiation. However, studies on the galling development during extended sliding and the corresponding quantitative measurement still lack depth. In the present study, a tribological test is established to study the galling development under press hardening conditions. The tribological test set-up aims to simulate extended sliding between the Al–Si-coated boron steels and the tool die material. The contact conditions in the interface are studied by a numerical model of the tribological test. The friction coefficients and material transfer are discussed taking into account the variation of the different test conditions. Using the results from the tribological tests, the galling simulation is performed in the numerical model. A geometry-updated sample based on the galling (transferred material build-up) height is simulated and the consequent pressure fluctuation is obtained in the numerical model. This contributes to the explanation of the severe transferred material accumulation during the test.

Tribological behaviour of Al-Si-coated ultra-high-strength steel during interaction with tool steel at elevated temperatures: Influence of tool steel surface topography parameters on galling

Galling is a severe form of adhesive wear encountered in metal forming operations. In hot stamping, an Al-Si coating is normally applied onto the ultra-high-strength steels to prevent decarburisation and to improve the corrosion resistance of the steel. Material transfer occurring from the coated ultra-high-strength steel to the tool surface has been identified as major issue in hot stamping. This transferred material impairs the quality of the produced parts and at the same time, it increases the costs of maintenance of the tools. This work focuses on the understanding of surface topography parameters and their effect on galling. Surface roughness level and orientation of the surface lay on tool surface have been studied. The results showed that a single parameter of the surface topography is not enough to describe the resistance to galling. Parameters such as Rv, Rp and Rsk are also important to consider in order to rank the galling resistance of the surface. The sliding direction with respect to the surface lay also had a significant influence on galling; sliding in the direction parallel to it resulted in substantially reduced material transfer.