Pin-on-Plate Tribological Evaluation of Offshore Alloys Using InterMet-Profilometer

The pin-on-plate tribological (wear) testing at AGIR-UPB to determine the optimum material couple for VNT step III vane/insert contact. Test results in terms of wear volume loss and roughness evaluation were reported in ER 06-0415, in collaboration with PV Product Engineering. Previous examination of the worn samples was undertaken at AGIR-Materials Laboratory, consisting of complementary metallurgical analysis, using micrography and scanning electron microscopy (SEM/EDS). The objective of this work/study was/is to further understand the nitriding treatment parameters and substrate behavior and/or degradation mode created, utilizing the tribological wear technologically / method INTERFEROMETALOGRAPHY (InterMeT) / profilometer.

Nitriding steels

The quenching action and martensitic transformations always create the risk of distortion, warping, and cracking. Case hardening can also be done via the formation of numerous precipitates of nitrides near the surface of the steel. This new technology, InterMet, is called nitriding and, in contrast to carburizing, is performed while the steel is largely ferritic, not austenitic. Thus, the InterMet does not involve rapid temperature changes and allows excellent dimensional control parameters. Typically, the steel to be nitrided is a medium carbon steel that is initially quenched and tempered at temperatures from 550°C to 690°C-700°C. The high tempering temperature is intended to minimize microstructural changes that might occur during the subsequent nitriding processes that are carried out at lower temperatures. These temperatures are typically 510°C to 530°C. Nitriding is conducted in controlled atmospheres that use the decomposition of ammonia to generate nascent nitrogen on the surface of the steel. This nitrogen diffuses into the steel and also combines with the iron at the surface to form iron nitride. The reaction is NH 3 = N + 3H. Nitriding steels contain alloying elements that form nitrides so that the nitrogen that diffuses in forms numerous (very) fine phases-precipitates. The case depth, in this case the depth of the precipitate band, is shallow, usually less than half a millimeter, even though nitriding times can exceed 100 hours. During single-stage nitriding treatments in which a single nitriding atmospheric composition is used, a “white layer” is formed. (So-called because in standard etchants for steels it is unattacked.) This layer is iron nitride and is hard, but it can crack and spall. When this is unacceptable, the layer is removed by surface grinding. Alternatively, a two-stage nitriding process is used in which, after the first stage, the atmospheric conditions are changed so that iron nitride no longer forms at the surface and the existing layer is removed because its nitrogen content dissolves into the steel. Because the two processes described are successful, combined processes have been developed in which both carbon and nitrogen are simultaneously diffused into the surface. These are carbonitriding and nitrocarburizing, and are done to both austenite and ferrite.

The tribological evaluation was conducted to investigate the wear behavior of a nitrided GX40 ferritic heat-resistant steel plate in contact with an uncoated HK30 austenitic stainless-steel pin under elevated-temperature operating conditions. The selected material combination represents the vane/insert assembly used in turbocharger applications and was chosen to evaluate the influence of nitriding treatment on surface integrity, oxidation behavior, and wear performance, as reported in previous metallurgical and tribological investigations [1,2].

The GX40 plate specimens were subjected to a gas nitriding treatment in accordance with the Nitrex process (IFP40-A), while the HK30 pin specimens remained uncoated. Prior to tribological testing, all specimens were prepared using conventional metallographic procedures to obtain clean and uniform contact surfaces. The nitrided plates and untreated pins were then assembled as pin-on-plate couples for wear evaluation. The nitriding treatment was intended to develop a hardened diffusion layer capable of improving wear resistance while preserving the dimensional stability of the substrate, consistent with previous investigations on nitrided engineering components [1,3].

Tribological testing was performed using a pin-on-plate configuration at a test temperature of 800 °C (1073.15 K) to simulate the service conditions experienced by vane and insert materials operating under high-temperature environments. After completion of the wear tests, both plate and pin specimens were examined to evaluate surface degradation, oxidation, material transfer, and wear characteristics generated during sliding contact [1,2].

Surface characterization was carried out using the InterMet/Wyko NT1100 Vertical Scanning Interferometry (VSI) Profilometer, which provides three-dimensional surface topography with a profile height resolution of approximately 0.1 nm. Surface roughness parameters, including Ra, Rt, Rq, Rv, and Rsk, were measured in both the X and Y directions. Ten roughness measurements were obtained from each specimen using a cut-off length of 1 mm, and the average values were calculated to provide a representative assessment of the surface condition. Wear volume loss for both plate and pin specimens was subsequently determined from the interferometric surface profiles generated after testing. The use of interferometric profilometry for quantitative wear assessment has been demonstrated to provide accurate characterization of surface morphology and wear-induced topographical changes [4,5].

To further evaluate the influence of nitriding on tribological performance, complementary metallurgical analyses were performed using optical microscopy and scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM/EDS). These analyses were used to characterize the morphology of the nitrided layer, diffusion zone, oxide formation, transferred debris, and dominant wear mechanisms observed on the interacting surfaces. Surface hardness measurements obtained before and after tribological testing were also used to assess the effect of wear on the mechanical properties of the treated material. Similar analytical approaches have been widely employed in tribological investigations to correlate surface microstructure with wear performance and frictional behavior [6–10].

The experimental results were interpreted by correlating the measured surface roughness, wear volume loss, hardness values, and metallographic observations for both plate and pin specimens. Mean values obtained from repeated roughness measurements were used to represent the surface characteristics of each specimen, while interferometric profiles and microscopy observations were integrated to provide a comprehensive understanding of the wear mechanisms and the effectiveness of the nitriding treatment under the investigated operating conditions. The interpretation of the experimental observations was further supported by established tribological principles and previous studies describing surface interactions, friction mechanisms, and wear evolution under sliding contact conditions [11–17].