Showing papers on "Maraging steel published in 2020"

A trade-off between powder layer thickness and mechanical properties in additively manufactured maraging steels

Abstract: In this paper, a comprehensive study on the microstructure and mechanical properties of an additively manufactured 18Ni-300 maraging steel (with the brand name MS1), fabricated through the laser-powder bed fusion (LPBF) technique is presented. The influence of powder layer thickness and the characteristics of feedstock powder as the input in the LPBF process is investigated on the microstructure and mechanical properties of solid cubes and cylindrical rods. Relative density and hardness are measured through the depth of the manufactured cubes. The study of porosity and hardness through the depth of LPBF-MS1 cubes proves homogeneous properties in the core of the material in comparison with more heterogeneous properties closer to the subsurface layers. X-ray diffraction techniques both on the powder and the as-built samples are then performed to identify phases in the fabricated samples. A correlation between lower austenite content and higher strength is observed for the tensile samples manufactured with lower powder layer thickness. Texture analysis shows a directional grain growth along the building direction resulting in a weak texture, while the material induces a stronger texture with an increased amount of austenite after the deformation. Studying the effects of powder layer thickness shows slightly lower strength and ductility for the samples manufactured with higher powder layer thickness, while the energy consumption, as well as the manufacturing time, are reduced.

A Review of Factors Affecting the Mechanical Properties of Maraging Steel 300 Fabricated via Laser Powder Bed Fusion

Abstract: Maraging steel is an engineering alloy which has been widely employed in metal additive manufacturing. This paper examines manufacturing and post-processing factors affecting the properties of maraging steel fabricated via laser powder bed fusion (L-PBF). It covers the review of published research findings on how powder quality feedstock, processing parameters, laser scan strategy, build orientation and heat treatment can influence the microstructure, density and porosity, defects and residual stresses developed on L-PBF maraging steel, with a focus on the maraging steel 300 alloy. This review offers an evaluation of the resulting mechanical properties of the as-built and heat-treated maraging steel 300, with a focus on anisotropic characteristics. Possible directions for further research are also identified.

Microstructure Evolution, Mechanical Properties and Deformation Behavior of an Additively Manufactured Maraging Steel

Abstract: In this work, the microstructure and mechanical properties of an additively manufactured X3NiCoMoTi18-9-5 maraging steel were determined. Optical and electron microscopies revealed the formation of melt pool boundaries and epitaxial grain growth with cellular dendritic structures after the laser powder bed fusion (LPBF) process. The cooling rate is estimated to be around 106 °C/s during solidification, which eliminates the nucleation of any precipitates. However, it allows the formation of austenite with a volume fraction of about 5% and dendritic structures with primary arm spacing of 0.41 ± 0.23 µm. The electron backscatter diffraction analysis showed the formation of elongated grains with significant low-angle grain boundaries (LAGBs). Then, a solutionizing treatment was applied to the as-printed samples to dissolve all the secondary phases, followed by aging treatment. The reverted austenite was evident after heat treatment, which transformed into martensite after tensile testing. The critical plastic stresses for this transformation were determined using the double differentiation method. The tensile strength of the alloy increased from 1214 MPa to 2106 MPa after the aging process due to the formation of eta phase. The experimental data were complemented with thermodynamic and mechanical properties simulations, which showed a discrepancy of less than 3%.

Effect of Heat Treatment Condition on Microstructural and Mechanical Anisotropies of Selective Laser Melted Maraging 18Ni-300 Steel

Abstract: 18Ni-300 maraging steel produced by the selective laser melting (SLM) process has a unique microstructure that is different from that of the same alloy processed by conventional methods. In this paper, maraging steels were fabricated by the selective laser melting process and their microstructures and mechanical properties were investigated in terms of post heat treatment conditions. Moreover, the effect of different heat treatments on the mechanical anisotropy was studied in detail. The micro Vickers hardness in the as-built state was around 340 Hv and could be increased to approximately 600 Hv by aging heat treatments. It was found that the solution heat treatment was not necessary to obtain a fully hardened state. From tensile tests of the maraging steels heat treated with different conditions, it was found that the highest strength was achieved by aging and solution treatment (ST) temperatures lower than the commonly used temperatures. In the direction parallel to the laser scanning, the highest ultimate tensile strength was obtained when 450 °C aging was done without solution heat treatment. In the other two directions tested, i.e., directions normal to the building and 45 degrees to the laser scanning direction, the highest tensile strength was obtained when aging was done at 450 °C after 750 °C solution treatment.

Tailoring the nanostructure of laser powder bed fusion additively manufactured maraging steel

Abstract: Grade 300 maraging steel specimens were synthesized using the additive manufacturing technique laser powder bed fusion (LPBF). The influence of heterostructures, i.e. cellular/dendritic regions with micro-segregations, formed during LPBF processing on the microstructure evolution, particularly the formation of nano-precipitates were investigated. We applied aging and solution- aging post heat-treatments to allow the precipitation in presence and after elimination of the heterostructures, respectively. A model describing the precipitation mechanisms and sequence for both heat-treatment routes is proposed based on the three-dimensional elemental distribution at near-atomic scale and the quantification of nano-scale segregated regions performed by atom probe tomography (APT). We observed that the heterostructures favored the austenite retention in the as-built condition, while they promoted the martensite-to-austenite reversion in the aged condition. On the contrary, the retention and reversion processes are suppressed in the solution-aged condition. The formation of nanosized (Ni,Fe,Co)3(Ti,Mo,Al) and (Fe,Ni,Co)7(Mo)6 precipitates was confirmed for both post heat-treatment routes. However, the nano-precipitates were smaller in size and higher in number density in the solution-aged condition, as compared to the aged state. The current work demonstrates that the elimination of heterostructures in the solution-aged condition, especially Ni micro-segregations, led to a high supersaturation and a concomitant increase in number density of (Ni,Fe,Co)3(Ti,Mo,Al) precipitates. The corresponding Ni-depleted matrix inhibited the martensite-to-austenite reversion. In contrast, the partial retention of heterostructures and the presence of retained austenite during aging without prior solution treatment resulted in a reduced number density of (Ni,Fe,Co)3(Ti,Mo,Al) precipitates, which can potentially trigger the reversion transformation supported by the rejection of Ni in front of (Fe,Ni,Co)7(Mo)6 precipitates.

Effect of build orientation on microstructure and tensile behaviour of selectively laser melted M300 maraging steel

Abstract: Additive Manufacturing (AM) is an emerging technology for fabrication of structural components, which refers to the processing through layer by layer addition of material using sliced CAD model of the desired geometry. Maraging steels are especially used in aerospace and tool industries due to their excellent combination of strength and fracture toughness. The present work aims at studying the effect of build orientation on microstructure and tensile behaviour of maraging steel (M300) processed by one of the AM techniques, namely, Selective Laser Melting (SLM). Initially, AM plates were processed in orientations of 0°, 45° and 90° and were further given solution treatment and aging. Measurement of density, surface roughness, hardness, residual stress and microstructural characterization of all samples were carried out and compared with those of conventionally melted (CM) samples. Transmission Electron Microscopy (TEM) confirmed the presence of Ni3Ti needle shaped precipitates and Fe2Mo globular precipitates after AM processing and also after heat treatment. Tensile testing of samples was conducted at a strain rate of 1 × 10−3 s−1. Better strength with reasonable ductility was observed in the samples built in 45° orientation compared to those at other build orientations and the properties in CM conditions. Solution treatment followed by aging decreased microstructural anisotropy caused by layer-wise effect due to laser processing. Surprisingly, most of the AM samples after heat treatment showed increase in strength without significant loss of ductility. Larger degree of work hardening and reversion of martensite to austenite might be the reasons for the observed behaviour. Typical ductile fracture featuring dimples due to microvoid coalescence were observed in all samples. Heat treatment not only improved tensile properties but also reduced anisotropy and residual stresses introduced during material processing.

The phase transitions in selective laser-melted 18-NI (300-grade) maraging steel

Abstract: Dilatometric studies in 18-Ni steel components fabricated by selective laser melting technique were carried out to determine the influence of heating rate on transitions occurring during the heating cycle. SLM components have been examined in controlled heating and cooling cycles. For analysis, heating of the analysed materials was carried out at heating rates of 10, 15, 20, 30 and 60 °C min−1. During the heating process, two solid-state reactions were identified—i.e. precipitation of intermetallic phases and the reversion of martensite to austenite. A simplified procedure based on the Kissinger equation was used to determine the activation energy of individual reactions. For precipitation of intermetallic phases, the activation energy was estimated 301 kJ mol−1, while the martensite to austenite reversion was determined at the activation energy 478 kJ mol−1.

Microstructural evolution and mechanical properties of a newly designed steel fabricated by laser powder bed fusion

Abstract: The M789 steel was designed by voestalpine group based on Grade 250 maraging steel to maintain good printability and increase the corrosion resistance. In this work, the M789 steel was manufactured by laser powder bed fusion process. The microstructural evolution of additively manufactured M789 steel was studied using optical and electron microscopy techniques. In the as-printed condition, the epitaxial grain growth with cellular dendritic structure was detected. The primary dendrite arm spacing was measured to be 0.42 ± 0.12 μm, suggesting the cooling rate is in order of 106 °C/s during solidification. This high cooling rate eliminates the occurrence of precipitation. The electron backscatter diffraction maps indicate the formation of elongated columnar grains with significant low-angle grain boundaries. After solution annealing and aging treatment, large needle-like martensitic structures were observed with fewer LAGBs than the as-printed condition. Spherical ETA-Ni3(Ti,Al) precipitates (η-phase) were formed during the heat treatment, leading to a significant increase in hardness, tensile strength, and yield strength compared to the as-printed condition. The tensile strength and yield strength along the building direction are 1798 ± 4 and 1714 ± 13 MPa, respectively. However, the elongation and toughness in the heat-treated condition are lower than that of the as-printed condition. Thermodynamic and mechanical properties simulations were also carried out, and the results are consistent with the experimental data.

Selective Laser Melting of 18NI-300 Maraging Steel.

Abstract: In the present study, 18% Ni 300 maraging steel powder was processed using a selective laser melting (SLM) technique to study porosity variations, microstructure, and hardness using various process conditions, while maintaining a constant level of energy density. Nowadays, there is wide range of utilization of metal technologies and its products can obtain high relative density. A dilatometry study revealed that, through heating cycles, two solid-state effects took place, i.e., precipitation of intermetallic compounds and the reversion of martensite to austenite. During the cooling process, one reaction took place (i.e., martensitic transformation), which was confirmed by microstructure observation. The improvements in the Rockwell hardness of the analyzed material from 42 ± 2 to 52 ± 0.5 HRC was improved as a result of aging treatment at 480 °C for 5 h. The results revealed that the relative density increased using laser speed (340 mm/s), layer thickness (30 µm), and hatch distance (120 µm). Relative density was found approximately 99.3%. Knowledge about the influence of individual parameters in the SLM process on porosity will enable potential manufacturers to produce high quality components with desired properties.

Microstructural heterogeneity and mechanical anisotropy of 18Ni-330 maraging steel fabricated by selective laser melting: The effect of build orientation and height

Abstract: Distinguished by a marked combination of high strength and high fracture toughness, 18Ni-300 maraging steel (MS) is widely used for intricate tool and die applications. MS is also amenable to the powder bed fusion additive manufacturing process, providing unique opportunities to make small features and incorporate cooling channels in molds. In this study, tensile test samples were fabricated using selective laser melting to investigate the effects of built height and orientations on the evolution of the microstructure and the mechanical properties of the samples. The microstructure of the as-fabricated samples consists of the primary a-martensite phase and fine cellular microstructure (~0.66–0.83 µm) with the retained austenite ?-phase aggregated at the boundaries of the cells, resulting in an enhanced mechanical performance compared with traditional counterparts under the same condition (without post-heat treatments). Random grain orientations with weak textures are revealed in all samples. The XY-built samples display better tensile performance when compared to the Z-built samples due to the fine grain sizes and the retained ? phase. The bottom of the Z-built sample exhibits a higher hardness than other parts of the sample, which could be attributed to its finer cellular structure.

Hybrid parts produced by deposition of 18Ni300 maraging steel via selective laser melting on forged and heat treated advanced high strength steel

Abstract: Long production times, the associated high costs of the products and product size limitations belong among current issues of selective laser melting (SLM) technology. Hybrid products containing small and complex-shaped parts deposited by SLM on the forged, rolled or hot stamped semi-products could offer a practical solution to these limitations. Cylindrical hybrid parts were additively manufactured by depositing 18Ni300 maraging steel on the cylindrical semi-products of CMnAlNb low-alloy advanced high strength steel (AHSS). The AHSS was used either in forged and air cooled condition or after heat treatments typically used for inducing the TRIP (transformation induced plasticity) effect. Various post-build heat treatments of the hybrid parts were performed. The mechanical properties of the hybrid parts were determined by hardness measurement across the interface and by a tensile test of the dissimilar joints. All tensile samples fractured in the high-strength steel side, several millimetres from the interface. Microstructure analysis of both materials and the interface region was carried out using light and scanning electron microscopes. The hybrid parts had the ultimate tensile strengths of 840−940 MPa, with total elongations of 12–19 %. The best combination of tensile strength and elongation was obtained with two-step heat treatment of the TRIP steel prior to additive manufacturing with no post-build heat treatment of the hybrid part.

Investigation on the cracking resistances of different ageing treated 18Ni maraging steels

Abstract: The cracking resistance is an important indicator concerned by the industry. In this study, the cracking resistances (impact toughness, fracture toughness and fatigue crack growth resistance) of different ageing treated 18Ni maraging steels were investigated. Meanwhile, the differences in microstructures and fractographies were compared. Finally, the key factors affecting the cracking resistances were discussed and confirmed. It is found that, with the increment in ageing temperature, the size and spacing of precipitated phases would augment, as well as, the impact and fracture toughness, and fatigue crack growth resistance would improve. The fracture morphologies of impact and fracture toughness specimens display that, with the improvement of toughness, the size of dimple would gradually upgrade, and the characteristic of cleavage fracture would gradually disappear. The key factor influencing the cracking resistances of maraging steel is the interparticle spacing. Augmenting the interparticle spacing would cause the improvement of cracking resistances for maraging steel.

Mechanical Properties of 3D-Printed Maraging Steel Induced by Environmental Exposure

Abstract: Changes in the mechanical properties of selective laser melted maraging steel 300 induced by exposure to a simulated marine environment were investigated. Maraging steel samples were printed in three orientations: vertical (V), 45° (45), and horizontal (H) relative to the print bed. These were tested as-printed or after heat-treatment (490 °C, 600 °C, or 900 °C). One set of specimens were exposed in a salt spray chamber for 500 h and then compared to unexposed samples. Environmental attack induced changes in the microstructural features and composition were analyzed by scanning electron microscopy and energy dispersive spectroscopy respectively. Samples printed in the H and 45° directions exhibited higher tensile strength than those printed in the V direction. Corrosion induced reduction in strength and hardness was more severe in specimens heat-treated between 480 °C and 600 °C versus as-printed samples. The greatest decrease in tensile strength was observed for the 45°-printed heat-treated samples after exposure. A comparison between additive and subtractive manufactured maraging steel is presented.

Relationship between the Size and Inner Structure of Particles of Virgin and Re-Used MS1 Maraging Steel Powder for Additive Manufacturing.

Abstract: Additive manufacturing (AM) is today in the main focus-and not only in commercial production. Products with complex geometry can be built using various AM techniques, which include laser sintering of metal powder. Although the technique has been known for a quite long time, the impact of the morphology of individual powder particles on the process has not yet been adequately documented. This article presents a detailed microscopic analysis of virgin and reused powder particles of MS1 maraging steel. The metallographic observation was performed using a scanning electron microscope (SEM). The particle size of the individual powder particles was measured in the SEM and the particle surface morphology and its change in the reused powder were observed. Individual particles were analyzed in detail using an SEM with a focused ion beam (FIB) milling capability. The powder particles were gradually cut off in thin layers so that their internal structure, chemical element distribution, possible internal defects, and shape could be monitored. Elemental distribution and phase distribution were analyzed using EDS and EBSD, respectively. Our findings lead to a better understanding and prediction of defects in additive-manufactured products. This could be helpful not just in the AM field, but in any metal powder-based processes, such as metal injection molding, powder metallurgy, spray deposition processes, and others.

Densification Behavior and Influence of Building Direction on High Anisotropy in Selective Laser Melting of High-Strength 18Ni-Co-Mo-Ti Maraging Steel

Abstract: The mechanical properties, physical properties and electrochemical behavior of metal components produced by selective laser melting can be influenced by the relative density and building direction. To this end, the optimization of the building process was conducted by identifying the influence of process parameters on the relative density and determining the ideal combination of parameters using the Box–Behnken design response surface methodology to achieve a relative density of 99.303 pct. With the ideal process parameters, material strength, thermal, and electrochemical performance were evaluated in a series of experiments. Anisotropic characteristics were displayed due to the differences in build-direction, microstructural features, and phase composition. The 0 deg possessed the highest tensile strength measured to be 1263.03 ± 8.71 MPa, while the 45 deg demonstrated the highest ductility with an elongation of 13.21 ± 0.34 pct. Thermal expansion was governed by the heat treatment process, such that anisotropic traits were eliminated after solution treatment. Strip melt tracks on the X–Y plane differed from the strip and arcuate melt tracks observed in the X–Z and Y–Z planes, leading to significant deficiencies in electrochemical reactance with an open circuit potential of − 645.8 mV in comparison to the latter measured at − 397.7 and − 396.7 mV, respectively.

Experimental study on grinding-induced residual stress in C-250 maraging steel

Abstract: Residual stress plays a significant role in the performance of a part, while the residual stress in the ground maraging steel, which is often used in the manufacture of precision parts, is rarely mentioned. In order to understand the variations of residual stress in ground maraging steel and provide insight into the controlled-stress grinding process of the steel, the surface and subsurface residual stress distributions in ground C-250 maraging steel (3J33) were studied. The results show that the mechanical effects dominate the thermal effects in the dry grinding process, indicated by only compressive residual stress generated in the ground workpiece. Furthermore, more insights into the residual stress distribution were provided by proposing four residual stress distribution parameters including surface residual stress, peak compressive residual stress, the depth of peak compressive residual stress, and residual stress penetration depth. The variations of these parameters were comprehensively studied. Results show that the surface residual stress and peak compressive residual stress depend greatly on the grinding speed and higher grinding speed generates larger compressive residual stress, while the depth of peak compressive residual stress varies slightly with the grinding parameters. The residual stress penetration depth increases with the increase of the grinding speed and grinding depth, and decreases with the increase of the workpiece speed. The results in this study can be used to assist in controlled-stress grinding applications for high performance critical parts of maraging steel.

Effects of cutting parameters on roughness and residual stress of maraging steel specimens produced by additive manufacturing

Abstract: Additive Manufacturing of metallic parts by powder bed fusion (PBF) has great potential to build complex geometries with innovative materials in a broad field of applications; however, it also presents some limitations as residual stresses, porosities, microcracks, and high roughness that restrict your plateau of productivity. Therefore, an alternative to improve the surface condition of PBF parts is the post-processing as milling. Maraging steel 300 is an important material used in the PBF process, considering its application in different segments, like automotive, tooling, and aerospace. Although there are a few works that investigated the effects of cutting parameters on the surface condition of maraging steel 300 components produced by PBF, this work investigated the effects of different cutting speeds (vc) and feed per tooth (fz) on average roughness Ra and residual stress of maraging 300 specimens. The lowest roughness level of Ra = 0.31 μm was obtained with fz = 0.02 mm/tooth and vc = 250 m/min. Furthermore, the cutting speed had a relevant effect on the compressive behavior of residual stresses. The feed per tooth combined with the cutting speed improved the surface roughness and the compressive residual stress of the specimens, showing the importance of considering both these parameters in the milling process planning of PBF maraging steel parts.

Enhancement of the surface properties of selective laser melted maraging steel by large pulsed electron-beam irradiation

Abstract: The present work aimed to decrease the surface roughness of maraging steel (MS) by selective laser melting (SLM) using large pulsed electron-beam (LPEB) irradiation as a post-treatment. The MS samples were fabricated using different combinations of laser power, scanning speed, hatch distance, and build angle. The morphological features, surface roughness, phase content, and corrosion resistance of the MS samples in their as-fabricated (ASF) state were compared after LPEB irradiation. The ASF SLM-MS samples exhibit the presence of partially melted particles that spread over the entire surface and many cracks in both the longitudinal and transverse directions. A higher arithmetical mean height (Sa: 2−17 μm), large variations in Sa measured at various locations, and a strong dependence of Sa on build angle were also observed. Post-treatment by LPEB irradiation removed the partially melted particles, while reflow of the molten mass filled the cracks and voids and facilitated the formation of a uniform surface with a bright metallic finish. This has resulted in a significant decrease in Sa (0.50–4.50 μm) and a smaller variation in Sa measured at different locations. Body-centered cubic α-martensite was the predominant phase for the ASF SLM-MS samples, along with a small fraction face-centered cubic γ-austenite phase. After LPEB irradiation, the martensite was reverted to the austenite phase. The corrosion resistance of the LPEB-irradiated samples was moderately better than that of the ASF SLM-MS samples. The uniform surface morphology, removal of partially melted particles, absence of pores and cracks, decrease in Sa, and moderate improvement in corrosion resistance suggests that LPEB irradiation can be used as a post-treatment for SLM-MS samples.

Microstructure and mechanical properties of maraging 18Ni-300 steel obtained by powder bed based selective laser melting process

Abstract: The purpose of this paper is to maraging 18Ni-300 steel fabricate by powder bed based selective laser melting (SLM) process. Microstructure and mechanical properties of the maraging steel part before and after heat treatment at a slow cooling rate were investigated.,The microstructure of the printed part was observed by optical microscopy and scanning electron microscopy. The phases were determined by X-ray diffraction. The surface roughness of the part was recorded by a profilometer. The tensile properties and microhardness of the parts before and after heat treatment were characterized by an electronic universal tensile testing machine and a Vickers hardness tester, respectively.,Maraging 18Ni-300 steel part comprised of the martensitic phase and a small fraction of austenite phase. After heat treatment, the volume fraction of austenite slightly increased. The surface roughness of the part was about 96 µm. The printed part was dense, but irregular pores were present. The yield strength, ultimate tensile strength (UTS), elongation and Young’s modulus of as-fabricated parts were 554.7 MPa, 1173.1 MPa, 10.9% and 128.9 GPa, respectively. The yield strength, UTS, elongation and Young’s modulus of as-treated parts were 2065 MPa, 2225 MPa, 4.2% and 142.5 GPa, respectively. The microhardness values of surface and cross-section of the as-fabricated part were 407.1 HV and 443.0 HV, respectively. After short-time heat treatment, the microhardness values of the surface and cross-section of the part were 542.7 HV and 567.3 HV, respectively. After long-time heat treatment, the microhardness values of the surface and cross-section of the part were 524.4 HV and 454.8 HV, respectively. The microhardness and tensile strength increased significantly with decreasing elongation due to the changes in phases and microstructure of the parts after heat treatment.,This work studied the effect of heat treatment at 550°C combined with a subsequent slow cooling rate on microstructure and mechanical properties of maraging 18Ni-300 steel obtained by the powder bed based SLM process.

Surface Modification of Additively Manufactured 18% Nickel Maraging Steel by Ultrasonic Vibration-Assisted Ball Burnishing

Abstract: The scope of this research is to characterize and optimize the vibration-assisted ball burnishing of additively manufactured 18% Nickel Maraging steel for tooling applications. We evaluate the suitability of vibration-assisted ball burnishing as an alternative method to post-process additively manufactured tool steel. To do so, we assessed a single pass post-processing technique to enhance surface roughness, surface micro-hardness, and residual stress state. Results show that ultrasonic burnishing after age hardening functionalizes additively manufactured surfaces for tooling applications creating a beneficial compressive residual stress state on the surface. The surface micro-hardness (HV1) varied between 503 and 630 HV1, and the average surface roughness (Ra) varied between 1.31 and 0.14 µm, depending on process parameters with a maximum productivity rate of 41.66 cm2/min making it an alternative approach to functionalize additively manufactured tool components.

Friction and Wear Characteristics of 18Ni(300) Maraging Steel under High-Speed Dry Sliding Conditions.

Abstract: 18Ni(300) maraging steel, which has exceptional strength and toughness, is used in the field of aviation and aerospace. In this paper, using a high-speed tribo-tester, tribological behaviors of 18Ni(300) maraging steel were investigated under high-speed dry sliding conditions. Morphology of the worn surfaces and the debris was analyzed by scanning electron microscope, and the oxides of worn surfaces caused by friction heat were detected by X-ray diffraction. The experiment results reveal that the friction coefficient of frictional pairs declines with increasing load and speed. With the speed and load increasing, oxides of the worn surfaces of 18Ni(300) maraging steel change from FeO to Fe3O4 and the wear mechanism converts from adhesive wear into severe oxidative or extrusion wear.