Thermal fatigue behavior of medium carbon steel with striated non-smooth surface

Imitating the body surface morphology of some biological systems, the medium carbon steel specimens with biomimetic surface were manufactured by laser processing technique. The key feature of this biomimetic surface is to arrange a various spacing of striations to create a non-smooth working surface. The thermal fatigue behavior of specimens with different surfaces, i.e. the specimen with 2-mm-spacing striations, 4-mm-spacing striations and smooth surface, was investigated and compared. It was found that the biomimetic surface has a beneficial effect on improving the thermal fatigue behavior of medium carbon steel, and that the specimen with 2-mm-spacing striations had the highest resistance to thermal fatigue. Compared with the base material, the striation zone on the non-smooth surface was mainly characterized by the refinement of grains and the alteration of microstructure.

Based on such strengthening mechanism as well as the emergence of huge compressive residual stress in the striation zone, the pile–nail effect, which is proposed initially and can be explained by crack retardation mechanism together with stress counteraction mechanism, is the main reason for enhancing the thermal fatigue resistance of medium carbon steel with biomimetic surface.     

Experimental investigations on wear resistance characteristics of alternative die materials for stamping of advanced high-strength steels (AHSS)

Newer sheet alloys (such as Al, Mg, and advanced high-strength steels) are considered for automotive body panels and structural parts to achieve lightweight construction. However, in addition to issues with their limited formability and high springback, tribological conditions due to increased surface hardness and work hardening necessitate the use of alternative microstructurally improved die materials, coatings, and lubricants to minimize the wear-related die-life issues in stamping of advanced high-strength steel grades. This study aims to investigate and compare the wear performances of seven different, uncoated die materials (AISI D2, Vanadis 4, Vancron 40, K340 ISODUR, Caldie, Carmo, 0050A) using a newly developed wear testing device. DP600 AHSS (advanced high-strength steel) sheets were used in these tests. It was concluded that the tested die materials demonstrated higher wear resistance performance when compared to the conventional tool steel AISI D2 die material.     

Elevated temperature wear behaviors of a Co–Mo–Si ternary metal silicide alloy

Wear resistant Co–Mo–Si ternary metal silicide alloy consisting of Co₃Mo₂Si primary dendrite and the interdendritic ductile Co-base solid solution (Co_ss) was designed and fabricated by the laser melting process. The alloy exhibited strong abnormal load- and temperature-dependence of wear under elevated temperature metallic sliding wear test conditions.     

Abrasive wear behaviour of laser clad and flame sprayed-melted NiCrBSi coatings

In this work, the influence of the processing conditions on the microstructure and abrasive wear behaviour of a NiCrBSi hardfacing alloy is analysed. The hardfacing alloy was applied in the form of coatings onto a mild steel substrate (Fe–0.15%C) by different techniques: laser cladding (LC) and flame spraying (FS) combined with surface flame melting (SFM). In both cases, the appropriate selection of the process parameters enabled high-quality, defect-free NiCrBSi coatings to be obtained. The microstructure of the coatings was analysed by scanning electron microscopy (SEM), with attached energy dispersive spectroscopy (EDS) microprobe, and by X-ray diffraction (XRD). Their tribological properties were evaluated by micro-scale ball cratering abrasive wear tests using different abrasives: diamond, SiC and WC. Microstructural characterisation showed that both coatings exhibit similar phases in their microstructure, but the phases present differ in morphology, size distribution and relative proportions from one coating to another. Wear tests showed that in three-body abrasive conditions, despite these microstructural differences, the wear behaviour is comparable for both coatings. Conversely, in two-body wear conditions with diamond particles as the abrasive, it was observed that the specific wear rate of the material is sensitive to microstructural changes. This fact is particularly apparent in LC coatings, in which the zones of the layers with higher proportions of very small hard particles present a lower wear resistance. These results indicate that it is important to have good microstructural control of this material, in order to obtain coatings with an optimized and homogeneous tribological behaviour.     

Dry sliding wear in case-hardened niobium microalloyed steels

Low-carbon steel samples containing a small addition of niobium singly or in combination with nitrogen have been carburized in a natural Titas gas atmosphere at 950 °C and 15 psi gas pressure for different time periods. At the end of the predetermined time period, the specimens were pre-cooled to 860 °C in the furnace and quenched in 10% brine. One set of the quenched specimens was tempered at a low temperature of 160 °C and the other set was sub-zero treated at −195 °C in liquid nitrogen, followed by tempering at the same tempering temperature. Surface hardness was measured by Rockwell hardness testing machine and optical microscopy was performed on etched samples. Using a pin-on-disc type apparatus, wear test was carried out under dry sliding condition to assess the beneficial effect of niobium and niobium with nitrogen on the wear properties of the carburized and hardened low-carbon steels in relation to the resulted surface hardness and microstructures.

It has been found that niobium with or without nitrogen improves the wear resistance under both the heat treatment conditions. Niobium with nitrogen is more effective than niobium in improving the wear resistance. Whatever was the heat treatment condition, the wear rate of the specimens increases for all the steels as the carburizing time increases. It has also been found that samples with sub-zero treatment always have higher wear resistance than that of samples without sub-zero treatment. Niobium singly or in combination with nitrogen has been found as a modifier of the wear mechanism.     

Surface finishing of die and tool steels via plasma-based electron beam irradiation

The plasma-based electron beam (PBEB) irradiation with a maximum diameter of 60 mm has been used to smoothen the workpiece surface. After PBEB irradiation, the machined surface roughness is reduced, and its corrosion resistance is improved. However, for die and tool steels, some craters still exist on the surface after PBEB irradiation, which remain a big problem for the roughness improvement and quality control of the machined surfaces. In this study, four types of work materials have been tested during PBEB irradiation. The effects of non-metallic inclusions (MnS and carbide) and impurities on the generation of craters were investigated. In addition, craters formed on the work materials with different nitrogen concentrations have also been observed after PBEB irradiation. Finally, a case study is given, and it is found that PBEM irradiation can achieve good surface finish of die steel materials.     

In situ observation of wear particle formation on lubricated sliding surfaces

The topography of a Cu surface was monitored in situ during a sliding experiment with lubrication. Fe and steel pins were used as counter-faces and poly alpha olefin (PAO-8) was utilized as lubricant. 3-D images of a specific area within the wear track were acquired after every cycle. These experiments were carried out with a state-of-the-art tribometer, which uses a holographic microscope in immersion to observe the Cu surface online. The results show the evolution of the wear track, with a particular focus on the generation of flake-like wear particles. For the first time, these phenomena are correlated with the dynamic behavior of the surface. It is shown that the generation of wear particles results from various processes that occur on the surface, apart from crack propagation. The areas where wear takes place spread within the wear track during the experiment. Additional investigations of the subsurface structure were performed by means of focussed ion beam cross-sections. These revealed a gradient from nanocrystalline to ultrafine-crystalline grain structure beneath the Cu surface, as well as an orientation effect when sliding in one direction, linking the present work with the results published in the literature. Finally, X-ray photoelectron spectroscopy depth profiles were obtained from the counter-faces. These indicated material transfer from the softer to the harder surface, but no deposition from the sliding pins onto the Cu surface.     

Application of numerical simulation for wear analysis of warm forging die

Warm forging process has better forming precision than hot forging process and has better formability than cold forging. But warm forging die sustains higher temperature and working pressure, the die wear is faster than those of hot forging and cold forging. The purpose of this research is to combine the experimental techniques, wear model and numerical simulation method to predict the wear of warm forging die. The non-isothermal ring compression test was adopted to estimate the friction coefficient in different temperatures and the on-line temperature recording system was setup to correct the heat transfer coefficient of the interface. The wear coefficients in different temperatures were acquired from high temperature wear experiment. Finally, the Archard wear theory and DEFORM, a FEM code, were used to analyze the warm forging of automotive transmission outer-race and predict the die wear condition.     

Friction and wear of MoS2 films on laser textured steel surfaces

Incorporation of solid lubricant into micro-reservoirs produced by Laser Surface Texturing (LST) and its effect on the tribological properties of surfaces under dry friction is studied. The density of the dimple reservoirs and the height of the bulges around them are investigated in terms of the longevity of solid lubricant films burnished on LST steel surfaces. Friction tests were performed using a ball-on-flat device. Optimum density (40-50%) of the dimples is revealed. It is shown that the adhesion of solid lubricant in the space between the dimples is provided by mechanical engagement of particles in the rough surface and by smearing the solid lubricant around the dimples. Best results are obtained with the surfaces that were lapped to half of the height of bulges. Long wear life of burnished film on LST steel surfaces is apparently provided by preservation of thin MoS₂ film around the bulges and by supply of solid lubricant from the dimples to the surface.     

Laser engineered surfaces from glass forming alloy powder precursors: Microstructure and wear

Fe-based metallic glass forming powders have been deposited on mild steel substrates using high power laser cladding. Coatings microstructures have been analysed by scanning- and transmission-electron microscopy and at varying substrate dilutions, have been found to comprise a 100 to 500 nm interdendritic austenitic phase and a dendritic dual-phase of ferrite/martensite. The application of double layer coatings has shown microstructural refinement. This leads to a needle-like microstructure resulting in a nanoindentation tested hardness increase from ~ 11 GPa up to almost 15 GPa. The layers have been subjected to both dry sliding wear and 3-body microscale abrasive wear testing. The dry sliding results show the layers to exhibit excellent wear resistance - particularly at high speed (50 cm s^− 1) with wear rate values of ~ 1 × 10^− 8 mm³/Nm being recorded for the double layer coatings. The single layer coatings reveal a micro-wear mechanism connected with the slip between the ferrite and martensite in the dendritic dual-phase. Microscale abrasive wear testing also reveals that the layers have a good wear resistance, with wear scars exhibiting characteristic material removal by micro-chipping. There is no preferential abrasion of any one phase, nor are track over-lap areas, cracks or pores found to result in varying wear scar dimensions.     

Microstructure, mechanical and wear properties of laser processed SiC particle reinforced coatings on titanium

Laser processing of Ti-SiC composite coating on titanium was carried out to improve wear resistance using Laser Engineered Net Shaping (LENS™) — a commercial rapid prototyping technology. During the coating process a Nd:YAG laser was used to create small liquid metal pool on the surface of Ti substrate in to which SiC powder was injected to create Ti-SiC metal matrix composite layer. The composite layers were characterized using X-ray diffraction, scanning and transmission electron microscopy equipped with fine probe chemical analysis. Laser parameters were found to have strong influence on the dissolution of SiC, leading to the formation of TiSi₂, Ti₅Si₃ and TiC with a large amount of SiC on the surface. Detailed matrix microstructural analysis showed the formation of non-stoichiometric compounds and TiSi₂ in the matrix due to non-equilibrium rapid solidification during laser processing. The average Young's modulus of the composite coatings was found to be in the range of 602 and 757 GPa. Under dry sliding conditions, a considerable increase in wear resistance was observed, i.e., 5.91 × 10^− 4 mm³/Nm for the SiC reinforced coatings and 1.3 × 10⁻³ mm³/Nm for the Ti substrate at identical test conditions.     

铸造碳化钨粉末物性对激光熔覆陶瓷颗粒增强Fe基复合材料耐磨性能的影响

为研究铸造碳化钨粉末物性对激光熔覆陶瓷颗粒增强Fe基复合材料耐磨性能的影响,将不同制备方法和粒径的铸造碳化钨粉末添加到Fe基合金粉中,在45号钢表面进行激光熔覆以获得高硬度和高耐磨的合金化层。利用金相显微镜、扫描电子显微镜(SEM)、X射线衍射仪(XRD)、硬度计分别分析了合金化层的显微组织、物相组成以及显微硬度。利用轮式磨损试验机测试了其常温下的耐磨性能,并进行了比较。结果表明:熔覆层主要由莱氏体组成,碳化钨粉末的制备方法和粒径差异对复合材料的耐磨性能具有重要影响。等离子旋转电极雾化法制备的碳化钨粉末能起到最好的增强耐磨作用,粒径细的碳化钨粉末比粒径粗的粉末增强耐磨效果要好。     

激光相变硬化在模具表面强化中的应用研究

介绍了激光表面相变硬化加工的特点及应用、材料表面激光相变硬化工艺的研究———激光相变硬化机制、激光相变硬化层的影响因素、激光相变硬化过程的温度场研究、激光相变硬化后残余应力场的研究以及激光相变硬化在模具表面强化中的应用     

Monte Carlo simulation of extrusion die life

Due to the high cost of metal forming tools (especially in hot extrusion), one of the major goals in tool design is a longer service life. Estimation and prediction of tool life thus becomes critically important for performance evaluation of the tools. The two most dominant failure mechanisms for extrusion dies (solid, hollow, and semi-hollow dies all taken together) are fracture and wear. In the first part of the paper, a fracture mechanics based fatigue life prediction model is described. A similar treatment is then presented for wear-related failures. Fracture and wear usually coexist as failure modes, and final die breakdown occurs due to the mechanism that becomes dominant. Therefore, a competing fracture–wear model has been later developed to represent the complete die failure situation. Attempt has been made to correlate the stochastic nature of various fatigue and wear related die parameters to die life. Monte Carlo simulation has been used to predict the life distribution of a die for a given set of manufacturing conditions and mechanical properties. In comparison with actual life data from the industry, the simulated life yields very realistic predictions.     

Development of in-situ composite surface on mild steel by laser surface alloying with silicon and its remelting

In the present study, an attempt has been made to develop in-situ iron silicide dispersed surface on mild steel substrate by laser surface alloying with silicon using a high-power continuous wave CO₂ laser. The effect of laser surface remelting of the alloyed surface using argon and nitrogen (with and without graphite coating) as shrouding environment has also been studied. The microstructure of laser surface alloyed mild steel with silicon consists of uniformly dispersed iron silicide in grain refined α-iron matrix with an improved microhardness to 575 VHN as compared to 150 VHN of as-received mild steel substrate. Surface remelting in Ar atmosphere coarsened the microstructure and reduced the area fraction of silicide and hence, reduction in the microhardness to 450 VHN. Surface remelting in nitrogen increased the microhardness to 740 VHN due to the formation of iron nitrides in addition to the presence of silicides. Graphite coating prior to remelting improved the microhardness to 800 VHN due to the presence of martensites along with nitrides and silicides. A maximum enhancement in wear resistance was achieved when remelting was done in nitrogen environment with carbon deposition. The mechanism of wear was found to be predominantly abrasive in nature as compared to adhesive and oxidative in as-received mild steel.     

Mechanical and wear behaviour of high-speed steels reinforced with TiCN particles

Metal matrix composites (MMCs), based on M3/2 high-speed steel (HSS) and reinforced with two different percentages of TiCN (2.5% and 5% by wt), were manufactured following a conventional powder metallurgy (P/M) route: mixing, compacting and sintering. The carbide and base material powders were dry mixed and uniaxially compacted at 700 MPa. After this, vacuum sintering was carried out at 1275 °C, determined as optimal sintering temperature in a previous sinterability study. Sintered materials were characterised by measuring hardness, transverse rupture strength (TRS) and wear behaviour. The study is completed with a microstructural analysis by scanning electron microscopy (SEM) along with energy dispersive X-ray analysis (EDXA).     

Oxidation and wear behaviors of Ti-based thin films

The degradation of Ti-based coatings is known to be due to the formation of titanium oxide (TiO₂) at their surfaces. In this study, wear and thermal oxidation behaviors of various magnetron sputtered Ti-based thin films were studied after static oxidation and sliding wear. The oxidized surfaces after the static oxidation and the wear debris generated from pin-on-disc wear tests with alumina ball were characterized to identify the compounds, particularly titanium oxides, to gain a better understanding of the tribochemical reactions. The coatings that were examined include TiN, TiCN (N rich), TiCN (C rich), TiAlN, AlTiN, TiSiN, and TiCNO thin films. These coatings were characterized using Raman spectroscopy, scanning electron microscopy, and X-Ray diffractometer. The results show that TiSiN and AlTiN have the highest oxidation resistance, comparing with other coatings. As for the analyses of wear debris, all of the Ti-based coatings are worn by the mechanism of forming TiO₂, except AlTiN. AlTiN is worn by ploughing wear.     

奥氏体中锰钢的磨料磨损性能

在ML-100型销-盘式磨料磨损试验机上,通过不同钢种之间的对比试验,研究了奥氏体中锰钢（BTW钢）石英砂磨料及煤矸石磨料下的磨损性能,并采用SEM分析了其磨损机制。结果表明,在高硬度的石英砂磨料中,BTW钢加工硬化明显,有效硬化层深度达900 μm,耐磨性能优于其他钢种,而在质地较软的煤矸石磨料中,其耐磨性降低;BTW钢在不同磨料下的磨损机制均为犁削,但形貌差异较大,石英砂磨料下磨损表面较为均匀,犁沟深度较浅、宽度较窄,脊缘部分较薄,脊缘在反复磨损中断裂成屑的数量较多,而煤矸石磨料中,犁沟存在于整个磨损表面且变形较小,几乎没有发现切削存在。     

Microstructure and wear property of laser-clad Co3Mo2Si/Coss wear resistant coatings

Novel wear resistant Co₃Mo₂Si/Co_ss coatings consisting of a microstructure of hard and strong Co₃Mo₂Si intermetallic phases embedded in the ductile Co-base matrix were fabricated on austenite stainless steel by the laser cladding process from the Co-Mo-Si powder blend precursor. The microstructures of the coatings were characterized by optical microscopy (MO), X-ray diffraction (XRD), and scanning electron microscopy (SEM) with an energy-dispersive X-ray analysis (EDX). The wear resistance of the coatings was evaluated in a dry sliding wear test condition at room temperature. Results indicated that the laser-clad Co₃Mo₂Si/Co_ss coatings exhibited very excellent wear resistance against abrasive and adhesive wear.