Microstructure and mechanical properties of laser welded dissimilar DP600/DP980 dual-phase steel joints

The use of dual phase (DP) steels in the automobile industry unavoidably involves welding and dynamic loading. The aim of this investigation was to evaluate the microstructural change and mechanical properties of laser welded dissimilar DP600/DP980 steel joints. The dissimilar joints showed a significant microstructural change from nearly full martensite in the fusion zone (FZ) to the unchanged ferrite-martensite dual-phase microstructure in the base metal. The welding resulted in a significant hardness increase in the FZ but the formation of a soft zone in the heat-affected zone (HAZ). The dissimilar welded joints were observed to exhibit a distinctive unsymmetrical hardness profile, yield-point-like phenomenon, and single-stage work hardening characteristic, with yield strength and work hardening rate lying in-between those of DP600 and DP980 base metals, and ultimate tensile strength equivalent to that of DP600 base metal. Although the welded joints showed a lower fatigue limit than the base metals, the fatigue life of the welded joints at higher stress amplitudes was almost the same as that of the DP600 base metal. The welded joints failed in the soft zone at the DP600 side under tensile loading and fatigue loading at the higher stress amplitudes. Fatigue crack initiation occurred from the specimen surface and crack propagation was characterized by typical fatigue striation together with secondary cracks.     

Holographic testing of possible mechanical effects of laser cleaning on the structure of model fresco samples

Custom-made fresco samples covered in artificial crust were laser cleaned, and the consequent mechanical effects were tested with digital holographic speckle pattern interferometry (DHSPI). DHSPI proved capable of detecting the structural alterations in the fresco. It was shown that the laser cleaning process caused no structural damage to the intact samples and has not worsen the damages in case such were originally present. Quantitative evaluation of the fringe patterns revealed that the potential risk of laser cleaning is negligible on the mechanical structure of the frescos. Consolidation with Remmers KSE 300 has improved the laser cleaning efficiency in the UV.     

The influence of mechanical and CO2 laser cut-edge characteristics on the fatigue life performance of high strength automotive steels

The cut-edge characteristic properties of automotive structures formed during the mechanical blanking and laser-cutting processes significantly influence fatigue life performance. This factor is becoming increasingly important as S355MC and DP600 high strength steels (HSS) grades under investigation exhibit an increased sensitivity to fatigue cracks initiating from cut-edge regions. It was determined that by manipulating the critical cutting process parameters, clearance in terms of mechanical blanking, and by controlling the interrelationship between power and traverse cutting speed during the laser cutting process can result in optimised fatigue lives being achieved. Optimal fatigue lives were attained by minimising the cut-edge surface damage and by controlling the near edge microstructural deformations during each cutting process. It is the critical significance of fatigue which is the limiting factor towards being able to effectively downgauge steel grades used as automotive chassis and suspension components. This research is some of the first that considers the influence of mechanical and laser cut-edge surface quality and internal properties, which has then been partnered to the critical fatigue performance of HSS grades.     

INVESTIGATION OF INFLUENCE OF HYDROGEN ATTACK ON FATIGUE PROPERTIES OF PRESSURE VESSEL STEELS

1.IntroductionLowalloysteelsexposedtohydrogenatmoderatepressuresandtemperaturessufferalossinmechanicalandfatigueproperties.Thisphenomenon,calledhydrogenattack(HA)['],isduetothemethanegeneratedbychemicalreactioll:abC. 2H2~ac ACHe.Methaneformationmayoccureitheratafreesurface(surfacedecarburization)oratinternalsiteinthemetal(internaldecarburiatioll)suchasgrainboundariesanddefects.Internalmethaneisunabletodiffuseoutofthesteel,anditmaybllildupintheinterllalvoidsandgrainboundaries,growandcoalesce…     

Laser-induced thermal damage detection in metallic materials via acoustic emission and ensemble empirical mode decomposition

A detection method of laser-induced thermal damage–surface burn, rehardening and residual stress, was studied in this work. Artificial thermal damage was produced to various steels, e.g., AISI 1045, ASTM A36 and AISI 304, by virtue of laser irradiation. The aim of the present work is to identify thermal damage through sensor and feature extraction techniques. Acoustic emission(AE) sensor and ensemble empirical mode decomposition(EEMD) method were employed for this purpose. A de-noising method was proposed to eliminate noises from original AE signals, based on EEMD. Quantified thermal damage features were obtained. Results evidenced a strong correlation between AE features, i.e., RMS value of the reconstructed acoustic emission signal, and surface burn, residual stress value, as well as hardness of steels. The present work could be used as a potential and indirect approach for thermal damage detection.     

Effect of the cutting process on the fatigue behaviour of press hardened and high strength dual phase steels

Press hardened 22MnB5 steels are steady broadening their applications in vehicles since they allow to meet the increasing demands for weight reduction and safety standards. Press hardened parts have been typically applied as structural reinforcements where high rigidity and crash resistance are required. However, the need for further weight reductions in vehicles leads to potential applications in chassis areas, where parts must face up to cyclic loads. The fatigue behaviour of the press hardened 22MnB5 steel and the effect that post-forming processes (such as trimming or punching that usually follows press hardening) on the fatigue behaviour is scarcely known. Thus, the aim of this work is to analyse the fatigue behaviour of 22MnB5 press hardened steels cut using different strategies. The fatigue limit has been obtained in tensile samples of 22MnB5 with an Al–Si coating. Samples were cut by laser and shearing with two different clearance values. Results are compared to those obtained with a high drawing quality mild steel and a dual phase steel with a tensile strength of 1000 MPa, DP1000. It is shown that the fatigue behaviour of press hardened 22MnB5 steels and DP1000 is governed by the defects introduced in cut edges, while the behaviour of the mild steel is almost independent of the cut edge quality. This finding indicates that high strength steels are markedly sensitive to pre-existent defects, such as burr, cracks at the cut edge or surface cracks, and they can be considered as low damage tolerant steels. On the contrary, mild steels are highly damage tolerant, i.e. the fatigue life is mainly independent of the initial defect size. In press hardened 22MnB5, even when cut edges have a high surface quality, the fatigue limit is still limited by the presence of cracks in the brittle Al–Si coating. Crack propagation due to the low damage tolerance capacity of the press hardened steel has been successfully rationalized through a fracture mechanics approach. Thus, if coated press hardened 22MnB5 steels are to be applied in vehicle components subjected to cyclic loads, they must be designed following fracture mechanics concepts to state the safe loading conditions for adequate fatigue resistance.     

Reviews on factors affecting fatigue behavior of high-Mn steels

A variety of factors affect the fatigue behavior of high-Mn steels, which include both extrinsic (i.e., loading type, R ratio, specimen type, surface condition, temperature, and environment) and intrinsic (i.e., chemical composition, grain size, microstructure, stacking fault energy) factors. Very often, the influence of extrinsic factors on the fatigue behavior is even greater than that of intrinsic factors, misleading the interpretation of fatigue data. The metallurgical factors influence the initiation and propagation behaviors of fatigue by altering the characteristics of slip that is prerequisite for fatigue damage accumulation. It is however not easy to separate the effect of each factor since they affect the fatigue behavior of high-Mn steels in complex and synergistic way. In this review, the fatigue data of high-Mn steels are summarized and the factors complicating the interpretation are discussed.     

Surface roughness monitoring using computer vision

In this paper, a comparative experimental surface roughness measurement method based on the speckle pattern caused by a laser beam on a rough surface is presented. Surfaces with known surface roughness are measured using this method to obtain a calibration curve. This information is used to measure surfaces produced by surface grinding, and the results compared with stylus measurements. The online use of this method for tracking the roughness of a workpiece being processed on a surface grinder and to monitor the condition of the grinding wheel is also reported.     

Laser-structured grinding tools – Generation of prototype patterns and performance evaluation

Laser ablation with ultrashort pulsed lasers enables high geometrical flexibility and allows for fabrication of micro scale structures in e.g. ultrahard materials that are hardly possible with any other process. In this work a novel laser structuring method employing picosecond lasers is proposed and applied to produce various micro patterns on the surface of hybrid bonded (metal-vitrified) CBN grinding tools. Grinding performance and wear behaviour of the structured tools are tested and compared to the results of a non-structured tool.It is found that the type of surface pattern has a strong influence on the grinding results. The laser structured tools generally allow for a reduction of grinding forces by up to 54%. However, depending on the pattern geometry, a significant increase in workpiece surface roughness and elevated wear rates (in terms of surface roughness) have to be taken into consideration. Despite this fact, the study does not reveal a significant increase in radial profile wear of the structured tools.     

EXPERIENCE WITH THERMOMECHANICAL FATIGUE UNDER SERVICE-TYPE LOADING

The thermomechanical fatigue behaviour of different high temperature alloys has been investigated and is under investigation respectively. The creep-fatigue behaviour of heat resistant steels was investigated by long-term service-type strain cycling tests simulating thermomechanical fatigue (TMF-) loading conditions at the heated surface of e.g. turbine rotors. Single-stage as well as three-stage cycles leads to similar results at the application of the damage accumulation rule. Life prediction which simulates typical combinations of cold starts, warm starts and hot starts has been established successfully for isothermal service-type loading and will be exceeded for thermomechanical loading. Long-term thermomechanical fatigue testing of Thermal Barrier Coating systems show typical delamination damage. An advanced TMF cruciform testing system enables complex multiaxial loading.     

Laser ablation of titanium alloy under a thin and flowing water layer

Underwater laser ablation has become an alternative machining process that is able to reduce the thermal damage in work materials caused by lasers. However, the disturbance of water to the laser beam is a crucial concern for the ablation performance in water and cut surface quality obtained. In this study, a new laser ablation technique has been proposed, in which a waterjet was applied to impinge the top workpiece surface in order to form a thin and flowing water layer. With the assist of such water layer during the laser ablation, the redeposition and heat-affected zone can be minimized. Titanium alloy (Ti–6Al–4V) selected as a work sample was grooved by using a nanosecond-pulse laser under different machining conditions. The cut geometry and heat-affected zone were observed and analyzed to justify the process performance. The metallurgical change and cracks that occurred on and underneath the groove surface were also investigated in this study. The experimental results revealed that a clean cut with less thermal damage can be obtained when the workpiece was ablated by a laser under the flowing water layer. In addition, a narrower and deeper groove can be fabricated when a higher waterjet flow rate was applied. The laser ablation under the flowing water layer developed in this study could be a potential method for machining titanium alloy or even other thermal-sensitive materials.     

固溶氮原子对不锈钢单轴及多轴低周疲劳特性的影响

对３１６Ｌ和３１６ＬＮ不锈钢进行了单轴拉压及多轴非比例加载低周疲劳实验研究,分析了固溶的氮原子对单轴及多轴非比例加载低周疲劳特性及其微结构的影响。结果表明,固溶的氮原子增大了不锈钢铁的单轴循环软化程度及非比例循环附加强化程度,延长了不锈钢的单轴拉压低周疲劳寿命,但却缩短了不锈钢的多轴非比例加载低周疲劳寿命,固溶的氮原子对单轴及多轴低周疲劳密度位错结构的形成具有明显的抑制作用,利用Ｍｏｅｓｓｂａｕｅ     

Contribution of research to the understanding and mitigation of environmentally assisted cracking in structural components of light water reactors

Environmentally assisted cracking (EAC) is a potential threat to the safety and integrity of water-wetted components in operating water-cooled nuclear power plants. Two forms of EAC are commonly distinguished, depending on the form of loading contributing to damage: stress corrosion cracking (SCC) and corrosion fatigue. A number of instances of in-service degradation due to EAC have occurred in operating plants worldwide, often leading to unplanned plant outages. Understanding the causes of EAC is essential to minimise the loss of plant availability due to its occurrence and to avoid the possibility of catastrophic failure, for example, if a crack grew to a critical size in a major pressure boundary component. This paper will describe some examples of these phenomena in the main materials of construction of pressure boundary and other critical components in pressurised and boiling water reactors (BWRs). Over the last several decades, substantial research programmes have been carried out in a number of laboratories worldwide, aimed at further understanding of the processes leading to EAC to manage occurrences in plant and minimise future failures. Selected areas of research on EAC in light water reactor environments are discussed. Corrosion fatigue in low-alloy pressure vessel steels was the subject of considerable attention in the 1980s and early 1990s because of its potential threat to pressure vessel integrity and the publication of data, suggesting that there is a major influence of environment on fatigue crack growth in some laboratory tests. The author’s research provided insight into the conditions under which the major environmental effects occur and contributed to the development of an ASME Code Case for pressurised water reactor (PWR) conditions which provided a means of screening based on steel sulphur content and loading conditions. More recently, the research focus in this area has moved to austenitic stainless steels, again providing support to Code Case development and furthering mechanistic understanding. A recent review of knowledge gaps for EPRI provides a basis for future research on environmentally assisted fatigue and will inform the development of new assessment methodologies. A key area of the current study concerns differences in loading conditions between specimens in laboratory tests and plant components subject to transient loading. In the case of SCC, stainless steels have shown the greatest propensity to cracking in BWRs, while Alloy 600 has been a major cause of in-service failures in PWRs, both on the primary side, as recognised by Coriou in the early 1960s, and in secondary environments where a number of different corrosion-related failure processes have been identified. High-strength alloys, such as Alloy X-750 used for fastener applications, have also caused failures in both reactor types. For austenitic materials, SCC susceptibility is enhanced by irradiation, resulting in failures in core internals components. Ferritic stainless steels also undergo SCC under some specific circumstances but are generally more resistant than the lower chromium austenitic materials.     

Influence of substrate microstructure on the contact fatigue strength of coated cold-work tool steels

The contact fatigue behavior of three microstructurally distinct tool steels coated with a physical vapor deposited TiN film is studied. Substrate microstructural differences come from variations in either chemical composition or processing route. Experimental procedure is based on determining critical applied loads and pressures, under both monotonic and cyclic spherical indentation loading conditions, for emergence and evolution of distinct damage modes at the coating surface: circumferential cracking, cohesive spallation and interfacial decohesion. Experimental results indicate that all coating/substrate systems evaluated are susceptible to mechanical degradation associated with repetitive contact loading. This is clearly discerned from the fact that some damage mechanisms, such as cohesive spallation at the coating and adhesion failure at the interface, are exclusively observed under cyclic loading. Substrate microstructure effects are evidenced by consideration of coating detachment as the critical damage mechanism. In this regard, crack nucleation resistance of primary carbides is pointed out as the main reason for the distinct response against decohesion observed under cyclic contact loads. Hence, finer and tougher, as well as less irregular and more homogeneously distributed primary carbides are pointed out as key microstructural features for enhancing contact fatigue strength of coated cold-work tool steels.     

Laser-assisted replication of large-area nanostructures

We proposed and demonstrated a high-throughput fabrication method for large-area nanostructured polymers. The mold used consists of a quartz substrate and a nanostructured diamond-like carbon (DLC) thin film. A laser is irradiated from the back of the mold and only the DLC surface is directly heated. Then the surface of a polymethyl methacrylate (PMMA) film pressed by the mold is melted and the nanostructures are replicated. In this method, replication can be achieved with a low amount of heat and a short cycle time compared with conventional thermal replication. The effects of the laser power density, irradiation time, and environmental temperature on the replication area were experimentally investigated via the spot irradiation of a laser. Furthermore, the temperature distribution around the surfaces of the mold and polymer was investigated by performing numerical simulations. By scanning the laser, we successfully demonstrated the replication of a 500-nm-pitch pattern on a PMMA film with an area of 10 × 10 mm² in about 10 s. This technique is expected to lead to the high-throughput and low-energy fabrication of large-area nanostructured optical films.     

MULTIAXIAL CREEP-FATIGUE LIFE EVALUATION UNDER PROPORTIONAL LOADING

A new method was proposed for the multiaxial creep-fatigue life evaluation under proportional loadings. Because this method was derived from the strain range partitioning method with a multiaxiality factor, it was possible to consider the influence of both creep-fatigue interaction and multiaxial stress state on fatigue life. In order to predict the combined axial-torsional fatigue life the damage under combined loading was defined as linear summation of the damages under axial loading and torsional loading. Axial-torsional creep-fatigue tests were carried out using tubular specimens of 316LC austenitic stainless steel and the ferritic rotor steel. This rotor steel was developed for the permanent magnet type eddy current retarder in heavy trucks. Experimentally obtained lives of both steels were well corresponded with the lives predicted by the proposed method. It was found that the proposed method was effective in multiaxial fatigue life evaluation under proportional creep-fatigue Ioadings.     

回火温度对4Cr5MoSiV1钢和8407钢热疲劳性能的影响

采用自约束热疲劳试验方法,研究对比了不同回火温度对4Cr5MoSiV1、8407钢热疲劳性能的影响。观察分析了热疲劳裂纹形貌,采用热疲劳损伤因子定量地研究了两种钢的热疲劳过程,结果表明,两种钢的热疲劳裂纹萌生发生在冷热疲劳循环次数为100-200次之间,在1600次冷热循环前,二者的热疲劳损伤程度无明显的差别,在1600次冷热循环后,8407钢的热疲劳损伤程度低于4Cr5MoSiV1钢。在较低的回火温度条件下,8407钢的热疲劳抗力稍优于4Cr5MoSiV1钢。而在高温回火时8407钢的热疲劳抗力高于4Cr5MoSiVl钢。分析了这两种钢的热疲劳机制,指出决定材料热疲劳裂纹抗力的是钢的热稳定性和钢的强度或硬度。     

Effect of Residual Stress on Fatigue Failure of Carbonitrided Low-Carbon Steel

The effect of residual stress on fatigue behavior and mechanisms of carbonitrided AISI 1015 steel under uniaxial cyclic loading has been experimentally studied. By progressive removal of thin surface layers using an electropolishing technique and subsequent residual stress measurements using an x-ray diffraction technique, the compressive residual stress at the surface was approximately 900 MPa. The stress decreased toward the center, and became stable tensile residual stress of approximately 20 MPa. The fatigue resistance of carbonitrided AISI 1015 steel was higher than that of AISI 1015 steel due to the presence of compressive residual stress in case layer. The fatigue limit of AISI 1015 steels with and without carbonitriding was 340 and 300 MPa, respectively. Subsurface cracks initiated at the case-core interface, i.e. approximately 400 μm from the surface. With increasing number of stress cycles, the subsurface cracks coalesced and propagated intergranularly through the case layer. After some incubation cycles, the subsurface cracks reached the surface of specimen, and became a main crack. During this stage, the stress increased, and caused the formation of voids in core material. Consequently, the crack propagated through the core material, interacted with voids, and caused complete fracture.     

Microstructure of reaction zone in WCp/duplex stainless steels matrix composites processing by laser melt injection

The laser melt injection (LMI) process has been used to create a metal matrix composite consisting of 80μm sized multi-grain WC particles embedded in three cast duplex stainless steels. The microstruture was investigated by scanning electron microscopy with integrated EDS and electron back-scatter diffraction/orientation imaging microscopy. In particular the search of the processing parameters, e.g. laser power density, laser beam scanning speed and powder flow rate, to obtain crack free and WC_p containing surface layer, has been examined. Before the injection of ceramic particles into remelted surface layer, the influence of processing parameters of laser surface remelting on the microstructure and properties of selected duplex steels was also investigated. Although after simple laser surface remelting the austenitic phase is almost not present inside remelted layer, in the case of LMI the austenite was observed in vicinity of WC particles, due to increase of carbon content acting as austenite stabilizer. The diffusion of carbon in the reaction zone results also in a formation of W₂C phase in the neighborhood of WC particles with a strong orientation relationship between them. The maximum volume fraction of the particles achieved in the metal matrix composite layer was about 10% and a substantial increase in hardness was observed, i.e. 575 HV0.2 for the matrix with embedded particles in comparison to 290 HV0.2 for untreated cast duplex stainless steels.     

腐蚀疲劳裂尖材料损伤研究

金属环境断裂过程依赖于材料,应力与介质参数间的复杂交互作用,这些交互作用的澄清是实现材料断裂控制的重要基础,而裂纹尖端单元损伤过程的研究则是联接微观机制与宏观断裂规律的桥梁。通过对钝化和活化典型腐蚀体系下恒速长大腐蚀疲劳裂纹受力学与电化学参数扰动后的扩展动力学响应规律的系统研究,并结合多种其它分析手段,考察了不同腐蚀因素对裂尖材料的损伤作用机制。在实验基础上,对腐蚀导致裂纹闭合变化,裂纹扩展过载延