Desensitization of sensitized 304 stainless steel by laser surface melting

Laser surface melting was used to desensitize the surface layer of sensitized 304 stainless steel. The degree of sensitization was determined quantitatively for sensitized and sensitized then laser surface melted samples from the modified ASTM-262 practice E test to be 45% and 0%, respectively. Grain-boundary melting which occurs in the heat-affected zone is believed to contribute the desensitization in the solid. X-ray diffraction results did not show any phase transformation in the melted layer or in the heat-affected zone. The results of the tensile tests indicate that sensitized stainless steel regains its corrosion resistance properties and, in addition, its mechanical properties seem to be enhanced by the laser surface melting.     

Pitting corrosion resistance of laser surface alloyed 304 stainless steel

Abstract

The pitting corrosion resistance of AISI 304 stainless steel (SS), which was laser melted in both nitrogen and argon atmosphere, was studied using the potentiodynamic anodic polarisation method. An attempt was made to introduce nitrogen onto the surface layers by melting in nitrogen atmosphere and argon atmosphere using a continuous wave CO₂ laser as the heat source at a power output of 3·06 kW at the laser head. The pitting corrosion resistance was determined by measuring the critical pitting potential during anodic polarisation. Secondary ion mass spectrometry (SIMS) was carried out on the laser melted surface to characterise the chemical composition. It was found that the pitting corrosion resistance of 304 SS was improved when laser surface melting was carried out in argon and was improved further when melting was carried out in nitrogen atmosphere. However, the improvement in pitting corrosion resistance in laser melted material was observed only in the reabraded condition and the pitting corrosion resistance in the as melted condition was lower than for the unmelted 304 SS. The improvement in localised corrosion behaviour was attributed to the increase of nitrogen content which was incorporated onto the surface layer during laser surface melting and this was confirmed using SIMS.

MST/1530     

Hardening of 316L stainless steel by laser surface alloying

The goal of this study is to investigate different hardening routes for 316L stainless steel by laser surface alloying. We have investigated the formation of iron-chromium carbides by SiC or carbon incorporation, the alloying with submicronic particles of TiC and the precipitation of titanium carbide from mixtures of Ti and SiC. For each hardening route we present the microstructures and the hardness of the processed surface alloys and the conditions leading to the best compromise between highest hardness, best homogeneity and lowest occurrence of cracks. From these results it can be reasoned that hardening by iron-chromium carbides is the best hardening route and that this surface alloy might be a good candidate for tribological applications.     

Surface interactions of a W-DLC-coated biomedical AISI 316L stainless steel in physiological solution

The corrosion stability of a W-DLC coated surgical AISI 316L stainless steel in Hanks’ solution has been evaluated. Particle induced X-ray emission (PIXE) measurements were performed to evaluate the incorporation of potentially bioactive elements from the physiological solution. The film structure was analyzed by X-ray diffractometry and micro-Raman spectroscopy. The wear behavior was assessed using the sphere-on-disc geometry. The in vitro biocompatibility of the W-DLC film was evaluated by cytotoxicity tests. The corrosion resistance of the stainless steel substrate decreased in the presence of the PVD layer. EIS measurements suggest that this behavior was closely related to the corrosion attack through the coating pores. PIXE measurements revealed the presence of Ca and P in the W-DLC film after immersion in Hanks’ solution. This result shows that the PIXE technique can be applied to identify and evaluate the incorporation of bioactive elements by W-DLC films. The film showed good wear resistance and biocompatibility.     

激光功率对不锈钢表面清洗效果影响的研究

利用激光清洗技术对不锈钢表面进行清洗试验,研究不同激光功率(300W、400W、500W)对清洗效果的影响。通过SEM和EDS分析不锈钢表面清洗前后的表面形貌及成分分布;利用白光干涉仪检测不锈钢表面粗糙度及清洗厚度。结果表明,随着激光功率的增加,不锈钢表面氧化物逐渐分解剥落,清洗厚度不断加深,在500W时达到50pm,并且造成基体部分损伤;粗糙度值先降低后增加,在400 W时达到最低值0.38 pm。激光清洗的清洗阈值近似为3.96×103 W/cm2,基体损伤阈值在5.52×103 W/cm2左右,不锈钢表面氧化层在400W时达到最佳激光清洗效果。     

An impedance study of two types of stainless steel in Ringer physiological solution containing complexing agents

The effect of complexing agents EDTA and citric acid on the electrochemical behaviour of AISI 304 and orthopaedic stainless steels in Ringer physiological solution was investigated by potentiodynamic polarization method and electrochemical impedance spectroscopy (EIS). The content of Mo has a pronounced effect on the corrosion resistance, as evident by the broader passive range of the orthopaedic stainless steel containing Mo. The addition of complexing agents induces significant changes in polarization and impedance characteristics, i.e., the shift of corrosion and breakdown potentials in a more negative direction, an increase in current density, and a significant decrease in charge transfer resistance. The results were interpreted by the formation of soluble complexes of metal ions with chelating agents, especially EDTA, which suppressed the formation of the outer Fe(III) layer of the passive film. The impact of complexing agent on the electrochemical parameters was found to be related to its concentration in electrolyte and the stability constant of the complex formed with the related metal ion.     

模具钢表面激光沉积316L不锈钢的组织转变及差异性

为了揭示Cr系注塑模具钢表面采用异种材料沉积过程的组织演变机制,在两种模具钢表面沉积316L不锈钢,利用扫描电镜、电子探针、X射线衍射仪等表征微观组织及硬度特征,并结合熔凝动态过程及Schaeffler相图分析界面及沉积层区组织转变机理。结果表明:由于基体元素组成及组成相分布的差异,导致在沉积层界面的组织转变呈现明显差异性。高Cr莱氏体钢组织过渡区厚度为200μm,过渡区组织形态和组成相明显区别于沉积层其他区域,碳化物在过渡区经历了先局部聚集性长大、之后枝晶间均匀分布等复杂变化;而P20钢沉积层的组织过渡区厚度小于20μm,组织以枝晶方式生长且与沉积层非过渡区连通,同时枝晶内发现了弥散分布的颗粒状碳化物。在熔池中C,Cr,Ni等元素的动态变化作用下,两种钢沉积层的非过渡区组成相也呈现明显差异,高Cr莱氏体钢和P20钢沉积层的基体相分别为奥氏体相与马氏体相,由此导致其硬度值变化范围分别为295~325HV_0.2和500~575HV_0.2。     

316L不锈钢选区激光熔化成型非水平悬垂面研究

为了提高选区激光熔化直接成型非水平悬垂面的能力,通过实验研究了影响悬垂面极限成型角度的工艺因素.实验使用316L不锈钢粉末,设计了不同倾斜角度的悬垂平面模型和悬垂曲面模型.结果表明:提高预置粉末密度可以提高悬垂面成型的极限角度;在25 μm的成型厚度下,激光扫描能量输入在0.15～0.2 J/mm时悬垂面成型精度最高,...     

响应面分析法优化不锈钢激光切割工艺参数

为了获得良好的不锈钢激光切割质量,确定合理的切割工艺参数,本文以3 mm厚304不锈钢为研究对象,采用响应面法进行试验方案的设计和分析,利用超景深显微镜进行了试样表面切缝宽度、表面纹理最大峰值、挂渣量的测量,利用最小二乘法进行数据处理,研究了激光功率(X₁)、切割速度(X₂)、离焦量(X₃)以及辅助气体压力(X₄)对不锈钢切割表面切缝宽度、表面纹理最大峰值、挂渣量的影响规律,并基于响应面法得到了3个响应目标的预测函数.实验结果表明:X₂=2.48 m/min,X₃=-1.05 mm,X₄=1 MPa时,随着激光功率的增加,切缝宽度不断增大;X₁=300 W, X₄=1.2 MPa时,随着切割速度的加快切缝宽度逐渐减小,随着离焦量的增大切缝宽度先减小后增大;X₁=300 W,X₃=0 mm,X₄=1.40 MPa时表面纹理的最大波峰值RZ随着激光功率、离焦量以及切割速度的增大先减小后增大。以切缝宽度最窄、表面纹理最大峰值最小、挂渣量最少为响应目标确定了3 mm厚304不锈钢激光切割的最佳工艺参数为X₁=365.86 W,X₂=2.75 m/min,X₃=0 mm,X₄=1.4 MPa。试验验证发现:切缝宽度、表面纹理最大峰值、挂渣量的预测误差应分别控制在8.4%~12.7%、21%~24.9%、16.7%~19.5%。     

飞秒激光不锈钢表面陷光微结构的制备与性能研究

利用飞秒激光在高真空环境下,在316L不锈钢表面两次交叉扫描制备了周期性微纳结构,并研究了微纳结构对波长范围200~900nm的光波的吸收增强能力。样品表面微结构形貌与成分采用扫描电子显微镜(SEM)和X射线衍射仪测试。第1次扫描采用高能流激光,获得了微米级锥状钉结构,表面覆盖了典型的激光诱导周期性表面结构(LIPSS)。然后将样品旋转90°,采用能流为0.02J/cm2的激光进行第2次扫描,路径与第1次扫描相交。第1次扫描的结构中的LIPSS被第2次低能流激光打断纳米颗粒,从而与锥状钉结构结合形成双尺度微结构。反射率测试结果表明,这种双尺度微结构表面的平均反射率约为2.28%,为光滑表面平均反射率的3.42%。结合XRD分析结果,不锈钢表面获得强陷光性能主要归因于飞秒激光制备的微结构。     

Modelling surface ridging in ferritic stainless steel

Abstract

A new model is proposed to account for the occurrence of surface ridging (parallel corrugations) in ferritic stainless steel. In the absence of notable texture component clustering, it is shown that local anisotropy in plastic behaviour can still occur on a scale considerably larger than the grain size. The construction of a simple finite element model incorporating plasticity data from microtexture measurements demonstrates that parallel surface corrugations can be simulated during uniaxial tensile straining. It is shown that the corrugation profile is the result of the superimposition of a number of differential transverse strains that contribute to the overall deformation induced bending.     

Anticoagulant surface of 316 L stainless steel modified by surface-initiated atom transfer radical polymerization

Polished 316 L stainless steel (SS) was first treated with air plasma to enhance surface hydrophilicity and was subsequently allowed to react with 2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane to introduce an atom transfer radical polymerization (ATRP) initiator. Accordingly, the surface-initiated atom transfer radical polymerization of polyethylene glycol methacrylate (PEGMA) was carried out on the surface of the modified SS. The grafting progress was monitored by water contact angle measurements, X-ray photoelectron spectroscopy and atomic force microscopy. The polymer thickness as a function different polymerization times was characterized using a step profiler. The anticoagulative properties of the PEGMA modified SS surface were investigated. The results showed enhanced anticoagulative to acid-citrate-dextrose (ACD) blood after grafting PEGMA on the SS surface.     

Grain boundary engineering of 304 austenitic stainless steel by laser surface melting and annealing

In order to improve the intergranular corrosion resistance of 304 stainless steel, laser surface remelting experiments were conducted using a 2 kW continuous wave Nd: YAG laser. The grain boundary character distribution (GBCD) and microstructures of the materials were analyzed using EBSD, SEM and OM. The experimental results showed that combination of laser surface melting and annealing on 304 stainless steel resulted in a high frequency of twin boundaries and consequent discontinuity of random boundary network in the materials, which led to an improvement of resistance to intergranular corrosion. The maximum CSL density could reach 88.6% under optimal processing conditions: 1220 K and 28 h.     

Effect of surface modification on laser direct joining of cyclic olefin polymer and stainless steel

Surface modification pretreatment on the laser-bonded joint between a cyclic olefin polymer (COP) and stainless steel (SUS304) was studied to determine its effect on improving the laser-bonded joint strength. The joint strength between the surface-modified COP and SUS304 was significantly improved compared with that of an equivalent un-treated joint. This improvement is caused by the generation of oxygen functional groups on the COP surface resulting in the improved adhesion of these groups with the oxide film formed on the SUS304 surface.As for the surface pretreatment of COP, the generation of bubbles in ultraviolet (UV)–ozone processing due to thermal degradation of the COP was more noticeable than with plasma pretreatment. Excessive surface modification of the COP, causing a decrease in joint strength was found to correlate with the surface energies of COP and SUS304.     

Hardfacing of stainless steel with laser melted colmonoy

Stainless steel substrates were hardfaced with laser melted Colmonoy applying a continuous wire injection technique using a CO₂ laser. The microstructures of single pass laser clad deposits were examined and related to workpiece traverse velocity at constant laser output. Particular attention was paid to the effect of dilution of the cladding alloy due to convectional mixing with the partially melted substrate material. The solidification microstructure of the rapidly cooled cladded layer consisted predominantly of nickel rich cellular dendrites and interdendritic lamellar eutectic borides.     

High-brightness laser welding of thin-sheet 316 stainless steel

Photolytic iodine laser (PIL), a new industrial laser in the market, offers much higher brightness than existing Nd:YAG and CO₂ lasers. PIL has also a unique wavelength (1315 nm) that has not yet been tested for welding applications. In this work, the capabilities of PIL for precision seam welding of 0.1-mm thick sheet of AISI 316 stainless steel in the lap-joint configuration were evaluated. The weld performance data of PIL laser were compared with Nd:YAG and CO₂ lasers. The astounding benefits of PIL weld are narrow seam, extremely fine solidification cell structure, fully austenitic microstructure, and small heat-affected-zone (HAZ). These benefits are attributed to the PIL's high brightness that in turn enables achieving small spot size and energy transport through plasma rather than by heat conduction. In contrast, the welds produced by Nd:YAG and CO₂ lasers exhibited wider seams, coarser solidification structures, duplex microstructures of austenite and ferrite, and larger HAZ due to slow cooling of the melt, and lateral heat diffusion. Despite the narrow seam, the PIL weld carried a high tensile load (92% that of base metal) and was harder than the base metal. Microstructural analysis revealed that PIL welds exhibited fully austenitic structures and were free from hot cracking. These advantages are consequences of the rapid solidification effects including large undercooling, minimal segregation of impurities to the grain boundaries, and fine grain size.     

Laser surface hardening of AISI 420 stainless steel treated by pulsed Nd:YAG laser

The AISI 420 martensitic stainless steel was surface-hardened by a pulsed Nd:YAG laser. The influences of process parameters (laser pulse energy, duration time and travel speed) on the depth and hardness of laser treated area and its corrosion behavior were Investigated. In the optimum process parameters, maximum hardness (490 VHN) in the laser surface treated area was achieved. The pitting corrosion behavior was studied by potentiodynamic polarization technique in 3.5% NaCl solution at 25 °C. Metallographical and electrochemical corrosion studies illustrated beneficial effects of laser surface hardening by refining the microstructure and enhancing the pitting corrosion resistance of the martensitic stainless steel. The pitting corrosion resistance of laser surface treated samples in 3.5% NaCl solution depends on the overlap ratio clearly. The pitting potential (E_pp) decreased significantly by increasing the ratio of pulse overlapping.     

Effect of laser surface hardening on the hydrogen embrittlement of AISI 420: Martensitic stainless steel

The susceptibility of stress corrosion cracking (SCC) of AISI 420 which was surface transformed hardened by a pulsed Nd:YAG laser, was investigated in 5% sodium chloride + 0.5% acetic acid solution by the U-Bend method, in the range of pH value from 3.5 to 6, in the absence and presence of 1 ppm thiosulphate ion, at 25 and 60 °C. The results showed that the laser-treated areas are more susceptible to SCC than the base metal. Hydrogen embrittlement (HE) is the main cause of crack propagating, mostly effective on the grain boundaries and the interface between carbide particles and second phases; tempered martensite or ferrite.     

Investigation of crack closure in stainless steel by laser interferometry

The effect of strain-induced martensite transformed during fatigue on the fatigue crack propagation rate near ΔK _th, as well as low-cycle fatigue behaviour of three differently heat-treated stainless steels, was investigated. The heat treatments were chosen so that austenite stability during fatigue was different. The crack closure stress during fatigue crack propagation near the ΔK _th region was measured using laser interferometry. The sensitized specimen showed the highest value of closure load ratio (K _cl/K _max), which was considered to be due to the roughness-induced crack closure caused by intergranular facets. The specimen with the lowest austenite stability showing the largest amount of strain-induced martensite during fatigue, showed the highest crack growth rate. The effect of brittle fracture through the harder strain-induced martensite was larger than that of possible transformation-induced crack closure.