Effects of surface preparation on pitting resistance, residual stress, and stress corrosion cracking in austenitic stainless steels

Surface finishing treatments such as shot blasting and wire brushing can be beneficial in improving the integrity of machined surfaces of austenitic stainless steels. These operations optimize in-service properties such as resistance to pitting corrosion and stress corrosion cracking (SCC). In this study, ground steel surfaces were subjected to a series of sand blasting and wire brushing treatments. The surfaces were then characterized by their hardness, surface residual stress state, and resistance to stress corrosion and pitting corrosion. Some samples were selected for depth profiling of residual stress. It is found that surface hardening and the generation of near-surface compressive residual stress are the benefits that can be introduced by sand blasting and brushing operations.     

Experimental study of nonlinear Rayleigh wave propagation in shot-peened aluminum plates—Feasibility of measuring residual stress

Shot-peening is widely used in the aerospace industry to enhance the resistance of structural components to fatigue damage and stress corrosion by putting the outside layer of a component under an initial, residual compressive stress. The ability to measure these near-surface residual stresses is useful from a quality control and certification perspective, and can help predict the fatigue life of shot-peened components. This paper presents experimental results to examine the feasibility of measuring near-surface residual stresses using nonlinear Rayleigh surface waves. Experiments are conducted on aluminum alloy (AA 7075) samples shot-peened at different peening intensities and thus with different levels of residual stresses. The surface roughness of these samples is also measured. The nonlinear ultrasonic results show a large increase in the acoustic nonlinearity parameter, indicating the potential of nonlinear ultrasonics for the in situ measurement of near-surface residual stresses. The effects of surface roughness and the driving frequency on the measured acoustic nonlinearity parameter are briefly discussed. Finally, a preliminary model is used to interpret some experimental results. Future work to evaluate the separate contributions of cold work, residual stress and surface roughness to the total measured nonlinearity is also discussed.     

Einfluß der Oberflächenbeschaffenheit auf die Beständigkeit nichtrostender,austenitischer Chrom-Nickel-Stähle gegen transkristalline Spannungsrißkorrosion

Influence of surface conditions on the resistance of stainless austenitic bromium nickel steels to transcrystalline stress corrosion Grinding weld seams may result in an increased susceptibility to stress corrosion cracking of the areas treated in that way. This susceptibility may be eliminated by an ultimate pickling step; the thickness of the layer to be removed is 0.15 to 100 μm, depending on surface roughness. The susceptibility to stress corrosion cracking of the ground zones can be determined, however, only in the boiling 42% MgCl₂-solution; no stress corrosion cracking could be produced in solutions containing from 10 to 3O% MgCl₂ or 10 to 2O% and even 40% CaCl₂. Another possibility to eliminate stress corrosion cracking susceptibility is an ultimate sand blasting which produces compressive residual stresses in the ground surface.     

Influence of Ultrasonic Rolling on Corrosion Resistance of 7075 Aluminum AlloyEI北大核心CSCD

表面纳米化处理是一种有效改善耐腐蚀性能的手段,但受表面粗糙度和残余应力等因素的影响,其相关机制并不清晰。 运用透射电镜(TEM)和扫描电镜(SEM)研究经超声表面滚压工艺(USRP)处理后 7075 铝合金的组织和性能。结果表明： 经 1 道次和 15 道次 USRP 处理后,7075 铝合金表面粗糙度减小并且引入了残余压应力。滚压 15 个道次的试样表面能获得平均晶粒尺寸为 52 nm 的纳米晶。相较于未处理试样,经 1 道次和 15 道次 USRP 处理后试样的耐腐蚀性能均显著提高。其中, 滚压 15 个道次试样的耐腐蚀性能提升更为显著。这主要是因为纳米晶可以使材料表面形成更加致密的钝化膜,导致其耐腐蚀性能显著提高,而表面粗糙度降低和引入残余压应力是提升耐腐蚀性能的次要因素。对比分析残余应力、表面粗糙度和表面纳米晶对 7075 铝合金耐腐蚀性能的影响,揭示了 7075 铝合金经表面纳米化处理后耐腐蚀性能提升的机制。     

Effect of Turning and Ball Burnishing on the Microstructure and Residual Stress Distribution in Stainless Steel Cold Spray Deposits

In this paper, an experimental study of influence of machining by turning and ball burnishing on the surface morphology, structure and residual stress distribution of cold spray 17-4 PH stainless steel deposits is provided. It is shown that cold spray deposits could be machined by turning under parameters closed to turning of bulk 17-4 PH stainless steel. Ball burnishing process permits to decrease surface roughness. Cross-sectional observation revealed that the turning and ball burnishing process allowed microstructure changes in the coating near-surface zone. In particular, significant particle deformation and particle boundary fragmentation is observed. Measurements of residual stresses showed that residual stresses in the as-spray deposit are compressive. After machining by turning, tensile residual stresses in the near-surface zone were induced. Further surface finishing of turned coating by ball burnishing allowed the establishment of the compressive residual stresses in the coating.     

激光冲击处理后X80焊接接头表面质量对腐蚀性能的影响

利用激光冲击波对X80管线钢焊接接头进行强化处理,用金相显微镜、光学轮廓仪分析激光冲击处理前后金相组织与表面粗糙度,并用应力测试仪测试了激光冲击处理前后残余应力和残余奥氏体的变化,分析了激光冲击波改善焊接接头表面质量的机理.结果表明,激光冲击处理后焊接接头表面晶粒细化,表面粗糙度有所提高;激光冲击处理形成了残余压应力,残余奥氏体向马氏体转化;激光冲击处理后表面粗糙度和残余奥氏体对慢拉伸性能的影响起主要作用,内积功下降3.8%;激光冲击处理后应力腐蚀敏感指数I_SCC由50.94%下降至45.10%,残余压应力和晶粒细化是提高其抗应力腐蚀的主要机制.     

Stress corrosion cracking of gas pipelines - Effect of surface roughness, orientations and flattening

The primary corrosion mitigation of the external surface of high pressure steel gas pipelines is protective coatings with secondary protection usually by cathodic protection. Adhesion and resistance to cathodic disbondment of the coating is critical for its integrity and grit blasting is an important process in achieving this adhesion. The effect of surface roughness, from grit blasting, on the intergranular stress corrosion cracking resistance of X70 gas pipelines was investigated using slow strain rate testing in carbonate/bicarbonate solution at 75 °C. The effect of orientation of test pieces with respect to the axial direction of pipes was also investigated.Time to failure ratios decreased with increasing surface roughness indicating reduced stress corrosion cracking resistance. The reduced resistance to cracking with increasing roughness would be predominantly associated with stress concentration effects related to the surface roughness resulting from the grit blasting. Crack concentration decreased with increasing roughness, which is likely to be associated with the concentration of surface damage from the grit blasting using varying sized grit. As formed pipe surfaces, with no grit blasting, resulted in some of the lowest time to failure ratios and hence some of the lowest resistances to stress corrosion cracking. These also showed some of the deepest cracks. The influence of roughness and residual stresses on threshold stress is currently being investigated.Time to failure ratios indicated a greater resistance to stress corrosion cracks for circumferentially orientated test pieces compared to those longitudinally orientated. Whilst further testing would be required for confirmation, the current results suggest that flattening the test pieces had only a minor, if any, effect on stress corrosion cracking susceptibility as measured by slow strain rate testing to fracture.     

Influence of a laser rapid heat treatment on the corrosion resistance of the high nitrogen steel X30CrMoN15 1 + 0.3% N

By rapid heat treatments surface layers with properties that are distinctly different from those of the initial microstructure can be generated. In case of high nitrogen steels the rapid heat treatment aims at generating a hard surface with compressive residual stresses and high corrosion resistance. Here, the microstructure, the residual stresses and especially the corrosion behaviour of the rapid heat treated high nitrogen steel X30CrMoN15 1 and the reference steel X39CrMo17 1 are presented in dependence of the maximum heat treatment temperature and the heating rate.     

Effects of laser shock processing on electrochemical corrosion resistance of ANSI 304 stainless steel weldments after cavitation erosion

Effects of laser shock processing (LSP) on electrochemical corrosion resistance of weldments after cavitation erosion were investigated by X-ray diffraction (XRD) technology, scanning electron microscope (SEM), roughness tester and optical microscope (OM). Some main factors to influence erosion and corrosion of weldments, residual stresses, surface roughness, grain refinements and slip, were discussed in detail. Results show that LSP impacts can induce compressive residual stresses, decrease surface roughness, refine grains and generate the slip. Thus, the erosion and corrosion resistance with LSP impacts is improved.     

喷丸结合振动光整加工工艺对钛合金疲劳性能的影响

目的提高钛合金的疲劳性能。方法采用喷丸结合振动光整加工工艺对TC4钛合金进行了表面加工处理。对未加工试样、经过喷丸处理后的试样和经过喷丸与振动光整加工工艺处理后的试样,分别进行了表面粗糙度、表面层残余应力测试,并对三种状态下的试样进行了旋转弯曲疲劳试验。对比了不同工艺处理后试样的表面粗糙度、表层残余应力及疲劳强度。结果与喷丸工艺相比,采用喷丸结合振动光整加工工艺对试样进行处理后,试样的表面残余压应力值提高了39 MPa,残余压应力峰值、残余压应力层的厚度略有降低。喷丸结合振动光整加工工艺在不明显改变试样残余压应力场的条件下,使试样的表面粗糙度得到大幅降低。疲劳试验结果表明,喷丸工艺使TC4钛合金的疲劳强度提高了16.3%,喷丸结合振动光整加工工艺使TC4的疲劳强度提高了23.8%,比喷丸后TC4钛合金的疲劳强度高出7.5%。结论在喷丸工艺的基础上,喷丸结合振动光整加工工艺通过改善TC4钛合金的表面完整性,使TC4钛合金的疲劳强度得到进一步提高。     

Influence of Laser Peening on Phase Transformation and Corrosion Resistance of AISI 321 steel

The objective of this study is to investigate the influence of laser peening without coating (LPwC) on austenitic to martensitic (γ → α′) phase transformation and corrosion behavior of austenitic stainless steel AISI 321 in 3.5% NaCl environment. Results indicate that LPwC induces a large compressive residual stresses of nearly ?854 MPa and γ → α′ phase transformation of about 18% (volume fraction). Microstructures of peened surface confirmed the γ → α′ phase transformation and showed no grain refinement. Hardness increased slightly with a case depth of 900 μm. Despite the smaller surface roughness introduced, corrosion resistance improved after peening due to compressive residual stresses.     

微喷砂时间对陶瓷刀具表面完整性影响研究北大核心

针对陶瓷刀具高速切削高温合金时刀具磨损严重的问题,采用微喷砂处理技术对Sialon基陶瓷刀具表面进行处理,研究喷砂时间对Sialon基陶瓷刀具表面完整性(微观形貌、表面粗糙度、维氏硬度、断裂韧度、残余压应力)的影响规律,提升陶瓷刀具表面完整性,从而提高刀具耐磨性。研究表明:在喷砂4 min、喷砂压强0.4 MPa的条件下,陶瓷表面微观缺陷得到改善,断裂韧度提升32%,残余压应力提升12%,可以获得综合水平较高的表面完整性,为陶瓷刀具微喷砂处理参数的选择提供理论性指导,有助于提升陶瓷刀具表面完整性。     

Residual stresses and durability in cold drawn eutectoid steel wires

Prestressing steel wires have excellent mechanical properties but there is a need to improve their durability in aggressive environments. In this work, the influence of residual stresses on the environmentally assisted cracking of these wires is studied. A good correlation has been found between residual stresses at the surface of the wires and the time to rupture during stress corrosion test proposed by the International Federation of Prestressing. Wires with the same microstructure, surface quality and mechanical properties show very different behaviour in aggressive environments depending on their residual stress state. Research shows that environmentally assisted cracking can be improved significantly by acting on the surface residual stresses produced by wire drawing. In addition, in this study a post-drawing treatment to generate compressive residual stresses at the surface of the wires is proposed.     

Effect of near-surface residual stress and microstructure modification from machining on the fatigue endurance of a tool steel

This study concerns the effect of machining on the fatigue life of an EN X155CrMoV12 tool steel (SAE J438b), with regard to the generation of near-surface residual stress and microstructural modification of the machined surface. Two possible methods for machining tool steels were compared: electro-discharge machining (EDM), a high energy density process, and milling, a more conventional cutting process. Particular attention was given to characterization of the surface roughness, microstructure, and residual stress, using a combination of microstructural analysis, crack observation, scanning electron microscopy (SEM), x-ray diffraction (XRD), and chemical composition changes by energy-dispersive x-ray. A decrease of around 35% in the fatigue limit was observed for the EDM samples, compared with the milled samples. This was attributed to a tensile residual stress state after EDM, combined with significant phase transformation and hydrogen embrittlement. The milled surfaces showed no microstructural transformation or surface cracking and contained compressive residual stresses, all of which contributed to an improved fatigue resistance.     

Alumina grit blasting parameters for surface preparation in the plasma spraying operation

This paper examines how the grit blasting process influences the surface roughness of different sub-strates, the grit residue, and the grit erosion. The influence of grit blasting conditions on induced sub-strate residual stresses is also discussed. Aluminum alloy, cast iron, and hard steel were blasted with white alumina grits of 0.5,1, and 1.4 mm mean diameters. Grit blasting was performed using either a suction-type or a pressure-type machine equipped with straight nozzles made of B4C. The influence of the follow-ing parameters was studied: grit blasting distance (56 to 200 mm), blasting time (3 to 30 s), angle between nozzle and blasted surface (30°, 60°, 90°), and blasting pressure (0.2 to 0.7 MPa). The roughness of the substrate was characterized either by using a perthometer or by image analysis. The grit residue remain-ing at the blasted surface was evaluated after cleaning by image analysis. The residual stresses induced by grit blasting were determined by using the incremental hole drilling method and by measuring the de-flection of grit-blasted beams. Grit size was determined to be the most important influence on roughness. The average values of R_a and Rt and the percentage of grit residue increased with grit size as well as the depth of the plastic zone under the substrate. An increase of the pressure slightly increased the values of Ä_a and Rt but also promoted grit breakdown and grit residue. A blasting time of 3 to 6 s was sufficient to obtain the highest roughness and limit the grit breakdown. The residual stresses generated under the blasted surface were compressive, and the depth of the affected zone depended on the grit diameter, the blasting pressure, and the Young’s modulus of the substrate. More-over, the maximum residual stress was reached at the limit of the plastic zone (i.e., several tenths of a mil-limeter below the substrate surface).     

喷丸除锈过程及效果的数值模拟研究

目的模拟研究喷丸除锈工艺的过程和处理效果,为优化工艺参数提供指引。方法以建筑用低碳钢及其锈蚀产物为研究对象,利用单元去除技术和周期性胞元方法建立有限元模型,模拟多个丸粒随机撞击和氧化层去除的过程,并与文献中的实验数据进行了对比验证,从氧化层去除速率、去除后的表面粗糙度和表层残余应力三个角度分析喷丸除锈工艺的效果。结果提高除锈效果可从增大丸粒入射速度和丸粒直径来入手,氧化层越厚喷丸除锈处理的时间越长。结论喷丸除锈工艺能有效去除锈蚀层,同时改善表面粗糙度和引入残余压应力,具有较广的应用前景。有限元模拟能为该工艺优化提供便捷的研究途径。     

Microstructure, mechanical properties and oxidation behavior of the TiAl(Si,Ag) coating on near-α titanium alloy

In order to improve oxidation resistance of TIMETAL834, protective coating based on γ-TiAl intermetallic alloy with Si and Ag admixtures was produced by magnetron sputtering. Analytical scanning and transmission electron microscopy were used for detailed analyses of a microstructure and chemical composition of the coating. It was found that the coating contains two sublayers with different microstructure and morphology. Energy-dispersive synchrotron radiation diffraction was applied for stress analysis. The results show that there are tensile residual stresses present within the TIMETAL834 substrate and compressive residual stresses within the γ-TiAl sublayer. It was established that the surface treatment applied in this work essentially improves the alloy oxidation resistance.     

吹砂对DD6单晶高温合金表面完整性和高周疲劳强度的影响EI北大核心CSCD

将标准热处理的试样分别采用粒径为150、124和100μm白刚玉砂在0.5 MPa压力下吹砂,研究吹砂对第二代单晶高温合金DD6表面完整性的影响;对未吹砂和粒径150μm吹砂试样分别进行760和980℃旋转弯曲高周疲劳性能测试,研究吹砂对DD6合金疲劳性能的影响。结果表明:吹砂会破坏单晶高温合金的表面完整性,使表面出现砂粒切削造成的不规则凹坑,改变表面形貌;砂粒粒径增加,表面粗糙度和显微硬度均增大;吹砂使大量位错在γ相通道中滑移,靠近表面区域位错密度较大;并且,大量位错剪切γ’相,形成反相畴界和层错;吹砂造成形变强化、引入残余应力;150μm、0.5 MPa吹砂对DD6合金760℃旋转弯曲疲劳性能基本无影响,但会降低合金980℃疲劳性能,对低应力幅区疲劳寿命影响较大,使疲劳强度下降约7.3%。缺口效应、氧化损伤、形变强化和残余压应力的耦合作用导致吹砂与不吹砂试样疲劳寿命产生差异。     

Filiform corrosion behaviour on machined AA7150 aluminium alloy

The microstructure and the filiform corrosion behaviour of machined AA7150 aluminium alloy were investigated using scanning and transmission electron microscopies combined with potentiodynamic polarization and filiform corrosion testing, respectively. It is found that the grain refinement, redistribution of alloying elements, and elements segregation at grain boundaries are evident within the near-surface region on the machined AA7150 aluminium alloy. The corrosion susceptibility of machining introduced near-surface deformed layer is significantly improved caused by the modified microstructure associated with severe deformation. Filiform corrosion resistance on the machined surface is obviously decreased, due to the surface roughness associated with machining tracks and the presence of the electrochemically more active near-surface deformed layer introduced by machining.     

Effect of pitting corrosion on fatigue performance of shot-peened aluminium alloy 7075-T651

Pitting corrosion has a major influence on aging of structural elements made of high-strength aluminium alloys as corrosion pits lead to earlier fatigue crack initiation under tensile dynamic loading. A cause of fatigue crack initiation in a corrosive medium is a stress concentration at a corroded area. In order to improve material resistance to corrosion fatigue it is necessary to reduce pit-tip stresses. To eliminate or reduce pit stresses, cold surface hardening by shot peening was proposed. The objective of the present study was to investigate the effect of surface hardening by shot peening on electrochemical stability and corrosion fatigue properties of high-strength aluminium alloy 7075-T651 in the corrosive environment of a chloride solution. The results obtained show a favourable influence of shot-peening treatment on corrosion fatigue properties. Induced compressive residual stresses in the surface layer retard the initiation of fatigue cracks, and so the fatigue life improvement of structural elements made of high-strength aluminium alloys was observed.