冶金因素对钢点蚀扩展的影响

选择六种冶金因素各有特点的低碳钢和一种低合金钢,通过模拟闭塞腐蚀电池实验,结合冶金特点和腐蚀形貌分析,研究冶金因素对孔蚀扩展的影响. 结果表明,钢中固溶氧量、夹杂物、钙处理、磷含量及其偏析和合金元素镍铬等对点蚀扩展有重要影响. 钢中固溶氧量高、钙处理、高磷量和高镍铬量有利于抑制蚀坑扩展.     

Effect of hydrogen on the stress corrosion cracking behavior of X80 pipeline steel in Ku'erle soil simulated solution

Hydrogen was a key factor resulting in stress corrosion cracking (SCC) of X80 pipeline steel in Ku'erle soil simulated solution. In this article, the effect of hydrogen on the SCC susceptibility of X80 steel was investigated further by slow strain rate tensile test, the surface fractures were observed using scanning electron microscopy (SEM), and the fracture mechanism of SCC was discussed. The results indicate that hydrogen increases the SCC susceptibility. The SEM micrographs of hydrogen precharged samples presents a brittle quasi-cleavage feature, and pits facilitate the transgranular crack initiation. In the electrochemical impedance spectroscopy (EIS) measurement, the decreased polarization resistance and the pitting resistance of samples with hydrogen indicate that hydrogen increases the dissolution rate and deteriorates the pitting corrosion resistance. The potentiodynamic polarization curves present that hydrogen also accelerates the dissolution rate of the crack tip.     

Why stainless steel corrodes

Stainless steels are used in countless diverse applications for their corrosion resistance. Although they have extremely good general resistance, they are nevertheless susceptible to pitting corrosion. This localized dissolution of an oxide-covered metal in specific aggressive environments is one of the most common and catastrophic causes of failure of metallic structures. The pitting process has been described as random, sporadic and stochastic and the prediction of the time and location of events remains extremely difficult. Many contested models of pitting corrosion exist, but one undisputed aspect is that manganese sulphide inclusions play a critical role. Indeed, the vast majority of pitting events are found to occur at, or adjacent to, such second-phase particles. Chemical changes in and around sulphide inclusions have been postulated as a mechanism for pit initiation but such variations have never been measured. Here we use nanometre-scale secondary ion mass spectroscopy to demonstrate a significant reduction in the Cr:Fe ratio of the steel matrix around MnS particles. These chromium-depleted zones are susceptible to high-rate dissolution that 'triggers' pitting. The implications of these results are that materials processing conditions control the likelihood of corrosion failures, and these data provide a basis for optimizing such conditions.     

钝化304不锈钢单晶在硫酸含氯介质中的孔蚀

采用交流阻抗技术,研究钝化304不锈钢单晶体在pH1.0,0.25mol/lNa_2SO_4含氯介质中的孔蚀行为。对孔蚀过程中的阻抗频谱特征进行了分析,提出孔蚀的发生和发展过程中都有复合物(MOHC1)形成。复合物进一步吸附更多的氯离子,导致溶解反应发生。并提出一个孔蚀的的反应机理模型,它与304不锈钢多晶体在硫酸含氯介质中的孔蚀机理没有本质的差别。     

超低氧条件下钢液脱氧与氧化物夹杂尺寸

研究真空感应熔炼过程中因炉衬材料热分解造成的脱氧元素烧损及其全氧含量的变化规律。在此基础上，探讨脱氧元素过饱和度对脱氧产物尺寸分布规律的影响，以及超低氧钢全氧含量与氧化物夹杂数量和尺寸的关系。结果表明，在真空感应熔炼条件下，残留在钢中的氧化物夹杂数量随脱氧元素铝烧损量增加而增加．熔炼超低氧钢的关键是避免炉衬热分解；脱氧产物尺寸随脱氧元素过饱和度增加而增加；超低氧铜的大颗粒夹杂主要来自炉衬材料。     

Effects of the shape and size of rectangular inclusions on the fatigue cracking behavior of ultra-high strength steels

The fatigue cracking behavior of ultra-high strength steels containing rectangular inclusions of small sizes were investigated based on in situ observations by scanning electron microscopy (SEM). The size and shape of rectangular inclusions affect markedly the initiation site and propagation path of a fatigue crack. Especially, the initiation site of a fatigue crack depends strongly on the angle between the long-axis of a rectangle inclusion and the loading direction, and the length/width ratio of this rectangle inclusion because the residual stress distribution fields vary with these conditions. The results coincide very well with those of finite element analysis.     

Incubation and development of corrosion in microstructures of low alloy steels under a thin liquid film of NaCl aqueous solution

Electrochemical measurement, optical microscopy, and scanning electron microscopy were employed to investigate the corrosion behavior of some low alloy steels. The steels were held under a thin liquid film of 0.5wt% NaCl aqueous solution. It is found that the steels with the same chemical composition but different microstructures exhibit obviously different corrosion behaviors. However, the corrosion behavior of the steels with different compositions but the same microstructures may be similar in the present investigation. The corrosion rate of bainite is slower than that of ferrite and pearlite. The corrosion products of bainite are uniform and fine. The size of carbon-rich phases produces a great impact on the corrosion of the steels, whether in the initial stage or in the long term. It is easy to induce large pitting for carbon-rich phases with large size, which damages the compactness of the rust layer.     

Pitting corrosion and crevice corrosion behaviors of high nitrogen austenitic stainless steels

Pitting corrosion and crevice corrosion behaviors of high nitrogen austenitic stainless steels (HNSS) were investigated by electrochemical and immersion testing methods in chloride solution, respectively. The chemical constitution and composition in the depth of passive films formed on HNSS were analyzed by X-ray photoelectron spectrum (XPS). HNSS has excellent pitting and crevice corrosion resistance compared to 316L stainless steel. With increasing the nitrogen content in steels, pitting potentials and critical pitting temperature (CPT) increase, and the maximum, average pit depths and average weight loss decrease. The CPT of HNSS is correlated with the alloying element content through the measure of alloying for resistance to corrosion (MARC). The MARC can be expressed as an equation of CPT=2.55MARC-29. XPS results show that HNSS exhibiting excellent corrosion resistance is attributed to the enrichment of nitrogen on the surface of passive films, which forms ammonium ions increasing the local pH value and facilitating repassivation, and the synergistic effects of molybdenum and nitrogen.     

Effects of precipitates and inclusions on the fracture toughness of hot rolling X70 pipeline steel plates

In order to investigate the fracture toughness, crack tip opening displacement (CTOD) experiments were conducted on two X70 pipeline steel plates with different rolling processes. After the experiments, optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to observe the microstructure and fracture morphology. The effects of precipitates on the fracture toughness and the crack initiation mechanism induced by inclusions were analyzed. The CTOD result shows that the steel with a lower finishing cooling temperature has a higher fracture toughness. Inclusions with different shapes and two kinds of precipitates with different sizes were observed. It can be concluded that precipitates with different sizes have different effects and mechanisms on the fracture toughness. Distinguished from the earlier researches, inclusions enriched in silicon can be also served as the crack initiation.     

Effects of chloride ion concentration and pH values on the corrosion behavior of Cr12Ni3Co12Mo4W ultra-high-strength martensitic stainless steel

The effects of Cl- ion concentration and pH values on the corrosion behavior of Cr12Ni3Co12Mo4W ultra-high-strength martensitic stainless steel (UHSMSS) were investigated by a series of electrochemical tests combined with observations by stereology microscopy and scanning electron microscopy. A critical Cl- ion concentration was found to exist (approximately 0.1wt%), above which pitting occurred. The pitting potential decreased with increasing Cl- ion concentration. A UHSMSS specimen tempered at 600°C exhibited a better pitting corrosion resistance than the one tempered at 400°C. The corrosion current density and passive current density of the UHSMSS tempered at 600°C decreased with increasing pH values of the corrosion solution. The pits developed a shallower dish geometry with increasing polarization potential. A lacy cover on the pits of the UHSMSS tempered at 400°C accelerated pitting, whereas corrosion products deposited in the pits of the UHSMSS tempered at 600°C hindered pitting.     

Ultrahigh cycle fatigue fracture mechanism of high-quality bearing steel obtained through different deoxidation methods

The mechanism of oxide inclusions in fatigue crack initiation in the very-high cycle fatigue(VHCF)regime was clarified by subjecting bearing steels deoxidized by Al(Al-deoxidized steel)and Si(Si-deoxidized steel)to ultrasonic tension-compression fatigue tests(stress ratio,R=−1)and analyzing the characteristics of the detected inclusions.Results show that the main types of inclusions in Si-and Al-deoxidized steels are silicate and calcium aluminate,respectively.The content of calcium aluminate inclusions larger than 15μm in Si-deoxidized steel is lower than that in Al-deoxidized steel,and the difference observed may be attributed to different inclusion generation processes during melting.Despite differences in their cleanliness and total oxygen contents,the Si-and Al-deoxidized steels show similar VHCF lives.The factors causing fatigue failure in these steels reveal distinct differences.Calcium aluminate inclusions are responsible for the cracks in Al-deoxidized steel.By comparison,most fatigue cracks in Si-deoxidized steel are triggered by the inhomogeneity of a steel matrix,which indicates that the damage mechanisms of the steel matrix can be a critical issue for this type of steel.A minor portion of the cracks in Si-deoxidized steel could be attributed to different types of inclusions.The mechanisms of fatigue fracture caused by calcium aluminate and silicate inclusions were further analyzed.Calcium aluminate inclusions first separate from the steel matrix and then trigger crack generation.Silicate inclusions and the steel matrix are closely combined in a fatigue process;thus,these inclusions have mild effects on the fatigue life of bearing steels.Si/Mn deoxidation is an effective method to produce high-quality bearing steel with a long fatigue life and good liquid steel fluidity.     

Effects of heat treatment on the corrosion resistance of carbon steel coated with LaMgAl11O19 thermal barrier coatings

LaMgAl11O19 thermal barrier coatings (TBCs) were applied to carbon steels with a NiCoCrAlY bond coat by plasma spraying. The effects of heat treatment on the corrosion resistance of carbon steel coated with LaMgAl11O19 TBCs were investigated in 3.5wt% NaCl solution using polarization curves, electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), and X-ray diffrac-tion (XRD). The results show that a large number of cracks are found in the LaMgAl11O19 TBCs after the samples are heat-treated, including some through-thickness cracks. The corrosion forms of the as-sprayed and heat-treated TBCs are uniform corrosion and pitting corrosion, respectively. The as-sprayed TBCs exhibit three EIS time constants after being immersed for less than 7 d, and then a new time constant ap-pears because of steel substrate corrosion. When the immersion time is increased to 56 d, a Warburg impedance (W) component appears in the EIS data. The EIS data for the heat-treated TBCs exhibit only two time constants after the samples are immersed for less than 14 d, and a new time constant appears when the immersion time is increased further. The heat treatment reduces the corrosion resistance of carbon steel coated with LaMgAl11O19 TBCs. The corrosion products are primarilyγ-FeOOH and Fe3O4.     

不锈钢裸表面在氯化物溶液中的点腐蚀

研究了不锈钢去膜表面在氯化镁介质中的点腐蚀现象。去膜表面发生点蚀的临界电位低于膜覆盖表面发生点蚀的临界电位。去膜表面的点蚀主要在晶界和夹杂起源。点蚀形貌是敏锐的条纹状花样。根据作者提出的裸表面与氯化物介质反应步骤模型讨论了点蚀特征电位的意义以及裸表面点蚀形成的过程。     

回火对微合金管线钢疲劳裂纹扩展行为的影响

采用MTS858电液伺服万能试验机、扫描电镜及透射电镜研究回火对一种高强度微合金管线钢疲劳裂纹扩展行为的影响。研究结果表明:回火可提高微合金管线钢疲劳裂纹扩展的门槛值,降低疲劳裂纹扩展速率,但对裂纹扩展稳态区的扩展速率影响不大;回火使碳氮化物沉淀析出、晶间马氏体/奥氏体(M/A)组元由岛状转变为点状及细条状,形成马氏体薄膜结构,阻碍变形和裂纹在材料中扩展,增加裂纹的偏折程度;在控轧控冷终冷温度进行2～4 h回火热处理,可以提高微合金管线钢强韧性和抗疲劳裂纹扩展能力。     

Effects of silty sand on CO<Subscript>2</Subscript> corrosion behavior of low-Cr tubing steel

An experimental study was carried out to assess the effects of silty sand on the CO 2 corrosion behavior of 1 wt% Cr (1Cr) and 3 wt% Cr (3Cr) tubing steel under 0.5 MPa CO 2 at 100°C and 1.5 m/s flow velocity.The 1Cr and 3Cr specimens both suffered general corrosion,but the surface was coarser in the pure CO 2 corrosion environment.Under silty sand conditions,severe pitting corrosion occurred on the 1Cr specimens and some acicular pitting appeared on the 3Cr specimens.The average corrosion rates of 1Cr and 3Cr steels increased by factors of 3 and 1.6,respectively.The corrosion products were analyzed by scanning electron microscopy (SEM),energy dispersive spectroscopy (EDS),X-ray diffraction (XRD),and electrical impedance spectroscopy (EIS).The results show that silty sand acts as an inclusion in corrosion product films and reduces the homogeneity and density of the products,rather than abrading the corrosion film.Ion-diffusion channels may build up around the irregular silty sand;this would degrade the protective capabilities of the product films and aggravate corrosion.     

钝化304不锈钢在弱碱性含氯介质中的点蚀机理

采用交流阻抗技术研究了钝化304不锈钢在pH8.4,H_3BO_3(0.2Mol/t-Na_2B_4O_7(0.05mol/1)及pH9.2,Na_2B4O_7(0.05moI/I)的含氯介质中的点蚀行为;提出了点蚀的反应机理及发展阶段的界面等效电路模型,并认为在点蚀的发生和发展阶段都可能有复合物(MOHCl)及其进一步的吸附产物(MOHCl-Cl_n~-)形成。     

服役U71Mn钢轨疲劳裂纹萌生及扩展分析

采用HV-1000型显微维氏硬度计、光学显微镜、扫描电子显微镜和能谱仪等手段,研究了服役后U71Mn钢轨表层力学性能、轨距角处的显微组织以及裂纹萌生和扩展机理.结果表明:钢轨表层轨距角处的硬化程度最严重,并且该区域沿列车运行方向呈现出较为严重的剪切塑性变形,裂纹容易在该处萌生.钢轨轨距角处的裂纹萌生受到表层切应力分布状态、表面摩擦因数以及表层夹杂物等的综合影响,钢轨表层受到循环应力的反复作用,在加工硬化的作用下材料局部发生硬化,塑韧性降低,另外钢轨表层有大量长条状的MnS夹杂物,这些因素都极大地促进了疲劳裂纹的萌生.轨距角处的裂纹沿着钢轨表层剪切塑性变形的方向扩展,主要方式是穿晶扩展,裂纹间隙中较多的氧化物对裂纹扩展有较大的促进作用.     

Microstructure and mechanical properties of F<Subscript>GBA</Subscript>/B<Subscript>G</Subscript> air cooling bainitic steels containing niobium

The effect of different Nb additions on the mechanical properties and microstructure evolution of grain boundary allotriomorphic ferrite (F_GBA) / granular bainite (B_G) air cooling bainitic steels was investigated. The results indicate that the tensile strength and yield strength increase by 157 and 97 MPa, respectively with the addition of 0.02wt% Nb. The steel acquires superior strength and toughness with the addition of 0.06wt% Nb. The results of scanning electron microscopy and transmission electron microscopy reveal that the addition of Nb not only refines the size of granular bainite but also increases the volume fraction of granular bainite in F_GBA/B_G steels. The refinement effect of granular bainite is improved with the increase of Nb content.     

钢中冶金因素与裂纹扩展阻力特性

本文研究了三种中碳锰钢中不同程度的硫化锰夹杂和晶粒尺寸等冶金因素对材料裂纹扩展阻力特性的影响.结果表明:造成材料裂纹扩展阻力各向异性的主导因素是沿轧向分布的条片状硫化锰夹杂,而不是通常所认为的带状组织;随(高温正火)晶粒长大,材料纵、横向阻力经历了不同程度的降低并达到相近水平后,使得各向异性减弱或消失;不同程度的硫化锰夹杂使得裂纹扩展阻力曲线上下移动,而对阻力曲线的斜率无明显影响;晶粒粗大及晶间网状铁素体强烈降低材料的裂纹扩展阻力,并极易导致裂纹的失稳扩展.