EFFECT OF LASER CLADDING ON HYDROGEN EMBRITTLEMENT RESISTANCE OF AUSTENITIC STAINLESS STEEL

A plasma spraying plus laser remelting technique has been performed. onaustenite stainless steel (22Cr-13Ni-5Mn ) with a newly developed hydrogen resistantcoating material. The results show that the surface cladding layer can effectively reducethe hydrogen content increasing of the stainless steel under the atmosphere of high pres-sure (30MPa), high temperature (300℃) and high purity (99. 995%) hydrogen andgreatly improve the hydrogen embrittlement resistance of the stain1ess steel. Throughanalysis of microstructure, a mechanism of hydrogen embrittlement resistance is presentedthat at room temperature, the surface oxidation films, both existing on the surface ofcoated and uncoated specimens, inhibit the adsorption and diffusion of hydrogen molecu-lae. However, at high temperature, it is the surface cladding layer with relatively low sol-ubility and Permeability for hydrogen that significantly reduces the amount of hydrogenentering into the interior of the material and improves its hydrogen embrittfement resis-tance.

Revisiting Stainless Steel as PWR Fuel Rod Cladding after Fukushima Daiichi Accident

In the past, stainless steel was utilized as cladding in many PWRs （pressurized water reactors）, and its performance under irradiation was excellent. However, stainless steel was replaced by zirconium-based alloy as cladding material mainly due to its lower neutron absorption cross section. Now, stainless steel cladding appears as a possible solution for safety problems related to hydrogen production and explosion as occurred in Fukushima Daiichi accident. The aim of this paper is to discuss the steady-state irradiation performance using stainless steel as cladding. The results show that stainless steel rods display higher fuel temperatures and wider pellet-cladding gaps than Zircaloy rods and no gap closure. The thermal performance of the two rods is very similar and the neutron absorption penalty due to stainless steel use could be compensating by combining small increase in U-235 enrichment and pitch size changes.

Cracking Phenomenon in Spot Welded Joints of Austenitic Stainless Steel

The spot welds nugget cracking of austenitic stainless steel at temperatures between 700°C - 1010°C was investigated. Traditionally, the cracks have been observed around the spot nugget in welded temperature. Actually, these cracks are developed due to incomplete melting and inappropriate electrode pressure, which causes an expulsion of molten metal. These cracks start to grow and cause either the interface or plug fracture according to the loading type. In this work, the micro-cracks in the weld nugget were indicated for this type of steel at elevated temperature. Cracks appear in a certain range of temperature;about 700°C - 750°C. The cracks like defect and cavitations were presented. According to the fracture mechanics point of view, these cracks reduce the mechanical strength. Therefore, these cracks have to be taken into account with a certain precaution. Moreover, considering the working temperature and reducing the element may develop ferrite particles.

Effect of Preheating Temperature on Surface Cracking of High Nitrogen CrMn Austenitic Stainless Steel

18Mn18Cr0.5N steel specimens were preheated at the temperatures from 1100 to 1250°C for 5 min,and then cooled to 950°C and compressed.The cracking behaviors were investigated using optical microscopy and scanning electron microscopy.The results showed that the hot workability of 18Mn18Cr0.5N steel gradually decreased with increasing preheating temperature between 1100 and 1200°C,and quickly deteriorated up to 1250°C.Above 1200°C,delta ferrite particles appeared in 18Mn18Cr0.5N steel,promoted cavity coalescence on grain boundary,and accelerated surface crack formation during the hot working process.

Role of Microstructural Constituents on Surface Crack Formation During Hot Rolling of Standard and Low Nickel Austenitic Stainless Steels

The effect of alloy segregation and delta （δ） ferrite contents on surface cracking of three standard （i.e. AISI 304L, AISI 310S and AISI 321） and two low nickel （i.e. LNi-1 and LNi-0.3） austenitic stainless steels （ASS） during hot roiling was investigated using optical microscopy （OM）, automatic image analyzer, scanning electron microscopy （SEM）, energy dispersive X-ray spectroscopy （EDX） and electron probe micro analyzer （EPMA）. It was observed that the amount of 6-ferrite varied among different grades and also distributed heterogeneously across the width of the steel plates. In general, low nickel ASS showed higher amount of 6-ferrite compared to the standard ASS grades. The tendency to surface cracking during hot rolling gradually increased with increasing 6-ferrite content. Interestingly, carbon and nitrogen exerted maximum effect on 6-ferrite formation. The higher carbon and nitrogen content in the steel decreased 6-ferMte content. In addition, the segregation of Cu and Mn plays significant role in low nickel ASS and Ni-Cr in case of standard ASS has profound effect on surface cracking of the steel plates. A possible cause of surface crack formation/origination in steel plates during hot rolling was discussed.

Microstructures of Austenitic Stainless Steel Produced by Twin-Roll Strip Caster

The microstructures of austenitic stainless steel strip were studied using color metallographic method and electron probe micro analysis （EPMA）. In the cast strips, there are three kinds of solidification structures： fine cel- lular dendrite in the surface layer, equiaxed grains in the center and fine dendrite between them. The solidification mode in the surface layer is the primary austenite AF mode because of extremely high cooling rate, with the retained ferrite located around the primary cellular austenite. In the fine dendrite zone, the solidification mode of molten stainless steel changes to FA mode and the residual ferrite with fish-bone morphology is located at the core of the dendrite. The retained ferrite of equiaxed grains in the center is located in the center of broken primary ferrite dendrite with vermicular morphology.

Effect of copper on edge cracking behavior and microstructure of rolled austenitic stainless steel plate

Cu is known to affect the edge cracking characteristics of austenitic stainless steel as it causes embrittlement.The hot rolling test of four kinds of austenitic stainless steel with different copper content(0,2.42,3.60 and 4.35 wt.%)was carried out to examine the effect of hot rolling cracks on steel containing different copper contents.The evolution of crack and microstructure was analyzed using the scanning electron microscope,energy-dispersive spectrometer,electron back scattered diffraction and transmission electron microscope.Experimental results showed an upward trend in edge cracking degree when Cu content was 4.35%,and the crack extended from the edge of the steel plate to the middle by about 14 mm.Besides,severe oxidation was observed inside the crack by fractography.With the increase in copper content at 1250℃,the content of{110}<112>brass and{112}<111>copper textures decreased.When the content of copper was 4.35%,the decrease was most significant,and{112}<111>copper texture content decreased to only 0.5%.Generally,the textures of 2.42%Cu and 3.60%Cu 304L steel changed little,while a large change in the texture of 4.35%Cu 304L steel was observed.To conclude,the increase in rolling temperature can prevent edge crack and its propagation effectively.

Constituent phases of the passive film formed on 2205 stainless steel by dynamic electrochemical impedance spectroscopy

The passive film formed on 2205 duplex stainless steel（DSS） in 0.5 M NaHCO3＋0.5 M NaCl aqueous solution was characterized by electrochemical measurements,including potentiodynamic anodic polarization and dynamic electrochemical impedance spectroscopy（DEIS）.The results demonstrate that there is a great difference between the passive film evolutions of ferrite and austenite.The impedance values of ferrite are higher than those of austenite.The impedance peaks of ferritic and austenitic phases correspond to the potential of 0.15 and 0.25 V in the low potential range and correspond to 0.8 and 0.75 V in the high potential range.The evolutions of the capacitance of both phases are reverse compared to the evolutions of impedance.The thickness variations obtained from capacitance agree well with those of impedance analysis.The results can be used to explain why pitting corrosion occurs more easily in austenite phase than in ferrite phase.

Cavitation Erosion Behavior of CrMnN Duplex Stainless Steel in Distilled Water and 3% NaCl Solution

The cavitation erosion (CE) behavior of CrMnIM ferrite-austenite duplex stainless steel in distilled water and 3% NaCI solution was investigated by using a magnetostrictive-induced CE facility. The damaged surfaces were observed by scanning electron microscope (SEM). It was found that the CE resistance of CrMnN steel was higher than that of OCrl3Ni5Mo steel. The mass loss rate of CrMnN steel in distilled water was similar to that in 3% NaCI except at the early stage of CE. The failure mode of ferrite phase was brittle fracture, which had adverse effect on the resistance to CE, while the failure of austenite phase was a ductile failure in CrMnN steel. The excellent resistance to CE was related to the good mechanical properties of austenitic phase and the consumption of CE energy by plastic deformation involving slip and twinning.

High performance ferritic stainless steel B445R for architectural applications

Traditionally,austenitic stainless steels 304 and 316 have been employed in coastal regions as roofing materials unfortunately,they are expensive and not fully resistant to pitting corrosion under severe coastal corrosive environment.A ferritic stainless steel B445R was developed.Compared with austenitic 316L,B445R is①less costly;②uperior corrosion resistant with minor maintenance for long-term service;③insusceptible to thermal distortion in the welding seam. B445R sheet shows a higher yield strength and lower tensile strength,lower elongation and lower work-hardening than austenitic 316L.It can be easily fabricated and deformed just like plain carbon steel.After bending 180o,there is no occurrence of 'cracking' or noticeable 'orange peel'.The formability of the welding seam is also satisfactory.The pitting potential of B445R is 650 mV,larger than that of 304 and 316L,as shown in Fig.1. The corrosion rate of B445R submerged in 6%FeCl,solution is 0.3 -0.56 g/(m^2 ? h^(-1)),much lower than that of 316L,as shown in Fig.2.The superior pitting corrosion resistance of B445R can be ascribed to synergetic effect of higher Cr and Mo. Dull-finished B445R sheets have been employed as the roofing materials for Guangzhou Asian Games Arena,as shown in Fig.3.About 380 t of 1.0 mm B445R with dull finish was used for roofing panels.About 100 t of 0.8 mm B445R with hairline or fluororesin paint finish was used for side wall panels.The composite roof build-up (from up to bottom) includes:①shingles of ferritic stainless steel B445R;②Kalzip-type standing seam of austenitic 304;③water-proof DFM;④structural steel;⑤75 mm thick insulation;⑥secondary purlin of 150 mm×100 mm×4.5 mm of galvanizied Q235;⑦0.8 mm thick profile deck of galvanizied Q235;⑧acoustic insulation.The roofing shingles or panels with the same width but different length were formed by bending four sides and fixed to a 'L' shape reinforcing frames of stainless steel by fasteners.The 'L' frames was connected to ribs of the standing seam by a clamping fixture made of aluminum.

Metallurgical characteristics of armour steel welded joints used for combat vehicle construction

Austenitic stainless steel(ASS) and High nickel steel(HNS) welding consumables are being used for welding Q&T steels, as they have higher solubility for hydrogen in austenitic phase, to avoid hydrogen induced cracking(HIC) but they are very expensive. In recent years, the developments of low hydrogen ferritic steel(LHF) consumables that contain no hygroscopic compounds are utilized for welding Q&T steels. Heat affected zone(HAZ) softening is another critical issue during welding of armour grade Q&T steels and it depends on the welding process employed and the weld thermal cycle. In this investigation an attempt has been made to study the influence of welding consumables and welding processes on metallurgical characteristics of armour grade Q&T steel joints by various metallurgical characterization procedures. Shielded metal arc welding(SMAW) and flux cored arc welding(FCAW) processes were used for making welds using ASS, LHF and HNS welding consumables. The joints fabricated by using LHF consumables offered lower degree of HAZ softening and there is no evidence of HIC in the joints fabricated using LHF consumables.

Effect of welding processes and consumables on fatigue crack growth behaviour of armour grade quenched and tempered steel joints

Quenched and Tempered(Q&T) steels are widely used in the construction of military vehicles due to its high strength to weight ratio and high hardness. These steels are prone to hydrogen induced cracking(HIC) in the heat affected zone(HAZ) after welding. The use of austenitic stainless steel(ASS) consumables to weld the above steel was the only available remedy because of higher solubility for hydrogen in austenitic phase. The use of stainless steel consumables for a non-stainless steel base metal is not economical. Hence, alternate consumables for welding Q&T steels and their vulnerability to HIC need to be explored. Recent studies proved that low hydrogen ferritic steel(LHF) consumables can be used to weld Q&T steels, which can give very low hydrogen levels in the weld deposits. The use of ASS and LHF consumables will lead to distinct microstructures in their respective welds. This microstructural heterogeneity will have a drastic influence in the fatigue crack growth resistance of armour grade Q&T steel welds. Hence, in this investigation an attempt has been made to study the influence of welding consumables and welding processes on fatigue crack growth behaviour of armour grade Q&T Steel joints. Shielded metal arc welding(SMAW) and Flux cored arc welding(FCAW) were used for fabrication of joints using ASS and LHF consumables. The joints fabricated by SMAW process using LHF consumable exhibited superior fatigue crack growth resistance than all other joints.

Si合金化对10Cr铁/马钢在液态铅铋共晶环境中的液态金属脆化敏感性的影响

铁素体/马氏体钢是第四代液态铅铋冷却快堆燃料包壳和其他堆内构件的重要候选结构材料。Si合金化是目前国内外改善铁/马钢在铅铋环境中的腐蚀性能的关键技术手段,但Si的加入对应力腐蚀开裂敏感性(即“液态金属脆化”)的影响规律还有待深入研究。本文通过开展慢应变速率拉伸实验,对比研究了4种不同Si含量对10Cr铁素体/马氏体钢在350℃、贫氧和饱和氧铅铋以及氩气环境中的拉伸断裂行为,并结合断口分析,确定了Si对液态金属脆化敏感性的影响规律。结果表明,在所有Si含量下,铅铋对屈服强度和最大抗拉强度均没有显著影响,铅铋的影响主要体现在延伸率出现了明显下降;屈服强度随Si含量的变化基本保持不变,最大抗拉强度则呈现小幅度增大趋势;在氩气环境中,延伸率随Si含量的增加而增大。在贫氧铅铋环境中,延伸率的下降幅度与Si含量大体呈正相关。这说明Si含量越多,10Cr铁素体/马氏体钢的液态金属脆化敏感性越大。

不锈钢中亚稳奥氏体-马氏体相变行为的原位分析

采用原位EBSD分析方法,系统研究铁素体不锈钢淬火配分(quenching and partitioning,Q&P)工艺后残余奥氏体特征对其相变行为的影响。结果表明:变形过程中残余奥氏体的相变行为与其晶粒尺寸、分布和形貌有关,其影响程度按降序排列。与马氏体间奥氏体和马氏体与铁素体间奥氏体相比,三叉晶界处的奥氏体和孪晶奥氏体在变形早期更容易发生马氏体相变,这与变形过程中不同分布残余奥氏体所受不同的应变或应力密切相关。与大尺寸奥氏体相比,小尺寸奥氏体在变形后期开始转变,有助于延长均匀伸长率。这可能是由于小尺寸奥氏体具有较高的界面能和平均C含量,并且由于细晶强化效应需要较大的宏观应力/应变来诱导马氏体相变。拉长/等轴残余奥氏体在变形早期容易转变,而薄膜状残余奥氏体的转变主要集中在变形后期,有助于进一步提高塑性。不同的转变行为是由于C和N含量的差异以及层错、位错和滑移等缺陷的存在。

热处理对激光熔覆GX4CrNi13-4显微组织和力学性能的影响

激光熔覆凭借低成本、高效率被激光再制造领域重点关注,激光熔覆制造的GX4CrNi13-4马氏体不锈钢是一种核电站中应用广泛的结构材料。为改善激光熔覆制件低塑性问题,需进行热处理组织结构调控,改善其力学性能。采用激光熔覆技术制备了GX4CrNi13-4不锈钢样品,对其热处理组织开展调控研究。首先通过热膨胀试验推导出该合金的奥氏体相变开始温度为620℃,作为热处理工艺开发的基准参考温度。分别制定了固溶时效(1050℃保温1 h+550℃保温4 h,简称固溶时效处理)和单时效(620℃保温2 h,简称单时效处理)两种热处理工艺,对比研究了热处理对覆层金属显微组织和力学性能的影响作用。然后采用X射线衍射仪、光学显微镜、扫描电子显微镜、透射电子显微镜对热处理后的显微组织结构和物相分布等进行表征,并对热处理前后的样品进行室温拉伸性能测试。结果表明:激光熔覆GX4CrNi13-4马氏体不锈钢沉积态样品基体组织主要为马氏体/铁素体双相组织,铁素体相呈连续网状结构,沿马氏体晶界析出,此外还存在少量残余奥氏体。经固溶时效热处理后,基体仍主要由马氏体和铁素体组成,但连续网状铁素体发生分解,且出现大量微米级马氏体晶内析出物,这导致材料塑性略有提升,但强度显著下降。对覆层样品进行单时效热处理,由于温度处于奥氏体相转变临界温度,样品中产生了逆变奥氏体相,该相在拉伸过程中引发相变诱发塑性(Transformation Induced Plasticity,TRIP)效应。此外,单时效处理后沿马氏体析出的网状铁素体进一步得到分解,呈离散分布。TRIP效应和铁素体分解的共同作用下,有效改善了激光熔覆GX4CrNi13-4不锈钢的塑性同时使得强度被较好地保持。激光熔覆工艺在修复和再制造领域具有广泛的应用前景,但熔覆过程的高冷却速度、复杂的热循环对材料的组织结构产生影响,使得修复件往往具有高强度但塑韧性不足。开展合适的热处理组织性能调控是改善材料综合力学性能的有效手段,在激光熔覆GX4CrNi13-4不锈钢的热处理工艺研究中,选择奥氏体相变温度作为时效温度,利用逆变奥氏体TRIP效应和网状铁素体分解的联合作用,获得强度-塑性匹配的良好力学性能。

热老化对铸造奥氏体不锈钢的环境疲劳寿命影响

铸造奥氏体不锈钢材料在一回路高温高压水环境下长期服役存在热老化和疲劳问题。本文对典型的核电厂主管道材料Z3CN20-09M开展400℃的加速热老化试验,利用透射电子显微镜分析热老化试样的结构,在高温空气和模拟一回路高温高压水环境下研究热老化后Z3CN20-09M的疲劳寿命变化规律,分析热老化时间对Z3CN20-09M疲劳寿命的影响。结果表明,Z3CN20-09M在热老化5 000 h后其(011)晶面上发生晶格畸变。随着热老化时间延长至5 000 h,Z3CN20-09M在高温空气和模拟一回路高温高压水环境下的疲劳寿命下降,ASME疲劳设计曲线对于评价Z3CN20-09M热老化后的疲劳寿命仍具有足够的安全裕量。

应急柴油机组电动泵M3402不锈钢泵壳精密铸造工艺研究

针对应急柴油机组电动泵用奥氏体-铁素体不锈钢泵壳的材料成分、力学性能要求和结构尺寸,设计了泵壳精密铸造成型初始工艺,首次试制后对浇注系统进行了优化,对优化工艺后泵壳的成形过程开展了ProCAST铸造工艺模拟仿真和缺陷预测,以验证该工艺的可行性,最后采用该优化工艺开展了试制生产。结果证明:采用ProCAST铸造工艺仿真有效预测了缺陷位置,验证了浇注系统优化后工艺的可行性。泵壳铸件尺寸满足精度要求,无裂纹、缩松等缺陷,材料的铁素体含量约为13.5%,各项力学性能符合M3402铸件力学性能的检测要求。

冷成形奥氏体不锈钢封头硬度和铁素体含量超标及异种钢焊接裂纹案例分析

冷成形奥氏体不锈钢封头硬度、铁素体含量超标,异种钢焊接裂纹是压力容器监造过程中遇到的比较典型的制造质量问题。本文结合监造设备案例,对质量问题产生的原因进行分析,提出了处理措施,为后续监造过程中预防、处理同类质量问题提供借鉴。

双相不锈钢在含硝酸体系中的优选腐蚀行为研究

研究了双相不锈钢在氧化性的HNO3 +HCl、HNO3 +NaCl、HNO3 +FeCl3 3种体系中两相的腐蚀差异 .通过动电位极化实验 ,用SEM和EDS对表面形貌进行观测和两相的鉴别 ,发现奥氏体相是优先腐蚀的相 .在HNO3 +HCl和HNO3 +NaCl中随着Cl-浓度的增加 ,两相的优选腐蚀更加明显 ,但在HNO3 +FeCl3 中 ,低浓度的FeCl3 会掩盖两相的优选腐蚀 ,而高浓度的FeCl3 不存在这种反常行为 .

Cr-Mn-N奥氏体-铁素体不锈钢的空蚀行为

利用超声振荡空蚀实验机对Cr-Mn-N奥氏体-铁素体不锈钢进行了空蚀研究.结果表明,低硬度Cr—Mn—N不锈钢的抗空蚀性能高于高硬度的0Cr13Ni5Mo不锈钢.空蚀过程中, Cr—Mn—N奥氏体-铁素体不锈钢中的铁素体优先脱落,空蚀表面的铁素体含量由19％迅速降低到6.9％后保持稳定.奥氏体易于在铁素体脱离后形成的蚀坑处开始脱落.加工硬化是该不锈钢吸收空蚀冲击能量的一个重要途径,是其硬度较低却表现出良好抗空蚀性能的重要原因.