EFFECT OF DYNAMIC STRAIN AGING ON LOW CYCLE FATIGUE BEHAVIOUR OF 18-8 AUSTENITIC STAINLESS STEEL

Studies were made of the symmetric tensile-compressive low cycle fatigue behaviour and the influence of dynamic strain aging(DSA)pre-treatment of 18-8 austenitic stainless steel. Within the testing amplitude range of strain.±0.5 % to±1.5 %,the three processes of cyclic hardening,cyclic saturation and cyclic softening were observed.In the same amplitude of strain,the peak stress of the samples pre-treated by DSA is higher than that of solid-solu- tion and cold working pre-treatment,but no remarkable differences of the fatigue lives of them were found.TEM observation shows that the uniform and stable dislocation networks with high density form after DSA pre-treatment,which increases the cyclic peak stress.The cyclic softening results from the low dislocation density and elongated cell structure with low energy.

Investigations on the Corrosion Behaviour and Structural Characteristics of Low Temperature Nitrided and Carburised Austenitic Stainless Steel

The wear resistance of austenitic stainless steels can be improved by thermo-chemical surface treatment with nitrogen and carbon. However, it is possible that the corrosion resistance will be impaired by the precipitation of chromiumnitrid or -carbide. The present contribution deals with investigations of the corrosion behaviour and structural characteristics of a low temperature nitrided and carburised austenitic stainless steel. The material investigated was AISI 316L (X2CrNiMol7-12-2) austenitic stainless steel. A commercial plasma-nitriding unit (pulsed dc) was used for the nitriding and carburising process. Additional samples were treated by the gasoxinitriding process for a comparison between plasma- and gasoxinitriding. The nitrided and carburised layer of austenitic stainless steel consists of the nitrogen or carbon S-phase (expanded austenite), respectively. X-ray diffraction investigations show the typical shift of the peaks to lower angles, indicating expansion of the fee lattice. Also the X-ray diffraction technique was employed to study the residual stresses in the nitrogen and carbon S-phase. The corrosion behaviour of surface engineered samples was investigated with electrochemical methods. Anodic potentiodynamic polarisation curves were recorded for testing the resistance against general corrosion (in H2SO4) and pitting corrosion (in NaCl).

Effect of processing temperatures on characteristics of surface layers of low temperature plasma nitrocarburized AISI 204Cu austenitic stainless steel

The influence of processing temperatures on the surface characteristics of AISI 204Cu austenitic stainless steel was investigated during a low temperature plasma nitrocarburizing.The resultant layer was a dual-layer structure,which comprises a N-enriched layer on the top of C-enriched layer.The surface hardness and the layer thickness increase up to about HV 0.05 1000 and 20μm with increasing temperature.The specimen treated at 400°C shows a much enhanced corrosion resistance compared to the untreated steel.A loss in corrosion resistance was observed for specimens treated at temperatures above 430°C due to the formation of Cr2N.

Researches on the problems in the production of low nickel austenitic stainless steel

Problems encountered in the production of low nickel austenitic stainless steel have been studied. These problems primarily include the changes to the microstructure of the slab during the heating process, the formation and removal of deformation - induced martensite during cold rolling, and the effects of the annealing process on the surface oxide structure. A reasonable manufacturing process has been proposed on the basis of the research results and high-quality cold-rolled strips of low nickel austenitic stainless steel have been produced.

Control of Al_2O_3 inclusions in austenite stainless steel

In the present study, samples were extensively collected throughout the stainless steel manufacturing process. The three-dimensional morphology of inclusions was revealed by non-aqueous solution electrolysis. The high concentration of aluminum in ferrosilicon caused the increment of [Al]s in steel and Al2 O3 in inclusions, which led to the higher melting temperature of inclusions. It was concluded that the application of low Al ferrosilicon and calcium treatment could prevent the formation of Al2 O3-rich inclusions.

Effect of the heat input on microstructure and properties of submerged arc welded joint of 08Cr19MnNi3Cu2N stainless steel

SAW308L submerged arc welding wire and SJ601A submerged arc welding flux were selected to weld the 12 mm 08Cr19MnNi3Cu2N low nickel and high nitrogen austenitic stainless steel plates with three different welding heat input,and microstructure,tensile properties,microhardness and corrosion properties of the welded joints were studied.The results show that no defects are found in the three groups of welded joints,and the welded joints have better performance.The tensile strength of 08Cr19MnNi3Cu2N stainless steel welded joints with different heat input is slightly lower than that of the base metal,and fracture occurs in the weld zone,and the hardness of the weld zone is lower than that of the base metal.The weld microstructure of stainless steel welded joints with different heat input is composed of austenite+δferrite,and ferrite is uniformly distributed in austenite.With the increase of the welding heat input,the ferrite content in the weld zone decrease gradually,the grain size in the thermal affected zone increase gradually,and the impact toughness reduce.

Application of Grain Boundary Engineering to Improve Intergranular Corrosion Resistance in a Fe–Cr–Mn–Mo–N High-Nitrogen and Nickel-Free Austenitic Stainless Steel

Optimization of grain boundary engineering(GBE) process is explored in a Fe–20Cr–19Mn–2Mo–0.82N high-nitrogen and nickel-free austenitic stainless steel, and its intergranular corrosion(IGC) property after GBE treatment is experimentally evaluated. The proportion of low Σ coincidence site lattice(CSL) boundaries reaches 79.4% in the sample processed with 5% cold rolling and annealing at 1423 K for 72 h;there is an increase of 32.1% compared with the solution-treated sample. After grain boundary character distribution optimization, IGC performance is noticeably improved. Only Σ3 boundaries in the special boundaries are resistant to IGC under the experimental condition. The size of grain cluster enlarges with increasing fraction of low ΣCSL boundaries, and the amount of Σ3 boundaries interrupting the random boundary network increases during growth of the clusters, which is the essential reason for the improvement of IGC resistance.

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.

焊接方法对奥氏体不锈钢焊接接头低温冲击性能的影响

为了研究奥氏体不锈钢焊接接头的低温冲击性能对低温压力容器的安全性影响,本文通过-196℃的低温冲击试验,对比研究了钨极气体保护焊(GTAW)、熔化极气体保护电弧焊(GMAW)、埋弧焊(SAW)3种焊接方法对S30408和S31608两种奥氏体不锈钢焊接接头低温冲击性能的影响。对于S30408焊接接头,热影响区的低温冲击性能小于母材与焊缝区,成为薄弱位置;3种焊接方法中,GTAW获得的焊接接头在热影响区的冲击性能处于最优水平。对于S31608焊接接头的低温冲击性能,焊缝、热影响区与母材的差异较小;3种焊接方法中,SAW获得的焊接接头在冲击吸收能量上与母材结果相近,能够获得焊接接头较好的综合冲击性能。通过对比不同焊接工艺下焊接接头断口形貌特征表明,焊接接头不同区域冲击断口形貌特征与低温冲击性能的结果相吻合。本文研究工作对于奥氏体不锈钢低温压力容器焊接工艺的选择具有参考意义。

低温液体氧氮化对奥氏体不锈钢磨损行为的影响

[目的]奥氏体不锈钢被广泛用于制造各种航空电子装备零部件,但其硬度和耐磨性欠佳。[方法]采用低温液体氧氮化技术对304奥氏体不锈钢进行表面改性处理。采用扫描电子显微镜、透射电子显微镜、维氏硬度计等分析了所得复合改性层的微观组织和截面显微硬度分布,通过摩擦磨损试验探究了表面改性层的耐磨性。[结果]低温液体氧氮化表面改性层由外侧致密的Fe_(3)O_(4)相和内侧富氮S相构成。S相是含氮过饱和固溶体,含有大量位错、层错和孪晶,因此硬度较高。载荷和温度会影响不锈钢样品的磨损行为,温度升高和载荷增大都会使304奥氏体不锈钢样品和低温液体氧氮化样品的平均摩擦因数轻微下降,磨损体积损失增大。不过低温液体氧氮化处理能够缩短304奥氏体不锈钢样品初始磨损阶段的持续时间,使其在载荷10 N、温度200℃条件下的最大磨损体积损失由1.086 mm^(3)降至0.144 mm^(3)。[结论]低温液体氧氮化处理能够显著提高304奥氏体不锈钢的耐磨性。

奥氏体不锈钢焊缝金属低温性能影响因素综述

随着低温系统向77 K以下应用发展,低温工程对奥氏体不锈钢焊接接头力学性能提出了更加苛刻的要求,焊缝金属的低温强度、低温冲击韧性及抗氢脆性成为低温工况安全运行的关键性能。因此,该文详细阐述低温对奥氏体不锈钢焊缝金属强度、冲击韧性的影响及不同温度导致的两种氢脆行为,并深入介绍了铁素体相、马氏体相变、氮含量对焊缝金属低温强度的提升作用及这些因素与析出相、夹杂物对低温冲击韧性进一步降低作用,然后着重论述了奥氏体相的高稳定性对焊缝金属抗氢脆性的积极作用及铁素体相的不利作用。在实际应用中焊缝金属的性能同时受多种因素地共同作用,因此对于相同因素对不同性能的研究将有助于提高低温下奥氏体不锈钢焊缝金属的使用安全性。

核燃料筒体部件焊缝的承载性能研究

不锈钢电子束焊通常被认为是手工GTAW焊的最佳升级方案,然而在某些有剪切强度要求的焊接结构中,电子束焊缝较窄的特性可能会影响其承载性能。为保证堆芯产品焊接结构可靠性,首先对比分析了电子束(EB)焊和手工GTAW焊缝的几何尺寸和承载面积,发现EB焊缝的宽度仅为GTAW焊缝的53.5%,承载面积仅为GTAW焊缝的48.8%。然后通过理论计算和工况分析,得出筒体部件焊缝的最大承载力为134765 N,而EB焊缝和GTAW焊缝的理论承载力均远大于这一值。最后,通过实物试验验证,EB焊缝和GTAW焊缝的承载性能相当,且均大于筒体部件本身结构的承载能力。最终证明EB焊缝可满足正常工况及事故工况下产品承载性能的设计要求,为核燃料国产化研究的顺利推进提供了支持。

奥氏体不锈钢全奥氏体组织熔敷金属中Ni含量对力学性能影响

为了研究Ni含量对全奥氏体组织熔敷金属力学性能的影响,采用铬当量相近、Ni质量分数不同的6种E316LMn-15焊条,通过控制其它化学成分,避免金属间化合物的形成并减弱了热裂倾向。分别对SMAW施焊后熔敷金属进行光学金相组织观察、铁素体测量、室温拉伸试验和-196℃冲击试验。结果表明,熔敷金属均为以奥氏体模式凝固的全奥氏体组织,具有良好的室温拉伸性能,抗拉强度均在520 MPa以上,屈服强度均超过380 MPa,断后伸长率也都在35%以上;同时熔敷金属均具有较高的低温韧性,-196℃平均冲击吸收能量均超过60 J。Ni的质量分数在15.58%~16.04%范围内,-196℃平均冲击吸收能量提升幅度大于Ni质量分数在16.04%以上的熔敷金属,且Ni质量分数在16.27%以上的熔敷金属低温冲击吸收能量趋于稳定,Ni质量分数为16.85%的试样-196℃平均冲击吸收能量高达90.4 J。

华龙一号主管道电渣锭成分偏析控制

华龙一号核电主管道采用X2CrNiMo 18.12(控氮)超低碳奥氏体不锈钢制造,该钢种碳含量低,铬、镍含量高且范围窄,尤其是N含量控制难度大,对质量要求苛刻;同时主管道锭型达到百吨级,易产生成分偏析,通过对大型主管道电渣锭成分偏析的分析与研究,阐述了大型单相电渣锭成分均匀性的控制方法,并取得了良好的效果。

低加热速率下ZG06Cr13Ni4Mo低碳马氏体不锈钢回火过程的相变研究

利用热膨胀仪、透射电子显微镜(TEM),X射线能谱仪(EDX),X射线衍射仪(XRD)等对ZG06Cr13Ni4Mo在0.05℃/s低加热速率下回火过程中逆变奥氏体的产生机理进行了研究.结果表明:低加热速率下ZG06Cr13Ni4Mo在A_s—A_f区间回火产生的逆变奥氏体中富集了奥氏体化元素Ni,且不含高密度位错,马氏体向逆变奥氏体的转变是扩散型相变,高于600℃回火得到的逆变奥氏体不稳定,在随后冷却过程中部分发生马氏体转变,一次回火在620℃时能得到最大量的逆变奥氏体;620—660℃一次回火生成的逆变奥氏体在冷却过程中产生大量弥散分布的马氏体,增加了600℃二次回火时逆变奥氏体的形核位置,使二次回火后逆变奥氏体含量显著增加。

高氮低镍奥氏体不锈钢的研究进展

高氮低镍奥氏体不锈钢是一种以氮代镍来获得稳定奥氏体组织的新钢种,它不但可以提高不锈钢的综合性能、节约镍资源,而且可以解决含镍较高的不锈钢用于人体时造成的镍过敏问题,在生物医学领域应用潜力巨大。综述了高氮低镍奥氏体不锈钢的发展历史和现状、不锈钢中氮的作用及高氮钢的主要制备工艺。

奥氏体不锈钢低温渗氮层的组织与耐磨性

利用低压等离子体弧离子源在350～450℃之间对奥氏体不锈钢进行快速渗氮表面强化,在适当的工艺条件下,处理90min即可得到厚度10μm左右的高硬度的氮过饱和奥氏体固溶体强化层.采用X射线衍射、电子探针及俄歇谱仪等对渗层进行结构分析表明,在处理温度低于450℃时,渗层为单相氮过饱和奥氏体固溶体层,表面氮可以达到15%,显微硬度达到1200HV,与原基体材料相比。

奥氏体不锈钢低温低压等离子体渗氮

利用低压等离子体弧源离子渗氮技术在低压（４×１０^(－１)Ｐａ）、低温 （３５０～４００℃）条件下进行奥氏体不锈钢表面渗氮处理，可在奥氏体不锈钢表面形成硬度高、耐蚀性好，厚度１０μｍ左右的氮在奥氏体中的过饱和固溶体氨化层，最高表面氮浓度达到４５％左右。

不锈钢冶炼及凝固过程氮的控制

总结了氮在不锈钢中有害和有利正反两方面的作用。通过热力学计算和实测数据分析了温度和氮分压对不锈钢熔体中氮溶解度的影响,理论分析了不锈钢熔体吸氮和脱氮的动力学,指出了真空和高压分别是生产超低氮和高氮钢的主要方法。结合以往的研究成果和生产实践提出了生产超低氮铁素体不锈钢和高氮不锈钢的具体工艺技术措施。