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Effects of cold rolling deformation on microstructure,hardness,and creep behavior of high nitrogen austenitic stainless steel

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Chinese Physics B 2014年第2期23卷 379-384页

作者：孙世成孙贵训江忠浩季长涛刘家安连建设 Key Laboratory of Automobile Materials College of Materials Science and EngineeringJilin UniversityNanling Campus Key Laboratory of Advanced Structural Materials Ministry of EducationCollege of Materials Science and EngineeringChangchun University of Technology

Effects of cold rolling deformation on the microstructure, hardness, and creep behavior of high nitrogen austenitic stainless steel （HNASS） are investigated. Microstructure characterization shows that 70% cold rolling deformation results in significant refinement of the microstructure of this steel, with its average twin thickness reducing from 6.4 μm to 14 nm. Nanoindentation tests at different strain rates demonstrate that the hardness of the steel with nano-scale twins （nt-HNASS） is about 2 times as high as that of steel with micro-scale twins （mt-HNASS）. The hardness of nt-HNASS exhibits a pronounced strain rate dependence with a strain rate sensitivity （m value） of 0.0319, which is far higher than that of mt-HNASS （m = 0.0029）. nt-HNASS shows more significant load plateaus and a higher creep rate than mt-HNASS. Analysis reveals that higher hardness and larger m value of nt-HNASS arise from stronger strain hardening role, which is caused by the higher storage rate of dislocations and the interactions between dislocations and high density twins. The more significant load plateaus and higher creep rates of nt-HNASS are due to the rapid relaxation of the dislocation structures generated during loading.

关键词： high nitrogen austenitic stainless steel cold deformation nanoindentation tests creep behavior

Effect of cold deformation on corrosion fatigue behavior of nickel-free high nitrogen austenitic stainless steel for coronary stent application

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Journal of Materials Science & Technology 2018年第4期34卷 660-665页

作者： Jun Li Yixun Yang Yibin Ren Jiahui Dong Ke Yang Institute of Metal Research Chinese Academy of Sciences School of Materials Science and Engineering University of Science and Technology of China

Due to the excellent mechanical properties, good corrosion resistance, high biocompatibility and nickel- free character, the high nitrogen nickel-free austenitic stainless steel （HNASS） becomes an ideally alternative material for coronary stents. Stent implantation works in harsh blood environment after a balloon dilatation, i.e., the material is used in a corrosive environment with a permanent deforma- tion. The present study attempts to investigate effects of pre-straining on high-cycle fatigue behavior and corrosion fatigue behavior of HNASS in Hank＇s solution and the relevant mechanism for coronary stents application. It is found that higher pre-straining on HNASS results in higher strength and maintains almost same corrosion resistance. Fatigue limit of 0% HNASS is 550 MPa, while corrosion fatigue limit is 475 MPa. And improvement in fatigue limit of 20% and 35% pre-strained HNASS is in comparison with the 0% HNASS, while corrosion would undermine the fatigue behavior of HNASS. In a suitable range, the pre- straining had a beneficial effect on corrosion fatigue strength of HNASS, such as nearly 300 MPa improved with 20% cold deformation. This result provides a good reference for predicting the life of HNASS stent and as well its design.

关键词： Cold deformation high nitrogen austenitic stainless steel high-cycle fatigue Corrosion fatigue

Aging Precipitation Behavior of 18Cr-16Mn-2Mo-1.1N high nitrogen austenitic stainless steel and Its Influences on Mechanical Properties

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Journal of Iron and steel Research International 2012年第8期19卷 43-51页

作者： LI Hua-bing, JIANG Zhou-hua, FENG Hao, MA Qi-feng, ZHAN Dong-ping (School of Materials and Metallurgy, Northeastern University, Shenyang 110819, Liaoning, China) School of Materials and Metallurgy Northeastern University Shenyang 110819 Liaoning China

The solution-treated (ST) condition and aging precipitation behavior of 18Cr-16Mn-2Mo-1.1N high nitrogen austenitic stainless steel (HNS) were investigated by optical microscope (OM), scanning electron microscope (SEM), and transmission electron microscope (TEM). The results show that the ST condition of 18Cr-16Mn-2Mo-1.1N HNS with wN above 1% is identified as 1100 ℃ for 90 min, followed by water quenching to make sure the secondary phases completely dissolve into austenitic matrix and prevent the grains coarsening too much. Initial time-temperature-precipitation (TTP) curve of aged 18Cr-16Mn-2Mo-1.1N HNS which starts with precipitation of 0.05% in volume fraction is defined and the 'nose' temperature of precipitation is found to be 850 ℃ with an incubation period of 1 min. Hexagonal intergranular and cellular Cr2N with a=0.478 nm and c=0.444 nm precipitates gradually increase in the isothermal aging treatment. The matrix nitrogen depletion due to the intergranular and a few cellular Cr2N precipitates induces the decay of Vickers hardness, and the increment of cellular Cr2N causes the increase in the values. Impact toughness presents a monotonic decrease and SEM morphologies show the leading brittle intergranular fracture. The ultimate tensile strength (UTS), yield strength (YS) and elongation (El) deteriorate obviously. Stress concentration occurs when the matrix dislocations pile up at the interfaces of precipitation and matrix, and the interfacial dislocations may become precursors to the misfit dislocations, which can form small cleavage facets and accelerate the formation of cracks.

关键词： solution-treated condition aging precipitation behavior time-temperature-precipitation curve high nitrogen austenitic stainless steel mechanical property

Precipitation Kinetics of Cr_2N in high nitrogen austenitic stainless steel

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Journal of Iron and steel Research International 2008年第6期15卷 72-77页

作者： SHI Feng WANG Li-jun CUI Wen-fang LIU Chun-ming School of Materials and Metallurgy Northeastern University Shenyang 110004 Liaoning China

The precipitation behavior of Cr2 N during isothermal aging in the temperature range from 700 ℃ to 950 ℃ in Fe-18Cr-12Mn-0.48N （in mass percent） high nitrogen austenitic stainless steel, including morphology and content of precipitate, was investigated using optical microscopy, scanning electron microscopy, and transmission electron microscopy. The isothermal precipitation kinetics curve of Cr2 N and the corresponding precipitation activation energy were obtained. The results show that Cr2N phase precipitates in a cellular way and its morphology is transformed from initial granular precipitates to lamellar ones in the cell with increasing aging time. The nose temperature of Cr2 N precipitation is about 800 ℃, with a corresponding incubation period of 30 min, and the ceiling temperature of Cr2N precipitation is 950℃. The diffusionactivation energy of Cr2 N precipitation is 296 kJ/mol.

关键词： high nitrogen austenitic stainless steel isothermal aging precipitation kinetics precipitation activation energy

Optimization of Grain Boundary Character Distribution in Fe-18Cr-18Mn-0.63N high-nitrogen austenitic stainless steel

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Acta Metallurgica Sinica(English Letters) 2013年第5期26卷 497-502页

作者： Feng SHI Xiaowu LI Yutong HU Chuan SU Chunming LIU Institute of Materials Physics and Chemistry College of Sciences Northeastern University School of Materials and Metallurgy Northeastern University

Grain boundary engineering（GBE） is a practice of improving resistance to grain boundary failure of the material through increasing the proportion of low Σ coincidence site lattice（CSL） grain boundaries（special grain boundaries） in the grain boundary character distribution（GBCD）. The GBCD in a cold rolled and annealed Fe-18Cr-18Mn-0.63N high-nitrogen austenitic stainless steel was analyzed by electron back scatter difraction（EBSD）. The results show that the optimization process of GBE in the conventional austenitic stainless steel cannot be well applied to this high-nitrogen austenitic stainless steel. The percentage of low ΣCSL grain boundaries could increase from 47.3% for the solid solution treated high-nitrogen austenitic stainless steel specimen to 82.0% for the specimen after 5% cold rolling reduction and then annealing at 1423 K for 10 *** special boundaries of high proportion efectively interrupt the connectivity of conventional high angle grain boundary network and thus achieve the GBCD optimization for the high-nitrogen austenitic stainless steel.

关键词： high nitrogen austenitic stainless steel Grain boundary character distri-bution CSL grain boundary EBSD

Precipitates in an isothermally aged Fe-18Cr-12Mn-0.04C-0.48N high-nitrogen austenitic stainless steel

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Rare Metals 2007年第S1期26卷 185-190页

作者： SHI Feng,WANG Lijun,CUI Wenfang,REN Yuping,LI Hongxiao,and LIU Chunming School of Materials & Metallurgy,Northeastern University,Shenyang 110004,China Northeastern Univ Sch Met & Mat Shenyang 110004 Peoples R China

Vertical section of Fe-18Cr-12Mn-0.04C-N system phase diagram varying with nitrogen content at 1×105 Pa was calculated using Thermo-Calc software and thermodynamic *** morphology and crystallography information of precipitates in Fe-18Cr-12Mn-0.04C-0.48N high-nitrogen austenitic stainless steel during isothermal aging at 800 ℃ after austenization was investigated using optical microscopy(OM),and transmission electron microscopy(TEM) with energy distribution spectrum(EDS).The experimental results show that three precipitates,(Cr,Fe,Mn)2(N,C),(Cr,Fe,Mn)23(C,N)6 and σ phase exist in this steel,which is consistent with the thermodynamic calculation,indicating that thermodynamic calculation can provide instructions for alloy composition design,heat treatment and prediction of precipitation sequence in Fe-18Cr-12Mn-0.04C-N system.

关键词： precipitate isothermal aging thermodynamic calculation phase diagram high nitrogen austenitic stainless steel

high nitrogen austenitic stainless steels Manufactured by nitrogen Gas Alloying and Adding Nitrided Ferroalloys

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Journal of Iron and steel Research International 2007年第3期14卷 63-68页

作者： LI Hua-bing JIANG Zhou-hua SHEN Ming-hui YOU Xiang-mi School of Materials and Metalllargy Northeastern University Shenyang 110004 Liaoning China

A simple and feasible method for the production of high nitrogen austenitic stainless steels involves nitrogen gas alloying and adding nitrided ferroalloys under normal atmospheric conditions. Alloying by nitrogen gas bubbling in Fe-Cr-Mn-Mo series alloys was carried out in MoSi2 resistance furnace and air induction furnace under normal atmospheric conditions. The results showed that nitrogen alloying could be accelerated by increasing nitrogen gas flow rate, prolonging residence time of bubbles, increasing gas/molten steel interfaces, and decreasing the sulphur and oxygen contents in molten steel. nitrogen content of 0.69% in 18Crl8Mn was obtained using air induction furnace by bubbling of nitrogen gas from porous plug. In addition, the nickel-free, high nitrogen austenitic stainless steels with sound and compact macrostructure had been produced in the laboratory using vacuum induction furnace and electroslag remelting furnace under nitrogen atmosphere by the addition of nitrided alloy with the maximum nitrogen content of 0.81%. Pores were observed in the ingots obtained by melting and casting in vacuum induction furnace with the addition of nitrided ferroalloys and under nitrogen atmosphere. After electroslag remelting of the cast ingots, they were all sound and were free of pores. The yield of nitrogen increased with the decrease of melting rate in the ESR process. Due to electroslag remelting under nitrogen atmosphere and the consequential addition of aluminum as deoxidizer to the slag, the loss of manganese decreased obviously. There existed mainly irregular Al2O3 inclusions and MnS inclusions in ESR ingots, and the size of most of the inclusions was less than 5 um. After homogenization of the hot rolled plate at 1 150℃ × 1 h followed by water quenching, the microstructure consisted of homogeneous austenite.

关键词： nitrogen gas alloying nitrided ferroalloy high nitrogen austenitic stainless steel vacuum induction melting electroslag remelting

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

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International Journal of Minerals,Metallurgy and Materials 2009年第5期16卷 517-524页

作者： Hua-bing Li Zhou-hua Jiang Yan Yang Yang Cao Zu-rui Zhang School of Materials and Metallurgy Northeastern University Shenyang 110004 China

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.

关键词： high nitrogen austenitic stainless steel pitting corrosion crevice corrosion nitrogen critical pitting temperature syner-gistic effect

Influence of chemical composition and cold deformation on aging precipitation behavior of high nitrogen austenitic stainless steels

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Journal of Central South University 2013年第12期20卷 3354-3362页

作者：李花兵姜周华冯浩朱红春张祖瑞 School of Materials and Metallurgy Northeastern University

The influence of chemical composition and cold deformation on aging precipitation behavior of *** （HNS-A）, 18Cr-16Mn-I.3N （HNS-B）, 18Cr-18Mn-2Mo-0.96N （HNS-C） and 18Cr-18Mn-2Mo-0.77N （I-INS-D） high nitrogen austenitic stainless steels was investigated. The results show that the ＂nose＂ temperatures and incubation periods of the initial time-temperature-precipitation （TTP） curves of aged HNSs are found to be 850 ℃, 60 s; 850 ℃, 45 s; 850 ℃, 60 s and 900 ℃, 90 s, respectively. Based on the analysis of SAD patterns, the coarse cellular Cr2N precipitate which presents a lamellar structure has a hexagonal structure of a=0.478 nm and c=0.444 nm. The Z phase corresponding to a composition of Fe36Cr^2Mo10, is determined to be a body-centered cubic structure ofa=0.892 nm. The precipitating sensitivity presents no more difference with the nitrogen content increasing from 0.77% to 0.96%, but exhibits so obviously that the cellular precipitates nearly overspread the whole field. The addition of Mo element can restrain the TTP curves moving left and down, which means decreasing the sensitivity of aging precipitation. With increasing the cold deformation, the sensitivity of precipitation increases obviously.

关键词： high nitrogen austenitic stainless steel aging precipitation time-temperature-precipitation curve chemical composition cold deformation