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.

High Nitrogen Austenitic Stainless Steels Manufactured by Nitrogen Gas Alloying and Adding Nitrided Ferroalloys

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.

Fabrication of high nitrogen austenitic stainless steels with excellent mechanical and pitting corrosion properties

A series of high nitrogen austenitic stainless steels were successfully developed with a pressurized electroslag remelting furnace. Nitride additives and deoxidizer were packed into the stainless steel pipes, and then the stainless steel pipes were welded on the surface of an electrode with low nitrogen content to prepare a compound electrode. Using Si3N4 as a nitrogen alloying source, the silicon contents in the ingots were prone to be out of the specification range, the electric current fluctuated greatly and the surface qualities of the ingots were poor. The surface qualities of the ingots were improved with FeCrN as a nitrogen alloying source. The sound and compact macrostructure ingot with the maximum nitrogen content of 1.21wt% can be obtained. The 18Cr18Mn2Mo0.9N high nitrogen austenitic stainless steel exhibits high strength and good ductility at room temperature. The steel shows typical ductile-brittle transition behavior and excellent pitting corrosion resistance properties.

PRECIPITATION BEHAVIOR OF M_(2)N IN A HIGH-NITROGEN AUSTENITIC STAINLESS STEEL DURING ISOTHERMAL AGING

The precipitation behavior of M2N and the microstructural evolution in a Cr-Mn austenitic stainless steel with a high nitrogen content of 0.43mass% during isothermal aging has been investigated using optical microscopy （ OM）, scanning electron microscopy （SEM）, and transmission electron microscopy （TEM）. The aging treatments have led to the decomposition of nitrogen supersaturated austenitic matrix through discontinuous cellular precipitation. The precipitated cells comprise alternate lamellae of M2N precipitate and austenitic matrix. This kind of precipitate morphology is similar to that of pearlite. However, owing to the non-eutectoidic mechanism of the reaction, the growth characteristic of the cellular precipitates is different from that of pearlite in Fe-C binary alloys. M2N precipitate in the cell possesses a hexagonal crystal structure with the parameters a = 0.4752nm and c = 0.4429nm, and the orientation relationship between the M2V precipitates and austenite determined from the SADP is [01^-10]M2N//[101]γ, [2^-1^-10]M2N//[010]γ.

Intergranular corrosion behavior of high nitrogen austenitic stainless steel

The intergranular corrosion （IGC） behavior of high nitrogen austenitic stainless steel （HNSS） sensitization treated at 650-950℃ was investigated by the double loop electrochemical potentiodynamic reactivation （DL-EPR） method. The effects of the electrolytes, scan rate, sensitizing temperature on the susceptibility to IGC of HNSS were examined. The results show that the addi-tion of NaCl is an effective way to improve the formation of the cracking of a passive film in chromium-depleted zones during the reactivation scan. Decreasing the scan rate exhibits an obvious effect on the breakdown of the passive film. A solution with 2 mol/L H2SO4＋1 mol/L NaCl＋0.01 mol/L KSCN is suitable to check the susceptibility to IGC of HNSS at a sensitizing temperature of 650-950℃ at a suitable scan rate of 1.667 mV/s. Chromium depletion of HNSS is attributed to the precipitation of Cr2N which results in the susceptibility to IGC. The synergistic effect of Mo and N is suggested to play an important role in stabilizing the passive film to prevent the attack of IGC.

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

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.

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

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 database.The 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.

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

The influence of chemical composition and cold deformation on aging precipitation behavior of 18Cr-16Mn-2Mo-I.IN （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.

Influence of welding parameters on nitrogen content in welding metal of 32Mn-7Cr-1Mo-0.3N austenitic steel

The transfer behavior of nitrogen into the welding metal during gas tungsten arc welding process of 32Mn-7Cr-1Mo-0.3N steel was investigated. The effects of gas tungsten arc welding process variables, such as the volume fraction of nitrogen in shielding gas, arc holding time and arc current on the nitrogen content in the welding metal were also evaluated. The results show that the volume fraction of nitrogen in gas mixture plays a major role in controlling the nitrogen content in the welding metal. It seems that there exhibits a maximum nitrogen content (depending) on the arc current and arc holding time. The optimum volume fraction of nitrogen in shielding gas is 4% or so. The role of gas tungsten arc welding processing parameters in controlling the transfer of nitrogen is further (confirmed) by the experimental results of gas tungsten arc welding process with feeding metal.

Strain rate and cold rolling dependence of tensile strength and ductility in high nitrogen nickel-free austenitic stainless steel

The tensile strength and ductility of a high nitrogen nickel-free austenitic stainless steel with solution and cold rolling treatment were investigated by performing tensile tests at different strain rates and at room temperature. The tensile tests demonstrated that this steel exhibits a significant strain rate and cold rolling dependence of the tensile strength and ductility.With the increase of the strain rate from 10^-4s^-1to 1 s^-1, the yield strength and ultimate tensile strength increase and the uniform elongation and total elongation decrease. The analysis of the double logarithmic stress–strain curves showed that this steel exhibits a two-stage strain hardening behavior, which can be well examined and analyzed by using the Ludwigson equation. The strain hardening exponents at low and high strain regions（n2and n1） and the transition strain（εL） decrease with increasing strain rate and the increase of cold rolling RA. Based on the analysis results of the stress–strain curves, the transmission electron microscopy characterization of the microstructure and the scanning electron microscopy observation of the deformation surfaces, the significant strain rate and cold rolling dependence of the strength and ductility of this steel were discussed and connected with the variation in the work hardening and dislocation activity with strain rate and cold rolling.

Development and application of nitrogen-alloyed austenitic stainless steels in Baosteel

In recent years,nitrogen-alloyed stainless steels have been a research hotspot in the field of stainless steel product and technology. Nitrogen-alloyed austenitic stainless steels developed by Baosteel and their applications are introduced. These steels are nitrogen-controlled products 304 N and 316 LN,nitrogen containing economical products BN series and high-nitrogen stainless steel（ HNS） series. The results show that the presence of nitrogen can significantly improve the strength and corrosion resistance of steel produced. By nitrogen alloying,economical austenitic stainless steels w ith considerably less nickel than 304 can be obtained; the corrosion resistances of these steels are almost the same as 304. Furthermore,by a scientific approach of nitrogen alloying,high-nitrogen steel of0. 8% nitrogen content is fabricated under the non-pressurized conditions,and the pitting potential of this steel is ＆gt;1. 0 V. At present,nitrogen-alloyed steels developed by Baosteel are w idely utilized in the manufacture of cryogenic storage containers,transportation containers,and many household w ares.

Aging precipitation and recrystallization in high-nitrogen austenitic stainless steel

The interaction between precipitation and recrystallization in cold deformed Fe-18Cr-12Mn-0.48N high-nitrogen austenitic stainless steel was investigated by means of hardness test, optical microscopy (OM) and transmission electron microscopy (TEM). The results show that the recrystallization of the steel begins at about 750℃ . When aging at 750℃ , the precipitation occurs prior to recrystallization. Large numbers of the second phases nucleate in dislocation, grain boundary and subgrain boundary. Precipitation of the second-phase particles hinders the formation of recrystallization nucleus.

Mechanical Properties and Microstructure Evolution of Cold-deformed High-nitrogen Nickel-free Austenitic Stainless Steel during Annealing

The mechanical properties and microstructure evolution of cold-deformed CrMnN austenitic stainless steel annealed in a temperature ranging from 50 ℃ to 650 ℃ for 90 min and at 550 ℃ for different time were investigated by tensile test, micro hardness test, and Transmission Electron Microscope （TEM）. The steel was strengthened when it got annealed at temperatures ranging from 100 ℃ to 550 ℃, while it was softened when it got annealed at temperatures ranging from 550 ℃ to 650 ℃. Annealing temperature had stronger effect on mechanical properties than annealing time. TEM observations showed that nano-sized precipitates formed when the steel was annealed at 150 ℃ for 90 min, but the size and density of precipitates had no noticeable change with annealing temperature and time. Recrystallization occurred when the steel was annealed at temperatures above 550 ℃ for 90 min, and its scale increased with annealing temperature. Nano-sized annealing twins were observed. The mechanisms that controlled the mechanical behaviors of the steel were discussed.

Effect of strain-induced martensitic phase transformation on the formability of nitrogen-alloyed metastable austenitic stainless steels

Strain-induced martensitic phase transformation and its influence on the forrnability of newly developed nitrogen-alloyed metastable austenitic stainless steels were systematically investigated. Yield strength for the as- received steels bearing low nickel content was around 300 MPa and their elongation ratios varied from 55.2% to 61.7%. Erichsen numbers of these samples differed from 13.82 to 14.57 mm. Although its Cu content was lower than that of other samples, steel D2 exhibited better plasticity and formability, which was attributed to ~/--,c~＇ martensitic phase transformation. EBSD, XRD, and magnetism tests showed that increases in deformation ratio gradually increased the α＇ martensite phase of a sample ,thereby contributing to its strain and inducing the optimal transformation-induced plasticity effect. An Meeo/5o temperature of around 20 ℃, which is close to the deformation temperature,provided the austenite with adequate stability and gradually transformed it into martensite, thereby endowing lean ASS with better formability.

Effects of Nitrogen Concentration on Microstructure and Antibacterial Property of Copper-Bearing Austenite Stainless Steels

Austenite antibacterial stainless steels have been found to have wide applications in hospitals and food industries. In recent years epsilon copper precipitation in antibacterial stainless steels has obtained much research interest due to its antibacterial action. The objective of this study was to determine the effects of nitrogen concentration on the precipitation of epsilon copper and antibacterial property. Two kinds of austenite antibacterial stainless steels containing copper and different nitrogen concentration （0.02 and 0.08 wt pct, respectively） were prepared and the microstructures were characterized by a combination of electron microscopy and thermodynamic analysis. A mathematical expression was deduced to predict the effect of nitrogen concentration on the activity coefficient of copper, In（fCu/f^0cu）=0.53524＋4.11xN-0.48x^2N. Higher nitrogen was found to increase the free energy difference of copper concentration distribution between precipitation phase and austenite matrix, stimulate the aggregation of copper atoms from austenite, increase the precipitation amount and consequently enhance the antibacterial property of steel.

Improvement of the Corrosion Resistance of High Alloyed Austenitic Cr-Ni-Mo Stainless Steels by Solution Nitriding

Characteristic features of austenitic steel grades combine a good corrosion resistance with a low hardness, wear resistance and scratch resistance. An interesting possibility for improving the wear behaviour of these steels without loss of their corrosion resistance lies in enriching the near surface region with nitrogen. The process of a solution nitriding allows the rise of the solution of nitrogen in the solid phase. On this state nitrogen increases the corrosion resistance and the tribilogical load-bearing capacity. The aim of the study was, to investigate the improvement of the pitting corrosion behaviour by solution nitriding. A special topic was to observe the effect of nitrogen by different molybdenum content. So austenitic stainless steels (18% Cr, 12% Ni, Mo gradation between 0.06 to 3.6%) had been solution nitrided. The samples could be prepared with various surface content of nitrogen from 0.04 to 0.45% with a step-by-step grinding. The susceptibility against pitting corrosion of these samples had been tested by determination of the stable pitting potential in 0.5M and 1M NaCl at 25°C. For the investigated steel composition and the used corrosion system there is no influence of molybdenum on the effectiveness of nitrogen. The influence of nitrogen to all of the determined parameters can be described well by PRE = Cr + 3,3 * Mo + 25 *N. XPS analysis of the sample surfaces support the results of the pitting corrosion tests.Additionally surface investigations with an acid elektolyte (0,1M HC1 + 0,4M NaCl) were performed. In this case the passivation effective nitrogen content increases markedly with rising molybdenum concentration of the steel. Obviously an interaction of Mo and N is connected with a strongly acid electrolyte.

Effect of short-time low-temperature austenitizing on microstructure and mechanical properties of DT300 ultra-high strength steel fabricated by laser powder bed fusion

To address the inhomogeneous microstructure and improve the mechanical properties of DT300 ultra-high strength steel specimens fabricated by laser powder bed fusion,different post-heat treatment schedules are performed.With the increase in austenitizing temperature and time,the migration rate of austenite grain boundaries continuously increases with the dissolution of nano-carbides,and the formation of nano-oxides and twin martensite is also inhibited accordingly.The rapid growth in the size of prior austenite grains and martensite laths,as well as the decrease in the content of nano-oxides and twin martensite,led to a rapid decrease in the strength(yield strength and ultimate tensile strength)from HT2 to HTF specimens.The HT1 specimens(austenitizing at 830℃for 30 min,then oil quenching and tempering at 300℃for 120 min and finally air cooling)display excellent mechanical properties of yield strength of 1572 MPa,ultimate tensile strength of 1847 MPa,elongation of 9.84%,and fracture toughness of 106 MPa m^(1/2),which are counterparts to those of conventional DT300 steel forgings after heat treatment.

Welding of nickel free high nitrogen stainless steel: Microstructure and mechanical properties

High nitrogen stainless steel(HNS) is a nickel free austenitic stainless steel that is used as a structural component in defence applications for manufacturing battle tanks as a replacement of the existing armour grade steel owing to its low cost, excellent mechanical properties and better corrosion resistance.Conventional fusion welding causes problems like nitrogen desorption, solidification cracking in weld zone, liquation cracking in heat affected zone, nitrogen induced porosity and poor mechanical properties.The above problems can be overcome by proper selection and procedure of joining process. In the present work, an attempt has been made to correlate the microstructural changes with mechanical properties of fusion and solid state welds of high nitrogen steel. Shielded metal arc welding(SMAW), gas tungsten arc welding(GTAW), electron beam welding(EBW) and friction stir welding(FSW) processes were used in the present work. Optical microscopy, scanning electron microscopy and electron backscatter diffraction were used to characterize microstructural changes. Hardness, tensile and bend tests were performed to evaluate the mechanical properties of welds. The results of the present investigation established that fully austenitic dendritic structure was found in welds of SMAW. Reverted austenite pools in the martensite matrix in weld zone and unmixed zones near the fusion boundary were observed in GTA welds. Discontinuous ferrite network in austenite matrix was observed in electron beam welds.Fine recrystallized austenite grain structure was observed in the nugget zone of friction stir welds.Improved mechanical properties are obtained in friction stir welds when compared to fusion welds. This is attributed to the refined microstructure consisting of equiaxed and homogenous austenite grains.

Microstructure and pitting corrosion of shielded metal arc welded high nitrogen stainless steel

The present work is aimed at studying the microstructure and pitting corrosion behaviour of shielded metal arc welded high nitrogen steel made of Cromang-N electrode. Basis for selecting this electrode is to increase the solubility of nitrogen in weld metal due to high chromium and manganese content. Microscopic studies were carried out using optical microscopy(OM) and field emission scanning electron microscopy(FESEM). Energy back scattered diffraction(EBSD) method was used to determine the phase analysis, grain size and orientation image mapping. Potentio-dynamic polarization testing was carried out to study the pitting corrosion resistance in aerated 3.5% NaCl environment using a GillAC electrochemical system. The investigation results showed that the selected Cr-Mn-N type electrode resulted in a maximum reduction in delta-ferrite and improvement in pitting corrosion resistance of the weld zone was attributed to the coarse austenite grains owing to the reduction in active sites of the austenite/delta ferrite interface and the decrease in galvanic interaction between austenite and delta-ferrite.

核电厂定位销局部干法水下TIG焊接工艺

以核电厂定位销水下焊接为研究对象,研制双层气体保护的局部干法水下TIG旋转焊枪,运动轨迹通过直流电机带动钨极绕固定直径转动实现.对核级材料Z2CN19-10控氮奥氏体不锈钢进行焊接,研究局部干法水下TIG焊接焊缝成形,优化工艺参数,结合热循环曲线及电弧形态,分析焊接接头显微组织及力学性能.结果表明,当焊枪的内层和外层保护气均通氩气时,焊缝成形良好,电弧形态稳定;增大焊接电流或减小焊接速度均使焊缝熔深增大,熔宽增大;对比水下焊接接头和陆上焊接接头发现,水的快速冷却作用会促使熔合线附近的铁素体的形态由树枝状转变为板条状,奥氏体含量减少,焊缝中心晶粒细化;水下焊接接头的显微硬度和力学性能略高于陆上焊接接头.