Nitrogen Stainless Steel Research Articles

Synergistic effect of Ni and N on improvement of pitting corrosion resistance of high nitrogen stainless steels

The pitting corrosion resistance of Fe18Cr10Mn(0·33–0·69)N, Fe18Cr10Mn1Ni(0·33–0·84)N, and Fe18Cr10Mn0·35N(0–3)Ni alloys were investigated. The pitting potential increased as the N content increased in both Fe18Cr10Mn(0·33–0·69)N and Fe18Cr10Mn1Ni(0·33–0·84)N alloys. The rise in the pitting potential was more pronounced in Fe18Cr10Mn1Ni(0·33–0·84)N alloys than in Fe18Cr10Mn(0·33–0·69)N alloys. However, it was found that Ni alone had no effect on the pitting corrosion resistance of Fe18Cr10Mn0·35N based alloys. Thus, it was concluded that the alloyed N worked synergistically with Ni to promote the pitting corrosion resistance in Fe18Cr10Mn based alloys. Analyses of passive films of Fe18Cr10Mn(0·33–0·69)N and Fe18Cr10Mn1Ni(0·33–0·84)N alloys revealed that N was incorporated into the passive film, with N enriched at the film/metal interface. However, the alloyed N increased the Cr cation fraction in passive films of Fe18Cr10Mn1Ni(0·33–0·84)N alloys, whereas N decreased in that of Fe18Cr10Mn(0·33–0·69)N alloys. This difference was considered as the reason for the synergistic effect between N and Ni in Fe18Cr10Mn based alloys.

Production of High Nitrogen Stainless Steel X8CrMnN18-18 through EAF-AOD-LRF-CC-Steckel Mill Route and its Hot Deformation Study by Gleeble Thermomechanical Simulator

High nitrogen containing austenitic stainless steel X8CrMnN18-18 exhibits attractive combination of high strength, toughness and corrosion resistance.This grade containing more than 5000 ppm of nitrogen was produced commercially through EAF-AOD-LRF-CC-Steckel mill route and its microstructure and mechanical properties were studied. Excellent combination of strength,ductility and toughness is achieved in the entire range of 6mm to 50mm thick hot rolled plates. Uniaxial compression tests were carried out to understand the hot deformation behavior by varying temperature and strain rate. Softening behavior during deformation was analyzed from flow stress strain curves and microstructural analysis. Dynamic recrystallization (DRX) behavior of the material was observed during thermo mechanical processing. Critical strain related to DRX and Avrami kinetics of DRX was calculated by analyzing the flow curve data. Microstructural characterization was done by optical microscopy and EBSD analysis. Extensive grain refinement can be achieved by thermo-mechanical processing controlled by DRX. Keywords: High nitrogen stainless steel, Strength and toughness, DRX, Grain refinement, TMCP

A Comparative Study of Semiconducting Behavior of Passive Film of High Nitrogen and Ni and Mn Free Stainless Steels in 3.5 wt. % NaCl

The semiconducting property of passive films formed on Alloy 1 (18Cr-2Mo-1N)], Alloy 2 (17.5Cr-3Mo-0.5N)] and Alloy 3 (Type 316 SS) were studied by using the Mott-Schottky (M-S) approach in 3.5 wt. % NaCl solution of pH 2, 7 and 12. The M-S analysis shows that the film acts as n - type and p - type semiconductors across the potential range. The donor density of Alloy 1 has been found to be lower by about 31 %, 11 % and 6 % as compared to that of Alloy 2 at pH 2, 7 and 12 respectively. However; Alloy 3 has higher donor density 44 %, 27 % and 30 % in comparison with Alloy 1. The donor density of Alloy 3 found to be greater about 21 %, 18 % and 25 % to the Alloy 2 at pH 2, 7 and 12 respectively. These results indicate that the sensitivity of electrolyte composition and the presence of alloying elements like nitrogen and nickel on the donor density of passive film. High nitrogen stainless steels exhibited a lower donor density that corresponds to good protectiveness, more stable passive film which is in agreement with a low passive current density (ipass), higher pitting potential (Epit) and polarization resistance (Rp).

Comparative Evaluation of 316L and N-Bearing Ni-Free Austenitic Stainless Steel as a Potential Cost-Effective Replacement for Body Implants

Currently austenitic stainless steels, cobalt-chromium alloys and titanium alloys are used in body implants. As per ISO 5832-1, Cr-Ni-Mo alloy 316L with minimum 13% nickel is widely used for body implants. ASTM standard F 2229-07 also permits nitrogen strengthened essentially Ni-free Cr-Mn-Mo alloy (UNS S 29108) for this purpose. Nitrogen as austenite stabilizer is able to substitute nickel. It serves the dual purpose of increasing the strength as well as pitting corrosion resistance. This paper compares the corrosion behaviour of these two grades. Cyclic potentiodynamic tests were carried as per ASTM F2129 in Simulated Body Fluids (SBFs) like Ringers, Hanks and Phosphaste Buffer Saline solution at 37 °C, which corresponds to the human body temperature. The pitting potential was significantly higher for Ni free grade S29108 as compared to 316L. In addition, re-passivation potential of the S 29108 was also far superior than 316L. The reverse scan indicated that the breakdown of the passive film was not reached in S 29108, whereas a hysteresis loop was observed in 316L. The strength of annealed S 29108 is far superior and meets the property requirement of ISO 5832-1 for 316L under cold rolled conditions. Thus this alloy could replace annealed as well as cold rolled 316L as per ISO 5832-1. This promising alloy has an added advantage of being significantly cheaper as compared to 316L and other Ti, Co based alloys to enable cost effective medical care to common man. Keywords: High nitrogen stainless steel, 316L, Bio-Implants, Potentio-dynamic tests.

Corrosion Behavior of High Nitrogen Nickel-Free Fe-16Cr-Mn-Mo-N Stainless Steels

The purpose of the current study is to develop austenitic nickel-free stainless steels with lower chromium content and higher manganese and nitrogen contents. In order to prevent nickel-induced skin allergy, cobalt, manganese, and nitrogen were used to substitute nickel in the designed steel. Our results demonstrated that manganese content greater than 14 wt pct results in a structure that is in full austenite phase. The manganese content appears to increase the solubility of nitrogen; however, a lower corrosion potential was found in steel with high manganese content. Molybdenum appears to be able to increase the pitting potential. The effects of Cr, Mn, Mo, and N on corrosion behavior of Fe-16Cr-2Co-Mn-Mo-N high nitrogen stainless steels were evaluated with potentiodynamic tests and XPS surface analysis. The results reveal that anodic current and pits formation of the Fe-16Cr-2Co-Mn-Mo-N high nitrogen stainless steels were smaller than those of lower manganese and nitrogen content stainless steel.

Research on Powder Metallurgy High Nitrogen Stainless Steels

High nitrogen Fe-18Cr-12Mn-3Mo-xN austenitic stainless steels were prepared by mechanical alloying and powder metallurgical pressing-sintering techniques. The microstructures, nitrogen contents and sintering properties of the milled powders and sintered bodies had been also investigated. The results show that high nitrogen and nearly spherical composite powders with good compressibility can be obtained by high-energy vibration ball milling and high temperature nitriding. It is also found that the sintering temperature has a significant effect on the density and the nitrogen content of sintered specimens, and the optimal sintering temperature are determined to be 1250°C, at which a favorable combination of the relative density as high as 97.1% and the final nitrogen content of 0.79wt% in sintered specimens can be obtained. The sintered bodies mainly consist of γ-austenite, α-ferrite and the lamellar Cr2N phase. After solution-annealing at 1150°C for 1.5h followed by water-quenching, high nitrogen austenitic stainless steels reveal a fully austenitic structure.

Influence of Deformation and Quenching Temperature on Structures and Properties of High Nitrogen Stainless Bearing Steel

The influence of deformation and quenching temperature on microstructure and properties for high nitrogen stainless bearing steel was studied using the calculation of Thermo-Calc software and relevant Fe-based database and some experiments. The results showed that there was a small precipitation amount of carbide in the sample after a large deformation. The value of impact toughness (AKU) was increased from 2.0 J to 12.0 J with the deformation increasing from 71% to 83%. When the quenching temperature reached 1050, the value of the hardness was up to the tiptop. Additionally, according to the calculation, it was found that there was a linear functional relation between the precipitation of carbide M23C6(M) and quenching temperature (T). And the relational model could be expressed as:MM23C6=55.34 - 0.05T(T1000).

Effects of pre-precipitation of Cr2N on microstructures and properties of high nitrogen stainless steel

Aging precipitation and solid solution heat treatment were carried out on three steels which have chromium content of 18%, manganese content of 12%, 15%, 18%, and nitrogen content of 0.43%, 0.53%, 0.67%, respectively. The mechanisms of precipitation and solid solution of high nitrogen austenitic stainless steel were studied using the scanning electron microscopy, transmission electron microscopy, electron probe micro analysis and mechanical testing. The results show that, Cr2N is the primary precipitate in the tested stainless steels instead of Cr23C6. Cr2N nucleates at austenitic grain boundaries and grows towards inner grains with a lamellar morphology. By means of pre-precipitation of Cr2N at 800 °C, the microstructure of the steels at solid solution state can be refined, thus improving the strength and plasticity. After the proposed treatment, the tensile strength, the proof strength and the elongation of the tested steel reach 881 MPa, 542 MPa and 54%, respectively.

Characteristics of Dynamic Recrystallization during Hot Deformation for High Nitrogen Stainless Steels

Dynamic recrystallization was studied for the stainless steels with nitrogen contents of 0.56% to 1.08% during hot deformation at temperatures of 900~1200 with strain rates ranging from 0.003 to 42 s-1. It was found that flow stress could increase remarkably with increasing nitrogen content. Flow curves during the deformation by 0.1~42/s at temperatures of 900~1200°C show a single peak, indicating the occurrence of dynamic recrystallization during deformation. The peak strain seems to decrease with increasing N content, suggesting that higher content of N facilitates dynamic recrystallization. The quenched microstructures were analyzed by optical microscopy, EBSD and TEM. The recrystallized grain sizes on the quenched specimens were measured and its dependence on temperature and strain rate was analyzed. At high temperature, continuously dynamically recrystallized microstructures were observed; whilst at low temperature, necklace-like partially recrystallized microstructures were found. Key words: High nitrogen stainless steel; dynamic recrystallization; stress-strain curves

Direct observation of the passive layer on high nitrogen stainless steel used as bipolar plates for proton exchange membrane fuel cells

The purpose of this report is to directly observe the passive layer and concentration behavior of nitrogen interstitially incorporated in Ni-saving high nitrogen stainless steel (HNS) using an aberration corrected scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) or – electron energy loss spectroscopy (EELS). The thickness of the passive layer barely changed after 1000h single cell operation, compared with the as-polished state. The observed passive layer was thin (3nm) and mainly composed of chromium oxide, as confirmed by STEM-EDS. It was confirmed that nitrogen was not present in the passive layer, but was concentrated at the interface between the passive layer and the metal bulk. The concentrated area ranged approximately 2nm to steel bulk from the interface. With help of the STEM-EDS and EELS, we were able to understand the nature of the passive layer for Ni-saving HNS, which caused remarkable improvement of the cell performance due to superior corrosion resistance.

Some Theoretical Problems Analysis of Melting High Nitrogen Stainless Steels

High nitrogen stainless steels with excellent mechanical and corrosion resistance properties is focused on to develop a new class of engineering material. The manufacture process of high nitrogen stainless steels under high nitrogen pressure is complicated. There are some theoretical problems to be solved for large scale melting and cast high nitrogen stainless steels. The thermodynamic calculation models of nitrogen solubility in the liquid phase, δ ferrite phase and γ austenitic phase built can well predict the nitrogen solubility and analyze the effect of temperature, alloy components and nitrogen pressure on the nitrogen solubility. The minimum nitrogen pressure for preventing the nitrogen porosity formation has been calculated by the micro-segregation. By the analysis nitrogen behavior of melting and cast high nitrogen stainless steels, it is possible to produce high nitrogen stainless steels in a large scale in China.

Some Theoretical Problems Analysis of Melting High Nitrogen Stainless Steels

High nitrogen stainless steels with excellent mechanical and corrosion resistance properties is focused on to develop a new class of engineering material. The manufacture process of high nitrogen stainless steels under high nitrogen pressure is complicated. There are some theoretical problems to be solved for large scale melting and cast high nitrogen stainless steels. The thermodynamic calculation models of nitrogen solubility in the liquid phase, δ ferrite phase and γ austenitic phase built can well predict the nitrogen solubility and analyze the effect of temperature, alloy components and nitrogen pressure on the nitrogen solubility. The minimum nitrogen pressure for preventing the nitrogen porosity formation has been calculated by the micro-segregation. By the analysis nitrogen behavior of melting and cast high nitrogen stainless steels, it is possible to produce high nitrogen stainless steels in a large scale in China.

Analysis of Magnetism in High Nitrogen Austenitic Stainless Steel and Its Elimination by High Temperature Gas Nitriding

Stable austenitic structure in medical stainless steels is basically required for surgical implantation. A weak magnetism was found in a high nitrogen nickel-free austenitic stainless steel for cardiovascular stent application. This magnetic behavior in high nitrogen stainless steel was investigated by optical microscopy, X-ray diffraction (XRD), electron probe microanalysis (EPMA) and superconducting quantum interference device (SQUID). The results showed that the magnetism came from the composition segregation of ferrite formation elements such as Cr and Mo in the steel and some delta-ferrites were locally formed during the pressurized electroslag remelting process. The magnetism of high nitrogen stainless steel could be eliminated by a proper high temperature gas nitriding (HTGN).

High voltage retainable Ni-saving high nitrogen stainless steel bipolar plates for proton exchange membrane fuel cells: Phenomena and mechanism

In the present paper, we elucidate the possible reasons for high voltage retention during operation when a Ni-saving high nitrogen stainless steel (hereafter referred as to Ni-saving HNS) was used as bipolar plates for PEMFCs. A typical type 316L was also employed as bipolar plates to compare the cell performance. The Ni-saving HNS bipolar plate adopting cell obviously exhibited better cell performance at various current densities. After 1000 h of cell operation, the tested metallic bipolar plates were analyzed by scanning electron microscopy, inductively coupled plasma-mass spectrometry, X-ray photoelectron spectroscopy, transmission electron microscopy–energy-dispersive X-ray spectroscopy, and transmission electron microscopy–electron energy loss spectroscopy to prove the superiority of the Ni-saving HNS bipolar plates. As a result, the surface state of the bipolar plates exposed to the PEMFCs environment is the critical factor that affects cell performance. Protection of the metal surface mainly with chromium oxide-based surface layer, which was induced from a reaction of N from the Ni-saving HNS and H 2O generating proton, H +, that lowers local surface pH, was thus substantially effective to retain the voltage upon operation.

Case study: Failure of a high nitrogen stainless steel femoral stem

This paper presents a case study summarising the failure analysis of a high nitrogen stainless steel femoral stem. The failure of the stem occurred after only 4.5 years in service by fatigue failure across the proximal region. The experimental work consisted of a visual inspection, macroscopic inspection, fractography and microscopic examination, surface profilometry and chemical analysis of the material. All metallurgical tests revealed the material to be satisfactory in terms of microstructure, hardness and composition. Inspection of the introducer slots revealed mechanical damage created during surgery to install the stem. The fatigue crack was shown to have initiated from a point of mechanical damage present in one of the introducer slots. This study highlights the detrimental effect of surface damage to initiate fatigue cracks under cyclic loading conditions and the need for greater awareness regarding the possibilities of creating stress raisers such as scratching, scoring and impacts which can lead to premature failure of prostheses.

The influence of concave dimples on the metallic counterface on the wear of ultra-high molecular weight polyethylene

The aims of this study were to investigate how the dimples on a metallic counterface affect the wear rate of ultra-high molecular weight polyethylene (UHMWPE) in a multi-directional reciprocating pin-on-plate machine. The plates were high nitrogen stainless steel, representing the femoral head material. The pins used in this study were of ultra-high molecular weight polyethylene GUR 1120, which represents the material used in an acetabular cup. Three different cases were investigated: a smooth surface without dimples as a reference case, a surface with medium dimples with an average valley of 2.3 µm and a surface with large dimples with an average valley of 5.9 µm. It was found that all the dimples investigated did not improve the lubrication, but instead increased the surface roughness; consequently, the wear of UHWMPE was slightly increased.

Influence of Partial Replacement of Nickel by Nitrogen on Microstructure and Mechanical Properties of Austenitic Stainless Steel

A new modified austenitic stainless steel has been developed through partial replacement of nickel by nitrogen. Nitrogen stainless steel was produced in 10 kg induction furnace under nitrogen pressure, while reference one, AISI 316 steel grade, was produced in open-induction furnace. Both were cast and hot forged, and the total nitrogen was determined. Furthermore, the produced forged steels were subjected to solution treatment at different temperatures. The microstructure of produced stainless steels was observed. The X-ray diffractmeter and Mossbauer effect spectroscopy were used to follow the phase change in reference and modified steels after different heat treatment temperatures. The influence of grain-size, soluble, and insoluble nitrogen on tensile strength and hardness was investigated. The major phase in the modified steel has a fcc structure similar to the reference one, but with finer grains and more expanded lattice. The yield strength and hardness of the nitrogen-modified stainless steel are higher than the reference steel. On the other hand, the increase of nitrogen content deteriorates the steel ductility.

Influence of hot working on microstructure and mechanical behavior of high nitrogen stainless steel

The relationship between microstructures and mechanical properties of a high nitrogen stainless (HNS) steel (0.65N–1.8Ni, wt%) manufactured by argon oxygen decarburization and continuous casting was investigated in this article. The plates with different thicknesses were obtained by thermo-mechanical control process. The results revealed that the plates prepared by hot rolled and solution treated possessed good balanced mechanical properties, i.e., satisfactory strength and higher toughness. Compared with hot-rolled and solution-treated plates, the hot-rolled plate had much higher tensile strength, but its impact toughness was extremely low. Furthermore, with the increase of deformation strain, the plate with finer grains, more precipitates and higher strength was achieved, but its plasticity decreased obviously. The worm-like carbides formed along the grain boundary during the finish rollings at 850 °C, which are detrimental to the toughness of hot-rolled plates. In addition, small amount of ferrite precipitated in the steel due to the non-equilibrium solidification during continuous casting, and to ensure full austenitic structure, composition design method was recommended.

Effects of Surface Active Element on the Biocompatibility of High Nitrogen Stainless Steel

Nickel-free high nitrogen stainless steel has become more important due to its impressive mechanical and corrosion properties. In contrast to high pressure processes, melting plasma in a normal atmosphere is an alternative way of obtaining high nitrogen content at low cost. However, melting in such an atmosphere will bring some surface impurities, like sulfur and oxygen, into the stainless steel through the refractories or the materials themselves. Therefore, this research aims the relationship between the sulfur and nitrogen content, and their influence on biocompatibility. Thermo-Calc is first used to design the composition of Fe-Cr based austenitic nitrogen stainless steel, and melted with different melting techniques, including plasma and an induction melting furnace. The results indicate that the nitrogen content varies with different sulfur content. It is also found that α-Fe (Ferrite) plates are observed near the grain boundaries when the sulfur content reaches a certain level. Besides, the Sulfur content has an obvious influence on the biocompatibility rather than nitrogen contents.

Manufacturing High Nitrogen Austenitic Stainless Steels by Pressurized Induction Furnace

A 25kg pressurized induction furnace with maximum nitrogen pressure 6MPa was invented to manufacture high nitrogen stainless steels. The suitable process was explored, and the analysis of radiography and macrosegregation of nitrogen of high nitrogen steel ingot was performed. The mechanical properties and inclusions of the forging ingot were also investigated. The suitable solidification nitrogen pressure can effectively prevent the formation of nitrogen porosity and macrosegregation of nitrogen. It is very important to control the purity of the nitrogen gas to a higher level for avoiding the manganese loss and decreasing oxygen content in the steel during the nitrogen gas pressurized melting process. The sound and compact macrostructure high nitrogen austenitic stainless steel with nitrogen content above 1.0 wt % has been manufactured by the pressurized induction melting method. There are mainly nonmetallic inclusions with the size less than 5μm in the HNS-A ingot. The HNS-A exhibits excellent mechanical properties.