High Nitrogen Austenitic Stainless Steel Research Articles

On the crystal structure of Cr2N precipitates in high-nitrogen austenitic stainless steel. III. Neutron diffraction study on the ordered Cr2N superstructure

The ordered structure of Cr(2)N precipitates in high-nitrogen austenitic steel was investigated utilizing high-resolution neutron powder diffractometry (HRPD). On the basis of the Rietveld refinement of neutron diffraction patterns, the ordered Cr2N superstructure was confirmed to be trigonal (space group P31m), with lattice parameters a=4.800 (4) and c=4.472 (5) A, as suggested in previous transmission electron microscopy studies [Lee, Oh, Han, Lee, Kim & Takaki (2005). Acta Cryst. B61, 137-144; Lee, Kim & Takaki (2006). Acta Cryst. B62, 190-196]. The occupancies of the N atoms in four crystallographic sites [1(a), 1(b), 2(d) and 2(c) Wyckoff sites] were determined to be 1.00 (5), 0.0, 0.74 (9) and 0.12 (3), respectively, reflecting a partial disordering of N atoms along the c axis. The position of the metal atom was specified to be x=0.346 (8) and z=0.244 (6), corresponding to a deviation from the ideal position (x=0.333 and z=0.250). This deviation caused the ((1/3 1/3)(0))-type superlattice reflection to appear. A comparison between the ideal and measured crystal structures of Cr2N was performed using a computer simulation of selected-area diffraction patterns.

Cytotoxicity study of plasma-sprayed hydroxyapatite coating on high nitrogen austenitic stainless steels

Stainless steel has been frequently used for temporary implants but its use as permanent implants is restricted due to its low pitting corrosion resistance. Nitrogen additions to these steels improve both mechanical properties and corrosion resistance, particularly the pitting and crevice corrosion resistance. Many reports concerning allergic reactions caused by nickel led to the development of nickel free stainless steel; it has excellent mechanical properties and very high corrosion resistance. On the other hand, stainless steels are biologically tolerated and no chemical bonds are formed between the steel and the bone tissue. Hydroxyapatite coatings deposited on stainless steels improve osseointegration, due their capacity to form chemical bonds (bioactive fixation) with the bone tissue. In this work hydroxyapatite coatings were plasma-sprayed on three austenitic stainless steels: ASTM-F138, ASTM-F1586 and the nickel-free Böhler-P558. The coatings were analyzed by SEM and XDR. The cytotoxicity of the coatings/steels was studied using the neutral red uptake method by quantitative evaluation of cell viability. The three uncoated stainless steels and the hydroxyapatite coated Böhler-P558 did not have any toxic effect on the cell culture. The hydroxyapatite coated ASTM-F138 and ASTM-F1586 stainless steels presented cytotoxicity indexes (IC50%) lower than 50% and high nickel contents in the extracts.

Microstructural thermostability of high nitrogen austenitic stainless steel

A high nitrogen austenitic stainless steel, Fe–24Mn–18Cr–3Ni–0.62N, was isothermally annealed from 750 to 900 °C for different times to investigate the thermostability of its microstructures. Results show that the precipitates were Cr 2N and initially formed along the grain boundary. The time–temperature–precipitation diagram was established, according to which the critical cooling rate of this material with less than 0.1–0.5 vol.% of precipitated Cr 2N was specified as 30 °C/min. The microhardness of the matrix in samples annealed at different temperatures decreased with the diffusion of nitrogen from the matrix to grain boundaries.

On the crystal structure of Cr2N precipitates in high-nitrogen austenitic stainless steel. II. Order–disorder transition of Cr2N during electron irradiation

The crystal structure and order-disorder transition of Cr2N were investigated utilizing transmission electron microscopy (TEM). Based on the analyses of selected-area diffraction (SAD) patterns, the crystal structure of the ordered Cr2N superstructure was confirmed to be trigonal (P31m), characterized by three sets of superlattice reflections (001), ((11/33)0) and ((11/33)1). During electron irradiation, the superlattice reflections gradually disappeared in the regular sequence (001), ((11/33)0) and ((11/33)1), indicating that the order-disorder phase transition of Cr2N occurred. The convergent-beam electron diffraction (CBED) observation revealed that the space group of disordered Cr2N is P6(3)/mmc, which corresponds to an h.c.p. (hexagonal close packed) sublattice of metal atoms with a random distribution of N atoms in six octahedral interstices. The redistribution model of N atoms through the order-disorder transition is discussed based on the characteristics and disappearing sequence of superlattice reflections.

加圧溶解高窒素オーステナイト鋼の特性

加圧溶解高窒素オーステナイト鋼の特性

In situ SEM tensile test of high-nitrogen austenitic stainless steels

Using tensile test and in situ SEM, the microstructures of two kinds of high-nitrogen austenitic stainless steels were investigated. Deformation mechanisms, especially slip bands and deformation twins, are discussed. Results show that the slip and twinning both occur during the tensile testing. Slip bands stretch across each grain and are broadened during the tensile test and finally become microcracks. Solid solution strengthening is induced by nitrogen-alloying, which enhances the mechanical properties of austenitic stainless steels.

Effect of nitrogen on the plasma (ion)-carburized layer of high nitrogen austenitic stainless steel

Austenitic stainless steel or high nitrogen austenitic steel are used extensively as structural non-magnetic materials. The former, because of their low surface hardness, have low wear resistance. Recently developed plasma (ion)-carburizing improves the surface of austenitic stainless steel by removing the oxide scale. In contrast, high nitrogen austenitic stainless steel are expected as wear-resistance material because the hardness is higher than that of conventional austenitic stainless steel.In this study, two kinds of austenitic stainless steel were plasma (ion)-carburized in an atmosphere of CH4+H2 plasma at 1303 K for 14.4 ks. The metallurgical characteristics were then investigated using XRD (X-ray diffraction method), OM (optical microscopy), hardness testing, glow discharge optical emission spectroscopy (GDOES) and a scanning electron microscopy (SEM).The results show that the plasma (ion)-carburized low nitrogen austenitic stainless steel had the layer with structure similar to the common austenitic stainless steel. The plasma (ion)-carburized high nitrogen austenitic stainless steel had the carbide layer at the top surface, carbide dispersed layer in the matrix, and then lamellar structure of carbides toward the inner layer.

Vertical-up and -down Laser Plasma Powder Hybrid Welding of a High Nitrogen Austenitic Stainless Steel

Although laser hybrid welding is increasingly introduced into the manufacturing process of several industrial branches, such technologies have up to the present only been qualified for simple horizontal positions. But, particularly in the shipbuilding and offshore as well as in the automotive and transportation industry there is a high demand to incorporate such innovative procedures also to out-of-position welding of assembly sections. Laser plasma powder arc welding (LPPAW) has already been proven to be a stable hybrid process and less susceptible to production related incidents than other hybrid processes. For instance, misalignment and root gap displacements can most smoothly be compensated, due to separation of the filler material feeding from the arc energy input. As a further step towards the application of LPPAW to real production welding, first results of vertical-up and vertical-down welding of 4mm thick high nitrogen austenitic stainless steel (1.4565) plates are presented in the present contribution. Several welding parameters have been varied and their effects on the weld cross sectional shape have been studied with the most important result, that the laser plasma powder process can principally be applied to out-of-position conditions. Similar to previous investigations of LPPAW in the horizontal position, the results of vertical-up and vertical-down welding are significantly dependent on the geometric configuration of the plasma torch towards the laser beam. Aside from these geometric parameters, the influences of the welding speed and of the plasma powder arc welding current have been studied.

On the crystal structure of Cr2N precipitates in high-nitrogen austenitic stainless steel

The crystal structure of Cr(2)N precipitates in high-nitrogen austenitic stainless steel was investigated by transmission electron microscopy (TEM). Based on the analyses of selected area diffraction (SAD) patterns, the crystal structure of Cr(2)N was confirmed to be trigonal (P31m) and was characterized by three sets of superlattice reflections: (001), ((11/33)0)and ((11/33)1). These could be explained in terms of the epsilon-type occupational ordering of nitrogen. The static concentration waves (SCWs) method was applied to describe the ordered superstructure of Cr(2)N. The occupation probability function (OPF) for describing the distribution of N atoms in the Cr(2)N superstructure was derived based on the superlattice reflections obtained in the SAD patterns and could be expressed as: n(r)=c-1/6eta1cos2piz+4/3eta3cos(2pi/3)(x+y+3z). The crystallographic models for epsilon-type ordering, mainly suggested in the Fe-N system, were discussed in comparison to the present model.

Numerical simulation of Cr2N age-precipitation in high nitrogen stainless steels

At the temperature raging from 700 to 950 °C, the Cr2N age-precipitation in high nitrogen austenitic stainless steels Fe24Mn18Cr3Ni0.62N was investigated in this paper. A qualitative mathematical model of Cr2N age-precipitation, ln tS = f (Me,1/T), was established based on the thermodynamics and kinetics and phase transformation theories. Satisfactory results were obtained by means of the test of artificial neural network. This mathematical model can be applied to the calculation design and predication of Cr2N age-precipitation in high nitrogen stainless steels.

Numerical simulation of Cr2N age-precipitation in high nitrogen stainless steels

Abstract At the temperature raging from 700 to 950 °C, the Cr2N age-precipitation in high nitrogen austenitic stainless steels Fe24Mn18Cr3Ni0.62N was investigated in this paper. A qualitative mathematical model of Cr2N age-precipitation, ln tS = f (Me,1/T), was established based on the thermodynamics and kinetics and phase transformation theories. Satisfactory results were obtained by means of the test of artificial neural network. This mathematical model can be applied to the calculation design and predication of Cr2N age-precipitation in high nitrogen stainless steels.

Influence of atmospheric humidity and grain size on the friction and wear of high nitrogen austenitic stainless steel

Research on steels is still active and is motivated by the need to make further improvements in their properties. Among the different steel types, austenitic stainless steels possess good corrosion resistance and formability. However, they also have a relatively low yield strength. One method of increasing the yield strength is by alloying the steel with nitrogen. Such nitrogen-alloyed austenitic stainless steels exhibit attractive properties such as high strength and ductility, good corrosion resistance and reduced tendency to grain boundary sensitation [1]. The high austenitic potential of nitrogen allows the reduction of nickel content in steel. This offers additional advantages such as cost savings and makes the steel more suitable for stainless steel applications involving contact with human skin for people with nickel allergies. Due to the above-mentioned properties, in recent years significant efforts have been devoted to the production of high nitrogen austenitic stainless steels, and metallurgists have been very active in research concerning this class of steels. Particularly, considerable attention has been paid to solidification mechanisms and recrystallization processes [2–4]. Nitrogen in solid solution produces, in the austenitic stainless steel, many desirable properties: it stabilizes the austenite γ phase and increases the resistance to intercrystalline and pitting corrosion, but the most important property is the increase of the yield strength (RP0.2) without a corresponding decrease in ductility. There is another way of increasing the yield strength without severely affecting the ductility. This can be achieved by grain refining. Since austenitic stainless steels do not undergo phase transformation at typical annealing temperatures, the only way to refine the grain size is by recrystallization after cold rolling. An alternative way to obtain fine grains is by applying an austenite-martensite-austenite transformation. In previous publications [5, 6], we examined the influence of grain size and chemical composition on the mechanical properties and corrosion resistance of this family of steels.

Wear Resistance of Fine-Grained High Nitrogen Austenitic Stainless Steel Coated with Amorphous Carbon Films: The Soft X-ray Spectroscopy Approach

The wear resistance dependence on grain size of a high-nitrogen-alloyed austenitic stainless steels (HN) is investigated and compared to measurements for the same samples coated with amorphous carbon (a-C:H) and nitrogenated amorphous carbon (a-C:H(N)) films, deposited by means of plasma-enhanced chemical vapour deposition. A synergic effect between the grain refining and the film in the case of nitride amorphous carbon overcoats is observed in terms of increased low-friction performance duration. The temperature dependence of the wear resistance of a micro crystalline HN stainless steel coated with carbon films is also investigated. An overall decrease of the films' wear resistance is found with increasing temperature. Furthermore, a higher wear resistance is found in the a-C:H coated steel with respect to the a-C:H:N material. High-lateral-resolution photoemission microscopy reveals that inhomogeneities within the film after wear testing are correlated to an increase of the number of N-sp2 C-bonded sites. The study of energy-distribution curves and high-lateral-resolution images on the nitrogenated samples shows that a modification of the surface chemistry occurs by mechanical action; in particular this implies that existing N-sp2 C sites are beginning to cluster as temperature increases.

Nanograin size, high-nitrogen austenitic stainless steels

Abstract A new class of high-strength steels has been developed by combining solid solution hardening and grain boundary hardening in high-nitrogen austenitic stainless steels. Average grain diameters from 50 to 250 nm can be achieved, and yield strength levels of more than 2000 MPa in the solution-annealed condition have been measured. The effect of nitrogen solid solution hardening and grain boundary hardening is systematically explored.

Sliding wear behaviour of an electrochemically modified austenitic high-nitrogen steel surface

Wear debris from artificial metallic implant joints is known to provoke detrimental foreign-body reactions in the surrounding human tissue. Although commonly used biotolerant metals generate only a little amount of particles, wear is still a major cause for concern. It is the aim of this work to evaluate the sliding wear resistance of a topologically modified high-nitrogen austenitic stainless steel. A disc-on-pin test in self-mating contact is performed in distilled water. Submicron particles are trapped by the structured topography and form together with the plastically deformed metal a hybrid surface which has the potential to significantly improve the tribological behaviour of the tested high-nitrogen steel.

Formation of nitrogen bubbles during the solidification of 16–18% Cr high nitrogen austenitic stainless steels

Nitrogen alloyed austenitic stainless steels (NAASS) exhibit attractive properties as high strength and ductility, good corrosion resistance and reduced tendency to grain boundary sensitisation. However, slabs produced by these steels with nitrogen content above about 0.2 mass% show the presence of small pores located just below the slab surface, with consequent downgrading of the product. The origin of the pores is to be attributed to the formation of gas bubbles due to segregation/supersaturation phenomena occurring during solidification of gas forming solutes. Thermodynamic methods and mathematical analysis of solidification have been used to predict the composition adjustments, still remaining in the NAASS grade, which permit to avoid the formation of bubbles. Predictions have been tested against results of laboratory scale solidification trials and found satisfactory.

Cavitation erosion resistance of a high nitrogen austenitic stainless steel as a function of its grain size

Cavitation erosion resistance of a high nitrogen austenitic stainless steel as a function of its grain size

Fatigue behavior of a high nitrogen austenitic stainless steel as a function of its grain size

Fatigue behavior of a high nitrogen austenitic stainless steel as a function of its grain size

Wear resistance of a high-nitrogen austenitic stainless steel coated with nitrogenated amorphous carbon films

In this paper, the dependence of wear resistance on grain size of nitrogen-alloyed austenitic stainless steel is investigated and compared to measurements for the same samples coated with nitrogenated amorphous carbon [a-C:H(N)] film, deposited by means of plasma-enhanced chemical vapour deposition. To this aim, ball-on-disk (BoD) tests were performed to investigate the wear stability of coated and uncoated substrates. Long-time test results on the uncoated material show that no improvement in the low-friction performance (LFP) duration was found with decreasing grain size of the substrate. On the other hand, an improvement in the low-friction performance duration with substrate microstructure refinement was found in the case of nitrogen-doped overcoats. An effect of the film structure on the LFP duration was also detected in the case of the finest-grained samples, indicating a synergic effect between the ultrafine grain structure and the nitrogen-doped carbon coating.

Wear resistance of a high nitrogen austenitic stainless steel coated with amorphous carbon films: influence of grain size and film composition

In this paper, the wear resistance of nitrogen alloyed austenitic stainless steels is, for the first time, investigated and compared to that measured for the same samples coated with amorphous carbon (a-C:H) films deposited by means of plasma enhanced chemical vapour deposition. Ball on Disk (BoD) tests are performed in order to investigate the wear stability of coated and uncoated substrates with different grain sizes. For the uncoated samples, the BoD results show a strong dependence of the wear track volume versus grain size. Furthermore, a strong improvement of the wear resistance is found in a-C:H coated specimens with respect to the uncoated ones with the same substrate microstructure. The effects of nitrogen incorporation into the films are also investigated showing an improvement of the tribological properties of the coated stainless steel. In particular, a synergic effect is detected between the grain refining and the film in the case of nitride amorphous carbon overcoats with a consequent increase of the wear resistance.