Nitrogen-alloyed Austenitic Stainless Steel Research Articles

Mechanical and Erosion Characterization of Untreated and Solution-Treated Nitrogen-Alloyed (23-8N) Austenitic Stainless Steel

Mechanical and Erosion Characterization of Untreated and Solution-Treated Nitrogen-Alloyed (23-8N) Austenitic Stainless Steel

Slurry Erosion Study on Nitrogen-Alloyed Austenitic Stainless Steel and Weld Beads

Slurry erosion study on nitrogen-alloyed austenitic steel (NA-ASS) 23-8-N has been conducted. Solution annealing of base metal at two different temperatures 950 °C and 1150 °C dissolved the carbides precipitations and improved its strength and impact toughness. Bead-on-plate welds were made using gas metal arc welding process and these were solidified with 5–10% ferrite. Solution annealing results in homogenous microstructure with fine residual carbides and higher erosion resistance. Higher impact toughness and strength of 1150 °C solution annealed samples resulted in higher strain hardening with a minor loss of hardness. Solution treatment enhanced the erosion resistance of the base metal. Cumulative weight loss of bead-on-plate weld was relatively higher than that of the base material due to lower hardness and compositional difference. The weight loss from the surface due to erosive action was minimum for 1150 °C at 90° impingement angle. Wear and material loss occurred through micro-cutting and ploughing.

Weld joint properties of nitrogen-alloyed austenitic stainless steel using multi-pass GMA welding

The welding of nitrogen (0.29 wt%)-alloyed austenitic steel (grade 23-8-N) was performed with gas metal arc welding process. Solution treatment was performed at 950 °C and 1150 °C on base metal prior to weld. Base metal after second treatment has maximum ultimate tensile strength of 942 MPa and impact toughness 66 J. The microstructures of different zones of the weld joint were characterized using an optical microscope and field scanning electron microscope (FESEM). The microhardness, tensile and impact toughness tests of the weldments were conducted along with weld ferrite evaluation. ER2209 duplex filler metal used for welding has lower C and N content which changed the weld solidification mode. Weld has microstructure containing austenite + ferrite. Being a strong austenite former, nitrogen caused minimum ferrite near weld–HAZ interface while maximum ferrite content was observed at weld centre. Weld metal has minimum while base metal has maximum microhardness. UTS (892 MPa) and impact strength (96 J) of weld made on 1150 °C solution-treated base metal were maximum as compared to other weld joints.

Hot Deformation Characteristics of 18Cr-5Ni-4Cu-N Stainless Steel Using Constitutive Equation and Processing Map

The hot deformation of 18Cr-5Ni-4Cu nitrogen-alloyed austenitic stainless steel was tested with a Gleeble-1500D simulator in the temperature range of 1273–1473 K and in the strain rate range of 0.01–10 s−1. The Zener-Hollomon parameter method was used to construct a constitutive equation for high-temperature plastic deformation. The energy dissipation diagram of the material was calculated based on dynamic material modelling (DMM). The microstructural variations were characterized via X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) with energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM). The thermodynamic calculation results showed that the addition of nitrogen to 18Cr-5Ni-4Cu steel promoted the formation of Cr2N and gas phases and expanded the austenite phase region; these results were consistent with the XRD test results of the solid solution sample. The hot deformation activation energy after nitrogen addition was 556.46 kJ∙mol−1. The processing map predicted that the optimum hot working regimes were in the temperature range of 1416–1461 K, where ln ε ˙ was 0.75–1 on the power dissipation map. At high temperatures and a small strain rate, dynamic recrystallization easily occurred. The TEM analyses showed that nano-scale M23C6 and Cr2N precipitated at the grain boundary, and NbC with a diameter of approximately 150 nm appeared along the grain boundary, resulting in grain boundary strengthening. The phase precipitation results were consistent with the Thermo-Calc calculation results. The nitrogen solid solution in the steel promoted the precipitation of nitrides, which caused grain boundary strengthening. Thus, the grain boundary stress increased and wedge-shaped grain boundary cracks formed.

Processing of gas-nitrided AISI 316L steel powder by laser powder bed fusion – Microstructure and properties

This work investigated the processing of high nitrogen-alloyed austenitic stainless steels by laser powder bed fusion (L-PBF). Prior to L-PBF processing, the AISI 316 L steel powder was nitrided at a temperature of 675°C in a 3 bar nitrogen atmosphere, thus achieving a N content of 0.58 mass-%. By mixing nitrided 316 L powder with untreated 316 L powder, two different powder mixtures were obtained with 0.065 mass-% and 0.27 mass-% nitrogen, respectively. After nitriding and mixing, the powder was characterized in terms of its flow properties and chemical composition. The nitrided steel powder was then processed by L-PBF, and the microstructure as well as the chemical composition were investigated by means of scanning electron microscopy and carrier gas hot extraction. It was shown that nitriding of steel powders in an N2 atmosphere can be used to significantly increase the nitrogen content of the powder without impairing its flow properties. With increasing nitrogen content of the powder, the porosity within the L-PBF built specimens increased. However, both the yield strength and the tensile strength were greatly improved without a marked reduction in the elongation at fracture of the respective steels. This work shows that nitrogen-alloyed austenitic stainless steels can be processed by L-PBF and the mechanical properties can be improved.

Cracking the Initiation Mechanism of High Cu-Bearing Nitrogen-Alloyed Austenitic Stainless Steel in the Process of Hot Deformation

The cracking initiation mechanism of high Cu-bearing nitrogen-alloyed austenitic stainless steel was systematically investigated by using a Gleeble-1500D simulator under different strains and deformation temperatures in the hot deformation process. The cracking initiation process and microstructure variations were characterized by optical microscopy (OM), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) with energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM). In the deformation process, Cu-rich and Cr-rich phases were found around the microscopic crack at the strain of 0.5. Cu content was found to be higher at the grain boundary than inside the grain. The equilibrium phase diagram calculated by Thermo-calc shows that Cu precipitates out in the form of an elementary substance below 1022 °C, when the Cu mass fraction reaches 5%. Meanwhile, dislocation walls and twin crystals were observed by TEM. The results show that the synergistic effect of the secondary phases, such as M23C6 precipitated along the grain boundary and stress concentration, lead to crack generation, which is lower at high temperature and low temperature and is higher at 1100 °C and increase as the strain increases.

Cavitation Erosion and Solid Particle Erosion Behaviour of a Nitrogen Alloyed Austenitic Stainless Steel

Cavitation Erosion and Solid Particle Erosion Behaviour of a Nitrogen Alloyed Austenitic Stainless Steel

Solid Particle Erosion Behaviour of Martensitic and Nitrogen Alloyed Austenitic Stainless Steel

In the present work, comparison of solid particle erosive behaviour of 13/4 steel and Nitrogen alloyed austenitic stainless steel (23/8N steel) have been studied using air jet erosion testing. Air jet erosion testing has been performed at various angles for as received samples at 31 m/s velocity and a discharge rate of 3 gm per minute by alumina particles with angular shape and 53-63μm size. Microstructure, eroded surface characteristics, mechanical properties and erosion rate are characterized by using scanning electron microscope, hardness and tensile testing. 23/8N steel shows good hardness coupled with high tensile toughness and work hardening ability, which leads to improved erosion resistance. The surface morphology analysis of the worn out surfaces, cumulative weight loss measurements and steady state of erosion rate results were used to understand the effect of nitrogen on the degradation mechanisms. The results show that 23/8N steel exhibits excellent resistance to erosion in comparison to13/4 steel.

Constitutive modeling of nitrogen-alloyed austenitic stainless steel at low and high strain rates and temperatures

In this paper, microstructures-based constitutive relations are introduced to simulate the thermo-mechanical response of two nitrogen-alloyed austenitic stainless steels; Nitronic-50 and Uranus-B66, under static and dynamic loadings. The simulation of the flow stress is developed based on a combined approach of two different principal mechanisms; the cutting of dislocation forests and the overcoming of Peierls–Nabarro barriers. The experimental observations for Nitronic-50 and Uranus-B66 conducted by Guo and Nemat-Nasser (2006) and Fréchard et al. (2008), respectively, over a wide range of temperatures and strain rates are also utilized in understanding the underlying deformation mechanisms. Results for the two stainless steels reveal that both the initial yielding and strain hardening are strongly dependent on the coupling effect of temperatures and strain rates. The methodology of obtaining the material parameters and their physical interpretation are presented thoroughly. The present model predicts results that compare very well with the experimental data for both stainless steels at initial temperature range of 77–1000K and strain rates between 0.001 and 8000s−1. The effect of the physical quantities at the microstructures on the overall flow stress is also investigated. The evolution of dislocation density along with the initial dislocation density contribution plays a crucial role in determining the thermal stresses. It was observed that the thermal yield stress component is more affected by the presence of initial dislocations and decreases with the increase of the originated (initial) dislocation density.

The Influence of the Shielding Gas to the Static and Dynamic Strength Properties of Laser Welded Workhardened Nitrogen Alloyed Austenitic Stainless Steel

As an interstitial atom, nitrogen strengthens the structure of austenitic stainless steel (ASS). It therefore has been used to increase the strength of ASS. On the other hand, work hardening of ASS is a common method to increase the strength of the sheet product. When a work-hardened structure is welded, the strength properties decreases at the melted zone and the heat-affected zone (HAZ) of the weld. The nitrogen content can also be reduced by the effect of the heat input of the weld. Because the width of the soft area of the HAZ depends on the energy input of the weld, the strength of the weld depends on energy input. Therefore, laser welding provides better strength to the welded structure. The role of the shielding gas is also significant. Argon shielding gas is inert, but nitrogen used as a shielding gas can strengthen the weld metal and HAZ microstructure. In this study, the effect of different shielding gases in the laser welding of AISI 201 LN TR type work-hardened ASS are tested and the results are reported. Both non-destructive material and destructive material tests are performed. According to the results of the tensile test, the use of nitrogen as a shielding gas strengthens the laser-welded structure. The results of the low-cycle fatigue test show that fatigue strength improves when nitrogen is used as the shielding gas.

Cyclic deformation behavior in a nitrogen-alloyed austenitic stainless steel in terms of the evolution of internal stress and microstructure

In order to understand the physical nature of cyclic deformation behavior in 316LN under uniaxial loading, the evolution of dislocation structure and density was explored by transmission electron microscopy (TEM) observation on specimens loaded with different cycles. Particular attention was paid to the evolution of internal stress which was evaluated by partitioning the hysteresis loops, and further decomposed into intergranular and intragranular components by taking into account quantitatively the heterogeneous dislocation structure. For medium and high strain amplitudes, planar dislocation structures were observed in most grains during the initial hardening stage, in which the intergranular internal stress dominates the cyclic hardening behavior. The activation of cross slip in the following softening stage not only acts to reduce intergranular internal stress, but also promotes the formation of heterogeneous dislocation distributions which acts to improve the intragranular internal stress. For low strain amplitude, no dislocation rearrangement was observed and the evolution of internal stress was determined only by its intergranular component throughout the entire cycling process.

Temperature Dependence of Tensile Behaviors of Nitrogen-Alloyed Austenitic Stainless Steels

The temperature dependence of tensile behaviors of two nitrogen-alloyed austenitic stainless steels, an annealed 316LN steel and a high-nitrogen austenitic stainless steel (Fe-Cr-Mn-0.66% N), was investigated by tensile test at different temperatures from 293 K down to 77 K. It was found that strength of the two steels increased with decrease of temperature. With a decrease in temperature, the uniform elongation increased for the 316LN steel, whereas it increased followed by a decrease for the high-nitrogen steel. A three-stage hardening behavior occurred in the 316LN steel, but not in the high-nitrogen steel, with decrease of temperature. The strain-induced martensite transformation in the 316LN steel could retard void nucleation and increase the strain-hardening rate, resulting in much higher tensile stress and higher uniform elongation of 316LN steel. It was analyzed that stacking fault energy of the high-nitrogen steel decreased with decrease of temperature, which promoted the twinning and planar slipping in the steel, and resulted in brittle fracture at cryogenic temperatures.

Shielding Gas Oxygen Additions as a Means of Curbing Nitrogen Degassing During the Autogenous Arc Welding of Nitrogen-Alloyed Stainless Steel

This study examined the influence of oxygen additions (0.5% to 2.0%) to argon-rich shielding gas on nitrogen degassing during the autogenous arc welding of a high-manganese nitrogen-alloyed austenitic stainless steel, previously commercially available under the trade name of Cromanite. Autogenous arc welding of this steel in inert shielding gas results in considerable nitrogen losses from the weld pool, characterised by an unstable arc, spattering and violent metal expulsion from the weld pool. Oxygen additions to the shielding gas stabilise the arc and curb nitrogen-induced porosity, but at least 2.0% oxygen (by volume) is required to maintain the weld metal nitrogen content at the level of the parent material prior to welding. The beneficial effect of oxygen additions to the shielding gas is attributed to the formation of a solid MnCr2O4 spinel phase on the weld pool surface during welding, which retards nitrogen degassing by reducing the area available for the adsorption of nitrogen atoms prior to their recombination to form N2. This layer has a granular, irregular appearance and presents a less effective barrier to nitrogen degassing than the continuous, uniform liquid slag layer that forms when the Cr-Ni 300-series austenitic stainless steels are welded in oxygen-containing shielding gas mixtures.

Plastic deformation and fracture behaviors of nitrogen-alloyed austenitic stainless steels

The plastic deformation and fracture behaviors of two nitrogen-alloyed austenitic stainless steels, 316LN and a high nitrogen steel (Fe–Cr–Mn–0.66% N), were investigated by tensile test and Charpy impact test in a temperature range from 77 to 293 K. The Fe–Cr–Mn–N steel showed ductile-to-brittle transition (DBT) behavior, but not for the 316LN steel. X-ray diffraction (XRD) confirmed that the strain-induced martensite occurred in the 316LN steel, but no such transformation in the Fe–Cr–Mn–N steel. Tensile tests showed that the temperature dependences of the yield strength for the two steels were almost the same. The ultimate tensile strength of the Fe–Cr–Mn–N steel displayed less significant temperature dependence than that of the 316LN steel. The strain-hardening exponent increased for the 316LN steel, but decreased for the Fe–Cr–Mn–N steel, with decreasing temperature. Based on the experimental results and the analyses, a modified scheme was proposed to explain the fracture behaviors of austenitic stainless steels.

Dynamical behaviour and microstructural evolution of a nitrogen-alloyed austenitic stainless steel

The aim of this work is to study the mechanical properties of a nitrogen austenitic stainless steel (Uranus B66) and their relation to its microstructural evolution. Quasi-static (10 −3 s −1) and quasi-dynamic (1 s −1) compression tests have been carried out with a universal servo-hydraulic testing machine. Dynamic (>10 3 s −1) compression tests have been performed on a classical split-Hopkinson bar apparatus. These tests, which cover a wide range of plastic strain, show that the material has a high-strain hardening rate, a good ductility and a great strain rate sensitivity. The temperature sensitivity has been determined over a large range, going from 77 K to 673 K. Transmission electron microscopy (TEM) observations have been conducted in order to correlate the microstructure to the mechanical behaviour. Uranus B66 undergoes basically the same structure evolution during both quasi-static and dynamic compression tests. The plastic deformation is governed initially by planar gliding, followed by mechanical twinning when the dislocation density is saturated.

Effects of temperature cycling and nitrogen on the stability of microstructures in austenitic stainless steels

The influence of temperature cycling from room temperature to 77 K on nitrogen-alloyed austenitic stainless steels, Fe24Mn13Cr0.4N, Fe24Mn18Cr3Ni0.6N, Fe1Mn19Cr8Ni0.2N and Fe17Mn14Cr1Ni0.4N, has been studied by optical microscopy and scanning electronic microscopy (SEM). Results show that the phase transformation from austenite to ε martensite takes place due to the temperature cycling and stress concentration, whereas nitrogen can stabilize the austenitic microstructures greatly. Finally, the mechanism of the phase transformation and the structure of ε martensite are discussed in detail.

Mechanical behaviour of nitrogen-alloyed austenitic stainless steel hardened by warm rolling

This investigation deals with the hardening effect, by warm rolling, of the alloy Uranus B66 ®, a nitrogen-alloyed austenitic stainless steel produced by Creusot-Loire Industrie. The warm rolling process leads to substructural changes from the appearance of planar slips at low deformation, the micro-twins formation followed by sequences of their bending, breaking and disappearance at intermediate deformation, and finally to the formation of heavily deformed domains at the highest warm rolling reduction. The mechanical behaviour of the warm rolled Uranus B66, under quasi-static tensile and quasi-static and dynamic compression tests, has been analysed. Warm rolling increases the mechanical resistance to a saturation level and decreases the ductility when compared to that of the as-received material. The dynamic flow stress after warm rolling up to 85% increases to such a level that brittle fracture occurs after small plastic deformation. The origin of the strength saturation is related to the terminal microstructure derived from the warm rolling deformation.

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.

Nitrogen-Alloyed Austenitic Stainless Steels for Wire and Cable Applications

Nitrogen-Alloyed Austenitic Stainless Steels for Wire and Cable Applications

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.