High Nitrogen Austenitic Stainless Steel Research Articles

Microstructure and mechanical properties of flash butt welding joint of high nitrogen austenitic stainless steel

The microstructure and properties of high nitrogen stainless steel welded joints under different flash currents were studied by flash butt welding. The results show that, the flash welding parameters have obvious influence on the joint structure and mechanical properties. The joint is organized into symmetrical distribution, and the grains obtained in the interface area are small, mostly pearlite and ferrite. With the increase of flash current, grain coarsening in overheated zone (OZ) and welding zone (WZ) of joint, which was mainly attributed to the increase of welding heat input. Martensite exists on both sides of the weld, in which the hardness reaches the peak. The tensile strength of welded parts can reach up to 97% of the base material, and the fracture is in the form of ductile fracture.

Effect of Sensitizing Treatment on the Microstructure and Susceptibility to Intergranular Corrosion of High-Nitrogen Austenitic Stainless Steel

This experiment investigated the effect of different sensitizing treatment times (2–48 h) at 800 °C on the microstructure and intergranular corrosion of high-nitrogen austenitic stainless steel by using scanning electron microscopy, energy-dispersive spectroscopy, double loop electrochemical potentiokinetic reactivation, and an oxalic acid etch test. The results indicate that with increasing sensitizing time, the austenite grains grew significantly and the carbide quantity began to increase at the grain boundary. The lamellar pearlite-like Cr2N was precipitated when the specimens were aged at 800 °C for 12 h. DL-EPR testing showed that IGC susceptibility of high-nitrogen austenitic stainless steel is mainly caused by the precipitation of carbide and increases when sensitization time is increased from 2 to 48 h.

Friction and wear behaviors of a high nitrogen austenitic stainless steel Fe-19Cr-15Mn-0.66N

The friction and wear behaviors of a Fe-19Cr-15Mn-0.66N high nitrogen austenitic stainless steel (HNSS) were investigated. Tribological investigations were carried out under different applied loads of 5 N, 10 N, 15 N, and 20 N. Scanning electron microscope (SEM) and laser scanning confocal microscope (LSCM) were used to understand the wear mechanisms under different loads and the reasons for the improved wear resistance. The lower friction coefficient and improved wear resistance were observed with the increase in applied loads. Under a higher load, the friction enhanced the work hardening ability of HNSS, which in turn improved its surface hardness and thus the increased wear resistance of HNSS.

Research on Microstructure Evolution of Deposited Metal of Low Nickel High Nitrogen Austenitic Stainless Steel

In this paper, in situ observation of the cooling process of the deposited metal of low nickel high nitrogen austenitic stainless steel obtained by laser‐arc hybrid surfacing welding with shielding gas containing only Ar and only N2, respectively, is carried out using the ultra‐high‐temperature confocal laser scanning microscope (CLSM). An in‐depth analysis of the microstructural changes is conducted with SEM, EDS, and EBSD. The results showed that the surface substructure is refined during crystallization with shielding gas Ar. However, grains are seriously coarsened in the δ phase area. Widmanstatten γ and intragranular γ are formed as a result of δ ⟶ γ phase transition through the shear‐diffusion mechanism. In the cooling process with shielding gas N2, the temperature range of each phase area narrowed obviously. Blocky γ began to appear because of the δ ⟶ γ phase transition through the diffusion mechanism. Generally, Ar and N2 have a strong effect only on the very shallow range from the surface. The area with a larger local misorientation with shielding gas Ar is at lath γ on the surface. With shielding gas N2, the large local misorientation area is the last contact position of γ during the δ ⟶ γ phase transition on the surface and cross section.

Effect of N2 content in shielding gas on formation quality and microstructure of high nitrogen austenitic stainless steel fabricated by wire and arc additive manufacturing

The composition of shielding gas exerts a crucial influence on high nitrogen austenitic stainless steel (HNASS) deposited by gas metal arc additive manufacturing. The proportion of N 2 in shielding gas greatly affected the formation quality and microstructure of deposited parts. As N 2 proportion improved, the formation quality became worse gradually and the nitrogen concentration of deposition metal increased. The maximum nitrogen concentration of deposition metal was more than 1%. However, N 2 porosity in the deposition metal became serious when the N 2 content was over 10 %. The microstructure of the deposition metal changed from two phases of austenite and ferrite in shielding gas of 0∼10 %N 2 to complete austenite with 15 %∼25 %N 2 content in shielding gas. With optimal shielding gas of 5%N 2 +2%O 2 +10%He+83%Ar, the deposited parts of HNASS demonstrated good formation and excellent properties. The average ultimate tensile strength of 920 MPa was achieved and the elongation was 27.7%.

Cathodic and Anodic Stress Corrosion Cracking of a New High-Strength CrNiMnMoN Austenitic Stainless Steel

A new high-nitrogen austenitic stainless steel with excellent mechanical properties was tested for its resistance to stress corrosion cracking. The new conventional produced hybrid CrNiMnMoN stainless steel combines the excellent mechanical properties of CrMnN stainless steels with the good corrosion properties of CrNiMo stainless steels. Possible applications of such a high-strength material are wires in maritime environments. In principle, the material can come into direct contact with high chloride solutions as well as low pH containing media. The resistance against chloride-induced stress corrosion cracking was determined by slow strain rate tests and constant load tests in different chloride-containing solutions at elevated temperatures. Resistance to hydrogen-induced stress corrosion cracking was investigated by precharging and ongoing in-situ hydrogen charging in both slow strain rate test and constant load test. The hydrogen charging was carried out by cathodic charging in 3.5 wt.% NaCl solution with addition of 1 g/L thiourea as corrosion inhibitor and recombination inhibitor to ensure hydrogen absorption with negligible corrosive attack. Slow strain rate tests only lead to hydrogen induced stress corrosion cracking by in-situ charging, which leads to total hydrogen contents of more than 10 wt.-ppm and not by precharging alone. Excellent resistance to chloride-induced stress corrosion cracking in 43 wt.% CaCl2 at 120 °C and in 5 wt.% NaCl buffered pH 3.5 solution at 80 °C is obtained for the investigated austenitic stainless steel.

Kinetics of Cr2N Precipitation and Its Effect on Pitting Corrosion of Nickel-Free High-Nitrogen Austenitic Stainless Steel

In the present work, hot-rolled nickel-free high-nitrogen austenitic stainless steels (HNSS) were solution annealed and aged at 700, 800 and 900 °C for 14 h. Precipitation behavior of Cr2N at the grain boundaries (GB) of HNSS was studied using TC-PRISMA simulation and TEM analysis. Pitting corrosion behavior of HNSS at solution annealed and aged conditions were studied using cyclic polarization and electrochemical impedance spectroscopy (EIS) analysis. TC-PRISMA simulation results showed that equal volume fraction of chromium nitride (Cr2N) precipitates was observed in GBs of austenite in all three aging conditions. However, large size precipitates were observed in samples aged at 900 °C compared to samples aged at 700 and 800 °C. TEM investigations showed the presence of discrete needle like Cr2N precipitates at the GBs and coarse lamellar austenite in regions adjoining the GBs in samples aged at 700 °C aging temperature. Disk-type Cr2N precipitates were observed in GBs of austenite in 900 °C aging temperature. Besides, sample aged at 900 °C exhibited relatively higher repassivation behavior compared to solution annealed and samples aged at 700 and 800 °C.

Enhancement of low-cycle fatigue life of high-nitrogen austenitic stainless steel at low strain amplitude through ultrasonic shot peening

Austenitic stainless steel stabilized by nitrogen has a very low content of undesired nickel (0.1 wt%). Such steel is a much better alternative for bioimplants than that stabilized with a high nickel content (≥ 8 wt%). For biomedical applications, in particular for orthopedic implants, resistance of the implant material to cyclic loading is an essential criterion. This investigation considers the effect of surface treatment by ultrasonic shot peening (USP) on low-cycle fatigue behavior, with 3 mm shots for varying durations of 3–18 min, at total strain amplitudes ranging from ±0.4% to ±0.8%. It was observed that, while the fatigue life was reduced due to USP at the highest strain amplitude of ±0.8%, it was enhanced by ≈ 18 times at the lowest strain amplitude of ±0.4% following 18 min of USP treatment.

Effects of nitrogen content on pitting corrosion resistance of non-magnetic drill collar steel

High-nitrogen (N) austenitic stainless steel (Cr–Mn–N series) is commonly used for non-magnetic drill collars, which exhibits excellent mechanical properties and corrosion resistance. The effects of N content (0.63 to 0.86 wt.%) on the pitting corrosion behavior of the experimental non-magnetic drill collar steel were investigated using the electrochemical tests and immersion tests. Besides, X-ray photoelectron spectroscopy was used to analyze the constitution of the passive film. The results show that with the enhancement of N content from 0.63 to 0.86 wt.%, the metastable pitting corrosion sensitivity of the tested materials in 3.5 wt.% NaCl solution decreased and the pitting corrosion resistance increased. Meanwhile, the corrosion rate in 6 wt.% FeCl3 solution at 30 °C decreased from 10.40 to 4.93 mm/a. On the other hand, nitrogen was concentrated in the form of ammonia (NH4+ and NH3) on the outermost surface of the passive films. The contents of Cr2O3 and Fe2O3 raised in the passive films, together with the content of CrN, at metal/film interface increased as N content increased from 0.63 to 0.86 wt.%, which facilitated protective ability of the passive films, thus contributing to higher pitting corrosion resistance.

Work-Hardening Mechanism in High-Nitrogen Austenitic Stainless Steel

The remarkably high work-hardening rate in high-nitrogen austenitic stainless steels is generally believed to be due to the promotion of dislocation accumulation by nitrogen addition. However, analysis of dislocation accumulation behavior by the modified Williamson-Hall/Warren-Averbach method reveals that no difference to the increment of the dislocation density during deformation exists between austenitic steels with and without nitrogen. Since cross slipping is markedly suppressed in high-nitrogen steels, the moving dislocations are back-stressed by the planar dislocation arrays. This induces the deformation resistance and the high work-hardening rate.

Effect of forge pressure on the microstructure and mechanical properties of high nitrogen austenitic stainless steel joints by continuous drive friction welding

The high nitrogen austenitic stainless steel joint was fabricated by continuous drive friction welding (CDFW). The aim of this work is to study the effect of forge pressure on the microstructure and mechanical properties of austenitic stainless steel joints. The microstructure, the hardness, and the tensile properties of joint were analyzed using optical microcopy, scanning electron microscopy (SEM), a computerized Buehler hardness tester and universal testing machine. The results showed that the welding process resulted in a remarkable microstructure change across the joint. The band [Formula: see text]-ferrites were found in the weld zone (WZ), and it decreased with increasing the forging pressures. The precipitated phase ([Formula: see text]) was observed in the WZ and thermomechanical affected zone in the joint, and it increased with increasing the forging pressure. The lowest hardness was found in the WZ, and it didn’t change significantly at various forging pressure. But the width from the WZ to the heat affected zone (HAZ) increased slightly. With the increase of the forging pressure, the strength increased, the hardening capacity and strain hardening exponent decreased. In addition, all the joints exhibited stage III and IV, unlike the base metal, only stage III appeared. All the tensile specimens failed in the weld center and the fracture surfaces was characterized by a typical dimple at different forge pressure.

Effects of heat input and cooling rate during welding on intergranular corrosion behavior of high nitrogen austenitic stainless steel welded joints

Intergranular corrosion of welded joints using multi-strand winded welding wire was investigated using the double loop electrochemical potentiokinetic reactivation (DL-EPR) tests, intergranular corrosion tests and scanning Kelvin probe force microscopy (SKPFM) tests. The results revealed welded joint in high heat input and low cooling rate shows the highest DOS value, and undergoes the most important weight loss during intergranular corrosion tests. While welded joint in low heat input and cooling rate has the highest corrosion resistance. SKPFM also indicated that welded joint in the low cooling rate and heat input presents low potential and is not easy for electrochemical reaction.

On nitrogen diffusion during solution treatment in a high nitrogen austenitic stainless steel

The generation and microstructural characteristics of the surface layer formed during solution treatment at 1200°C of a high nitrogen austenitic stainless steel, have been investigated using optical microscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS). Contrasting with a bulk microstructure of austenite and ferrite, a multi-layer surface structure is obtained, which consists of a white outer-surface layer which is ferrite and a grey subsurface layer with austenite. Formation of the multi-layer structure occurs through phase transformations between austenite and ferrite due to the various levels of nitrogen diffusion. With increasing solution treatment time, the thicknesses of both the surface and the subsurface layer increase. Hardness tests show that the microhardness increases from the surface layer, to the subsurface layer and then to the bulk. This is attributed to the combined effects of solid solution strengthening from nitrogen and the grain size of austenite or ferrite.

The Microstructure and Mechanical Properties of Multi-Strand, Composite Welding-Wire Welded Joints of High Nitrogen Austenitic Stainless Steel.

A multi-strand composite welding wire was applied to join high nitrogen austenitic stainless steel, and microstructures and mechanical properties were investigated. The electrical signals demonstrate that the welding process using a multi-strand composite welding wire is highly stable. The welded joints are composed of columnar austenite and dendritic ferrite and welded joints obtained under high heat input and cooling rate have a noticeable coarse-grained heat-affected zone and larger columnar austenite in weld seam. Compared with welded joints obtained under the high heat input and cooling rate, welded joints have the higher fractions of deformed grains, high angle grain boundaries, Schmid factor, and lower dislocation density under the low heat input and cooling rate, which indicate a lower tensile strength and higher yield strength. The rotated Goss (GRD) ({110}〈10〉) orientation of a thin plate and the cube (C) ({001}〈100〉) orientation of a thick plate are obvious after welding, but the S ({123}〈63〉) orientation at 65° sections of Euler’s space is weak. The δ-ferrite was studied based on the primary ferrite solidification mode. It was observed that low heat input and a high cooling rate results in an increase of δ-ferrite, and a high dislocation density was obtained in grain boundaries of δ-ferrite. M23C6 precipitates due to a low cooling rate and heat input in the weld seam and deteriorates the elongation of welded joints. The engineering Stress–strain curves also show the low elongation and tensile strength of welded joints under low heat input and cooling rate, which is mainly caused by the high fraction of δ-ferrite and the precipitation of M23C6.

Formation mechanism and application potential of Σ1 boundary in grain boundary engineering of high nitrogen austenitic stainless steel

Σ3 boundaries improve the intergranular corrosion resistance of high nitrogen austenitic stainless steels. However, the regeneration of these boundaries is, unfortunately, accompanied by significant grain coarsening. 18Mn18Cr0.5N and 18Mn18Cr0.6N steels were thermo-mechanically processed and the corresponding microstructures were evaluated. Formation mechanisms were proposed for Σ1 boundaries in different R-Σ3-Σ1-type junctions. These boundaries formed during grain growth accompanying Σ3 formation and reaction. After 30% cold tension and cyclic heating between 500 °C and 1100 °C, numerous Σ1 boundaries were formed and disrupted the high angle grain boundary network efficiently. Moreover, a small grain size was maintained. Σ1 boundaries have significant application potential in grain boundary engineering.

Nanocrystallisation of Nickel Free High Nitrogen Austenitic Stainless Steel through Ultrasonic Shot Peening

Ultrasonic Shot Peening (USP) - a novel route of surface modification was employed on biomedical grade Nickel free high nitrogen stainless steel (18Cr-21Mn-0.65N-balance Fe) to obtain a surface nanostructure without changing its chemical composition and microstructural phase transformation. Hardened steel shots of diameter 2 mm and 3mm were repeatedly impacted on the specimen surface at a constant frequency of 20KHz for 2 and 8 minutes duration. Coarse surface grains of size 36±6µm transformed into nanocrystalline grains of size 13-18 nm. The deformed layer resulted by USP treatment increased with increase in shot diameter and duration of USP. The microstructure was characterized by using optical microscope, SEM, XRD and TEM technique. The hardness and roughness of the treated surface was also found to be strongly dependent on the USP process parameters.

Wetting and brazing characteristic of high nitrogen austenitic stainless steel and 316L austenitic stainless steel by Ag–Cu filler

In the present work, wetting behaviour of Ag–Cu eutectic filler on high nitrogen austenitic stainless steel (HNS) and 316L austenitic stainless steel (316L) base metals was firstly studied using a sessile drop method in high vacuum. The Ag–Cu eutectic filler exhibits completely different dynamic wetting process on the surface of HNS and 316L. The equilibrium contact angle of Ag–Cu/HNS system is lower and its wetting time is shorter than that of Ag–Cu/316L system. Subsequently, brazing experiments were carried out for HNS and 316L base metals using the same filler at 840 °C for a holding period of 5–30 min under high vacuum. Microstructural analysis shows that the 316L/316L joints are composed of α-Cu and fine Ag–Cu eutectic. The HNS/HNS joints consist of β-Ag and coarse Ag–Cu eutectic. The optimal shear strength of HNS/HNS joints (290 MPa) is significantly higher than that of 316L/316L joints (175 MPa) because of the formation of diffusion layer. Therefore, HNS has tremendous potential to take the place of the traditional austenitic stainless steels for industry application.

Effect of Solution Treatment Time on Microhardness of High Nitrogen Austenitic Stainless Steel

High nitrogen stainless steel (HNSS) is becoming an increasingly important engineering material because of its excellent corrosion resistance and good mechanical properties. A hot rolled Cr19Mn19Mo2N0.7 HNSS, having a microstructure consisting of banded ferrite and dotted/dendritic nitrides dispersed in the austenite matrix, was solution treated at 1160oC for various times. It is shown that the nitride can be dissolved, and the shape of ferrite can be changed by the solution treatment. The microhardness of both austenite and ferrite decreases with increasing solution treatment time. This is attributed to an increased grain size and a homogeneous distribution of alloying elements. The results are helpful in controlling the thermomechanical processing of HNSSs in commercial practice.

Hot-Deformation Behavior of High-Nitrogen Austenitic Stainless Steel under Continuous Cooling: Physical Simulation of Surface Microstructure Evolution of Superheavy Forgings during Hot Forging.

Superheavy forgings are increasingly used in the nuclear industry. The strain rate is extremely low during hot forging due to the huge size of the superheavy forging; in fact, the surface temperature of the forging decreases obviously during each deformation step. Hot-deformation behavior differs from that of isothermal deformation. In this study, 18Mn18Cr0.6N steel was selected as a model material. Hot-compression tests were conducted using a Gleeble 3800 simulator at a strain rate of 10−4 s−1 and continuous cooling rates of 0.0125 Ks−1 and 0.025 Ks−1. The microstructure was observed using electron backscatter diffraction analysis and transmission electron microscopy. The flow stress increased with increasing strain: the higher the cooling rate, the higher was the hardening rate. Continuous cooling inhibited dynamic recrystallization by delaying its nucleation. The subgrain/cell size increased linearly with increasing final temperature of deformation in the temperature range 1273 to 1448 K. An intense <001> texture formed in 0.8-strained specimens and the matrix exhibited a low Taylor factor orientation. Most dislocations were separately distributed in subgrains and did not entangle with each other or with the subgrain boundary. Dislocation arrays transferred easily through boundaries and dislocation accumulation at boundaries was weak. This study contributes to understanding the hot-forging process of superheavy forgings.

Explosive Welding of Copper to High Nitrogen Austenitic Stainless Steel

In this work, T2 red copper and high nitrogen austenitic stainless steel (HNASS) were explosively welded for the first time. The welding window was theoretically developed, and the experiment was designed by considering the effect of explosive loading on welding quality. To evaluate the welding quality, the microstructure and mechanical properties of the composite material were systematically investigated. The results showed that the welding quality was in good agreement with the results predicted by the welding window. The micromorphology of the welding interface changed with a varied welding parameter, and no intermetallic compounds were found at the welding interface by EDS analysis. The microhardness of the two raw materials both increased near the welding interface, and the enhancement increased with the explosive loading. The failure pattern of the welding interface in the tension-shear experiments was dominated by the strength failure of the red copper.