High Nitrogen Austenitic Stainless Steel Research Articles

Grain-boundary plane crystallography and energy in austenitic steel

The presence of grain boundaries in polycrystalline materials affects the materials properties and performance. Recently it has been realized that boundaries can be manipulated to give better properties, and the design and control of grain boundaries is now an area of strong research interest in the search for high performance engineering materials. Grain boundaries can be classified using the Coincident Site Lattice Model (CSL), which defines the periodicity, i.e., the degree of fit between the two lattices which constitute the boundary. Using this model it is possible to divide boundaries into categories: low angle (up to 15{degree} misorientation), CSL and random i.e., high angle non-CSL. Some CSL boundaries have been shown to have special properties: an example from recent research in the same program as that currently reported has shown that twin boundaries ({Sigma} = 3 in CSL notation) in High Nitrogen Austenitic Stainless Steels do not favor the formation of Cr{sub 2}N precipitates. The research presented here examines grain boundary inclinations of surface grains in austenitic steel specimens which have been isothermally aged at higher 700 C or 800 C. Grain boundary plane crystallography has also been obtained for the 800 C aged sample.

High-nitrogen austenitic stainless steels with high strength and corrosion resistance

An alloy design model has been used to develop an austenitic stainless steel containing 25–28 wt.% Cr, 22 wt.% Ni,6 wt.% Mn, 4–8wt.%Mo, and O.6–O.9 wt.% N. The steel is produced by rapid-solidification powder metallurgy with subsequent consolidation by hot isostatic pressing. The steel exhibits high yield strength and corrosion resistance, both of which increase with increasing nitrogen content, yet high ductility and impact strength are maintained when properly annealed.

High Nitrogen Steels. Sensitivity of a High Nitrogen Austenitic Stainless Steel to Fatigue Crack Initiation.

The low cycle fatigue behaviour of a 0.9%NCr-Mn austenitic stainless has been investigated at room temperature and at 77 K at total strains ranging from Δεt=0.6 to 2.5%. The macroscopic behaviour, cyclic stress amplitude and accommodation period, is strongly influenced by the test temperature. The fatigue resistance has been associated to surface plasticity and to crack initiation, Cycling at low temperature or at room temperature at high strain amplitude favours crack initiation at grain boundaries which is detrimental for fatigue resistance. However, it is found that the fatigue cracks tend to initiate transgranularly which is beneficial for fatigue resistance when the alloy is previously aged at 350°C.

耐食・耐熱材料 超高窒素オーステナイト系ステンレス鋼の温間加工法と引張特性

耐食・耐熱材料 超高窒素オーステナイト系ステンレス鋼の温間加工法と引張特性

High Nitrogen Steels. Effect of Nitride (Cr2N) Precipitation on the Mechanical, Corrosion, and Wear Properties of Austenitic Stainless Steel.

High-nitrogen austenitic stainless steels are thermally unstable and susceptible to nitride precipitation (primarily Cr2N) during elevated temperature exposure. This paper describes the effect of Cr2N precipitation on the mechanical, corrosion, and abrasive wear properties of a high-nitrogen austenitic stainless steel, nominally Fe-19Cr-5Mn-5Ni-3Mo-0.02C-0.7N. In the annealed state, Cr-rich nitrides (Cr2N) precipitate sequentially as intergranular, cellular, and finally, intragranular precipitates. Cold working, prior to aging, increases grain boundary and intragranular precipitation kinetics, but retards cellular phase formation. Nitride precipitation has only a minor influence on the yield and ultimate tensile strength of annealed materials, but causes embrittlement which is enhanced by high strain rate conditions such as impact testing and by the accelerated pricipitation kinetics associated with prior deformation. Nitride precipitation results in sensitization which is also accelerated by prior deformation. The degree of sensitization, as measured by electrochemical potentiokinetic reactivation (EPR) testing, correlates to intergranular, cellular, and intragranular Cr-depletion. Although the degree of sensitization is defined by both the Cr-minimum and width of Cr-depleted region, a Cr level below approximately 14 wt% is required for sensitization. Precipitation also results in a greater corrosion susceptibility as determined by potentiodynamic polarization scans. The wear behavior of the 0.7 wt% N alloy is little affected by the precipitation of Cr2N since the volume fraction and size (compared to the abrasive) of the Cr2N precipitates is insufficient to alter the wear resistance of the alloy.

A study of the dislocation core structure in high nitrogen austenitic stainless steel by weak beam EM

The large strengthening effect of nitrogen interstitials and its strong influence on the dislocation distribution of Fe-Cr-Ni austenite have been rationalized by Owen et al. using a short-range-order-based model. However, the model cannot explain the high population of screw dislocations resulted from a small amount of plastic deformation. Grujicic suggested that nitrogen addition to Fe-Cr-Ni austenitic stainless steels causes spreading of the screw dislocation core into two or more non-parallel planes which results in low dislocation mobility. Higher stress and higher temperature are thus required to overcome the energy barrier for the movement of dislocations. The core structure of dislocations in nitrogen-containing stainless steels is of fundamental importance for understanding the strengthening mechanism of this material. The weak beam dark field imaging technique allows the direct observation of partial dislocations and stacking faults bonded by the partials. When the optimum imaging condition is satisfied, relatively intense, narrow images of less than 2 nm wide can be obtained.

Passivity of high-nitrogen stainless alloys: the role of metal oxyanions and salt films

Abstract Surface-analytical studies of high-nitrogen austenitic stainless steels exposed to de-aerated 0.1 M HCl have revealed that nitrogen alloying additions influence the composition of salt layers and the passive film/alloy interface. It was shown that nitrogen, nickel and molybdenum additions stimulate selective dissolution of iron, resulting in a significant enrichment of chromium beneath the passive film. The build-up of a protective ferrous molybdate layer was seen to be most strongly enhanced with additions of nickel and, to a lesser extent, nitrogen. While the primary kinetic barrier to anodic dissolution of high-nitrogen stainless steels is a chromium-oxide-based passive film, it appears that a mixed nitride surface layer and an ultra-thin layer of ferrous molybdate act as secondary kinetic barriers.

Influence of nitride (Cr 2N) precipitation on the plastic flow behavior of high-nitrogen austenitic stainless steel

Aging at 700 °C results in grain boundary nitride precipitation only, while aging at 900 °C results in grain boundary and cellular precipitation (40 vol%). These thermal treatments have a small but positive effect on YS, no effect on the UTS, but dramatically reduce the ability of the material to deform under localized plastic deformation (necking), leading to reduced tensile ductility. The plastic flow behavior of all annealed and aged high-nitrogen samples were modeled using the modified Ludwik relation (equation 2). Below the UTS, grain boundary nitride precipitation at 700 °C has no measurable effect on the plastic flow behavior of the material, and the modeling parameters n 1, K 1, n 2, and K 2 have values which do not deviate significantly from the behavior of the un-aged material. Cellular precipitation, caused by aging at 900 °C, does significantly affect the plastic flow behavior of the material at both low and high strains. Cellular precipitation causes both increased strengthening of the matrix in the low strain regime (0.001 < ε < 0.03) and systematic decreases in the strain hardening exponent (n 1) and strength coefficient (K 1) with increased aging. The rate of strain hardening ( dσ dε ) measured from the σ-ε plots is unaffected by isothermal aging and nitride formation.

Sensitization of High-Nitrogen Austenitic Stainless Steels by Dichromium Nitride Precipitation

Abstract High-nitrogen (N) stainless steels (SS) are receiving increased attention because of their strength advantages over carbon (C)-alloyed materials, but they have been found susceptible to dichromium nitride (Cr2N) precipitation during thermal exposure between ∼ 600°C and 1,050°C. Sensitization susceptibility of a high-N, low-C austenitic SS by Cr2N precipitation at 700°C and 900°C was determined using the single-loop electrochemical potentiokinetic reactivation (EPR) test. High-N SS was found susceptible to sensitization caused by grain boundary (GB) precipitation of Cr2N, with the degree of sensitization increasing systematically with aging time at 700°C. Sensitization of high-N materials did not require the concomitant precipitation of chromium (Cr)-rich metal carbide (M23C6). Materials aged at 900°C were not sensitized, although the rate of precipitation was greater than at 700°C. This indicated the minimum Cr level in the Cr-depleted zone of the matrix associated with nitride precipitation at 9...

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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.

Cellular precipitation in a nitrogen alloyed stainless steel

AbstractPrecipitation in a high nitrogen austenitic stainless steel of the Nitronic series has been studied. Above a critical nitrogen concentration, cellular precipitation is observed together with formation of Cr2N as a general precipitate and intragranular precipitation of Z phase (Cr2Nb2N2). The nature of cellular precipitation has been studied using constant activity aging in nitrogen and the characteristics of the process have been examined using X-ray diffraction and electron microscopy. The cellular precipitation process is shown to be consistent with a model based on long range diffusion of nitrogen and equilibrium between the gas phase and steel matrix. The solubility of nitrogen in austenite in equilibrium with the gas phase or with Cr2N precipitates is consistent with published data.MST/917

Linear-Elastic Fracture of High-Nitrogen Austenitic Stainless Steels at Liquid Helium Temperature

Abstract Four commercial Fe-Cr-Ni-Mn austenitic stainless steels containing 0.14, 0.26, 0.28, and 0.37 wt% N were fractured in liquid helium at 4 K, and measurements of the linear-elastic plane-strain stress-intensity factor, KIc, were made. Interstitial nitrogen significantly strengthens these steels at low temperatures so that brittle fractures occur under plane strain conditions at 4 K despite moderate ductility in uniaxial tension. The brittle fracture mechanism at 4 K involves a form of cleavage or slip-band cracking as evidenced by the formation of transgranular facets on {111} planes.

Structure and properties of high-nitrogen corrosion-resistant austenitic steels

1. Alloying of austenitic stainless steels with as much as 0.8% N makes it possible to raise the ultimate strength to 100 kgf/mm2 and the yield strength to 70 kgf/mm2. 2. With slow cooling from 1250° the structure of these steels is of the itrogenous pearlite type. 3. High-nitrogen austenitic Cr-Ni stainless steels and Cr-Ni-Mn steels are not susceptible to ICC after quenching (tests by the AM method, GOST 6032-58). Sensitizing of the steel with >0.54% N induces susceptibility to ICC.