Synthesis of austenitic stainless steel powder alloys by mechanical alloying

Fe–18Cr–xNi (x = 8, 12, 13, 15, and 20 wt%) blended elemental powders were subjected to mechanical alloying in a high-energy SPEX shaker mill. The milled powders were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy and transmission electron microscopy techniques. It was shown that the sequence of phase formation in the Fe–18Cr–8Ni, Fe–18Cr–12Ni and Fe–18Cr–13Ni compositions was ferrite in the early stages of milling and then formation of austenite, which eventually transformed to stress-induced martensite on continued milling. The time for the formation of the austenite phase was shorter for the 12Ni and 13Ni powder blends than for the 8Ni powder. However, in the Fe–18Cr–15Ni and Fe–18Cr–20Ni compositions, the initial phase to form was ferrite and then a fully austenitic structure had formed on milling the powder for 10 h. No martensitic transformation occurred in this case on continued milling. The phase formation and microstructural features were confirmed by X-ray diffraction and transmission electron microscopy and diffraction techniques. A new metastable phase diagram was proposed outlining the stability of the austenite phase in ternary Fe–Cr–Ni alloys.     

Influence of N on precipitation behavior,associated corrosion and mechanical properties of super austenitic stainless steel S32654

The influence of N on the precipitation behavior,associated corrosion,and mechanical properties of S32654 were investigated by microstructural,electrochemical,and mechanical analyses.Increasing the N content results in several alterations:(1) grain refinement,which promotes intergranular precipitation;(2) a linear increase in the driving force for Cr2 N and Mo activity,which accelerates the precipitation of intergranular Cr2 N and π phase,respectively;(3) a linear decrease in the driving force for σ phase and Cr activity,which suppresses the formation of intragranular σ phase.The total amount of precipitates first decreased and then increased with the N content increasing.Furthermore,the intergranular corrosion susceptibility depended substantially on the total amount of precipitates and also first exhibited a decreasing and then an increasing trend as the N content increased.In addition,aging precipitation caused a considerable decrement in the ultimate tensile strength(UTS) and a remarkable increment in the yield strength(YS).Both the UTS and YS always increased with N content increasing throughout the solution and aging process.Whereas the elongation was considerably sensitive to the aging treatment,it exhibited marginal variation with the N content increasing.     

Influence of hot rolling and heat treatment on structure and properties of HSLA steel explosively clad with austenitic stainless steel

Abstract

The present work aims at studying structure–property correlations in an explosively clad HSLA steel with austenitic stainless steel of AISI 304L grade. The clad plate was subjected to hot rolling followed by a quenching and tempering treatment to achieve better mechanical properties in the base plate. Optical microscopy studies revealed that the interface between the two steels was wavy in the as clad plate and the waviness decreased substantially due to hot rolling. Subsequent heat treatment has not shown any significant effect either. The base plate had tempered martensite/bainite structure in as clad or heat treated conditions and ferrite-pearlite-bainite structure in hot rolled condition. The grains were finer and elongated near the interface. The stainless steel exhibited equiaxed grain structure in as clad, hot rolled or heat treated plates. Tensile properties and charpy impact energy of the base plate were lowered due to hot rolling and then increased substantially due to heat treatment. The microhardness was observed to be a maximum at the bond interface for all three conditions studied. The shear bond strength was the highest in the as clad condition and decreased for the rolled as well as heat treated conditions. Scanning electron microscopy fractography on shear bond specimens revealed the presence of predominantly equiaxed dimples with few regions of rubbed fracture. Quantitative electron probe microanalysis across the bond interface indicated linear change in concentrations of nickel, chromium and manganese between the levels appropriate to the clad layer and base metal.     

Influence of grain boundary carbides on mechanical properties of high nitrogen austenitic stainless steel

Precipitation behavior of grain boundary carbides and its influence on mechanical properties and fracture mechanism of the high nitrogen austenitic stainless steel produced by different processing methods were studied. The simulation software Thermo-calc was applied to analyze the effects of element content on precipitation of carbides. The results show that hot-rolled plate has higher strength, but solution-treated one followed by water quenching has excellent combination of strength and ductility (toughness). M₂₃C₆ is the main precipitate and deteriorates the toughness of the steel obviously when it precipitates along grain boundaries. In this case, intergranular fracture is the predominant failure mechanism and the fracture surface is characterized by the shape of rock candy. The toughness at −40 °C is decreased by 53% when small amount of carbides precipitates during sand cooling process after solution treatment. The simulation results exhibit that with the decrease of C content, both the precipitation quantity and precipitation temperature of M₂₃C₆ decrease. Cr and N have no influence on precipitation quantity of M₂₃C₆, but the precipitation temperature will increase with the increase of Cr and the decrease of N.     

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.     

Cracking behavior and mechanical properties of austenitic stainless steel parts produced by laser metal deposition

The cracking behavior, microstructure and mechanical properties of austenitic stainless steel parts produced by laser metal deposition (LMD) are presented. The existing criteria for evaluating the solidification cracking sensitivity during welding of stainless steels have been adapted. Apart from the presence of sulfur and phosphorous, the presence of silicon was found to have a detrimental effect on cracking resistance. Cracking was not observed if the total content of sulfur, phosphorous and silicon was kept low enough, even not for stainless steels with austenitic solidification mode. Three-dimensional parts produced using optimal process parameters and feedstock powder composition have been investigated in more detail. The parts have a density of 99.6%. The microstructure consists of fine columnar dendrites, which coarsen as a function of height along the building direction. This is accompanied by a decrease in hardness. The tensile strength and elongation are in general higher than for annealed wrought material. The tensile strengths are higher while the elongation is lower for samples loaded perpendicular to the build-up direction than for those loaded parallel.     

Influence of hydrogen on mechanical properties of nitrogen supersaturated austenitic stainless steels

Abstract

Alloying austenitic stainless steels with nitrogen up to a concentration of 1 wt-% improves yield strength, tensile strength, and ductility. Further increase in the nitrogen concentration results in chromium nitride precipitation at the grain boundaries and a decrease in the ductility with a change in the fracture mode from ductile to intergranular. Hydrogen charging causes high reversible dilatation in the lattice and remarkable reduction in the ductility. The ductility losses caused by hydrogen are more pronounced at higher nitrogen concentrations and a change of the fracture mode from intergranular to transgranular is observed in steels with more than 1 wt-% nitrogen. Chromium nitride precipitates are shown to have an insignificant role in the hydrogen embrittlement. Hydrogen charging steels with nitrogen concentrations of below 1 wt-% enhances the strengthening effect of nitrogen but, at higher nitrogen concentrations, hydrogen is shown to be detrimental to the strength.     

Influence of crystallographic textures on tensile properties of 316L austenitic stainless steel

Role of cold rolling texture on the tensile properties of the cold rolled and cold rolled and annealed AISI 316L austenitic stainless steel is described here. The solution-annealed stainless steel plates were unidirectionally cold rolled to 50, 70 and 90% of reduction in thickness. The cold rolled material was annealed at 500–900 °C annealing temperatures. X-ray diffraction technique was employed to study the texture evolution in cold rolled as well as cold rolled and annealed conditions. The texture components that evolved were translated into slip transmission number ‘λ’ and Schmid factor ‘μ’. These two parameters were correlated with the tensile properties of the material. The tensile properties were evaluated under all processing conditions. Softening of the cold rolled material was observed after annealing with increasing annealing temperatures. From the stress–strain curves, strain hardening coefficient ‘n’ and strain hardening rate ‘θ’ were determined. It was found that the effect of texture on tensile behaviour could be understood clearly by strain hardening rate. Out of the two parameters, ‘n’ and ‘θ’, strain hardening rate was found to be more sensitive to type of texture in the material.     

Irradiation behavior of nanostructured 316 austenitic stainless steel

In order to get information about radiation resistance of ultrafine grained austenitic stainless steels, a 316 steel was deformed by high pressure torsion. The mean diameter of the grain after deformation was 40 nm. This material was annealed at 350 °C for 24 h or irradiated with 160 keV iron ions at 350 °C. Changes in the microstructure during annealing or irradiation were characterised by transmission electron microscopy (grain size) and laser assisted tomographic atom probe (solute distribution). Results indicate that this annealing has no influence on the grain size whereas the grain diameter increases under irradiation. Concerning the solute distribution, atom probe investigations show evidence of radiation-induced segregation at grain boundaries. Indeed, after irradiation, grain boundaries are enriched in nickel and silicon and depleted in chromium. On the contrary, no intragranular extended defects or precipitation are observed after irradiation.     

Study of the processing map and hot deformation behavior of a Cu-bearing 317LN austenitic stainless steel

The hot-working behavior of a Cu-bearing 317LN austenitic stainless steel (317LN–Cu) was investigated in the 950–1150 °C temperature and 0.01–10 s^− 1strain rate range, respectively. The effects of different deformation parameters and optimum hot-working window were respectively characterized through analyzing flow stress curves, constitutive equations, processing maps and microstructures. The critical strain for dynamic recrystallization (DRX) was determined by the inflection point on θ-σ and −∂θ/∂σ-σ curves. The peak stress was found to increase with decrease in temperature and increase in strain rate. Typical signs of DRX over a wide range of temperatures and strain rates were observed on the flow stress curves. The power dissipation maps in the strain range of 0.1–0.4 were basically similar, indicating the insignificant effect of strain on the power dissipation maps of 317LN–Cu. However, the instability maps showed strong strain sensitivity with increasing strain, which was attributed to the flow localization. The optimum hot-working window for 317LN–Cu was obtained in the temperature range 1100–1120 °C and strain rate range 0.01–0.018 s^− 1, with a peak efficiency of 38%. Microstructural analysis revealed fine and homogenized recrystallized grains in this domain.     

Microstructure and mechanical properties of resistance upset butt welded 304 austenitic stainless steel joints

Resistance upset welding (UW) is a widely used process for joining metal parts. In this process, current, time and upset pressure are three parameters that affect the quality of welded products. In the present research, resistance upset butt welding of 304 austenitic stainless steel and effect of welding power and upset pressure on microstructure, tensile strength and fatigue life of the joint were investigated. Microstructure of welds were studied using scanning electron microscopy (SEM). X-ray diffraction (XRD) analysis was used to distinguish the phase(s) that formed at the joint interface and in heat affected zone (HAZ). Energy dispersive spectroscopy (EDS) linked to the SEM was used to determine chemical composition of phases formed at the joint interface. Fatigue tests were performed using a pull–push fatigue test machine and the fatigue properties were analyzed drawing stress-number of cycles to failure (S–N) curves. Also tensile strength tests were performed. Finally tensile and fatigue fracture surfaces were studied by SEM. Results showed that there were three different microstructural zones at different distances from the joint interface and delta ferrite phase has formed in these regions. There was no precipitation of chromium carbide at the joint interface and in the HAZ. Tensile and fatigue strengths of the joint decreased with welding power. Increasing of upset pressure has also considerable influence on tensile strength of the joint. Fractography of fractured samples showed that formation of hot spots at high welding powers is the most important factor in decreasing tensile and fatigue strengths.     

Mechanical properties of austenitic stainless steel with high niobium contents

To better understand the influence of the microstructural characteristics on the mechanical properties of Fe-Cr₂₂-Ni₂₅-Nb_x austenitic stainless steel (ASS-Nb_x), the mechanical properties were investigated. Nb addition was conducted with four different amounts: 0, 0.29, 0.58 and 0.86%. With the increasing Nb content, the mean grain size for the Fe-Cr-Ni-based alloys decreased, while the size of Nb precipitate increased. Owing to the different microstructural characteristics, their mechanical properties were altered. The highest tensile strength was obtained for ASS-Nb_0.29 alloy. However, with the increasing Nb content in ASS-Nb_0.29–0.86 alloys, the tensile strength decreases despite the grain refinement. The mechanical properties of the ASS-Nb alloys were influenced by the Nb precipitation hardening and the grain boundary cohesive strength, arising from the size of Nb precipitations.     

Strain-rate effects on the mechanical behavior of the AISI 300 series of austenitic stainless steel under cryogenic environments

A series of uni-axial tensile tests were carried out under various low temperatures and strain-rate ranges for AISI 300 austenitic stainless steel. The strain-rate dependencies of the materials under investigation were evaluated at temperatures ranging from ambient to cryogenic. Non-linear mechanical behavior such as phase transformation, discontinuous yielding and micro-damage of four kinds of commercial stainless steel-based material were quantitatively investigated by measuring transformation induced plasticity (TRIP) and threshold strain for 2nd hardening. In this study, the main properties of each material were analyzed and compared based on the conditions of strain-rates and temperature. Test results showed that all the test materials were strongly dependent on temperature and strain rate. It is expected that the findings in this study could be used for the cryogenic design and further research of structure materials under cryogenic environments.     

Interaction of residual stress with mechanical loading in an austenitic stainless steel

Single edge notched bend, SEN(B), fracture specimens fabricated from AISI Type 316H austenitic stainless steel and containing a residual stress field were used to quantify the interaction of a residual stress field on subsequent fracture behaviour when the specimens were subjected to an applied load. Autogenous welding (where no additional filler material is used) was used to impart the residual stress field following a procedure which had been extensively characterised numerically and experimentally. Crack growth resistant curves were obtained for the specimens, and for similar specimens containing no residual stress field. It was observed that the residual stress field had negligible impact on the fracture behaviour of the specimens, in contrast to recently reported work which demonstrated a large influence of a residual stress field when the test specimens were fabricated from a ferritic steel. A numerical programme was conducted to consider the results in the context of a structural integrity assessment. The test results were assessed using both the procedures described in R6, a well-known structural integrity assessment procedure, and by explicit calculation of a modified J -integral via two-dimensional cracked body finite element analysis. It was shown that the assessments were pessimistic, all predicting an influence of the residual stress field on fracture when none was observed experimentally.     

Study on hot deformation behaviour of 316LN austenitic stainless steel based on hot processing map

Abstract

316LN is a type of austenitic stainless steel whose grain refinement only depends on hot deformation. The true stress–strain curves of 316LN were obtained by means of hot compression experiments conducted at a temperature range of 900–1200°C and at a strain rate range of 0·001–10 s^?1. The influence of deformation parameters on the microstructure of 316LN was analysed. Both the constitutive equation for 316LN and the model of grain size after dynamic recrystallisation were established, and the effect of different deformation conditions on the microstructure was analysed. The results show that the suitable working region is the one with a relatively higher deformation temperature and a lower strain rate, in which the dynamic recrystallisation is finely conducted. Moreover, the working region that should be avoided during hot deformation was indicated.     

Influence of alloying on the free energy of austenitic grain boundaries in steel

We compute the conditional energy σ, of austenitic grain boundaries and analyze changes in boundary energy at elevated temperatures for low-carbon, chrome-manganese and chrome-manganese-molybdcnum steels. It is established that σ, varies within the range 1.0–1.3 J/m² and that, in almost all cases of alloying, the effect caused by a decrease in the surface energy of iron is so strong that even in the presence of chorophobic elements σ, decreases at elevated temperatures. For multicomponent steels, we analyze variations of the temperature coefficients of boundary energy. Philadelphia, United States of America. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 32, No. 4, pp. 63–68, July–August, 1996     

Influence of alloying on the free energy of austenitic grain boundaries in steel

We compute the free energy of austenitic grain boundaries for steels whose carbon content is equal to 0.2%, for chrome-manganese steels with various concentrations of molybdenum and phosphorus, and for chrome-manganese-molybdenum steel. On the basis of regression analysis, we develop an interpolational model which enables one to estimate (both qualitatively and quantitatively) not only the effects of each element on the free energy of grain boundaries but also changes in these effects caused by the presence of other elements with different surface activities. It is shown that changes in the influence of alloying elements on the boundary energy of multicomponent systems can be explained by the interaction of different elements. Philadelphia, USA. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 32, No. 3, pp. 55–62, May–June, 1996.     

Effect of loading direction on hot ductility of super austenitic stainless steel with columnar crystals

Journal of Materials Science - High-temperature tensile deformation characteristics of a 00Cr25Ni19Mn3Mo7NCe super austenitic stainless steel (SASS) at deformation temperatures of...     

Influence of alloying on the free energy of austenitic grain boundaries in steel

We compute the free energy of boundaries of austenitic grains and analyze the effect of alloying elements (Mo, Ni, V, W, Nb, Ce, Cr, Ti, Al, Si, B, and Cu) on the boundary energy of low-carbon, chromemanganese, and chrome-manganese-molybdenum steels. By using regression analysis, we develop interpolational models for the qualitative and quantitative evaluation of the effect of each element on the free energy of grain boundaries. Philadelphia, USA. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 32, No. 2, pp. 24–34, March–April, 1996.