Cr-Mn Austenitic Stainless Steel Research Articles

Sequential solution annealing of a CrMnCN austenitic stainless steel

The influence of various solution annealing treatments on the microstructure and mechanical properties of a new Cr–Mn austenitic stainless steel with 1.15 wt% (C + N) and 0.51 C/N ratio has been investigated. A sequential solution annealing (SSA) treatment is designed to sufficiently dissolve and lower numerous precipitates including carbides and nitrides in the austenite matrix. In this process, the hot-forged steel is firstly solution heat treated at 1120 °C for 2 h, then continuously heated to 1180 °C for 0.5 h, and further heated to 1200 °C for 0.5 h, followed by water quenching. The experimental results show that the austenite grain size increases, and number of precipitates decreases, with increasing temperature and time of solution annealing treatments. The highest impact energy is obtained after this SSA treatment due to a more homogeneous grain distribution, a smaller amount of carbides remaining on the grain boundary, and solid solution strengthening of interstitial (C + N) atoms.

Microstructure, mechanical properties and sensitization of ultra-low nickel Cr–Mn austenitic stainless steels

The present study deals with the effect of shielded metal arc welding (SMAW) and gas metal arc welding (GMAW) processes with different welding speeds on microstructural and mechanical properties including intergranular corrosion susceptibility of ultra-low nickel chrome-manganese austenitic stainless steel (ASS). The microstructural analysis was performed by an optical microscope. The Fisher Ferritscope was used for determining ferrite content in the weldment. The microstructural analysis shows that the size of dendrites of δ ferrite and the width of heat affected zone (HAZ) decreases when welding speed increases. It was found more δ-ferrite in SMAW weld compared to GMAW weld. It was also found that when welding speed increases, tensile strength, impact strength and hardness values decrease for both welding processes. From the U-bending test, it was found that the welded joints are strong enough to withstand bend at 1800 without any cracks for both welding processes. It was also found that Cr-Mn ASS weld becomes susceptible to intergranular corrosion on welding. For both welding processes, the degree of sensitization value decreases with an increase in welding speed for both the welding processes as evaluated by the double loop electrochemical potentiodynamic reactivation (DLEPR) test. Also, the weld joint made using the GMAW process is more susceptible to intergranular corrosion than that made using the SMAW process.

The machining parameter optimisation during the machining of the Cr-Mn austenitic stainless steel by the topsis method

The Cr-Mn stainless steel (austenitic) was dry turned in a lathe. A carbide tool (Indoloy) was used for the machining. The speed (v), feed (f), and depth of cut (DOC, d) were the input parameters. The experimental work was done following the Taguchi L 27 design plan. The chip reduction coefficients (CRC) and the von Mises stresses (VMS) were determined for all the experimental conditions. The TOPSIS (Technique for Order of Preference by Similarity to the Ideal Solution) method was used to optimise the process parameters. The optimal conditions were attainable at high speed (v = 100 m/min.), moderate feed (f = 0.1 mm/rev.), and moderate DOC (d = 1 mm). At optimized machining conditions, the VMS was 3429.5 MPa and the CRC was 1.6. The VMS was higher for the lower speed, and the VMS was lower for the higher speed. The higher VMS was attributed to the formation of strain-induced martensite (SIM). The lower VMS was due to the dynamic recrystallization (DRX) at the higher temperature for the higher speed. For the higher speed, the DRX and the dynamic precipitation (DP) occurred at the chip surface (under).

The machining behaviour of Cr-Mn austenitic stainless steel

The dry turning was done on the Cr-Mn austenitic stainless steel. The carbide cutting tool (Indoloy) was used for the machining. The experiments were done applying the Taguchi decoupled design of experiment (DOE) methodologies. Speed (v, m/min), feed (f, mm/rev.), and depth of cut (DOC, d, mm) were the input parameters, and chip reduction coefficient (CRC) and von Mises stress (VMS) were the output parameters. The VMS was greatly influenced by the speed and feed. The DOC negligibly affected the VMS. The VMS was influenced due to the strain-induced martensite (SIM) formation for the specific parameters. The VMS was also influenced due to the strain rate hardening, dynamic recrystallization (DRX), and dynamic precipitation (DP) under different experimental conditions. The chip formation occurred due to strain-induced martensite formation at the low speed, low feed, and low DOC. The SAW tooth mechanism formed the chip for the high speed, feed, and DOC. The sizes of the DRX grains and carbide precipitates (DP) were heavily influenced by the experimental conditions.

Dynamic Plastic Deformation Induced by Repetitive Hammering on Cr-Mn Austenitic Stainless Steel

Austenitic stainless steels have advantages, such as high ductility and good corrosion resistance. The cold working process can increase the hardness and strength of the material. However, because a metastable austenite phase occurs in that material, there is a phase change of γ austenite to α’-martensite and ε-martensite, which will reduce the ductility and its corrosion resistance. The strengthening process with dynamic plastic deformation (DPD) can prevent the formation of martensitic phases through repeated impact at high strain rates. This study analyzed microstructures and hardness evaluation on Cr-Mn austenitic stainless steel due to dynamic plastic deformation through the repetitive hammering method. Repetitive hammering with a strain rate of 6,2 s-1 on Cr-Mn austenitic stainless steels was carried out on five specimens with variations in the impact of 50, 100, 150, 250, and 350 times with impact energy of 486 J/cm2, 2.207 J/cm2, 2.569 J/cm2, 6.070 J/cm2, and 11.330 J/cm2 respectively. Microstructure, hardness, and XRD (X-ray diffraction) analyses were carried out on specimens before and after repetitive hammering. Metallography was carried out to observe the microstructure using an optical microscope. The hardness was tested through the Rockwell A hardness test. XRD examination was used to identify the phases formed and indications of nano-twins. The repetitive hammering process up to 350 times has succeeded in increasing hardness from 53.5 HRA to 71.6 HRA. Plastic deformation introduced by repetitive hammering produced slip bands, cross bands, wavy bands, and indication of nano-twins formation and increased the hardness.

Effect of filler and autogenous welding on microstructure, mechanical and corrosion properties of low nickel Cr-Mn ASS

ABSTRACT This experimental study investigates the effects of welding low nickel Cr-Mn austenitic stainless steel with gas tungsten arc welding process with and without filler (autogenous) weld by varying the welding currents (120A, 130A and 140A). The microstructural examination of weld has been carried out with optical microscope and mechanical properties with Vickers microhardness and tensile tester. The corrosion behaviour of the welded joints are analysed by double loop electrochemical potentio-kinetic reactivation test to obtain degree of sensitisation. The microstructural examination of welded joints revealed that, as the welding current increases, the inter-dendritic spacing as well as the dendrite size increases. The tensile strength and hardness decrease with increasing welding current for 308 L filler and autogenous weld. The degree of sensitisation increases with increasing welding current for both the welds. The results reveal that the autogenous welds have higher tensile strength, higher hardness and lower degree of sensitisation as compared to welds with 308 L filler.

Electrochemical evaluation of cr-mn austenitic stainless steel in aqueous sulphuric acid and influence of thiocyanate ions

PurposeLow nickel austenitic stainless steel (ASS) has attracted much attention worldwide because of its economical price. This study aims to investigate the effect of different corrosive environments on the corrosion behavior of chrome-manganese (Cr-Mn) ASS. The tests were carried out as a function of H2SO4 concentrations, temperature and addition of ammonium thiocyanate (NH4SCN) (0.01 M). Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques were used to study the corrosion behavior of Cr-Mn ASS. It was observed that with increasing H2SO4 concentration, temperature and with the addition of NH4SCN solution, icorr, icrit and ipassive values increased. EIS data show decreasing charge transfer resistance value with increasing concentration and temperature. Higher corrosion rate with increasing temperature and concentration of H2SO4 is related to the anions (SO42−), which is responsible for reducing the stability of passive films. With the presence of 0.01 M NH4SCN thiocyanate (SCN− anion), there is a higher dilution of the passive film resulting in a higher corrosion rate. Energy-dispersive spectroscopy (EDS) analysis reveals the adsorption of sulfur on the surface in NH4SCN containing a solution. The significant presence of counter ions and the adsorbed sulfur species on the steel surface play a vital role in corrosion behavior.Design/methodology/approachAll the experiments were performed on a 3 mm thick sheet of Cr-Mn ASS (202 ASS) in hot rolled condition. The samples were then annealed at 1,050°C for 1 h, followed by water quenching. For microstructural examination, they were electrochemically etched in 10 Wt.% oxalic acid solution at 1 amp for 90 s. A computer-controlled Potentiostat (Biologic VMP-300) was used. After the cell was set up, the working electrode (WE) was electrostatically cleaned at −1 V vs saturated calomel electrode (SCE) for 30 s to remove the air-formed film. Then, WE were allowed to attain stable open circuit potential (OCP) for 1 h, following by the EIS test and potentiodynamic polarization test. The polarization test was started from a cathodic potential (−1.2 V vs SCE) and continued up to an anodic potential (1.6 V vs SCE) a scan rate of 0.1667 mV/s. EIS experiment was conducted on the same instrument by using a sinusoidal AC signal of 10 mV in a frequency range of 1,000,000 to 0.01 Hz at OCP.FindingsPotentiodynamic polarization graph shows that with the increase in sulphuric acid concentration. Increasing temperature from 20°C to 80°C in 0.5 M H2SO4 solution increases the corrosion rate (icorr) of Cr-Mn ASS. On the addition of 0.01 M NH4SCN to the sulfuric acid solution (0.1, 0.5 and 1 M) the corrosion rate increases drastically almost four to five times. EDS and XRD analysis shows the presence of sulfur over the oxide film and preferential site for dissolution of Cr and Mn at the steel surface when NH4SCN is added to the sulfuric acid solution.Originality/valueA study on the corrosion behavior of Cr-Mn ASS is scanty according to the author’s knowledge. Therefore, the present study will investigate the corrosion behavior of Cr-Mn ASS on SO4−2 anions, temperature and the addition of SCN− ion in sulfuric acid.

On the role of nitrogen on hydrogen environment embrittlement of high-interstitial austenitic CrMnC(N) steels

This work investigates the susceptibility of high-interstitial CrMn austenitic stainless steel CN0.96 to hydrogen environment embrittlement. In this context, an N-free model alloy of CN0.96 steel was designed, produced, and characterized. Both steels were subjected to tensile tests in air and in a high-pressure hydrogen gas atmosphere.Both steels undergo severe hydrogen embrittlement. The CN0.96 steel shows trans- and intergranular failure in hydrogen, whereas the N-free model alloy shows exclusively intergranular failure. The different failure modes could be related to different deformation modes that are induced by the presence or absence of N, respectively. In the CN0.96 steel, N promotes planar dislocation slip. Due to the absence of N in the model alloy, localized slip is less pronounced and mechanical twinning is a more preferred deformation mechanism. The embrittlement of the model alloy could therefore be related to mechanisms that are known from hydrogen embrittlement of twinning-induced plasticity steels.

Effect of thermal aging on IGC of cold worked Cr-Mn austenitic stainless steel

PurposeThe purpose of this study is to investigate the effect of prior cold work after annealing and thermal ageing on intergranular corrosion or sensitization of Cr-Mn austenitic stainless steel (ASS) is necessary. Such a study is particularly important because ASS are mostly used and welded in mill-annealed condition, which is equivalent to fully annealed material with some cold worked (CW).Design/methodology/approachThe effect of 15% CW of 202 ASS were investigated using microstructural (optical microscope), mechanical (grain size and hardness) and electrochemical methods (double loop electrochemical reactivation [DLEPR]) followed by thermal ageing (800°C, 900°C and 1000°C).FindingsX-ray diffraction analysis shows the presence of martensite in CW samples. The increase in martensite formation (800°C and 900°C) can be observed with the variation of thermal ageing (TA) duration (1, 2 and 3 h). However, there was decreased in the formation of martensite at the temperature of 1000°C because of martensitic reversal. The DLEPR test result shows higher degree of sensitization (DOS) for 800°C and 900°C but for 1000°C, there was re-homogenization of samples which leads to lower DOS (thermal ageing for 1, 2 and 3 h).Originality/valueFor 300 series steel, there are various literature available for the effect of cold work on mechanical properties and DOS. However, no one has investigated the effect of cold work and thermal ageing on the sensitization of 202 Cr-Mn ASS.

Enhanced corrosion resistance of Cr-Mn ASS by low temperature salt bath nitriding technique for the replacement of convectional Cr-Ni ASS

Purpose The purpose of this paper is to enhance the corrosion resistance of Cr-Mn austenitic stainless steel (ASS) via low temperature salt bath nitriding and to replace the convectional Cr-Ni ASS with newly developed enhanced corrosion resistive Cr-Mn ASS. Design/methodology/approach The low temperature salt bath nitriding was performed on Cr-Mn ASS at 450°C for 3 h in potassium nitrate salt bath. Findings The present paper compares the corrosion resistance of salt bath nitrided Cr-Mn ASS with convectional Cr-Ni ASSs (316 L and 304 L ASSs) in 3.5 per cent NaCl by electrochemical techniques. The electrochemical impedance spectroscopy result shows the increase in film resistance and potentiodynamic polarization results show the enhanced corrosion resistance of nitrided Cr-Mn ASS, which is almost equivalent to that of 316 L and 304 L ASSs. This is attributed to the formation of nitrogen supersaturated dense nitride layer. The present results therefore suggest that the nitrided Cr-Mn ASS may replace costly convectional Cr-Ni ASSs for commercial and industrial applications. Originality/value Ever-increasing price of nickel (Ni) is driving the industries to use Ni-free or low-Ni austenitic stainless steels (ASSs). But its corrosion resistance is relatively poor as compared to conventional Cr-Ni ASSs. However, its corrosion resistance can be improved by nitriding. The low temperature salt bath nitriding of Cr-Mn ASS and its electrochemical behavior in 3.5 per cent NaCl has not been studied. The present research paper is beneficial for industries to use low cost Cr-Mn, enhance its corrosion resistance and replace the use of costly conventional Cr-Ni ASSs.

Microstructural, mechanical and intergranular corrosion behavior of cold work and thermal ageing of low nickel austenitic stainless steel

In the present investigation, microstructural, mechanical and electrochemical properties of cold rolled Cr-Mn austenitic stainless steel were studied. Heat treatment (800 °C, 900 °C, 950 °C and 1000 °C) and thermal ageing (one, two and three hours) of surface were investigated using an optical microscope, scanning electron microscope. Mechanical characteristics like hardness and tensile strength were evaluated. Electrochemical test were carried out using double loop electrochemical potentio kinetic reactivation tests (DLEPR) for finding the degree of sensitivity (DOS). The results showed that without cold rolled, thermal aged for one, two and three hours, there was high sensitization at the temperature of 850 °C and 900 °C, but at the temperature of 950 °C and 1000 °C there was re-homogenization. The grain size increased with increase in temperature. With cold rolled samples, there was a reduction in grain size, increased in strength and hardness in the samples, which is due to the formations of strain induced martensite (SIM). With the increase in thermal duration (1, 2 and 3 h) there was an increase in martensite formation at temperature of 850 °C and 900 °C. At temperature of 950 °C and 1000 °C, work hardening diminished. Also, it was observed that increase in strain induces martensite which in turn increases degree of sensitization (% DOS).

Influence of Cold Rolling Reduction on Microstructure and Mechanical Properties in 204C2 Austenitic Stainless Steel

The present study investigated the effect of cold rolling reduction on microstructure and mechanical properties of a 204C2 Cr–Mn austenitic stainless steel which contained 16%Cr, 2%Ni, 9%Mn and 0.083 %C). The 204C2 austenitic stainless steels were cold rolled at multifarious thickness reductions of 10%, 20%, 30%,40% and 50%, which were compared with the solution-treated one. Microstructure of them was investigated by means of optical microscopy, X-ray diffraction technique and scanning electron microscopy. For mechanical properties investigations, hardness and tensile tests were carried out. Results shows that the cold rolling reduction induced the martensitic transformation (γ→α ́) in the structure of the austenitic stainless steel. With the increase of the rolling reduction, the amount of strain-induced martensite increased gradually. Hardness, ultimate tensile strength and yield strength increased with the incremental rolling reduction in 204C2 stainless steels, while the elongation decreased. At the thickness reduction of 50%, the specimen obtained best strength and hardness. Hardness of 204C2 stain steel reached 679HV. Ultimate tensile strength reached 1721 MPa. Yield strength reached 1496 MPa.

Effect of Strain Induced Martensite Reversal on the Degree of Sensitization of Metastable Austenitic Stainless Steel

Effect of reversion of strain induced martensite on degree of sensitization (DOS) of Cr-Mn Austenitic stainless steel (ASS) have been examined. The martensite reversion is known to be effective in refining the grain size of metastable ASS. However, martensitic reversal takes place only after severe cold rolling reductions. In this study, the effect of degree of cold rolling and subsequent thermal ageing on the microstructure, mechanical, and sensitization properties of a metastable Cr-Mn ASS was investigated. Samples were cold worked (25% and 45%) and then thermal aged at different temperature of 700°C and 900°C for 2 hrs. Microstructures were subjected to optical microscopy. Microstructural study shows that with increasing thermal ageing temperature, strain induced martensite undergoes reversal to austenite and is associated with grain refinement. XRD analysis has revealed that at 25% cold work, volume fraction of martensite was 24%, and as cold work was increased to 45%, the volume fraction of martensite was 42%. Whereas after thermal ageing at 700°C, part of strain induced martensite undergo reversal to austenite and only 18% martensite is left in the matrix. On thermal ageing at 900°C, only 2% strain induced martensite was left in the matrix. Hardness also follows the same trend as observed for volume fraction of martensite during cold work and subsequent thermal ageing. As the cold work percentage increases, hardness and volume fraction increases but as thermal ageing temperature increases, both hardness and volume fraction of martensite decrease rapidly. Degree of sensitization (DOS) decreases with increasing thermal ageing temperature.

Effect of heat on microstructural, mechanical and electrochemical evaluation of tungsten inert gas welding of low-nickel ASS

PurposeThe purpose of the study is to evaluate Cr-Mn ASS weld using different heat inputs for its microstructure, mechanical properties and electrochemical behavior. The microstructural examination used optical and scanning electron microscopy. It was observed that ferrite content decreases with increasing heat input. The length of dendrites, inter-dendritic space and volume of lathy ferrite increase with increasing heat input. The increasing heat input caused grain coarsening near the fusion boundary and produced wider heat-affected zone (HAZ). It also decreases hardness and tensile strength. This is attributed to formation of more δ ferrite in the weld. The electrochemical evaluation suggested that the δ ferrite helps in improving the pitting potential in 3.5 per cent NaCl solution saturated with CO2. Whereas in 0.5-M H2SO4 + 0.003-M NaF solution, higher passivation current density was observed because of dissolution of dferrite. The interphase corrosion resistance decreased with increasing heat input.Design/methodology/approachThe Cr-Mn austenitic stainless steel or low-nickel ASS was procured in form of 3-mm sheets in rolled condition. The tungsten inert gas welding was performed at three different heat inputs (100 A, 120 A and 140 A), argon as shielding gas with a flow rate of 15 L/min. Different welded regions were observed using optical microscope and scanning electron microscope. Electrochemicals test were performed in solutions containing 3.5 per cent NaCl with saturated CO2 solution and 0.5 M sulfuric acid + 0.003 M NaF at a scan rate of 0.1667 mV/s at room temperature (30 °C ± 1 °C) using a potentiostat.FindingsThe test steel Cr-Mn ASS is suitable with the selected electrode (308 L) and it produces no defects. Vermicular ferrite and lathy ferrite form in welds of various heat inputs. The increase in heat input reduces the formation of lathy ferrite. The width of HAZ and un-mixed zone increases with increase in heat input. The weld zone of low heat input (LHI) has the highest hardness and tensile strength because of higher δ ferrite content and small grain size in the weld zone. The hardness at high heat input (HHI) is found to be lowest because of grain coarsening in the weld. With increase in δ ferrite, the pitting resistance increases. In 0.5-M sulfuric acid + 0.003-M NaF, the increase in ferrite content reduces the passivation current density. Interphase corrosion resistance increases with increase in δ ferrite content as higher per cent degree of sensitization was observed in LHI welds as compared to medium heat input and HHI welds.Originality/valueThis work focuses on welding of ASS by tungsten inert gas welding at different heat inputs. Welding is a critical process for joining metals in most of the fabrication industries and proper heat input is required for getting desired microstructure in the weld metal. This would highly affect the strength and corrosion behavior of the alloy. This paper would give an understanding of how the change in heat input by tungsten inert gas welding affects the microstructural and corrosion behavior in the weld metal.

Effects of initial grain size and strain on grain boundary engineering of high-nitrogen CrMn austenitic stainless steel

18Mn18Cr0.5N steel with an initial grain size of 28–177 μm was processed by 2.5%–20% cold rolling and annealing at 1000°C for 24 h, and the grain boundary character distribution was examined via electron backscatter diffraction. Low strain (2.5%) favored the formation of low-Σ boundaries. At this strain, the fraction of low-Σ boundaries was insensitive to the initial grain size. However, specimens with fine initial grains showed decreasing grain size after grain boundary engineering processing. The fraction of low-Σ boundaries and the (Σ9 + Σ27)/Σ3 value decreased with increasing strain; furthermore, the specimens with fine initial grain size were sensitive to the strain. Finally, the effects of the initial grain size and strain on the grain boundary engineering were discussed in detail.

The effect of cold work by rolling on electrochemical behaviour of Cr-Mn austenitic stainless steel in chloride environment

The effect of cold work (CW) on electrochemical behaviour of Cr-Mn austenitic stainless steel (AISI 202) in 3.5% NaCl solution was studied through microstructure, x-ray diffraction (XRD), Mott-Schottky (M/S) techniques, Potentiodynamic polarization (PDP), and Electrochemical Impedence Spectroscopy (EIS). The XRD spectrum was used to estimate volume fraction of martensite and dislocation density, whereas EIS and PDP plots were used to evaluate electrochemical behaviour. The M/S plots were used to estimate defect densities of passive film. It is found that as the cold work is increased, austenite transform to martensite and dislocation density increases. It is also found that corrosion resistance of the material decreases as the cold work is increased. When cold work is upto 15%, the material shows passivity. At higher cold work the material looses passivity. It is also found that defect density of passivating film is more when Cr-Mn stainless steel is subjected to more cold work. Most importantly, a relationship between dislocation density of the substrate and defect density of the film has been noticed.

Pitting Corrosion Behavior of Cr–Mn Austenitic Stainless Steel with Addition of Molybdate and Tungstate Under Stagnant and Flow Condition in NaCl Solution

Molybdate and tungstate inhibitors were introduced in stagnant and flowing conditions for determining pitting corrosion resistance of Cr–Mn SS in chloride-containing media. Corrosion behavior was investigated using potentiodynamic and electrochemical impedance spectroscopy measurements in different NaCl concentrations (1, 2% and 3 wt.%). It was found that increase in NaCl concentration increases the corrosion rate. The addition of 0.01 M inhibitors decreased the corrosion rate in Cr–Mn SS. Better corrosion resistance was found with the addition of molybdate in stagnant conditions. However, the opposite behavior was observed in flowing conditions, but there was an increase in E pit with addition of molybdate. Additionally, the mechanism of the corrosion attack developed on the material surface after polarization was analyzed by scanning electron microscopy and energy-dispersive spectroscopy (EDS mapping and point scan). The inhibition effect of the additions was due to a more stable passive film against Cl ions. Mn in the steel may cause opposite effect (initiation of the pits on steel), mainly due to the presence of MnS inclusions which acted as pitting initiators.

Dependence of corrosion resistance on grain boundary characteristics in a high nitrogen CrMn austenitic stainless steel

Processing schedules for grain boundary engineering involving different types of cold deformation (tension, compression, and rolling) and annealing were designed and carried out for 18Mn18Cr0.6N high nitrogen austenitic stainless steel. The grain boundary characteristic distribution was obtained and characterized by electron backscatter diffraction (EBSD) analysis. The corrosion resistance of the specimens with different grain boundary characteristic distribution was examined by using potentiodynamic polarization test. The corrosion behavior of different types of boundaries after sensitization was also studied. The fraction of low-Σ boundaries decreased with increasing strain, and it was insensitive to the type of cold deformation when the engineering strain was lower than 20%. At the strain of 30%, the largest and smallest fractions of low-Σ boundaries were achieved in cold-tensioned and rolled specimens, respectively. The fraction of low-Σ boundaries increased exponentially with the increase of grain size. The proportion of low-angle grain boundaries increased with decreasing grain size. Increasing the fraction of low-Σ boundaries could improve the pitting corrosion resistance for the steels with the same grain size. After sensitization, the relative corrosion resistances of low-angle grain boundaries, Σ3 boundaries, and Σ9 boundaries were 100%, 95%, and 25%, respectively, while Σ27 boundaries, other low-Σ boundaries and random high-angle grain boundaries had no resistance to corrosion.

Influence of Mn-Ni alloying elements on surface film and pitting resistance of Cr-Mn austenitic stainless steels

This research focused on surface film and corrosion resistance of three commercial type 200 austenitic stainless steels including L4, L1 and LH with different Mn/Ni mass ratio in Chinese market. X-ray photoelectron spectrum (XPS) results showed that Fe, Cr and Mn oxide layer thickness and passive film thickness were reduced as Mn/Ni mass ratio increased. Mn enrichment was only found in LH passive film. Mn oxide percentage in LH passive film was also the highest. These disadvantages deteriorated pitting resistance of LH, which was proved by anodic polarization result. Mn/Ni mass ratio should not be higher than 1.7 to ensure good corrosion resistance and low raw material cost.

Intergranular Corrosion Behavior of Low-Nickel and 304 Austenitic Stainless Steels

Intergranular corrosion (IGC) susceptibility for Cr-Mn austenitic stainless steel and 304 austenitic stainless steel (ASS) was estimated using electrochemical techniques. Optical and SEM microscopy studies were carried out to investigate the nature of IGC at 700 °C with increasing time (15, 30, 60, 180, 360, 720, 1440 min) according to ASTM standard 262 A. Quantitative analysis was performed to estimate the degree of sensitization (DOS) using double loop electrochemical potentiokinetic reactivation (DLEPR) and EIS technique. DLEPR results indicated that with the increase in thermal aging duration, DOS becomes more severe for both types of stainless steel. The DOS for Cr-Mn ASS was found to be higher (65.12% for 1440 min) than that of the AISI 304 ASS (23% for 1440 min). The higher degree of sensitization resulted in lowering of electrical charge capacitance resistance. Chronoamperometry studies were carried out at a passive potential of 0.4 V versus SCE and was observed to have a higher anodic dissolution of the passive film of Cr-Mn ASS. EDS studies show the formation of chromium carbide precipitates in the vicinity of the grain boundary. The higher Mn content was also observed for Cr-Mn ASS at the grain boundary.