Grain-boundary penetration of type 316 stainless steel exposed to cesium or cesium and tellurium

When 20 pct cold-worked Type 316 stainless steel is exposed to Cs at 700°C under controlled oxygen-chemical potential environment, Cs penetration into the stainless steel grain boundaries occurs at oxygen potentials ΔG_o2 ≥ -96 kcal per mole. At lower oxygen potentials (~ΔG_o2 ≤ —110 kcal per mole), no corrosion occurs. Under the same experimental conditions, when the stainless steel is exposed to Cs:Te (2:1, atomic), corrosion occurs and penetration morphology appears to depend strongly on the oxygen-potential environment. The stainless steel suffers intergranular corrosion by Te (in the presence of Cs-Te) under conditions where chromium oxidation is not expected to occur. The kinetics of grain-boundary penetration by Te have been studied at temperatures between 550 and 700°C. The depth of the penetrated zone varies as (time)^1/2, and the process has an activation energy of 34 kcal per mole. The results are discussed, and the effects of stainless steel microstructure and externally applied stress on corrosion reactions are also described.     

Grain-boundary penetration of type 316 stainless steel exposed to cesium or cesium and tellurium

When 20 pct cold-worked Type 316 stainless steel is exposed to Cs at 700°C under controlled oxygen-chemical potential environment, Cs penetration into the stainless steel grain boundaries occurs at oxygen potentials ΔG_o2 ≥ -96 kcal per mole. At lower oxygen potentials (~ΔG_o2 ≤ —110 kcal per mole), no corrosion occurs. Under the same experimental conditions, when the stainless steel is exposed to Cs:Te (2:1, atomic), corrosion occurs and penetration morphology appears to depend strongly on the oxygen-potential environment. The stainless steel suffers intergranular corrosion by Te (in the presence of Cs-Te) under conditions where chromium oxidation is not expected to occur. The kinetics of grain-boundary penetration by Te have been studied at temperatures between 550 and 700°C. The depth of the penetrated zone varies as (time)^1/2, and the process has an activation energy of 34 kcal per mole. The results are discussed, and the effects of stainless steel microstructure and externally applied stress on corrosion reactions are also described.     

Temperature and strain rate dependence of cyclic deformation response and damage accumulation in ofhc copper and 304 stainless steel

The effects of temperature and strain rate on deformation behavior and dislocation structure were investigated for OFHC copper and type 304 stainless steel. It is shown that the cyclic stress response is inversely related to the cell size for copper cycled at different temperatures ranging from -75 to 650°C. Type 304 stainless steel underwent a change from a planar to a wavy slip character as the temperature was changed from room temperature to 760°C. At elevated temperatures, cells were observed and the size of the cells tended to increase with increase in temperature. The effects of temperature on the cyclic stress-strain parameters were investigated for copper, type 304 stainless steel and Ferrovac “E” iron. On studying the effects of temperature and strain rate on the fracture mechanisms it was found that a time dependent fracture mode was dominant at high temperature levels and low strain rates. However, at high strain rates the life was insensitive to temperature. The role of grain boundary migration on the fracture process was investigated. Grain boundary migration was found to be dependent on strain rate for copper. However, for type 304 stainless steel, the grain boundary migration was inhibited at high temperature (760°C) due to the presence of precipitates at the grain boundaries. In strain cycling of OFHC copper and type 304 stainless steel, it was found that the addition of creep-type damage to fatigue damage resulted in a total damage which was not equal to unity for failure when these different modes were imposed sequentially. The sense of the damage accumulation appeared to have no effect on this summation.     

Applicability of bond percolation theory to intergranular stress-corrosion cracking of sensitized AlSl 304 stainless steel

A model system (isothermally sensitized 304 stainless steel in the SL-EPR test environment) was examined in order to assess the significance of the percentage of active grain boundaries on intergranular stress-corrosion cracking (IGSCC) susceptibility. The relationship between the primary passivation potential and Cr concentration was exploited in order (1) to determine the population distributions of grain boundary minimum Cr contents and (2) to subsequently control the quantity of active grain boundaries in a constant extension rate (CER) test. A significant decrease in resistance to IGSCC was observed when greater than 23 pct of the grain boundaries were active during the CER test, regardless of the distribution of Cr-depletion levels among the individual grain boundaries. Such an abrupt decrease in IGSCC resistance at greater than 23 pct active grain boundaries has been interpreted within the context of bond percolation theory. This critical value corresponds to a one-dimensional bond percolation threshold for a three-dimensional array of tetrakaidecahedra and confirms the results of Wells and co-workers. The implications of a possible threshold in the case of IGSCC in sensitized 304 stainless steel (304SS) exposed to oxygenated water at 288 °C are examined. Here it is believed that the 23 pct criterion is a necessary but not sufficient criterion for IGSCC. Other considerations are discussed.     

The sintering of 304L stainless steel powder

An examination of the shrinkage kinetics for a 304L stainless steel powder showed that initial densification is controlled by a strain assisted volume diffusion mechanism. At temperatures above 1330 K, grain growth reduces the shrinkage rate; however, at lower sintering temperatures, the shrinkage rate is temporarily increased by the proximity of the moving grain boundaries to the interparticle necks. The activation energies of volume diffusion (240±20 kJ/mol) and grain growth (285±35 kJ/mol) were in good agreement with prior results.     

The sintering of 304L stainless steel powder

An examination of the shrinkage kinetics for a 304L stainless steel powder showed that initial densification is controlled by a strain assisted volume diffusion mechanism. At temperatures above 1330 K, grain growth reduces the shrinkage rate; however, at lower sintering temperatures, the shrinkage rate is temporarily increased by the proximity of the moving grain boundaries to the interparticle necks. The activation energies of volume diffusion (240 ± 20 kJ/mol) and grain growth (285 ± 35 kJ/mol) were in good agreement with prior results.     

Tribological Investigation of Effect of Grain Size in 304 Austenitic Stainless Steel

Stainless steels are widely used in a variety of engineering applications such as food appliances, surgical instruments, nuclear reactors and cryogenic applications. The properties of stainless steel are greatly affected by the grain size. The present study investigates the effect of grain size on sliding wear behavior of AISI 304 stainless steel. The sliding wear properties are measured using a Pin-on-Disc machine. Annealing heat treatment process varies the grain size of steel at 1100 °C. The wear test is performed on different grain sizes of AISI 304 steel at various sliding speeds under dry condition. The wear rate of the steels at different sliding distances is plotted as a function of grain size. The maximum wear rate is obtained at an intermediate grain size. It is noted that frictional force and temperature initially increases and then reaches the saturation plateau. The results are used to establish a correlation between the grain size and sliding wear properties of stainless steel. The present study is useful in enhancing the life of various components made of the stainless steel.     

The effects of cold working on sensitization and intergranular corrosion behavior of AISI 304 stainless steel

The effects of prior cold rolling of up to an 80 pct reduction in thickness on the sensitization-desensitization behavior of Type AISI 304 stainless steel and its susceptibility to intergranular corrosion have been studied by electrochemical potentiokinetic reactivation (EPR) and Strauss-test methods. The results indicate that the prior deformation accelerated the sensitization as compared to the undeformed stainless steel. The deformed Type 304 stainless steel experienced desensitization at higher temperatures and times, and it was found to be enhanced by increased cold deformation. This could be attributed to the increased long-range chromium diffusion, possibly brought on by increasing pipe diffusion and vacancies. The role of the deformation-induced martensite (DIM) and texture, introduced by uniaxial cold rolling, on the sensitization-desensitization kinetics has also been discussed. This study could not reveal any systematic relationship between texture and the degree of sensitization (DOS) obtained. The effect of DIM on DOS seems to be pronounced at 500 °C when the steel retained significant amounts of DIM; however, the retained DIM is insignificant at higher sensitization times and temperatures.     

Twin-Boundary cavitation during creep in aged type 304 stainless steel

A transition from grain-to twin-boundary cavitation was observed in Type 304 stainless steel specimens aged at 593 and 649°C (1100 and 1200°F) and creep tested at 593°C (1100°F) and 207 MPa (30 ksi). Evidence of twin-boundary cavitation was also observed in unaged specimens tested at 649°C (1200°F) and 193 MPa (28 ksi). This same behavior was also found in aged Type 316 stainless steel. Several possible reasons have been suggested for the absence of frequently observed grain-boundary cavitation and its transition to twin-boundary cavitation.     

Differences in oxides on large-and small-grained 304 stainless steel

The oxides formed on large-grained (∼40 °m) and small-grained (∼4 μm) 304 stainless steel oxidized in air at 800 °C have been examined and compared by Auger electron spectroscopy to learn more about the role of grain boundaries in the oxidation of the materials. For vacuum preannealed specimens, relatively thick iron oxides formed over the grains and thin, chromium-rich oxides formed over grain boudaries of large-grained material. The oxide formed over the entire surface of the small-grained material was a thin chromium-rich layer similar to that formed over grain boundaries of the large-grained samples. The oxidation of both small- and large-grained samples was consistent with selective formation of Cr₂O₃ at grain boundaries followed by a lateral diffusion of Cr and spreading of Cr₂O₃. A protective Cr₂O₃ layer formed readily on small-grained material but not on large-grained material. In contrast to the differences in oxide morphology for small- and large-grained preannealed specimens, oxide morphologies were similar for small- and large-grained material when the outer surface layer was removed by polishing after annealing and before oxidation tests. Surface differences, not adequately defined by Auger and SEM studies, caused marked changes in oxide morphologies for large-grained material. The difference in oxidation behavior before and after polishing was attributed to enhanced oxidation at grain boundaries during the vacuum preannealing treatment and to differences in defect concentrations in the surface region.     

Recrystallization following hot-working of a high-strength low-alloy (HSLA) steel and a 304 stainless steel at the temperature of deformation

The recovery and recrystallization behavior of two commercial quality steels, a Cb(Nb) strengthened high-strength low-alloy (HSLA) steel and a 304 stainless steel, was studied following hot-working. Specimens were deformed in tension at a constant head velocity of 2 in.Js to reductions-in-area of 30 to 50 pct at temperatures in the austenite range from 1600° to 1900°F. The subsequent annealing behavior was observed at the temperature of deformation. Decreasing recrystallization rates with decreasing temperature andJor deformation were observed. It is suggested that CbC precipitation occurred during annealing of the HSLA steel and accounted for an arrest in the softening behavior. For the 304 stainless steel it is concluded that dynamic recrystallization took place during deformation, that thermal microtwinning was an active recovery mechanism during annealing, and that there was a preference for grain boundaries as nucleation sites for recrystallized grains. These conclusions regarding the annealing behavior of 304 stainless steel were supported by metallographic analysis of specimens water quenched from the temperature of deformation. Formerly with Rensselaer Polytechnic Institute,Troy, New York. Formerly Professor, Rensselaer Polytechnic Institute. This paper is based upon a thesis submitted by T. L. CAPELETTI in partial fulfillment of the requirements of the degree of Doctor of Philosophy at Rensselaer Polytechnic Institute.     

Nitriding of steels

The influence of nitriding temperature (900 to 1000°C) on the microstructure of low carbon steel and 304 stainless steel was studied. Nitriding was done in steps of 25°C with diffusion times between 3 and 9 hours. Metallographic technique was used to study the process kinetics. The case depth increases with temperature following an exponential relationship (parabolic). A quantitative relationship between diffusion layer thickening and nitriding temperature was determined.     

节镍无磁不锈钢Cr18Ni6Mn3N的组织及性能

研究了节镍无磁不锈钢Cr18Ni6Mn3N的热轧及固溶后的力学性能和耐蚀性能,分析了其固溶和时效析出后的组织演变规律、冷变形过程中形变诱发马氏体相变及其磁性能.结果表明:该不锈钢的固溶组织为单相奥氏体,其力学性能和耐蚀性能均高于SUS304不锈钢;800℃保温4 h后,在晶界析出粒状氮化物,随着保温时间延长,逐渐沿晶界凸起片层状析出物并向晶内生长,保温20 h后,凸出的片层状析出物直径达20μm.冷轧压下率18.3%时尚未发现形变诱发马氏体组织,随着变形量增大,马氏体含量增多,磁导率上升,但与相同条件下的SUS304不锈钢相比,冷轧板固溶后相对磁导率可降至1.002,因此可用于低成本无磁不锈钢领域.      

Flow stress and microstructural evolution during hot working of alloy 22cr-13ni-5mn-0.3n austenitic stainless steel

The stress-strain behavior and the development of microstructure between 850 °C and 1150 °C in an austenitic stainless steel, 22Cr-13Ni-5Mn-0.3N, were investigated by uniaxial compression of cylindrical specimens at strain rates between 0.01 and 1 s^-1 up to a strain of one. The measured (anisothermal) and corrected (isothermal) flow curves were distinctly different. The flow stress at moderate hot working temperatures, compared to a number of other austenitic alloys, was second only to that of alloy 718. Both static and dynamic recrystallization were observed. Recrystallization was sluggish in comparison to alloy 304L, apparently due to the presence of a fine Cr- and Nb-rich second-phase dispersion, identified as Z phase, which tended to pin the high-angle grain boundaries even at a high temperature of 1113 °C. Recrystallization may also be retarded by preferential res-toration through the competitive process of recovery, which is consistent with the relatively high stacking-fault energy for this alloy. It is concluded that this alloy must be hot worked at temperatures higher than usual for austenitic stainless steels in order to minimize flow stress and refine grain size.     

Transformation of Oxide Inclusions in Type 304 Stainless Steels during Heat Treatment

Heat treatment of Type 304 stainless steel in the range of 1273 K (1000 °C) to 1473 K (1200 °C) can transform manganese silicate inclusions to manganese chromite (spinel) inclusions. During heat treatment, Cr reacts with manganese silicate to form spinel. The transformation rate of inclusions depends strongly on both temperature [in the range of 1273 K to 1473 K (1000 °C to 1200 °C)] and inclusion size. A kinetic model, developed using FactSage macros, showed that these effects agree quantitatively with diffusion-controlled transformation. A simplified analytical model, which can be used for rapid calculations, predicts similar transformation kinetics, in agreement with the experimental observations.     

Elevated temperature mechanical properties of shock-strengthened austenitic stainless steel

The mechanical properties of explosively shock-strenghened (320 kbar) Type 304 stain-less steel were investigated. Tensile tests were performed on the shock-strengthened material at temperatures ranging from room temperature to 650°C. The shock hardening caused large increases in yield strength, moderate increases in ultimate strength, and reductions in ductility. Annealing studies on the shock-strengthened stainless steel indicated that considerable amount of the shock-induced strength stainless steel indicated that considerable amount of the shock-induced strength increase is retained after long time anneals at 700 and 750°C. However, metallographic studies on the annealed material indicated the presence of fine carbide particles which may partly account for the strength of the material. Creep studies indicated that the minimum creep rate at 650°C was reduced by a factor of 1000 due to shock strengthening.     

Chromium depletion and martensite formation at grain boundaries in sensitised austenitic stainless steel

A microstructural and compositional investigation of grain boundary precipitation and martensite formation in sensitised 304 stainless steel has been conducted. Grain boundary depletion of chromium has been quantified in terms of sensitisation time, temperature and boundary type by energy dispersive X-ray microanalysis in the transmission electron microscope. Chromium depleted profiles measured in grain boundary vicinities are sometimes asymmetrical and correlate with the expected profiles generated by growth of semicoherent and incoherent carbide interfaces. The depletion of chromium promotes martensite formation within near-grain boundary regions and this transformation has been directly studied by in situ cold stage microscopy down to − 150°C. Transformation occurs at the most severely depleted boundaries and initiation is favoured at slip band-boundary intersection points and along grain boundaries whose plane orientation matches that of the martensite habit plane. The preferential formation of grain boundary martensite could be an important factor in the stress corrosion and environment sensitive failure of this material.     

The oxidation resistance of fine-grained sputter-deposited 304 stainless steel

The air oxidation of free standing 0.1 cm thick sputter-deposited and wrought 304 stainless steel specimens was studied and the long term oxidation weight gains of the sputterdeposited material were much less than weight gains for the wrought material at 800, 900, and 1000°C. The amount of scaling was also much less on the sputtered material and a thin, adherent oxide formed. The oxide on the sputtered material was more uniform in composition and was higher in chromium and manganese compared to oxide on wrought stainless and eventually formed MnCr₂O₄ after long periods of exposure. No stratified oxide layers, as typically observed in wrought stainless steel, formed on the fine-grained sputtered material. The improved scaling resistance of the sputter-deposited steel was attributed to a combination of grain boundary enhancement of chromium diffusion, reduced stresses in the oxide and mechanical keying of the oxide to closely spaced grain boundaries. The stability of grain size for the sputtered material (grain size ≤ 6 μm) also contributed to the better oxide adherence.     

Effect of Flowing Lead–Bismuth Eutectic on the Mechanical and Structural Properties of Stainless Steel 304L

Stainless steel 304L is being considered as a structural material for a component in a critical facility in an environment of molten lead–bismuth eutectic (LBE) at a temperature between 250 and 350 °C. This paper gives results on the effect, of 10,000 h exposure to non-isothermal liquid LBE at temperatures of 250 and 350 °C, on the mechanical and structural properties of SS 304L. The dissolved oxygen concentration in the molten eutectic was 4 × 10^?10 wt% and flow velocity was 16 cm/s. In order to assess the changes in mechanical properties tensile tests were carried out in air at 25 °C and fractography of fractured surface was observed by scanning electron microscopy. The results showed no change in the mechanical properties of SS 304L after 10,000 h exposure to LBE at 250 and 350 °C. Electron probe microanalysis of the interface of SS 304L with LBE showed that there was no penetration of LBE into the grain boundaries nor preferential dissolution of any of the components of steel in LBE after 10,000 h exposure at either temperature. No oxide layer was observed on the surface of SS 304L.     

Oxidation behavior of a fine-grained rapidly solidified 18-8 stainless steel

The cyclic oxidation behavior of a fine-grained, rapidly solidified 303 stainless steel was determined at 900 °C in pure oxygen. The rapidly solidified alloy exhibited superior resistance to oxidation compared with that of a wrought 304 stainless steel; its oxidation resistance was as good as that of a wrought 310 stainless steel, even though the latter alloy contained more Cr and Ni. The matrix of the rapidly solidified steel contained a uniform dispersion of fine MnS precipitates (0.2 to 0.5 μm), which were effective in inhibiting grain growth at elevated temperatures. The enhanced resistance to oxidation of the rapidly solidified alloy is attributed to two factors: (1) the formation and growth of protective Cr₂O₃ and SiO₂ scales were promoted by the fine alloy grain size (5 to 8 =gmm) and by the presence of the MnS dispersion, and (2) the adherence of the scale was increased by the formation of intrusions of SiO₂ from the external scale into the alloy, which formed around MnS precipitates and along closely-spaced alloy grain boundaries, and which acted to key the scale mechanically to the alloy.