Mo Austenitic Stainless Steel Research Articles

Comparative Study Between Sensitization Degree of the 0.4% Mo Austenitic Stainless Steel and UNS S31803 Duplex Stainless Steel

This study aims at assessing the degree of susceptibility to intergranular corrosion of the UNS S31803 duplex stainless steel and the 0.4% Mo austenitic stainless steel through the double-loop electrochemical potentiodynamic reactivation (DL-EPR) test. Both steels were subjected to isothermal treatments at 650ºC and 900ºC in order to compare the effect of temperature on the respective degrees of sensitization. It was observed that the 0.4% Mo austenitic stainless steel presented a higher degree of sensitization and, consequently, a greater susceptibility to the intergranular corrosion process when compared to the UNS S31803 duplex stainless steel in the two temperatures studied. It was also observed that, for both steels, the heat treatment at 900ºC resulted in a higher degree of sensitization when compared to the heat treatment at 650ºC.

Effects of molybdenum on the composition and nanoscale morphology of passivated austenitic stainless steel surfaces.

Surface analysis by time-of-flight secondary ion mass spectrometry, X-ray photoelectron spectroscopy and scanning tunnelling microscopy has been applied to provide new insight on Mo effects on the composition and nanostructure of the passive films grown in sulfuric acid on well-controlled Fe-17Cr-14.5Ni-2.3Mo(100) austenitic stainless steel single crystal surfaces. A duplex hydroxylated oxide matrix, 1.8-1.9 nm thick, is formed with a strong partition between Cr(iii) and Fe(iii) in the inner and outer layers, respectively. Cr(iii) is increasingly enriched by preferential iron oxide dissolution upon passivation and ageing. Ni, only present as oxide traces in the film, is enriched in the alloy underneath. Mo, mostly present as Mo(iv) in the Cr-rich inner layer prior to anodic polarisation, becomes increasingly enriched (up to 16% of cations) mostly as Mo(vi) in the Fe-rich outer layer of the passive film, with ageing promoting this effect. Metallic Mo is not significantly enriched below the passive film produced from the native oxide covered surface. Mo does not markedly impact the nanogranular morphology of the native oxide film nor its local thickness variations assigned to substrate site effects on Cr(iii) enrichment. Site specific preferential passivation still takes place at the (native) oxide-covered step edges of the alloy surface, and transient dissolution remains preferentially located on the terraces. Nanostructures, possibly Mo-containing, and healing local depressions formed by transient dissolution during passivation, appear as a specific effect of the Mo presence. Another Mo effect, observed even after 20 h of passivation, is to prevent crystallisation at least in the Fe-rich outer part of the passive film where it is concentrated mostly as Mo(vi) (i.e. molybdate) species.

Dependence of critical cleavage stresses as a function of orientation and temperature in single crystals of Fe-18% Cr-14% Ni-2% Mo austenitic stainless steel containing hydrogen

The effect of electrolytic hydrogen saturation on the critical cleavage stress τ cr was studied as a function of test temperature, duration of hydrogen saturation, and orientation of the tension axis in single crystals of Fe-18% Cr-14% Ni-2% Mo austenitic stainless steel.

高窒素オーステナイトステンレス鋼の組織と機械的性質に及ぼす時効の影響

The effect of aging on microstructures and mechanical properties of 1%N-23%Cr-4%Ni-2%Mo austenitic stainless steel aged at 600∼800°C for up to 1000 h were investigated with electron microscope, analysis of extracted residues, ferrite scope, and mechanical tests such as hardness, tensile, and Charpy impact. Chromium nitrides precipitated on grain boundary at the aging temperature below 650°C, while lamellar precipitates, which were composed of plate like Cr2N and martensite, were formed above 700°C. σ phase precipitated at 700°C for 1000 h and above 750°C for longer than 100 h. Martensite was also observed around σ phase, and the total amount of martensite was 40-50% after aging at 700-800°C for 1000 h. Age hardening was observed above 700°C and hardness, which increased with aging time and temperature, reached about Hv 500 after aging at 800°C for 1000 h. Increase in tensile strength by aging was higher than that in proof strength, because of pronounced increase in work hardening rate by aging. Strengthening by aging depended mainly on the amount of lamellar precipitates and additionally on σ phase and its surrounding martensite at higher temperature. Tensile ductility and impact toughness were decreased by aging due to lamellar precipitates and σ phase.

Laser beam welding of steel plates using consumable inserts

Summary The aim of this study was to identify the appropriate parameters for laser welding of steel plates. The mechanical characteristics and metallurgical microstructures of the joints were analysed experimentally. Welded joints were made using two different types of steel (Cr–Mo low-alloy steel and austenitic stainless steel). Samples were cut from the welds and examined macrographically and micrographically; tensile and microhardness tests were also performed. Observation of these samples showed the beneficial effects of narrower molten and heat-affected zones and the consequent advantageous reductions in distortions and residual stresses. On the basis of these experimental results we conclude that both ferritic and austenitic steels can be properly welded, with good mechanical and metallurgical results, using consumable inserts pre-fitted to the edges before welding, provided that suitable welding parameters are chosen.

High dilution submerged arc welding of Cr–Ni–Mo austenitic stainless steel

Improvements in the submerged arc welding (SAW) of Cr–Ni–Mo austenitic stainless steel have been achieved from a combination of simplified edge preparation, intermediate heat input, and high dilution (75–80%). The cost benefit is 83% compared with the previously used practice. Mechanical properties of the weld metal in a square edge preparation showed a decrease in yield and tensile strengths, but a significant improvement in toughness compared with the Y preparation weld metal. Thin foil transmission electron microscopy confirmed the presence of χ phase in some areas of the Y preparation weld metal, and M23C6 in some areas of the square edge preparation. This has been related to the incidence of fewer reheated and cooled areas in the square edge preparation weld. The reduced precipitation has influenced the toughness characteristics of the square edge preparation weld metal.

High dilution submerged arc welding of Cr–Ni–Mo austenitic stainless steel

Improvements in the submerged arc welding (SAW) of Cr–Ni–Mo austenitic stainless steel have been achieved from a combination of simplified edge preparation, intermediate heat input, and high dilution (75–80%). The cost benefit is 83% compared with the previously used practice. Mechanical properties of the weld metal in a square edge preparation showed a decrease in yield and tensile strengths, but a significant improvement in toughness compared with the Y preparation weld metal. Thin foil transmission electron microscopy confirmed the presence of χ phase in some areas of the Y preparation weld metal, and M23C6 in some areas of the square edge preparation. This has been related to the incidence of fewer reheated and cooled areas in the square edge preparation weld. The reduced precipitation has influenced the toughness characteristics of the square edge preparation weld metal.

The effect of temperature on the low cycle fatigue properties of a 15Cr-15Ni, Ti modified austenitic stainless steel

A titanium modified 15Cr-15Ni-2.5 Mo austenitic stainless steel designated as Alloy D9 is currently a favored material in liquid metal cooled fast breeder reactors for fuel clad and sub-assembly wrappers and is also being considered as a primary candidate alloy for fusion reactor first wall applications. The choice of this material in the nuclear industry is primarily based on its excellent resistance to radiation induced void swelling. In nuclear reactor applications the components are often subject to temperature gradient induced thermal stresses which are cyclic in nature as a result of start-ups, shutdowns and transients. Therefore there is a need for comprehensive understanding of both the macroscopic cyclic deformation behavior and the micromechanisms which influence such behavior. Further this understanding must include not only the behavior at maximum operating temperature but also the behavior at lower temperatures encountered during transients. Till date the cyclic stress response and fatigue life of Alloy D9 over a wide temperature range has not been examined and there is no information available on the deformation-induced microstructure during fatigue loadings. The purpose of this paper is to report some recent observations pertaining to the temperature dependence of cyclic stress response and evolving deformation substructure of Alloymore » D9 between 300 and 923 K.« less

Gouging of Cr–Ni–Mo austenitic stainless steel welds

High integrity austenitic stainless steel welds within the structure of chemical carriers may require to be are air gouged to ensure that the weld achieves full penetration. The gouging process leaves a coating on the gouged surface which, traditionally, has been ground off. Such grinding is time consuming and expensive. Using X-ray photoelectron spectroscopy, the composition of the gouged layer was determined and the thickness determined to be no greater than 60 nm. Leaving the layer in place was found to have no detrimental effects on the parent plate/weld interface. This was established using optical microscopy, transmission electron microscopy, and scanning electron microscopy.

Gouging of Cr–Ni–Mo austenitic stainless steel welds

High integrity austenitic stainless steel welds within the structure of chemical carriers may require to be are air gouged to ensure that the weld achieves full penetration. The gouging process leaves a coating on the gouged surface which, traditionally, has been ground off. Such grinding is time consuming and expensive. Using X-ray photoelectron spectroscopy, the composition of the gouged layer was determined and the thickness determined to be no greater than 60 nm. Leaving the layer in place was found to have no detrimental effects on the parent plate/weld interface. This was established using optical microscopy, transmission electron microscopy, and scanning electron microscopy.

A novel technique for characterizing annealing behaviour

The ultrasonic testing technique is widely used for obtaining information about the microstructural and mechanical properties of metals in recent years. An advantage of using ultrasonic testing over that of traditional testing methods, such as hardness testing, is that it enables the extraction of information about the structural state of the material through volume measurement averaged throughout the thickness of the sample without recourse to destructive testing. In the present work, isothermal annealing of 20% cold worked Ti-modified 15Cr-15Ni-2.2Mo austenitic stainless steel often referred to as alloy D-9 was characterized by carrying out ultrasonic velocity measurements. Furthermore, variation in ultrasonic velocity with aging time was compared with that of hardness measurements. Alloy D-9, which conforms to ASTM A771 (UNS S38660), has been chosen as the prime candidate material for the fuel clad and hexagonal wrapper for the fuel subassemblies of the Prototype Fast Breeder Reactor. This material would be used at elevated temperatures in cold worked condition. At the elevated service temperatures, microstructural degradation and loss of prior cold work may occur. Metallography and hardness testing techniques could be used to obtain information about the degradation of microstructure of these core components during the post-irradiation examination. However, these techniques wouldmore » be cumbersome when applied to testing of highly radioactive materials. Hence developing simple techniques to study the changes in the microstructure of cold worked material exposed to elevated temperatures and irradiation environment assumes importance.« less

Effect of ageing on the microstructural stability of cold-worked titanium-modified 15Cr-15Ni—2.5Mo austenitic stainless steel

A titanium-modified 15Cr-15Ni-2.5Mo austenitic stainless steel conforming to ASTM A 771 (UNS S 38660), commercially called Alloy D9, is being indigenously developed for application as material for the fuel clad and the hexagonal wrapper for fuel subassemblies of the Prototype Fast Breeder Reactor. As this material would be used in the cold-worked condition and would be subjected to prolonged exposure to elevated service temperatures, the effect of ageing on the microstructural stability was studied as a function of the amount of cold work. The material was given 2.5–30% prior cold work and then aged at temperatures in the range 923 to 1173 K for times ranging from 0.25 to 1000 h. Hardness measurements made before and after ageing were correlated with the Larson-Miller parameter to determine the highest stable prior cold-work level. Optical microscopy was used to study the microstructural changes. The influence of prolonged exposure for two and three years at the operating temperatures of clad and wrapper, on the elevated temperature tensile properties of a 20% prior cold-worked Alloy D9 was also studied through accelerated ageing treatments based on the present parametric approach.

Cryogenic properties of new austenitic stainless steel for fusion reactor superconducting magnets

A non-magnetic austenitic stainless steel has been developed, which has an adequate combination of yield strength and fracture toughness at liquid helium temperature, as a candidate structural material for fusion reactor superconducting magnets. The salient feature of the steel is its low-temperature toughness preserved after aging equivalent to the wind-and-react heat treatment of Nb 3Sn superconducting coil. The steel also keeps its toughness after electron beam welding. Its composition is characterized by addition of vanadium and nitrogen to 25Cr-14Ni-0.5Mo austenitic stainless steel. Microscopic observations have suggested that finely dispersed precipitates, presumably vanadium carbonitrides, are responsible for the excellent cryogenic mechanical properties.

Experiences with a high‐alloyed stainless steel under highly corrosive conditions

AbstractThe paper emphasizes experiences from the chemical industry with a highly alloyed stainless steel, Avesta 254 SMO.A number of installations are reported where this 6 Mo austenitic stainless steel has been selected since conventional stainless steels have shown inadequate corrosion resistance. These installations are at the production of – aluminium fluoride – chlorate, sodium and potassium – diamines – synthetic fibres – soda and at the recovery of – solvents, viz. chlorinated hydrocarbons – sulphur dioxide Further the paper summarizes some experiences from the fabrication point of view.

16Cr-15Ni-2.5Mo鋼の高温強度特性におよぼすPの役割

16Cr-15Ni-2.5Mo鋼の高温強度特性におよぼすPの役割

On the corrosion behaviour of a ferritic 18 Cr‐2 Mo‐steel — Part I

1 Introduction Some interesting new developments have taken place in recent years in the field of ferritic stainless steel (1–16). As a material for chemical apparatus, the common highly alloyed chromium steels as listed in national standards (e.g. in the German Standard DIN 17440) have only found limited applications. The reasons are sensitivity of several of these chromium alloyed stainless steels to intergranular corrosion (especially after welding), lower corrosion resistance compared to austenitic stainless steels, and difficulties in fabricating (especially welding). It has been shown (1–16) that the intrinsic drawbacks of customary ferritics can be overcome by metallurgical measures, primarily keeping the amount of carbon and nitrogen extremely low. The solubility in the ferrite for these two elements in rather low, both occupying interstitial sites. Stainless steels of the type dealt with in this paper are therefore sometimes termed Extra Low Interstitial (ELI)‐ferritic stainless steels. At sufficiently low concentrations of carbon and nitrogen (and some other elements), the sensitivity of ferritic stainless steels to intergranular corrosion is definitely lowered, and their ductility at ambient temperature is increased, i.e. the transition temperature is lowered. An advantage of these steels is their resistance to stress corrosion cracking. They have, so far, shown no sensitivity against chloride stress corrosion cracking under realistic operating conditions. For this reason, cooling water systems using river water with a high chloride content represent a suitable field of application for these steels. They can be welded up to a wall thickness of 3mm without sensitisation and undue loss of impact strength so that tubes for heat exchangers can be made of these steels. Their development has led to alloys ranging from 18 Cr‐2 Mo‐0 Ni to about 28 Cr‐2 Mo‐4 Ni. The present paper will only deal with the 18 Cr‐2 Mo steel because this material can be compared in price and properties with the standard 18 Cr‐9 Ni‐2 Mo austenitic stainless steel. In addition, the material in question has now become available in the form of pipe and sheet.

Testing austenitic-ferritic superheater tube transition welds under combined “two-shifting” and bending stresses

Three 309 stainless transition welds between 21/4 Cr-1 Mo and 321H austenitic stainless steel superheater tube sections have been subjected to overtemperatureaccelerated simulated “two-shift” cycles involving cyclic temperature and pressure, together with externally applied bending stresses. After 2960 cycles, a failure, similar to those experienced in service welds, occurred in the weld which had experienced the highest temperature and intermediate bending stress; the weld with intermediate temperature and highest bending stress was damaged; the third weld, with lowest temperature and stress was damage-free. Detailed metallographic examination showed that, in the damaged weld, internal damage involved cavitation with intercavity linkage by “lace-work” cracking, believed to be fatigueinduced. Such damage is typical of that observed in service-damaged transition welds. In the failed weld, the apparent fatigue component was absent, the cavitation being more typical of creep damage. The relative importance and likely synergistic effects of the damaging parameters, ie, maximum temperature, cyclic loading, and extraneous stress remain to be determined.

Hot workability of a 6% Mo austenitic stainless steel

AbstractThe addition of 6% molybdenum to austenitic stainless steel, although very beneficial to localized corrosion resistance, leads to hot workability difficulties. Hot short cracking at the interdendritic boundaries during initial breakdown of the cast ingot has been observed to be the principal failure mode. A hot working schedule has been developed which takes advantage of a peak in the ductility curve of this alloy at 1850°F (1010°C). An initial moderate reduction can be accomplished at this temperature without cracking which promotes a complete recry-stallization of the ingot during subsequent heating to rolling temperatures. The resultant fine-grained structure can be further processed to sheet without hot short cracking. Resume L'addition de 6% de molybdene a l'acier inoxydable austenitique augmente grandement sa resistance ala corrosion localisee mais engendre des problemes au cours du corroyage. On a observe que le principal mode de rupture, lors des premieres passes du degrossissage des lingo...

Hot workability of a 6% Mo austenitic stainless steel

Hot workability of a 6% Mo austenitic stainless steel