M.D. Mathew

Influence of carbon and nitrogen on the creep properties of type 316 stainless steel at 873 K

Abstract This paper is concerned with the creep properties of a nitrogen-alloyed type 316L (316LN) stainless steel (SS) at 873 K in the stress range 215–335 MPa. A comparison has been made with the creep properties of type 316 SS having a similar grain size and chemical composition with respect to the major alloying elements. Type 316LN SS showed a higher rupture life, a lower steady state creep rate and a higher rupture ductility at all stress levels. The improvement in creep properties has been attributed to the combined influence of precipitation strengthening by fine intragranular and intergranular carbides and strengthening arising from the nitrogen in solid solution. Correlations between metallographic observations and the creep properties as well as the role of interstitial elements in these properties are discussed.

Finite element analysis of type IV cracking in 2.25Cr–1Mo steel weldment based on micro-mechanistic approach

Creep studies were carried out on 2.25Cr–1Mo steel base metal and its fusion-welded weldments at 823u2009K over the stress range 100–240u2009MPa. The weldment possessed lower creep rupture strength than the base metal due to type IV failure at the outer edge of the heat-affected zone (HAZ). Premature failure of the weldment was associated with pronounced creep cavitation accompanied with localized creep deformation in the soft intercritical region of the HAZ that was sandwiched between relatively higher creep deformation-resistant microstructural regions. The cavitation was associated with coarse intergranular precipitates in the intercritical region of the HAZ. The type IV cracking in the intercritical region of the HAZ was found to initiate deep inside the weldment and propagate towards the specimen surface. Finite element analysis of stress and strain distributions across the weldment was carried out considering the micro-mechanical strength inhomogeneity across it to explain the observed features of type IV cracking. The estimated higher von-Mises and principal stresses deep inside the intercritical region of the HAZ of the weldment led to the localized creep deformation and preferential cavity nucleation and growth, resulting in type IV failure of the weldment. The role of intergranular precipitate particles in the intercritical region of the HAZ in facilitating creep cavity nucleation by the exhaustion of creep ductility of the material close to the precipitate was corroborated from finite element analysis of stress and strain distribution around the precipitates.

Prediction of creep parameters of type 316 stainless steel under service conditions using the π-projection concept

Abstract The creep behaviour of type 316 stainless steel has been studied under constant loading conditions in the temperature range from 823 to 923 K and for rupture lives up to 50 000 h. The θ-projection concept has been applied to the creep curves and was found to yield accurate predictions when interpolating and predicting creep data under service conditions. The shape of the creep curves, as well as the minimum creep rate, the time to reach a limiting strain, and the time to fracture, were considered.

A Comparison of Creep Rupture Strength of Ferritic/Austenitic Dissimilar Weld Joints of Different Grades of Cr-Mo Ferritic Steels

Evaluations of creep rupture properties of dissimilar weld joints of 2.25Cr-1Mo, 9Cr-1Mo, and 9Cr-1MoVNb steels with Alloy 800 at 823xa0K were carried out. The joints were fabricated by a fusion welding process employing an INCONEL 182 weld electrode. All the joints displayed lower creep rupture strength than their respective ferritic steel base metals, and the strength reduction was greater in the 2.25Cr-1Mo steel joint and less in the 9Cr-1Mo steel joint. Failure location in the joints was found to shift from the ferritic steel base metal to the intercritical region of the heat-affected zone (HAZ) of the ferritic steel (type IV cracking) with the decrease in stress. At still lower stresses, the failure in the joints occurred at the ferritic/austenitic weld interface. The stress-life variation of the joints showed two-slope behavior and the slope change coincided with the occurrence of ferritic/austenitic weld interface cracking. Preferential creep cavitation in the soft intercritical HAZ induced type IV failure, whereas creep cavitation at the interfacial particles induced ferritic/austenitic weld interface cracking. Micromechanisms of the type IV failure and the ferritic/austenitic interface cracking in the dissimilar weld joint of the ferritic steels and relative cracking susceptibility of the joints are discussed based on microstructural investigation, mechanical testing, and finite element analysis (FEA) of the stress state across the joint.

A comparative study of creep rupture behaviour of modified 316L(N) base metal and 316L(N)16-8-2 weldment in air and liquid sodium environments

Abstract Creep rupture behaviour of modified type 316L(N) stainless steel base metal and weldments prepared with 16-8-2 filler wire has been investigated in air and flowing sodium environments at 823 K. No adverse environmental effects have been noticed due to sodium on the creep rupture behaviour of these weldments for tests up to 10 000 h. Rupture lives of the weldment were higher in the sodium environment than those in air. Rupture lives of the weldments were found to be lower than those of the base metal by a factor of two to five in both air and sodium environments. Minimum creep rates were essentially the same for the weldment as well as for the base metal in both the environments, whereas rupture strain was usually lower for the weldment than that of the base metal. The reduction in area of the weldment specimens increased with increase in stress. Failures in the specimens of weldmens were in the weld metal region. Microstructural studies carried out on failed weldment specimens after the creep rupture tests revealed extensive cavitation in the weld metal region in air tested specimens predominantly at the austerite/δ-ferrite interphase. However, no cavitation was observed in specimens tested in sodium.

Tensile deformation behaviour of AISI 316L(N) SS

Abstract Tensile deformation behaviour of AISI 316L(N) stainless steel was investigated over the range 298 to 1023 K. The yield and tensile strength were found to meet the minimum values mandated by RCC-MR, the French design code for fast reactors. The tensile flow behaviour was modelled using Ludwigson and Voce equation. Ludwigson equation could model the flow behaviour adequately before the onset of saturation at temperatures >823 K. Voce equation could model the flow behaviour between 298 and 1023 K. The physical basis for the variation with respect to strain rate and temperature of the constants obtained by modelling the flow curve using Ludwigson and Voce equation is discussed.

Analysis of Creep Rupture Behavior of Cr-Mo Ferritic Steels in the Presence of Notch

Effect of notch on creep rupture behavior of 2.25Cr-1Mo, 9Cr-1Mo, and modified 9Cr-1Mo ferritic steels has been assessed. Creep tests were carried out on smooth and notched specimens of the steels in the stress ranging 90 to 300xa0MPa at 873xa0K (600xa0°C). Creep rupture lives of the steels increased in the presence of notch over those of smooth specimens, thus exhibiting notch strengthening. The strengthening was comparable for the 9Cr-1Mo and 2.25Cr-1Mo steels and appreciably more in modified 9Cr-1Mo steel. The strengthening effect was found to decrease with the decrease in applied stress and increase in rupture life for all the steels. The presence of notch decreased the creep rupture ductility of the steels significantly and the 2.25Cr-1Mo steel suffered more reduction than in the other two 9Cr-steels. Finite element analysis of stress distribution across the notch was carried out to understand the notch strengthening and its variation in the steels. The variation in fracture appearance has also been corroborated based on finite element analysis. Reduction in von-Mises stress across the notch throat plane resulted in strengthening in the steels. Higher reduction in von-Mises stress in modified 9Cr-1Mo steel than that in 2.25Cr-1Mo and 9Cr-1Mo steels induced more strengthening in modified 9Cr-1Mo steel under multiaxial state of stress.

Effect of stress on the transformation behaviour of delta-ferrite in type 316 stainless steel weld metal

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Effect of Prior Cold Work on Tensile Flow and Work Hardening Behavior of a Titanium Bearing Modified Austenitic Stainless Steel

The tensile flow and work hardening behavior of 14Cr-15Ni-2.2Mo-Ti austenitic stainless steel at 298 K and 973 K with different prior cold worked levels ranging from 16% to 24% were analyzed using the Ludwigson, Swift and Voce constitutive equations. Influence of prior cold work on various Ludwigson parameters is discussed. While the Swift equation is used to predict the prior cold work, the yield strength is estimated using the Voce equation. The work hardening analysis using plot showed stage II hardening in the case of the material without coldwork. A two stage hardening behavior consisting of rapidly decreasing transient stage followed by gradually decreasing stage III hardening is observed at all cold work levels.

Anomalous creep behaviour of two heats of type 316 stainless steel at 823 K

The creep deformation behaviour of two heats of nuclear grade type 316 stainless steel has been studied at 823 Κ at stress levels of 216, 245, 275 and 294 MPa, with test durations exceeding 84000 hours. Normal creep behaviour was observed at all stress levels except at 216 MPa. The creep deformation behaviour at 216 MPa was characterised by renewed primary and secondary creep stages for both heats of material. This anomalous behaviour is found to depend on carbon content and grain size of the material.