R. K. Dayal

On the pitting corrosion resistance of nitrogen alloyed cold worked austenitic stainless steels

Pitting corrosion studies were carried out on cold worked (5%, 10%, 15%, 20%, 30% and 40%) nitrogen-bearing (0.05%, 0.1% and 0.22% N) type 316L austenitic stainless steels in neutral chloride medium. Potentiodynamic anodic polarisation study revealed that cold working up to 20% enhanced the pitting resistance, and thereafter a sudden decrease in pitting resistance was noticed at 30% and 40% cold working. Increase in nitrogen content was beneficial up to 20% cold work in improving the pitting corrosion resistance, beyond which it had a detrimental effect at 30% and 40% cold working. The role of nitrogen in influencing the deformation band width and dislocation configuration is explained based on the results of transmission electron microscopy investigations. Scanning electron microscopy observation of the pitted specimens indicated decreasing size and increasing density of pits, along the deformation bands with increasing nitrogen for 40% cold worked specimens. The macrohardness values increased as the cold working increased from 0% to 40%. X-ray diffraction studies revealed the increased peak broadening of austenite peak {0 2 2} with increase in cold working. The relationship between pitting corrosion and deformation structures with respect to nitrogen addition and cold working is discussed.

Corrosion studies on materials of construction for spent nuclear fuel reprocessing plant equipment

Abstract Corrosion studies on specimens of commercial Type 304L stainless steel (SS), nuclear grade type 304L SS, extra low-carbon nitric acid grade (NAG) Uranus-16 SS, NAG Uranus-65 SS, Ti, Ti-5% Ta, Ti-0.25% Pd, Zircaloy-2, weldments of Ti and of Ti-5% Ta, and surface-modified (thermally oxidised and anodised) Ti were carried out to assess their corrosion resistance in nitric acid medium. The results indicated that Zircaloy-2, Ti-5% Ta, Uranus-16 SS and Uranus-65 SS have excellent corrosion resistance in boiling nitric acid solution. Specimens of Zircaloy-2, Ti-5% Ta and thermally-oxidised Ti showed excellent corrosion resistance also in a simulated uranium-containing reprocessing medium in a concentrated nitric acid solution. SEM and XRD analyses were carried out on the tested specimens to examine the scale morphology and phases present on the surface.

Influence of metallurgical variables on sensitisation kinetics in austenitic stainless steels

Abstract Austenitic stainless steels are the most favoured construction materials for various components required in chemical, petrochemical, fertiliser and nuclear industries. However, these steels are prone to sensitisation. In the sensitised condition, the steels are quite susceptible to intergranular corrosion and intergranular stress corrosion cracking in chloride and caustic environments resulting in the premature failure of the fabricated components during precommissioning and service periods. The topic of sensitisation has been of interest in studies worldwide. The studies included mechanism of sensitisation, test methods, data generation, and influence of several variables and protection methods. This article covers a review of important contributions to this topic with special emphasis on the influence of metallurgical variables. The present state of understanding is discussed in the following areas: mechanisms, test methods, sensitisation diagrams, influence of chemical composition, cold work and grain size, low temperature sensitisation, modelling and laser surface treatment. The need for further investigation in certain areas is highlighted.

Influence of chemical composition, prior deformation and prolonged thermal aging on the sensitization characteristics of austenitic stainless steels

Abstract Sensitization behaviour of austenitic stainless steels are greatly influenced by several metallurgical factors such as chemical composition, degree of prior deformation, grain size, aging temperature–time. The need for generation of data on sensitization kinetics for specific composition of stainless steels to take care of the heat to heat variation for the fabrication of critical components is often questioned. An attempt was made in this investigation to understand this aspect by establishing time–temperature–sensitization diagrams, continuous-cooling–sensitization diagrams and critical cooling rate for three sets of AISI 316 stainless steel in which as the wt% of carbon decreases, that of nitrogen increases so as to encompass the normal span of concentration range usually encountered in different heats. A systematic trend is observed in these experimentally determined sensitization data of these typical stainless steels. This would eliminate the need for the independent generation of sensitization data for stainless steel of specified composition which is within the range investigated here. The database reported for these alloys will also help to recommend the limits of critical cooling rate to avoid sensitization during fabrication.

Influence of microstructure on the hydrogen permeability of 9%Cr–1%Mo ferritic steel

The influence of microstructure of 9%Cr‐1%Mo steel on the hydrogen diAusivity, solubility and hence the permeability was investigated using electrochemical permeation technique. This steel was austenitised and cooled at various cooling rates to produce diAerent microstructures. Tempering behaviour was also studied by heat treating for diAerent durations at 1023 K. Characterisation of microstructures was carried out using scanning electron microscopy and analytical transmission electron microscopy. A fully martensitic product was obtained during fast cooling and a mixture of proeutectoid ferrite and martensite during slow cooling. Tempering the normalised steel resulted in the formation of fine intragranular M2X precipitates and M23C6 on the boundaries. The hydrogen diAusivity and solubility showed a regular trend with the amount of strain in the lattice. Lattice defects and precipitates act as trap sites for hydrogen. Increase in lattice strain either due to increase in defect density, substructure or coherent precipitates resulted in decrease in diAusivity due to increase in trap sites. Martensite structure oAered the maximum resistance to hydrogen diAusivity and tempered martensite the least resistance due to the annihilation of defects during tempering. ” 1999 Published by Elsevier Science B.V. All rights reserved.

Studies on hydrogen permeability of 2.25% Cr–1% Mo ferritic steel: correlation with microstructure

Abstract The influence of microstructure on the hydrogen permeability, diffusivity and solubility in 2.25% Cr–1% Mo ferritic steel was investigated using electrochemical permeation technique. Varieties of microstructures ranging from martensite in water-quenched (WQ) steel to a predominant ferrite structure in annealed steel were characterised using analytical transmission electron microscopy. In the tempered structures, continuous precipitation of a variety of carbides of different morphologies and sizes was also characterised. The hydrogen diffusivity showed a continuous increase as the structure changed from martensite to ferrite and also with increasing extent of tempering. Solubility showed a corresponding decrease. The trends have been understood in terms of the number of reversible traps available for hydrogen in these different structures. Accordingly, martensite structure offered the maximum resistance to hydrogen diffusivity and tempered structure the least resistance due to the annihilation of defects during tempering and reduction in the solute content of the matrix due to precipitation.

Influence of prior deformation on the sensitization of AISI Type 316LN stainless steel

Abstract The sensitization behaviour of a nuclear grade AISI 316LN stainless steel (SS) was studied for various cold-work levels ranging from 0% (mill-annealed) to 25% reduction in thickness. ASTM standard A262 Practices A and E were adopted to detect the susceptibility to intergranular corrosion. The results obtained in these tests were used to construct time-temperature-sensitization (TTS) diagrams. Using these data, the critical linear cooling rate was calculated, above which there is no risk of sensitization. In order to predict the sensitization behaviour during practical cooling conditions, Continuous-cooling-sensitization (CCS) diagrams were established utilising the TTS diagrams by a mathematical method. The influences of prior deformation and nitrogen in the alloy on the sensitization kinetics are discussed. It was found that nitrogen addition retards the sensitization kinetics and that t min (minimum time required for sensitization at nose temperature) increases by two orders of magnitude in Type 316LN SS compared to that of Type 316 SS at the different prior deformation levels. Cold-working up to 15% accelerates the onset of carbide precipitation and on further cold working there is not much difference in the kinetics. Desensitization is faster in highly cold-worked material, especially at high temperatures.

Influence of thermal aging on the intergranular corrosion resistance of types 304LN and 316LN stainless steels

Intergranular corrosion (IGC) resistance of types 304LN and 316LN stainless steels (SS) thermally aged at 823, 873, and 923 K for various durations was assessed by ASTM A262 practice A test (electrolytic etch test) and electrochemical potentiodynamic reactivation (EPR) test. The results indicated that the type 316LN SS has significantly improved IGC resistance compared to 304LN SS. Based on the results of these tests, time-temperature-sensitization (TTS) diagrams were developed for both alloys. The secondary precipitates formed during thermal aging treatments were electrochemically extracted and analyzed by X-ray diffraction (XRD) to determine the types of precipitates formed during the aging treatments. The results indicated that the precipitates were mostly of M23C6 carbides.

Influence of deformation on sensitization kinetics and its microstructural correlation in a nuclear grade 316 stainless steel

Abstract This paper deals with the influence of morphological features of grain boundary carbides on the sensitization behaviour in a nuclear grade 316 stainless steel. Sensitization kinetics was studied in the temperature range of 825 to 1175 K for different degrees of cold work, ranging from 5 to 25%. It was found that the temperature of the nose of the time-temperature-sensitization (TTS) diagrams at 1025 K, remained unaltered with cold work, whereas the rate of sensitization at 1025 K increased with deformation up to ~ 15%, and remained constant thereafter. In order to understand the cause of the enhanced rate of sensitization with deformation, transmission electron microscopy was carried out. The morphology and the size of grain boundary M23C6 carbides, which are responsible for sensitization, were characterised for samples aged at 1025 K for ageing durations corresponding to the onset of sensitization. Transmission electron microscopy was employed to measure the average diameter - D , the intercarbide spacing \ gl, and the shape factor - S (the ratio of length to width). These parameters were found to decrease with deformation, reaching a saturation at 15% cold work. The role of these microstructural variables in enhancing the sensitization kinetics with deformation, is discussed in the light of the well known chromium depletion theory.

Improving intergranular corrosion resistance of sensitized type 316 austenitic stainless steel by laser surface melting

An attempt was made to modify the surface microstructure of a sensitized austenitic stainless steel, without affecting the bulk properties, using laser surface melting techniques. AISI type 316 stainless steel specimens sensitized at 923 K for 20 hr were laser surface melted using a pulsed ruby laser at 6 J energy. Two successive pulses were given to ensure uniform melting and homogenization. The melted layers were characterized by small angle X- ray diffraction and scanning electron microscopy. Intergranular corrosion tests were carried out on the melted region as per ASTM A262 practice A (etch test) and electrochemical potentiokinetic reactivation test. The results indicated an improvement in the intergranular corrosion resistance after laser surface melting. The results are explained on the basis of homogeneous and nonsensitized microstructure obtained at the surface after laser surface melting. It is concluded that laser surface melting can be used as an in situ method to increase the life of a sensitized component by modifying the surface microstructure.