S. Rajendran Pillai

Effect of External Stress on the Behavior of Oxide Scales on 9Cr-1Mo Steel

Tensile specimens of 9Cr-1Mo steel weresubjected to oxidation in air at a temperature of 973 Kfor periods of 25, 47, 70, 97, 120, and 140 hr. Theintegrity of the oxide scale was examined by an in-situ technique which involved the recording of theacoustic-emission activity associated with the breakingof the scale. Specimens were also subjected to oxidationfor the same times after subjecting them to an external stress of 40 MPa. The applicationof the external stress was found to alter the integrityof the scale. Spalling of the scale occurred to a lesserextent when external stress was applied due to partial release of growth stresses becauseof the elongation of the specimen. The scale buckledbefore spalling when no external stress was applied. Onthe other hand, the scale developed wedge-type cracks before spalling when subjected toexternal stress during oxidation. Postoxidationexamination of the stressed specimen revealedsubstantial segregation of silicon and chromium at thegrain boundaries (oxide ridges). The specimens, which wereoxidized without external stress, revealed a reductionin the concentration of chromium on the surface. Thisbehavior was attributed to enhanced spalling in the case of the latter specimen compared to theformer.

Carbon potential and carbide equilibrium in 18/8 austenitic steels

Abstract Austenitic stainless steels are in contact with high temperature liquid sodium in a fast reactor. In order to understand the carbon transport occurring between various regions of these materials through liquid sodium, an accurate knowledge of the carbon activity-concentration relationship in the austenitic steels is essential. A new method involving equilibration of the steel sample with liquid sodium coupled with the measurement of carbon activity in sodium, employing an electrochemical carbon meter, is reported for the measurement of the carbon potential of the steel at different carbon concentrations in the temperature range of 860–960 K. The results obtained are discussed and a new expression is proposed relating carbon activity with the composition of steel. The thermodynamics of carbide equilibrium in the steel are studied by isolating and characterising the carbide phase. These data have been analysed using a regular solution model for the carbide phase, M 23 C 6 and based on this the observed composition of M 23 C 6 as well as the variation of the carbon potential of steel with temperature have been explained.

Formation of NaCrO2 in sodium systems of fast reactors and its consequence on the carbon potential

Abstract The most important corrosion product in sodium loops is NaCrO 2 . In this study, the threshold oxygen level in sodium, for the formation of NaCrO 2 when in contact with austenitic (AISI 304 SS, 316 SS and D9 alloy) and ferritic (Fe-9Cr-1Mo and Fe- 2 1 4 Cr-1Mo) steels at temperatures between 573 and 1023 K were evaluated. By a weighted regression analysis of the reliable measurements reported in the literature, the Gibbs energy of formation of NaCrO 2 , Δ f G ° = −913762.8 + 237.24 T J/mol, was obtained. Using this relation, the threshold oxygen levels ( C O ) were evaluated and fitted by regression analysis into expressions of the form ln C O (ppm) = A + B T . Based on the nature of the carbides formed in AISI 304 SS and 316 SS, the effect of depletion of chromium on the carbon activity of these alloys was analysed by a thermodynamic model.

An electrochemical meter for measuring carbon activity in liquid sodium

Abstract This paper reports the development of an improved electrochemical carbon meter which gives better long term performance compared to similar meters described by earlier workers. The initial problem of the meter was identified to be due to decarburization of the Fe/Fe 3 C reference electrode. The new reference electrode consisting of a graphite-filled nickel capsule was found to give satisfactory performance. However, the test in commercial grade sodium did not give the expected temperature dependance of emf values. This is attributed to the influence of carbon bearing impurity species present in commercial grade sodium. Experiments with purified sodium gave emf response with change in temperature in accordance with the established solubility relationship of carbon in sodium. The long term test of performance of the meter in liquid sodium is described. Its utility for measuring the carbon activities of various metal carbide systems is also pointed out.

Diffusion annealing and laser surface alloying with aluminium to enhance oxidation resistance of carbon steels

Abstract The oxidation behaviour of a carbon steel was investigated after modifying the surface in three different ways, namely: (a) physical vapour deposition of aluminium by a resistance heating evaporation technique; (b) diffusion annealing of predeposited aluminium; and (c) laser surface alloying with predeposited aluminium. While diffusion annealing at a temperature of 1023 K was found to result in the formation of Al13Fe4 and Al5Fe2 phases, laser surface alloying resulted in a case containing Al13Fe4 and Al2Fe phases. The oxidation behaviour of all the above specimens was studied at 873 K for a total duration of up to 200 h. The mass change and morphological modifications to the surface were monitored at regular intervals of 25 h. It was observed that aluminising results in a substantial enhancement of the high temperature oxidation resistance in comparison with the untreated base material, by virtue of the superior oxidation resistance of the intermetallic phases formed on the surface. Furthermore, laser surface alloying was found to result in better oxidation resistance, in comparison with diffusion annealed and as deposited specimens.

Corrosion behavior and tensile properties of AISI 316LN stainless steel exposed to flowing sodium at 823 K

Austenitic stainless steel of the grade AISI 316 LN was exposed to flowing sodium in a loop at 823 K for 6,000 h to examine the corrosion and mass-transfer behavior. The specimens were incorporated in specially designed sample holders in the loop. These were retrieved and examined by various metallurgical techniques. Specimens were also subjected to thermal aging in the same sample holder to aid in separating the consequences of exposure to sodium from those cause by mere thermal effects. Microstructural investigations have revealed that thermal aging caused the precipitation of carbides at the grain boundaries. Exposure to sodium caused the leaching of elements such as chromium and nickel from the specimen. Loss of nickel from the austenite phase promoted the generation of ferrite phase. Microhardness investigation revealed the hardening of the sodium-exposed surface. Analysis using an electron Probe Microanalyzer revealed that the surface of the steel was both carburized and nitrided. Tensile tests indicated that there is no appreciable difference in the yield strength (YS) and ultimate tensile strength (UTS) of the thermally aged and sodium-exposed specimens when compared with the material in the as-received condition. However, the thermally aged and sodium-exposed specimens showed a decrease in the uniform elongation and total elongation at rupture, perhaps due to carburization and nitridation.

Carburization of stainless steel clad by uranium-plutonium carbide fuel

Carburization of stainless steel clad by the fuel in fast reactor fuel pins, containing uranium-plutonium mixed carbide, would adversely affect the mechanical properties of the clad. The extent of carburization depends on the relative carbon potentials of the fuel and the clad. The measurement of the carbon potential of stainless steel as a function of temperature in the operating range of the fuel pins was recently reported from this laboratory. The method is based on equilibration of SS specimens with liquid sodium while simultaneously monitoring the carbon potential of sodium with an electrochemical carbon meter. In the present work, the same procedure was extended to the fuel and results obtained with both UC and (U,Pu)C in the range of 840 to 960 K are reported. Using the present results and the earlier data on carbon potential of SS, the possibility of clad carburization is analysed.

Carbon Transfer in Sodium System Under Thermal Cycling-Possibility of “Metal Dusting”

High temperature sodium systems encounter thermal cycling during operation. Under this condition, the behavior of carbon dissolved in sodium needs special attention. Carbon chemistry of sodium is very complex because of the existence of multiple carbon-bearing species. In addition, carbon exists both in the dissolved (“active”) and undissolved (“inactive”) forms. Under thermal cycling conditions, as the temperature is lowered, carbon may precipitate as sodium acetylide. The equilibrium carbon activity imparted by this species is high enough to cause the precipitation of iron carbide (Fe3C) in ferrous alloys. The Fe3C may be destabilized at lower activities of carbon in the environment (when the temperature is increased) and may decompose to a fine dispersion of metal and graphite. This phenomenon of “metal dusting” is extremely detrimental to the components of sodium systems. The article analyses the possibility of “metal dusting” in an operating sodium system.

A new method for the measurement of the diffusivity of carbon in alpha-iron using an electrochemical cell

A new method has been employed to measure the diffusivity of carbon in alpha iron. The method involves the measurement of ionic current of a carbon concentration cell which employs an iron cylinder as the anode. The design of the cell is such that when a constant external potential is applied, the ionic current is determined by the concentration polarization of carbon in the two electrodes. From the values of ionic current, the diffusivity of carbon (Dc) is calculated by the application of Ficks law. The results in the temperature range 854–975 K have been fitted by regression analysis to obtain the expression:Dc = 2.448 × 10−5 exp(−102900.7/RT) withDc in m2 s−1 andR in JK−1 mol−1. The results agree well with data in the literature.