P. Shankar

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.

Electron microscopic study of cr2n formation in thermally aged 316ln austenitic stainless steels

The formation of dichromium nitride phase in low nitrogen austenitic stainless steels has been studied by transmission electron microscopy (TEM). The electron diffraction patterns unambiguously confirm the formation of Cr2N phase on aging these steels in the range of 1023 to 1123 K, for time durations up to 100 hours. It is found that while no perceptible microstructural features could be recognized, formation of dislocation pairs is an important characteristic that could be associated with the nitrogen ordering in the matrix. The precipitation sequence processes have been discussed on the basis of stress-induced interactions that are predominant in interstitial alloys. Further, certain aspects of me-chanical behavior are explained on the basis of our study.

Clustering and ordering of nitrogen in nuclear grade 316LN austenitic stainless steel

The high misfit strains associated with nitrogen addition and the high chemical affinity with chromium result in a significant driving force for the formation of Cr-N clusters in austenitic steels. The microstructural signatures of cluster formation in nuclear grade 316LN austenitic stainless steel during the early stages of aging at 1123 K are discussed. Aging beyond 25 h results in transformation of fcc clusters to hexagonal Cr 2 N precipitates. Strain induced ordering of nitrogen in Cr 2 N precipitates due to continued ageing is also highlighted.

Applications of Nanomaterials

Historically, there are several recorded instances of technologies that have revolutionized human civilization. From the invention of automobile wheels to the printing press, technological revolutions have resulted in remarkable improvement in the quality of life and have eventually led to societal transformations. With nanotechnology promising to impact almost every sector (Fig. 4.1), it is popularly believed that this could be the next revolution.

Characterization of aluminide coatings formed by diffusion alloying on nitrogen-containing type 316L stainless steels

Abstract Diffusion alloying of pre-deposited aluminum film in nitrogen-bearing 316L stainless steels resulted in the formation of intermetallic phases like FeAl 2 and Al 13 Fe 4 . The high affinity of nitrogen for aluminum resulted in surface enrichment of nitrogen, as well as increased AlN precipitation, particularly in the high nitrogen steels.

Unique Properties of Nanomaterials

As we approach nanoscale dimensions, we move closer to the atomic or molecular scales. Atoms are the building blocks of all matter. They can be assembled in many ways to obtain the desired product. Both the chemistry and the geometric arrangement of atoms can influence the properties of the material. Hence, if we have the ability to construct matter, atom by atom, we would be able to perform wonders. For example, we know that both graphite and diamond are made of pure carbon. Thus, in principle, if we are able to rearrange the atoms (carbon) in graphite at our discretion, it would be possible to make diamond! Or, if we could rearrange the atoms (silicon and oxygen) in sand (and add a few other trace elements), it should be possible to make a computer chip! Engineering at the nano-level can bring about large changes in the properties of the products. In Chapter 1, we saw how the high defect concentration in nanomaterials results in novel and unique physical, chemical, elastic and mechanical properties of this class of materials. A few of these are highlighted in this chapter.

The Big World of Nanomaterials

Materials have been of great interest to human beings since time immemorial. A few million years ago, it was found that rocks could be used to break things that were impossible to break with bare hands. Stones were the first tools and even today they are still in use in kitchens and laboratories to pound and grind, or as mortars and pestles. Around 5000–6000 years ago, it was accidentally discovered that when a rock containing copper was placed on a fire, molten copper could be collected. This discovery led to the reduction of metal ores to produce metals for the fabrication of items from ploughshares to swords. New materials with greater hardness and longer use than stone became available for making tools. Our growth and progress have paralleled the development of metals and metallurgy.

Microstructural and electrochemical studies in thermally aged type 316LN stainless steels

AbstractThe effects of nitrogen (680 and 1600 ppm) on the microstructure and electrochemical behaviour of thermally aged type 316LN stainless steels is discussed. Electrochemical potentiokinetic reactivation tests indicated a decrease in chromium depleted regions with increasing nitrogen addition in austenitic stainless steels. Secondary precipitates developed in these alloys during aging at 873 K for 500 h were extracted by an electrochemical method. The precipitates were analysed by X-ray diffraction method. Further TEM investigation on 1600 ppm nitrogen steel was also carried out to help understand the precipitation behaviour. The presence of nitrogen resulted in precipitation of mostly Cr2 N and χ (chi) phases in the alloy that contained 1600 ppm of nitrogen, in contrast to M23C6 precipitates in the alloy that contained 680 ppm of nitrogen. The influence of the microstructural evolution and its effect on chromium depletion observed in the present investigation is discussed.

Nanostructured Materials with High Application Potential

Materials development has remained the backbone of human civilization and will continue to be the anchor for all future developments. The development of new materials and advanced material technologies has served as the cradle for most engineering developments. Revolutions in the communication, computing, energy, chemical, transport and engineering industries have been possible only due to credible advances in materials technology and the development of new classes of materials. As discussed in earlier chapters, nanomaterials are slowly beginning to make their presence felt in science and technology. The potential engineering applications of nanomaterials are vast. This chapter will describe only a few typical nanostructured materials of current interest. One extreme end of nanostructures is single electron transistors. Figure 6.1 shows an example of a single electron transistor with niobium leads and aluminium islands.

Corrosion Behaviour of Nitrogen Bearing Austenitic Stainless Steel Surface Diffusion Alloyed with Ti/Al Precoated Films

Abstract The present work investigates the aqueous corrosion behaviour of type 316L stainless steel (SS) containing various matrix nitrogen contents (0·015, 0·1, 0·2 and 0·56%N), surface modified by diffusion annealing of a precoated film of titanium/aluminium. Type 316L SS specimens were precoated with a Ti/Al multilayer by the electron beam deposition method and surface diffusion annealed at 1173 K for 1 h in vacuum. X-ray diffraction analysis indicated the formation of Ti3Al, Al5Ti2, Al2Ti and Al13Fe4 intermetallic phases. Nitrides such as Ti2N were also observed, particularly in high nitrogen steels. The interaction between the titanium/aluminium coating and the matrix constituents, particularly with nitrogen, was characterised by secondary ion mass spectrometry (SIMS). The nitrogen content at the modified surface increased with increase in the nitrogen content of the substrate matrix. SEM observation of cross-sectionally mounted surface modified alloys indicated the formation of thick adherent layers. The role of such intermetallic phases in corrosion resistance in both 0·5 M H2SO4 and 0·5 M NaCl is discussed in detail based on open circuit potential-time measurements, potentiodynamic polarisation studies and electrochemical impedance spectroscopy (EIS) investigations. The role of matrix nitrogen in the formation of intermetallic coatings and its role in corrosion resistance in acidic and chloride media are investigated.