Advanced Materials Research Vol. 794

Abstract: Fabrication of various sub assemblies and items from imported austenitic stainless steel material of SA 240 - 304L (32 mm to 85 mm thick) was started in the year 1990 and work on these assemblies was stopped in 1993 due to unavoidable circumstances. Items and sub-assemblies were either in partially welded / machined or formed condition. Sub assemblies / items were stored in open yard for 13 years after applying plastic peal coating compound and covered with tarpaulin. The storage yard was near to the sea (roll on & off Jetty) as well as next to petrochemical industries. Sub assemblies have seen different extreme weathers conditions, saline and corrosive atmosphere for all these years and during this storage time base material and welded joints were subjected to contamination.

Abstract: Stainless steels such as ferrritic, austenitic, martensitic and duplex stainless steels are well known for their corrosion resistance to varying extents. Among these, austenitic stainless steels exhibit superior corrosion resistance and better ductility for formability. Therefore, the ability to give simple to intricate shapes in this grade of steel brings their potential for a wide range of applications. However, the meta-stable austenite in AISI 304 is known to undergo a strain induced martensitic (SIM) transformation during conventional rolling at room temperature. This strain induced martensite causes reduction in ductility and limits formability of stainless steel. Therefore, wavy rolling technique was developed to strengthen the stainless steel through microstructural refinement. In the current study, wavy rolling with 1.5 mm amplitude was conducted on 1 mm thick stainless steel sheet to different cycles ranging from 1-4. These rolled samples were characterized by optical and Atomic Force Microscopy (AFM) with resolutions down to the nanolevel. This AFM tool is in a position to bring out the details of grain refinement and topographical roughness emerging from crystalline and microstructural defects like orientation, precipitation, stacking faults, deformation bands, slip lines and shear bands with progress in rolling as referred by the number of rolling cycles here. The structural development is semi-quantitatively related to the degree of deformation and its effect on tensile properties during wavy rolling cycle. Keywords: Structural properties; Roughness; Deformation; Wavy rolling.

Abstract: Surface mechanical attrition treatment (SMAT) technique has became popular to develop a nanostructured surface layer on metallic materials for upgrading their overall properties and performance. In this paper, we have presented the SMATing behavior of low stacking fault energy material like AISI 304 using optical microscopy, SEM, microhardness measurement and XRD analysis. SMATing was performed for 15, 30, 45, 60, 75, 90 min by using hardened bearing-steel balls (size: 5.7 mm diameter, hardness: 500HV_0.1) at 50 Hz vibrating frequency. XRD analysis indicated the lowest grain-size of about 8.6 nm in the surface region of specimen SMATed for 60 min. In comparison with the non-SMATed specimen, 17 times increase in the dislocation density and 4 times increase in the micro-strain were observed in this SMATed specimen. Improvement in the surface-hardness due to the SMAT was almost two times hardness before SMAT was 190 HV_0.1 and after SMAT it was 400 HV_0.1. There is a gradual decrease in the hardness value across the cross-section of the specimen, and core-hardness value was reached after 300 μm depth below the surface. XRD results indicated the possibility of martensitic phase transformation at the surface during SMATing of AISI 304 steel. SMATed AISI 304 specimens showed good thermal stability at 550°C for 6 h which was confirmed by microhardness measurement

Abstract: AISI 304 austenitic stainless steel is generally “difficult-to-cut” material than other types of steel on account of their high strength, high work hardening tendency and poor thermal conductivity. The focus of the paper is on the dry, high speed machining which is ecologically desirable and cost effective. It is also the future of machining and called as green machining. PVD multilayered TiN/TiAlN and TiAlN/TiSiN coated inserts were used for dry, high speed turning of AISI 304 austenitic stainless steels material. TiN/TiAlN coating was deposited using “Cathodic Arc Evaporation” (CAE) technique where as TiAlN/TiSiN coating was deposited using “Closed-Field Unbalanced Magnetron Sputtering” (CFUBMS) technique. Coatings are deposited on K-grade (K-20) cemented carbide insert. Scanning Electron Microscopy (SEM), microhardness tester and scratch tester were used to examine microstructure, microhardness and adhesion of coating. The thickness of the both coating was found to be 3.8 ±2 µm. TiN/TiAlN coating demonstrated micro-hardness value 34 GPa where as TiAlN/TiSiN coating shows 37 GPa. The adhesion strength of the TiAlN/TiSiN coating is 86 N and that of TiN/TiAlN coating is 83 N.The turning tests were conducted in dry machining environment at cutting speeds in the range of 100 to 340 m/min, feed in the range of 0.08 to 0.20 mm/rev keeping depth of cut constant at 1 mm. The influences of cutting speed, feed and tool coating were investigated on the machined surface roughness, flank wear and cutting force. TiAlN/TiSiN coated tool showed better performance and exhibited lower cutting forces than TiN/TiAlN coated tool. Built-up edge was not observed during using coated tool due to better thermal stability of the coating. The research work findings will also provide useful economic machining solution in case of dry, high speed turning of AISI 304 stainless steel, which is otherwise usually, machined by costly PCD or CBN tools. The present approach and results will be helpful for understanding the machinability of AISI 304 stainless steel during dry, high speed turning for the manufacturing engineers.

Abstract: This lecture presents the authors personal views on the landmark events that have strongly affected the welding of stainless steels over their lifetime. Although 1913 is commonly recognized as the birth of stainless steels with the commercialization of the martensitic alloy of Harry Brearly and the austenitic alloy of Eduard Maurer and Benno Straus, the story can be considered to begin as long ago as 1797 with the discovery of chromium by Klaproth and Vauquelin, and the observation by Vauquelin in 1798 that chromium resists acids surprisingly well. From the 1870s onwards, corrosion resisting properties of iron-chromium alloys were known. One might mark the first iron-chromium-nickel constitution diagram of Maurer and Strauss in 1920 as a major landmark in the science of welding of stainless steels. Their diagram evolved until the outbreak of World War II in Europe in 1939, and nominally austenitic stainless steel weld metals, containing ferrite that provided crack resistance, were extensively employed for armor welding during the war, based on their diagram. Improved diagrams for use in weld filler metal design and dissimilar welding were developed by Schaeffler (1947-1949), DeLong (1956-1973) and the Welding Research Council (1988 and 1992). Until about 1970, there was a major cost difference between low carbon austenitic stainless steels and those austenitic stainless steels of 0.04% carbon and more because the low carbon grades had to be produced using expensive low carbon ferro-chromium. Welding caused heat affected zone sensitization of the higher carbon alloys, which meant that they had to be solution annealed and quenched to obtain good corrosion resistance. In 1955, Krivsky invented the argon-oxygen decarburization process for refining stainless steels, which allowed low carbon alloys to be produced using high carbon ferro-chromium. AOD became widely used by 1970 in the industrialized countries and the cost penalty for low carbon stainless steel grades virtually vanished, as did the need to anneal and quench stainless steel weldments. Widespread use of AOD refining of stainless steels brought with it an unexpected welding problem. Automatic welding procedures for orbital gas tungsten arc welding of stainless steel tubing for power plant construction had been in place for many years and provided 100% penetration welds consistently. However, during the 1970s, inconsistent penetration began to appear in such welds, and numerous researchers sought the cause. The 1982 publication of Heiple and Roper pinpointed the cause as a reversal of the surface tension gradient as a function of temperature on the weld pool surface when weld pool sulfur became very low. The AOD refining process was largely responsible for the very low sulfur base metals that resulted in incomplete penetration. The first duplex ferritic-austenitic stainless steel was developed in 1933 by Avesta in Sweden. Duplex stainless steels were long considered unweldable unless solution annealed, due to excessive ferrite in the weld heat-affected zone. However, in 1971, Joslyn Steel began introducing nitrogen into the AOD refining of stainless steels, and the duplex stainless steel producers noticed. Ogawa and Koseki in 1989 demonstrated the dramatic effect of nitrogen additions on enhanced weldability of duplex stainless steels, and these are widely welded today without the need to anneal. Although earlier commercial embodiments of small diameter gas-shielded flux cored stainless steel welding electrodes were produced, the 1982 patent of Godai and colleagues became the basis for widespread market acceptance of these electrodes from many producers. The key to the patent was addition of a small amount of bismuth oxide which resulted in very attractive slag detachment. Electrodes based on this patent quickly came to dominate the flux cored stainless steel market. Then a primary steam line, welded with these electrodes, ruptured unexpectedly in a Japanese power plant. Investigations published in 1997 by Nishimoto et al and Toyoda et al, among others, pinpointed the cause as about 200 ppm of bismuth retained in the weld metal which led to reheat cracking along grain boundaries where the Bi segregated. Bismuth-free electrode designs were quickly developed for high temperature service, while the bismuth-containing designs remain popular today for service not involving high temperatures.

Abstract: Based primarily on microstructure, five stainless steel types are recognized: ferritic, martensitic, austenitic, duplex and precipitation-hardening. The major problem in ferritic stainless steels is the tendency to embrittlement, aggravated by various causes. During welding, control of heat input is essential and, in some cases, also a postweld heat treatment. The austenitic type is the easiest to weld, but two important issues are involved in the welding of these steels: hot cracking and formation of chromium carbide and other secondary phases on thermal exposure. The nature of the problems and remedial measures are discussed from a metallurgical perspective. Duplex stainless steels contain approximately equal proportions of austenite and ferrite. The article discusses the upset in phase balance during welding both in the weld metal and heat-affected zone and the formation of embrittling secondary phases during any thermal treatment. Martensitic stainless steels are susceptible to hydrogen-induced cracking. Welding thus involves many precautions to prevent it through proper preheat selection, postweld heat treatment, etc. In the welding of precipitation-hardening stainless steels, it is usually necessary to develop in the weld metal strength levels matching those of the base metal. This is achieved by applying a postweld heat treatment appropriate to each type of alloy.

Abstract: Many critical applications in chemical equipment, aircraft and ordinance demand a material of construction with high strength and good corrosion resistance. Frequently the strength requirement exceeds that obtainable with austenitic or ferritic stainless steel and it is necessary to use one of the martensitic stainless steels. Since martensitic stainless steels are structural materials, weldability has been an important consideration in their development. AISI 431 is one of the most potentially attractive steels in this class used extensively for parts requiring a combination of high tensile strength, good toughness and corrosion resistance. Although this material has been used for many years, little information is available on the welding behavior of these steels. Further, data on electron beam (EB) welding and solid state welding process like friction welding are scarce. The lack of knowledge constitutes a potential drawback to the more widespread use of these steels. Hence, a study has been taken up to develop an understanding on the electron beam welding and friction welding aspects of martensitic stainless steel type AISI 431. Various kinds of post weld heat treatments (PWHT) were investigated to determine their influence on microstructure and mechanical properties. Weld center in EB welding resulted a cast structure consists of dendritic structure with ferrite network in a matrix of un-tempered martensite. In friction welding, the weld center exhibited thermo-mechanical effected structure consists of fine intragranular acicular martensite in equiaxed prior austenite grains. In both the welding processes, post weld tempering treatment resulted in coarsening of the martensite which increases with increase in tempering temperature. In the as-weld condition, welds exhibited high strength and hardness and poor impact toughness. Increase in impact toughness and decrease in strength and hardness is observed with an increase in tempering temperature. The hardness of EB welds increased with increase in the austenitizing temperature up to 1100 °C and a marginal decrease thereafter was observed. Double austenitization after double tempering resulted in optical mechanical properties i.e., strength, hardness and toughness.

Abstract: Austenitic stainless steel is the major structural material for the primary and secondary sodium systems (except for the steam generators) for the currently operating and planned fast reactors all over the world. The boundaries of sodium systems of Prototype Fast Breeder Reactor (PFBR) is designed so as to have an extremely low probability of leakage, rapidly propagating failure and gross rupture under the static & dynamic loads expected during various operating conditions.The degradation of material properties (e.g. effect of sodium, temperature and irradiation), transients, residual stresses, flaw size etc. are the important considerations, which shall be taken into account. The principal material of construction for PFBR is austenitic stainless steel of grade 316LN/304LN. The scope of welding and fabrication of PFBR components is too large due to versatile types of systems with varieties of components with complex constructional features. High operating temperature of various systems causing high stresses are to be minimized by designing thin walled structure. Most of the Nuclear Steam Supply System (NSSS) components are thin walled and require manufacturing in separate nuclear clean hall conditions to assure the quality.The welding with stringent tolerances along with high distortion in stainless steels due to high thermal expansion and low thermal conductivity makes the fabrication extremely challenging.The welding standards and acceptance criteria of PFBR equipment is stringent compared to other industrial specification. Manufacture of over dimensional components (diameter greater than 12m and thickness upto 40mm) such as MainVessel, Safety Vessel, Inner Vessel involves die pressing of large size dished end & conical petals. The solution annealing of cold worked petals is a mandatory requirement if strain exceeds 10%. Innovative welding techniques and many trials were conducted on mock up for establishing the process parameters. The forming techniques, bending methods and welding procedures were qualified with stringent non-destructive and destructive examinations and testing before taking up the actual job. Thermal Baffle has two large concentric cylindrical shells, inner and outer shells of about 12.4m diameter and fabrication is a challenging task. PFBR also involves dissimilar joint welding between carbon steel (A48P2) and austenitic stainless steel (316LN) at integration location of roof slab & main vessel. This welding is carried out by combination of Gas Tungsten Arc Welding (GTAW) & Shielded Metal Arc Welding (SMAW) processes using ER 309L & E 309-16 welding consumables with controlled heat input to minimize the dilution of carbon & distortion. The weld between primary pipe & grid plate cannot be accessed for in-service inspection and therefore requires extra-ordinary skilled welders. Space constraints & lack of accessibility makes the welding & inspection challenging. This paper highlights the welding and fabrication aspects of few major, over dimensional and critical equipment of 500MWe Prototype Fast Breeder Reactor. Keywords: Stainless Steel, Main Vessel, Safety Vessel, Inner Vessel, Grid Plate, PFBR, SS welding, distortion.

Abstract: Stainless steels are regularly used as one of the preferred material of construction in the pressure vessels and heat exchangers manufactured by welding for process plants and energy sector at BHEL, Tiruchirappalli. They are considered mainly because of their corrosion resistance and high temperature suitability. But the practising welding engineers have to face innumerable challenges with stainless steel with regard to defects minimization, distortion control and dimensional stability on large and complex assemblies. Use of Nickel based filler wires or development of welding procedures simulating the true configuration of the product and by conducting specific tests and NDE are followed for weld defects control. Sequence welding, development of special fixtures, etc have come as handy options for welding distortion control of SS. Innovative inspection techniques for the certification of dimensions including geometrical tolerances especially on large constructions using SS welding and evolution of special Helium leak testing procedures for certain in-process checks in critical products are inevitable in the SS fabrication industry as a part of Quality Assurance programme. As a pioneer in SS fabrication, some of our challenging experiences pertaining to these three areas are discussed in this paper.

Abstract: Stainless steels are one of the versatile materials available in five grades viz., austenitic, ferritic, martensitic, duplex stainless steels and precipitation hardenable variety, having applications in various industrial sectors covering thermal power plants, nuclear, fertilizer, urea processing plants, cryogenic industries, aerospace & defence, etc. Each grade of stainless steels has its own unique characteristics in terms of strength, corrosion resistance, hardening behavior etc. Power beam processing using lasers or electron beam can be effectively utilized to process almost all the grades of stainless steels to enhance the performance for intended application. The non-contact and autogenous nature of the process coupled with precise and low heat input processing offers greater benefits compared to processing with conventional processes. This paper describes the application and advantages of power beam processing of different grades of stainless steels. Keywords: laser processing, electron beam processing, stainless steels, welding, cladding, hardening.