Advanced Materials Research Vol. 794

Abstract: Railways form the lifeline of the transport network in our country. Indian Railways command a large market share of the over-land passenger and freight traffic. To sustain this, rail industry is required to technologically and commercially challenge superior highways and advanced technology of road vehicles and also the competitive airlines. Thick and thin sheet metals are widely used in wagon and rail car body fabrication respectively. Rail coach manufacturing sector is in the new challenging phase due to acquirement and adoption of German and Korean technologies and arrival of multinational companies viz M/S Bombardier & M/S Alstom. Thin sheet metal fabrication demands aerodynamic profiles larges size bodies with high level of aesthetic and dent free surface. Thick sheet metal fabrication demands larges size bodies with high level of structural rigidity and dimensional stability .The outer skin of the rail coach body is generally made through thin sheet metal parts and stiffened with moldings, pressings and stamped sections. In the recent past rail car manufactures has been switching over from HSLA steel to stainless steel to acquire the benefit of the later: primarily corrosion resistance Capability and durability and ease of maintenance. The manufacturing practices, challenges addressed during fabrication are brought out.

Abstract: Dissimilar metal joints of stainless steel to titanium find extensive industrial applications especially in the nuclear industry. However, it is well known that fusion welding of stainless steel to titanium is difficult because of the formation of brittle intermetallic compounds and the associated problems. To avoid this, welding processes or techniques with high reliability and productivity for these dissimilar materials are demanded. In the present work, joints comprising of 304 stainless steel and commercially pure titanium were produced by friction welding using nickel as interlayer. Investigation on the mechanical properties of the joints shows the occurrence of highest hardness value at the interface of titanium and nickel interlayer. X-ray diffraction studies confirmed the presence of various types of intermetallic compounds at the interface of the welded joint. The tensile strength of the joint varies with the thickness of nickel interlayer used. Joints having maximum strength equals to 72% of that of titanium base metal could be produced. In all the joints, tensile failure occurred at Ti-Ni interface due to the presence of the intermetallic compounds at this interface. Fracture surface analysis reveals that the tensile fracture path is along the intermixing zone of titanium and nickel interlayer.

Abstract: Distortion is a perennial problem faced by engineers engaged in welding fabrication. The shape change deformations and change in the dimensions that occur after welding is termed as distortion, leading to various undesirable consequences. So there exists a necessity to control distortion within limits. When distortion exceeds acceptable limits, correction of distortion after the complete fabrication results in major reworking operation that consumes both fabrication time and cost. Distortion control in complex structures has always been a challenge to fabrication engineers, dealing with stainless steels. To arrive at an appropriate control method, an in-depth analysis of the shape change deformation behavior of the component and adaptation of appropriate methods to control distortion are essential. In this study, two circular components made of stainless steel have been taken up for control of distortion. The first structure is the labyrinth used in hydro turbine assembly that consists of a ring type of flange made of martensitic stainless steel (ASTM grade A240 S41500) welded with a shell type web using grooved fillet welds. The size of the welded assembly is huge and the quantum of weld is very high leading to heavy distortion that often required to be corrected by way of weld build-up and subsequent machining. The second one is the vortex finder assembly which is a part of cyclone separator that is used to separate the fine particles of coal from coarse ones for use in the combustion chamber of typical Circulating Fluidised Bed Combustion (CFBC) boilers. This is again a cylindrical structure with three shells joined with three flanges and supported by stiffeners. The structure is made of special grade Austenitic stainless steel to withstand high temperature. In both the structures, most of joints are of Tee-joints with fillet welds. During fabrication of these assemblies, the angular tilt, local waviness and the diameter variation were the major issues. Correction after fabrication is not viable considering the configuration and shape of the component. This calls for control of distortion during fabrication itself. Towards this, a suitable welding procedure using an appropriate weld sequence was evolved after studying the weld details of the components. This weld sequence was implemented in-situ in actual components. During fabrication on-line monitoring was carried out and course corrections were provided. After implementation of the weld sequence, it was found that the distortion of the component could be reduced and contained well within the permissible limits. This made the extra rework redundant thus leading to a significant reduction in the cycle time of manufacture of the component. The details of the methodology adopted are described in this paper. Keywords: Labyrinth assembly, vortex finder assembly, stainless steel welding, distortion, T joint, grooved fillet weld, angular tilt, weld sequence.

Abstract: The paper presents advanced ultrasonic and eddy current NDE techniques developed in the authors laboratory for nondestructive evaluation of austenitic stainless steel welds. The paper discusses the performance and comparison of 2D discrete wavelet transform (DWT) and de-noising methods applied on eddy current images obtained from stainless steel weld pad with machined longitudinal notches and a systematic approach for eddy current defect characterisation in weld pads by neural network. The simulation and experimental results on the effect of elastic anisotropy on ultrasonic phased array inspection in austenitic stainless steel weld are also discussed. A guided wave based ultrasonic method developed for detection of defects in stainless steel welds and its validation with complimentary techniques such as radiography and in-situ metallography are also presented.

Abstract: Stainless steels (SS) possess excellent corrosion, creep and high temperature oxidation resistance and are invariably used in refinery for construction of heater tubes, tube supports, Heat exchanger bundles, piping and internal lining of pressure vessels. Ferritic stainless steel type 405 is used for column strip-lining, martensitic stainless steel type 410 is used for column trays and heater tubes and austenitic stainless steel family is used very extensively for lining, piping, heat exchanger, heater tubes and tube supports. On-stream and turnaround condition monitoring of plant and equipment are carried out for health assessment and mitigation of premature failure. However, catastrophic failures of stainless steel due to stress corrosion cracking, thermal fatigue and stress relaxation cracking are encountered in addition to bulging and cracking of strip-lining. Field repairs of these components are required to be done. Stainless steels are difficult to weld due to low thermal conductivity, higher coefficient of thermal expansion, fissuring and solidification cracking problem during welding. Lower heat input and fast cooling facilitate the welding process. Welding of service exposed stainless steels is more challenging, as it has already undergone metallurgical degradation. Welding of stainless steels is carried out using TIG and SMAW process with matching electrode after establishing the welding specification procedures and welders qualification. Field repairs of stainless steels components are also attempted with original procedures and in case of difficulties, a buttering layer of inconel (ERNiCr3) or ER 309Mo is provided on the welding surface before using matching electrodes. Quality assurance of weld joint is ensured by stage-wise inspection and non-destructive testing. Dye penetrant test of root run and radiographic examination of final weld joint are most common. Post weld heat treatment is done as per code requirement. This Paper highlights three case studies on field repairs of stainless steel components in refinery. 1. Welding procedure followed for repair of bulged and cracked SS 316 strip-lining and cladding on carbon steel backing material. It is a dissimilar welding of SS 316L with degraded carbon steel. 2. Field welding of SS 347 Piping components, which has undergone thermal relaxation cracking at fillet joints. 3. Welding repair of SS 310 cast heater tube support conforming to A 297 Gr HK 40. The Paper also presents brief failure analysis with reasons and remedies.

Abstract: Stainless steels are engineering materials capable of meeting a wide range of design criteria. They exhibit excellent corrosion resistance, strength at elevated temperature, toughness at cryogenic temperature and fabrication characteristics, and they are selected for a broad range of consumer, commercial, and industrial applications. In the fabrication of stainless steel products, components, or equipment, manufacturers employ welding as the principal joining method. Stainless steels possess good weldability and a welded joint can provide optimum corrosion resistance, strength, and fabrication economy provided reasonable care is exercised during welding. L&T's Heavy Engineering (HE) has established a reputation for quality products in the global market with its strong engineering capabilities and state-of-the-art manufacturing facilities. It manufactures and supplies various critical equipments like reactors, vessels, heat exchangers and inter-connecting piping to Fertilizer, Refinery, Petrochemical, Chemical, Oil & Gas, Power, Nuclear and allied Strategic sectors. The wide spectrum of equipments mentioned involves fabrication of various grades of Stainless Steel (SS)like Austenitic, Ferritic, Martensitic, Duplex, Super Duplex etc. This paper discusses some of the high productivity welding processes and the techniques being used in manufacturing Stainless Steel vessels at Larsen & Toubro’s Heavy Engineering such as: Narrow groove welding of high thickness SS joints by Submerged Arc Welding(SAW), High deposition SS weld surfacing using Electro Slag Strip Cladding (ESSC), Hotwire GTAW for joining & surfacing of SS, SS Liner welding by GTAW for critical Urea Service applications, Automatic Tube to Tube sheet Welding etc.

Abstract: The present investigation is aimed at to study the effect of four welding processes namely friction stir welding, gas tungsten arc welding, laser beam welding and electron beam welding on fatigue behavior of the ferritic stainless steel conforming to AISI 409M grade. Rolled plates of 4 mm thickness were used as the base material for preparing single pass butt welded joints. The fatigue life and fatigue crack growth behavior were evaluated using hourglass and centre cracked tension (CCT) specimens respectively. A 100 kN servo hydraulic controlled fatigue testing machine was used under constant amplitude uniaxial tensile load with stress ratio of 0.1 and frequency of 15 Hz. Fatigue properties are correlated with the tensile, impact toughness, micro hardness, microstructure, fracture surface morphology and residual stress of the welded joints. It is found that the joint fabricated by friction stir welding process showed superior fatigue life and fatigue crack growth resistance compared to other joints. This is mainly due to the synergetic effect of dual phase ferritic-martensitic microstructure, superior tensile properties and favorable residual stress, which inhibit the growth of cracks compared to other joints.

Abstract: Understanding of deformation, fracture or fatigue behaviour of AISI 304LN grade stainless steel with reference to in-situ evolution of deformation induced martensite (DIM) is important for the structural integrity of numerous critical engineering components made of this steel. The primary objective of this report is to present a concise overview on the state-of-the-art of these aspects based on a series of investigations by the authors and their co-workers through over more than a decade. The major experiments involved are determination of tensile, fatigue and fracture behaviour of the steel using standard testing procedures. The associated structural and sub-structural changes in the deformation volume or at local regions such as fracture surfaces or crack tips are characterized. The nature and amount of DIM have been detected through microstructural analysis, X-ray diffraction, hardness measurement, ferrofluid based technique, ferritoscope assessment and TEM, in addition to extensive fractographic analysis by SEM. The major highlights of the investigations centre on revelations of the role of DIM on tensile deformation of 304LN stainless steel at various strain rates and temperatures, illustrating the association of DIM with constrained and unconstrained deformation ahead of crack tips in monotonic and cyclic fracture tests, and examination of the extent of DIM transformation during stress controlled and strain controlled cyclic loading and fatigue crack growth, with an underlying theme of continuously emphasizing the nature, location and amount of DIM formed.

Abstract: High nitrogen containing austenitic stainless steel X8CrMnN18-18 exhibits attractive combination of high strength, toughness and corrosion resistance.This grade containing more than 5000 ppm of nitrogen was produced commercially through EAF-AOD-LRF-CC-Steckel mill route and its microstructure and mechanical properties were studied. Excellent combination of strength,ductility and toughness is achieved in the entire range of 6mm to 50mm thick hot rolled plates. Uniaxial compression tests were carried out to understand the hot deformation behavior by varying temperature and strain rate. Softening behavior during deformation was analyzed from flow stress strain curves and microstructural analysis. Dynamic recrystallization (DRX) behavior of the material was observed during thermo mechanical processing. Critical strain related to DRX and Avrami kinetics of DRX was calculated by analyzing the flow curve data. Microstructural characterization was done by optical microscopy and EBSD analysis. Extensive grain refinement can be achieved by thermo-mechanical processing controlled by DRX. Keywords: High nitrogen stainless steel, Strength and toughness, DRX, Grain refinement, TMCP

Abstract: Low cycle fatigue (LCF) and Creep-fatigue interaction (CFI) behavior of 316LN austenitic stainless steel alloyed with 0.07, 0.11, 0.14, .22 wt.% nitrogen is briefly discussed in this paper. The strain-life fatigue behavior of these steels is found to be dictated by not only cyclic plasticity but also by dynamic strain aging (DSA) and secondary cyclic hardening (SCH). The influence of the above phenomenon on cyclic stress response and fatigue life is evaluated in the present study. The above mentioned steels exhibited both single-and dual-slope strain-life fatigue behavior depending on the test temperatures. Concomitant dislocation substructural evolution has revealed transition in substructures from planar to cell structures justifying the change in slope. The beneficial effect of nitrogen on LCF life is observed to be maximum for 316LN with nitrogen in the range 0.11 - 0.14 wt.%, for the tests conducted over a range of temperatures (773-873 K) and at ±0.4 and 0.6 % strain amplitudes at a strain rate of 3*10^-3 s^-1. A decrease in the applied strain rate from 3*10^-3 s^-1 to 3*10^-5 s^-1 or increase in the test temperature from 773 to 873 K led to a peak in the LCF life at a nitrogen content of 0.07 wt.%. Similar results are obtained in CFI tests conducted with tensile hold periods of 13 and 30 minutes. Fractography studies of low strain rate and hold time tested specimens revealed extensive intergranular cracking.