ULTRA-NARROW GAP P-GMA WELDING FOR THICK SECTION OF 304LN STAINLESS STEEL TO HSLA STEEL

Abstract

Dissimilar metal welding of thick wall austenitic stainless steel (γ-SS) and high strength lowalloy steel (HSLA) plates are used in defense, power generation, chemical, petrochemical and nuclear industries where the temperature elevates up to a range of the order of 350-450°C. Fusion welding is one of the most commonly used process; it is also largely used for the joining of dissimilar metals. The success of narrow groove multi-pass welding is primarily governed by manipulation of narrow welding torch inside weld groove by keeping enough room for the minimum required angle of attack to the groove wall to ensure its required fusion and successful inter-pass cleaning of weld bead. But the successful use of GMAW process in a practically narrowest gap butt welding of thick section (≥25mm) largely depends upon manipulation of a narrow torch nozzle relevantly deep inside a narrow weld groove along with a centrally placed single bead per layer of multi-pass weld deposition. Thus, to produce a lowest possible narrow groove GMA weld of thick section it is imperative to design a narrowest torch nozzle that can be successfully accommodated inside a narrowest possible weld groove to produce a sound weld. However, the narrowing down of weld groove may start influencing the flow of shielding gas inside the groove and create turbulence that can adversely affect the weld quality by introducing porosity and oxide inclusion in it. In this investigation, a model of shielding gas flow dynamics and its flow rate in case of employing newly developed GMAW nozzle has been studied at different projection angle of torch nozzle head inside the narrow groove of butt joint by using ANSYS-CFX (14.5) software. The utility aspect of such a narrow torch nozzle head from the view point of smooth flow of shielding gas inside a close fitted ultra-narrow weld groove has also been studied. The outcome of the analytical studies has been used to produce a defect free ultra-narrow multi-pass weld by employing P-GMAW process with vertically placed electrode depositing centrally placed single bead per layer in weld groove. The effect of various pulse parameters of P-GMAW process to produce ultra-narrow gap weld of thick dissimilar section of austenitic stainless steel to HSLA steel has also been studied. To deal with the complicated critical conditions of controlling the process parameters and thermal behaviour of weld deposit to produce a sound multi pass narrow groove dissimilar weld with centrally located single bead per layer weld deposition, a thermal model has been developed and analysed. In the light of this model analysis appropriate welding parameters have been designed to produce sound multi pass single bead per layer (MPSBPL) welding of thick austenitic stainless steel to HSLA steel plate. The characteristics of ultra-narrow gap dissimilar weld ii joint have been studied with respect to weld size, transverse shrinkage and bending stresses, mechanical and metallurgical properties of the weld. The characteristics of the ultra-narrow groove dissimilar weld have been compared with those of conventional groove dissimilar multi pass multi bead per layer (MPMBPL) weld joint of austenitic stainless steel to HSLA steel. In view of the above an effort has been made to design a narrowest possible torch nozzle head of GMAW that can allow narrowing down the weld groove up to a limit of just accommodating the nozzle head in it during thick section welding. The utility aspect of such a narrow GMAW torch nozzle head from the view point of smooth flow of shielding gas inside a closely fitted narrow weld groove has also been studied. The outcome of these studies has been used to produce a defect free ultra-narrow multi-pass weld of thick dissimilar steel section of an austenitic stainless steel and high strength low alloy (HSLA) structural steel by employing PGMAW process with vertically placed electrode depositing single bead per layer in weld groove. The welds of same dissimilar steel plates were also prepared by using commonly known technique of MPMBPL of weld deposition in conventional V-groove with the help of P-GMAW process. This is basically planned to study the effectiveness of the newly developed ultra-narrow gap welding procedure over the conventional groove welding with respect to the characteristics of weld joint. The background and prospect of the present work and observations of the above studies are analysed in the light of their objectives are presented in different chapters. Chapter 1 contains introduction of the area under discussion justifying the necessity to carry out studies on MPSBPL of weld deposition in narrow gap dissimilar welding of thick austenitic stainless steel to HSLA steel plates using P-GMAW process. The importance of P-GMAW and narrow gap welding has been discussed and the problems associated with respect to its practical implications in use are also briefly explained. The methodology which can be followed for practical implementation of MPSBPL of weld deposition in narrow gap P-GMA welding of relatively thick sections has been justified to address in present investigation. Chapter 2 begins with the survey of existing literature outlining the evolution of arc welding process and procedures used for joining of dissimilar thick section of austenitic stainless steel to HSLA steel. The existing criticality of dissimilar weld joint during welding of thick section of austenitic stainless steel and HSLA steel is also discussed. The state of the art knowledge on thermal influence of welding processes on various weld joint characteristics with respect to its metallurgical and mechanical properties have been critically analysed. Further the understanding on influence of welding procedure on stress distribution across the weld joints has been carefully examined. The deficiency of knowledge regarding the production of sound weld joint of thick steel plate using the technique of multi pass weld deposition in narrowest possible gap has been iii consolidated. The support of thermal model to understand the influence of various pulse parameters on characteristics of multi pass weld of thick austenitic stainless steel to HSLA steel plate has been critically identified. In this regard it is realised that the application of P-GMAW process in consideration of thermal and geometrical aspects of weld pool as a function of pulse parameters may play a considerable role to achieve the objective of the present investigation. Chapter-3 describes designing and development of an advanced nozzle head for narrow gap GMAW torch nozzle to facilitate the application of P-GMAW process in MPSBPL of weld deposition in ultra-narrow gap welding of thick dissimilar plates by considering the thermal influence on torch nozzle through various sources. To facilitate this, a narrowest possible gap system has been arranged. The performance of designed nozzle head system without any damage of burning during its use in MPSBPL of weld deposition in ultra-narrow groove welding has been reported and justified. The simulated characteristics of GMAW for arcing and argon shielding gas flow at a given flow rate inside the ultra-narrow groove of 25 mm thick butt weld are analyzed by 3D CFD modeling using ANSYS-CFX (14.5) software. The effect of variation in intensity of gas flow on the arcing and flow characteristics of shielding gas have been studied at different projection angle of the newly designed nozzle tip to its wall varied from 00 to 600. Chapter 4 deals with the thermal model for producing of lack of groove wall fusion free narrow gap weld during multi pass centrally placed single bead per layer of weld deposition in ultra-narrow groove of thick dissimilar plates with no angle of attack to the groove wall. The thermal model has been developed in consideration of thermal and geometrical aspects of the weld pool. Chapter 5 presents the experimental procedures for bead on plate weld deposition as well as preparation of ultra-narrow groove weld joints, of dissimilar thick (25mm) section of austenitic stainless steel to controlled rolled HSLA steel plate using the P-GMAW process employing solid filler wire. Both the studies of bead on plate weld deposition and preparation of dissimilar weld joint has been carried out by using the welding parameters confirmed by the thermal model applicable to preparation of ultra-narrow gap weld joint by centrally placed MPSBPL weld deposition in narrow weld groove. It also details the experimental procedure of preparation of dissimilar weld joint of austenitic stainless steel using MPMBPL weld deposition in commonly used conventional groove. Various testing of weld bead on plate deposition and weld joints has been described. The welding parameters and procedures used in this investigation with respect to the groove design and welding processes have been thoroughly described so that various aspects of weld characteristics can be suitably realised in the light of it. iv The experimental techniques of studying the shrinkage, bending and residual stresses, microstructures of different regions and mechanical properties of dissimilar weld joints prepared at different welding parameters and procedures have also been described. The testing has been planned to study the characteristics of weld joint by correlating them with  and Im at a given heat input (Ω). This is to establish the basic understanding of superiority of P-GMAW to produce ultra-narrow gap weld joint of 25 mm thick austenitic stainless steel to HSLA steel plate using MPSBPL weld deposition technique. The testing have also been planned to compare the properties of the ultra-narrow groove P-GMA weld to the conventional groove weld joint. Chapter 6 describes the results of various experiments presented in the preceding chapter and demonstrates the basic analyses of different facets of the investigation having major features explained briefly as stated below. This chapter also present the analysis and validation of proposed thermal model for preparation of sound ultra-narrow gap weld by MPSBPL of weld deposition technique. Similar to the earlier observations of several investigators here also it is understood that summarized influence of pulse parameters defined by the factor  maintains significant correlation with Im and  to control the P-GMAW process for required characteristics of weld bead deposition. The process control is basically realised through systematic variation of thermal and geometrical aspects of weld pool as well as microstructure of weld deposit and HAZ adjacent to fusion line as a function of , Im and . It is observed that comparatively higher values of Im and  are appropriate for MPSBPL of weld deposition in ultra-narrow gap dissimilar welding of thick sections of austenitic stainless steel to HSLA steel. Results of simulated model analysis is critically studied to realize the possibility of manipulation of GMA welding torch in a narrowest possible groove of thick section welding and to produce a practically porosity and oxidation free sound weld with no lack of groove wall and inter pass fusion. Accordingly, the attention is basically put forward to study the characteristics of shielding gas flow inside the ultra-narrow groove primarily to avoid turbulence and the arc characteristics promoting a weld pool efficient enough to give required groove wall fusion at different gas flow rate and nozzle tip angle. The arc characteristics are studied mainly with respect to its length and width (spread) especially to realize the thermal distribution in weld pool inside the narrow groove. The gas pressure at outlet of the welding torch as well as the velocity and turbulence kinetic energy of shielding gas flow at different zone has been estimated. The thermal model based on consideration of thermal and geometrical aspects of weld pool is able to produce a dissimilar weld joint free from lack of fusion by appropriate selection of pulse parameters, whereas a deviation in values from the specified range of the parameters results v a defective weld joint. The adverse situation arises because an unsuitable combination of parameters results in reduction of weld pool temperature than that required for proper groove wall fusion. It also gives unfavourable bead geometry with respect to that requires for required heat transfer to groove wall giving rise to an unsound MPSBPL narrow gap weld joint. The P-GMAW process has unique capacity to control the amount of heat and its distribution to the weld at a given heat input by appropriate selection of pulse parameters. Thus, it is possible to get a sound weld joint using P-GMAW process in case of MPSBPL narrow gap welding of thick plates. It is observed that MPSBPL narrow gap P-GMA weld joint is having considerably lower shrinkage, bending and residual stresses improved mechanical properties along with refined microstructure than those of conventional groove dissimilar weld joint prepared by using MPMBPL of weld deposition. It has been further observed that the P-GMA welds prepared at higher  and Im at a given Ω result into comparatively better weld joint characteristics. Chapter 7 presents conclusion of the investigation by identifying several innovative findings and understandings over the influence of the pulse parameters defined by their summerised influence in the hypothetical factor  and Im at a given Ω they are decided in consideration of the thermal model, on characteristics of weld joint produced by using P-GMAW process. Superiority of the MPSBPL ultra-narrow gap dissimilar weld joint with respect to several characteristics of conventional groove MPMBPL weld joint of thick austenitic stainless steel and HSLA steel plate is established. Various meritorious aspects with suitable nozzle head of the MPSBPL bound PGMAW process employed with newly designed GMAW torch nozzle.