NARROW GAP PULSE CURRENT GAS METAL ARC WELDING OF THICK WALL 304LN STAINLESS STEEL PIPE

Abstract

The acceptability of weld joint is primarily governed by required fusion of base metal through formation of arc crater of sufficient extent in it, at an optimum thermal exposure giving rise to a minimum undesirable heat affected zone (HAZ) and residual stresses. The control of microstructure and residual stresses of weld joint becomes quite critical in case of joining of thick sections of austenitic stainless steel by multipass welding procedure due to its low thermal conductivity and high coefficient of thermal expansion. The arc welding of such steel is generally carried out by Shielded metal arc welding (SMAW), Gas tungsten arc welding (GTAW) and Gas metal arc welding (GMAW) processes, where they influence severity of weld thermal cycle in different manner depending upon amount of weld deposition, welding parameters and shielding environment. The severity of thermal and mechanical effects of welding can be considerably minimized by reducing the amount of weld metal in a joint by using narrow gap welding technique. The preparation of narrow groove weld joints by SMAW and GTAW processes generally requires welding at a lower angle of attack to groove wall by a skilled welder and the process automation is highly critical. SMAW has further limitation with respect to slag entrapment resulting in poor mechanical and fracture mechanics properties, while in GTAW process welding speed is significantly lower than SMAW and GMAW processes. The solution to these limitations of SMAW and GTAW processes can be satisfactorily addressed by the merits of GMAW process using spray mode of metal transfer which offers better ease of operation primarily due to dominating electromagnetic force resulting in projected transfer of the droplet. But, the convenient use of GMAW process in spray mode of metal transfer at high welding current increases heat input to the weld and consequently affects the size, temperature and solidification behaviour of weld pool resulting an adverse influence on grain coarsening of ii heat affected zone (HAZ), residual stresses of weld joint and carbide precipitation along the grain boundaries. These undesirable conditions which are created by comparatively higher heat flow during welding can be controlled by two methods, first by reducing the amount of weld metal i.e. by narrow gap welding and secondly by modification of thermal and solidification behaviour of weld deposit, through control of welding process and parameters. The difficulties in manipulation of relatively large conventional torch head of the order of 26mm O.D. in narrow weld groove deters the narrow gap GMA welding of thick section necessitating the development of a narrow torch head feasibly justified in order to have desired protection of metal deposition and groove wall fusion. In this context the possibility of using the merits of pulsed current GMAW (P-GMAW) process primarily with respect to its more efficient characteristic of metal deposition has been studied to facilitate the narrowing down of weld groove without compromising the acceptable weld quality. The ability of P-GMAW process to combine a comparatively low work-piece heating along with high arc stiffness and strong mechanism of metal transfer depending upon pulse parameters allowing improved fusion and penetration in groove wall has been explored for preparation of narrow gap GMA weld having atleast a comparable or improved weld quality than that of conventionally produced narrow gap SMA weld. In the context of criticality and complexity in selection of simultaneously interactive pulse parameters like peak current (In), base current (Ib), pulse duration (tp) and pulse frequency (f), it is decided to consider the summarized influence of pulse parameters through hypothetically derived dimensionless factor 0 = [(Ib/Ip)ftb], where tb is expressed as [(1/f)— tp]. In P-GMAW process during metal transfer primarily occurring at the peak current (Ip), two heat sources of different natures act simultaneously on weld pool. One is continuous heat source (arc heat source) of double ellipsoidal nature acting at the surface of the base plate, which melts and produces an initial weld pool in the base metal. The other one is an interrupted iii heat source supplying superheated filler metal, considered as a point heat source dictating the size and geometry of weld pool over that initially developed by the arc heating. Thus, depending upon variation of 0, total heat transferred to the weld pool (QT) is primarily attributed to the initial arc heating (QAw) followed by the deposition of superheated filler metal (Qf) affecting the thermal behaviour of the weld metal deposited per pulse. Thus in addition to the hypothetical factor 4), the variation of heat input (S2) to the system as a function of mean current (I,,,) , arc voltage (V), welding speed (S) and heat transferred to the weld pool (QT) also influences the weld characteristics. Hence the planning of appropriate use of 4), S2 and QT has been planned in light of its reported utility to control P-GMAW process. In view of the above the present investigation on welding of thick wall 304LN stainless steel pipe has been carried out using pulse current gas metal arc welding (P-GMAW) in narrow weld groove. The studies have been systematically planned in order to gain sufficient knowledge to establish a welding technique superior to some conventionally used welding processes and weld groove design. 1. To design and develop a narrow GMA torch nozzle device which facilitates the application of P-GMAW process in narrow gap welding of thick wall pipes. 2. To study the efficient use of factor 0, S2 and QT, by analysing its influence on sensitization and other characteristics of weld through bead on plate deposition. This is in order to develop a systematic knowledge for selection of appropriate range of welding parameters and procedure which may be effectively used in pulse current narrow groove welding of thick section. 3. Development of narrow gap welding procedure by P-GMAW process for joining of thick sections by appropriate reduction in number of weld passes and amount of weld metal. iv 4. To study the effect of variation in pulse parameters considered by their summarized influence through the dimensionless factor 4> on metallurgical, mechanical and fracture mechanics properties as well as residual stresses of weld joint. 5. To study the acceptability of weld joint with respect to corrosion susceptibility of its heat affected zone. 6. To establish suitability of narrow gap P-GMA welding by comparing it's utility to improve the properties of weld joint with respect to those prepared by commonly used SMA welding with and without narrow weld groove. The entire details of investigation carried out in this work to achieve these objectives have been reported in six chapters as outlined below. Chapter-1 contains the introductory remarks about the thermal influence of welding process and procedure on weld joint characteristics of thick austenitic stainless steel sections. The importance of P-GMAW and narrow gap welding has been categorically addressed and the problems associated with respect to its practical implementation are also briefly discussed. Finally, the methodology which can be followed for practical implementation of narrow gap P-GMA welding in thick sections has been highlighted and justified to address in this work. Chapter-2 begins with the survey of the existing literature outlining the evolution of arc welding process and procedures used for joining of thick austenitic stainless steel sections. In this chapter the existing knowledge on thermal influence of welding processes on various weld joint characteristics with respect to its metallurgical, mechanical, fracture mechanics and corrosion properties has been critically analysed. Further, the influence of welding procedure on residual stress distribution across the weld joints has been carefully examined. The deficiency of knowledge with respect to influence of various pulse parameters on weld joint properties in P-GMAW process, v limiting its practical applications, has been identified and the problem proposed to be investigated has been outlined in this context. Chapter-3 presents the development of narrow torch nozzle device suitable for narrow gap GMA welding of thick pipes/plates along with proper consideration of the characteristics of shielding jacket at the outlet of torch head protecting the contamination of weld deposit from environmental reaction. The performance of newly developed torch with respect to arc stability and weld bead oxidation characteristics has been reported justifying the -suitability of the narrow torch nozzle for use in narrow gap GMAW of thick section. Chapter-4 describes the experimental procedures followed to prepare the conventional V-groove and narrow groove weld joints of austenitic stainless steel pipes using gas metal arc welding (GMAW), pulsed current GMAW with solid filler wire and Shielded metal,, arc welding (SMAW) for characterisation of the weld joints. The welding parameters and procedure used in this investigation with respect to the groove design and welding process have been thoroughly described, so that various aspects of weld characteristics of concern can be appropriately realised in the light of it. The weld joints have been characterised primarily by studying the influence of heat input (S)) on its residual stress distribution, susceptibility to intergrannular corrosion and initiation fracture toughness along with its conventional mechanical and metallurgical properties. In case of P-GMA welds the influence of different 4 on weld joint properties has also been characterised to identify and analyse the causes behind its variation. The advantage of P-GMAW with due reference to SZ and 4, over the SMAW process with respect to various weld joint properties have been compared. Chapter-5 presents the results of various experiments described in the preceding chapter and demonstrates the different facets of the present work, broadly classified into two vi major features. Firstly, the influence of P-GMAW process on conventional groove weld joint characteristics have been analysed by varying 4), Q and QT. Secondly, by using narrow gap welding procedure, the influence of groove design on weld joint characteristics have been established. In both the cases to establish the superiority of P-GMAW process, weld joints produced with SMAW process have also been analysed at almost similar groove design. For better understanding on influence of P-GMAW process on weld joint properties, pulse parameters have been correlated further with the amount of weld metal varied by changing groove design. Finally, by analysing the characteristics of weld joints with respect to their mechanical, metallurgical, corrosion and fracture mechanics properties along with residual stress distribution, the necessary control of welding parameters and narrow gap welding procedure in P-GMAW process has been established in order to improve the weld quality. Chapter-6 concludes the investigation by several innovative knowledge and understandings over the influence of hypothetical factor 4) on characteristics of weld joint produced by P-GMAW process. The advantage of narrow gap P-GMA welding of thick SS 304LN pipes with suitable modification in GMAW torch nozzle device have also been broadly realised.