PULSED CURRENT NARROW GAP GAS METAL ARC WELDING OF THICK HSLA STEEL PLATE

Abstract

The welding of high strength low alloy (HSLA) steels largely dominates their use in fabrication of various components of modem applications starting from manufacturing of automobile vehicles dealing with sheet metal to heavy engineering employing thick sections. According to their chemical compositions often these steels maintain good weldability due to low carbon and alloying content (3-5 Wt. %). However, because of its comparatively complex weld thermal behaviour the multi-pass welding of thick sections of HSLA steel becomes critical in order to satisfy desired joint properties. Welding of thick sections of HSLA steels are generally carried out largely by consumable electrode welding processes such as shielded metal arc welding (SMAW) and gas metal arc welding (GMAW). They influence severity of weld thermal cycle in different manner primarily depending upon amount of weld deposit, welding parameters and shielding environment. The amount of weld deposition can be considerably reduced by using narrow gap welding technique. However preparation of narrow groove weld joints by SMAW requiring appropriate manipulation of electrode at a low angle of attack can be done only by a skilled welder and as such the process automation practically not become feasible. SMAW has further limitation with respect to slag entrapment. The limitations of SMAW processes can be well addressed by the merits of GMAW process considering its advantage of producing relatively fast, long, clean and continuous weld at any position and introducing automation. In GMAW process the characteristics of metal transfer, wire melting rate and size of weld pool primarily dictate the weld thermal cycle depending upon use of various welding parameters. Out of three basic modes of metal transfer operate in GMAW process the spray mode of metal transfer offers high deposition rate and ease ii of operation in all position of welding. However, depending upon material and size of filler wire and shielding environment the spray transfer is achieved at high welding current which enhances heat input to the weld. The increase in heat input consequently increases the weld pool size as well as temperature and fluidity of molten metal and thus significantly affects the solidification behaviour of weld and results transverse residual stresses of appreciable magnitude especially in thick sections. The adverse conditions created by comparatively higher heat flow during welding of thick sections can be largely controlled by two methods. This is by reducing the amount of weld deposit using narrow gap welding technique and by application of pulse current gas metal arc welding (P-GMAW) process giving rise to weld deposition under intermittent pulses of high current for a small duration: High quality multi-pass and multi seam narrow gap welding of thick sections requires appropriate manipulation and positioning of electrode with respect to groove wall especially to achieve desired fusion in it, which continuously becomes more difficult with the filling of weld groove due to shrinkage of weld deposit closing down the groove width. It adversely affects the introduction of semi-automatic or automatic welding process like GMAW in narrow gap welding. Thus, production of sound narrow gap weld joint of thick section requires positioning of high heat intensity at proper location of required fusion with low heat input to minimise distortion in weld groove. Thus, success of producing sound narrow gap weld largely depends upon control of angle of attack between the electrode and groove wall and the geometry of weld bead affecting the thermal distribution necessary for fusion of groove wall and earlier deposited weld metal. However, the control of geometry and size of weld bead with respect to desired thermal distribution in welding process is largely governed by the welding parameters. iii In consideration of the above it is assumed that the P-GMAW process can be more effectively used than other welding processes for introducing high heat intensity in desired location at low heat input. It is because of its unique capacity of manipulation of energy distribution in the process by appropriate control of pulse parameters which helps in more precise control of weld thermal behaviour necessary for narrow gap welding as stated above. But the large number of pulse parameters, like peak current (Ip), base current (Ib), pulse on time (tp), pulse off time (tb), pulse: frequency (f) and their simultaneous interaction amongst themselves during welding, creates complexity in selection of pulse parameters. However, the complexity in selection of pulse parameters can be largely solved by using a summarized influence of pulse parameters proposed earlier and defined by a dimensionless hypothetical factor; 0 = Ih x f x th where, th = 1 — tp derived on the basis of energy balance concept of P f the system. In addition to the variation of heat input (S) to the system as a function of mean. current (I.), arc voltage (V) and welding speed (S) the heat transfer to the weld pool (QT) through arc and metal deposition dictated by the factor 4 also influences the characteristics of P-GMA weld deposit. Thus, it is imperative to understand the nature of variation in characteristics of weld bead on plate as a function of 0, I,,,, S2 and QT to use of weld deposition effectively for successful narrow gap welding. In consideration of this the performance of P-GMAW process has been explicitly studied with respect to thermal behaviour and geometry of weld bead on plate deposit and compared with those of the conventional GMAW process. Further, the empirical correlations amongst the 4, Im, S2 and QT with various geometrical and metallurgical characteristics of weld iv bead have also been worked out in order to develop a clear understanding on superior use of P-GMAW process in welding of HSLA steel for improved weld properties. In present investigation welding of controlled rolled thick (25mm) HSLA steel plate has been carried out using pulse current gas metal are welding in narrow weld groove with the help of the basic understanding acquired from the bead on plate studies. The studies have been systematically planned in order to gain sufficient knowledge to establish superior welding technique with respect to that used with conventional welding process and weld groove design. The primary aspects of the studies are as follows. I. To study the efficient use of the factor 0, Im and c2 by analysing its influence on thermal behaviour of weld and HAZ, weld bead surface appearance as well as weld pool geometry and microstructure of weld and HAZ through bead on plate deposition. This is in order to develop necessary understanding for selection of appropriate range of welding parameters and procedure, which may be effectively used in pulse current narrow groove welding of thick section of the HSLA steel. 2. 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 deposit. 3. To study the effect of variation in pulse parameters considered by their summarized influence as the factor 4 on metallurgical, mechanical and fracture mechanics properties as well as shrinkage stress and bending stress of weld joints. 4. To establish suitability of narrow gap P-GMA welding by comparing its utility to improve the properties of weld joint with respect to those prepared V by commonly used GMAW and SMAW with and without narrow weld groove. The entire details of investigation carried out in this work to achieve these objectives have been reported in five chapters as outlined below. Chapter 1 contains the introduction revealing a critical view on state of art of the subject matter justifying the necessity to carry out the studies on narrow gap of thick HSLA steel plate welding using P-GMAW process. The importance of the objective of the studies on such welding technique in order to understand the effect of various welding parameters on certain primary characteristics of weld joint with respect to its practical implications as a superior to the conventional welds has been logically stated. h` Chapter 2 begins with a survey of existing literature outlining the evolution of arc welding process and procedures used for joining of thick HSLA steel. In this chapter the existing knowledge on thermal influence of welding processes on various weld joint characteristics with respect to its metallurgical;,. mechanical and fracture mechanics properties have been critically analysed. Further the influence of welding procedure on stress distribution across the weld joints has been carefully examined. The deficiency of knowledge primarily regarding influence of various pulse parameters on preparation of sound narrow-gap weld of thick HSLA steel plate having desired joint properties has been critically identified in order to facilitate the application of P-GMAW process in it. In this context it is realised that thermal behaviour of welding process as well as geometry and amount of weld deposition as a function of pulse parameters may play a significant role to achieve the objective of the work. vi Chapter 3 describes the experimental procedures of bead on plate weld deposition and preparation of conventional V-groove and narrow groove weld joints of controlled rolled thick (25mm) HSLA steel plate using with and without pulse current gas metal arc welding (GMAW) employing solid filler wire and shielded metal arc welding (SMAW) and various testing of bead on plate and welded specimens. 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 appropriately realised in the light of it. The bead on plate (BOP) studies have been primarily carried out in order to investigate the effect of 4), In, and S2 on thermal behaviour and characteristics of weld deposition affecting the bead surface as well as geometry and microstructure of weld deposit and HAZ adjacent to fusion line. The characteristics of bead on plate P-GMA weld deposition have been analysed at a given arc voltage of 24±1 V under varied 0, Im and SZ in the range of 0.07-0.23, 160-230A and 5.5-10.6kJ/cm respectively. Whereas, the characteristics of GMA bead on plate weld deposit has been analysed at a given arc voltage and welding current of 24±1V and 230A respectively under different SZ of 8.28, 11.58 and 15.46kJ/cm. Based on the understanding of empirical correlation of the 4), mean current (I,n) and heat input (S2) with variation of different characteristics of weld bead the narrow gap P-GMA weld joints have been prepared by appropriate weld deposition at different parameters giving rise to sound welds. To compare the characteristics of these welds, narrow gap GMA weld is also prepared at high mean current resulting desired fusion and penetration. In this context a narrow gap SMA weld has also been prepared by employing the conventional practices in use and compared. vii The experimental techniques of studying the shrinkage and bending stresses, microstructures of different regions as well as mechanical and fracture mechanics properties of weld joints prepared at different welding parameters and procedures have also been described. The characteristics of weld joint have been studied by correlating them with the , Im and E2 to establish the basic reasons of superiority of P-GMAW over the GMAW and SMAW processes to produce narrow gap weld joint of 25 mm thick HSLA steel plate having relatively improved properties. Chapter 4 presents the results of various experiments described in the preceding chapter and demonstrates the basic analyses of different facets of the subject matter. Major features of those aspects are summarised below. In agreement to the observations of earlier investigators here also it is noted;that summarized influence of pulse parameters defined by the factor 0 maintains significant correlation with Im and 0 to control the P-GMAW process in bead on plate weld deposition. The process control is basically realised through systematic variation of thermal behaviour, bead surface as well as geometry and microstructure of weld deposit and HAZ adjacent to fusion line as a function of 0, I,,, and Q. It is marked that comparatively higher values of 4 and Im and lower S2 of the . order of 0.23±0.05, 230±4A and 5.5±0.5kJ/cm respectively are suitable for multi-pass and multi seam narrow gap welding of thick sections of HSLA steel whereas, a deviation in values from the specified range of the parameters results a defective weld. This is because of an inappropriate combination of parameters lying out of the prescribed range primarily reduces the temperature of weld deposit and gives rise to its unfavourable geometry and amount resulting an unsound narrow gap weld. The P-GMAW process has the capacity to control the amount of heat transferred to the weld pool (QT) by varying mass, velocity and heat content of the viii droplets resulting into achieving equivalent amount of metal deposition at a significantly lower heat input (Q) in comparison to that occurs in SMAW and GMAW processes. Thus in narrow gap P-GMA welds considerable reduction in shrinkage and bending stresses along with refined microstructure in multi-pass and multi seam deposit have been observed in comparison to those of the GMA and SMA weld joints. Consequently it also improves mechanical and fracture mechanics properties of the weld. It has been further observed that the P-GMA welds prepared at higher c), I,,, and lower S2 result relatively better weld joint characteristics than those of the GMA and SMA weld joints. Chapter 5 concludes the investigation by identifying several innovative knowledge and understandings over the influence of the controlled pulse parameters with respect to the hypothetical factor 4), I,n and Q on characteristics of weld joint produced by P-GMAW process. The advantage of narrow gap P-GMA welding of thick HSLA steel plate with suitable modification in weld groove size has been broadly realised as follows. The use of P-GMAW process with narrow gap welding procedure reduces the shrinkage stress as well as distortion and bending stress of weld joint by about 40-45% than those of conventionally produced welds using GMAW and SMAW processes. Such a narrow gap welding produces uniformly distributed finer microstructure in multi-pass deposition and reduces the width of heat affected zone (HAZ) by about 40-55% with respect to that observed in conventionally produced GMA or SMA weld joints. In this regard the use of narrow gap P-GMA weld joints at relatively higher 4), I,,, and lower S2 increases the tensile properties by 10-30%, Cv impact toughness at -30°C by 15-40% and initiation fracture toughness (JQ) by 40% than those of the conventional GMA and SMA weld joints.