Badri K. Narayanan

Welding Consumable Developments in the Aftermath of the Northridge Earthquake

The earthquake that struck Northridge, California, on 17 January 1994, caused collapse of a number of reinforced concrete and wood frame structures. Fracture occurred in welded connections in some steel frame buildings, though none collapsed. Self-shielded flux cored electrodes are popular for steel frame building erection because they are largely unaffected by welding in windy conditions. These electrodes are generally “killed” with aluminium, and the aluminium recovered in the weld deposit affects the microstructure and, therefore, the properties. The paper describes the evolution of steel weld metal microstructures from self-shielded flux cored electrodes, as a function of aluminium content.

Effect of Strain Ageing on Yield Strength and Post Yield Behavior of FCAW-G Ferritic Weld Metal

Ferritic weld metal was deposited with gas shielded flux cored arc welding (FCAW-G) process. The nitrogen level in the deposited weld metal varies typically between 20 to 100 ppm. Nitrogen is a fast diffusing solute element that is known to cause strain ageing affecting both strength and toughness. Weld metal was produced with intentional additions of two strong nitride formers, titanium and vanadium. All-weld metal tensile samples were subjected to varying levels of strain, aged at 170°C for 20 minutes and reloaded to failure. Both the yield and tensile strength increased with increase in pre-strain confirming the presence of strain ageing. The strain hardening rate is also seen to change with strain ageing. There is also a corresponding decrease in the uniform elongation with increase in nitrogen and prestrain. The effect of strain ageing treatment on weld metal toughness was also evaluated. A nominal 2%–3% strain was imposed on the weld metal by straining it in the direction of welding and Charpy V-Notch toughness of the weld was measured. The ductile to brittle transition temperature (DBTT) of the weld metal was estimated by measuring the percent shear and the weld metal toughness at different temperatures. The DBTT of the weld metal is seen to shift slightly to higher temperatures with increase in pre-strain. However there was a dramatic drop in the upper shelf energy and a consistent decrease in the average toughness of the weld metal at all temperatures. The as-welded and reheat microstructure of the weld metal was characterized using optical and electron microscopy techniques. The possible implications of strain ageing on pipeline girth weld procedure qualification and inservice integrity are discussed.Copyright

Calculation of energy balance and efficiency in Laser Hot-Wire (LHW) cladding process

Laser Hot-Wire (LHW) processing is a subset of freeform welding, which in turn is an extension of the well-established wire-based robotic cladding. The process is adaptable as a viable additive manufacturing (AM) technology. In LHW two separate heat sources are combined - a laser impinges on the surface of the workpiece and the feed wire is resistively heated through an external DC circuit. The two materials used in the experiments reported here are Ti-6Al-4V and Nickel 625. Advantages of the process are high deposition rate, high energy efficiency, and the ability to produced controlled unique microstructures. Preliminary calculations of energy efficiency and energy balance of the process are reported.

Variables Affecting Fracture Toughness of Welds in T-K-Y Connections

Fracture toughness of tubular welded joints is one of the critical factors affecting the structural integrity and reliability of offshore structures, such as platforms and subsea pipelines. The factors affecting the design fracture toughness of these structures are related to, both, the welding process as well as the chemical composition of the weld metal.The welding process in this application typically comprises of depositing weld metal in the tubular joints of varying thicknesses through series of weld passes. The number of weld passes required for welding these joints subjects the weld metal to repetitive cycles of heating and cooling. The effect of the thermal cycling introduces significant heterogeneity in the microstructure. This is further exacerbated by the presence of micro-alloying elements such as Niobium (Nb) and Vanadium (V) that form complex carbides, nitrides and carbo-nitrides during post weld heat treatment (PWHT).The focus of this work is to evaluate the effect of micro-alloying elements on the ductile to brittle transition temperature and the mode of fracture at temperatures relevant to offshore applications. A threshold Nb and V level has been determined for achieving acceptable weld metal toughness. The improvement in the fracture toughness using this approach has been quantified by Charpy V-Notch (CVN) and Crack Tip Opening Displacement (CTOD) measurements. The Ductile to Brittle Transition Temperature (DBTT) has been shown to be shifted to lower temperatures by 25 °C after post weld heat treatment in the welds where the total amount of Nb and V are controlled to less than 40 ppm.A wet precipitate extraction technique was used to extract precipitates from the welds to establish the presence of fine Nb rich precipitates in the welds with the higher DBTT. The weld deposited with controlled levels of Nb and V was further tested in different joint configurations and base plate thickness. The fracture toughness was evaluated by CTOD testing of the weld in two different thicknesses (50 mm and 70 mm). Increased specimen thickness resulted in lower CTOD values.Copyright

Effect of Strain Ageing on Mechanical Properties of Pipeline Girth Welds

Pipeline girth welds for on-shore and off-shore pipelines use a variety of arc welding processes. The trend towards strain based designs for line pipe installation and the effect of coatings for off-shore pipelines have resulted in evaluation and testing of pipe material subjected to strain ageing. However, very little work has been done to systematically study the effect on ferritic weld metal. This work details some initial work done on evaluating the effect of strain ageing on ferritic weld metal deposited with a 1.2 mm diameter flux cored wire under 75% Ar −25% CO2 shielding gas. Pipeline girth welds were welded on API Grade X-70 pipe and tested to get all weld metal tensile and Charpy V-Notch properties. The weld metal strength overmatched the base material by 7–9%. The ductile to brittle transition temperature for the weld metal was −40°C. The effect of strain ageing on weld metal properties was evaluated. All weld metal tensile samples were subjected to varying levels of pre-strain and ageing treatments to evaluate the effect on yield strength and post-yield behavior. An increase in yield strength after straining and ageing as well as the re-appearance of yield point is observed. Increase in pre-strain decreases elongation. Increase in ageing temperature delays the appearance of dynamic strain ageing. The activation energy for the increase in strength after strain ageing has been measured by assuming a diffusion controlled mechanism. Charpy V-Notch samples were taken to generate transition curves of weld metal after strain ageing and compared to the as-welded condition.Copyright

Effect of Deposit Composition on the Mechanical Properties and Cracking Tendency of Cellulosic-Covered SMAW Weld Deposits

This investigation utilizes test electrodes manufactured with boron at different levels (including no boron). The design of these electrodes is identical with the exception of the intentional changes highlighted. Gapped bead on plate (GBOP) testing is used to determine the relative propensity of the electrodes for weld metal cracking. Test electrodes are also evaluated for deposit composition, CVN impact toughness, strength, and hardness on pipe joints. This work also uses a non boron-containing test electrode whose deposit composition has been modified such that its carbon equivalent is the same as one of the boron-containing electrodes. This serves to separate the influence of the specific element boron from the influence of general carbon equivalent/hardenability on the tendency for cracking. The results indicate that the effect of changes in boron and carbon equivalent over the range tested and in this specific electrode design is very slight. In most cases, the effect is not significant when compared to the amount of variation observed in the testing. In essence, the signal was lost in the noise. In terms of susceptibility to hydrogen assisted cold cracking (HACC) — the area of most concern — there appear to be other factors that are much more influential than those tested. If the goal is to minimize the cracking sensitivity of cellulosic weld metal, simply eliminating the use of boron is not the answer. More work is required to identify these other factors and quantify their effect.© 2010 ASME

Characterization of Weld Metal Deposited With a Self Shielded Flux Cored Electrode for Pipeline Girth Welds and Offshore Structures

Pipeline girth welds deposited with a self-shielded flux cored electrode process (FCAW-S) have been characterized to assess the effect of micro-alloying elements on microstructure and precipitate evolution and correlate it to strength and toughness. A 2.0 mm diameter electrode was used to deposit weld metal in a 12.7 mm thick API grade X-70 pipe joint. The weld metal properties were characterized and shown to overmatch the pipe. The DBTT of the weld metal has been determined through Charpy V-Notch toughness measurements. The effect of heat input and welding procedure has been assessed over a range of heat inputs (1–1.5 kJ/mm.). The effect of dilution from the base plate on toughness has been assessed by measuring the sensitivity of weld metal toughness to changes in carbon content. The as-welded region of the weld has been characterized using different characterization techniques. Ferritic weld metal deposited with a self-shielded arc welding process has intentional additions of aluminum, magnesium, titanium and zirconium. This results in a complex precipitation process that has been characterized with a combination of electron microscopy techniques. The effect of micro-alloying additions on the variant selection during the austenite to ferrite transformation and microstructure evolution has been studied with electron back scattered diffraction (EBSD) in conjunction with orientation imaging microscopy (OIM). Transmission electron microscopy (TEM) was used to characterize the precipitate evolution in these welds. The evidence shows that the formation of a spinel oxide is critical for the nucleation of nitrides of zirconium and titanium and prevents the agglomeration of aluminum rich oxides and the formation of large aluminum nitrides. The evolution of precipitate formation is critical to limit large inclusions and improve weld metal toughness. The presence of titanium and zirconium increases the fraction of high angle grain boundaries within the microstructure resulting in increased resistance to crack propagation. The characterization of the microstructures at two different carbon contents indicates the greater propensity to form twin related variants with increase in carbon content. This suggests a lower transformation temperature of austenite and may be the reason for poor toughness.Copyright

Semi-Automatic Gas-Less Process for Girth Welding X-80 Line Pipe

A new process (M2M™) to girth weld API Grade X-80 line pipe with a gas-less technology is presented. This process combines innovations in controlling arc length and energy input with microstructure control of the weld metal deposited to achieve high strength (over matching 550 MPa yield strength) and Charpy V-Notch toughness of over 60 Joules at −20°C. This paper will concentrate on the metallurgical aspects of the weld metal and the systematic steps taken to achieve high strength weld metal without sacrificing toughness. The development of an appropriate slag system to achieve the best possible microstructure for high toughness weld metal is discussed. The indirect effects of the slag system on the weld metal composition, which in turn affects the microstructure and physical properties, are detailed. In order to achieve sound weld metal without gas protection using a semi-automatic process, a basic slag system with minimal acidic components is used to improve the cleanliness of the weld metal without sacrificing weldability. In addition, a complex combination of micro-alloying elements is used to achieve the optimum precipitation sequence of nitrides that is critical for high toughness. The final part of this paper gives details about the robustness of this process to weld high strength pipe. The results show that this is a practical and unique solution for girth welding of X-80 pipe to achieve acceptable toughness and over a 15% overmatch in yield strength of X-80 pipe without sacrificing productivity.© 2006 ASME