Welding is a Fundamental Technology – Image Courtesy of the United States Air Force Retrieved From https://en.wikipedia.org/wiki/File:GMAW.welding.af.ncs.jpg
Radically Shifting Welding Landscape
Welding is a fundamental technology without which the manufacturing industry cannot operate at the current levels of sophistication. Properties of weld joints, rather than those of the welded materials, often determine the performance of a structure.
Like most other technologies, welding is steadily evolving. Research has significantly altered the landscape of welding with transitions over the past two decades being particularly noteworthy.Arc welding with conventional machinery and work practices still remains the main method of connecting heavy plates that make up large steel structures. Narrow gap welding, high heat input, and automatic welding are boosting the efficiency of arc welding.
Laser welding is exhibiting greater application for connecting thin steel sheets. With laser welding utilizing power sources of greater capacity, lasers are also joining heavy plates.
The Transformations in Welding
Please note, there is some degree of overlap in the account that follows because developments in composition and form of welded materials are inherently linked to advances in welding processes and technologies. As such, it is not possible to completely segregate them.
Evolution of Welding Materials & Processes
Revised requirements are bringing about a transformation in the composition and structure of materials used for welding. The processes, personnel, and equipment of welding are evolving to keep pace. We analyze here the changes under the following heads:
- Composition of Weld Materials
- Structure / Form of Weld Materials
- Sector-Specific Changes in Welding Processes
- Welding Process
- Welding Equipment
- Composition of Weld Materials
Researchers have developed materials with novel compositions:
- thicker, high-strength steel plates for colossal container ships as such plates enable higher transportation efficiency
- eco-friendly steels and structures capable of delivering hyper efficiency even in the punishing Arctic winter
- weatherproof, nickel-containing steels resistant to saltwater corrosion for the construction of coastal bridges and other marine structures
- heavy plate, fire-resistant steels that maintain strength at elevated temperatures
- high-strength but thin steel plates for bridges
- corrosion-, acid-, and wear-resistant steels and other alloys
- high-toughness steel that resists deformation for buildings in earthquake-prone areas
- eco-friendly sheets of high-tensile steels for the automobile industry deliver better transportation efficiency
Stainless Steels score over low-alloy steels on numerous fronts. Research looks to replace austenitic stainless steels with ferritic ones in response to the rising cost of nickel and molybdenum. It also aims to further improve their corrosion resistance and customizability.
S-TEN1 is a steel resistant to hydrochloric and sulphuric acid. Industrial machinery demands such steels.
Titanium offers the merits of exceptional corrosion resistance with high strength and low weight. Applications of titanium are expanding and it is now available as thin sheets, welded pipes, heavy plates, and wire rods.
Heavy titanium plates find application in shipbuilding and offshore structures. These are welded using Electron Beam Welding and Metal Inert Gas (MIG) Welding in addition to processes used earlier viz. Plasma Welding and Tungsten Inert Gas (TIG) Welding.
A common thread here is the increased use of high-strength steels. Stress concentration at the weld toe and tensile residual stresses bring down the fatigue strength of welds. And, you cannot improve the same by hiking the strength of base metals.
This is a huge issue for welding high-strength materials. Barriers for coherent welding of high-strength steels and remedies include:
- Heat Affected Zone (HAZ) Embrittlement because the carbon equivalent of steel increases with its strength causing stress concentration at the weld joint
Controlling the steel composition, post-welding cooling rates, and selecting the proper flux addresses this issue
- Decreased Fatigue Strength of the Weld Joint is countered by introducing compressive residual stress at the bead end and expanding the bead end radius through re-melting
- HAZ Softening as the heat of welding tempers out martensite and bainite. The softened portion fails under tensile load. Use of spot welding and softening-resistant steels checks such softening
- Hydrogen Embrittlement is the trapping of hydrogen inside the weld pool that makes the joint brittle
Research for boosting HAZ toughness has traditionally focused on improving its microstructure. HTUFF steels use titanium oxide, magnesium oxide, and nitride to improve the HAZ microstructure making them useful for high-heat-input welding. Shipbuilding and building construction use such steels.
Instead of improving the HAZ toughness, the Non Brittle Fracture Welding (NBFWTM) method utilizes innovative pass sequences and heat input techniques in the final layer of multi-layer welds.
The method minimizes stress concentration in the weld toe by controlling the shape of the smooth weld bead. And hammer peening introduces compressive stresses to counter the tensile residual stresses.
- Structure / Form of Weld Materials
Advances in the form of material used include:
- Heavy Plates were designed to provide better strength and toughness, higher operational efficiency, lesser fabrication costs, and superior resistance to corrosion and fatigue while saving labor.
Since the 1980s, the Thermo Mechanical Control Process (TMCP) has helped make heavy steel plates with 490 MPa tensile strength, high weld-ability, and a carbon equivalent similar to that of mild steel.
Toughness decreases near the weld zone for steels with greater content of carbon or alloying elements because austenitic grains become coarse. High heat input aggravates such coarsening and gives a brittle joint. TMCP gives high-strength steels with low carbon and alloying elements.
- Thin Sheet steels were developed to address twin requirements of the automotive industry – lower weight of automobiles without compromising on passenger and pedestrian safety.
High cost and poor workability of aluminum and magnesium sheets limits their utility. And so evolved high-strength steel sheets with tensile capacity of 590 MPa or even 1,470 MPa.
III. Steel Pipes find application in numerous industries. Except seamless pipes, all other pipes viz. spiral welded, butt welded, electric welded, and UOE (uing and oing pipes) are manufactured by welding.
UOE makes large diameter, longitudinally welded pipes – plates are first formed into a U shape and thence into an O shape before welding. Developments include:
- workable and ultra heat-resistant ferritic stainless steel for exhaust manifold pipes
- corrosion-resistant steels for muffler pipes
- super-austenitic stainless steel pipes for water desalination plants and for plants that process salt-containing foods
- high-strength and high-corrosion-resistance steels for boiler pipes
- S-TEN1 that resists sulphuric and hydrochloric acid for pipes in acidic environments
- Sector-Specific Changes in Welding Processes
Sector-wise revamps include:
- Shipbuilding is moving towards the use of large, heavy plates of YP390MPa steels of thickness 50 mm or more. Such plates improve the transportation efficiency and speeds of container ships whose sizes are expanding by the day.
Single-Pass Submerged Arc Welding (SAW) is prolifically used for welding these plates. A novel technology for supplying high heat input without coarsening the weld grain structure is the Single-Pass, Two-Electrode, Electrogas Arc Welding method.
Container ships and high-rise buildings are employing thicker, 80-100 mm steel plates. Welding processes that utilize high heat input and multiple passes have come to the fore to weld heavy gauge plates viz. YP460 in shipbuilding and HBLTM440 and HBLTM385 in construction.
Electro Gas Welding (EGS) with two electrodes is one such process and is increasingly employed to weld hatch side coamings and other similar components on ships. CO2 Arc Welding is used to weld longitudinal stiffeners.
EGS requires high heat input and sophisticated technology while CO2 arc welding is expensive and time consuming. Narrow Gap Welding Technologies evolved to overcome the limitations of both. These provide deep penetration without requiring high heat input.
J-STARTM Welding is one such process. A CO2 arc welding process, it facilitates droplet transfer through a spray to address these requirements. The process utilizes electrode negative polarity (EN) instead of the usual electro positive (EP) polarity.
What lends high economy and efficiency to the J-STARTM Welding process is the fact that it can execute narrow gap welding while only using conventional welding power sources.
- Pipelines and Hydraulic Power Plants prefer X-120, a low carbon steel with fine acicular ferrite, low bainitic structure, and good weldability. High tensile steels with 780 MPa and even 950 MPa strengths are also used.
Pipelines have to be more eco-friendly, transport oil from deeper waters, provide greater transportation efficiency, and deliver high operational levels to the energy sector.
Single-Pass SAW with large heat input makes pipes from X-120 sheets and MAG Automatic Welding connects these pipes on site. Welding high-tensile steels requires pre heating, high heat input, and consumables that prevent low temperature cracking.
Welding pipes demands processes capable of welding thicker, high-tensile steels. And merely hiking the heat input does not work for it lowers the toughness of the HAZ.
Engineers now use SAW with Small Diameter Wire to comply with such requirements. This method provides better deposition efficiency and penetration without increasing the current. The high-current, high-speed SAW with multiple electrodes is no longer used.
III. Marine Structures are now expanding in size and making deep inroads into the bone chilling cold of the Arctic. High-strength steels with the ability to resist ultra cold environments are needed here and so are the aforementioned welding processes to join them.
- Bridges prefer high-strength steel as it minimizes deadweight. But you cannot reduce the thickness of even such steels for fear of failure from fatigue brought on by variable cycles of loading-unloading – traffic load varies. Steels of 570 MPa or even 780 MPa are used here.
Welding such steels requires processes that provide high heat input without substantial preheating. Evolution in welding processes requires changes in consumables as well. Copper precipitation enables proper welding of high-strength steels.
- Tanks and Pressure Vessels require steels with corrosion resistance, acid resistance, and high strength at high temperature. SCMV4 and A387 Gr. 91 provide these properties up to 5750C while (9-12)Cr-Mo(-W) do so up to 6000C.
These steels contain chromium and molybdenum that harden easily. Pre-heating and post-welding heat treatment prevents their low-temperature cracking.
Austenitic stainless steels such as NF709 and XA 704 serve as pipe material in boilers because they can withstand inter-granular corrosion for up to 7000C.
Resistance to fractures and their propagation is an important qualification for low-temperature tank steels storing LPG and LNG. Designers are now using 50 mm thick plates for such tanks instead of the earlier 30 mm.
Aluminum-killed steels such as SLA under JIS G 3126 and nickel steels such as SLA under JIS G 3127; and 2.5%-, 3.5%-, 5%-, and 9%-nickel steels are useful here. These require nickel-based consumable systems for welding.
- Automobiles now need lightweight materials that do not contain environment damaging materials such as lead. High-tensile steels are therefore replacing mild steel.
Stringent standards in the automotive industry require the weld-ability of high-tensile steel to be at least as much as that of mild steel to ensure equivalent joint strength.
This has triggered changes in the arc, spot, seam, and projection welding, and soldering processes the industry has conventionally used for joining sheet metals.
The industry now prefers Laser Welding and Laser Brazing because these offer better work efficiency, higher compatibility with continuous welding, and easy one-sided welding of closed section components.
While laser welding is fast and provides excellent bead appearance, it requires accurate gap control. Laser welded joints also suffer from low reliability. And although arc welding allows large gap tolerance, it gives poor bead appearance.
JFE Steel’s Intelligent SpotTM Welding alters welding current and electrode force when welding automotive parts. This is in contrast to the conventionally used constant current and force.
Such flexibility provides the necessary nugget diameter and weld strength that ensure correct welding of three-sheet lap joints made from high-strength steels that are increasingly used in automobile construction.
JFE Steel also developed the Pulse SpotTM Welding technology. It does not allow a fall in the cross tensile strength of weld joints between very high-strength steels. It uses a high-voltage, short-duration current to control strength distribution and segregation of weld joints.
In the near future, laser welding may replace resistance spot welding in the automobile sector. This is a welcome possibility for laser welding can deliver continuous weld joints vis-à-vis discontinuous spot / point joints given by resistance spot welding.
VII. Construction and Industrial Machinery requires corrosion and wear resistant steels with high tensile strength such as HT950 and HT590. Wear-resistant steels are however prone to weld cracks. Innovative consumables address this issue.
- Welding Processes
Process-specific additions in welding processes according to the 2006 paper Recent Developments in Welding Technology by GS Booth et all. cover the following processes:
- Advanced Arc Welding
- Friction Stir Welding (FSW)
- Laser Welding
- Advanced Arc Welding is preferred for welding offshore structures mainly because you can use it on-site. There has not been any fundamental change in arc welding technologies, but developments have boosted the efficiency and productivity of existing processes:
- Inexpensive Activating Fluxes for Gas Tungsten Arc Welding (GTAW) improve the arc’s penetration when applied on the joint before welding
Fluxes restricts the areal spread of the arc thereby boosting its current density and penetration vis-à-vis the broad, shallow arcs of conventional GTAW
Stainless steel, C-Mo steel, C-Mn steel, and nickel-based alloys are welded using these fluxes. The industry did not adopt them earlier because of their high cost, operational complexity, and inferior surface finish
- Fresh Keyhole Plasma Welding Procedure has successfully overcome the earlier cost, reliability, and control issues with keyhole plasma welding and allowed fabricators to harness the high-speed and deep-penetration potential of this process
This has also enabled welding of alternative joint types in 2.5mm thick austenitic stainless steels not possible with GTAW and gas metal arc welding (GMAW). All in all, this procedure has lend better flexibility for the design and production of thin sheet parts
Although the laser beam process offers the same benefit of low distortion in a greater measure, the improved keyhole plasma welding procedure comes with lesser investment
- Automated Robotic Welding of Mega Structures is a consequence of welder shortage and the requirement for increasing customization and shorter delivery schedules
Noteworthy is the European framework project named NOMAD – Autonomous, Flexible Robot for Welding Manufacture of Large Steel Structures. It developed an autonomous robot in 2004-05
This robot successfully moved around large, stationary structures of 5-50 ton weight and completed weld joints without jigs, fixtures, special tools, or dedicated handling equipment
Offline programs create path data while laser sensors detect the start and end positions. The robot can also weld numerous structures in offshore settings
Also developed over the past two decades are robots capable of narrow gap welding of heavy plates and on-site girth welding of gas pipelines
- Friction Stir Welding (FSW) was developed in 1991. FSW is now applicable for materials other than aluminum, various steels in particular. This makes it useful for the offshore industry.
Totally mechanized with repeatable quality, it is free from the defects of porosity and hot cracking because of being a solid state, non-fusion process.
It does not require filler metal, is energy efficient, and does not generate spatter, fumes, and radiation. However, the rotating probe leaves an exit hole. Then again, the process requires sturdier fixtures and greater control over the joint fit up.
Choice of the tool material is critical for the tool is subjected to extreme stresses and temperatures of around 12000C. Materials presently in use include ceramics, particularly polycrystalline cubic boron nitride (PCBN), W-Re based refractory material, and alternative ceramics.
III. Laser Welding offers deep, narrow welds. This boosts speed, quality, joining rates, design and production flexibility, and reproducibility while minimizing distortion, consumable costs, and manning levels.
Nd:YAG, CO2, and Yb:YAG Fiber Lasers are used for industrial welding. Hybrid Laser Arc Welding that combines arc welding with Nd:YAG, CO2, fiber laser or diode offers better penetration, traverse speed, heat input, tolerance to fit up gap, and energy coupling while lowering hardness.
All laser welding processes require close control over steel composition and fit up. Hybrid welding requires setting up of fresh parameters to make the two constituent processes mutually compatible.
- Welding Equipment
Following is a short description of the progress in welding equipment and training:
- Transformer Machines to Invertors: some of these invertors are fool-proof, an important attribute because many in the welding industry regard conventional welding as a rather uncontrolled process.
This is because there was too much dependence on the welder – there were procedures in place but it was the welder’s discretion whether or not to follow them.
- Programmable Weld Machines eliminate the time, cost, and effort spent in guiding welders with simple tasks such as setting the correct feed rate and welding voltage for a fresh task.
By programming the welding setup for diverse tasks, you also nip in the bud the possible human errors and their lethal consequences. And there are many to speak of.
And the fact that the U.S. manufacturing industry is plagued with a severe shortage of skilled welders as also of skilled machinists means any automation is more than welcome.
III. Virtual Welding is a simulation training system for rookie welders. It has a replay function that even seasoned professionals find useful for refining their skills and detecting minor flaws that may have subconsciously crept into their techniques.
Broadly speaking, virtual instructors guide the trainee on weld parameters such as tip-to-work distance, optimum welding speed, and the torch / electrode tilt angle.
Simulators also create welding noises to make the training as real-life as possible. Traffic signal-like lights provide feedback on the trainee’s performance and allow immediate course correction.
Industry leaders have welcomed virtual welding on account of its numerous inherent merits. One, the trainee gets a feel of the process without the trainer interrupting him for course correction.
Then, you can measure the results, get consistent high quality training, and save on resources cost. The recording function notes the entire welding sequence including correction notes for future reference. Virtual Welding somewhat simplifies the onerous task of training fresh welders.
Because it joins materials at the molecular level, welding is an unparalleled joining process. Recent developments further stamp its authority over the manufacturing world.
Visit our blog for more such updates on welding.