Any welded assembly requiring resistance to externally applied forces or moments but not intended to resist pressures above 103 kPa (15 psig) and requiring resistance to externally applied forces or moments is considered a structure.
Structural welding has progressed, starting at its original application in the building and bridge industries to a wide spectrum of industrial uses. Fabricationsas diverse as offshore oil and gas platforms, gain silos, missile launchers, crane booms, and amusement rides depend on welding for strength andintegrity.There are threeessential phases to any engineering and construction project: design, fabrication or erection, and inspection.
Structural Design- Design typically involves the arrangement of structural elements to adequately resist external loads.A major aspect of design therefore involves the calculation of stresses and sizing of members and connections. Computer models and hand calculations, used in conjunction with design specifications, are used by the structural engineer to accomplish this task. An additional aspect of design that is important to the structural engineer,fabricator and erector is the selection of base metal. The engineer should be aware of several properties necessary for adequate service and construction qualities.
Construction Steels- Yield strength is the property of most interest to engineers. A steel’s yield strength primarily depends on its carbon content, alloy content, heat treatment, mechanical processing, or any combination of these. High carbon or alloy content can produce steel with high hardenability, which can result in hydrogen cracking.
This susceptibility can necessitate special welding requirements, such as high preheat and interpass temperatures or even postweld heat treatment. The “conventional” construction steels, such as ASTM A36 and ASTM A572 Grade 50, have an excellent history of ready weldability when welded in accordance with prescribed limits, such as are found in the ANSYAWS D1.l, Structural Welding Code-Steel. Such steels use carbon or low alloys (such as vanadium) or both to provide strength in the as-rolled or normalized condition.Modern steel making technique shave created a new generation of steels with optimum weldability,because of low carbon and alloy contents. In order for such steels, such as ASTM A841 ASTM A913 or API ZW, to achieve the strength levels expected of the more traditional construction steels, controlled rolling and thermal processing (i.e.,thermomechanical processing) are used in the steel millto finish the wrought material. Weldability of the materials, as measured by the irrelatively low hardenability, and resistance to hydrogen cracking, is excellent; low preheat is required (or frequently none) to produce high yield strength welds with excellent notch toughness and low cracking sensitivity.
While weldability and yield strength are always considerations for any type of welded structure, notch toughness typically is a primary concern for dynamically loaded structures. Dynamic loads can range starting at low stress cyclically applied over a long period of time to a single impact at high velocity. Bridges and offshore platforms exemplify the first category, making fatigue a primary consideration in design. In order to minimize the probability of fracture under dynamic stress and in the presence of a discontinuity, a specified notch toughness is usually required for base metal, filler metal, deposited weld metal, base metal heat-affected zone (HAZ) or any combination of these. Base metal notch toughness is a metallurgical property usually obtained through control of chemistry, deoxidizing (killing), heat treatment (e.g., normalizing), or thermomechanical processing. Filler metals usually rely on alloys, such as nickel, for enhancing toughness as demonstrated by the electrode manufacturer’s certification tests.
For deposited weld metal and the heat-affected zone (HAZ), a filler metal and base metal with a specified notch toughness obviously must be employed, but technique also becomes important in ensuring that the fused joint has the required minimum toughness. This is demonstrated in the welding procedure specification (WPS) qualification test. Typical techniques include low heat input and the use of small weld beads that temper or grain-refine previously deposited passes.
Although the science of fracture mechanics has introduced mathematical rigor to the subject of fracture resistance, the Charpy-V notch test remains the method of choice for assessing notch toughness. Typically expressed as impact energy required for specimen fracture at a specified test temperature, Charpy V-notch values are a qualitative assessment of toughness.
It is important for designers to understand the relative nature of these values; whereas it can be stated with confidence that a material tested at 27 joules at -18°C (20 ft-lbs at 0°F) is tougher than a material tested at 27 joules at 21°C (20 ft-lbs at 70°F), it would not be appropriate to assume that the 27 joules at -18°C (20 ft-lb at 0°F) material could successfully resist an impact of 27 joules at -18°C (20 ft-lb at 0°F) in actual service. This is because several factors influence notch toughness which are not accurately reflected in the Charpy V-notch test. Temperature, loading rate, severity of stress risers and degree of restraint against plastic flow are all key factors in degrading fracture resistance, and it is the interaction of these factors that is critical to this property.
Fracture mechanics has attempted to rationalize these factors through the use of formulae and tests, which are generally expensive and time-consuming.
However, even fracture mechanics methods cannot exactly replicate actual service conditions. The engineer must still use judgement in assessing how much toughness is adequate for any given application.
Service Environments- The bridge and offshore industries have long histories with their particular structures and dynamic loading environments; therefore, the incidences of brittle fracture that occur now in these applications are usually due to poor design details or fabrication that allows severe stress concentration sites for cracking to initiate. Until the Northridge, California, and Great Hanshin (Kobe), Japan, earthquakes of 1994 and 1995, respectively, steel moment frame buildings designed to resist seismic loading had a flawless track record of fracture-free sirvivability. However, these two earthquakes severely undermined confidence in many assumptions about seismic structural design, material properties and testing methods.
Many welded connection and base metal fractures were observed in buildings, though no fracture resulted in structural collapse. As a result, extensive research and debate continues as to the best method of resisting fracture in a seismic event. Earthquakes, representing nature’s power at its most awesome, will continue to challenge all assumptions and predictions. However, the performance of welded steel structures, while also not conforming to predicted behavior, have succeeded in their most vital requirement, the preservation of human life. In comparison to structures of other materials that collapsed and did result in deaths, welded steel is still viewed by many as the optimum seismic material.
Material Selection- Though steel constitutes the bulk of structural metal used by industry, aluminum is the second most popular structural metal, primarily because of its low weight to strength ratio and resistance to corrosion. The marine and aerospace industries in particular find welded aluminum to be an attractive alternative to steel. Alloys such as 6061 are readily weldable and frequently used in the as-welded condition, even though the joint strength is less than the base metal strength. When higher strengths as well as corrosion resistance are requirements, stainless steel competes with other metals such as nickel and titanium. These metals have their own peculiar weldability requirements that challenge engineers and fabricators. Other materials may be introduced into the structural markets of the future, posing fabricators with the challenge of welding with a variety of processes.
Welding Processes- Process selection is, in fact, a vital concern of fabricators faced with production and quality demands as well as the need to control costs. In the past, the flexibility offered by the SMAW process made it an overwhelming favorite for shop, field and repair welding alike. Although the cellulosic and rutile SMAW electrodes (e.g., E6010) have traditionally been popular for their contribution to weldability, the large quantity of diffusible hydrogen contained within their deposited weld metal can promote hydrogen cracking unless strictly controlled.
With the productivity improvements made in automatic and semi-automatic processes, the popularity of shielded metal arc welding (SMAW) has declined significantly. Shop welding provides an ideal environment to make the gas metal arc welding (GMAW) and flux cored arc welding (FCAW) processes popular for welding sheet steels as well as structural thicknesses, 3.2 mm (0.125 in.) and up.
Thick materials, on the order of 50 mm (2 in.) and greater, are more efficiently shop welded with the submerged arc welding (SAW), electroslag welding (ESW) or electrogas welding (EGW) processes. The ESW process offers the highest potential productivity, but extra care must be taken to avoid mid-welding stoppages and excessive HAZ grain growth. While SMAW is still frequently used in shop welding, its use is declining for general production welding.
Field welding, with its exposure to wind and differing accessibility situations, limits the process types available to a contractor. Here again SMAW has been the process of choice and remains popular; however, the improved reliability, productivity and portability of the FCAW-S process have made significant inroads into SMAW’s popularity. The self-shielded flux core process is, in fact, very often selected in the building and marine industries because of its high arc duty cycle, high productivity and lower defect rejection rate than SMAW. Repair welding remains the one area where SMAW will probably maintain its popularity.
Procedure Qualification- Qualification of welding procedure specifications (WPS) and personnel (welders, welding operators and tack welders) is an essential aspect of any fabricator’s quality assurance and quality control (QNQC) program. Fabricators can economize by promoting the use of prequalified WPSs which are mechanical testing exempt when performed in accordance with ANSUAWS D 1.1.This requires staying within prescribed limits which could limit a fabricator’s maximizing productivity. It is frequently more efficient to take the expense of qualifying a WPS in order to use parameters that are more productive than permitted for WPS requalification.
Welder Qualification- Whereas WPS tests are intended to demonstrate metallurgical and mechanical compatibility between base metal and filler metal, personnel qualification tests the welder’s skill and competence to deposit sound weld metal. Qualified personnel are the first line of defense against welding defects. Fabricators can benefit by ensuring that their welding personnel are well trained for their job function.
Inspection- Inspection occurs before, during and after welding to ensure conformance with contract requirements (e.g., drawings, specifications). The owner of a structure may choose to select a verification inspector, who is typically a third party agency, to oversee a fabricator’s work. The fabricating company will have its own inspectors to supervise qualifications, material certifications, joint fit-ups, electrode and base metal preparation, and all other activities required to deliver a quality product.
Inspection is sometimes an area much neglected by engineers, who should take into account the diversity of available inspection methods. The engineer should ascertain prior to bid document release what kinds of welds require which type of inspections. Typically, critical connections subject to tension require a nondestructive testing (NDT) method suitable for probing below the weld surface. Radiographic testing (RT) and ultrasonic testing (UT) are the most popular methods for this task, with RT competing with UT in the shop environment, but with UT being overwhelmingly popular in the field. Both methods can detect discontinuities within the volume of the weld, through visual indications on exposed film (RT) or acoustic reflections displayed on a screen (UT).
For less critical connections, such as joints in compression or shear, surface NDT methods are less expensive and easier to implement. Magnetic particle testing (MT) and liquid penetrant testing (PT) are the preferred methods, though MT is limited to steels with a predominantly ferritic or martensitic microstructures. Only surface or near-surface discontinuities are visually detectable, but since it is usually surface defects that result in crack initiation, these NDT methods are adequate for non-tension welds.
Repair welding is necessitated when unacceptable discontinuities are discovered by the inspection. If the Engineer of Record refuses to accept the defective weld, gouging out of the offending discontinuity is required, followed by cleaning of the gouged area, rewelding and reinspection. However, it is always the Engineer’s prerogative to accept a Code-defective weldment if the intended design application is not adversely affected. Frequently, the prospects of successfully rewelding are poor, and this may influence the Engineer’s decision.
Fitness-for-purpose methods are available that attempt to couple the NDT-detected discontinuities with fracture-mechanics analyses to ascertain crack sensitivity. Since many codes, such as ANSYAWS D 1.1, are primarily workmanship-based rather than service load-based, requirements that are not complied with in construction do not necessarily imply that a weld will not adequately resist their design loads. It is up to the Engineer to determine this acceptability if he or she so chooses. Otherwise, Code requirements prevail and potentially costly weld repair will be required.
Cost and Structural Integrity- Economy is an important factor in any engineering project, of course, but the Engineer’s, primary concern is structural integrity and safety, reflected by adherence to quality requirements. It is the harmonious marriage of these virtues that make structural welding such a popular method of providing civilization with safe, affordable structures within which we can work, play and enjoy life in general.
References- Specific information on structural welding is available through the American Welding Society (AWS) in its document, ANSVAWS D1.l-96, Structural Welding Code-Steel. Computerized access to this document is available on a compact disk, (CD- ROM), and one version includes full-page images of the 25 AWS publications referenced in the document.