What’s in a Weld?
To fuse metal by the application of heat is the simple definition of welding. When the pieces of metal become boom sections stretching 200-feet over a jobsite, the welding process becomes ultra critical. Because welding is a broad discipline, it’s good to know the procedures that apply to concrete pumps. Knowing your concrete pump is welded together properly is the peace of mind you deserve from a manufacturer.
Several things that make a good weld start before man or machine strike an arc. Proper heat is essential to a good weld. The two pieces to be joined are actually bonded by melting the metal to be joined and then providing filler material to any voids in the weld. The quality of the steel must be checked to assure that the welded joint will meet the strength requirements. Two pieces of steel welded together properly can result in higher strength than a single piece of steel.
Good welds start with good welders, so experience counts. And repeatability is the watchword in manufacturing so inspection of the finished welds is critical to maintaining quality standards. Responsible manufacturing means that welding inspection and quality control are maintained and documented because the stresses are high and the margin for error slim.
With rigorous certification requirements and strict regulations as their guidelines, responsible manufacturers maintain the highest industry standards. Continuing education and recertification every two years is a desirable standard because of the importance of the work.
At Schwing America’s White Bear, MN plant, the process begins the moment the raw materials hit the holding area in the weld shop. First, because all material is purchased in accordance to specific material standards, information regarding mill and heat lot origin must be checked, verified and recorded for traceability. At this point, Quality Department inspectors conduct several non-destructive tests to assure the proper chemical components are present.
“If you’re striving for a top quality product, you start with the best materials,” says Schwing America Weld Shop Manager Steve Donohue. “Our inspectors assure that we’re doing just that.”
The raw material is fabricated into the many piece parts that will eventually become the subframe, boom pedestal and outriggers of a finished boom pump. The parts are also inspected by the weld shop Quality Department and verified against the prints for all features including material types, size, bevels, and bend lines. After piece parts have passed inspection, they are labeled with a steel plate containing information regarding material type and in-house information for proper processing in the Weld Shop where certified welders and regularly calibrated machinery turn parts into state-of-the-art machines.
Tack welds, notch-free corner welds, and notch-free butt welds are the three main processes utilized to produce a structurally sound concrete pump. Tack welds are utilized to produce enough of a bond to hold two pieces of steel together before another process is introduced. Notch-free corner welds occur where two piece parts are joined at an angle. Both of these types of welds are utilized to create the box-like sections that will eventually become a boom or housing for a telescoping outrigger.
Schwing welders who match certification and training with programming knowledge have the ability to operate and supervise fully mechanized welding equipment. To complete the notch-free corner welds, two Weld Shop employees work with the manufacturer of a mechanized robot to determine the rate of speed and other factors that will ensure quality corner welds. The welders operate and supervise the robot as it moves within the subframe creating a seamless bond and a structurally sound weldment.
Donohue recalls past methods of welding parts like the outrigger housing. “It used to be that someone would have to crawl inside and make all four welds from in there.” The robot not only increases job safety and production; it also eliminates the potential for human error and achieves a strong, quality corner weld. The use of robots and accurate fixtures assure dimensional repeatability and ease of welding, so that each piece part is manufactured the same way.
Butt welds are needed when two pieces of steel meet to form a consistent, single sheet. To assure a quality, notch-free butt weld, piece parts are cut off at a 30-degree angle to allow more surface area coverage during the weld. This is an art with welding done by hand. A concave support piece allows the weld to build-up on both sides of the union.
According to Donohue, the number one boom section on every Schwing concrete boom pump undergoes 17 processes within the weld shop, each of which is inspected, stamped and recorded through electronic storage or hand written reports for traceability. Over the course of construction, each weldment is stamped two to three times by the welders themselves and any number of department inspectors.
“We manage this shop through stringent quality control, but also by maintaining an environment where questions are welcome and suggestions are always considered.”
According to Donohue, an open forum for questions and suggestions promotes improvement in both production-oriented and industrial engineering areas. It also promotes a real passion for the product and the technology that enables them to create it.
In order to meet the highest industry standards, Schwing’s welders are required to undergo recertification every two years for every type of welding available, including fully and partly mechanized welding procedures as well as those requiring tools such as active gas and covered electrodes.
To meet that same high bar, quality inspectors are recertified in two-year increments and are tested in all types of non-destructive testing. American Welding Institute certification is also required every nine years.
Even the machinery, such as Weld Shop robots, aren’t immune from the watchful eye of an inspector. All measuring and testing equipment used for manufacturing is cleaned and calibrated on an annual basis.
Schwing Quality Assurance and Engineering departments undergo recertification testing through the German Welding Institute once every three years. The department is inspected for welding standards procedures and material types. Working weld tests are also conducted annually to test all operations, materials and personnel.
Despite reliable employees and accurate equipment and fixtures, Quality Department inspectors are responsible for testing all materials and weldments as they make their way through the processes. Even during final machine inspection, employees conduct several different types of non-destructive testing including visual, ultrasonic, dye penetrant and magnetic particle processes.
Ultrasonic testing involves the use of high frequency sound waves to test for material density and soundness. This type of testing is most rigorously conducted on butt welds to insure there are no imperfections in the materials or the finished weld.
In dye penetrant processing, the visible dye is applied to the surface of the part and left on the surface for a period of 10 to 30 minutes. Excess penetrant is removed from the part surface, and developer powder is used to indicate material deficiencies.
The magnetic particle detection process involves the use of a colored iron powder, which is applied to the inspection area. A high-powered magnet applied from the opposite side is used to attract the powder. Indication of a crack or other structural damage is apparent when the powder is attracted to a void.
To test the quality of Weld Shop work, responsible manufacturers also conduct destructive testing on welded joints. These tests serve as a type of internal quality control, assuring that each step of construction has been executed to the highest standards.
When considering your next boom pump purchase, it is important to educate yourself about a manufacturer’s welding procedures, certifications, inspections and testing within their weld shop. As contract specifications call for longer booms, higher output and smaller footprints, this knowledge is your peace of mind for a productive day from your pump and people.