Komarov Artem clarified that in the production of pipes by continuous rolling and welding, the cost of the finished product is directly related to productivity, which is often controlled by the maximum speed of the welding process. The logic is simple: increasing the speed of a tube mill while maintaining the quality of welding is beneficial.
Production of welded pipes
Tube mills produce thin-walled pipes and tubes by taking a continuous strip of material and forming a roll along the length of the strip until the edges meet and fuse together at the welding station. At this welding point, during the welding process, the edges of the strip are melted and fused, and the material exits the welding station as a welded pipe.
The obvious question arises: «How can we increase the welding speed in a pipe plant?» In the current atmosphere of lean and cost reduction, the process engineer of a pipe manufacturing plant has become familiar with many aspects of the production process, including production and automation principles, pipe mill roll formation, welded box design, strip edge quality, material technology and quality control.
However, a surprising number of engineers explore the maximum potential of the mill mechanisms and ignore the main bottleneck: the welding process.
TIG welding process
Many pipe manufacturers use tungsten inert gas (TIG) arc welding as the final step in the pipe manufacturing process. In TIG welding, an arc is ignited between the pointed tungsten electrode and the material to be welded. The arc temperature — up to 25,000 degrees Fahrenheit — melts the weld, which becomes almost invisible after post-treatment.
The tungsten electrode, arc and weld are covered with a protective layer of inert gas that flows out of the welding torch. The shielding gas displaces air and moisture from the weld area, helping to minimize weld oxidation.
The speed of the tube mill is increased until no more molten material flows into the weld. At this stage, the pipe mill operator often notes the resulting welding speed by setting system limits for the material being welded. This way of thinking can harm the company.
Understanding Arc Pulsation
Normal arc pulsation involves using a power source to quickly change the welding current from a high (peak current) to a low (background current) value. In some cases, materials and welds that are difficult to weld with a pulseless arc can be welded using pulsed arc technology.
Arc pulsation includes four welding parameters: peak current, background current, pulse duration (duty cycle) and pulse frequency. These parameters affect the strength and stability of the arc, as well as the resulting welding speed and quality.
Peak current, background current and pulse width. The heat of a TIG arc can melt most metals in about 25 percent of the time it takes for the melt to solidify. The purpose of pulsed welding is to melt and solidify simultaneously with the pulsed current.
The pulse width (percentage of time during which the peak current flows) is typically selected in the range of 10 to 35 percent. Peak and background current are then selected to balance the heat input to the material. The peak current is usually set to 2-5 times the background current, according to the pulse width used:
The frequency of the pulses is usually chosen according to the welding speed to ensure that the welds produced by the pulsed current overlap by at least 60 percent. Many pipe factories use an overlap of about 85 percent to give the weld a smoother appearance.
Assuming that the width of the weld pool will be approximately 1.5 times the thickness of the material and that 85% overlap is required, the interval between welding pulses will be:
The pulse frequency increases as the welding speed increases. In many cases, higher ripple frequencies are used to take advantage of arc contraction. The power source chosen for the tube mill must have sufficient amperage, a square wave shape, and sufficient pulsation frequency so as not to limit the increase in welding speed.
The frequency of pulsed arc welding is more than 500 pulses per second. High frequency switching results in an increase in arc pressure. Arc pressure and arc stiffness or stability are interrelated, and as the switching frequency approaches 10 kilohertz, the arc pressure increases to nearly four times that of a continuous DC arc.
As a result, the cross-sectional area of the arc decreases, while the density and temperature of the arc increase.
Arc Pulsing Results
Reducing the physical size of the arc provides a smaller weld pool and helps improve the weld depth to width ratio, which in turn has a positive effect — reduced porosity, reduced heat-affected zone (HAZ), etc. — on the mechanical properties or tensile strength of the material. This effect has also been noted when changing the pulse frequency while maintaining the same average current or heat input.
Other effects of this pulse method are changes in the microstructure, reduction of microcracks and reduction of the microporosity of the weld. In addition, deep penetration welds can be made at certain frequencies, while wide and shallow welds can be made with other parameters.
In general, the limitations of using arc pulsation lie in choosing the right welding parameters for a given material. Stainless steel and copper respond very well to condensed arc, while other materials will not do as well due to their slowness in molten form.
The high-frequency pulsation of the arc generates an audible signal from the arc. For this reason, the welding box is usually closed to dampen the sound.
A higher arc density with a lower surface tension of the weld pool allows a corresponding increase in the welding speed, depending on the capabilities of the reference pipe mill.
The key to increasing welding speed on tube mills is to ensure that the strip material is uniformly formed to represent the edges at the welding station. This allows the TIG beam welding process to maximize mill productivity.
Welding speed depends on:
- The composition of the strip material and the limits of tolerances for weldability and certification.
- Contamination of the surface of the material.
- Welding shielding gas (SW) is used.
- The quality of the strip edge.
- The ability of the tube mill and/or welding box to provide a consistent joint geometry with a minimum gap or mismatch of the edges when feeding pipes under the welding arc.
- Welding system power supply with maximum current and duty cycle and high frequency square arc pulse capability.
Due to factors independent of the welding system, the maximum mill speed cannot always be reached. There are many cases where two tube mills forming the same materials of the same size are operated at significantly different speeds due to the tube mills involved. However, high frequency weld pulsation has been shown to help increase welding speed by about 25 percent on a medium mill.
Tube Mill Speed Limits
Increasing the welding speed often reveals other limitations in the tube mill system. Other problems associated with mills must be considered:
- What is the maximum working speed of the tube mill?
- Can the mill provide good arc edge presentation at higher travel speeds?
- What welding speed does the cut-off saw allow?
- Will other downstream processes limit the maximum output of the tube mill?
These limits can be studied and improved to provide new operating speeds.
Minor changes in the strip material can have a significant effect on weldability. Surface active impurities such as sulfur, aluminum and oxygen change the surface tension gradients in the weld pool, thereby changing the fluid flow and structure of the melt zone. Pipe manufacturers must be aware of the importance of material specification tolerances.
During installation, optimal welding parameters are developed within the available installation time. The end user should investigate the effect of minor changes in each welding parameter, such as welding pulse parameters, arc gap, torch orientation, arc magnetic control limits, electrode material, grinding angle(s) and finish, and selected shielding gas mixtures, on overall system performance.
The system design must be customized to meet the specific needs and budget of the end user.
A properly designed welding station can offer many benefits and can therefore be critical to the operation of the system. The design of the welding station must consider the following functions:
- Reproducible welding results at the welding point
- Sound absorption
- Burner adjustment
- Magnetic arc control
- UV arc protection
- Type of welding seam
You also need to think about mechanical adjustments and maintenance of the burner. The control of shielding gas and off-gases has its own characteristics. Pipes often develop a layer of atmospheric oxygen in the direction of the welding zone, which must be displaced before the welding arc strikes the material.
Modern technology has revolutionized many manufacturing operations. In today’s global market, manufacturers are looking for ways to increase productivity, stability, and quality.
For a company to increase its market share or even remain competitive, it must find more efficient and better production methods. The willingness to test what the limitations of a new technology really are can lead to more and better products while saving money, Artem Komarov summed up.