TPT March 2022

AR T I C L E

Thermatool Corp

A path to a better high frequency weld By Mick Nallen, president/CEO, Thermatool Corp

The standard, simple equation for weld heat input for fusion welding applications (ie TIG) is very basic. For high frequency induction welding, the weld heat input formula is more complicated – with additions to the formula that include power, frequency, vee angle, vee length, and impeder performance. The primary method used to calculate HF weld heat input is based on process data recorded over time.

High frequency (HF) welding is proven to be a reliable, fast, and cost-efficient way of manufacturing tube and pipe. As the industry grows so does the technology behind high frequency welding. HF welding can achieve the best quality welds, but a lot of parameters must be precisely controlled. Fortunately, Thermatool has the technology to ensure high weld quality. The success of the welding operation depends on a lot of key process parameters such as power, frequency, vee length, vee angle and the material being welded. While power is the means of putting the heat into the work piece, frequency control is the tool to control the distribution of heat (power). Weld power Every high frequency (HF) induction welder output power is measured in kilowatts (kW) to an induction coil surrounding a closed continuously roll formed tube or pipe. The power output needs to be set accurately, not too high, not too low – to make a high-quality weld. From day to day, run to run, Thermatool HF Welders put out selected power (kW) with very high accuracy and precision, and hold each setting stable to within one per cent of the process set point. Weld frequency High frequency (HF) induction welder output power also has an AC sine wave frequency measured in kilohertz (kHz) or cycles per second. The welding frequency selected for a specified power output also needs to be set accurately, not too high, not too low – to make a consistently high-quality weld. Thermatool HF Welders produce weld power at a selected frequency (kHz) with very high accuracy and precision, with frequency stable to within one percent of the process set point. Weld heat input This is a result of the Joule Heating Effect, Skin Effect, Proximity Effect, and Thermal Conduction [1] . All of these electromagnetic phenomena are going to be influenced by the tube or pipe material properties; however, you can take advantage of these phenomena by adjusting weld power and frequency.

Welding Heat Input Required Formula H i = A * V * 0.06 s H i – heat input required in ampere volts min/mm A – welding current A in Amps V – arc voltage in volts s – welding speed in m/min

The relationship between voltage and current (amperes) is Ohm’s law V=IR. In other words, voltage is equal to current multiplied by resistance. Resistance is determined by the geometry and material of the tube or pipe being welded. Always remember, current does the heating work. Mill speed adds in the time component, in direct relation to the fusion welding heat input calculation. Vee angle Recommended at 4 to 7 degrees and dependent on use of welding frequencies over 200kHz to take advantage of proximity effect. Vee length Recommended at one times the diameter of the tube or pipe being produced, this number is highly variable depending of the weld box design used on the mill. The forge weld roll diameter normally determines the weld Vee length, and in general small diameter weld rolls always result in lower weld heat input values.

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MARCH 2022