TPT November 2007

Technology Update

predict weld quality through changes in power and frequency to ultimately improve profitability for the company through a repeatable superior weld. For instance, when a mill operator increases the mill’s speed, the model will calculate what changes are necessary in power and frequency in order to maintain the same HAZ characteristics. One way these changes can be communicated is through an operator display providing graphical representation of the two descriptive qualities of the HAZ, namely weld heat, depicted on the Y-axis and HAZ width, on the X-axis. Software can then be used to graphically illustrate the method by which an operator can achieve a high quality weld. The ideal, desired HAZ, as defined through experience and past welding history, is specified to be at the X, Y origin or ‘best weld’ location. During normal operation the actual HAZ characteristics are located on the graph. The operator can then adjust both the frequency and the power to bring it to the origin. For example, as the weld changes, whether due to an increase in mill speed or as a result of the slow decay of an impeder, the actual weld indicator will begin to move away from the central ‘best weld’ location and begin to show a widening or a narrowing of the HAZ width and a lowering or raising of the applied heat, ie weld power. The mathematical model calculates and displays the ‘best weld’ location, thereby giving the operator a road map to adjust welder power and frequency. The welder operator will be able to restore the ideal HAZ characteristics by aligning the target with the centre of the HAZ display. The simplicity of the user interface helps the operator understand how minor changes at the mill can greatly influence the quality of the end product. This enables the operator to make the best weld possible with the highest level of repeatability.

€ From research to production – the heat affected zone (HAZ)

Taking a look at these together, one can see that the HAZ is described and effectively controlled by the following parameters: material characteristics (melting point, permeability etc), mill setup (vee length), mill speed, power, and frequency. This is not, however, an exhaustive list of variables. The mechanical attributes of the electrical reference depth and the thermal reference depth are, to some degree, intuitive. In practice, however, the relationships of these variables are complicated. Occasionally operators will attempt to modify these reference depths by modifying a single variable, such as vee length. Unfortunately this practice is very limiting and sometimes reveals very sensitive aspects of the HF weld setup. In HF welding, the shortest possible vee length is desired to maximise welding efficiency. A short vee length also minimises ‘vee breathing,’ the mechanical process variation of the vee angle during HF welding. As much as possible a company should resist variation from their ideal vee length. In effect, the best way to adjust the quality of a weld is not through physical changes of vee length or mill setup, but instead through electrical means like adjusting frequency and power. If the goal was to increase production by 10 per cent, how should one best modify frequency and power to achieve the same desired weld at the increased mill speed? Computational software can be utilised to solve this problem. After the operator enters basic mill setup data (vee length, diameter, and wall) a model can help

The area of the base metal which has had its microstructure and properties altered by welding is called the heat affected zone (HAZ). If one were to remove a cross section of welded pipe the HAZ would appear as a discoloured hourglass at the weld point. The first step to understanding how to control the HAZ is to learn how to describe it. This hourglass shape can be best described with the use of two values: vee apex heat and HAZ width. Vee apex heat is the heat energy at the centre of the tube wall at the point of welding measured in Joules/mm 2 . HAZ width is the physical width of the heat affected zone (the width of the HF weld hourglass) and is determined by HF welding frequency and time. These weld parameters are characteristics of both HF current distribution (frequency) and thermal conduction (time). HF current distribution is essentially how deep an induced current penetrates the strip edge and is calculated from the electrical reference depth. This is a function of material resistivity, frequency, and material permeability.

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Electrical reference depth =

As the heat from induced current distributes in the vee edge, thermal conduction pulls that heat through the material. This process is best described with the thermal reference depth which is a function of material diffusivity, vee length, and mill speed.

Thermal reference depth =

This article was supplied by Mr Michael DiDonato, mechanical engineer, Thermatool Corp.

fi HAZ screen display panel

Thermatool Corporation – USA Fax : +1 203 468 4281 Email : info@ttool.com Website : www.thermatool.com

fi A diagram of welding power and weld power vee length

Inductotherm HWT (Thermatool Europe) – UK Fax : +44 1256 467224 Email : info@ihwtech.co.uk Website : www.inductotherm-hwt.co.uk

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N ovember /D ecember 2007