Heat Treat Industry News
April 07, 2010
Induction for better productivity and reliability
The pipes or tubes stand out for their excellent straightness and minimum ovalityINDUCTION heating and heat treating is a key process technology for the metal fabricating, automotive, and engine building industries.
Tube manufacturers have achieved better productivity, quality, and quality control with reduced energy consumption through the innovative application of various induction heating technologies.
Inductive high frequency linear welding of ferrous and non-ferrous tubing is a resistance pressure weld process using noncontact energy transfer. In this process an inductor coil and impeder assembly generate a well controlled alternating electromagetic field, which in turn induces dense electrical currents along the strip edges. This electrical current is densest at the contact point where the roll forming process brings the opposing strip edges together to form a closed tube or pipe. At this contact point the current heats the strip edges to the melting temperature just as they are pressed together by the forming rolls to form a tightly controlled uniform weld. No welding rod is needed to make this high quality weld.
Linear induction welding is used to make tube ranging from a few millimetres up to 26" in diameter, with wall thicknesses up to 25mm. These tubes are used in a variety of applications, such as heat exchangers, exhaust pipes, structural tubing, precision tubing with micron level concentricities, window glass framing/spacers, and pipe for fluid and gas transport. Contemporary transistorised induction welders operate at 50 to 2,000kW and 50 to 600kHz. Power and frequency settings are selected according to process and product requirements (alloy, tube dimensions, process speed and so on).
Due to the growing product spectrum of tube and pipe mills, flexible welding systems are increasingly essential. These welders accommodate a broad spectrum of new materials and their alloying elements, particularly high strength steels and stainless steels, in a wide variety of dimensions.
Through the optimum configuration of system parameters and welding frequencies, a single induction welder reliably produces high quality weld seams across a broad range of products.
Through a combination of limit value acquisition and online process and quality control, trendsetting tubular products are reliably produced with high value linear welds. These welded tubular products are made from flexibly rolled strip: for example strip with defined thickness profiles in the length direction. These ‘tailor rolled products’ allow automakers to reduce material cross section thickness in uncritical locations, thereby reducing material costs, reducing vehicle weights and improving fuel economy. This technology is enabled by a top performance welder capable of very high speed automatic response to changing wall thickness.
Heat treating is performed on tubes with certain alloy compositions and/or certain applications requirements. The coarse grain microstructure in the weld can be normalised through a single annealing partial seam heat treatment. Tube quality can be further increased by double annealing, ie cycling the weld seam through the ferrite–austenite transition twice. This double annealing achieves higher notch strength values.
Welded stainless steel tubing is typically submitted to full body heating. To minimise oxidation resulting from black (atmospheric) annealing, the bright annealing process is done in an inert shield gas. With an installed base of nearly 200 bright annealing systems around the world, SMS-Elotherm supports tube and pipe makers with comprehensive process know-how and experience.
Induction heating brings important benefits to the quench and temper process. The high flexibility of today’s manufacturing lot sizes for tubular and bar products is a decisive advantage and a main reason for the success of induction heating technology in practice. Unlike conventional furnaces, induction heating systems facilitate quick and efficient production setup and changeover from one product to the next. Fast and simple adaptability of the entire production chain to the new specifications is assured.
The inductively heated material remains in the austenitisation range for a short time, preventing unwanted decarburisation. Special handling after induction heating keeps the tubes straight – often with better straightness than the final product.
A separate straightening process is not required, and the total capital investment and operating cost are dramatically reduced.
Production equipment suppliers are encouraged to pursue modular assemblies. However, they are also asked to specify and continue development of individual components in response to customer requirements.
Source: http://www.itatube.org/newsdetail.php?ID=3248
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