Induction preheating of welded joints

What is preheating used for?

Preheating is used when it is provided by the technological documentation, project or welding procedure. It can reduce the cooling rate of the compound, help remove moisture and reduce the risk of unwanted structures and cold cracks. For some processes, it is required to maintain a minimum temperature during welding or control the interlayer temperature.

The induction system does not determine the allowable temperature instead of the welding technologist. Steel grade, carbon equivalent, thickness, welding method, additive material and regulatory documentation requirements can significantly change the mode. The task of the installation is to provide the calculated heat supply to the specified zone and reproduce it in the accepted sequence.

ring heating of pipes and chandeliers before welding the joint;

heating the area along the straight or curved seam;

pre-heating of massive cast and forged parts;

maintaining the temperature according to the program during welding.

What to report for equipment selection

For a preliminary proposal, you need a drawing of the node with the designation of the welded connection and the required heated zone. If the mode is set by the technologist, you should attach a welding card or an extract from it. For work on the site, the distance from the installation to the inductor, access around the part, the ability to place cables and liquid cooling are additionally taken into account.

brand of the main metal and additive material;

wall thickness, diameter, mass and product configuration;

type and length of welded connection;

initial, required and maximum permissible temperature;

the width of the heating zone on each side of the seam;

allowable time of access to the mode and duration of maintenance;

method of measuring the temperature and installation points of sensors;

conditions in the shop or at the facility, the presence of thermal insulation and drafts.

Which series is suitable for welding heating

LN is convenient for repair and installation work: the remote inductor can be brought to the product, and the power part remains close to the workplace. AVB and AVZ are applicable to stationary posts and repetitive parts of average mass. AVN is chosen for thick-walled pipes, massive connections and deeper heating. AVVS can be used for small products and narrow local areas. A specific series is determined by calculating and checking for details.

For a closed joint, a ring or detachable inductor is used, for an extended seam, a contour inductor or a sequential treatment of sections. When choosing the step, clearance and position of the turns take into account the required width of the zone, access to the seam and the location of the thermal sensors. Thermal insulation around the heated area often reduces losses and increases uniformity, but its material and installation must comply with customer technology.

Example of an approximate heat calculation

An indicative example for an initial estimate. Assume that the calculated metal mass in the weld preheating zone is 25 kg and the temperature must increase from 20 to 180 °C. With an average steel specific heat capacity of 0,5 kJ/(kg·°C), useful energy is 25 × 0,5 × 160 = 2000 kJ. Assuming an overall process efficiency of 55 %, approximately 3636 kJ is required from the mains. At an average mains input power of 40 kW, the idealised time is 3636 / 40 = 91 seconds.

Real time is usually longer due to heat dissipation in the neighboring metal, tool heating, wind, low ambient temperature, and limited inductor coverage. If the heating is carried out sequentially along the seam, the speed of movement and overlap of zones are additionally calculated. The power, frequency, inductor, thermal insulation and program are tested on the product or process sample.

Solution development and commissioning

UralInduktor develops the concept of the complex and forms a technical solution. The Chinese factory performs detailed design and manufacture of equipment. A simple inductor is usually included in the price; detachable, extended, multi-zone and replaceable inductors are calculated separately as agreed.

The assembled plant is inspected with the inductor ordered. In Miass, re-checking without load is carried out with full connection. Management and documentation package are delivered in Russian. The commissioning works are carried out by the customer with tests on its details and registration of acts. Warranty - 18 months with the beginning of the term under the terms of the contract: from shipment or from the signing of the commission act. Price on request.

What to send for selection

Send the connection drawing, the grade and thickness of the metal, the required temperature, the width of the zone and the extract from the welding technology. We will offer a series of installation, preliminary power and inductor design. The actual temperature regime will be confirmed by tests using the customer's technology.