Pre and Post-weld Stress Relief with Induction Heating

Induction heating is used to heat conductive materials. Developed in the early 20th century, it quickly became a popular choice for hardening military equipment parts during wartime. Because of induction heating’s controllability, speed and consistent output, its popularity continued to grow as new manufacturing and production methodologies were developed. Today, induction heating has become a popular technology for the welding industry to provide pre and post-weld stress relief.

Induction heating is unique because it uses molecular excitation as its source of heat, as opposed to open flames or external electric elements. Conduction heaters consist of a few primary components: An electromagnet and an electronic oscillator that passes a high-frequency alternating current (AC) through the electromagnet. RF (radio frequency) energy is transferred into the workpiece via electromagnetic waves. These alternating magnetic waves penetrate the object, creating electric eddy currents. These eddy currents (Foucault currents) flow through the target piece and produce heat. Its important to note that the heat is created from inside the object itself, with no open flame or external electric heat source.

Induction heating works very quickly, and since there is no contact with the target piece, there are far less concerns about part contamination.  Many industrial processes use induction heating when very high temperatures and uniform control is desired.

Pre and post-weld heat treating (stress relieving) is a growing market for induction heating systems because it offers significant benefits such as excellent heat placement and distribution, lower cycle times, safety, ease of use, and efficiency.

Induction heaters provide temperatures and cycle times hard to achieve otherwise. By virtue of their high temperature capabilities, very fast heat up times, precise application of heat, excellent controllability, and ease of setup / breakdown, the use of induction heating has been know to cut 30% to 50% of total weld cycle time in real-life welding applications.

For more information about induction heaters for pre and post-weld heat treating contact:

Hotfoil-EHS, Inc.
2960 East State Street Ext.
Hamilton, NJ 08619
Phone # 609.588.0900
Fax # 609.588.8333
www.hotfoilehs.com

Postweld Heat Treatment

Some welding operations require postweld heat treatment

Welding is a process of joining materials, usually metal or thermoplastic, by causing a fusion at the junction of two or more physical parts. The fusion is accomplished through the creation of specific conditions at, and adjacent to, the joint area. Welding is a heat based process, but there are other heat based processes used for joining metals. The welding process differs from lower temperature joining operations, such as soldering or brazing, that do not require melting of the base metal.

There are numerous welding methods, as well as specific procedures that are applicable to different metal alloys and intended use of the finished product. Some of these procedures involve controlled heating and/or cooling of the weld and the closely adjacent area of the parts being joined. This is generally termed heat treatment, and more specifically, preheating and postweld heat treatment.

Heat treatment, in any form, is a time consuming and costly operation. Strength, toughness, corrosion resistance, and residual stress of the joint are all impacted by the proper application of heat treatment. Many application codes and standards require specific protocols for heat treating, which is utilized to achieve a specific resulting condition in the subject material or to restore material characteristics altered by production processes such as welding.

Stress relief is a benefit and goal of postweld heat treatment. During the welding process, base materials near the weldment, deposited weld metal, and the heat affected zones exhibit different levels of metallurgical phase transformation. Various degrees of material hardening occurs, particularly in the heat affected zone adjacent to the weld metal deposit, with high stresses due to melting and solidification. Stress relief through heat treatment is accomplished by controlled heating and cooling of the affected material area to relieve a significant amount of the imposed stresses. This postweld heat treatment can reduce the hardness and increase ductility, lessening the incidence of cracking in weldments. The process generally involves maintaining the target area at a specific temperature (soaking), then executing a controlled cooling schedule.

One respected source, TWI, sums up post weld heat treatment (PWHT).....

The necessity for PWHT depends on the material and the service requirements. Other factors that influence the need for PWHT are the welding parameters and the likely mechanism of failure. In some standards, PWHT is mandatory for certain grades or thicknesses, but where there is an option, cost and potential adverse effects need to be balanced against possible benefits. The energy costs are generally significant due to the high temperatures and long times involved, but costs associated with time delays may be more important. Detrimental effects include distortion, temper embrittlement, over-softening and reheat cracking, which means that control of heating and cooling rates, holding temperature tolerances and the times at temperature are extremely important, and must be carefully controlled in order to realise the full benefit of the process.

The precise level of heat control, as well as the substantial quantity of heat, required to successfully accomplish a PWHT protocol calls for the use of special purpose equipment intended for PWHT. Hotfoil EHS is a recognized manufacturer of industrial application heating equipment and specializes in the design and manufacture of power consoles, ceramic mat heaters, and accessories for pre and post-weld heat treatment. Share your application challenges with them and work together to develop a cost effective solution.