Unravelling the Mystery of Heat Treatment
This article originally appeared in Connector Magazine, Fall 2020 Edition
Erectors can save cost and time by taking this welding technique in-house
Large steel erection projects commonly require heat treatment for welding because of the thick materials used in structural steel construction. Proper implementation is critical; so often this work is outsourced. The perceived level of expertise to perform this work can make the prospect of using in-house personnel daunting. However, most welding contractors are actually fully capable of performing on-site heat treatment operations.
The two primary methods for heat treating metal are preheating and post-weld heat treatment (PWHT). Preheating is the process of heating the base metal to a specific temperature prior to welding to minimize the temperature difference between the welding arc and the base metal. This process helps to reduce internal stresses that occur as substantial temperature differences between the weld and the base metal normalize, which can cause cracking and distortion. Slowing the cooling rate also allows hydrogen to escape from the weld, which minimizes the potential for cracking.
Preheat requirements for steel are specified by applicable welding codes and are dependent on such factors as the chemical composition of the steel, section thickness, welding process employed, weld heat input, diffusible hydrogen, etc.
Post-weld heat treatment operations are usually performed to improve mechanical properties by minimizing hardness, increasing ductility and relieving weld-induced stresses. Typically, PWHT in structural applications is required when the steel will be exposed to environmental conditions like salt water or seismic activity.
A more efficient option for heat treatment
Outside contractors are often brought in to perform heat treatment operations due to the perceived complexity of the task, which can increase costs and potentially add delays to a project. Meanwhile, welders are often paid for unproductive time waiting for joints to be heated.
“There are numerous examples of heat treatment costs reaching ten times the original quoted price, as well as project delays, when working with heat treatment service providers”
– Lori Kuiper, Orbital, Heating and Pipe Welding Product Manager
“There are numerous examples of heat treatment costs reaching ten times the original quoted price, as well as project delays, when working with heat treatment service providers,” says Lori Kuiper, Orbital, Heating and Pipe Product Manager at Red-D-Arc Welderentals.
While it’s true that some applications may be too complex for beginners to perform, most are straightforward and easy to complete. Once a contractor overcomes the initial uncertainty of a new process, they find out how simple heat treating is compared to welding. With the proper equipment and assistance from an experienced technician, welders and other skilled workers can be trained to perform heat treatment operations effectively. This gives contractors the option for taking this process in house, thereby reducing costs associated with outsourcing.
Alternative heating methods
Open-flame heating (i.e. torches) is a simple method for heat treatment, but many companies have moved away from this method due to safety concerns and the large amounts of propane that must be stored. In addition, open-flame methods result in inconsistent heating, and temperatures can only be roughly verified, thereby reducing quality. It is also less efficient than more modern preheat and post-weld heat treatment processes (such as induction and resistance heating) since less energy is consumed with these processes.
The two leading alternatives to the open-flame method are induction heating and resistance heating. Induction heating uses non-contact heating to induce heat electromagnetically, which provides consistent heat throughout the base material. With this method, the weldment basically becomes its own heating element. It offers fast time-to-temperature, includes accurate temperature measurements and records, and is a very safe option for preheats, especially compared to the open-flame method. Induction heating, however, is limited to magnetic materials, which can restrict its use.
Resistance heating works by electrically heating a wire that passes through flexible interlocking ceramic heating elements (typically called beads or pads). These pads are positioned in contact with the base material, which allows the heat to transfer uniformly into the part. The ceramic pads are very durable and their serviceable life can be extended with proper treatment and handling. Resistance heating can achieve temperatures of 2000°F with standard heating elements and temperatures of 3000°F with high-temperature elements. Resistance heating is not limited to magnetic materials, which makes it the most versatile option for a wide range of heat treatment applications.
The right heating equipment
Red-D-Arc, an international welding equipment rental company, has decades of experience working with various heat treatment technologies as well as a history of collaborating with clients. The company has a full heating portfolio to meet any application, and their experts can tailor the best solution for any given application.
Customers looking for a highly portable, field-construction induction heating system may want to consider the Miller ArcReach Induction Heater. Its small footprint and built-in data recorder make it ideal for the field. With an 8 kW output, it can preheat up to 600°F depending on material size and thickness.
Miller Induction Heating Equipment
The Miller ProHeat 35 is the industry standard for larger induction heating applications. It’s also the workhorse for preheating large columns and bridge girders. This system features the ability to accurately monitor, control and record temperatures. With a 35 kW heating capacity and the flexibility of liquid-cooled cables, it can easily be used by contractors to preheat large assemblies.
Colin Brown is Senior Marketing Manager for Red-D-Arc Welderentals. Red-D-Arc’s customers can rent, lease or purchase equipment, and services also include end-to-end support, onsite training, troubleshooting and maintenance. For more information, visit reddarc.uk