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.
Pre-weld and post-weld heat treating is critical for many welding operations. Without proper thermal manipulation, welds and heat affected zones can have mechanical properties that are undesirable. Worse yet, inadequate heat treatment can result in cracks and devastating weld failures. While temperature and time are the primary concerns when heat treating a weld, the heating method should also be considered diligently when selecting a process. Induction heating is one of the most popular types of heat treating methods, and rightfully so. The benefits of induction heating are many, and Red-D-Arc has the equipment you need to successfully implement an induction heat treating operation for your projects.
Importance of Preheating in Welding – Equipment Options
Preheating reduces the risk of cracking in weld metal and heat-affected zone (HAZ) by:
- Lowering the cooling rate of the weld – prevents formation of brittle weld metal/HAZ, allows more time for hydrogen to escape the molten weld.
- Removing moisture (hydrogen source) from the part.
- Reducing shrinkage by lowering the temperature differential.
Preheat or not?
The requirement and level of preheating for steel is determined by the relevant welding code and is dependent on the weld heat input, chemical composition of steel (carbon equivalent), thicknesses, diffusible hydrogen etc. Non-ferrous materials generally require preheat because of their high thermal conductivity.
Scheduled for completion in July 2016, U.S. Bank Stadium, the new home of the Minnesota Vikings, is a fixed roof structure that will be nearly 30 stories tall at its highest point. During construction, the base plates of some of the structural support columns needed to be pre-heated to 400o prior to welding. Traditional heating by flame proved to be difficult. It was determined that induction heating was the right solution to safely and efficiently heat the steel to an exact temperature. Induction heating is the preferred method to address challenges associated with the pre-heating of structural steel because it provides a safe and accurate method of heating the metal to a precise temperature.
Red-D-Arc offers Miller ProHeat 35 Induction Heating Systems (IHS) available for rent, lease and sale. This highly efficient technology uses non-contact heating to induce heat electromagnetically rather than using a heating element in contact with the part.
UPDATE: This project was completed ahead of schedule.