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.
A customer involved in the rebuilding of a hydroelectric power generation station came to Red-D-Arc with a unique challenge: Use induction heat to help separate a massive rotor from its center shaft assembly. A traditional approach is to use a combination of open flame along with dry ice which would have taken several weeks to set up and also ran the risk of being ineffective due to the scale and complexity of the project.
Sil Nonis, an induction heating specialist at Red-D-Arc, suggested using 6 induction heating units activated in sequence to provide thermal growth in a uniform pattern; releasing the interference from the center shaft before lifting the rotor. The 6 Miller ProHeat 35 Induction Heating Systems powered two half circle assemblies, positioned at the bottom of the locking ring which was 5′ in diameter as well as two half circle assemblies positioned on a top locking ring which was 4′ in diameter. Massive rotor arms also had 4 pads attached to them, positioned in sequence.
Setup took approximately 2 days and removal of the rotor was completed in a single day. The same process will be used to reinstall the rotor after it has been rebuilt. Over the next few years two additional rotor assemblies will be removed, rebuilt and reinstalled using the same process.