Heat Treating

Video Presentation: Self Performing Heat Treatment

 

Reduce costs by performing induction heating operations in-house. Enter your information in the form below to gain immediate access to Lori Kuiper's 2021 presentation at the AWS Pipe Welding Expo

What is Preheating?

Preheating is defined as the application of heat to a base metal prior to welding and the desired temperature maintained throughout welding till the weld joint is complete. Preheating the weld joint and surrounding area serves a dual purpose: to reduce the amount of moisture in the base metal prior to welding, and to slow the cooling rate of a finished weld and base metal. This is accomplished by heating the base material to a temperature typically between 200 – 600 deg F depending upon the type of material and thickness, which is determined by the applicable welding code.

Sudden heating and cooling of a weld causes stresses in the neighboring area between the weld and base material (called the Heat Affected Zone or HAZ). The microstructure and properties in the HAZ region change so that they differ to that of the weld and the base material. These changes can be detrimental depending on the material, and potentially cause hydrogen induced cracking, higher or lower strength, reduced corrosion resistance or lower toughness. Typically, if a failure is going to occur, it will be in the HAZ.

Hydrogen induced cracking can be slow to take effect, and visible cracks may appear hours or days after the weld has been completed, which may result in costly repairs and downtime. Hydrogen easily migrates and tends to diffuse out of the weld zone and park along discontinuities that are present in the microstructure. Once enough hydrogen starts to group up it can cause stresses in the microstructure that lead to cracking.

The welding process, filler metal, and moisture in the base material are the main contributors to hydrogen being present in the weld. Using low-hydrogen filler metals and proper preheat methods will help minimize available hydrogen. Induction and resistance heating methods provide the most consistent heating control and will ensure that the interpass heating is maintained throughout the duration of the weld. These methods also allow for controlled cooling at the end cycle or to transition directly into a post-weld heat treatment if required.

How Does Induction Heating Work - Learn the Basics

Induction heating is a simple cost effective heating process that delivers fast and consistent heat when compared to other conventional heating process.