Preheat is a process of using controlled heat during the welding of metals. Code mandates that certain metals are required to be within a set temperature range before welding may begin. The process of pre weld heat treatment involves preheating each side of the weld joint to a predetermined temperature before welding. This is achieved by the application of thermocouples, electrical resistance heating elements or induction heating coils and insulation being placed around the welded area to achieve a controlled preheat during welding.
The heating elements/coils and thermocouples are connected to cabling (triple cable sets) that are connected to and controlled by a control console; most control consoles requires a primary power supply of 480 volts 90 amps 3 phase. The control consoles on the secondary side have outputs/zones that reduce the voltage output from the primary side through a series of coils/lumps down to 80 volt 45 amps 3.6 Kilowatt per heater being utilized. This makes the preheat operation lower voltage and a safer application for field heat treatment.
Pre heat treatment helps reduce the cooling rate of the weld puddle/pool between each weld pass and also helps drive out moisture in the parent metal. This in turn helps prevent hydrogen buildup and the potential for cracking during the welding phase. Maintaining required preheat and interpass temperature (the maximum allowable temperature before a new weld pass may begin) is critical, both for producing a softer, less brittle microstructure to allow hydrogen to diffuse out of the weld metal and HAZ (Heat Affected Zone) during welding.
These temperatures are often driven by code or in certain cases by proven and specified welding procedures, preheat weld treatment in some cases can be elevated to reduce the need for PWHT in certain metals and processes. Because the metals being used in the industries we service are highly dependent on the quality of the pre and post weld heat treatment as much as the quality of the weld, our customers utilize Axiom for a quality approach that helps with the ease and constructability of their projects. Helping keep them on track and on budget.