Written by Gerry McWeeney on November 8, 2021

Heat Treatment Methods and Application Information

Heat Treatment Applications

These are the common applications for Heat Treatment

Post Weld Heat Treatment (PWHT)

Post Weld Heat Treatment (PWHT), also referred to as stress relieving, is an application of controlled heat applied after the welding is complete.

PWHT is the relaxation of residual stress caused during the welding or in other processes such as high pressure piping and is also effective for hydrogen removal.

PWHT Time, Temperature and Thermocouple locations are driven by industry codes, Axiom’s knowledge and ability to meet or exceed these requirements are the reason our customers rely on us for a quality driven method.

Preheat

Preheating is a process of heating the metal to a desired temperature and maintaining this before and during the welding process.

Maintaining required preheat and interpass temperatures is critical, both for producing a softer, less brittle microstructure for allowing hydrogen to diffuse out of the weld metal and HAZ during welding and also helps with the cooling effect of the metal after each weld pass.

These temperatures are often driven by code or in certain cases by proven and specified welding procedures, preheat in some cases can be elevated to reduce the need for PWHT in certain metals and processes.

Bake Out (Hydrogen Bake Out) 

Bake Out is a heat treatment process that drives out any hydrogen that may have occurred during welding or has seeped into the parent metal in an in service process pipe, vessel or structure.

Hydrogen occurs in metals on a molecular level when hydrogen gets absorbed into the metal, if left undetected can lead to hydrogen cracking and may lead to mechanical failure of the pipe, vessel or structure.

Post-Heating

Post Heating is the maintenance of preheat after the weld has been completed, which allows hydrogen dissolution from the weld to occur. The post-heat temperature may be the same as, or greater than, the original preheat temperature specified

Often used when a weld has not been complete in heavier wall materials where pre heat does not require to be maintained and/or after welding in these heavier materials. Post Heating is also often used during and after welding in creep strength enhance ferritic steels (CSEF Steel)

Line Thaw

Line Thaw is applied heat to help liquify congealed or solidified product in a process system

Heat is applied externally in conjunction with some means of pressure internally to force the product towards an exit in the system or to free the product enough that the systems can go back to full operations.

Due to some complexities of this type of heating being aware of the product lower and upper critical temperatures to avoid any temperature over runs should be discussed prior to applying heat

Methods

Below are common methods of Heat Treatment

Electrical Resistance

Electrical Resistance heating uses low-voltage ( 80volt) flexible ceramic heating elements placed around the component controlled by multi point heat treatment control consoles.

Power to the consoles can be by temporary generator power or from a main power supply inside the facilities.   

The most utilized method in the industry due to the flexibility and sizes of heating elements (FCP) available and the types and geometry of the applications we work on resistance heating can give a more finite temperature control to the parts being treated.

With code changes in our midst electrical resistance may be the preferred method for many applications.

Induction

Induction heating uses an electromagnetic field to create a disturbance in the molecules of the metal that creates heat, the frequency of the electromagnetic field can be quicker than traditional electric resistance heating, the system is quicker to set up and is a good tool for some heat treatment applications.

Not all metals are equal, some do not take well to the induction frequencies, it is always good to check before proceeding with this method

Often used for bolt heating and stretching, induction rods can help expedite the opening of turbine casings, presses and heat exchanger systems.

These systems often provide quicker heating, faster set up and teardown, and can be a safer option due to cool coils in the vicinity of welders and other personnel.

Due care and attention should be taken with rapid heating in certain applications to prevent material overheating.

Combustion Heat

Is used to heat large structures inside temporary furnaces, inside vessels, lime kilns and other large structures because of its efficiencies. Utilizing gas and air to force the heat into the area requiring the heat treatment creates a uniformity that heats the intended area.

Typical uses are Temporary Furnaces, PWHT of Pressure Vessels and Refractory dry out of lined structures, vessels and lime kilns.

A preferred method for refractory dry out it helps drive heat through the material to remove any chemical water used in the formation of the refractory, also helps with chemical bonding and reduces the chance of spalling increasing the life and efficiency of the material.

Blended Heating and Cooling

Blended Heating & Cooling is a new Heat Treatment method utilizing the use of 1, 2 or all 3 heating methods simultaneously to help maximize temperature dissipation into the heated area and a cooling method once the metal design criteria have been met, reducing costs on the front end and improving your time back to production on the back end.


By utilizing Axiom’s experience and patented technology* we have the means to offer a compilation of heating and cooling methods that help you manage the budget and improve the schedule.
Axiom Heat Treatment is The World Leader in blended methods of heating and cooling

*Pending

Contact us to discuss or to schedule a lunch and learn or demo

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