Pre Weld and Post Weld Heat Treatment

Welding between insulated heating areas
Heating done by ceramic heaters (pink). 

There are several key reasons why it's important to preheat before welding. First, a preheated part cools more slowly, which slows the overall cooling rate of the welded part. This improves the metallurgical (crystalline) structure and makes it less prone to cracking. Additionally, hydrogen that may be present immediately after a weld is also released more efficiently, which further reduces the possibility cracking. Preheating also mitigates stress from the shrinkage at the weld joint and nearby metal. Finally, pre-heating reduces the possibility of fracture during fabrication due to brittleness.

An industry standard means of accurately managing the pre-heat and post-weld heat treatment is through the use of specialized electric heaters. Electric welding preheaters, known as "ceramic mat heaters", are rugged and flexible heating elements designed so that they conform uniformly around the weld and surrounding area.  Ceramic mat heaters are normally controlled by a power console that uses thermocouples and electronic controllers to regulate, monitor, and many times record, the preheat temperature profile.

Ceramic heaters on spool piece.

Welding codes, gathered from years of rigorously tested data, including metallurgical science, chemical properties of materials, and radiographic analysis, provide specific detail on the minimum preheat temperature, the soak time, and the welding process to be used.

Before deciding to preheat or post heat any welding job, consult with a heat treating expert to make sure proper welding code requirements are being followed.

Welding Heat Treatment Products from Hotfoil-EHS

Here's a little shameless, self-promotion from your friends at Hotfoil-EHS.

Applying Ceramic Mat Heaters for Pre and Post Weld Heat Treatment

Designed for pre and post weld heat treating, ceramic mat heaters function efficiently for both curved and flat applications. The flexibility of these heaters provides a convenient means to heat treat large metal surfaces in the field.

The application of ceramic mat heaters is straightforward - the heater is placed directly on the part to be welded, insulation is normally laid over the heater, and voltage is applied. A thermocouple is tack-welded near to monitor the part. The part is brought up to temperature and then the welding is done. After the weld, the part is cooled at a controlled rate.

Welding a large pipe with localized heating using
ceramic mat heaters.

Preparing the pipe for the weld and allowing
the heaters to raise the temperature of the pipe.

Wrapping ceramic mat heaters around
spool piece.

Close-up of ceramic mat heaters around
spool piece.

Ceramic Pad/Mat Heaters: The Tried and True Way to Heat Treat

Ceramic mat heater: Proven performance over time.

Heat treating large metal surfaces in the field before and after welding is usually a challenge. Over the years, products have been developed that perform well and withstood the test of time. One of these products is the ceramic pad heater.

Ceramic pad heaters (also known as mat heaters) are constructed of interlocking sintered alumina ceramic beads (usually pink or white in color) which insulate an 80/20 Nickel/Chrome multi-strand wire (the resistance element).

Ceramic pad heaters are designed for most pre-heat and post-heat treatment requirements on weld joined materials (stress relieving). Available in numerous sizes, shapes, configurations, voltages and wattages, they can be used up to 2150°F in various weld heat treating applications.

Various shapes and sizes.

They are designed for re-use in a wide variety of heating applications. Their flexible design allows for conformance and good heat transfer to the target part. Their rugged design is well suited for most industrial environments where shock and vibration is a reality. Ceramic pad heaters can be used with or without the application of insulation. The use of insulation will greatly speed up the heating
process as well as more efficiently retain the heat for soaking.

For more information on the use and installation of ceramic pad (mat) heaters, visit Hotfoil-EHS at www.hotfoilehs.com or call 609.588.0900.

New Hotfoil-EHS Catalog Available for Download

Electric Heating Systems, Inc. (EHS), established in 1994 in Robbinsville, NJ, is a designer and supplier of turnkey, permanent low temperature heating systems for the electric power and tank fabricating industries. Later, this division's name was changed to Hotfoil, and the EHS division began manufacturing ceramic mat (pad) heaters, power consoles, and accessories for the heat treating industry.

Hotfoil-EHS is now a major manufacturer and producer of standard and custom designed industrial heat treating products with an exhaustive inventory including custom control panels, heat treating power consoles, cables and splitters, temperature recorders, thermocouple attachment units and pin guns.

You can download the latest product catalog here.

New Video on Ceramic Mat Heaters for Welding Heat Treating

Just a short video on Hotfoil EHS pre and post weld heat treating mat heaters (also known as pad heaters).

Resistance Heating Mats for Welding Pre and Post Heat Treatment

Ceramic Mat Heater

Resistance heating mats (or pads) are constructed of ceramic beads strung on nichrome or Kanthol wire. These resistance heating elements accurately heat the workpiece to the proper temperature before, during, and after welding, assuring compliance with recommended preheat, interpass, and PWHT practices. Designed as semi-flexible ceramic heater pads, with interlocking beads and high temperature wire, so a proper fit to the workpiece. Capable of temperatures up to 1,850 degrees F., ceramic mat (pad) heaters have an additional benefit in that they can withstand welding temperatures and don’t have to be moved during welding.

Resistance Heating Mats for Welding Pre and Post Heat Treatment from Hotfoil EHS, Inc.