Welding pre-heat options reconsidered

27 Apr.,2023


In recent years the use of induction heating equipment has increased drastically, ever since a welding equipment manufacturer made the equipment more readily available on a retail level. The primary benefit of using induction is that it heats material quicker than methods using traditional electrical resistance. This can be very useful for a post-weld heat-treat cycle or for heating a hub on a shaft for easier removal. But what about for welding pre-heat?

In fact, professional field heat-treatment companies that have induction heating equipment in their fleets rarely use induction for welding pre-heat. There are three main reasons for this: equipment durability, temperature control, and equipment capacity.

Equipment Durability

The power supplies for electrical resistance equipment used for welding pre-heat typically comes in two forms: a self-contained generator rig (125 to 150 kVA) or a console (65 to 100 kW). A three-phase transformer provides power to heating elements through welding cables. Equipment failure rarely occurs with these heavy-duty transformers, and they can last 10 to 20 years or longer in field applications.

Induction heating equipment for welding pre-heat typically uses a 35-kW power supply with a power inverter similar to those used in welding machines. The inverter is full of many electronic components that are susceptible to failure, especially in field applications, where it is difficult to control temperature, humidity, and air quality.

While professional field heat-treatment companies frequently send back-up equipment to their job sites when using induction, this practice is rare while using electrical resistance.

Temperature Control

With the advancement of materials and welding procedures, requirements for controlled welding pre-heat are becoming more stringent. Minimum pre-heat and maximum interpass windows are getting tighter, especially for creep-strength-enhanced ferritic steels.

There is a lack of consensus in defining the phrase “temperature control” when discussing induction heating equipment. However, this discussion can be swayed when considering how induction machines are designed with only a single point of power output control.

Despite induction machines using multiple thermocouple inputs and having the ability to average or choose the controlling thermocouple, there is still only one single power output. While this power output can be paralleled into multiple heating blankets or coils, it does not allow for independent power output control. The only method to allow independent control of multiple induction coils or blankets is to have multiple inverters in a single unit.

So how do you put this to the test? Take any induction machine that runs multiple coils and wrap a 12-in. Sch. 40 pipe with one coil, and a 16-in. Sch. 160 pipe with the other coil. Very quickly you will begin to see how the temperatures cannot independently be controlled. The 12-in. Sch. 40 pipe will overheat very quickly, until the 16-in. Sch. 160 pipe reaches its set point, or the opposite will occur and the 12-in. Sch. 40 pipe will reach its set point temperature and the 16-in. Sch. 160 will not reach its set point temperature.

Electrical resistance equipment typically has 6 to 24 independent points of control. The transformer or generator supply switches the power to the heating elements through contactors. Each contactor allows for independent control of the heating elements.

Equipment Capacity

For weld pre-heating in the field, the most popular 35-kW induction machines are limited to covering 53 sq. ft. of surface area in a liquid-cooled setup. A standard six-way electrical resistance console can provide up to 144 sq. ft. of coverage, meaning that portable generating rigs that are used for pipeline welding pre-heats can cover two to four times that amount from a single unit.

Comparatively, an induction machine may heat a single 48-in., 1.20-in.-thick pipe to 300 degrees in 15 minutes instead of 30 minutes by electrical resistance. A portable generating rig will heat two to four of those joints at the same time, and they can be started remotely and heated ahead of time, thereby allowing for substantial time savings in the overall fabrication sequence.

Induction has many advantages to electrical resistance in various field heat-treating applications, but welding pre-heat is rarely one of them. Make sure that you are using the pre-heat process that makes the most sense for your application.

David Scott Fong is technical solutions specialist at KASI Technologies Inc, 3001 Buckingham Dr., #125, Sherwood Park, Alta., 780-297-7793, scottfong@kasi.ca, www.kasi.ca.

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