In many critical oil and gas welding applications, weld preheating and bakeout are necessary to avoid defects, cracks and distortion. Preheating and bakeout help ensure necessary weld quality by reducing the risk of hydrogen-induced cracking. These practices also allow the weld to cool slowly, reducing material distortion. For jobsite preheating, induction heating delivers efficiency benefits, consistent heating and safety when compared to other methods.
Preheating methods
In preheating, the area around the weld joint or the entire part is heated to a specific temperature before welding. This is often done to reduce the cooling rate of the finished weld and to reduce the hardness in the heat-affected zone (HAZ) – resulting in a less brittle and more ductile weld. The process also reduces the potential for hydrogen cracking.
The three methods frequently used on jobsites for weld preheating are induction, open flame and resistance.
- Induction uses cables or blankets and electromagnetic fields to generate heat within the base material, heating it from within. This method of preheating provides continuous, even heat.
- Open flame uses flame applied directly to the part to heat it. Operators burn a gas fuel using a torch, sometimes with compressed air.
- Resistance uses electrically heated ceramic pads, commonly called chiclets, placed on the base metal. The pads transfer heat through radiant and conductive heat where they touch the part.
Benefits of induction
Induction heating is newer to the jobsite, but it offers numerous benefits over open flame or resistance heating. Here are four reasons why construction and fabrication welders are turning to induction heating.
1. Reduced expenses. Labor is a significant expense with resistance heating because the time-consuming setup can take hours per weld joint. In addition, the ceramic pads used in resistance systems can break and require frequent replacement. Resistance heating power sources are heavy and inefficient, requiring large power drops on the jobsite. Every ceramic pad group requires a wire harness and thermocouple to power and control it. On some jobsites, wire harnesses are brought in by the truckload.
Flame heating also involves significant labor costs. Because flame has a slow time to temperature, it’s time-consuming to heat and reheat the part after operator breaks or shift changes. Flame heating also has the recurring costs of purchasing and handling the fuel. This method is also quite inefficient because most of the energy from the flame is reflected into the surrounding air rather than the part itself.
By comparison, induction heating delivers great overall efficiency, so it provides a strong return on investment in terms of energy costs, especially for operations that use preheating on a regular basis. And because of its fast time to temperature and easy setup process, induction heating also delivers labor cost savings.
2. Time saved in setup and heating. A preheating process that requires long setup and teardown times slows down the entire operation and can hold up next steps in the welding process. Setup time for resistance heating can take up to 3 hours per weld joint. Operators must wire each pad, and the configurations and cabling are complex. Teardown times are also lengthy because operators must wait for the equipment to cool down before they can remove it and move to the next joint.
With flame heating, the part begins to cool as soon as the flame is removed. Time is wasted when a part must be reheated after a break or shift change.
With induction heating, operators apply heating cables or blankets to the weld joint and connect the system and thermocouples. Setup is typically 20 min. or less per joint. Induction tools don’t become hot, so operators can move them immediately to the next joint without extended cool-down time.
3. Heating consistency and quality. With resistance heating, ceramic pad failures or outputs that are stuck on a certain temperature can cause cold or hot spots. If a heater pad over a thermocouple burns out, the other heater pads will work harder and grow hotter to try and bring that thermocouple up to the proper temperature, resulting in hot and cold spots.
Uneven heating is also a common problem with flame heating. The amount and concentration of heat depends on several factors, including the amount of fuel consumed, distance between the flame and weldment, manipulation of the flame via adjustment of the gas control, and control of heat losses to the atmosphere.
Induction heating offers an even heat profile, eliminating local hot spots in the weld joint and heating the part from the inside out – rather than forcing heat from the outside in. This uniform heating with minimal temperature variation helps ensure quality welds. In addition, induction systems allow for automatic temperature documentation – eliminating the inconsistency and time involved with doing it manually.
4. Safer environment. The ceramic pads used in resistance heating become hot, increasing the risk of burn hazards. Flame heating also has the obvious risk of working around open flame. Induction heating offers a safer environment because the heating tool stays cool – only the part becomes hot.
Operations can reduce costs, improve jobsite safety and optimize project efficiency by using induction heating technology on the jobsite. New induction heating technology designed specifically for jobsites provides additional portability, ease of use and flexibility for preheating – delivering greater control and flexibility that saves time and money.
