Pipes Induction Heater Applied to Pre-Weld Preheating, Post-Weld Heat Treatment

What Is the Purpose of Preheating Before Welding?

The purpose of preheating before welding is to reduce the cooling rate of the weld joint, to avoid the formation of hardened structures, and to reduce welding stress and deformation. It is an effective method to prevent welding cracks.

Preheating is often required for welding high-strength steels, steels prone to hardening, materials with excellent thermal conductivity, thick weld pieces, and when the ambient temperature around the welding area is too low. Generally, the welding of important components, alloy steels, and thick parts requires preheating. Preheating prior to welding can lower the cooling rate after welding while minimally affecting the duration at high temperatures, which is ideal. 

Therefore, when welding steels with a tendency to harden, the main process measure to reduce the cooling rate and hardening tendency is preheating, rather than increasing the line energy. Preheating the welding joint for high-strength steels (such as X80 steel) used in long-distance pipelines is a necessary process to ensure welding quality.

The Benefits of Preheating Before Welding

• Preheating can slow down the cooling rate of the weld joint after welding, which facilitates the escape of diffusible hydrogen in the weld metal and avoids hydrogen-induced cracks. It also reduces the hardness of the weld and the heat-affected zone, improving the crack resistance of the weld joint.

• Preheating can reduce welding stress. Uniform local preheating or overall preheating can reduce the temperature difference (also known as a temperature gradient) between the welded pieces. This, on one hand, lowers the welding stress, and on the other hand, reduces the welding strain rate, helping to avoid welding cracks.

• Preheating can reduce the restraint degree of the welded structure, particularly noticeable in reducing the restraint degree of the joint. As the preheating temperature increases, the crack incidence rate decreases.

Adopted Preheating Methods

• Flame heating method, where the combustion temperature and effect can vary greatly in actual operation, causing uneven heating temperatures around the circumference of the steel pipe. Additionally, the management of liquefied gas supply, explosion prevention, logistics, and storage increases the difficulty.

• Electric heating band, where the heating band uses resistance wires, which generate heat after electrification and then transfer the heat to the weld joint. This method is inefficient and, although the single set cost is low, the equipment is easily damaged and has a high failure rate.

• Electromagnetic induction heating, compared to traditional heating methods, offers non-contact, energy-saving, and precise temperature control advantages. It is especially suitable for pipeline engineering operations under long-distance and harsh environment conditions, thus widely used in petroleum and natural gas transport systems.

Advantages of Pre-Welding Induction Heating

• Energy saving: The energy utilization efficiency of pipes induction heater is 40%-50% higher than ceramic tiles, and 50%-70% higher than flame, with significant energy-saving effects.

• Fast heating speed: Under the same working conditions, induction heating is faster than ceramic tiles and flame heating. Induction heating equipment does not suffer from oxidation and burning loss on the workpiece surface, saving materials and costs.

• High automation level: Pipes induction heater can automatically and accurately control temperature in real-time based on the current temperature state of the heated workpiece. Through a temperature closed-loop control system, the output power of the power supply can be adjusted in real-time, allowing automatic adjustment of the heating temperature or distribution on the workpiece.

• Improved and protected environment: Induction heating does not produce exhaust gas or smoke, emits less radiant heat under insulating conditions, produces low noise, offers an eco-friendly work environment, and reduces operator labor intensity.

• Safe and reliable: Pipes induction heater does not produce an open flame, greatly reducing the risk of fires, explosions, and other dangerous incidents, thereby enhancing safety.

• Convenient use: The main component of pipes induction heater is an inverter induction heating power supply based on semiconductor power devices. It does not require preheating and can be started and shut down at any time. This not only makes it convenient to use but also saves time and energy.

• Small installation footprint: The power structure of pipes induction heater is compact, with a modular and standardized component composition. Compared to ceramic tiles and flames, it has a smaller volume and weight, saving installation space and site costs.

• Localized heating: Pipes induction heater can perform local heating of the workpiece to replace overall heating, offering flexible processes, and saving heating time and costs.

  
  

To meet the needs of high-quality, efficient, and safe pipeline construction and to solve the pre-weld heating issues at pipeline construction sites, JKZ has developed pipes induction heater through practice and research. Featuring small size, light weight, novel structure, advanced technical performance, it provides high reliability and safety required for use under special environmental conditions in field pipeline construction.