Induction Preheating

What’s Induction preheating?

The welding of important components, alloy steel, and thick parts requires induction preheating before welding. The process means of heating the whole welding part or part of the welding area before welding is called preheating. The process of heating the whole welding part or the part of the welding area before welding is called the induction preheating process.

For steels with high welding strength and hardening tendency, materials with especially good thermal conductivity, and welding parts with large thickness, and when the ambient temperature around the welding area is too low, it is often necessary to do the welding parts induction preheating process before welding. The main purpose of induction preheating is to reduce the cooling rate of the welded joint. Induction preheating not only can reduce the cooling rate, but also does not affect the residence time at high temperatures, which is ideal. Therefore, when welding the copper with a hardening tendency, the main induction preheating machine technological measure is to reduce cooling speed and hardening tendency rather than increasing wire energy.

When multilayer and multipass welding is carried out on the weldment, the lowest temperature of the front weld is called the interlayer temperature when the welding ends. For materials requiring induction preheating welding, when multilayer welding is required, the interlayer temperature should be equal to or slightly higher than the induction preheating temperature. If the interlayer temperature is lower than the induction preheating temperature, it should be preheated again.

Why use Induction Preheating?

Induction preheating can slow down the cooling rate after welding. It is beneficial to escape the diffused hydrogen in the weld metal and avoid hydrogen-induced cracks. At the same time, It also reduces the welding seal and heat-affected zone hardening level, the welded joint crack resistance is improved.
Induction preheating can reduce welding stress. The temperature difference (also known as temperature gradient) between welders in the welding area can be reduced by uniformly local or whole induction preheating. In this way, on the one hand, the welding stress is reduced, on the other hand, the welding strain rate is reduced, which is beneficial to avoiding welding cracks.
Induction preheating can reduce the welded structures constraint degree, it is especially obvious to reduce the constraint of the Angle joint. With the increase of induction preheating temperature, the crack incidence decreases.

Induction preheating temperature and interlayer temperature (Note: when multi-layer and multi-pass welding is carried out on the weldment, the lowest temperature of the front weld is called interlayer temperature when the post-weld is welded. For materials requiring induction preheating welding, when multilayer welding is required, the interlayer temperature should be equal to or slightly higher than the induction preheating temperature. If the interlayer temperature is lower than the induction preheating temperature, it should be induction preheated again.

In addition, the uniformity of induction preheating temperature in the direction of steel plate thickness and in the weld area has an important effect on reducing welding stress. The width of local induction preheating should be determined according to the constraint of the welder, generally three times the wall thickness around the weld zone, and not less than 150-200 mm. If the induction preheating is not uniform, not only will not reduce the welding stress but will increase the welding stress.

How to Find the Suitable Induction Preheating Solution?

When choosing the appropriate induction preheating equipment mainly consider the following aspects:

The heated workpiece’s shape and size.: Large workpiece, bar material, solid material, should be selected relative power, low-frequency induction heating equipment; If the workpiece is small, pipe, plate, gear, etc., the induction preheating equipment with low relative power and high frequency should be selected.
The depth and area to be heated: Deep heating depth, large area, overall heating, should choose large power, low frequency induction heating equipment; Shallow heating depth, small area, local heating, selection of relatively small power, high frequency induction preheating equipment.
The requried heating speed: If the heating speed is fast, the induction heating equipment with relatively large power and relatively high frequency should be selected.
Equipment continuous working time: Continuous working time is long, relatively select slightly larger power induction preheating equipment.
Distance between the induction heating head and the induction machine: Long connection, even the use of a water-cooled cable connection, should be a relatively large power induction preheating machine.

What is The Induction Preheating Machine Application?

These years, as technology is changing all the time, induction preheating machine is widely used in all kinds of industries. Normally includes the following induction preheating fields.

Induction preheating: induction preheating for welding, preheating for coating, preheating for assembly, induction preheating for mold, induction preheating for ship deck.
Post-weld induction heat treatment: post-weld heat treatment of pressure vessel, boiler, roll, valve, pipe, and other metal parts.
Medium induction heating and insulation: Crude oil pipeline, water pipeline induction preheating, reactive gas pipeline heating, wood drying, metal drying, and so on.

The induction preheating equipment is used in petrochemical, oil and gas pipelines, aerospace, shipbuilding, steel, boilers, shipbuilding, pressure vessel, steel frame structure, railway, bridge, mine construction, automobile manufacturing, nuclear power, mining industry, etc.

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