Preheating is a commonly used practice in welding. Welders apply heat after welding or between passes. Let’s understand both preheating and interpass temperature in detail.
What is Preheat Temperature in Welding?
Preheating refers to the process of increasing the temperature of the base or parent material on both sides of the joint. This temperature in welding should be higher than ambient. Basically, preheating is the minimum temperature that the parent material needs before welding.
The preheat temperature for welding is necessary for the following reasons:
Decrease shrinkage stresses in the weld and Heat Affected Zone (HAZ).
Improve microstructure of HAZ.
Slow down the cooling rate to reduce hardness.
Remove moisture to avoid hydrogen buildup, porosity and probability of cracking.
Reduce distortion that might occur due to thermal stresses.
Compensate for high heat loss.
Eliminate grease oil and scale for the joints to enable quick welding speeds.
What is the Preheating Temperature of Steel?
It should be 30-50 degrees centigrade above the upper transformation temperature.
What is the Preheating Temperature of Cast Iron?
It should be around 200-310 degrees centigrade.
What is Preheat Temperature for Carbon Steel?
It depends upon various factors like thickness,base mats,Chemistry etc .
However, please note that the preheat temperature may vary across the base metals and primarily depend on the following factors:
Chemistry of the base metal
Strength of the base metal
Thickness of the base metal
Use of low-hydrogen process
Joint type, thickness and restraint
The welders can consider the following methods to determine preheat temperature:
HAZ Hardness Control Method
Hydrogen Control Method
Slide Rule Preheat Calculator
Table 3.3 in American Welding Society’s D1.1 Structural Welding Code
What is Interpass Temperature in Welding?
Interpass temperature refers to the base material’s temperature before the next pass is laid. It is the temperature at which ensuing weld runs are deposited. It exists between two beads, layers or shifts.
It is indicated in terms of minimum and maximum temperature. The minimum interpass temperature should be at least as high as the maximum one. If specific mechanical weld metal properties are required, then it is important to control the maximum temperature.
There are few reasons to measure interpass temperature:
Control the microstructural development of weld metal.
Ensure the consistency in all welds made in the procedural qualification.
Reduce the loss of alloying elements in welds.
Improve the notch toughness properties.
Prevent deterioration of mechanical properties of weld metal and HAZ.
Lower the probability of distortion.
Minimize the risk of hydrogen, solidification and liquid cracking.
Maintain sufficient wetting of the molten pool onto the base material.
Both preheating and interpass temperatures are crucial variables in welding. They both determine the cooling rate. The welders need to strike a delicate balance between preheating and interpass temperatures to achieve the expected welding quality.