Weld Joint Preheating Requirements and advantages

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Weld Joint Preheating Requirements and advantages

Post by WeldingNDT »

What is preheating?
Heating the base metal (parent metal) to a specific desired temperature prior to starting welding a joint is called as preheating. The temperature at which the base metal will be heated is called as preheat temperature. The construction codes determine the requirement of preheating and preheat temperature. The preheat temperature shall be addressed in the applicable welding procedure specifications. You may have a doubt, what is construction code? A book with a set of mandatory requirements (rules and standards) and recommendations for the design, construction, and testing of a product. see below table it contents some popular construction codes and the reference for the preheat requirement as the respective construction code.
In the absence of code, the following factors shall be considered for determining the preheat temperature;
Parent Metal / Base metal thickness
The weather conditions of Welding Location (Surrounding temperature, humidity, etc.)
Base metal chemical composition (Carbon equivalent number)
Hydrogen content of the electrode
Service (Operational) requirements
However, in general, preheat is required for carbon steel materials above 25mm thickness, but other factors shall always be considered before determining the preheat temperature.

Why preheating is required or What are the advantages of preheating?
Preheating is carried out for the following reasons;
It slows down the cooling rate of weld metal, HAZ (heat affected zone) and adjacent base metals, which yields a good microstructure to the metal, prevents martensite formation at the microstructural level and prevents cracking of the weld metal and HAZ.
Preheating removes the diffusible Hydrogen from the base metal and hence prevents the chances of Hydrogen induced cracking (HIC).
It helps in reducing the expansion and contraction rate.
It burns the unwanted material or impurities (if any) present on the joint surface.
Preheating also helps in achieving better mechanical properties such as notch toughness.

Oxy-fuel heating torches (flame heating), Induction heaters, Infrared radiant panel heaters, and electrical heaters are commonly used for preheating a joint. It is recommended to preheat the joint from the backside (Otherside), to ensure that the entire volume of metal, surrounding the joint, has been heated.
Preheating with Quadral Oxy-Fuel heating Torch
Preheating with Induction Heating Coil
The preheat zone shall be at or above the specified minimum temperature in all directions from the point of welding for a distance of the larger of 75 mm (3 in.) or 1.5 times the greater nominal thickness.

Digital infrared pyrometers or temperature indication crayons (thermal chalks) can be used for checking the temperature of the preheated item. The tempilstik is a famous brand for temperature indicating crayons

The welding always shall be started immediately after the preheating is done. If the metal thickness is very high or the surrounding temperature is very less, the temperature should be checked during the welding, and the temperature to be maintained between the preheat and the inter-pass temperatures until complete the joint welding.

You can get the details for the preheat, inter-pass temperature, heating method, preheat maintenance requirement from the applicable welding procedure specification - WPS.

An example, you have a joint for welding, as per your Welding Procedure Specification, it requires minimum preheat temperature is 100°C, preheating maintenance is continuous, and the inter-pass temperature is maximum 250°C.

If you want to verify the preheat inter-pass temperature for the above condition with temperature indication crayons, you must have two temperatures indicating crayons one is 100°C, and the other one is 250°C.

Prior to starting the welding, heat the joint with the applicable heating source for few minutes (higher thick metal will take more time), then check both parent metal (each side of the joint) with 100°C temperature indicating crayon by making a line or tick mark in the preheat zone (like using chalk piece in blackboard). If the crayon makes a line or tick mark that means heating is not sufficient, so continue the heating, if the crayon melts while making a line or tick which means the base metal reached or exceeds the desired temperature. Start welding immediately and you need to verify the inter-pass temperature frequently until complete welding of the joint.

Now use the second crayon which is temperature rating is 250°C to verify the inter-pass temperature like as explained for preheating. If the crayon makes a line or tick mark you can continue the welding. If the crayon melts the temperature may exceed 250°C hence stop the welding, and give time to cool down then restart the welding. If you stop the welding for cooling, it shall not be cooled below the minimum preheat temperature.

Remember, the preheat temperature is minimum so crayon needs to start melting, but the inter-pass temperature is maximum so crayon melting not allowed.

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