𝐏𝐫𝐞𝐡𝐞𝐚𝐭𝐢𝐧𝐠 𝐢𝐧 𝐖𝐞𝐥𝐝𝐢𝐧𝐠

 𝐖𝐡𝐚𝐭 𝐢𝐬 𝐏𝐫𝐞𝐡𝐞𝐚𝐭𝐢𝐧𝐠 𝐢𝐧 𝐖𝐞𝐥𝐝𝐢𝐧𝐠?

Heating the base metal surrounding the weld joint, to a specific  minimum temperature (Preheat Temperature) prior to welding or performing tack welds.

Preheating can be either just before the welding or continuous preheat maintenance till the welding is completed.

𝗣𝘂𝗿𝗽𝗼𝘀𝗲 𝗼𝗳 𝗣𝗿𝗲𝗵𝗲𝗮𝘁𝗶𝗻𝗴:

Prevention of Hydrogen Cracking (Cold Cracking/ Delayed Cracking).

Reduce residual stresses and control distortion.

Reduce propensity of hard micro-structure formation.

Eliminate stress-oriented weld cracking & minimize welding porosity.

𝗪𝗵𝗶𝗰𝗵 𝗺𝗮𝘁𝗲𝗿𝗶𝗮𝗹𝘀 𝘀𝗵𝗮𝗹𝗹 𝗯𝗲 𝗽𝗿𝗲𝗵𝗲𝗮𝘁𝗲𝗱?

1- Depending on the thickness of base metals.

2- Base metal chemical composition & CE (Carbon Equivalent) value.

3- According to applied Code or SPECs.

4- Presence of hard micro-structures in the base metal or weld metal.

5-Type of welding electrodes & level of hydrogen in the welding rod.

6-Base material type- Low alloy requires preheat.

𝗠𝗲𝘁𝗵𝗼𝗱𝘀 𝗼𝗳 𝗽𝗿𝗲𝗵𝗲𝗮𝘁𝗶𝗻𝗴:

1) Induction

How it works: Induction creates a magnetic field that generates eddy currents within the base metal, heating it internally from within. 

2) Open flame

How it works: Operators use a fuel gas and compressed air torch (sometimes called rosebuds) to apply flame directly to the metal part.

3) Resistance heating

How it works: Resistance heating uses electrically heated ceramic pads placed on the base metal. The heated tiles transfer heat to the part through radiant heat and conductive heat where the pads are in contact with the part.

4) Ovens

How it works: Ovens used for welding preheat use convection heating. The entire part is placed inside the oven for preheating.