Practical use of Heat Input calculation for Manual MIG Welding

Cats Clause

Starting to get Involved

Under CARES certification requirements, Heat Input must be calculated and used on the approved welding procedures. (That is a defined set of settings for a given type of weld.) I am struggling to understand how this may be used as a practical benefit however, other than just to comply with the standard. (Once a procedure has been approved. IE all the relevant settings recorded and sample welds tested, welders often "tweak" process settings to obtain an optimum weld, but they do not re-calculate the resultant heat input every time.)

Any experience with this?

many thanks
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CARES as in "...CARES is the only certification body in the UK that is UKAS accredited to perform Product Certification for reinforcing steel and associated products..." ?

I can't help - Just wanting to make sure.

Cats Clause

Starting to get Involved
Hi Marc, yes that's the one!

Other than the CARES requirements, I believe heat input to be a common way to control weld quality, just like to understand any experience of practical applications for a manual process on the shop floor.


Heat input is an important factor to consider for a few reasons. It can affect material strength, corrosion or chemical resistance, and cause excessive distortion just to name a few.

When a welding procedure is developed, typically there is a min/max preheat or interpass temperature required. When welding on thicker sections of metal, it acts as a heatsink, therefore "quenching" the weld, which can cause internal cracking, that is why preheating is often required prior to welding and interpass temperature be maintained.

Metals with higher carbon content often times require preheat (typically) because the the grain structure in the material doesn't expand and contract as evenly as other materials, and thus can crack if cooled too rapidly.

Aluminum can be affected by excessive heat input. If it stays at high temperature for too long, the strength of the material is considerably compromised. Often times, there will be a limited amount of times an aluminum weld joint can be repaired.

Stainless steels are extremely sensitive to excessive heat. If proper care isn't taken, SST can lose it's chemical and/or corrosion resistance as the metal becomes sensitized.

Heat input isn't just about adjusting settings on the machine. Factors such as the welding process, joint fit up, groove angles, welding position, contribute to heat input.

Example; if you have a procedure for welding a single V groove joint, with a 60° included angle, 1/16 root opening, with the GMAW process, and someone decides to change the groove angle to 90° with 1/8 root opening, and uses the GTAW welding process. You will now be adding considerable more heat into the base metal. The 90° groove angle will now need more filler metal, as will the 1/8 root opening, which will likely require slower welding travel speeds and/or more passes to fill the groove all of which has the potential to drastically change the metallurgy. These affects can weaken the base metal and or weld joint resulting in a failure.

Remember, when a welding procedure is established, basically a "recipe" has been created to weld material "A" to material "B" using a given process and set of variables. When the variables are changed beyond the limits of the given welding code, there are many things that affect that recipe, and heat input is one of them. If someone has developed a procedure, and gone through the trouble of adding restrictions on heat input, there's probably a good reason for it.

Hope this helps. :2cents:
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