Heat Input Required for Welding of Metals

[RobsVette]

Hey guys,

One thing I've never had a clear understanding on as far as engineering fundamentals goes, is the required heat input needed when welding two pieces of a metal together. Here is an example of what I mean.

When welding 2 pieces of plate (standard carbon steel plate SA-516) together using an SMAW procedure you would want to use an amperage of approximately 100 amps and maybe 24VDC. 100 amps * 24V = 2.4 KJ/Sec. Pretty straightfoward, now add that to the required travel speed and you get your heat input for the weld.

The question I have is how do you determine what is the correct heat input for a particular kind of metal? Ie... if you went to P4, P5A or P8 what determines what kind of heat input is required?

Mettalurgy is not my strong point so if this is at all unclear or you need more information to provide an answer please let me know.

Thanks,
Rob

 

[metengr]

RobsVette;
For most welding applications, heat input is based on trial and error because the welder tries to optimize weld deposition with weld quality.

Control of heat input comes into play for certain metals where excessive heat from welding results in poor strength or corrosion resistance. So, for certain metals heat input may need to be determined and controlled in comparison to other metals.

 

[stanweld]

Your calculation for heat input is somewhat in error.
Heat Input = 60 x V x A /travel speed.

The minimum heat input to provide full fusion will not change for carbon steel or the low alloy steels for the same thickness; as thickness increases, heat input will need to increase. It will be different for stainless steel due to the diffeence in thermal conductivity of SS vs carbon and low alloy steels.

 

[RobsVette]

Hey guys, sorry for the confusion. My question would be what is the minimum heat input to provide full fusion on any particular metals and how would you go about finding this out?

From what you guys have stated it seems like it is more of an experience based determination, rather than a an acutal mathematical calculation with engineering principles behind it.

Thanks,
Rob

 

[stanweld]

The heat input required to provide full fusion is based on the material's melting point, thermal conductivity and thickness (effects cooling rate) and the preheating temperature applied and the welding process used. As far as I am aware, formulas have not been developed.