What do you mean by heat input? Are you talking about required weld power in amps to weld it or are you concerned about heat input limits for the weld procedure?
A crude rule of thumb that is often quoted is that 1 amp is required for every .001" of material thickness. At least, that gives a starting point for tube welding on most stainless, Inconel, Hastelloy, and Kovar at 5 ipm.
But, heat input by textbook definition is (Amps x Volts x 60) divided by travel speed in ipm. Usually the goal is to reduce heat input so as to have better (smaller) grain structure. Large grains and poor toughness and high heat inputs go together.
Ok, you said 1 amp is required for every .001" of material thickness at 5ipm travel speed. Is there a formula to help me figure this at faster travel speeds such as 40-50 IPM (automated welding)? Let's say a lap weld and both materials are around .070" thick at 40 IPM travel speed. However, is what I'm really wanting is to be able to figure is a target heat input value by the material thickness and travel speed. I know this will change depending on the type of material your welding, but stainless 300 series and 400 series is my main focus.
I would venture to say that there really isn't a generic target heat input target for materials. The heat input range desired depends on the material and the mechanicals you wish to achieve. The desired heat input is engineered. A desired heat input range or reference point can be gained either from manufacturers recommendations and/or your own empirical results.
To put succinctly, what is the intended service?
I have used the heat input forumula (amps x volts x 60/travel speed) to convert between say 5ipm and 7 or 10 ipm, but it would probably not work for drastic changes. The problem lies with issues related to heat transfer like thermal conductivity, and all those other heat flow variables that are non linear and which are discussed at great length in welding engineering theory. In other words, it would probably involve calculus and/or differential equations, and would only be applicable to one specific alloy. But 40-50 ipm? That is pretty drastic. I hope you are planning to use a laser.
By G.S.Crisi 
Date 10-06-2009 18:25
Edited 10-07-2009 19:50
I think Josh (2008642) is referring to the minimum base material temperature required before start welding.
This temperature changes according to the chemical composition of the material. For low carbon steel, for example, the base material must be "warm to the hand", which in practice means 30 - 35 Celsius (about. 80 - 90 F). For alloy steels the applicable code states which is the temperature.
Giovanni S. Crisi
Sao Paulo - Brazil
Comment written on October 8. Indeed, Josh is not referring to base material temperature
BS 5135 or EN1011-2 both give advice on the minimum heat input required to weld structural steels of varying carbon equivalence and diffusible hydrogen content in the welding consumable depending on the combined thickness, or to work out what preheat is required if the heat input is known. The general principle is that the higher the heat inout the slower the cooling rate and the less risk of hydrogen cracking due to the slower cooling rate.
This is however a very complex issue and, as others have noted, just increasing the heat input will decrease toughness.
