Heat input as a mathematical operator
One of the most discussed topics in the welding industry is the heat input. Irrespective of the discussion on the calculation method, i.e. the k-factor included (heat input) or the k-factor excluded (arc energy), the discussion on calculating the correct minimum, maximum and allowable ranges will always remain. Understanding the calculation of the heat input range is crucial for creating suitable production welds.
I have seen many different views and arguments on how to create the heat input range. For easy reading I will use “heat input” as a general term, referring to both calculation methods (heat input and arc energy). This first article will focus on the heat input as the mathematical operator. Part two will dive into creation of the allowable range based on codes, standards and specifications.
The importance of the heat input
Heat input, the amount of energy we put into our weld- and base metal, gives us an indication of the behaviour of the microstructure of the metals. Simplified you could say that as long as the same heat input is applied in our production weld compared to the (pre-)qualified weld, the weld and base metal structure should behave similar and material wise we may expect a suitable weld. To know what the heat input is under given circumstances, welding procedure qualifications (WPQ’s) are often performed. Allowing us to dictate correct heat input values in our production welding procedure specification (WPS).
The heat input is a mathematical operator and not a parameter you can change, see the heat input formula below, with the note that the arc energy formula is slightly different.
As you can see the parameters Volts (U), Amps (I) and the travel speed (v) are single values. By that I mean there is no minimum, maximum nor a delta between values stipulated in this formula.
Let’s look at an example. We have 25 Volts, 225 Amps, 30 cm/min and k = 1 which results in 1.12 kJ/mm. Welding with exactly these parameters might be possible, but process variations are inevitable and higher and lower values will be required. Obviously large ranges make production and monitoring of welds easier. You may ask the question; how do I know the correct higher and lower values of the weld parameters? The answer is to be found in the weld record of a WPQ. In a weld record, parameters such as lowest and highest volts, amps and travel speeds are recorded.
Discussions on how to use the recordings or data, which recordings are acceptable, using average values, low and high values and even statistical approaches have come across my desk. I relate this back to the fact that no code really defines this matter, codes simply refer to the heat input formula. Still we need to dictate the correct heat input range on the WPS. My preference is to use the minimum and maximum values of the weld record with peak values of starts and stops excluded.
By using these minimum and maximum values we can mathematically calculate the largest heat input range by using the formulas below. Within the industry this generally accepted method is referred as a “min min max” approach.
min in kJ/mm
max in kJ/mm
A practical approach
Despite the general acceptance of this method, some argue that this leads to unnecessary large ranges which do not occur during production. This may be true under particular circumstances. Proposals such as utilizing average values only or “min min min” might work out. I always suggest to be open and agree on the heat input range calculation method with the client. Do not forget to inform the welding inspectors on the chosen method. Then they will be able to calculate the heat input in the same manner as you did for your WPS.