The WelderDestiny Compass

Help With Heat Input - Issue #037

Wednesday, September 13, 2017 / Perth Australia / By Niekie Jooste

In this edition of "The WelderDestiny Compass":

  • Welding Heat Input
  • Heat Input Defined
  • Why Control Heat Input?
  • Controlling Heat Input In The Field

Welding Heat Input

Today we get back to talking about welding. In particular we will take a look at why it is important to control welding heat inputs in many applications.

The fundamental thinking behind the importance of the welding heat input, is that as the heat input increases, the cooling rate of the weld is reduced. With a reduced cooling rate, most materials will exhibit increased grain size in the weld and heat affected zones (HAZ) of the base metal. Increased grain size is associated with reduced impact properties.

Another effect of reducing the cooling rate is that certain hard brittle phases, such as martensite, can be prevented from forming. When welding materials that are prone to hardening, and suffering from hydrogen assisted cold cracking, then it may be important to increase the heat input above a certain minimum value.

In other words, sometimes we would like to make sure that we remain below a maximum heat input level, and sometimes we want to make sure that we remain above a minimum heat input level.

Many people consider the welding heat input in a very one dimensional way. Many people think that as long as the heat input remains the same, then the welding result will be the same. This is unfortunately not the case. There is definitely more to this variable than many people realise. 

Not only is it often important to control the heat input, but it is one of the most difficult parameters to control in the field, because we can only calculate the heat input for a weld run after the run has been deposited. If something is wrong at that point, then what?

Today I will also introduce you to a smartphone app that I developed to assist you in controlling heat input out in the field.

If you would like to add your ideas to this week’s discussion, then please send me an e-mail with your ideas, (Send your e-mails to: or complete the comment form on the page below.

Now let's get stuck into this week’s topics...

Heat Input Defined

In short, the welding heat input is defined as the amount of energy that is used by the welding process to deposit a specific length of weld.

The usual practice is to express the energy in terms of "Joules". In the equation, it will be shown as "J". Often this is changed to kilo Joules, (kJ) in which case the Joule value is divided by 1000. The length can typically be inches (in) or millimetres (mm) or centimetres. (cm) The most common usage is inches for those working in imperial units (e.g. USA) and millimetres for those working in SI units. (Most of the rest of the world!)

The most common equation used for calculating the welding heat input is:

Now, let us look at that equation, and see what it is telling us. In summary, it is showing us that we can keep a stable heat input (HI) while increasing the welding voltage or amperage, as long as our welding travel speed is proportionately faster. This in turn suggests that we can end up with the same weld microstructure and mechanical properties, even if we change the welding parameters, as long as we just keep the welding heat input steady. That assumption is not entirely correct. To find out more, see a broader discussion on the topic here...

Why Control Heat Input?

The typical reasons for controlling the heat input are as follows:

  • To reduce the probability of hydrogen assisted cold cracking (HACC) when welding certain steels. This is common when welding pipelines using cellulosic welding electrodes.
  • To ensure high enough toughness properties in ferritic materials.
  • To reduce the formation of constituents in the microstructure of stainless steels that can reduce their corrosion resistance. (e.g. Sensitization of stainless steels, or formation of inter-metallic phases in duplex stainless steels.)
  • To reduce surface oxidation during welding. Typically of stainless steels, titanium and nickel based alloys.

The welding code you are using will guide you on the necessity of controlling the heat input, and the appropriate ranges that are qualified by your welding procedure qualification coupon.

Controlling Heat Input In The Field

Welding heat input is one of the most difficult parameters to control out in the field, because we can only decide if the required heat input has been achieved in hindsight. During the welding, we can monitor the volts, amps and travel speed, but only after the weld run has been completed can we perform the calculation to see if we have actually achieved the required heat input limits.

Not only is this process tiresome, but it leads to all kinds of issues when we find that the deposited weld bead had been welded with a heat input value outside the allowable limits. What is the best way forward? Grind out the offending weld bead? There is no one single answer, because there are numerous situations where the control of the welding heat input is important for different reasons.

Given the difficulties of controlling welding heat input in the field, I have developed a smartphone app to assist. It is called the "Heat Input Helper". This app allows you to enter the target maximum and minimum heat inputs, and then entering the voltage and amperage the welder is using to weld. The start button is pushed when the welder strikes his/her arc, and the app shows how far the welder should have welded during any given time to remain within the heat input limits. It therefore, very speedily, becomes obvious when the travel speed is outside the required range. Remedial action can then be taken long before the weld run is completed.

The app has been written with a delay function so that Welders can actually use it to time themselves. This is useful when Welders want to "practice" to weld within a certain heat input range.

At this stage you can download the app from the Google play store...

You can give the Heat Input Helper a "test drive" here...

If you are interested in a more in-depth discussion of the heat input essential variable, click here...

Yours in welding

Niekie Jooste

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