Welding variables have minimal effect on the final welded joint for many applications. Many applications involve thin plain carbon steel such as A36 or similar grades. Variables become critical when welding higher-strength or alloyed base materials.

To produce high-strength materials with good toughness properties, two types of materials have different mechanisms used in their manufacture. High-strength, low-alloy (HSLA), and thermo-mechanically controlled processed (TMCP) steel.

High-strength, low-alloy (HSLA) steel is produced by micro alloying. This adds small amounts of alloy such as: 

  • Manganese
  • Molybdenum
  • Chromium
  • Nickel
  • Niobium
  • Titanium
  • Vanadium

Thermo-mechanically controlled process (TMCP) is produced through carefully controlled temperatures and rolling pressures during steel production. In some cases, both methods have high-strength, quality steel with good toughness properties.

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To answer the original question, the welding heat input calculation is defined by voltage, amperage, and travel speed. It is generally expressed as kilojoules per linear inch of the weld (kJ/in.).

The general heat input range recommended for welding various carbon steel and specialty alloys is 30 to 70 kJ/in. However, not all materials are bound by that range.

Heat input from welding can severely affect the weld mechanical toughness properties of the base material in the heat-affected zone (HAZ) and the weld metal itself. Therefore, the higher the base material’s strength and/or corrosion resistance, the higher the precaution.

In most cases, problems arise when the heat input is too low or too high. Heat input that is too high can slow your cooling rate, resulting in excessive grain growth. This excessive grain growth changes welding mechanical properties. It mainly decreases the material’s cold weather toughness.

Using incorrect welding parameters can cause small amounts of alloy to be lost in the arc. This directly reduces the effective welding mechanical properties of the welded joint. This is also true with stainless steel, resulting in degradation in corrosion resistance. For stainless steel, restrict heat input to 50 kJ/in. or less should prevent this from happening.

Slow travel speed usually occurs when you attempt to fill a large joint with only a few welds passes. This raises the heat input drastically. As a result, the weld pool’s solidification rate occurs slower. This promotes grain growth in the weld metal and HAZ. Large grain size tends to produce a weaker microstructure. This depends on filler metal and base metal chemistry, percentage of dilution, and peak temperatures.

Some HSLA steels have trace amounts of other elements added. These elements act as grain refiners. Boron or titanium can maintain good weld mechanical properties with acceptable weld heat input as high as 115 kJ/in.

Limiting heat input and controlling maximum inter-pass temperatures are critical. These are critical to maintaining the parent metal properties. This refers to welding on TMCP, quenches, and tempered (Q&T) steel such as A514. Steel that gains better weld mechanical properties from the Q&T process is likely to lose these gains in strength and toughness. Preheat and maximum interpass temperatures will be determined by material thickness. The heat input should be limited to no more than 60 kJ/in.

Excessive travel speed with low welding parameters can produce heat input values that are too low. Issues arise if this happens because of the rapid solidification of the weld bead. The resulting weld can have a very unrefined grain structure. This will produce a weld deposit with excessive yield and tensile strength values that overmatch the base material. To prevent this, maintain a heat input value greater than 20 kJ/in.

Preheating is done for two primary reasons. First is the desire to slow the cooling rate of the weld metal and HAZ. This is to prevent undesirable microstructures from forming, such as martensite. The second is to drive off condensate, found on the material’s surface, from ending up in the weld joint. This would cause porosity or increase the potential for hydrogen cracking. Preheating can also assist in reducing the number of residual stresses in the weld metal and weld joint. This can eliminate post-weld heat treatment (PWHT) in some applications.

Controlling interpass temperature is essential. It ensures that the weld’s cooling rate will be fast enough to deliver the required weld mechanical properties. For example, A514 interpass temperature should be kept under 400 degrees F. While NiCrMo steel such as 4340 can be as high as 650 degrees F. Keep in mind these values are thickness dependent.

The metallurgy determines the various possible outcomes. Getting familiar with: 

  • Time-temperature-transformation diagrams 
  • Continuous-cooling-transformation diagrams 

will provide an excellent visual for the phase changes occurring in the weld metal and HAZ.

If you’d like more information on welding mechanical properties or how PMET can help your business using our multiple welding services, please contact us today. We are experts in multiple weld services, including laser welding and TIG Welding.