Welder technique is not a stylistic choice; it is a metallurgical variable. A single deviation from the qualified method can change heat input, alter the microstructure, and compromise the entire structure.

Welders apply different techniques to execute their welding tasks. Fig. 1 presents 5 such these techniques in manual welding operations. While these techniques may appear similar in practice, they can have significantly different effects on heat input, weld microstructure, and compliance with the welding procedure specification.

 Welding Heat Input and Arc Energy

Every passage of welding arc across a metal surface introduces a defined quantity of energy into the parent metal. This energy,  commonly referred to as heat input, Q is measured in kilojoule per millimetre (kJ/mm) and is expressed in BS EN 1011-1 as:

Where k = thermal efficiency, U = arc voltage, I = welding current, v = travel speed in mm/s. It is apparent that the parameters in the equation are essential variables during pWPS qualification. The Q value is valid only when the welder travels linearly. For example, if the procedure requires a forward travel of 5 mm in 5 seconds, this progression must be achieved in a straight path. In practice, this is best achieved using a stringer bead welding technique (no. 1 in Fig. 1). When weaving techniques are introduced, the forward progression of the arc is no longer linear, leading to increased arc on time per unit length.

If a WPS is qualified using a stringer technique, deviation from this technique or the use of weaving technique as in no. 2 to 5 in Fig. 1 renders the weld non-compliant with the qualified WPS. Weaving increases effective heat input beyond the value qualified in the procedure, which can result in excessive reheating of the weld zone. Why is this the case?

Effect of Welding Heat Input on Microstructure and the HAZ

Heat input has a direct influence on weld metallurgy and the resulting microstructure of the parent metal. Both the magnitude of energy and the duration of exposure determine the extent of microstructural change within the weld and the HAZ.  

Welders who lack an understanding of welding metallurgy may not fully appreciate why repeated passage of arc energy across the same region is detrimental to weld quality, particularly in structural steel welding. During manual welding, the use of these weaving techniques allows the welding arc to cross a given area more than once. The immediate result is increased peak temperature in the HAZ of the parent metal and extended dwell time within that region. This action promotes grain growth in the coarse grain HAZ.

As grain size increases, the tendency for forming harder and more brittle phases during cooling also increases. Large austenite grains reduce nucleation sites during transformation and can therefore facilitate the formation of brittle transformation products, including martensite in susceptible steels under rapid cooling conditions. Even when martensite does not form, coarse grain growth significantly reduces toughness and overall weld performance.

 The Most Detrimental Welding Technique

Based on these metallurgical principles, no 2 in Fig. 1 represents the most detrimental welding technique and should be discouraged in all forms of structural welding. Its pattern promotes repeated reheating of the same material, elevated effective heat input, and enlarged heat affected zones, all of which can compromise the structural integrity of the welded component. Such techniques should not be taught or promoted in welding training environments.

 WPS Qualification and Production Welding Consistency

Now let us be clear here. If a particular technique must be used for WPS qualification due to its ability to produce desirable physical property such as penetration, this technique needs to be used by all the welders during production welding for the WPS to remain valid. For high integrity welding applications, where precise control of heat input per unit length is critical, stringer bead welding remains the most reliable method for delivering the heat input specified in the welding procedure.