The Essential Variables

The Essential variables are those variables that are essential for repeatability. Consider the following.

Joint Design
Joint Position
Material Specification
Process
Heat Treatment
Consumables, Electrodes, Gases
Process Controls
Technique
 

Joint design

The basis of a weld is the joint design, if there is anything wrong at that point, everything that follows will be a waste. Take the following butt welds; let us assume they are 10mm plate.

 

Figure 1

1

 

Figure 1

2

 

Figure 3

3

 

Figure 4

4

 

Figure 5

5

 

Figure 6

6

 

Externally each weld above will look much the same, but clearly there is a big difference in comparison to the material being joined. A design must be selected that guarantees equal volume as a minimum.

Figures 1 to 3 do not penetrate the full thickness of the parent material therefore the weld will not be equal to the parent material. The only way to rectify these is to remove all that has been done and prepare them as in 4 to 6.

The only way to produce welds equal to the parent plate is to prepare the joint design as in figures 4 to 6.

Figure 4 is a deposit onto a Backing bar. The time and preparation required for a backing bar is offset against the time it takes the welder to weld it, being able to use a large electrode and high energies for the initial weld.

Figure 5 a weld into a groove, the reverse side then gouged to clean metal and that side welded. This is the preferred joint design because there is half the deposited weld metal of figures 4 and 6, which means faster welding and less distortion.

Figures 4 & 6 are only used where there is insufficient access to the reverse side.

Figure 6 requires a greater degree of welding skill, closer tolerances, and the fit-up is much more time consuming.

Joint Position

Change of position, from flat to over head, does not necessarily require requalification. But it will do where impact properties are of concern.

Material Specification

A qualification in one Material specification covers a range of materials.

Process

A change in process (MIG, STICK, TIG) requires requalification of the procedure

Heat Treatment (Pre-heat, Inter-pass and Post-heat)

Some materials require cooling to be retarded, so we preheat . The more we need to retard the cooling the higher will be the pre-heat. It is the Carbon Equivalent (Ceq), the thickness and energy input (combination of amps, volts and travel speed) that dictate the preheat required.

Inter-pass temperatures are temperatures above which, welding cannot be restarted. The intention is the reverse of preheat, you don't want the cooling retarded too much.

Post weld heat treatment PWHT, are heat treatments that are applied after welding. They can be in the form of relief of stress, done to improve ductility in pressure retaining parts, or retain dimensions in structures when machining is done to heavy sections. Other forms of Post weld heat treatment are normalising and solution annealing.

Solution annealing improves corrosion resistance in stainless steels and normalising improves grain structure.

Changes in any of the above may require requalification depending on how it affects the finished weld.

Consumables, Electrodes, Gases

Electrodes need to be matched with the mechanical properties of the parent material. Change in class or size can required requalification. Change in Gasses can impact on the weld quality and usability.

Process Controls , Amps Volts, Travel Speed

Amps X Volts ÷ travel speed give you the energy input, energy input (also known as arc energy) is particularly important where impact properties are important.

Technique

The method of cleaning, the angle of electrode, the electrical stick-out, the size of gas cup, these form the technique. Some are essential and have a range some are not.