Each type of metal has very distinct properties, including how they respond to welding. It therefore makes sense that the base metal would be an essential variable within welding codes.
The problem is however that the way in which materials are covered in material specifications and materials designation systems are not uniform. Some material designation systems are based purely on the chemical composition. Some are based on product type, such as pipe, casting or forging. Some are based on the end use of the material, such as pressure equipment, building structures or ships.
It should therefore be clear that different welding codes that focus on different welding scopes will deal with the base metal essential variable in different ways.
None-the-less, we will take a big picture view of how the base metal affects our welding procedure and base our discussion on how the welding codes typically address the base metal essential variable.
Most welding codes will group base metals. The idea being that if you have demonstrated that you can successfully weld one of the base metals in the group, then that will be good enough to assume that you will get similar results when welding the other metals in that group. This saves us from having to qualify hundreds of different welding procedures on base metals that are essentially the same, or similar.
To achieve the outcome above, base metals are typically grouped on a combination of the following parameters:
The way in which this grouping is done, is different between different welding codes.
As a general rule, the American codes, such as ASME IX, lists every material specification that is to be included in a group. In the case of ASME IX, these groups are termed "P" numbers. When impact properties are also specified, then an additional grouping is added. This is called the "group" number.
The European codes tend to rather give the rules whereby different materials can be grouped. They also tend to give tables that show typical material groupings, but if you follow their rules, you can theoretically decide for yourself into which group the base metal falls. Many of the "EN" and "ISO" welding codes (such as ISO 15614) tend to reference ISO/TR 15608 for the "grouping rules". Interestingly, these material groupings are very much in line with the ASME P number grouping system, and is primarily based on chemical composition and strength factors.
The product specific welding codes such as AWS D1.1 or AS 1554 P1 have their own methods of assigning materials to groups. Due to the narrow scope of these codes regarding the materials covered, (structural steel in the two noted examples) their grouping systems are more unique to their requirements. The basic grouping mechanism is however based on:
There are obviously product specific welding codes dealing with widely differing materials such as aluminium, stainless steel and titanium, to name just a few. Each of these welding codes will have their own grouping system, so it is not possible to deal with them all here, but the principles are very much the same. They group materials together in a logical manner, to reduce the number of procedure qualifications that will be needed.
The broadest range of grouping in chemical composition will be based on the particular "alloy system" that is being welded. Typical alloy systems are:
While it looks like there is a 1:1 correlation between ASME IX and ISO/TR 15608, this is not necessarily the case. There are some notable differences, especially in the case of the high alloy Nickel Chrome alloys.
The later editions of ASME IX do have a column in their P number listing tables (Table QW/QB-422) showing the correlation between the P number and the "ISO 15608 Group". The advantage of the ISO/TR 15608 document is however that it tells you how to categorise any material yourself, so it does not exclude materials just because they may be produced to a foreign specification not included in a listing, like is the case with ASME IX.
More detailed grouping of the materials then takes place within these wider groups, based on the more specific alloying additions. For example, see the following:
If the weldability of the materials are the same, even if the alloy additions are different, then the welding codes may give you a wider qualified range that crosses over the different groups. For instance, if you qualified a welding procedure on a P4 to P4 material combination, ASME IX allows you to weld a P4 material to P4, P3 or P1 materials.
A metal's final properties can be changed significantly by heat treatments (thermal treatments) such as:
There are many more thermal treatments, but those mentioned above are sufficient for our present discussion.
It should be clear that a material that has enhanced properties due to a quenched and tempered thermal treatment will respond differently to welding than a material that was subjected to a precipitation hardening thermal treatment. For this reason, materials that have different thermal treatments are typically placed in different groups, or sub-groups, within the welding codes.
A metal's final properties can also be modified by mechanical treatments such as:
Welding will generally result in a recrystallization of a cold worked material in the heat affected zone (HAZ) of the weld. For this reason, materials with different mechanical treatments are placed in different material groupings or sub-groupings within the welding codes.
Some metals have their properties enhanced by a complicated combination of thermal and mechanical treatments. A typical example is of thermo mechanically controlled process (TMCP) steels.
At the steel mill, the TMCP steel undergoes a combination of hot and cold rolling to its final thickness. This means that the steel can have a high strength, without the typical micro alloy additions of the normalized steels. This reduced alloying content results in a lower hardenability, so welding is easier, because it rarely results in a hard brittle martensitic structure.
TMCP steels have their own issues, because they are more susceptible to weakening if the welding heat inputs are excessively high. In addition, elevated temperature treatments such as post weld stress relieving could result in excessive strength reduction.
In some instances, these steels are either placed in their own sub-grouping, or are addressed by some clause in the welding or construction code placing additional restrictions on these types of materials.
It is not the intention of this web page to give a definitive guide to all the different base metals possible. Rather, the intention is to describe why the base metal essential variable is included in the welding codes. In addition, I have attempted to introduce the fundamental reasons why the welding codes group the different materials into groups, and what factors are part of the grouping process.
It is important to appreciate that due to the different scopes of different welding codes, the specific grouping requirements can vary between codes.