Steels

Weldability of materials

In arc welding, as the weld metal needs mechanical properties to match the parent metal, the welder must avoid forming defects in the weld. Imperfections are principally caused by:

• poor welder technique;
• insufficient measures to accommodate the material or welding process;
• high stress in the component.

Techniques to avoid imperfections such as lack of fusion and slag inclusions, which result from poor welder techniques, are relatively well known. However, the welder should be aware that the material itself may be susceptible toformation of imperfections caused by the welding process. In the materials section of the Job Knowledge for Welders, guidelines are given on material weldability and precautions to be taken to avoid defects.

Material types

In terms of weldability, commonly used materials can be divided into the following types:

• Steels
• Stainless steels
• Aluminium and its alloys
• Nickel and its alloys
• Copper and its alloys
• Titanium and its alloys
• Cast iron

Fusion welding processes can be used to weld most alloys of these materials, in a wide range of thickness. When imperfections are formed, they will be located in either the weld metal or the parent material immediately adjacent tothe weld, called the heat affected zone (HAZ). As chemical composition of the weld metal determines the risk of imperfections, the choice of filler metal may be crucial not only in achieving adequate mechanical properties and corrosionresistance but also in producing a sound weld. However, HAZ imperfections are caused by the adverse effect of the heat generated during welding and can only be avoided by strict adherence to the welding procedure.

This part of the materials section of Job Knowledge for Welders considers the weldability of carbon-manganese (C-Mn) steels and low alloy steels.

Imperfections in welds

Commonly used steels are considered to be readily welded. However, these materials can be at risk from the following types of imperfection:

• porosity;
• solidification cracking;
• hydrogen cracking;
• reheat cracking.

Other fabrication imperfections are ) steel will have a lower alloy content than a normalised steel, and thus will be more readily weldable with regard to avoidance of HAZ hydrogen cracking and the achievement of maximumhardness limits. However, there is always some degree of softening in the HAZ after welding TMCP steels, and a restriction on the heat input used, so as not to degrade the properties of the joint zone (e.g. ≤2.5kJ/mm limits for 15mm plate).

Group 3. Quenched and tempered steels and precipitation hardened steels (except stainless steels), R _eH > 360N/mm ²

These are weldable, but care must be taken to adhere to established procedures, as these often have high carbon contents, and thus high hardenability, leading to a hard HAZ susceptibility to cracking. As with TMCP steels, there maybe a restriction on heat input or preheat to avoid degradation of the steel properties.

Groups 4, 5 and 6. Chromium-molybdenum and chromium-molybdenum-vanadium creep resisting steels.

These are susceptible to hydrogen cracking, but with appropriate preheat and low hydrogen consumables, with temper bead techniques to minimise cracking, the steels are fairly weldable. Postweld heat treatment is used to improve HAZtoughness in these steels.

Group 7. Ferritic, martensitic or precipitation hardened stainless steels.

When using a filler to produce matching weld metal strength, preheat is needed to avoid HAZ cracking. Postweld heat treatment is essential to restore HAZ toughness.

Group 8. Austenitic stainless steels.

These steels do not generally need preheat, but in order to avoid problems with solidification or liquation cracking upon welding, the consumables should be selected to give weld metal with a low impurity content, or if appropriate,residual ferrite in the weld metal.

Group 9. Nickel alloy steels, Ni ≤ 10%.

These have a similar weldability to Groups 4, 5 & 6.

Group 10. Austenitic ferritic stainless steels (duplex).

In welding these steels, maintaining phase balance in the weld metal and in the HAZ requires careful selection of consumables, the absence of preheat and control of maximum interpass temperature, along with minimum heat inputlevels, as slow cooling encourages austenite formation in the HAZ.

Group 11. High carbon steels.

These steels will be less weldable owing to their increased carbon content with respect to Group 1. It is likely that care over the choice of consumables and the use of high preheat levels would be needed.

It is important to obtain advice before welding any steels that you do not have experience in.

References

1. BS EN 1011-2:2001 'Welding - receommendations for welding of metallic materials - part 2: Arc welding of ferritic steels' British Standards Institution, March 2001.
2. PD CEN ISO/TR 15608:2005 'Welding - guidelines for a metallic material grouping system' British Standards Institution, October 2005.