Carbon-manganese (C-Mn) steels are the predominant structural steels in use in large quantities, in a diverse range of applications, throughout the engineering industry. C-Mn steel fabrications can generally be welded successfully, as long as the steel composition is known, appropriate precautions are taken and qualified procedures are followed.
C-Mn steels vary in their ease of welding, or weldability. Given the relevant combination of circumstances, hydrogen cracking, solidification cracking, reheat cracking, are all possible cracking mechanisms. Most modern compositions give good resistance to these types of problem, but in the case of hydrogen cracking, perhaps the most important consideration in structural steel applications, the welding procedure is very important.
Precautions necessary to avoid hydrogen cracking:
- Prevent the ingress of moisture or hydrogen. This can be done by ensuring the cleanliness and dryness of workpieces, and that consumables are clean and dry prior to welding. For critical applications, basic, low hydrogen, manual metal arc electrodes are commonly used. Low weld metal diffusible hydrogen levels are obtained by using electrodes from sealed packs or by baking them prior to use. Welding in high humidity environments is also likely to increase weld metal hydrogen levels, and additional measures may be required to avoid cracking.
- Characterise the chemical composition of the steel to be welded. The carbon equivalent (IIW CE) is used to characterise the effect of alloying elements on weldability, and represents the contribution of the composition to the hydrogen cracking susceptibility of steel. With increasing IIW CE value (more highly alloyed) and material thickness, provision must be made to mitigate the adverse effects. Preheat may be required, and consideration should be given to using low hydrogen welding consumables and processes. Generally, steels with an IIW CE value of <0.4 are not susceptible to hydrogen cracking, as long as low hydrogen processes are used. Guidance for the avoidance of hydrogen cracking is given in BS:EN 1011:2001 Part 2.
- Choose an appropriate electrode and welding process. One of the major differences between the arc welding processes is the way in which the molten weld pool is protected during welding. This is achieved by using either a flux that forms a protective molten slag or a shielding gas. For thin sections and low IIW CE materials, rutile flux coated electrodes are used for general fabrication in manual metal arc (MMA) and flux cored arc welding (FCAW), and fused fluxes are used in submerged arc welding (SAW). For larger sections and/or higher IIW CE steels, lower hydrogen consumables have to be considered. For MMA welding the electrodes are baked, and basic flux electrodes provide the lowest weld metal hydrogen characteristics. For SAW, basic agglomerated fluxes are often used. Metal inert/active gas (MIG/MAG) and tungsten inert gas (TIG) welding are lower hydrogen processes that use shielding gases rather than fluxes. Girth welds in pipeline materials are frequently made using cellulosic MMA consumables or automated TIG set-ups for increased welding speeds, although with cellulosic flux coatings hydrogen contents can be high, and care must be taken.
Precautions necessary to ensure satisfactory weld properties:
The required mechanical properties of a welded joint are readily achieved in C-Mn steels with the use of the appropriate welding consumables. However, the complex nature of the structural changes that occur during the weld thermal cycle does mean that some care is needed in assessing properties such as heat affected zone (HAZ) toughness and hardness.
- In-service degradation can occur by strain ageing or stress corrosion cracking in C-Mn steels in some applications. These types of problem are avoidable if the correct precautions are taken, e.g. C-Mn steels in sour service must meet specified maximum hardness values to remove the likelihood of sulphide stress corrosion cracking.
- Higher strength structural steels used in highly stressed conditions usually have to meet additional requirements on standard structural grades such as minimum toughness requirements and maximum hardness requirements, especially in the HAZ. Postweld heat treatment must also be considered in many cases if the hardness of the weld is a problem or if it is a requirement of the fabrication code. Quenched and tempered steels, used for example in oil and gas wells for drilling, require special care over the electrode drying procedures, preheat temperatures, heat input levels, postweld heat treatment and postweld non-destructive testing.
Procedure qualification:
- It is generally a requirement that a welding procedure is qualified and then followed to ensure that the weld will be completed satisfactorily, and that the joint properties obtained are appropriate for the needs of the particular application. Guidance for such procedure qualification are given in BS EN ISO 15607:2003 and BS EN ISO 15609-1:2004.
Related items on welding and weldability of C-Mn steels:
References
BS EN 1011-1:2009 Welding - Recommendations for welding of metallic materials.
Part 1- General guidance for arc welding.
BS EN 1011-2:2001 Welding - Recommendations for welding of metallic materials
Part 2- Arc welding of ferritic steels.
BS EN ISO 15607:2003 Specification and qualification of welding procedures for metallic materials. General rules (Replaces BS EN 288-1:1992)
BS EN ISO 15609-1:2004 Specification and qualification of welding procedures for metallic materials. Welding procedure specification. Arc welding (Replaces BS EN 288-2:1992)