Even within the welding industry, the term weldability remains fairly generic and, depending on the context, can have different meanings.

In the context of pipeline steel welding, the AS2885.2 defines weldability as “the ability of a metal to be welded under given fabrication conditions, in a specific weldment, and to perform satisfactorily in service”. Breaking this definition down further provides a basic understanding of weldability in the context of pipeline steels.

What makes a steel weldable?

There could be several reasons why a material cannot be welded. For example, the material may be too badly damaged (metallurgically) by the heat of welding, or the material can’t be welded using ‘normal’ techniques.

In the pipeline industry, an index called carbon equivalent is commonly used to help determine if a particular steel is able to be welded using common pipe welding techniques. The carbon equivalent is a value calculated from the chemical composition of the steel.

AS2885.2 provides the International Institute of Welding (IIW) equation for carbon equivalent, which incorporates the elements carbon, manganese, chromium, molybdenum, vanadium, copper and nickel. For modern pipeline steels, the lower the value of carbon equivalent, the more weldable the steel.

Commonly, low carbon pipeline steels with an IIW carbon equivalent less than 0.4 are considered to have good weldability. All Australian-made pipeline quality steel grades API 5L X42 through to X70, including those produced by BlueScope Steel, have a carbon equivalent less than 0.4 and therefore good weldability.

Care needs to be taken when dealing with steel grades of higher carbon content, like some of those used in pipeline fittings. While two steels may have the same carbon equivalent, the complete steel alloy design needs to be considered to assess the weldability.

Welding fabrication conditions

The conditions under which welding can occur vary significantly. Some steels may need to be welded in a workshop under closely controlled conditions, while others may tolerate much more adverse welding conditions, for example pipeline steel welding in the middle of the desert. The conditions under which the steel can be welded contribute to its weldability.

In the case of pipeline steels, they must exhibit excellent weldability under typical fabrication conditions, including:

• Welding in all positions around the pipe with cellulosic electrodes;
• No preheat;
• Adverse loadings (release of clamps, lifting and lowering); and,
• High production rate demands.

A steel may meet strength and toughness requirements, however it could be considered to have poor weldability if it cannot be welded under particular conditions. A detailed knowledge and understanding of the fabrication conditions are critical in developing steels suitable for the Australian pipeline industry.

Satisfactory performance

To perform satisfactorily in service, the weld and weld heat-affected zone must be fit-for-purpose. Key factors that define fitness-for-purpose in AS2885.2 are:

• Meeting the criteria of acceptance for girth weld discontinuities; and,
• Following the requirements for welding to avoid hydrogen-assisted cold cracking (HACC) in the heat-affected zone or weld metal.

It follows, then, that a common measure of the weldability of a pipeline steel is its resistance to cracking. There are several types of cracks that are possible in both the weld metal and the weld heat-affected zone. However, from the perspective of weldability of pipe steels, the resistance to heat-affected zone HACC (also referred to as hydrogen cracking, hydrogen-induced cracking or delayed cracking) is the most critical measure.

HACC occurs when three criteria are met:

• The presence of hydrogen;
• The presence of stress;
• The presence of a susceptible microstructure.

If one of these criteria is removed, heat-affected zone HACC cannot occur. Pipeline steel with excellent weldability is highly resistant to HACC because the steel alloy design ensures that under typical pipeline fabrication conditions a susceptible microstructure will not be produced.

Through the development of pipeline steel designs, the problem of HACC is now more likely to manifest in the weld metal. At present there is limited detailed understanding of the factors contributing to weld metal HACC, and this is currently the subject of a significant research effort within the new Energy Pipelines Co-operative Research Centre.

BlueScope Steel produces hot rolled coil feed for oil, gas and water pipelines. The company has been part of the Australian pipeline industry since 1968. Over this period, it has supplied approximately 1.3 million tonnes of steel for over 160 oil and gas linepipe projects.