Brittle issues

Tim Burgess, William Hackett

October 1, 2016

Balancing cost reduction with correct materials selection is key when it comes to problems such as hydrogen embrittlement, says William Hackett’s Tim Burgess.

Welded chain sling link cracked. Images from William Hackett.

Operators need to ensure that despite commercial pressures, the products used in the offshore environment are fully appropriate for their intended use.

Standards and guidelines for the sector have continued to advance, based on accumulated knowledge, resulting in best practices being updated and then the standards that are applied to our sector converging and becoming harmonized. The application of these industry standards and guidelines is focused on how the product is intended to be used. But, having a detailed understanding of the properties of chain and links is equally relevant within the lifting offshore environment. Without the proper understanding, the end result is increased risk to operations.

Indeed, over the past two years, there has been a worrying increase in cases of hydrogen embrittlement impacting both chain and links, which are being used in lifting within the offshore environment. This has been the result of inappropriate product being used offshore, which may have been driven by cost cuts. This presents a huge safety implication, which could result in a catastrophic failure. The Hydrogen Embrittlement chart illustrates the implication of a failure, which also further highlights the need to fully understand the integrity of the product and also the traceability of the grade of steel used in production.

Hydrogen embrittlement is the process by which metals such as steel become brittle and fracture due to the introduction and subsequent diffusion of hydrogen into the metal and can also be described as stress corrosion cracking. For hydrogen embrittlement to take place, three elements are required.

Hydrogen can enter steel through the manufacturing process: steel making, heat treatment and finishing processes; and through the evironment: corrosion in water, cathodic processes linked to the pH content of moisture in the air.

The susceptibility of steel to hydrogen embrittlement is related to its composition, microstructure, strength and hardness, otherwise known as the material condition. It is important to note that the material condition is always the root cause of hydrogen embrittlement while the hydrogen source and the mechanical stress are triggers in the process.

As the hardness of a steel increases there is an increased risk of hydrogen embrittlement.

When high-strength steels are subjected to sustained tensile loads under normal ambient temperatures, dissolved hydrogen is attracted to the regions of high tensile stress.

As it diffuses to high stress regions, it is absorbed on planes of weaknesses, such as grain boundaries, where it reduces the attractive forces between the iron atoms. When the force required for de-cohesion of these plane is reduced to less than that required to cause plastic flow, slow cracking occurs. This is called stress corrosion cracking and is normally a result of hydrogen embrittlement.

Mitigating the risk

While magnetic particle inspection can help to assess and in essence mitigate the risk of an incident occurring through hydrogen embrittlement, the risk is not eliminated and there is a cost attached to increased inspection schedules, which while safety is a critical concern, operators may not wish to bear.

The fundamental way to mitigate risk is to ensure that the product used offshore is suitable for that environment. This may appear an obvious statement to make, but it is critical to ensure that there is a clear understanding of how a product is going to be used and applied before it is exposed offshore.

For example, Grade 10, 12 or even higher master links are now being produced and all have a higher tensile strength and correlating hardness compared to lower grades. Using a higher grade product could reduce costs, if it were to replace a larger diameter product of a lower grade. The use of these products is totally appropriate for use in controlled industrial or construction environments, but not for the extreme conditions which steel is exposed to in the offshore environment, where they would face an increased susceptibility to stress corrosion attack.

To put this into context, a Grade 8 master link, when correctly heat treated, will provide toughness, tensile strength and resistance to shock absorption, at hardness levels that enable the steel within the product to withstand the extreme conditions of the offshore environment.

While the industry continues to face challenges of cost and operating in difficult environments, it is critical that there is a clear understanding on how products are being used to ensure that we all maintain the safety and performance within the sector.

Tim Burgess
is managing director of William Hackett. He has run the lifting business since 1989 and then acquired the group business in 2008. Tim also sits on the Board of LEEA – the Lifting Equipment Engineers Association, which is the leading representative body for all those involved in the lifting industry worldwide.