Miracle steels?

Materials World magazine
,
8 Apr 2015

Ahmed Shibli, from European Technology Development Ltd, analyses the problems caused by improperly treating high-temperature martensitic steels P91 and P92, and calls for more research into the issue.

The development of ferritic steels for power plant boilers has a remarkable history. They were developed from carbon steels to chromium (Cr), molybdenum (Mo) and vanadium (V) alloy steels in an effort to raise the boiler temperature and pressure, increasing power plant efficiency. Privatisation, competition and environmental concerns created a greater urgency for increasing plant efficiency and profits for shareholders. This led to the development of another remarkable series of steels known as ‘high Cr martensitic steels’, which increased creep rupture strength of the existing low alloy steels by a factor of between two and three, and the limits of boiler operating temperatures from about 540–568°C to 625°C, without increasing the material costs significantly. Parallel efforts were also made in the development of austenitic stainless steels but their use remains limited, mainly due to their much higher costs.

The development of this new generation of martensitic steels was a remarkable achievement of materials design in the late 20th Century. They can be formed and welded, and achieve acceptable rupture strength, creep ductility and fatigue strength. However, any new development, when introduced rapidly on an industrial scale, brings about its own practical problems and new unforeseen engineering challenges. 

The relatively new 9Cr martensitic steels P91 and P92 have helped to increase HRSG power plant temperatures and pressures – and, therefore, output and efficiency – and have helped with carbon reduction targets. They have also been used as replacement components for older power plants where the use of new steels has helped to increase plant flexibility, by using thinner section components and reduced component fabrication, transportation and other such costs. 

Unfortunately, there have been many incidents of P91 component cracking and failure. These failures have almost always occurred in the fine grain heat-affected zone (HAZ) – the region close to the base metal, which has the weakest creep strength. These are known as type IV failures and are common in the low-alloy ferritic steels used in the power industry. However, in P91 components, in some cases, these have occurred after only a couple of years in service, which has worried many plant operators. Usually, these have been blamed on incorrect heat treatment of the base, or the weld metal, or both, resulting in a weaker fine grain zone. 

Keeping the heat up

These steels can only achieve their design strength if they are heat-treated strictly to a specified temperature. As their high strength depends on achieving ideal martensitic microstructure, slight deviations from the specified temperatures can lead to disastrous consequences. In some cases, even where the heat treatment has been close to, but within, the limits of the specified temperatures, outcome has been poor, due to localised overheating. This is equally true of post-weld heat treatment temperature criticality. 

The need to keep manufacturing prices down has often made achieving such precision difficult. In many cases, manufacturers have used sub-contractors from companies and countries that are unfamiliar with the criticality of the heat treatment for these steels. As a result, today, P91 and P92 steel components are found in many power plants with different aberrant or abnormal microstructure variations, giving rise to failures worldwide. 

Many abnormal P91 components have only been found years after entering service, either during scheduled inspection outages or because there has been a premature failure. Not all of the abnormal components can be replaced immediately and many will have to remain in service indefinitely. This can present a serious problem, when plant owners or operators do not know how to treat these components in the absence of any material data or guidelines. The need now is to understand the behaviour of these aberrant steel components in service, and to predict their strength and safe remaining life so that power plants can operate safely.

Facing the flaws

High Cr martensitic steels are safer and stronger than most other high-temperature ferritic steels, thanks to their martensitic microstructure. However, if precise heat treatment is not carried out and martensitic microstructure does not fully materialise, these metals can be much weaker and behave like low-alloy steels, such as 2.25Cr1Mo or 0.5Cr0.5Mo0.5V, or other non-martensitic steels, such as the older and traditional 9Cr bainitic steel used in the nuclear industry. The aberrant microstructure with larger bainitic portion is usually produced because the heat treatment has been carried out above the Ac1 temperature of the steel. 

It has recently been shown that even when heat treatment is carried out below, but close to, the Ac1 temperature, it can still result in part bainitic microstructure, with loss in creep strength. Moreover, the Ac1 temperature can vary with the steel chemical composition, making precise heat treatment even more difficult. The same applies to the weld metal post-weld heat treatment. If the weld, the parent metals or both are over-tempered it can make the steel more vulnerable to early damage and failure. 

Furthermore, due to the bulk heat treatment and then cooling of the stacked pipes, the regions that sit on top of other pipes are slow to cool, giving rise to soft non-martensitic bands running longitudinally along the pipes. This makes these areas vulnerable to cracking. In addition, soft spots of non-martensitic microstructure have been found in components that have been heat-treated on site using electric blankets. 

Critical condition

Failures in industrial plants often require immediate weld repairs. Again, in the case of these martensitic steels, heat treatment of repair welds must be carried out on site with precise temperatures – a difficult task to accomplish. As a result, various weld repairs, configurations or repeat weld repairs have exhibited problems and, in the absence of data, plant operators have been at a loss to understand how to deal with such components.

Another welding issue faced in plants is low-alloy root runs, which some welding companies have introduced to avoid the purging needed with the high-alloy root runs. These root runs are notoriously difficult to detect and find. In addition, plant operators come across problems with dissimilar metal welds (both ferritic-to-ferritic and ferritic-to-austenitic) with a range of post-weld heat treatments with no long-term rupture data and, therefore, little information on their long-term integrity. 

There are two problems with these aberrant materials found in plants. One is the detection of creep cavitation damage, the main cause of failure in high-temperature plants. The damage develops in a different manner to that traditionally encountered in the low-alloy creep resistant, molybdenum, and vanadium steels used since the Second World War in high-temperature plants. Unlike the low-alloy steels’ creep cavitation, damage in martensitic steels first develops at a very slow rate, resulting in nanoscale cavitation that only grows to an observable microscale late in life (about 70–80% of life), when it becomes detectable in on-site inspection by traditional ultrasonic testing or replication techniques. Work is now in progress on the development of new techniques for the inspection of P91 and P92 components and interpreting damage in terms of the safe remaining life. 

The second problem is life assessment in the absence of creep strength data for the various aberrant P91 and P92 variants found in high-temperature plants. Again, work is now in progress for the study of these variants and determining their long-term rupture strength.

P91 and P92 steels are known to have achieved many power plant targets only dreamed of until recently. As a result, P91 is now widely used in conventional power plants and heat recovery steam generators worldwide. And its stronger version, P92, is following the same path. However, with any new materials come new issues and challenges, demanding new international efforts and resources to find solutions.and then cooling of the stacked pipes, the regions that sit on top of other pipes are slow to cool, giving rise to soft non-martensitic bands running longitudinally along the pipes. This makes these areas vulnerable to cracking. In addition, soft spots of non-martensitic microstructure have been found in components that have been heat-treated on site using electric blankets.