A number of industries utilise high temperature, high pressure processes as part of their production lines. As such, the materials engineered to harbour the process are also exposed to high temperatures and pressures. This can see key components such as pressure vessels and piping exposed to life limiting thermal degradation. Metallographic replication can be employed as an asset integrity inspection regime, to verify the condition of such components relative to the expected plant life. Well defined classification systems for thermal degradation of carbon and low alloy steels have been employed to this end for many years. A similar classification system for austenitic stainless steels did not exist until recently.

Austenitic stainless steels are used in a wide range of applications where increased high temperature strength and/or corrosion resistance are required relative to carbon and low alloy steels. In long-term high temperature applications, austenitic stainless steels show considerable microstructural development, with often life-limiting changes in mechanical properties. Currently, industry relies on conservative mechanical test data to assess remaining life without reference to the actual metallurgical condition. The impact of such conservative life estimates can result in materials being removed from service prematurely or premature failure.

Recently, R-TECH Materials has led an innovative programme of work which relates remaining life to actual metallurgical condition so that the remaining life of plant can be assessed much more accurately. This detailed study incorporated the metallographic assessment of in excess of 500 samples that had undergone creep testing and/or thermal ageing. The resulting data was utilised in order to relate the microstructural condition to the consumed life fraction. Subsequently R-TECH Materials have developed specialist metallographic replication techniques that enable the verification of the life fraction consumed relative to the current microstructural condition. This innovative programme of work provides a more secure basis for structural integrity evaluation and life prediction for austenitic steels in long-term high temperature service.

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