A new martensitic steel for power generation applications was developed: Tenaris High Oxidation Resistance (Thor) is an evolution of the popular ASTM grade 91, offering improved steam oxidation resistance and better long-term microstructural stability, with equal or better creep strength. Thanks to its design philosophy, based on consolidated metallurgical knowledge of microstructural evolution mechanisms, and an extensive development performed in the last decade, Thor was engineered to overcome limitations in the use of ASTM grade 91, above 600 °C, particularly related to scale growth and liftoff. After laboratory development, Thor was successfully validated at the industrial level. Several heats up to 80 metric tons were cast at the steel shop, hot rolled to tubes of various dimensions, and heat treated. Trial heats underwent extensive characterization, including deep microstructural examination, mechanical testing in the as-received condition and after ageing, long-term creep and steam oxidation testing. This paper presents an overview of metallurgical characterization performed on laboratory and industrial Thor material, including microstructural examination and mechanical testing in time-independent and time-dependent regimes. Data relevant to the behavior and the performance of Thor steel are also included.

A study of Grade 91 steel's creep rupture behavior at 600°C (up to 90,000 hours) and 650°C (up to 23,000 hours) reveals that static recovery of tempered martensite lath structures leads to decreased stress exponent and breakdown of creep strength. While M 23 C 6 and MX particles initially stabilize lath structures by hindering sub-boundary migration, the progressive aggregation of M 23 C 6 particles reduces their pinning force, triggering static recovery. Although Grade 91 steel shows better M 23 C 6 thermal stability compared to Grade 122 type steels (9-12%Cr-2W-0.4Mo-1Cu-VNb), coarsening of M 23 C 6 particles and subgrain width is expected to occur slightly beyond 100,000 hours at 600°C, potentially leading to creep strength breakdown.