Updated evaluation of creep damage tolerance in THOR® 115 parent material and weldments

Tenaris’ High Oxidation Resistance (THOR®) 115, henceforth T115, is a creep strength-enhanced (CSEF) steel that was introduced in the market in the last decade and has since been successfully used for construction of several new-generation, high efficiency power plants. The increased resistance to steam oxidation and long-term microstructural stability of T115 make this alloy a viable alternative to stainless steels when steam temperature increases. T115 is defined in multiple product standards, and ASME Boiler and Pressure Vessels Code (BPVC) permits its use for construction of power boilers through Code Case (CC) 2890.

In recent years, long-term creep damage tolerance of CSEF steels has been investigated by several authors. Accumulation of creep damage in components may result in early failure of boiler parts, and consequently cause not only a significant economic impact, but also raises significant safety concerns. The recently issued ASME BPVC CC 3048 provides rules for identifying creep damage intolerant behavior of materials by means of testing, and for safer design of components with CSEF steels that potentially show a creep damage-intolerant behavior. At the time CC 3048 was introduced, only Grade 91 Type 2 had been conclusively proven to reliably show a creep damage-tolerant behavior. Recently, results from testing the T115 steel were published to demonstrate that the material is similarly creep damage-tolerant according to the rules from CC 3048.

A typical location where creep damage accumulates is the heat-affected zone of weldments. Cross-weld creep behavior of T115 was thus also re-assessed through new test data: large, full thickness tests were purposefully taken from a 1.5 in (38 mm) thick pipe girth weld to provide representative information of components where plane strain conditions prevail. Location and distribution of creep damage was measured, correlated to hardness maps, and compared to the behavior of the well-known Grade 91 CSEF steel.