Most high temperature alloys suffer from a common disadvantage when used at sufficiently high temperatures - diffusion controlled microstructural changes, which result in impaired properties.
The most common type of reaction is the precipitation of non-desirable phases. Besides lowering the corrosion resistance by consuming beneficial alloying elements (above all chromium), this phenomenon leads to a reduced toughness/ductility of the material - especially at room temperature. The precipitates are often intermetallic phases such as sigma, chi, and Laves phase, but carbides and nitrides are also common.
At even higher temperatures, grain growth may occur, possibly increasing the creep strength somewhat, but simultaneously reducing the ductility substantially. Even if these microstructural changes lead to impaired materials properties, they can be acceptable as long as one is aware of them and consider them during service as well as at maintenance shutdowns.
Upon service exposure at elevated temperatures, most of the Outokumpu Stainless HT alloys become more or less embrittled. 4948, and especially 4878 are little affected, while the loss in toughness is substantial for 4828, 4833, and 4845, due to an extensive precipitation of intermetallic sigma phase. Contrary to these alloys, the main precipitates in 253 MA and 353 MA are carbides and nitrides, which may result in an equally low RT impact toughness. However, service experience indicates that the ductility is superior at lower deformation rates.
153 MA was developed as a leaner alloyed variant of 253 MA for applications where high demands are made on toughness. 153 MA will have a "sufficiently" high toughness after service at all temperatures