Effect of ferrite content on the creep strength of 12Cr-2Mo-0.08C boiler tubing steels

Abstract

The effect of ferrite content on the creep of a 12Cr-2Mo-0.08C, ferritemartensite, heatresisting steel was investigated. Creep tests were performed at 550, 600, and 650 °C for up to 10,000 hours on four steels containing, respectively, 51, 38, 23, and 12 pct ferrite. The variation in ferrite content was obtained through nickel additions of 0.6, 1.3, and 2.0 pct. Rupture time, minimum creep rate, elongation, and reduction in area are reported. The effect of ferrite content was found to be small but significant. At short rupture times, increasing ferrite content reduces creep strength. At long exposure times, however, increasing ferrite content increases creep strength. This effect is explained by the precipitation of Laves phase, Fe2Mo, and possibly Cr2N, in the ferrite during testing. This precipitation progressively strengthens the ferrite relative to martensite. In addition, martensite is continually weakening because of the coarsening of M23C6, the only precipitate found in the martensite. Nickel at the 2.0 pct level radically decreased creep resistance in these steels by accelerating aging of the precipitate structure in martensite.