Some Notes on the Structure and Impact Resistance of Columbium-Bearing 18—8 Steels After Exposure to Elevated Temperatures

Abstract

The influence of columbium on the impact properties of 18-8 steels has been studied in steels containing 8 to 16 per cent nickel and 0 to 3 per cent columbium. The addition of more than 2 per cent columbium decreases the toughness of the steel in the annealed condition due to the formation of M6C carbide and the compound Fe3Cb2. Long heating at 650 and 870 C. (1200 and 1600 F.) has no significant influence on impact toughness of steels containing up to 2 per cent columbium, but it is more detrimental to higher columbium steels. Positive X-ray identification of sigma phase was obtained after long periods at 650 C. (1200 F.) over the range 8 to 16 per cent nickel and 0.75 to 3 per cent columbium. However, the steels do not form large amounts of sigma even though strained at elevated temperatures. Cold working, followed by low-temperature annealing, accelerates the rate of precipitation of sigma phase and favors particle growth, whereas, high-temperature annealing inhibits precipitation and particle growth. The precipitation of sigma phase in a critical particle size is believed to have a beneficial effect on creep strength.

Phase transformations in Fe–Ni–Cr heat-resistant alloys for reformer tube applications

“…For instance, the so-called Z phase, which is a nitride of general formula Cr 2 Nb 2 N 2 (or CrNbN, but the unit cell actually contains six atoms), can form in austenitic [39] or martensitic steels. Atomic sites can be partly substituted by Fe, Mo, or V, giving for example Cr 2 (Nb,V) 2 N 2 [40].…”

Review of Z phase precipitation in 9–12 wt-Cr steels

“…The Z-phase has long been known to precipitate in austenitic steels in its Nb form, CrNbN, first reported in 1950 [6], although its crystal structure was first properly described in 1972 [7]. In contrast to the Z-phases usually seen in 9-12wt%Cr martensitic steels, the CrNbN precipitates very rapidly in low carbon austenitic steels as small and finely distributed rodlike particles, and is credited with beneficial strengthening effects, being thermally stable with a very low coarsening rate [8].…”

On precipitates in Fe–Ni base alloys used for USC boilers

“…13 Table 1 lists the weight fraction of some major precipitates calculated by a JMatPro software in several modern USC boiler materials. Some new kinds of strengthening precipitates such as Z phase, 14,15 Cu rich phase, 16,17 Laves 18,19 and α-Cr 20,21 can be found in alloys, while their fraction is relative small. Most of these precipitates have been suggested to induce negative effect on the alloy strength according to the existed theories.…”