Effect of Nb on the Proof Strength of Ferritic Stainless Steels at Elevated Temperatures.

Abstract

The effect of Nb on the high temperature proof strength of 0.46 % Nb-added ferritic stainless steel was studied from the viewpoints of solid solution strengthening, precipitation strengthening, and precipitation strengthening during tensile test and/or preceding hold time. The results obtained were as follows.(1) The increase in high temperature proof strength in Nb-added ferritic stainless steel is particularly remarkable at around 700°C in comparison with that in Ti-added steels.(2) After aging treatment for 2 h at 700°C, in addition to the approximately 0.14 % content of coarse Nb carbides and nitrides which had precipitated before this aging treatment, fine Fe 2 Nb Laves phase particles with a size of 0.02 mm precipitated out in an amount of approximately 0.3 %, and virtually no solid solution Nb existed in the steel. The 700°C proof strength in this condition showed a value near the 700°C proof strength before the aging treatment, that is, when approximately 0.3 % solid solution Nb was present.(3) When aging treatment is performed at 700°C for 2 h or more, virtually all the added Nb precipitates out. The coarsening of the Fe 2 Nb laves phase particles within the range of 0.02-0.05 mm during aging at 700°C greatly decreases the high temperature proof strength at 700°C.(4) Cold rolled steel sheets of Nb-added ferritic stainless steel are normally manufactured by annealing at a temperature of 900°C or higher in order to cause recrystallization. Accordingly, before measurement of the proof strength at 700°C, the steel contains a large quantity of solid solution Nb. The phenomenon of particularly high strength at 700°C is considered to be attributable to the precipitation of this solid solution Nb as a fine Fe 2 Nb Laves phase during the tensile test and/or preceding hold time.

Expressions for Solubility Products of Fe3Nb3C carbide and Fe2Nb Laves Phase in Niobium Alloyed Ferritic Stainless Steels

“…7,8) [ [6][7][8] were plotted. The data were experimentally obtained by the extracted residues from 0.01C-0.01N-18Cr-0.5Nb-2Mo mass% steel aged at 950°C for Fe 3 Nb 3 C 6) and 0.008C-0.007N-15Cr-0.46Nb-0.08Mo mass% steel aged at 700°C for Fe 2 Nb 7,8) respectively. The solubility products obtained here are good agreement with the experimental data in the previous literatures.…”

Effect of Lanthanum on the Precipitation of NbC in Ferritic Steels

“…[5][6][7] Generally, solute Nb is readily precipitated out as carbonitride when the steel used at high temperature for a long time, the high temperature strength can be affected due to the Nb(C,N) precipitation in Nb containing ferritic stainless steel. 8) As strong forming elements of carbide and nitride, the precipitation evolution of Nb and V depends on various factors, such as liquid/solid condition, solid solubility, and interaction of other elements. 9) The previous study show that the introduction of small amounts of rare earth (RE) elements, such as yttrium, cerium, lanthanum, is beneficial to improve the mechanical properties, the high-temperature oxidation resistance and the corrosion resistance in niobium containing steels, 10,11) these effects has been proved to be induced by RE and reactive element effect.…”

Effect of Laves Phase on High-Temperature Deformation and Microstructure Evolution in an 18Cr-2Mo-0.5Nb Ferritic Stainless Steel

“…Nabiran et al [5] reported that the contribution due to the rapid coarsening of Laves phase precipitates to the high-temperature strength was comparatively small. On the other hand, it was reported that Nb forms several different types of precipitates around 900°C that could degrade high-temperature strength and thermal fatigue resistance [6]. Morrie et al [8] noted that rapid coarsening of the Laves phase at high temperatures significantly reduces strength over 973K(700°C), and Sim et al [9] reported that coarse, rod-shaped Laves phase precipitates (Fe 2 Nb) formed at 973K(700°C) are very detrimental to high-temperature strength of Nbcontaining steels.…”

“…Niobium-(Nb) containing ferritic stainless steels were developed to overcome the lower high-temperature strength [1][2][3]. Niobium improves high-temperature strength and thermal fatigue resistance by solid-solution hardening and precipitation hardening [4][5][6][7][8][9]. Fujita et al [4] reported that fine Nb carbo-nitrides and Nb-based Laves phase precipitates formed during hightemperature deformation improve the strength of the alloy at 1173K(900°C).…”