Elsevier

Intermetallics

Volume 53, October 2014, Pages 120-128
Intermetallics

Accelerated precipitation in the AFA stainless steel Fe–20Cr–30Ni–2Nb–5Al via cold working

https://doi.org/10.1016/j.intermet.2014.04.018Get rights and content

Highlights

  • This alloy forms B2 NiAl and C14 Fe2Nb precipitates after aging up to 240 h at 800 °C.

  • After cold work, at 700 °C γ′-Ni3Al precipitates form along with B2 and C14 phase.

  • Cold work at 700 °C and 800 °C increased the precipitation kinetics of the alloy.

  • Aging temperature and cold work had a distinct influence on the material hardness.

  • With sufficient cold work, a finer dispersion of matrix particles could be attained.

Abstract

The effects of cold work on the microstructural evolution during aging of a solutionized alumina-forming austenitic stainless steel, Fe–20Cr–30Ni–2Nb–5Al (at.%), were investigated using scanning electron microscopy, transmission electron microscopy, and scanning transmission electron microscopy. Cold work prior to aging at either 700 °C or 800 °C facilitated the heterogeneous precipitation of both Laves phase and B2-type NiAl precipitates. While often co-located after cold work, these particles were distinct. γ′-Ni3Al precipitates were also observed in samples aged at 700 °C with 90% prior cold work. Compared to material that had not been strained, defects introduced by 50 and 90% cold work at 700 °C and 90% cold work at 800 °C not only caused a more rapid precipitation in the matrix but also an increase in the total volume fraction of precipitates as compared to material that had been simply aged.

Introduction

AFAs (alumina-forming austenitic stainless steels) are a new promising class of steels with potential for use in a number of energy-conversion applications [1], [2], [3], [4], [5], [6], [7], [8], [9]. For example, there is a current effort to develop power plant materials with excellent creep strength and corrosion resistance at temperatures >760 °C. Increasing the operating temperature improves efficiency with the added benefit of reducing CO2 emissions [10], [11], [12], [13]. The relatively low cost of ferritic steels make them desirable candidates, but they have yet to show the creep strength and oxidation resistance needed at high temperatures. Nickel-based superalloys can be used at temperatures in excess of 760 °C, but their high cost makes them less desirable for most applications [12], [13]. In order for an AFA to be a viable alternative, it must have a strength and creep resistance that are close to or exceed those of Ni-based alloys. AFAs have relied on MC carbides for strength [1], [3], [4], [14], but at temperatures greater than 800 °C there is concern that the carbides formed could coarsen and dissolve [4].

Further refinement of the Fe2Nb Laves phase particles has potential to improve the creep life and high temperature strength of AFAs. Fe2Nb Laves phase particles have shown long-term stability at high temperatures [14], [15], [16], [17]. However, previous studies that looked to Laves phase particles for strength have only seen low or moderate creep resistance [14], [17]. Decreasing the precipitate size and increasing the volume fraction would enable the precipitates to effectively pin dislocations and extend creep life. A Laves phase dispersion with particles less than 100 nm in diameter is recommended to obtain creep-levels on-par with MC carbide strengthened austenitic stainless steels [17].

One pathway to reducing the size and increasing the volume fraction of Laves phase precipitates in the matrix is to nucleate precipitates on dislocations. Nucleation on dislocations is an effect first modeled by Cahn [18] in 1957 and occurs because it lowers the total strain energy of an embryo. In order to effectively reduce the interfacial energy, precipitates that nucleate on dislocations should be well-matched on at least one matrix plane so they are able to form low-energy coherent or semi-coherent interfaces [19]. Images of Fe2Nb that has precipitated in austenite alloys that have the base components of AFAs (Fe–20Cr–35Ni–2Nb and Fe–20Cr–25Ni–2Nb) [15] show strain contrast in TEM images that is indicative of a semi-coherent precipitate-matrix interface, making Fe2Nb a promising candidate for precipitation on dislocations in AFA-type alloys.

Prestraining via cold work introduces dislocations that could dramatically affect the precipitation of the Laves phase, similar to what has been observed in previous studies of other precipitate systems [20], [21], [22], [23]. The effectiveness of prestraining before aging in changing microstructure and improving material properties depends on characteristics of the particular alloy system analyzed and the synergy of a number of different variables including, but not limited to: aging temperature, the amount of prestrain, and competing precipitation phenomena. For example, the improved hardness observed in a copper-bearing steel that had been pre-strained prior to aging at 300 °C was attributed to additional particle nucleation on dislocations or excess vacancies arising from prestraining, however at an aging temperature of 500 °C the prestrain had little effect on hardness or particle nucleation [21]. In aluminum alloys cold work has been shown to have different effects depending on the alloy system, with dislocations enhancing hardening by providing nucleation sites for precipitates in the Al–Cu, Al–Cu–Mg, and Al–Cu–Li systems, while not proving effective for hardening in other aluminum systems [24]. Severe plastic deformation by cold rolling at room temperature followed by subsequent aging has been shown to improve both strength and ductility in a high strength Al–Mg–Si alloy [23] as well as a Cu–Ag alloy [22]. An increase in precipitation after cold work has been observed with other precipitates in alloys more closely related to AFAs. HTUPS (high-temperature ultrafine-precipitation-strengthened steel) alloys, modified for Al2O3 formation, are cold worked to enhance the precipitation of nanoscale MC carbide precipitates during creep [1], [14], [25]. While aging studies on AFA-type alloys have been done in the past [16], Laves phase precipitates have yet to be targeted via a combined aging and cold working approach.

In this study, the effect of cold work on a solution-annealed AFA-type alloy, Fe–20Cr–30Ni–2Nb–5Al (at.%) was investigated by comparing samples that had received cold work prior to aging to those that were simply aged. Microstructural analysis was performed using a scanning electron microscope (SEM), a transmission electron microscope (TEM) and a scanning transmission electron microscope (STEM) and energy dispersive spectroscopy (EDS). In addition, the effect of cold work and aging on the hardness of the material was examined.

Section snippets

Experimental

Fe–20Cr–30Ni–2Nb–5Al (at.%) ingots were arc melted and drop cast into a copper mold 15.2 cm long × 2.5 cm in diameter under argon. Common cast features [26] were observed: there was some segregation and a central pore was created during the solidification process. The as-cast material was homogenized in vacuum for 24 h at 1250 °C, then water-quenched. This solutionizing treatment was used since SEM investigations confirmed that it enabled the formation of a solutionized single phase matrix.

Results

Fig. 1 shows bright-field TEM images with accompanying selected area diffraction (SAD) patterns of precipitates observed in Fe–20Cr–30Ni–2Nb–5Al (at.%). After the solution treatment the average grain size was 865 μm. Fig. 1(a) and (b) shows microstructural features after the alloy was aged at 800 °C for 24 h and Fig. 1(c) shows the alloy after 50% cold work prior to aging at 240 h. TEM examination confirmed that the alloy had an f.c.c. austenitic (γ) matrix (Fig. 1(a)) and that the lighter

Discussion

The above results show that cold rolling after solutionizing, but prior to aging, produces both more rapid precipitation of the Laves phase in the matrix and causes matrix precipitation of NiAl at 700 °C and 800 °C and of Ni3Al at 700 °C. With aging, the Laves precipitates show a trend of initially fast growth followed by subsequent slower coarsening as observed in a 9CrW steel [28]. As observed in the work of Yamamoto et al. after creep-rupture of this alloy at 750 °C [17], when this alloy is

Conclusions

Cold work resulted in the creation of a defect structure that facilitated the heterogeneous precipitation of both C14-type Fe2Nb-type and B2-type NiAl precipitates. Increasing the amount of cold work caused the B2 phase to precipitate out after shorter anneals and at a lower temperature. Both B2 and Laves phases experienced more rapid precipitation in the matrix compared to material that had not been strained prior to aging at 700 °C and 800 °C. γ′-Ni3Al precipitates were also noted after a 90%

Acknowledgments

Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund #49157-ND10 and the National Science Foundation Grant DMR 1206240 for support of this research. The authors thank Dr. E.P. George of the Oak Ridge National Laboratory, Oak Ridge, TN for providing the ingots.

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