Intermetallic phase formation in 25Cr-3Mo-4Ni ferritic stainless steel
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INTRODUCTION
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new grade of ferritic stainless steel, containing nominally (by weight percentage) 25Cr-3Mo-4Ni, has been developed recently for moderately severe corrosion service. This steel has excellent potential for application in seawater, pulp and paper production, and the chemical industry.~'2 New steelmaking processes have made it possible to reduce the interstitial content of ferritic stainless steels and thereby reduce the ductile-to-brittle transition temperatures to well below room temperature. 3 Stabilizing elements, such as A1, Nb, and Ti, are usually added to reduce further the ductileto-brittle transition temperature by combining with any residual carbon and nitrogen, l The 25Cr-3Mo-4Ni steels are generally ferritic at all temperatures below the melting point, and the strengthening and grain size are controlled by cold working and annealing. Two types of metallurgical transformations may occur in high Cr ferritic stainless steels. Both cause embrittlement and a reduction in corrosion resistance. One type of transformation consists of the precipitation of alpha prime (a') and is responsible for the phenomenon referred to as 475 ~ embrittlement. 4 Alpha prime is characterized by coherent chromium-rich clusters. The second type of transformation consists of the precipitation of intermetallic phases, such as sigma, chi, and Laves. l These phases form when highly alloyed ferritic and austenitic stainless steels are heated into or slowly cooled through the 500 to 1000 ~ range. The intermetallic phases are brittle and have higher Cr and Mo contents than the matrix. The structure and lattice constants of sigma, chi, and Laves phases are listed in Table I. The formation of these intermetallic precipitates is encountered commercially during cooling after hot working or during elevated temperature exposure. E.L. BROWN is Research Associate Professor, M.E. BURNETT is AMAX Foundation Fellow, and G. KRAUSS is AMAX Foundation Professor, Department of Metallurgical Engineering, Colorado School of Mines, Golden, CO 80401. P. T. PURTSCHER, formerly Research Assistant, Colorado School of Mines, is now with the Fracture and Deformation Division, National Bureau of Standards, Boulder, CO. Manuscript submitted January 5, 1982. METALLURGICALTRANSACTIONS A
The literature indicates that sigma, chi, and Laves phases should form in ferfitic stainless steels during aging in the 600 to 1000 ~ range. Work by Redmond, et al H'~2 on 18 pct Cr ferritic stainless steels with 0 to 5 pct Mo showed that increasing Mo additions shift the temperature-timeprecipitation curves for all three intermetallic phases to shorter times. At high aging temperatures (around 800 ~ the intermetallic phases were present as coarse allotriomorphs at the ferritic grain boundaries while at low aging temperatures (around 600 ~ the precipitation tended to be finely dispersed within the ferrite grains. In 18 pct Cr alloys containing up to 2 pct Mo, Laves phase was found to be the predominant phase both along grain boundaries and within grain interiors a
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