Introduction: Modified 9Cr–1Mo–V–Nb steel (Grade 91 or just Gr 91) is a 2nd generation creep-resistant ferritic/martensitic (F/M) steel with 9% Cr, 1% Mo and small additions of V and Nb to form fine carbo-nitrides. Gr 91 offers improved strength, creep life and fracture toughness with service temperatures up to ~650°C in power plants relative to low alloy steels. The goal of this work was to understand microstructural evolution of Gr 91 steels during additive manufacturing (AM) processing with different preheats to permit future process manipulation with a view toward optimizing microstructures and properties for potential nuclear reactor service.
Experimental Procedures: Small block (¾” x ¾” x ¾”) deposits were produced on an Optomec MR-7 LENS system (L-DED) using four different initial preheat levels: none (room temperature), 90°C, 180°C and 350°C, on a stainless-steel substrate. The 90°C and 180°C preheats were achieved using an industrial heating pad, while the 350°C level was realized using a laboratory heating plate wrapped in aluminum foil to avoid damage from loose powder. The blocks were produced using 425 W delivered to a beam in an unfocused condition with spot size of ~ 1 mm in a glove box with oxygen levels below 50 ppm. Powder was delivered from the hopper at 4 RPM through a four-tip nozzle assembly using a carrier gas of Ar at 5 lpm flow rate. Layer orientation was varied using a repeating 0°, 90°, 180°, 270° hatch pattern.
A custom heat of Gr 91 powder with -100/+325 size range was procured from Carpenter Surface Technologies. The composition of the powder complied with Gr 91 specifications except for the C content which was below spec (0.078 wt%). The PSD was determined using a Horiba laser scattering device. The D10, D50 & D90 sizes for the powder are ~52 m, ~78 m and ~123 m, respectively.
Samples for characterization were prepared using standard metallographic practices and etched with Vilella’s reagent. Microstructures were characterized across several length scales using LOM, SEM and XRD methods. Vickers microhardness measurements were made of the different regions of the deposit. LOM images were analyzed to estimate phase volume fractions using a commercial software (Image J). A predicted phase diagram was developed, and Scheil solidification simulation conducted for the alloy composition used here with a commercially available software (Thermo-Calc). DICTRA and finite difference simulations were conducted for the delta ferrite to austenite transformation.
Results: LOM and SEM characterization results indicated a bi-modal microstructure with large volume fraction of what appeared to be delta () ferrite with cellular solidification microstructure of light etching contrast surrounded by martensite with dark contrast. Microhardness results corroborated this interpretation with an average hardness of 375 VHN for the martensite phase and 210 VHN for the ferrite phase for positions near the bottom, middle and top of the deposits. In addition, measurements in the martensite at the mid-height of the deposits indicated greater hardness with increasing preheat temperature. XRD results Image analysis from the LOM micrographs and XRD results both showed a decrease in the volume fraction of the ferrite phase with increasing preheat temperature, although the values for the two methods differed significantly.
Scheil simulation results demonstrated primary solidification of this Gr 91 composition to ferrite up to 90% fraction solid (fs) with subsequent formation of austenite () and Nb(C,N) as terminal phases. DICTRA simulations using data from the calculated phase diagram and Scheil results with variable cooling rates demonstrated that the delta ferrite to austenite transformation can be limited at faster cooling rates resulting from lower preheat temperatures (and vise-versa).
Summary and Conclusions: The cause for the large volume fraction of remnant delta ferrite and the lower-than-expected volume fraction of martensite may be attributed to a kinetic limitation with the delta ferrite to austenite transformation during cooling in the solid-state. All results were consistent with the idea that the transformation did not approach completion owing to the rapid cooling rates associated with the L-DED process. The decreased volume fraction of delta ferrite and attendant increase in martensite results from lower cooling rates with increased pre-heat temperatures that provide longer times for the transformation to proceed further toward completion. Variations in mechanical properties relative to wrought Gr 91 may be anticipated for Gr 91 produced with AM methods. Differences in process-structure-property relations stem from the use of L-DED AM processes with Gr 91 relative to wrought and traditional welding methods.