Continuous Grain Size Gradients in Austenitic Incoloy 800H: Design, Processing, and Characterization
- PDF / 4,599,392 Bytes
- 13 Pages / 593.972 x 792 pts Page_size
- 16 Downloads / 173 Views
HE effect of microstructure on material performance is fundamental to materials engineering.[1] Tailoring microstructures in order to optimize properties includes creating spatially varying microstructures for optimum performance under applied fields such as stress,[2] temperature,[3,4] and electromagnetic fields.[5,6] Case hardening mild steel is a classic example.[7] Lee[8] used torsional deformation and annealing to obtain grain size gradients in AISI 1018 steel with improved tensile properties compared to a homogeneous grainsized alloy. Hwang[9,10] used oxide grain refiners and powder methods to obtain grain size gradients in nickel with improved fatigue resistance. Yan[11] used surface rotational rolling to obtain grain size and texture gradients in AZ31 magnesium alloy with concurrent CATHERINE M. BISHOP, SHAUN P. MUCALO and MILO V. KRAL are with the Department of Mechanical Engineering, University of Canterbury, Christchurch 8140, New Zealand. Contact e-mail: [email protected] DANIEL J. LEWIS is with the Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy NY 12180. Manuscript submitted September 23, 2019. Article published online January 21, 2020 METALLURGICAL AND MATERIALS TRANSACTIONS A
improvements in strength and ductility. Long[12] used cryogenic surface mechanical grinding to obtain nanograined surface regions for improved fatigue performance in pure copper. Cheng[13] used direct current electrodeposition to obtain gradient nano-twinned Cu structures with both high strength and high work hardening properties. Spatially varying microstructures can also be by-products of traditional manufacturing routes. One such example is the processing of steam reformer pigtail tubes that require 90 deg bends to enable the component to accommodate thermal expansion.[14] The bends are formed by rotary drawing Incoloy 800H, a high nickel austenitic steel. The plastic strain from the bending operation varies across the tube cross section, with outer fiber strain estimated to be 10 pct[15] at the intrados (inner bend radius) and extrados (outer bend radius) and minimal stored plastic work at the neutral bending plane. After a recrystallization anneal, the details of which are proprietary, the grain size is non-uniform with larger grains near the neutral plane and smaller grains near the intrados and extrados, Figure 1. Pigtails operate at elevated temperatures around 800 to 900 C and internal pressures approximately 1.9 MPa such that the dominant deformation mechanism is creep.[16]
VOLUME 51A, APRIL 2020—1719
Fig. 1—Macrostructure of cross section of 800H pigtail at bend with neutral plane indicated by vertical, white dashed line. Insets showing (below) large grains at bottom of section near the neutral plane and (right) small grains near region of maximum stored work.
Anecdotally, longitudinal creep cracks have been observed in the transition region between the largegrained and the small-grained zones, but there has been no systematic study of the effect of grain size
Data Loading...
