Observations on the relationship of structure to the mechanical properties of thin TD-NiCr sheet
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acterrzation." In the property characterization study, 5 the mechanical properties of four well-documented heats of thin TD-NiCr sheet were determined. The purpose of this paper is to correlate the mechanical properties which are reported in Ref. 5 for thin TD-NiCr sheet with the structural characteristics of that material. Emphasis is placed on tensile, stress-rupture, and creep behavior as related to microstructure and sheet thickness. In addition, creep damage effects as determined by room temperature tensile testing of as-crept specimens are reported; and evidence for the possible existence of a threshold stress for creep in TD-NiCr sheet is presented. EXPERIMENTAL Characteristics of Thin-Gage TD-NiCr Sheet The TD-NiCr sheet used in the property characterization study" was typical of the sheet product produced by Fansteel, Inc. following a standard commercial manufacturing prooess.I'" The heat numbers, nominal sheet size, and chemistry of the TD-NiCr sheet are listed in Table I. In accordance with standard practice, the 0.025 cm thick TD-NiCr sheet was finished by belt sanding and then cold rolled a few percent; the cold rolling was used to improve the sheet flatness. Previously, it was shown'i'" that the light cold-roll pass «5 pct) did not affect the microstructure or mechanical properties of the sheet. The 0.051 JOHN D. WHITTENBERGER is Materials Engineer, NASA-Lewis Research Center, Cleveland, OH 44135. Manuscript submitted April 17, 1975. METALLURGICAL TRANSACTIONS A
Table I. Nominal Sheet Size and Chemistry for TD·NiCr
ThO,
Ni
Nominal Sheet Size, m
0.0021 2.17 0.0019 2.14 0.0055 2.10 0003 2.10
bal bal ba! bal
0.5 X 0.9 0.45 X 0.9 0.45 X 0.9 0.45 X 1.05
Composition, Wt Pet
Heal Number
Cr
C
3636 3637 3697 3712
20.16 19.8 20.07 19.77
0.0214 0.0194 00248 0.025
S
Nominal Thickness, cm 0.051 0.025 0.025 0.051
cm thick TD-NiCr sheet was simply finished by belt sanding. The microstructure of the TD-NiCr sheet was composed of pancake-shaped grains with the short dimension parallel to the sheet thickness. The average grain thickness, grain length, grain size, grain aspect ratio, and sheet thickness/grain thickness ratio are reported in Table 11 for each heat as a function of sheet direction (parallel or normal to the sheet rolling direction). While the microstructure of Heat 3697 is composed of pancake-shaped grains, it should be noted that thickness of the grains is equal to the sheet thickness. Typical microstructures of these TD-NiCr sheet products are presented in ReI. 5. The microstructural differences noted in Table 11 are due to slight differences in the sheet rolling schedule during sheet manufacture. Manufacture of thin-gage, TD-NiCr sheet required warm pack-rolling (1000 K) which makes it difficult to precisely control the reduction per pass of each heat of material. As shown in the sheet manufacturing development studies/,3 a change in the average reduction per pass from about 5 pet to about 8 pet could produce significant differences in the recrystallized microstructure of TD-NiCr sheet
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