The fracture toughness of a ferrous maraging alloy containing manganese
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grades of nickel maraging steel i.e., approximately parallel to the (111).y. The Ms of these alloys is quite low i,e .. 32°C, the reaction effectively terminating at - 95°C. A-ray examination after solution-treatment and refrigeration at -196°C revealed that 15 pet retained austenite was present; this amount being unaffected by subsequent aging at 500°C, even after more than 300 h. The austenite was generally present as discrete colonies located between bundles of martensite needles (Fig. 1). The precipitate which developed on prolonged aging
EXPERIMENTAL DETAILS High-purity base materials were melted in a 70 kg vacuum induction furnace and cast in either the undeoxidized condition or after carbon-deoxidation, in order to investigate the influence of residual oxygen content. The chemical composition of the ingots is given in Table 1. The ingots were hot-rolled and cut into blocks 90 mm x 8 mm prior to solution treatment at 1150°C for 1 h. After this treatment the blocks were air-cooled (100°C/min) to room-temperature and then refrigerated for several hours at -196°C. Mter aging at 500°C for up to 1,000 h, the K 1c fracture toughness was determined using fatigue precracked single-edge-notch tensile specimens of 6.2 mm thickness, according to recommended practice." The microstructure and fractures of representative samples were studied by optical and electron microscopy. The retained austenite content was determined by an X-ray technique in which the diffracted intensities of prominent bcc and fcc peaks resulting from an incident beam of monochromatic Mo K Q radiation were directly compared.I'"
Table I. Chemical Composition of the Alloys
Pet Co High
Mo
Cr
Ni
ppm Mn
CON
20.0 5.08 0.05 0.18 14.5 0.002
70
Oxygen
8
Others ppm Si W VNb TiCu < 200
PS Al < 100
As Sn Sb < 50 Low
20.9 4.96 0.17 0.15 14.3 0.007
IS
Oxygen
RESULTS AND DISCUSSION Both the high- and low-oxygen materials had a martensitic structure in the solution-treated condition (Fig. 1), the acicular morphology becoming more clearly defined after aging. From the angles subtended by the martensite needles the habit plane appeared to be similar to that found in the commercial RAYMOND BROOK is a Lecturer in Metallurgy at the University of Sheffield. CHRISTOPHER MUSIOL, formerly a Research Student in the Department of Metallurgy, University of Sheffield, is now a Research Metallurgist at the British Rail Research Centre, Derby. England. Manuscript submitted June 4, 1975. METALLURGICAL TRANSACTIONS A
Fig. I-Solution treated 1 h at 1150°C, air-cooled and refrigerated at - 196°C, magnification 510 times. VOLUME 7A, MAY 1976-683
can be seen in Fig. 2 to be contained within the martensite laths as well as along their boundaries. Diffraction patterns obtained from the precipitate were too weak to interpret. A marked hardening response (Fig. 3) was obtained on aging, comparable to that noted by Suzuki" in a molybdenum-free, but otherwise similar, alloy system.
Suzuki attributed hardening to internal strains associated with solute segregation and /or
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