Absence of hydrogen influence on the mechanical stability of retained austenite in a 0.2c/12cr/1mo steel
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able I.
C 0.20
Cr 11.64
Mo 1.01
W 0.57
given in Table I. The steel was received as 1.6 cm thick plate which had been annealed at 750 °C after rolling. The room temperature tensile properties were determined for specimens with and without internal hydrogen in the as-quenched and quenched-and-tempered conditions. Specimen blanks were austenized at 1050 °C for thirty minutes and oil quenched. Tempering to increase the mechanical stability of the retained austenite was for one hour at 200 °C. Internal hydrogen was introduced before testing by cathodic charging at a current density of 0.006 A/cm 2 for one hour in a 4 pct sulfuric acid solution containing sodium arsenate as a cathodic poison. The specimens were plated with 0.003 cm of copper immediately after charging to retain the hydrogen and allowed to equilibrate for 24 hours before testing. During the tensile tests, the volume fraction of retained austenite in the gage section of each specimen was measured as a function of load using a magnetic saturation device. 5 Flat tensile specimens of the transverse orientation were used. These specimens were 0.127 cm thick with a width of 0.318 cm in the 2.85 cm gage section. Two specimens were tested for each condition at a strain rate of 0.018/min. After testing, the fracture surfaces of the tensile specimens were examined using SEM. The tensile properties of the as-quenched and tempered at 200 °C microstructures with and without internal hydrogen are given in Table II. The yield strengths of the two microstructures without internal hydrogen differed by only 7 pct. Hydrogen charging increased the yield strength of the tempered structure slightly and reduced the ductility substantially. Hydrogen charging caused the as-quenched structure to fail at a stress of about one-third its expected yield stress. As shown in Figure 1, hydrogen charging did not appear to affect the fracture mode of the as-quenched structure. Hydrogen charging did, however, dramatically alter the fracture mode of the quenched and tempered specimens. In the uncharged condition fracture occurred by microvoid coalescence, while in the hydrogen charged condition the fracture mode was similar to that observed for the hydrogen charged as-quenched material. Before testing, the as-quenched and the quenched-andtempered microstructures contained approximately 7.5 pct retained austenite of an interlath morphology; hydrogen charging did not alter these amounts. The volume fraction of retained austenite as a function of stress during the tensile tests is given in Figure 2. Because the ductility of the hydrogen charged specimens was limited, the retained austenite
Composition of HT-9*
V 0.30
Ni 0.51
Mn 0.57
Si 0.24
S 0.007
P 0.018
*compositions in wt pct
Table II.
Condition
As-quenched As-quenched, hydrogen-charged 200 °C temper 200 °C temper, hydrogen-charged 1876--VOLUME 17A, OCTOBER 1986
Flat Tensile Properties
Yield Strength, 0.2 Pct Offset
Fracture Stress
MPa
MPa
1475 failed at stress of 450 1378 1476
1501 450 1664 1500
Plastic Elongation 0.4
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