The effect of hydrogen gas on the plasticity/cleavage balance in Fe-3.5 Pct Si
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I.
INTRODUCTION
HYDROGEN embrittlement of ferrous alloys has been attributed to a number of different causes. When the lattice is supersaturated with hydrogen after cathodic charging or after quenching from a high temperature, there is no doubt that precipitation of hydrogen in internal cavities can pressurize the cavity sufficiently to cause cracking. Lz However, Petch and Stables 3 proposed that hydrogen could also assist the nucleation of cleavage by reducing the surface energy of iron. Indeed, a mechanism such as this, that does not involve pressure effects, is essential to explain the hydrogen embrittlement observed when testing in gaseous hydrogen at low pressures and temperatures. 4'5 In some high strength steels, hydrogen assisted cracking occurs at grain boundaries, and there is the possibility that hydrogen interacts with segregated impurities to give decohesion at the boundaries. 6 Despite this possibility, hydrogen assisted transgranular fracture is often observed, 7 and then it is possible that hydrogen acts alone without the aid of impurities in the alloy. The observations of fracture in grainoriented iron-3.5 pct silicon presented here show clearly that hydrogen gas can promote cleavage, mainly on the {100} planes, in circumstances where straining in air or in vacuum would not. A notable feature of the results is the occurrence of some cleavage apparently on the {110} plane.
II.
tween the specimen axes and the crystal directions of a grain with the preferred orientation is shown in Figure 1. When they are strained, specimens with the [001] orientation may respond by cleaving on the (001) plane or by yielding at the notch tip_b_), slip in the (111) directions. Slip in either the []11] or [111] direction results in plane-strain plastic deformation with no through-thickness component. Slip in the other two directions ([111] or [111]) gives plastic deformation with a large strain component in the throughthickness direction. Cleavage fracture is often considered to
t [001] ]'11
111
111
(001)
EXPERIMENTAL METHOD
tt 1]0]
The grain-oriented silicon iron employed in the tests was supplied in the form of transformer sheet. The material exhibits a sharp texture, with 90 pct of the_grains lying within 5 deg of the preferred orientation ((110) [001]). Strips, 76 • 18 • 0.34 mm, were cut either parallel to the rolling direction (the [001] orientation) or at 90 deg to the rolling direction (the [110] orientation). Slots 6 x 0.3 mm were machined into the long edge of the strip. The relation be-
1]1
9 J. T. EVANS, Lecturer, and D. HARDIE, Senior Lecturer, are both with the Department of Metallurgy and Engineering Materials, University of Newcastle upon Tyne, Newcastle upon Tyne, NEI 7RU, United Kingdom. R. J. HARRISON, formerly with the University of Newcastle upon Tyne, is now with The Anglo American Corporation, South America. Manuscript submitted May 27, 1982. METALLURGICALTRANSACTIONS A
'f
Fig. 1 --Orientation of a grain with the preferred orientation, relative to the specimen axes for the 2 kinds
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