Toichi Watanabe, Jiro Saga
The mechanical properties of two 18-8 type stainless steel wires were studied. Steel A contained 0.056%C, 0.039%N, 8.02%Ni, 16.77%Cr, while steel C 0.060%C, 0.033%N, 9.91%Ni, 18.80%Cr. The variations of the proportional limit and the yield point due to cold drawing and subsequent annealing were mainly investigated. The following conclusions may be drawn from the experimental results : (a) The steel A can be transformed nearly 100% into α phase only by drawing at room temperature. The γ → α transformation occurs most remarkably in the range of about 40~60% reduction, and takes place in the interior earlier than the exterior of the wire, while in steel C, the transformation is no more than 20% by drawing of 90% reduction. (b) Although the Vickers hardness (Hv) and the tensile strength (σB) increase with the increase of rate of drawing in both steels, the proportional limit (σP) does not. σP of cold-drawn wires, therefore, is very low in spite of their high Hv and σB. For steel C, this is more remarkable, and its σP is independent of the degree of cold working ranging from about 20 to 90% reduction. (c) The hardening by annealing after cold drawing proceeds in two stages: at the temperatures of 200° and 400° for steel A, and 250° and 450~550° for steel C. (d) Annealing after cold drawing is very effective for increasing σP, the effect being more pronounced in steel A in a highly cold drawn state. For example, σP is 120 kg/mm2 for steel A and 50 kg/mm2 for steel C, when they are cold drawn about 60% in reduction of area and annealed for 1/2 hr at 400° and 450° respectively.
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