INFLUENCES OF N & Cb ON THE PROPERTIES OF LCN-155 AND OF Cb ON THOSE OF S-816

Study on the Heat-Resisting Steels X-No. 1

Abstract

(1) The author made clear the influences of N & Cb on the properties of LCN-155, S-816 and the similar samples, studying their microstructures as cast and forged, the changes of hardness and structure by solution-treatment, ageing and hot-cold working, their mechanical properties at room and high temperatures, argon arc welding results with the similar electrodes, and researching the micro-hardness of their structure constituents and the electron microscopic structures, when necessary, at last he gave the equibrium-diagramatic aspects on these results. (2) At the melting of these samples, the mean of the yielding of Cb was 75%. (3) The influences of Cb on the micro-structures as cast are as follows; in LCN-155, the eutectics appear considerably when Cb was added over 1.0%, and in S-816, they increase remarkably as Cb increases and occupy the area of 1/4-1/3 of the observed field when 6% Cb was added. (4) In the samples as cast and forged, the micro-hardness of the deep etched primary austenite is equal to or a little lower than that of the light-etched austenite around the eutectic, and that of the eutectic, equal to or a little higher than that of the austenite around it. When N and Cb increase, the micro-hardness of each structure constituent increases. (5) When solution treatments at 1, 000-1, 200°C are carried for 10hr, a little decrease or no decrease of hardness at every temperature is observed in the samples with Cb compared with the ones without Cb, that is to say, the formers are better in stabilization. When aged respectively at 600-900°C for 30hr, the hardness decreases rather at 900°C frequently. The suppression of age-hardening by Cb are observed considerably, but there are some examples not so clearly observed. (6) The hot-cold work tests was made as follows; after tensile-loading appropriately at high temperatures on the test-pieces which are tapered at the place of parallel gauge parts, the loads per unit area, the contraction of area, the hardness and the grain size, at every cross-section of the tapered part, are measured. No monistic change between them is acknowledged, but the difficulties in working are felt a little by addition of N and almost always by increase of Cb. In the former the degree of hardening increases, and in the latter no notable differences or a little decrease are acknowledged as the degree of working. The grain sizes become not always finer as the working amount or N content increases, due to the influences of stress-relief-annealing. (7) According to the high temperature mechanical tests of LCN-155, the tensile strength at room temperature-800°C decreases about 5-2kg/mm² by addition of 1% Cb, but recovers generally by addition of about 0.12% N, and no remarkable change is seen when Cb was added moreover. Also, in S-816 and the similar samples, the strength at room temperature-900°C decreases by addition of Cb over about 4%. (8) The author illustrated the above results in considering that, to C, Cb has the most affenity and, to Cb, N has more affenity than C, and that, (Cr, Fe, W, Mo)₂₃ C₆ phase becomes more difficult to harden by ageing, when Cb solid-solves in. The results that the eutectics increase remarkably by Cb in the structures as cast was illustrated, too, by assuming the equibrium diagram. (9) By welding tests, the surfaces of the deposits as welded become more bright as Cb increases and have an appearance as like being polished electrolytically when Cb was added over 6%. To the hardness distribution of the sections of the deposits, Cb has no distinct influence. In the micro-structures, so much influences are not observed except that the eutectics in the deposits increase by Cb. The tensile strengths of the welded samples are considerably lower than that of the forged samples at room temperature, but the difference between them becomes smaller at higher temperature.