Seeing the macro-structure of electro-slag welded Cr-Mo steels, its grain size is finer as compared with ASTM A 302 B. This is because the plate thickness of Cr-Mo steel is thinner and its cooling velocity is faster than ASTM A 302 B. grain growth of deposit-metal which is shortcoming of electro-slag welding becomes more remarkably in proportion to increase of thickness. On the otherhand, we decided to do experiment to apply normalized & tempered heat-treatment to electro-slag welded joint of Cr-Mo steels in view of special steel and to appy severe condition.
As the result of normalized & tempered heat-treatment, impact value (especially notch toughness) has been increased. Improvement of deposit metal is most conspicuous among various structures of welded joint. Impact Vlalue of large size specimens which are 40 mm square show no difference between these steels and ASTM 302B as welded, but the former were as heat-treated. So that, to make conclusion from the impact value of 10 mm square specimen is supposed to be unreasonable. In near future, we are to make transiticn curves of these steels & ASTM 302B by large size impact specimens.
Changes of mechanical properties of electro-slag welded, normalized and tempered Cr-Mo steels by wires and fluxes which were on sale, were studied and following were found.
(1) Quantity of oxygen contained in electro-slag depositmetal increase according to its grain growth.
(2) Tensile-strength of electro-slag welded joint normalized by cooling velocity of faster than 300°C/h and tempered is satisfied with the minimum value of specification.
(3) Impact value increases according to the speed of cooling velocity in normalizing and its transition curve moves to lower temperature side. The best normalizing temperature of 1 Cr-0.5 Mo steel is 900°C
(4) Bend test of electro-slag welded specimens shows no crack when normalized and tempered but crack as welded.
(5) High temperature tensile strength of normalized and tempered welded joint is satisfied with specification as well as the strength of base metal.
(6) Fatigue test result of electro slag welded joint at the same value of base metal are satisfactory.
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