Comparison of the mechanical properties of ESR and directionally solidified AISI 4340 steel

Abstract

In a previous investigation by two of the present authors the mechanical properties of test specimens cut from directionally solidified AISI 4340 steel castings were determined.I It was found that the yield stress and ultimate tensile stress were essentially constant throughout the casting, independent of dendrite arm spacing. On the other' hand, the elongation and reduction in area were markedly sensitive to dendrite arm spacing, the maximum ductility being obtained with the minimum dendrite arm spacing. In this investigation, the mechanical properties of test specimens cut from AISI 4340 steel ingots, prepared by electroslag remelting, were determined. These results are presented and compared with the corresponding values for the directionally solidified ingots. The procedure used for producing directionally cast ingots is given elsewhere, 1 as well as data concerning the micro segregation of the alloying elements in this material. 2 The electroslag-remelted ingots were prepared on the UBC electroslag unit from air-melt bar stock of the same nominal composition as the directionally cast ingots. The 3 x 10-2 m diameter electrodes were melted using dc power and argon shielding, with the electrode negative and the mould insulated. The slag used was 70wt-%CaF2+30wt-% Ca(AI02h and was fused before melting. The melt rate was 1·5 gs-I, corresponding to an ingot solidification rate of 0·8 x 10-4 ms-I. The volume of metal which is molten at any time is approximately 6 x 10-5 m3 in the configuration used, with a 15kg ingot, and a 7·5 x 10-2m diameter mould. The same conditions were used to prepare two ingots. The dendritic structure across the electro slag-remelted ingots remained essentially constant along the ingot. The spacing of the primary dendrite branches was measured to be about 200 p,m midway between the ingot axis and the outside surface. Test specimens were cut from the ESR ingot both parallel and perpendicular to the ingot axis. They were heat treated and tested in the same manner as the directionally solidified test specimens 1 giving the results listed in Table 1. Two ESR ingots were examined and are indicated as A and B. The transverse and longitudinal test specimens, with respect to the ingot axis, are marked T and L respectively. The corresponding values for the directionally cast 22 kg heat of AISI 4340 steel (which were all transverse specimens) are included in the lower part of the table, as well as the ingot compositions. The results shown in Table 1 indicate that the ultimate tensile stress and the yield stress are highly reproducible for both ESR ingots and are similar to the values obtained for the directionally solidified ingots. The elongation and reduction in area values are not as consistent as the ultimate tensile and yield stresses, with six of the eight specimens in the range of 35-43% and two at low values of 20 and 12% for no apparent reason. There is no significant difference between specimens which were cut either parallel or perpendicular to the ingot axis in the ESR ingots. Comparing the elongation and reduction in area of the ESR specimens with the directionally solidified specimens, the electroslag ingots have appreciably better ductility,