Influence of sulfide inclusion on ductility and fracture behavior of resulfurized HY-80 steel
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I.
INTRODUCTION
T H E HY-80 steel is a high-strength low-alloy (HSLA) steel used in quenched and tempered condition. Its many attractive properties, like high specific strength, good formability, weldability, and corrosion resistance, have made this steel a good choice for applications in many engineering and marine constructions, including submarine pressure hulls. Despite these useful properties, the welding of this steel when not critically controlled has often posed problems, particularly in the shop floor conditions. The reason for these problems has been attributed to the presence of nonmetallic inclusions, particularly sulfide stringers. It has long been realized that nonmetallic inclusions have a profound influence on the ductility and fracture behavior of metals and alloys, tl,z,3J However, specific information as to the exact role of sulfur in controlling the fracture behavior of steel is rather sparse, because the quantitative assessment of these effects is not easy to determine. Ever since this steel was introduced in the mid-1960s, the relevant military specification has been updated several times in setting the limits of maximum permissible impurity and tramp elements like sulfur, phosphorus, antimony, and tin. taj This revision was made because these elements have a singular role in controlling the fracture behavior of HY-80 steel and its weldments. Although some information is available on the mechanistic effects, the quantitative relationship between the ductility and inclusion parameters in relation to its microstructure is elusive. The objective of this work is to establish a quantitative relationship between inclusion content and properties of resulfurized HY-80 steel, heat-treated to different strength levels, as a function of sulfur level and, further, to establish the micromechanism of fracture process in these steels. D.K. BISWAS, Metallurgy Division Head, M. VENKATRAMAN, and C.S. NARENDRANATH, are Scientists with the Naval Chemical and Metallurgical Laboratory, Naval Dockyard, Bombay 400001, India. U.K. CHATTERJEE, Professor, is with the Department of Metallurgical Engineering, Indian Institute of Technology, Kharagpur, India. Manuscript submitted August 13, 1990. METALLURGICAL TRANSACTIONS A
II.
EXPERIMENTAL
A. Materials and Heat Treatment
Several experimental heats of HY-80 steel with varying sulfur contents were produced in three batches. Two batches of 30 and 100 kg were air induction melted; they are designated as B and R, respectively. Another batch, designated M, of 30 kg was vacuum induction melted. The chemical composition of the HY-80 steel which was melted and resulfurized to various sulfur contents is given in Table I. Both the R and B batch were aluminum deoxidized, but the M batch was not AI killed, as it was melted in vacuum. The R batch was cast as square ingots of 100 m m in cross section, whereas the B and M batches were cast into cylindrical ingots of 100 m m in diameter and 250 m m in length, respectively. The desired sulfur levels in the melts were achieved by res
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