AN INVESTIGATION OF INTERNAL DEFECTS IN HIGH-CHROMIUM STAINLESS STEEL
LEE HSUN;HO CHAIN-CHU;CHANG CHIN-FANG;FUNG MUN-CHU;YAN SHO;CHAO HWEI-TIEN Institute of Metal Research; Academia Sinica
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LEE HSUN;HO CHAIN-CHU;CHANG CHIN-FANG;FUNG MUN-CHU;YAN SHO;CHAO HWEI-TIEN Institute of Metal Research; Academia Sinica. AN INVESTIGATION OF INTERNAL DEFECTS IN HIGH-CHROMIUM STAINLESS STEEL. Acta Metall Sin, 1957, 2(1): 1-18.
Abstract In the manufacture of stainless steel of the high chromium type, axialcracking or porosity in ingots and ghost lines in forged billets were frequentlyencountered. It is shown that these two kinds of defects are of the same origin, billets made from ingots free from axial cracking were also immune from ghostlines. The formation of axial cracking in ingots occurs during solidification, there-fore cooling conditions which affect the interval of solidification must play animportant role in assessing the extent of cracking. The use of refractory ingot-mould led to complete immunization of axial cracking, whereas the adoption ofother alternative means such as increasing the tapering of the ingot or decreasingthe wall thickness of the ingot-mould was not so effective. To certain extents axial cracking in ingots or ghost lines in billets wereassociated with the presence of nonmetallic inclusions in steel. Through an im-provement of deoxidation procedure during smelting, the nature and the distri-bution of inclusions were favourably changed. This, supplemented by the adoptionof thin-walled ingot-mould, has been tried in works practice, the severity of axialcracking in ingots or ghost lines in billets being greatly lessened. It is consideredthat the fluidity and surface tension of liquid steel may vary with the amount,the nature and the shape of inclusions present, any change in the latter whichwill lead to an improvement of the filling properties of the liquid steel must havea favourable influence towards the immunization of solidification defects. Smelting and casting in vacuum below certain pressure range resulted in per-fectly sound ingots, such ingots showed an external zone of columnar crystalsmuch smaller than those smelted and cast under similar conditions but in air.There may be many reasons for the above observation, for example, vacuum pro-motes degasification, the transfer of heat between ingot and mould is altered byvacuum, the properties of the liquid steel in vacuum and in air are different, etc.Some of these problems undoubtedly deserve further investigation. In the mean-time, it can be said that vacuum smelting and casting appear to be the bestmethod to ensure the production of high quality stainless steel.
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