Degradation of alloy properties by super purification and optimum design of advanced steels by the best use of impurity elements

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Some of alloy properties degrade by super purification. The best use of impurity elements such as S and B is needed for the optimum design of advanced heat-resisting steels. The objective of this research is to investigate the effect of impurity elements on the steam oxidation resistance. The results on the S impurity are summarized as follows ;( 1 ) It was observed that a mass loss occurred in the advanced high Cr ferntic steels m the early stage of steam oxidation at 650℃. However, this mass loss decreased significantly in the S-containing steels.( 2 ) As S was an impurity element to segregate on the surface of the stee at 650℃ it worked to assist the Cr enrichment on the surface owing to the strong affinity of these two elements As a result, Cr_2O_3 could form easily on the steel surface in the early stage of steam oSon a far as tne steel contained S.( 3 ) S existed under the Cr_2O_3 oxide layer or at the interface between the oxide and the base steel. The existence of S at the inte … More rface suppressed severe Cr depletion under the oxide, resulting in good oxidation resistance even for a long time exposure to steam.( 4 ) It was stressed here that even Cr_2O_3 oxide specimen itself showed a mass loss after steam oxidation at 650℃. However, the S doping into Cr_2O_3 decreased such a mass loss.( 5 ) As might be expected from these results, this S effect was not remarkable in the low Cr steels (e.g., 2.25%Cr steel).The mass loss obseryed ferritic steels and Cr_2O_3 oxide during steam oxidation was interpreted as due to the formation of a volatile hydroxide, CrO_2(OH)_2, as reported previously in austltic steels. The presence of S at an impurity level probably suppressed the formation of he volatile compound. This is probably the reason why S is effective in increasing the oxidation resistance to steam but not to air. The S effect became dominant if the steels contained 50ppm S. This is an allowable amount of S since it does not degrade mechanical properties of the steels. Therefore, for the optimum design, 50〜100ppmS should remain in advanced high Cr ferritic steels. Less