Deterioration in Surface Brightness of Vacuum Heated SUS 304, 316, and 440C Due to Surface Roughening

Abstract

Austenitic stainless steels (SUS 304 and 316) and a martensitic stainless steel (SUS 440C) were heated at temperatures from 900 to 1250°C (1173–1523 K) for the period of time up 300 min (0–18 ks) in a vacuum of 5.33 Pa. The steels heated were die cooled in the vacuum to prevent their surfaces from being stained with quenchants. Surface brightness was measured by use of an optical microscope equipped with a silicon photocell. Excess increases in temperature and/or time give rise to a deterioration in the brightness. A comparison of the surface appearance with the corresponding brightness shows that the deterioration results from occurrence of the following surface roughening: (1) relief formed by martensitic product on SUS 304 and 440C; (2) boundary groove on all the steels; (3) twin groove on SUS 304 and 316; (4) crevice at carbide-matrix interface and void brought about by disappearance of carbide due to growth of crevice on SUS 440C. Above all, the relief formed by martensitic products markedly deteriorates the brightness. In the case of the metastable austenitic stainless steels, SUS 304, evaporation of Cr and Mn induces the martensitic transformation in its surface layer. On the contrary, in the case of SUS 316, the transformation did not occur in spite of the evaporation at least under the present heating conditions. This is attributed to the natural stability of its austenite. No evaporation serves the obstruction or the suppression of all the above surface roughening and hence offers the excellent brightness to the stainless steels.