ac93ca6a_goya_Shi___Patent_applications__Description
[Patent application number] 20110155088 [Published] 06-30-2011 [Description] 
ENGINE VALVES - An engine valve according to the present teaching has a hot-forged head portion having an oxidized surface layer at a radially outer peripheral surface. The oxidized surface layer is formed during hot-forging of the head portion.
Yamada, Nagoya-Shi - Patent applications