Changes of Microstructure and Grainsize of Martensitic Stainless Steel during the Processes of Hot Reversed Extrusion, Broaching and Heat Treating

The field of high pressure steel cylinders (HPSC) used for the variety of applications, such as storage of technical gases, compressed natural gas (CNG), medical gases and special applications like scuba diving cylinders is still significantly expanding. With increasing safety requirements, the need of new techniques applied in field of HPSC structural and materials innovations comes into place.Speaking mainly about the field of scuba diving cylinders, the need of corrosion protection of inner surface comes in place due to the possible risk of valve blockage by metal corroded particles of inner surface. Even such a risk occurs in extremely rare occasions, there was until now no generally applied solution (such as inner wet, powder or Teflon coating, etc.) due to significantly high costs of such additional inner surface protection. Another problem that occurs so far are significant visual imperfections of scuba diving cylinders caused by the rough treating by scuba divers during the use and manipulation that causes chipping of outer surface painting and subsequent corroded areas.The breakthrough solution is the scuba diving cylinder made of a stainless steel by the processes of reversed extrusion and broaching from billet without any need of an additional coating. No such a product was made so far by these methods of production and no evidence of an evolution of the microstructure and grainsize changes during such manufacturing with subsequent heat treating was ever documented and analysed.