Effects of annealing temperature, time and air/CH4 ratio
on secondary oxide formation of AISI 304 stainless steel
Pattama Naewkanya1*, Ura Panchareon1, Pornchai Labcharoenwongsa2,
Gobboon Lothongkum3
1
Department of Chemical engineering, Faculty of engineering, Bangkok 10330
2
Thainox Stainless Public Company Limited, rayong, 21180
3
Department of Metallurgical engineering, Faculty of engineering, Bangkok 10330
*Corresponding Author: Tel. 08-4355-2420 Fax. (02) 218-6942
E-mail: pattama_oui@hotmail.com
Abstract
During hot-rolling, the primary oxide scales are continuously formed on both sides of
stainless steel slabs because of directly heating in a furnace by combustion of a fuel such as
natural gas, fuel oil, etc. The black or tandem coil was used as the raw material to produce the
cold-rolled stainless steel plate. In cold rolling process, the black or tandem coil was annealed
in the furnace using natural gas as fuel. Therefore, the surface of stainless steel is secondary
oxidized in the atmosphere of O2, CO2 and H2O. The secondary oxide formed on the surface
of steel are mechanically removed by scale breaking and shot-blasting, and chemically
immersed in acid pickling. This research studied on the oxidation of AISI 304 stainless steel
black coil or tandem coil. The samples were secondary oxidized in the gas atmospheres based
on methane combustion with air/CH4 ratios of 10, 11 and 12 at the temperatures of 1000ºC
and 1100ºC for 90 and 150 seconds. The primary (as-received state) and secondary oxide
films were investigated by OM, SEM equipped with EDX and XRD. The results showed that
the thickness of the primary oxide was about 5-6 m. The secondary oxide or scale was
composed of two layers: the outer layer was porous oxide and the inner layer was compact
oxide. The primary oxide consisted of two oxide phases: FeCr2O4, and Fe2O3. After
secondary oxidation, the weight changes of annealed samples at 1100C were more than at
1000C. The spallation of specimen increases with the oxidation time and temperature are
more than ratio of air/CH4 at 1000 oC. The thickness of secondary oxide after oxidation was
lower than before oxidation because porous oxide was spalled during cooling. The secondary
oxide consisted of two oxide phases; FeCr2O4, and Fe2O3 except for oxidation at temperature
of 1100ºC for 150 second and air/CH4 ratio of 10. It consisted of three oxide phases:
FeCr2O4, Fe2O3 and Cr2O3.
air/CH4 = 12, 1000 ºC, 90 s
air/CH4 = 10, 1100 ºC, 150 s
XRD patterns of samples oxidation at different conditions.
FeCr2O4, Fe2O3, Cr2O3, Fe,Ni