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ABSTRCT: This study deals with the effect of annealing temperature on the mechanical properties of SG 255 steel. Furthermore it aims to experimentally obtain better mechanical properties in particular ductility with lower annealing temperature and to minimize the cost of the manufacturing. The SG 255 steel is used in manufacturing of domestic LPG cylinder at Halala Company located in Sulaimanyah/Iraq. A number of samples were made according to ISO 6892, and then heat treated with different annealing temperature (850, 900 and 950 ºC). Moreover, the mentioned samples were broken by tensile machine to obtain the required mechanical properties, and to be compared with that of Halala factory. It was shown that higher elongation percentage 34.18%El with lower annealing temperature 900 ºC can be achieved and this leads to decrease the cost of manufacturing without degrading of cylinder's quality.

The investigation in this paper is based on the existence of carbon content and its role in the formation of Fe-Al intermetallic layer in unalloyed carbon steel C45 with a carbon content of 0.44 wt.%. Several sophisticated techniques such as PFIB SEM equipped with EDS and EBSD, GD-OES were employed for the in-depth surface analysis. The results of the metallographic examination reveal that the carbon would appear and could be well detected at the interface between the solidified aluminum and the solid iron-base steel substrate and got also incorporated in the top aluminum layer. Furthermore, due to the dissolution and outward diffusion of iron into the liquid aluminum melt during the HDA process, plus its involvement in the formation of the solid intermetallic surface layer, thus the carbon atoms gaining higher chemical affinity there will also be more likely to form carbide precipitates of different kinds like Fe3AlC and AlC inside these developing surface layers on the C45 type steel during the process of hot-dip aluminizing at 700 °C.

Sophisticated metallographic techniques helped reveal numerous new features developed at the inner interfaces of the hot-dip aluminized C45 carbon steel specimens. Multi-intermetallic phases composing of Fe 2 Al 5 , Fe 4 Al 13 , FeAl 3 , and FeAl were identified whereby it was also found that the FeAl 3 phase did not inherent from Fe 2 Al 5 , rather it is the Fe 4 Al 13 phase that exists at the outermost layer of Fe 2 Al 5 toward the aluminum topcoat where the Fe 2 Al 5 layer has the most prominent grain size. In addition, the Gibbs free energies are consistent and underline the preferential growth of the Fe 2 Al 5 phase having ΔG° = − 197.6 kJ/mol at 700°C.