Abstract
Different chemical compositions and heat-treatments of the C-Mn steels were investigated to characterise a susceptibility to cracking on cold bent tube. The C-Mn steels were categorised in terms of the content of free nitrogen. Ultimate tensile strengths, yield strengths and elongations were measured from the tensile testing at ambient, 250 and 360°C. Significant increase in ultimate tensile strength and decrease in elongation were observed in high free nitrogen material at 250°C the temperature at which free nitrogen is most active. From the uniaxial creep testing on high free nitrogen materials at 360°C increase in creep property, for example, lower minimum creep rate, was observed, however there was a decrease in creep ductility. This lower ductility of the high free nitrogen material has provided higher susceptibility to cracking in the creep crack growth tests at 360°C. Cracking in the high free nitrogen material A was approximately three times faster than the low free nitrogen material C at the same C* value. The creep cracking and rupture life in the high free nitrogen materials were more sensitive to the material condition, for example, pre-straining and/or heat treatment because of the role of the free nitrogen in the steels.
Published Version
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