Metallurgical Conditions Required for Superior Hot-workability of 13% Chromium Steels in a Seamless Rolling Process

• Published in: ISIJ International 2007, Vol.47(8), pp.1159-1167
• Main Authors: Kawakami, Akira, Asahi, Hitoshi, Ueno, Masakatsu
• Format: Article
• Language: English
• Subjects: AISI420; Applied sciences; CTG (Oil Country Tubular Goods); Exact sciences and technology; hot-workability; Metals. Metallurgy; phase diagram; seamless pipe; solution annealing; δ-ferrite
• ISSN: 0915-1559; 1347-5460
• DOI: 10.2355/isijinternational.47.1159

Hot-workability of 13% Cr steels was investigated in terms of microstructure by model elongator rolling tests and by tensile tests to develop modified types of 13% Cr steel seamless OCTGs (Oil Country Tubular Goods). Hot-workability deteriorated in both tests by the formation of δ-ferrite in austenite (γ) matrix; however, the deterioration occurred at lower temperatures in tensile tests than in model elongator tests. Materials are deformed more heavily in the latter than in the former, which is considered to be the cause of the difference. The result suggests that lower temperatures than the actual rolling temperature should be selected for the tensile tests in order to evaluate hot-workability of materials for seamless pipes. δ phase fraction during hot-working was almost equal to that estimated from the experimental phase diagram at a deformation temperature for plain C–13%Cr steels and at a primary heating temperature for Ni containing low C–13%Cr steels. This phenomenon is explained by a difference in the diffusion coefficient of C and that of Ni. Furthermore, it was clarified that isolated distribution and polygonal shape of δ prevented void-propagation during hot-working and was not harmful to hot-workability in plain C–13%Cr steels.