Abstract
In the pipe manufacturing and pipe processing industry, the demand for cost-effective pipes with high strength and good ductility is increasing. In the present study, the inductive longitudinal welding process was combined with a Quenching and Partitioning (Q&P) treatment to manufacture pipes with enhanced mechanical properties. The aim of the Q&P process is to establish a martensitic structure with increased retained austenite content. This allows for the beneficial use of both phases: the strength of martensite as well as the ductility of retained austenite. A 42SiCr steel, developed for Q&P processes, was joined at the longitudinal seam by a high-frequency induction (HFI) welding process and was subsequently heat-treated. The applied heat treatments included normalizing, austenitizing, quenching, and two Q&P strategies (Q&P-A/Q&P-B) with distinct quenching (Tq = 200/150 °C) and partitioning temperatures (Tp = 300/250 °C). Investigations of the microstructures revealed that Q&P tubes exhibit increased amounts of retained austenite in the martensitic matrix. Differences between the weld junction and the base material occurred, especially regarding the morphology of the martensite; the martensite found in the weld junction is finer and corresponds more to the lath-type morphology, compared to the base material in the circumference. In all zones of the welded tube circumference, retained austenite has been found in similar distributions. The mechanical testing of the individual tubes demonstrated that the Q&P treatments offer increased strength compared to all other states and significantly improved ductility compared to the quenched condition. Therefore, the approach of Q&P treatment of HFI-welded tubes represents a route for the mass production of high-strength tubular products with improved ductility.
Highlights
Due to the heat influx during induction heating, welding, and subsequent cooling, the pipe cross-section is subject to a process and material-specific, time- and position-dependent heat distribution which locally affects microstructural evolution
Due to the heat influx during induction heating, welding, and subsequent cooling, the pipe cross-section is subject to a process and material-specific, time- and position-dependent heat the distribution neutral axis werelocally analyzed different regions, representing themicrostructures weld junction, the coarsewhich affectsin microstructural evolution
The welding tests in combination with an offline quenching and partitioning (Q&P) treatment have demonstrated that 42SiCr can be High-frequency induction (HFI)-welded despite the high carbon equivalent and that a Q&P microstructure, comprising martensite and retained austenite, is formed throughout the welded tube
Summary
Since Speer et al first reported the partitioning of carbon from supersaturated quenched martensite into retained austenite in 2003 [1], quenching and partitioning (Q&P) has been widely investigated in terms of mechanisms, materials, routes, and processing. Due to their combination of high strength and ductility, Q&P steels have even been proposed to represent the third-generation advanced high-strength steels (AHSS) for the automotive sector [2,3]. In the year 2016, 1.6 billion tons of steel have been produced, of which 139 million t and ~9% were further processed to tubular products [4,5].
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