Based on the market demands regarding exploration in deep water fields, e.g. high pressures, low temperature and sour reservoirs, Vallourec & Mannesmann Tubes have developed new high strength steels up to grade X100 with low temperature toughness, sour service resistance and good weldability. This work is part of a development program for high performance heavy wall seamless pipes for special applications such as J-lay collars, buckle arrestors and risers. The paper presents the latest results on industrial studies on high strength heavy-wall pipes in grades X70 and X80 with wall thickness up to 80mm, which are manufactured by seamless hot-rolling and subsequent quench and temper treated. Vallourec & Mannesmann Tubes's alloying concept based on carbon content below 0.10% and micro-alloying of vanadium and niobium used for grades X60/X65/X70 was extended to heavy wall seamless pipes in grades X70/X80. Mechanical properties, advanced metallographic examinations and results of sour service resistance are reported. To demonstrate weldability, test welds were performed and examined. Introduction The market demands regarding exploration in deep water for oil and gas reservoirs lead to technological trends in using high strength steels (grade X65 to X80 and higher) with special requirements regarding e.g. corrosion, toughness. Increased yield strength and therefore thinner wall thickness enables the industry to reduce the costs of all components through the limitation of the string weight. On the same time the pipeline design includes special parts like J-lay collars for the pipe laying operation and buckle arrestors at regular intervals along the line for protection against an initiation and propagation of collapse and buckling [1]. All parts have to follow the demands for strength, ductility, toughness and sour service resistance combined with adequate welding performance [2, 3, 4, 5]. Over the last years Vallourec & Mannesmann Tubes have developed new high strength steels up to grade X100 [6]. Not only material development but also new technologies in the field of heat treatment capabilities allow the manufacturing of heavy wall seamless pipes with high strength and excellent corrosion properties. In this paper results of studies on X70 and X80 pipes with heavy wall (wall thickness between 40mm and 80mm) produced by the pilger mill are discussed. The pipes are quenched and tempered to create the suitable microstructure which provides good toughness and sour service properties. This work is a part of R&D program for high performance Q&T seamless pipes for deep water applications.
The continued shift of exploration and production to deep water fields will require the industry to develop alternative pipe solutions to cope with the challenging demands of these exploration regions. Because of the complexity of exploration conditions in deep water fields, e.g. high pressures, low temperature and sour reservoirs, higher grades and heavier wall thickness in combination with low temperature toughness and suitability for sour service are required. The Vallourec&Mannesmann Tubes’s alloying concept for line pipe steels based on low carbon concept [1] was extended to grades X70 and X80 with wall thicknesses up to 75mm. In this paper the latest results on industrial studies on high strength heavy-wall steels manufactured by seamless hot rolling and subsequent quench and temper treatment are presented. The work is a part of the development program for high performance heavy wall seamless pipes for special applications such as J-lay collars, buckle arrestors and risers. Mechanical properties, advanced metallographic examinations, results of the sour service resistance and weldability are reported.
-In order to characterize NiTi orthodontic wires microstructure, transformation temperatures, and deformation behaviour have been investigated. Various states of dislocation density were established by additional heat treatment of a work hardened material. The as recieved wire and the change in microstructure due to the chosen heat treatment were described by optical ligth microscopy. The stress induced transformation behaviour is related with the experimental results in transformation characteristic measurements and microstructural observations.
Martensitic transformation in Fe-Ni-based alloys is accompanied with an increase of volume (AVy-(r~ 3%). Lattice invariant shear leads to additional dilatometric changes for non-random transformations. As the result of predominantly AV,-, a large thermal hysteresis (AT,) originates. This leads to a diffusion-controlled a-y-transformation, i.e. the reverse reaction becomes crystallographically irreversible (a Ty). In the case of an aged Fe-Ni-Co-Ti alloy, AV,-, depends on the magnetic state of the austenite and on the volume fraction of the precipitated y'-particles. With increasing the volume fraction of y', AV,.,, is reduced. Dislocations at the former martensite/austenite interfaces indicate incomplete reversib'fity of the transformation cycles, even in the ausaged condition. However, drastic reduction of AT, is observed during an ausaging sequence for conditions close to maximum volume fraction and minimum size of y'.
Abstract:Martensitic transformation implies a change in crystal structure by lattice variant shear as well as the formation of lattice defects by various amounts of lattice invariant shear. Crystallographic reversibility defines conditions under which the reverse reaction leads to the restoration of a defect-free high temperature phase (y). This requires a lattice variant shear in opposite direction to the previous martensitic transformation. As alloys of iron are usually non-reversible a systematic study has been conducted to define the parameters which favour reversibility. Different Fe-Ni-based alloys have been studied to explore the effects of heating rates and alloying elements. The course of the retransformation is followed by DSC investigations. Optical and TEM investigations have been done to describe the change of macro-and microstructure after retransformation. IntroductionSince the shape-memory effect has been found in alloys of iron many investigations have been conducted to optimize the effect. However, less interest is spent on a general understanding of the origin of crystallographic reversibility in alloys of iron. The retransformation from martensite into austenite in Fe-based alloys is usually characterized by an irreversible course of the transformation. As a result, both macro-and microstructure become different compared with the initial one. But under certain conditions the ad-y-transformation will lead to a restoration of the high temperature phase (y) without any lattice defects. A prerequisite for this ad-y-transformation is a lattice variant shear in opposite direction to the previous martensitic one (y,-,,= y, , , ).The crystallographic reversibility is a prerequisite for the development of an iron-based shape memory alloy.A detailed knowledge is important to describe the conditions which favour the complete reversibility. Therefore investigations have been conducted systematically to explore different parameters which may influence the retransformation.The change in macro-and microstructure as well as the transformation temperature are discussed.Article published online by EDP Sciences and available at http://dx
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