Microstructural characterization and mechanical properties of 9%Ni steel welds by submerged arc welding process using nickel-base alloys
Natural Gas with more than 80% methane has a liquefaction temperature around -165 ° C. Temperature at which the gas reduces its volume by a factor of 600/1. This operating temperature makes the use of ferritic materials unfeasible because they have a brittle fracture mode below a critical temperatur...
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| Tipo de recurso: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2017 |
| País: | España |
| Institución: | CBUC, CESCA |
| Repositorio: | TDR. Tesis Doctorales en Red |
| OAI Identifier: | oai:www.tdx.cat:10803/462904 |
| Acceso en línea: | http://hdl.handle.net/10803/462904 |
| Access Level: | acceso abierto |
| Palabra clave: | Acer Acero Steel Soldadura Welding Resistència de materials Resistencia de materiales Strength of materials Ciències Experimentals i Matemàtiques 54 |
| Sumario: | Natural Gas with more than 80% methane has a liquefaction temperature around -165 ° C. Temperature at which the gas reduces its volume by a factor of 600/1. This operating temperature makes the use of ferritic materials unfeasible because they have a brittle fracture mode below a critical temperature, called transition temperature. For the construction of large containers, the most commonly used material is Steel A-553-T1 which has a nominal content of 9% nickel and whose crystalline structure is formed by a martensite matrix with some austenite reacted. This microstructure is achieved through double heat treatment; Tempering and tempering. To weld this steel, for this application, it is not possible to use materials of 9% Ni feed, in view of the impossibility of performing the necessary thermal treatments to achieve homogeneity of properties. On the other hand, the austenitic welding consumables present a ductile behavior with a high energy absorbed even to -196ºC and within these NiCrMo family nickel base alloys have a high mechanical strength, and a coefficient of thermal expansion close to the steel 9% Ni. Within this family of nickel base alloys, the Hastalloy C-276 alloy has been used, which increases its mechanical strength by solid solution, the main alloys being chromium and molybdenum both about 15% and with 2.5% of Tungsten and 5% iron. Although this is an alloy that is essentially single-phase Gamma, the last liquid is usually transformed into carbides or TCP phases as the pass Mu and P. These three phases have a very close composition so that their identification through the EDX is not possible . Currently, most of the tanks being built have a storage capacity between 150,000 and 200,000, so the sheet thickness of 1 was ferrule between 27 to 50mm, implying that the welds are multi-pass, Requiring between 16 and 30 passes to fill the joints of this first ferrule. Normally the vertical joints are welded with manual or semi-automatic processes while the horizontal joints are welded with automatic submerged arc process. According to the atmospheric tank design standards for cryogenic storage, the thickness of the sheets is determined by the Maximum Admissible Efforts that are calculated from the mechanical strength of the weakest structural element, the base metal or the weld joint. In the case of welding, the mechanical strength of the weld is determined from the cylindrical specimen tensile test obtained from the deposited metal, from the homologation coupon of the welding process. During the homologation of the manual or semi-automatic procedures, the results obtained in the tests of longitudinal and transverse traction are equivalent. In the case of automatic welding In addition to the low values obtained from the cylindrical tensions of the weld metal of the horizontal joints with respect to the transverse tensions, it is very often observed that an important difference in the resistance presented by the different tensile tests of the same welded specimen , These differences being much greater than the observed difference between two experimental conditions. Prior to this experimental work, 6 other tests and a few procedural approvals were carried out, in which sheets of 12, 21, 26.5 and 27 mm of thickness were used, with 2.4 mm and 1.6 mm threads always of the classification AWS A5.14 ER-NiCrMo-4, corresponding to the Hastalloy C-276 alloy, with different fluxes, stiffening levels, cylindrical probe diameter etc. This PhD work was carried out on the seventh test carried out in the summer of 2008 at Lincoln Electric Cleveland, where 4 fluxes, 2 wire diameters, alternating and continuous current and two voltage levels were tested, with an experimental design 23 with each flux. As all DOE tests were performed, 8 specimens were welded per flux, and a total of 32 specimens were welded. The purpose of this test was to select the best flux wire pair, and determine the optimum parameters to maximize the mechanical strength of the weld metal. The base material used in this experiment were A553 T1 steel sheets, with 9% Ni and annealed and tempered with a thickness of 21mm. The joint design of these specimens is asymmetrical and unbalanced "X" with a 1mm bead and a 2mm spacing. Following the actual joint design of the production plates. In order to prevent the melting bath from being picked up, a flux backing was placed. The tests performed on each specimen were as follows: Cylindrical traction welding metal: 4 per test pieces Charpy V Notch at -196 ° C Macro General Chemical Analysis, performed on the side faces of the macros. Chemical analysis on tensile specimens. Microhardness tests Vickers and Knoob. The wire-flux pair selected in these tests has been used for the welding of eight tanks: three in Spain: two in Gijón 2011-2013 and one in Bilbao 2014-2015; A tank in Chile, 2011-2013 and four other tanks in China, 2011-2013. With this pair, good results have been obtained in the approvals of welding procedures of these projects, both in the transversal tensions and in the cylindrical tensions, fulfilling the requisites of resistance necessary in each project. During the production, a welding metal with very few inclusions of slag has been deposited, presenting good degreasing and degassing. The objective of this research is to determine the factors that produce the variability of results in the tensile tests, correlating the structural and micro structural factors with the mechanical properties of the deposited metal, in order to maximize its mechanical resistance. |
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