| Popis: |
The subject of this dissertation is the study of the effects of fabrication history (prestraining, welding and heat treatment) on the mechanical properties of austenitic stainless steel thin wall boiler tubes. These tubes are usually cold bent to shape, and sometimes swaged, prior to interconnection by welding. The bends require solution heat treatment before welding. In addition, subsequent to welding, the residual stresses should be relieved. It is sometimes not practically feasible to follow these constructional practices strictly especially when a whole boiler is constructed as a single unit and becomes too large and complex and contains different tubing materials. As a result of this fabrication history, the mechanical properties of boiler tube materials can be significantly altered. Sample tubes simulating the fabrication steps were supplied by British Energy for this project. The primary aim of the study was to determine spatially resolved room-temperature tensile properties using digital image correlation (DIC) by testing cross-weld specimens machined from the thin wall welded tubes (with plain or prestrained base metal) before and after the heat treatment. The experimental procedure which is used to retrieve the tensile properties from these integrated tests was validated through finite element simulation. Digital image correlation, which is a full-field strain measurement technique, was implemented in order to obtain the local stress-strain curves from regions less than a square millimetre in area and to extract the corresponding local tensile properties such as offset proof strength. The variation of the 0.2% offset proof strength was successfully obtained along these specimens. Evidence of strain hardening due to constraint and weld thermomechanical cycles was found in the plain base metal near the weld pool and evidence of softening was seen in prestrained base metal. On the other hand, after the heat treatment, the effect of prestraining and welding is cleaned out and the strength along the specimen was almost homogenized. However, aswelded cross-weld specimens with prestrained base metal have demonstrated unusual local stress-strain behavior in the weld-affected region. For a better understanding of this behavior, tension test of a cross-weld specimen with a high strength mismatch between the weld metal and base metal was simulated using the finite element method. It was found that the strength mismatch in the specimen, in combination with the experimental procedure, may cause some anomalies in the local stress-strain curves. It was also confirmed that these anomalies are not very detrimental for the determination of the proof strength on the specimens with strength mismatch. Material characterization of the welds and detailed hardness surveys on crossweld specimens were performed. Plastic strain is known to be detrimental for high temperature performance of austenitic stainless steel tubes, therefore, the degree of the plastic deformation should be known before these tubes enter service. DIe, hardness, electron back-scattered diffraction and neutron diffraction (peak width and anisotropy strain) were used to determine the amount of plastic strain in the as-welded tubes. It was observed that there is a good agreement between the predictions of plastic strain in 20% prestrained and welded tube. |
