| Welded joints exhibit heterogeneity of mechanical properties. The high matching and low matching ware reflected between the weld metal and base metal, as well as the local softening of the heat affected zone of the fine grain steel. The testing of these kinds mechanical properties were different which have important influence on the strength and fracture toughness of the joints. In recent years, research had been focused on homogeneity of uneven joint mechanical properties of welding component. Also, on these fracture toughness parameters(CVN, CTOD and J) of fracture mechanics behavior and effect about brittle transition behavior, structure, defects assessment and performance. However, the study on the overall strength design and toughness evaluation of the mechanical properties used in the engineering was deficient. Therefore, the research on the evaluation method of the local welded joint’s mechanical properties was significance both on the design of the strength and toughness of the welded joint and the reliability assessment of the structure.The heat affected zone of welded joint was the weakest link of the whole joint. It’s width was only a few millimeters which also include the coarse grain zone and the fine grain zone. To establish a method for evaluating the performance of welded joints in micro-area, it was necessary to get the mechanical properties of the 0.3mm position accuracy. It is difficult to assess the accurate properties through of the conventional mechanical evaluation methods, such as small Punch, Micro-shear method, Indentation, etc. In the present paper, a new approach, that was double holes micro-shear test, was presented for determining the local plastic deformation properties in welded joints. In this test, the material between two small holes was sheared, and the strength and toughness were obtained. The local mechanical properties of the welded joint could be measured in the field, and the performance of the material is not affected. The main experiments and results were as follows:(1)According to the requirement of double holes micro-shear test and evaluation of welding local mechanical properties, automation and field measurements of the test device can be realized through the improvement of three kinds of structural design developed. The major components were composed of load unit, device for fixing unit, load-displacement measured unit and circuit control unit.(2)The double holes micro-shear test was operated on ferrite stainless steel for many times through changing the width of the bridge. The curves of the three parameters with the shear stress have been obtained respectively. The results show that the yield shear stress and ultimate shear stress were stable when the shear span is greater than 0.27 mm and the shear thickness was between 0.1~0.53 mm. In addition, the loading rate had little influence on shear stress.(3)Double holes micro-shear test with finite element model was established. The finite element simulation was consistent with the experimental load displacement curve. The double holes micro-shear from the mechanical behavior of plastic deformation until fracture was analyzed. The distribution of Mises and Stress triaxiality in the process of shear deformation were obtained. The occurrence of initial crack could be predicted. The influence of different shear clearance and shear thickness on the test precision was analyzed by using the finite element simulation.(4) The material yield strength and tensile strength using the method of finite element analysis were studied. The relationship between the double holes micro-shear test and the yield strength and tensile strength of the joints were established. The reliability of the relationship were verified. The yield strength error was less than 1.5%, and the error of work hardening index was less than 6.6%.(5)The micro structure of X70 pipeline steel coarse-grained heat affect zone(CGHAZ) with different toughness which under different welding heat inputs was obtained by using thermal simulation technique. The toughness of CGHAZ and the U71 rail steel base material was tested. Then, the micro-shear test device at low temperatures was designed and the double hole micro-shear test at a series of temperature was operated on thermal simulation specimen. Therefore, the shear load-displacement curves were obtained and the corresponding shear energy were calculated. Comparing with the charpy impact energy and shear energy, the results show that the temperature-shear energy and impact energy were no certain regularity at room temperature. The temperature shear energy curves were similar to the temperature impact energy curves which include the up plat form. The ductile-brittle transition temperature TDHM on the former curve is much lower.(6)The shear fracture morphology of X70 pipeline steel and U71 Mn rail steel were analyzed. With the decrease of temperature, the area of the fillet zone and shearing zone decreases. The area of the fracture zone of the ductile brittle transition temperature TDHM was about 60% and the shear zone area was about 40%.(7)The microscopic mechanism of the shear fracture process was studied and the deformation behavior of the material under the action of shear stress was predicted by using the finite element method. The stress triaxiality of material in the fracture(-0.1-1.1) was larger than the notch tip which further validate that the double holes micro-shear test is suitable for evaluating the local toughness of welded joints.(8)Welded joint of X70 pipeline steel, 430 ferrite stainless steel and U71 Mn rail steel under different welding methods were selected. The shear energy and temperature curves were obtained under different areas by micro-shear test. The research showed that the narrow gap of welding joint of X70 pipeline steel was low matching joint. The ductile brittle transition temperature of the weld center was the lowest(-80oC). 430 stainless steel base metal and welds were high matching. The TDHM of high temperature heat affected zone toughness was relatively high(-50 oC).The rail flash butt welding joint strength was lower than that of the parent material. The HAZ of TDHM induction the normalized was the highest(-12 oC). |
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