No abstract

Titanium/steel clad material with excellent mechanical properties and corrosion resistance has important application in the oil, gas and ocean equipment. Due to the metallurgical incompatibility of titanium and steel, the mechanical properties of weld joint would completely lose when the brittle intermetallic phase existed in the fusion welding process. Therefore, the gas tungsten arced welding (TIG) + metal inert-gas welding (MIG) + metal active-gas welding (MAG) with pure vanadium and pure copper composite filler metals for welding connection experiment in this study was carried out on Ø610mm×(14+2)mm TA1/X65 titanium/steel clad pipe fitting (titanium cladding with thickness 2 mm, X65 pipeline steel with thickness 14 mm). The microstructure, interface element distribution, main phase, microhardness distribution on cross section and mechanical properties of butt welds were researched by using OM, XRD, EDS element mapping, microhardness and tensile tests. The result showed that the use of pure vanadium and pure copper composite filler metals for welding connection could effectively avoid the production of intermetallic compounds in the process of titanium/steel composite pipe fusion welding. The deposited metal of titanium, vanadium, copper and steel had obvious zoning, the inter-diffusion melting phenomenon was not severe, which is by using solid solution phases to transit zonings of deposited metal. The microstructure of titanium and vanadium transition interface were composed of titanium-based solid solution, the microstructure of vanadium and copper transition interface was composed of vanadium-based solid solution, and the microstructure of copper and steel transition interface were composed of copper-based solid solution. The transition interface had no porosity, crack and other defects. The tensile strength of the weld was 546MPa, which is mainly contributed by the carbon steel layer. With pure vanadium and pure copper as transition filler metal for the welding connection TA1/X65 clad pipe was successfully realized by TIG+MIG+MAG welding method, and the strength index reached the desired effect.

This paper establishes a model to predict the fatigue behavior of coiled tubing subjected to variable total strain conditions. The approach based on nonlinear fatigue cumulative damage rule of effective hysteresis energy dissipation, but requires additional experimental results from fatigue tests that were performed under constant strain amplitude. Cyclic plastic strain energy is measured curve area of cyclic stress-strain curves. it is proved to be quite consistent between theoretical predictions and experimentl datas.

The Variation of the mechanical properties of coiled tubing with increasing of the bending cycles was studied based on the fatigue bending cycle machine. The results show that the tensile strength had no significance changes during cycle fatigue. The yield strength and the force value was significantly decreased after 300 cycles, and the corresponding decreased magnitude linearly increased with the increasing of bending cycles. The changing tendency of strain hardening exponent on compression surface is different from the one on extruded surface. When the cycles increase from 100 to 600, the exponent on compression surface shows increases firstly, and decreases afterwards, However, the one on extruded surface shows decreases firstly, and increases afterwards. The yield strengths of the compression surface and extruded surface show the same changing tendency as strain hardening exponent.

In the paper, the effects of thermo-mechanical control process on microstructure and properties of oil country tubular goods by electrical resistance welding are analyzed. The TMCP parameters are optimized by Gleeble-1500 thermo-mechanical simulator, and industrial trial is carried out. The microstructures in the welded joints of original ERW OCTG are mainly composed of proeutectoid ferrite, pearlite and widmanstatten. In contrast, the welded joints of ERW OCTG treated by TMCP consist only of refined ferrite and pearlite. The higher low-temperature impact toughness and the better resistance to the grooving corrosion of the welded joints of ERW OCTG treated by TMCP could be attributed to removing welding residual stresses, homogenizing microstructure, eliminating porosity and refining grain. TMCP facilitates the performance optimization of welded joints so that the quality of the finished product can be improved.