Microstructures and Plane Energy Spectra of X80 Pipeline Steel Welded Joints by Submerged Arc Automatic Welding

X80 pipeline steel was welded with submerged arc automatic welding, the microstructures, cavity sizes, fusion depths and plane scanning of chemical elements in the welded zone, fusion zone, heat affected zone and base steel were observed with OM（optical microscope） and SEM（scanning electron microscope）, respectively. The experimental results show that there is main acicular ferrite in the base steel and welded zone, the microscopic structure of fusion zone is a blocked bainite, and the heat affected zone is composed of multilateral ferrite and pearlite. M-A unit of the welded zone is the main factor to strengthen the welded zone, composed of acicular ferrites. The percentage of cavities in the welded joint is less than that in the base steel, which is beneficial to increasing its mechanical performance and corrosion resistance. The fusion depth in the fusion zone and welded zone is 101.13 μm and 115.85 μm, respectively, and the distribution of chemical elements in the welded zone is uniform, no enrichment phenomena.

Effects of Hot Bending Parameters on Microstructure and Mechanical Properties of Weld Metal for X80 Hot Bends

Mechanical properties of weld metal are the key factors affecting the quality of heavy-wall X80 hot induction bends. The effects of bending parameters on the mechanical properties of weld metal for hot bends were investigated by simulation conducted on a Gleeble 3500 thermal simulator. Continuous cooling transformation （CCT） dia- grams of the weld metal were also constructed. The influences of hot bending parameters （such as reheating temperature, cooling rate, and tempering temperature） on the microstructure and mechanical properties of weld metal were also analyzed. Results show that the strength of all weld metal specimens is higher than the value indicated in the technical specification and increases with the increase of reheating temperature, cooling rate, and tempering tempera ture. The impact toughness is apparently related to the variation of reheating temperature, cooling rate, and tempering temperature.

Microstructure and Mechanical Properties of Intercritical Heat-affected Zone of X80 Pipeline Steel in Simulated In-Service Welding

The intercritical heat-affected zone（ICHAZ） of X80 pipeline steel was simulated by using the Gleeble-3500thermal/mechanical simulator according to the thermal cycle of in-service welding.The microstructures of ICHAZ with different cooling rates were examined,and the hardness,the toughness and corresponding fractography were investigated.Results show that untransformed bainite and ferrite as well as retransformed fine bainite and martensite–austenite（M–A）constituents constitute the microstructure of ICHAZ.The two different morphologies of M–A constituents are stringer and block.Second phase particles which mainly composed of Ti,Nb,C,Fe and Cu coarsened in ICHAZ.Compared with normal welding condition,the toughness of ICHAZ is poor when the cooling time is short under in-service welding condition because of the large area fraction and size of M–A constituents that connect into chains and distribute at the grain boundaries.The Vickers hardness of ICHAZ that decreases with the increase in the cooling time is independent with the area fraction of M–A constituents.

Simulation study on heat-affected zone of high-strain X80 pipeline steel

The microstructure evolution and impact-toughness variation of heat-affected zone（HAZ）in X80 highstrain pipeline steel were investigated via a welding thermal-simulation technique,Charpy impact tests,and scanning electron microscopy observations under different welding heat inputs and peak temperatures.The results indicate that when heat input was between 17 and 25kJ·cm^（-1）,the coarse-grained heat-affected zone showed improved impact toughness.When the heat input was increased further,the martensite-austenite（M-A）islands transformed from fine lath into a massive block.Therefore,impact toughness was substantially reduced.When the heat input was 20kJ·cm^（-1） and the peak temperature of the first thermal cycle was between 900 and 1300°C,a higher impact toughness was obtained.When heat input was 20kJ·cm^（-1） and the peak temperature of the first thermal cycle was 1300°C,the impact toughness value at the second peak temperature of 900°C was higher than that at the second peak temperature of 800°C because of grain refining and uniformly dispersed M-A constituents in the matrix of bainite.

Effect of Nb on Mechanical Properties of HAZ for High-Nb X80 Pipeline Steels

The mechanical properties of heat affected zone （HAZ） of two commercial high-Nb X80 grade pipeline steels with different alloy elements were investigated using thermal simulation performed on a Gleeble-3500 thermal simulator. The results showed that the high-Nb steels have excellent weldability. Ernbrittlement regions appear in coarse grain heat affected zone （CGHAZ） and intercritically heat affected zone （ICHAZ） ~ Softening region appears in fine-grain heat affected zone （FGHAZ）, and the strength here was even lower than 555 MPa as required in the standard. Meanwhile, with the increase of heat input, the strength and the toughness of HAZ of steel with high Nb, C and lower alloy decrease notably. Therefore, take into account the welding procedure during manufacture of weld pipe, suitable amount of alloy elements, such as Cr, Ni, Cu, Mo and so on, is necessary for high Nb X80 heavy- thick steel plate.