Papers by Author: Yun Peng

Abstract: Nanoindentation technique was employed to examine the nanohardness and elastic modulus of weld joints of 1000MPa Class high strength low alloy (HSLA) steel. The distribution of micro scales mechanical behavior was analyzed combined with microstructure observation by OM, SEM and TEM. It was shown that weld metal and base metal were mainly of bainite and their nanohardness is about 4.0GPa. In the heat affected zone, fine grained heat affected zone and coarse grained heat affected zone with lath martensite structure had the maximum hardness (above 4.75GPa). From inter critical region, the hardness of HAZ decreased to 2.5 GPa. However, the elastic modulus of metallic material is not much sensitive to the microstructure.

Abstract: A transformation-induced plasticity steel was welded by gas tungsten arc welding. The microstructure of fusion zone was analyzed by means of optical microscopy and scanning electron microscopy with EDS. It is found that fusion zone may be classified into two zones, the completely melted zone and the partially melted zone. The microstructure of completely melted zone consists predominantly of martensite and bainite, and that of partially melted zone consists mainly of martensite, bainite and ferrite. The formation mechanism of fusion zone microstructure is analyzed. The micro-hardness distribution of the joint was measured by microhardness tester. Test results show that the partially melted zone is softened, which is resulted from the formation of 20.6% ferrite. During the bending test, crack occurred at 125 degree bending angle. It is found that the crack originates from the partially melted zone because of deformation concentration.

Abstract: Welding of 1000 MPa high strength alloy steel is difficult because of its welding cold cracking sensitivity and the difficulty in maintaining high joint toughness. In this paper the effects of alloy elements on welding cracking tendency are analyzed, and measures to prevent cold crack are proposed. Welding wires with high strength was deposited into weld metal and welded into joint. Tensile test, micro-hardness test and Charpy impact test were used to evaluate the strength and toughness of weld metal and heat affected zone. Optical microscope, transmission electron microscope and scanning electron microscope were used to analyze the microstructure. It is shown that the weld metal mainly consists of lath martensite, lath bainite, and residual austenite which exists between the laths. The strength of weld metal increases in a small degree with increasing carbon equivalent and its toughness and ductility are not related to carbon equivalent. The toughness and ductility are much sensitive to nonmetallic inclusions. The welded joint has tensile strength of higher than 1000 MPa when welded at heat input of 11 kJ/cm and 15 kJ/cm and the mechanical properties are little influenced by the amount of heat input in this range. The whole welded joint has good comprehensive properties.

Abstract: Thermal simulation and arc welding were carried out to test the weldability of atmospheric corrosion resistant steel 07MnCuPTiNb. In thermal simulation experiment, welding thermal cycles with different peak temperature and different cooling rate were adopted and the microstructure and impact toughness were analyzed. In arc welding experiment, different heat input was used and the microstructure, impact toughness, hardness distribution, tensile strength and bending ductility of welded joints were examined. When small heat input is used for welding, the impact toughness of HAZ remains to be good. The broken position of tensile test specimen is located in base metal zone. The joint also has good ductility.

Abstract: Aluminum alloy has being widely used in modern automobile and aeronautic industry. However, the welding of aluminum alloy, especially high strength aluminum alloy，is difficult. Porosities are usually brought in the weld metal. In this paper, MIG welding using mixed gas shielding is carried out. The characteristic shapes of porosity in weld metal are described, the mechanism of porosity formation is analyzed, and the factors that influence the tendency of porosity formation are studied. Experiment results indicate that by the use of mixed shielding gas of 38%He+62%Ar, the number of porosity is reduced, the width of HAZ and softened zone is decreased, and the mechanical properties of welded joint is increased.