Materials Science Forum Vols. 638-642

Abstract: 9%Ni steel similar composition weld wires are applied for constructions of liquefied natural gas storage tanks by gas tungsten arc welding and the weld metals show high toughness and strength level. In this study, the microstructures of 9%Ni steel similar composition weld metals have been investigated. It was found that fine microstructure was formed at the reheated zone and contributed to the high toughness level. The nucleation rate of blocks and packets from prior-austenite grain boundaries was larger at the reheated zone than the as-welded zone and resulted in the fine microstructure of the reheated zone. Our results showed that the lowering of Ni concentration at and near the prior austenite grain boundaries by the reverse transformation caused the larger nucleation rate at the reheated zone.

Abstract: Baosteel cooperated with the China’s welding consumable company and developed an electro-gas welding wire BH610-EG, which is a flux-cored welding wire and can match the high strength steel B610E. In this paper, the electro-gas welding technological tests are carried out by B610E with the thickness of 21 mm and 40 mm and BH610-EG wire with the diameter of 1.6 mm. And values of the heat input of electro-gas welding experiments are in the range of 80~100kJ/cm. The main conclusions can be obtained as follows: (1) In condition of high heat input, the electro-gas welding process using BH610-EG wire is very stable, the weld surface is good. (2) Various mechanical properties of the welded joints can meet the design requirements of the storage tanks. (3) The new developed electro-gas flux-cored welding wire BH610-EG can match with B610E steel and meet the requirement of high heat input of the crude oil storage tank.

Abstract: The ferrite grain refinement is a powerful mechanism to improve mechanical properties of low carbon steels providing steels with high strength and toughness at low temperatures and good weldability characteristics. The grain size refining is the only mechanism capable of to increase both mechanical strength and toughness. By refining the grain size of low carbon steel from 5 μm to 1 μm, its yield strength can be theoretically increased from 450 MPa to 650 MPa. In this way refining of ferritic grain is a very attractive processing route. This work aimed to investigate the characteristics of the heat affected zone of a microalloyed low carbon-manganese (0.11% C, 1.41% Mn, 0.028%Nb, and 0.012%Ti) steel with ultra-fine ferrite grain structure produced through quenching, warm rolling, followed by sub and intercritical annealing in laboratory. Four intercritical annealing treatments were performed after the same warm rolling processing to obtain different grain sizes with residual work hardening of the base metals. Specimens were TIG welded with 4 different levels of heat input. Cooling conditions during tests were recorded and used to evaluated the microstructure of the heat affected zones and their hardness. Cooling times between 800 and 500°C from 0.6 to 17 s were obtained. Martensite was observed in the heat affected zones for low-heat-input welding conditions. No softened zone was found in the heat affected zone in any of the performed tests.

Abstract: The accurate prediction of welding distortion is an important requirement for the industry in order to allow the definition of robust process parameters without the need to perform expensive experiments. Many models have been developed in the past decades in order to improve prediction. Assumptions are made to make the models tractable; however, the consequences are rarely discussed. One example for such an assumption is the strain hardening model, which is often a choice between either kinematic or isotropic hardening. This paper presents the results of tensile tests for DP600 performed from room temperature up to one thousand degrees and for different strain-rates. In order to employ a mixed isotropic-kinematic hardening model, the fractions of each hardening contribution have been determined by means of bend testing. The welding distortion of a DP600 overlap joint has been simulated and it is shown that such a mixed-hardening model results in more accurate and reliable results.

Abstract: Thanks to their oxide layer, aluminium alloys are remarkable for their ability to resist corrosion. However, in welding, this protective layer acts as a barrier which must be broken in order to succeed in the thermomechanical joining of aluminium. The chosen alloy (6082-T6 or AlSi1MgMn) has been subjected to various deformation path. The first of them consists in the channel-die (plane strain) compression of two cuboids, one above the other. Considering the configuration of the test, the surface size between the two samples rises, so that the fragmentation of the oxide layer creates welding bonds. However, the friction effects in the channel lead to a heterogeneous deformation, so that the contact surface undergoes different behaviors: a microscopic study then shows that the welds appear in areas with significant shear. Channel-die and uniaxial compressions of beveled samples confirm that more significantly than the global deformation, the shear strain is the most active phenomenon for achieving an effective thermomechanical joining. Another approach is the cumulative deformation as a result of a cyclic load: a tube is cut through its section and undergoes both a compression and cyclic torsion load. The contact surface between the two semi-tubes is under a shear behavior and the combination between plastic deformation and local heating leads to a fragmentation of the oxide layer: all this factors allow the thermomechanical joining of aluminium alloys.

Abstract: Time-resolved X-Ray Diffraction (TRXRD) experments were carried out to identify the phase transformation during welding in-situ. For the martensitic steel weld with different chemical compositions, the solidification behavior was directly analyzed in the time-resolution of 0.01 seconds. The halo pattern from the weld pool gives basis to observe the phase transformation during solidification process of weld. Furthermore, the latest development of TRXRD system was outlined. The importance of detector area was discussed and brand-new TRXRD system in real and reciprocal lattice space was presented.

Abstract: Friction stir processing (FSP) was applied to a cast aluminium alloy, A356-T6, and a cast magnesium alloy, AZ91-T5, and fully reversed axial fatigue tests have been performed using FSPed specimens. It was indicated that FCP exerted different influence on fatigue behaviour depending on alloy system. In A356-T6, the fatigue strengths of the FSPed specimens were lower than those of the as-cast ones in the finite life region, but the fatigue limit was significantly increased by FSP. The enhanced crack initiation resistance due to the elimination of casting defects by FCP resulted in the improvement of fatigue limit of the FSPed specimens, while the matrix softening due to the dissolution of precipitates by the heat input during FSP caused faster crack growth rates in the FSPed specimen, leading to the inferior fatigue strength of the FSPed specimens to the as-cast one in the finite life region. In AZ91-T5, both the fatigue strengths in the finite life region and fatigue limit were improved by FSP, because the hardness was increased and both the crack initiation and crack growth resistances were enhanced.

Abstract: Authors ultrasonically welded A6061 aluminum alloy sheet using two types of weld tips with the different contact face geometry, and investigated the effect of the weld tip geometry on the performance and the interface structure of welds. One type of tip has a cylindrical contact face without knurl, which is called C-tip in this study. The other type of tip has flat contact face with knurl, which is called K-tip in this study. The strength of the joints welded using C-tip was higher than that welded using K-tip and the C-tip could stably produce the higher strength joint.

Abstract: In the present study, the characteristics of Nd:YAG laser welded joints of 600 MPa DP steel (HDT580X), 2.4 mm in thickness, in respect of hardness, microstructures and mechanical properties were investigated. The test joints have been welded under a shielding gas on the stand for robotic Nd:YAG laser welding at the beam power of 1.5 kW and 2.0 kW. Three combinations of welding parameters were used: 2.0 kW - 2.1 m/min, 2.0 kW - 1.5 m/min and 1.5 kW - 1.2 m/min. Detailed examinations were performed on the joint welded at the highest speed. The microstructure was examined by the optical-, scanning- and transmission electron microscope. The heat affected zone (HAZ) was composed of ferrite, bainite and lath martensite, the weld contained lath martensite. The maximum hardness in the HAZ did not exceed 343 HV. The tensile strength of the welded joint was at the same level as that of the base material. The results of fatigue tests and residual stress measurement of laser welded DP steel joints are also presented. The fatigue strength of the welded joint is lower than that of the base material. The fatigue class FAT was determined, which is equal to 284 MPa - for the base material and 150 MPa – for the welded joint. By means of the modified hole drilling method the following residual stresses were measured: σmax = 573 MPa and σmin= -126 MPa.

Abstract: Cobalt based alloy with the addition of 5 and 10 wt% boron carbide have been deposited by PTA process on stainless substrates effectively. The dry sliding wear performances of the coated alloys have been studied at room as well as at elevated temperatures. Addition of boron carbide to satellite 6 resulted in refinement of grains and formation of complex carbides and borides, which has lead to increase in hardness and wear resistance of the hardfaced layers. Superior wear resistance has been found to be at the composition of Stellite 6 with 5 wt% addition of boron carbide.