Jijin Xu

Temperature distribution and residual stresses during multipass narrow gap welding of thick plates

Abstract In a multipass welding operation, the temperature distribution and residual stresses developed in the material change with each weld pass, and are therefore more complicated than those of single-pass welding. In the present study, two methods – testing and finite element analysis (FEA) – were used to determine the temperature distribution in thick plate with narrow gap during multipass submerged arc welding (SAW). The residual stresses on the top surface, bottom surface and along the thickness direction at some distance away from the weld line are presented. The results are compared with those calculated by FEA. These are used to validate the finite element model which is then used to appraise the weld performance.

Effects of additive elements on structural and electronic properties of Sn-based intermetallics by first principles

First-principles calculations are performed to study the structural and electronic properties of Sn-based intermetallic compounds (IMCs) with substitution elements. Cu atoms preferentially occupy the 2a sites in (Ni1 − x Cu x )3Sn4 structures. With the increase of Cu content, the heats of formation (H f) of (Ni1 − x Cu x )3Sn4 structures become higher. For the Ni3Sn4-based ternary structures with different substitution elements, a simple rule for the H f is obtained. Comparing with the substitution elements in group IB (Cu, Ag, Au), those from group VIIIB (Co, Ni, Pd, Pt) are more helpful for stabilizing the ternary structures. For substitution elements in the same group, the H f of these ternary structures becomes lower with the increase of atomic number. This rule is verified by calculating the H f of Cu6Sn5-based and AuSn4-based ternary structures.

Low stress welding technology without post-weld heat treatment

Abstract In order to assure the tightness and the service life of a full welded body ball valve, the temperature of grommet cannot exceed 150°C. Therefore, post-weld heat treatment cannot be used to relieve the welding residual stresses. In this study, a low stress welding technology was studied. The type of welding joint was optimised through the finite element method and vibratory welding was employed to decrease the residual stress. At the same time, the fracture toughness behaviour of weld zone and heat affected zone in terms of crack tip opening displacement was investigated to assess the safety of valve without PWHT in the active service. Finally, the results are discussed and analysed.

Reduction of Porosities in Pulse-MAG Welding of Galvanized Steel Sheets for a Zero-Gap Lap Joint Configuration

Pulse-MAG welding was conducted to joint lap fillet welds of galvanized steel sheets with zero-gap. During the welding process, metal transfer and molten pool were observed. A stable opening in the molten pool could become an escape path for zinc vapor. High current and welding travel speed could expand the opening. To obtain a stable opening, arc voltage needs to be controlled in a reasonable range. Besides, the exorbitant voltage and travel speed caused BCM humps and GRM humps respectively. With high current, moderate voltage and suitable travel speed, the stable opening of molten pool was created to promote the degassing of zinc vapor so that the amount and size of porosity in the weld beam were reduced.

A Prefabricated Strain-Slice-Style Speckle Pattern for Digital Image Correlation Method Under Air-Oxidation Condition at High Temperature up to 1000 °C

A prefabricated strain-slice-style speckle pattern applied to high-temperature digital image correlation (DIC) method is proposed and studied in this paper. By virtue of spot welding process, this proposed speckle pattern can be prepared directly and easily on specimen surface. A heating experiment was performed to investigate the high-temperature performance of this proposed speckle pattern under air-oxidation condition. Besides, the ability of this proposed speckle for high-temperature DIC method was verified in a thermal expansion experiment, during which thermal deformation and coefficient of thermal expansion (CTE) values were measured and subsequently compared with the reference data. Results show the effectiveness of this prefabricated strain-slice-style speckle pattern for full-field in-plane thermal deformation measurement with high accuracy using DIC method under air-oxidation condition at high temperature up to 1000 °C.

Grain Boundary Feature and Its Effect on Mechanical Property of Ni 690 Alloy Layer Produced by GTAW

Ni690 alloy surfacing layers were fabricated by gas tungsten arc welding (GTAW) with two different heat inputs, namely, large heat input (LHI) and small heat input (SHI). The high temperature performance of the surfacing layer was evaluated by employing Gleeble 3500 thermal/mechanical simulator. It is found that the ultimate tensile strength (UTS) of the LHI samples was higher than that of the SHI samples after reheat thermal cycles, regardless of the reheating temperature. The EBSD result shows that the proportion of high angle grain boundaries (GBs, >15°) in the LHI sample was obviously higher than that in the SHI sample. And more M23C6 particles were found to precipitate at the high angle GBs. The relations among UTS, GB angle distribution and M23C6 precipitations were analyzed. Moreover, the fracture modes were characterized by optical microscope (OM) and scanning electron microscope (SEM). The fracture mode was ductile fracture with deep dimples at 700 °C. While it changed to brittle intergranular fracture at 900 °C. As the temperature was enhanced to 1050 °C, the fracture returned to transgranular mode, with shallow dimples.

Study on Properties of LBW Joint of AISI 304 Pipes Used in Nuclear Power Plant

In this work, a girth weld of AISI 304 stainless steel pipes with a dimension of Φ89 mm × 250 mm × 14.5 mm was produced by laser beam welding. The microstructure was observed by optical microscopy. The microhardness test was carried out to study the mechanical properties of the welded joint. A slow strain rate test was performed to investigate the susceptibility to stress corrosion cracking (SCC) of the welded joint in the simulated pressurized water reactor environment. The results show that the values of microhardness in the weld and HAZ are close to those in the parent material. According to ultimate tensile strength-loss rate, elongation-loss rate and fractograph analysis, the welded joint has low susceptibility to SCC in the simulated pressurized water reactor environment.