Lianyong Xu

Investigation on Microstructure and Impact Toughness of Different Zones in Duplex Stainless Steel Welding Joint

This paper investigated on microstructure and impact toughness of different zones in duplex stainless steel welding joint. High-temperature heat-affected zone (HTHAZ) contained coarse ferrite grains and secondary precipitates such as secondary austenite, Cr2N, and sigma. Intergranular secondary austenite was prone to precipitation in low-temperature heat-affected zone (LTHAZ). Both in weld metal (WM) and in HTHAZ, the austenite consisted of different primary and secondary austenite. The ferrite grains in base metal (BM) presented typical rolling texture, while the austenite grains showed random orientation. Both in the HTHAZ and in the LTHAZ, the ferrite grains maintained same texture as the ferrite in the BM. The secondary austenite had higher Ni but lower Cr and Mo than the primary austenite. Furthermore, the WM exhibited the highest toughness because of sufficient ductile austenite and unapparent ferrite texture. The HTHAZ had the lowest toughness because of insufficient austenite formation in addition to brittle sigma and Cr2N precipitation. The LTHAZ toughness was higher than the BM due to secondary austenite precipitation. In addition, the WM fracture was dominated by the dimple, while the cleavage was main fracture mode of the HTHAZ. Both BM and LTHAZ exhibited a mixed fracture mode of the dimple and quasi-cleavage.

Characterization of creep crack-tip constraint levels for pressurized pipelines with axial surface cracks

Abstract Through extensive 3D finite element analyzes, the creep crack-tip constraint levels were characterized using a load-independent creep constraint parameter Q* for pipelines with axial surface cracks of different geometrical sizes, which involved in various crack depths and crack shapes. The Q* distribution along the crack front for axial internal surface cracks and axial external surface cracks exhibited the same distribution tendency. However, the constraint level of internal surface cracks was higher than external surface cracks. In addition, the constraint level was improved as the crack depth became deep; in contrast, the constraint level showed a reduction tendency as the crack shape ratio a/c increased from 0.2 to 1.0. The highest constraint levels for the axial surfaces cracks in pipelines approached to that of single-edge notched tension specimen. Finally, two empirical equations for estimating the constraint level were established as a function of crack depth ratio and crack shape ratio for axial surface cracks in pipelines.

NANOMECHANICAL PROPERTIES OF A Sn-Ag-Cu SOLDER REINFORCED WITH Ni-COATED CARBON NANOTUBES

In the present study, 0.05 wt.% of Ni-coated multi-walled carbon nanotubes (Ni-CNTs) were successfully incorporated into the 95.8Sn–3.5Ag–0.7Cu solder using the powder metallurgy technique, to synthesize a new lead-free composite solder. Its mechanical property (in terms of hardness) was investigated at room temperature using the nanoindentation method. The results revealed that the nanoindentation hardness increased by 14.3% with the incorporation of 0.05 wt.% of Ni-coated CNTs. This observation is in good agreement with the microhardness test results. Moreover, the addition of Ni-CNTs improved the creep resistance of the composite solder. The test results established that nanotechnology coupled with composite technology in electronics solders can result in the enhancement of mechanical properties. These advanced interconnect materials will thus benefit the microelectronics assembly and packaging industry.

Numerical analysis of the creep crack constraint effects and the creep crack initiation for pressurized pipelines with circumferential surface cracks

Abstract The creep crack constraint effects using a load-independent creep constraint parameter Q* and the creep crack initiation (CCI) times were characterized by 3D finite element method for pipelines with circumferential surface cracks of different geometrical sizes. The results revealed that the distribution regulation of Q* along the crack front for circumferential internal surface cracks and external surface cracks was similar. The maximum constraint level occurred near the deepest crack front part for cracks with smaller a/c, while it occurred near the free surface for cracks with larger a/c. The constraint values at the same position (2Φ/π) increased with the increasing of the crack depth when a/c kept constant. The circumferential internal surface cracks of pipelines were proved more dangerous than the external surface cracks with the same geometrical size. Furthermore, the CCI times were decided by the peak values of constraint, or the CCI firstly occurred at the position where the constraint level was maximum. In addition, the empirical relationships between the CCI times and crack sizes were fitted, which was also verified effectively.

Numerical Modeling of Weld Joint Corrosion

A numerical model is presented in this work that predicts the corrosion rate of weld joint. The model is able to track moving boundary of the corroding constituent of weld joint. The corrosion rates obtained from the model are compared with those estimated from mixed potential theory and two experimental techniques, namely immersion test and constant potential polarization test. The corrosion rate predicted using the model is within 10% of the estimate from the mixed potential theory, within 20% of that got from the immersion experiment and within 10% of that got from the constant potential polarization experiment for weld joint.

Finite Element Analysis of Calibration Coefficients for Residual Stress Measurements by the Ring Core Procedure

Abstract Finite element analysis (FEA) and experiments have been conducted to analyze the calibration coefficient for measuring residual stress in the ring core method. In this publication, firstly, the calibration coefficients were determined using FEA. Then, factors which affect the calibration coefficients were investigated, such as the tensile load applied to the components, the geometric sizes of the annular grove and the location of the strain gage. The results of FEA were consistent with the experimental results. The most appropriate geometric sizes of the annular groove and location of the strain gage were obtained. This study also defines the distortion energy parameter S to calculate and modify the calibration coefficients. Hence, it is reasonable to apply FEA to calibration coefficients in the ring core method.

Effect of Ni-coated carbon nanotubes on the microstructure and properties of a Sn-Ag-Cu solder

In this work, varying weight percentages of Ni-coated CNTs (Ni-CNTs) were incorporated into the Sn-Ag-Cu matrix to develop composite solders. The samples were extruded and characterized in terms of their thermal, wettability, microstructural, tensile and nano-mechanical properties. Our characterization results established that the composite technology coupled with nanotechnology in electronic solders can lead to 8% improvement in mechanical performance (in terms of 0.2% yield strength), 12% increase in the ultimate tensile strength and better creep behavior. With the addition of Ni-CNTs, there is no compromise on the melting point of the solder alloy and in fact a better wettability of the nanocomposite solders was observed. Thus, such a Ni-coated CNT filler in Sn-Ag-Cu solder provides a promising interconnect materials for microelectronics assembly and packaging industry.

Effects of charging conditions on the hydrogen related mechanical property degradation of a 3 Cr low alloyed steel

Abstract In this work, the susceptibility of a 3 Cr low alloyed steel to hydrogen degradation of its mechanical properties was investigated to simulate the outside pipeline environment condition by a series of electrochemical hydrogen permeation measurements, surface characterization and tensile tests. Results show that the amount of hydrogen charging into the 3 Cr low alloyed steel specimen is affected by the external charging conditions, such as the hydrogen charging time and charging current density. The diffusivity of hydrogen at room temperature in 3 Cr low alloyed steel has been determined as 2.87 × 10−8 cm2 × s−1. The reduction of ductility with charging time becomes obvious in the tensile experiments.

Corrosion behavior of pipeline steel welds in simulated produced water with different CO2 partial pressures under high temperature

Abstract In this work, the CO2 corrosion behavior of pipeline steel welds was systematically investigated. This was done by weight loss experiments in which welded samples were immersed in simulated produced water with different CO2 partial pressures. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), grazing incidence X-ray diffraction (GIXRD) and transmission electron microscopy (TEM) were used to effectively characterize the morphology, composition and phase of the corrosion products. The results showed that the corrosion rate of welded joints decreased as the immersion time increased. The corrosion rate of welded joints and the type of corrosion products formed also depend on the CO2 partial pressure. The layers of corrosion products had inhomogeneous distributions of iron, carbon, oxygen and calcium, with a higher concentration of iron in the internal corrosion film than that in the external corrosion film.

Effect of Ni-coated carbon nanotubes on interfacial intermetallic layer growth

In the present study, Ni-coated carbon nanotubes (Ni-CNTs) were incorporated into the Sn-Ag-Cu matrix, to form a composite solder. The interfacial intermetallic compound layer thickness formed on electroless nickel immersion gold (ENIG) metallized Cu substrate was determined under the as-soldered condition. It was observed that the addition of 0.01 wt.% Ni-CNTs into the Sn-Ag-Cu solder matrix, affected the formation of intermetallic compounds during the soldering reaction. For the reaction between the composite solder and the ENIG/Cu substrate, (Cu<inf>1−x</inf>Ni<inf>x</inf>)<inf>6</inf>Sn<inf>5</inf> and (Cu<inf>1−y</inf>Ni<inf>y</inf>)<inf>3</inf>Sn<inf>4</inf> were formed. The test results revealed that the thickness of interfacial IMC decreased from 2.30 μm to 1.84 μm with the addition of Ni-CNTs. Shear tests were also conducted on the as-soldered solder joints. The shear test results revealed that the composite solder joint exhibited a ~ 15% increase in yield strength and a ~ 17% increase in ultimate shear strength, as compared to its monolithic counterpart.