Effect of nickel and molybdenum additions on weld metal toughness in a submerged arc welded HSLA line-pipe steel

Abstract:

The effects of alloy additions of nickel (Ni), molybdenum (Mo), and Ni and Mo together on the impact toughness of an API HSLA-70 steel by submerged arc welding in the laboratory were investigated and micro-structural factors which affect the impact toughness were discussed. Ni additions resulted in a low impact toughness and an increased fracture appearance transition temperature (FATT) in weld metal (WM). The above influences of Ni should be attributed to the formation of acicular ferrite (AF) suppressed by increasing the Ni content. Conversely, the combined presence of Ni and Mo in the WM decreased the volume fractions of grain-boundary ferrite (GBF) and promoted formation of high toughness of AF. The increase of Mo content created an acicular ferrite-predominant weld metal microstructure with impressively improved toughness. Mo addition of 0.881 wt.% in the WM gave the optimal impact toughness at .45 .C with a microstructure of 77% AF and 20% granular bainite (GB).

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An upgraded brazing technique to manufacture ceramic-metal joints for UHV applications

Abstract:

The properties of an upgraded active vacuum brazing procedure, explicitly developed to manufacture ceramic-metal joints, are presented and some of the most significant applications described. The essential feature of the technique is the use of AgCuTi as brazing material, which permits the joining of alumina (Al2O3) very reliably with various metals, including stainless steel. The careful control of the brazing cycle and the quality of the alumina, in terms of purity and density, play an essential role in determining the final performances of the components. With the technology presented, several vacuum components have been developed and manufactured, among which the most interesting are electrical feedthroughs compatible with commercially available signal connectors and capable of operating at temperatures of 5001C or higher. In particular, a completely new model of a signal feedthrough, to be used with the Minisnap-Plug of Odu and presenting an alumina–stainless steel joint, has been developed and thoroughly tested in the laboratory. Its performances have been verified at more than 7001C, with temperature variations of up to 101/min. The latest version of these feedthroughs has shown good vacuum tightness and reliability up to 6501C and therefore presents better characteristics than most of the feedthroughs commercially available. In addition to being tested on the bench, some feedthroughs have been used in the nuclear fusion experiment Reversed Field experiment (RFX), without showing any sign of degradation or ageing.

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Modeling the effects of ultrasonic vibration on friction stir welding

Abstract:

A mathematical model is developed to analyze the effects of superimposed ultrasonic vibration on heat generation, temperature distribution and material flow in friction stir welding process. Ultrasonic softening plays a dominant role in ultrasonic vibration enhanced friction stir welding process, while the thermal effect of ultrasonic vibration is insignificant. Superimposing ultrasonic vibration in friction stir welding process can produce an enhanced plastic material flow, increase the welding speed and welding efficiency, and also improve the weld quality by enhancing the plastic material flow near the tool. The model is validated by a comparison of the calculated thermal cycles and thermo-mechanically affected zone boundaries with the experimentally measured ones.

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Single-sided laser beam welding of a dissimilar AA2024–AA7050 T-joint

Abstract:

In the aircraft industry double-sided laser beam welding of skin–stringer joints is an approved method for producing defect-free welds. But due to limited accessibility – as for the welding of skin–clip joints – the applicability of this method is limited. Therefore single-sided laser beam welding of T-joints becomes necessary. This also implies a reduction of the manufacturing effort. However, the main obstacle for the use of single-sided welding of T-joints is the occurrence of weld defects. An additional complexity represents the combination of dissimilar and hard-to-weld aluminium alloys – like Al–Cu and Al–Zn alloys. These alloys offer a high strength-to-density ratio, but are also associated with distinct weldability problems especially for fusion welding techniques like laser beam welding. The present study demonstrates how to overcome the weldability problems during single-sided laser beam welding of a dissimilar T-joint made of AA2024 and AA7050. For this purpose a high-power fibre laser with a large beam diameter is used. Important welding parameters are identified and adjusted for achieving defect-free welds. The obtained joints are compared to double-sided welded joints made of typical aircraft aluminium alloys. In this regard single-sided welded joints showed the expected differing weld seam appearance, but comparable mechanical properties.

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Compatibility of martensitic/austenitic steel welds with liquid lead bismuth eutectic environment

Abstract:

The high-chromium ferritic/martensitic steel T91 and the austenitic stainless steel 316L are to be used in contact with liquid lead–bismuth eutectic (LBE), under high irradiation doses. Both tungsten inert gas (TIG) and electron beam (EB) T91/316L welds have been examined by means of metallography, scanning electron microscopy (SEM-EDX), Vickers hardness measurements and tensile testing both in inert gas and in LBE. Although the T91/316L TIG weld has very good mechanical properties when tested in air, its properties decline sharply when tested in LBE. This degradation in mechanical properties is attributed to the liquid metal embrittlement of the 309 buttering used in TIG welding of T91/316L welds. In contrast to mixed T91/316L TIG welding, the mixed T91/316L EB weld was performed without buttering. The mechanical behaviour of the T91/316L EB weld was very good in air after post weld heat treatment but deteriorated when tested in LBE.

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Selection of parameters of pulsed current gas metal arc welding

Abstract:

Pulsed welding is a controlled method of spray transfer, in which the arc current is maintained at a value high enough to permit spray transfer and for long enough to initiate detachment of a molten droplet. Once the droplet is transferred the current is reduced to a relatively low value to maintain the arc. These periods of low current allow the average arc current to be reduced into the range suitable for positional welding, while periodic injection of high current pulses allows metal to be transferred in the spray mode. Parameters of these current pulses, such as Ip Ib, Tp and Tb have a distinct effect on the characteristics viz., the stability of the arc, weld quality, bead appearance and weld bead geometry. Improper selection of these pulse parameters may cause weld defects including irregular bead surface, lack of fusion, undercuts, burn-backs and stubbing-in. Therefore, it is important to select a proper combination of parameters of the pulsed current for welding, which will ensure that the process gives proper results in all the above aspects. However, arriving at such a combination of parameters without a rational base would be only a matter of chance with a fairly low probability for achieving desirable weld properties, since the complexity and interdependence of pulse parameters involved in this process. These difficulties of setting-up the welding conditions correctly has been one of the main reasons for the lack of popularity of pulsed GMA welding in industries. Hence, a detailed study is essential to arrive at a method of predicting the conditions that will give a good weld and this paper reviews various aspects of the pulsed GMA welding, the effects of pulse parameters and different methodologies adopted for selecting these parameters to obtain better quality welds.

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Hybrid welding with fiber laser and CO2 gas shielded arc

Abstract:

Argon-rich shielding gas was replaced by 100% CO₂ gas for cost reduction in fiber laser-GMA hybrid welding of double-side welded T-joints. The welding process using 100% CO₂ gas was characterized by a large number of spatters, while the penetration depth of a weld was increased and porosity was reduced. With the objective of obtaining a buried-arc transfer for the reduction of spatter formation, the welding parameters were optimized by observation with a high-speed video camera. Reduced arc voltage, arc leading arrangement and shortened wire extension were necessary to achieve a buried-arc transfer. A significant reduction in spatter generation could only be obtained by the procedure that the relative distances between the two heat sources in the X and Y directions were controlled to produce a proper profile of the arc cavity that could trap any spatters generated. A regulating action of a keyhole was observed to remove the disturbances in the melt flows caused by the arc short-circuiting, and high quality joints with good appearances and very few spatters could be produced.

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Performance of Welded High-Strength Low-Alloy Steels in Sour Environments

Abstract:

Sulfide stress cracking (SSC) susceptibility of gas metal arc welded API 5L X70 and X80 line pipe steel was tested using standard and modified NACE TM0177 (Method A) testing. Two applied stresses and three concentrations of dissolved hydrogen sulfide (H2S) were used. The effect of peak weld hardness was examined by using three weld conditions. Several welds meeting the HRC 22 requirement failed by SSC. Welds containing hardness exceeding 248 HV were resistant to SSC at low H2S concentrations. The center line segregation region (CSR) observed in the X70 pipe steel played an important role in the SSC and hydrogen-induced cracking (HIC) susceptibility of the welds. The X80 material was more susceptible at lower hardness values due to localized softening during welding, and subsequent plastic deformation upon loading. An extrapolation to in-service weldments indicated that the NACE MR0175 requirements may be conservative for high-strength low-alloy steel welds not in direct contact with the sour environment.

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Analysis of the weldability of ductile iron

Abstract:

In this report the weldability of ductile iron is established as a function of the chemical composition of different consumables. Manual shield metal arc welding (SMAW) and Oxyacetylene Welding (OAW) were used, with and without preheat, on casting samples of 300×90×5 mm without groove using two types of electrodes: Fe–Cr–Ni, Ni-98% and two types of grey iron rods. The suitability of each welding technique and consumable was evaluated through the study of the mechanical characteristics of the joints. These characteristics were obtained from tensile tests and were correlated with the microstructure of the joints. Finally, a comparison of the results is presented, including the influence of preheat for each technique and consumable.

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Influences of welding parameters on the quality and creep properties of friction stir welded polyethylene plates

Abstract:

This work aims to investigate the effects of main friction stir welding (FSW) parameters on the quality and creep properties of polyethylene plate joints. Welding variables for analysis were rotational speed, traverse speed, geometry and tilt angle of tool. Indentation creep tests were carried out on weld joints, using a constant load of 1 kgr for 0.5, 2, 5, 15, 30, 60, 90 and 120 min. Also, classical power law, Juhasz, Ashby-Sargent and Mulhearn-Tabor methods were applied for calculating stress exponent values, as a criterion for creep evaluation. The results showed that under special conditions creep resistance of welds are better than that of the base material. Meanwhile, it was found that Mulhearn-Tabor method is the most conservative estimation for assessing the indentation creep behavior of specimens.

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