Weld penetration

In this paper, the performanees of laser-ultrasound are estimated in order to identify lacks of weld penetration. The laser-ultrasonic technique is applied to cylindrical metallic strucmres (few mm thick) in a single-sided control. The results obtained for different materials (gold-nickel alloy and tantalum) are presented by B-sean views for which the control configuration is discussed with regard to the thermal effects at the laser impact. This testing is performed for different lacks of weld penetration (up to 0.5 mm for a thickness of 2 mm) even in the presence of the weld bead, which corresponds to an actual industrial problem. 

Then, the weld depths penetration are controlled in a pulse-echo configuration because the weld bead (of width 2 mm) disturbs the detection when the pump and the probe beams are shifted of 2.2 mm. The results are presented in figure 8 (identical experimental parameters as in figure 7). The slow propagation velocities for gold-nickel alloy involve that the thermal component does not overlap the ultrasonic components, in particular for the echo due to the interaction with a lack of weld penetration. The acoustic response (V shape) is still well observed both for the slot of height 1.7 mm and for a weld depth penetration of 0.8 mm (lack of weld penetration of 1.7 mm), even with the weld bead. This is hopeful with regard to the difficulties encountered by conventional ultrasound in the case of the weld depths penetration. 

Fig. 8 B-scan views of the artificial slot of height 1.7 mm (left image) and of the lack of weld penetration 1.7 mm (right image) in a pulse-echo configuration for gold-nickel alloy.

The weld depths penetration for gold-nickel alloy and tantalum cylinders have been well controlled by an entirely contactless ultrasound method. Nevertheless, the development of signal and image processing will allow to increase the resolution of the ultrasonic images. Moreover, in order to be able to size quite well the lacks of weld penetration, the simulation of the interaction beam-defect is presently developed in our laboratory. 

Welding Control. Plugs in the inner and outer capsules for the sales packages and industrial sources are seal-welded with an argon-shielded or helium-shielded tungsten electrode DC arc. The capsule is rotated under the automatically controlled arc to produce a minimum weld penetration of 1.27 mm. Each weld bead is visually inspected by periscope or by Questar telescope, and imperfectly formed welds are rejected. Weld quality is controlled by periodic metallographic examination of dummy capsules welded in the in-cell equipment. 

Figure 1.53 Designs to avoid crevices caused by incomplete weld penetration...

The method requires the use of a reference standard sample in order to set the sensitivity level before an unknown sample is tested for discontinuities. The type of reference discontinuities used in a particular application has been specified by the American Society for Testing Materials and the American Petroleum Institute. Some reference standards are depicted in Figure 2.10. Some of the discontinuities detected are seams, laps, cracks, slivers, scabs, pits, slugs, open welds, miswelds, improperly or misaligned welds, black or gray oxide weld penetrators, pinholes, hook cracks and surface cracks. 

A distinct increase in weld penetration results from an increase in axial pressure. The rotational speed yields a similar behavior as the weld penetration increases steadily with increasing rotational speed. The effects of normal force and rotation speed are also highly nonlinear. 

For a longer test duration, axial pressure and rotational movement are maintained for a longer period of time. This allows more melting and, thus, greater weld penetration for increased test duration. This increase in weld penetration with increasing test duration appears to be quite linear in most cases. The final weld penetration increases with increasing rotational speed and increasing axial pressure. 

The welds referred to are attachment welds, and there is no question of weld penetration. 

Use adequate clearance around penetrations, sealed with flexible, fireproof boots on the inside, weld penetrations to embedments on the inside and use soft bedding, on the outside, with flexible terminations or bellows. 

Circumferential weld failures due to comer drops were noted forboth caskslzes above a dr<9 height of 15 ft. However, no attempts were made to get hlgh-qirality welds and, in some cases, weld penetration was less than 50%. 

Wang, Y Shi, Q. Tsai, H.L. (2001). Modeling of the effects of surface-active elements on flow patterns and weld penetration. Metall. Mater. Trans. B, Vol. 32B, 145-161. 

Interfacial phenomena play an important role in materials processing. However, until recently, their importance has not been widely recognised. To date few processes have been designed to use interfacial phenomena to improve process control or product quality and knowledge of interfacial phenomena has been largely limited to the explanation of occurrences, such as variable weld penetration in TIG/GTA welding. However, as this review shows, interfacial phenomena affect a wide spectrum of materials processes. 

Figure 15 Schematic diagram illustrating the Heiple-Roper theory for variable weld penetration [25].

The link between weld penetration and surface tension of the alloy has been demonstrated by Mills et al [36] who used the levitated drop method to measure the surface tension of casts with good and bad penetration, i.e. high and low (DAV) ratios, respectively. A typical example is shown in Figure 17a. It was found that ... 

Mills and Keene [3] subsequently correlated (dy/dT) with the S contents of the steels and showed that the cross-over point where (dy/dT) = 0 occurred around 40 ppm (Figure 17b). Thus good weld penetration is obtained in steels with greater than 60 ppm S and poor penetration in casts with less than 30 ppm S. 

Effect of various elements on weld penetration Elements can be classified into the following three classes ... 

Recent work on the effect of sulphur (Shirali and Mills [38]) on weld penetration... 

Poor weld penetration is often accompanied by porosity. Kou and Wang [42] proposed that the direction of the flow in the weld pool could be responsible for the presence of pores in the weld. When the flow is radially inward the weld pool motion will assist the escape of bubbles away from the solidification front (Figure 23a). In contrast, when the flow is radially outward the bubbles will tend to be swept towards the solidification front (Figure 23b). It is obvious that the presence of a solid-slag film at the rear of the pool will tend to create a stagnant region which will assist the entrapment of gas bubbles by the solidification front, thus it is not... 

Process piping is typically all welded and nitric acid passivated. Welding is done with manual TIG or semi-automatic orbital welders. An argon or helium gas purge should be used during welding to prevent the formation of oxides and achieve fiill-weld penetration. 

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