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Addition of a small amount of denitrification elements, such as aluminium, is effective to prevent the porosity without forming the solidification crack ( Tsukamoto et al., 2004). This local expansion of the keyhole forms the abnormal cross section, leading to occurrence of the solidification crack. During welding, the inert gas enters the keyhole from the back surface and expands in the keyhole as shown in the x-ray transmission image of Fig. 2.25(b). This abnormal weld cross section is caused by a keyhole perturbation. However, the solidification crack is easily formed accompanied by the locally expanded weld cross section, which is very susceptible to the solidification crack, as shown in Fig. 2.25(a) ( Tsukamoto et al, 2003). The inert gas back shielding can effectively prevent this type of porosity. The nitrogen concentration in the molten pool then increases near the bottom, and if it exceeds the solubility of diatomic nitrogen, bubbles are formed in the molten metal. The plasma contains a lot of monatomic nitrogen, which can much more easily dissolve in the molten steel than diatomic nitrogen ( Dong et al, 2004).
#Webook nist gov heat solidication water full
During full penetration laser welding, the excessive laser energy, which penetrates through the steel plate, forms air plasma on the back surface as shown in Fig. 2.24(b). This is caused by supersaturated nitrogen in the molten pool. Bubbles are generated in the molten metal just behind the keyhole. Figure 2.24(a) shows the in-situ x-ray transmission image for full penetration CO 2 laser welding of 15 mm thick C-Si-Mn steel without back shielding. A typical example is shown in Fig. 2.23, where full penetration laser welding was carried out on C-Si-Mn steel without back shielding ( Tsukamoto et al., 2003). In single pass full penetration laser welding, the porosity caused by keyhole instability is less likely to occur, but it is formed via a different mechanism. The latter two are very difficult to measure because the gas sampling lines and analyzer must be heated to avoid condensation. NDIR analyzers are also commercially available to measure other IR-active gases and vapors such as NO, NO 2, HCN, HCl and H 2O. NDIR analyzers are widely used to measure concentrations of CO and CO 2. The difference between the two signals accounts only for the absorption due to CO and eliminates the effect of interfering gases. When the other filter is in position, absorption takes place and the intensity of the wavelengths characteristic to CO is reduced.
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When the nitrogen filter is in position, no absorption takes place and all IR radiation passes through to the sample cell. Two glass cells, one containing nitrogen and another containing CO are mounted on a wheel that rotates at constant speed between the collimating lens and the optical filter. The resulting interference can be eliminated with the gas filter correlation technique. The problem with this setup is that besides the gas that needs to be measured, in this case CO, the sample may contain other gases that absorb IR radiation at the same frequencies. Gas filter correlation non-dispersive infrared gas analyzer. The path length of the cell determines the lowest concentration that can be detected and a wide range of cells are available to suit most fire testing and research needs.Ģ.25. The reduced intensity measured by the detector is due to the absorption in the sample cell and is a direct function of the concentration of the absorbing gases in the sample. The beam then passes through a gas-filled sample cell, where some of the IR energy is absorbed, and is funneled through a condenser before it hits a detector. 2.24, such a filter would transmit radiation over a narrow range of wavenumbers between 23 cm −1. A lens is used to collimate the beam, which then passes through an optical filter that only transmits IR radiation over a limited range of frequencies that are within the absorption band of CO. The source produces a broad spectrum of IR energy. Figure 2.25 shows a schematic of a NDIR CO analyzer. NDIR analyzers are based on this observation. Figures 2.23 and 2.24 show that both CO 2 and CO absorb IR radiation in one or more distinct wavenumber regions or ‘frequency bands’.