Welds fusion line

DISTANCE FROM WELD FUSION LINE (mils)... 

Fig. 7.68 Effect of welding on sensitization as a function of distance from the weld fusion line of type 304 stainless steel as determined by the EPR test. Redrawn from Ref 104...

Fig. 9.42 Photomicrograph showing crack initiation from the pit at weld fusion line and propagation in transgranuiar mode with tree root morphoiogy 75 x [155].

All methods of welding stainless steel with a filler metal prodnce a weld fnsion bonndary consisting of base metal that has been melted, but not mechanically mixed with filler metal and a partially melted zone in the base metal. The weld fusion boundary lies between a weld composite consisting of filler metal diluted by base metal and the HAZ in the base metal. The width of the unmixed zone depends on the local thermal conditions along the weld fusion line. For a GTA welding process, the zone is most narrow at the weld face and is broadest near the middle of the weld thickness. 

Under certain conditions it is possible for a weldment to suffer corrosive attack which has the form of a fusion line crack emanating from the toe of the weld this is termed knifeline attack. It is occasionally experienced in welded stabilised steels after exposure to hot strong nitric acids. The niobium-stabilised steels are more resistant than the titanium-stabilised types by virtue of the higher solution temperature of NbC, but the risk may be minimised by limiting the carbon content of a steel to 0-06 Vo maximum (ELC steel). 

Knife-line Attack severe highly localised attack (resembling a sharp cut into the metal) extending only a few grains away from the fusion line of a weld in a stabilised austenitic stainless steel, which occurs when the metal comes into contact with hot nitric acid and is due to the precipitation of chromium carbides. 

L. Gas-oil hydrodesulfurization unit. Elbow cracked intergranularly and decarburized at fusion line between weld metal and heat-affected zone no postweld heat treatment. 

Corrosion is due to electrochemical potential differences (galvanic corrosion) between the HAZ/fusion line and the parent material, attributed to the unstable MnS inclusions produced during the welding cycle. It was observed that enhanced corrosion of the weld metal was due to electrochemical potential differences between the weld metal and the base metal, such that the weld metal is anodic in the galvanic couple. The potential difference may only be of the order of perhaps 30-70 mV, but the low surface area ratio of anode to cathode results in high corrosion rates (1-10 mm). (Bond)5... 

Figure 7.41 309 Unifuse overlay on 1-1 /4Cr-l /2Mo boiler tube in the wall blower region after service for about 7 yr in a supercritical unit of the coal-fired boiler (a) surface appearance of the weld overlay, weld beads are still clearly visible (b) cross-section of the weld overlay showing no corrosion and no cracking on the weld overlay at the fusion line (Reproduced from COM 1997 with permission from the metallurgy Society of CIM)...

Heat-affected zone (HAZ) is the volume of parent metal in which the mechanical properties and/or the microstructure have been changed by the heat of welding or thermal cutting. For most welds in carbon and low-alloy steels, the HAZ is a band, usually about 1/8 in. (3 mm) wide, adjacent to the fusion line of the weld. In austenitic stainless steels, a narrow, secondary HAZ may be generated some distance from the fusion line as illustrated in Figure 21.4. 

EPR measurements can be used to show the distribution of sensitization in the heat-affected zone of welded stainless steels. P values for test specimens cut parallel to and progressively away from the weld fusion zone of atype 304 stainless steel are shown in Fig. 7.68 (Ref 104). The shape of this curve is consistent with the time-temperature thermal histories shown in Fig. 7.61 and indicates that the steel was in the critical temperature zone for sensitization for the longest time at positions 100 mils (2.54 mm) from the fusion line. 

Pipe-Making. In many respects the tube-making methods used in Europe are the same as those used in the United States. There are some differences. Flash-butt welding of line pipe is unknown in Europe. More importantly electric resistance welded pipe, which can be obtained in quite large diameters in the United States, is sometimes available only in small diameter tube in Europe. If large size ERW pipe is ordered in Europe, fusion welded pipe may be delivered Fusion welded pipe may or may not be an acceptable alternative. 

Usually, welded joints of 5 and 9% Ni steels possess adequate workability within the range 300 to 77 K for static, dynamic, and alternating loads. In arc and electroslag welding, there is no need for the welded joints to be heat treated. In spite of the growth of primary austenite grains in the base metal at the fusion line, the subsequent y a transformation results in the formation of a disoriented cellular structure, which accounts for the satisfactory ductility at temperatures down to 77 K. 

Metallographic studies showed a continuous net of eutectic precipitations in the weld metal along the grain boundaries of the a-solid solution of copper in aluminum. Their coarse accumulations were also discovered in the heat-affected zone (HAZ) near the fusion line [ ]. 

In the weld metal, the solid solution of the two alloys contained 2.5 to 2.7% Cu and the eutectics contained 22 to 27% Cu. The distribution of Mn and Fe and Si additions in the solid solution was the same as the HAZ. In the eutectic interlayers of the weld (arrows 10 and 20), the Fe and Si concentration was one order higher than its average content in the alloy. In the melting and subsequent solidification of the weld metal, the Fe and Si additions were distributed more uniformly than in the HAZ, due to their larger quantity and to the dispersion of the secondary phases. Nevertheless, near the fusion line and in the HAZ, isolated eutectic clusters occurred (arrows 9 and 18), but contained less Fe and Si than the interlayers in the central part of the weld. 

Knife-line attack, immediately adjacent to the weld metal, is a special form of sensitization in stabilized austenitic stainless steels. Stabilizing elements (notably Ti and Nb) are added to stainless steels to prevent intergranular corrosion by restricting the formation of Cr-rich grain boundary precipitates. Basically, these elements form carbides in preference to Cr in the austenitic alloys. However, at the high temperatures experienced immediately adjacent to the weld fusion zone, the stabilizer carbides dissolve and remain in solution during the subsequent rapid... 

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