Welds solidification

Austenitic alloys also make use of the concept of stabilization. Stainless types 321 and 347 are versions of type 304 stabilized with titanium and niobium, respectively. These elements will preferentially combine with carbon that comes out of solid solution during weld solidification. Rather than a loss of corrosion resistance associated with formation of harmful chromium carbides, the carbides of titanium and niobium are not detrimental to corrosion resistance. The austenitic family of stainless also prompted another approach to avoiding the effects of chromium carbide precipitation. Because the amount of chromium that precipitated was proportional to the carbon content, lowering the carbon could prevent sensitization. Maintaining the carbon content to below about 0.035% vs. 

Dong, Z.B. Wei, Y.H. (2008). Three dimensional modeling weld solidification cracks in multipass welding. Theor. Appl. Tract. Mech., Vol. 46, 156-165. 

Avoiding porosity in welds is an important consideration in welding titanium alloys. If the joint and filler wire are properly cleaned and the tooling does not chill the weld too rapidly, porosity can be reduced or eliminated by using a lower welding speed, which will retand weld solidification and allow entrained gases to escape. 

Micro-fissures are caused by thermal contraction stresses during weld solidification and are a problem that plagues austenitic stainless steel fabrications. These weld metal cracks are more likely to form when phosphorus and sulfur levels are higher (that is, more than 0.015% P and 0.015% S), with high heat input welding, and in austenitic weld metal in which the a-ferrite content is low (< 3%). 

Solidification. The heat of the electric arc melts a portion of the base metal and any added filler metal. The force of the arc produces localized flows within the weld pools, thus providing a stirring effect, which mixes the filler metal and that portion of the melted base metal into a fairly homogeneous weld metal. There is a very rapid transfer of heat away from the weld to the adjacent, low temperature base metal, and solidification begins nearly instantaneously as the welding heat source moves past a given location. 

Fig. 6. Weld pool shape and resultant weld—metal solidification direction, (a) Slow welding speed, (b) Rapid welding speed.

Fig. 7. (a) Impurity elements are rejected into the Hquid between the dendritic solidification fronts, (b) Corresponding impurity concentration profiles. Cq, weld metal composition k, impurity partitioning coefficient in the Hquid maximum impurity soHd solubiHty eutectic composition at grain... 

General description. Porosity refers to cavities formed within the weld metal during the solidification process. Such cavities may form due to decreased solubility of a gas as the molten weld metal cools or due to gas-producing chemical reactions within the weld metal itself. At times, cavities can form a continuous channel through the weld metal (worm holes, piping), resulting in leaks (Case History 15.3). 

Critical factors. In general, porosity is caused by the entrapment of gas during the welding process or during solidification of the weld metal. Surface contamination may provide a gas source during the welding operation. 

Critical factors. Slag entrapment can occur if weld-metal temperature is too low or if solidification is too rapid. 

Elimination. Since slag is less dense than the weld metal, it will float to the surface if unhindered by rapid solidification. Therefore, preheating the components to be welded or high weld heat input may prevent slag entrapment. 

Stresses from welding result principally from the effects of differential thermal expansion and contraction arising from the large temperature difference between the weld bead and the relatively cold adjacent base metal. Shrinkage of the weld metal during solidification can also induce high residual stresses. Unless these residual stresses are removed, they remain an intrinsic condition of the weldment apart from any applied stresses imposed as a result of equipment operation. 

It is clear from examination of the fracture surface and weld cross sections that the weld was improperly formed, resulting in an irregular plane of unbonded metal. The smoothly rippled, spherical contours in some regions of the fractured area are evidence of solidification of the weld metal along a free surface that was not in contact with the plate. Substantial porosity is apparent. 

Pressure testing of the finned oil cooler in Fig. 15.29 revealed leaks. Examination of the interior of the cooler after sectioning in the vicinity of the leaks revealed a small cavity in the weld zone in the corner of some fins (Fig. 15.14) and porous areas inside the channel in the welded zone in other fins. Microstructural examinations of specimens cut through the sites revealed interconnected voids resulting from either shrinkage during solidification of the weld or lack of fusion of the base metal and weld metal. 

Solvent-Based Adhesives—In these the adhesive flows because it is dissolved in an appropriate solvent, and solidification occurs on evaporation of the solvent. Good bonds are usually formed if the solvent attacks or actually dissolves some of the plastic adherend to produce a solvent-welded bond. 

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