Cooling resistance welding

Because of their low thermal conductivity and higher electrical resistance stainless steels require 20-30% less heat input than carbon steels during resistance welding. The resulting slow cooling rate may lead to a lower corrosion resistance because of the precipitation of chromium carbides. 

In implant resistance welding, an electrically resistive element that is placed at the joint interface is heating by either direct or alternating current [2], The resistive implant may be as simple as a nichrome or stainless steel wire or mesh. More complex implants can be tapes of braided metallic wire with thermoplastic monofilaments or a composite of polymer matrix with electrically conductive particles or fibers. As shown in Fig. 26.26, during implant induction welding, the resistive implant is placed between the two parts. Electric current is then passed for a preset time through the resistance implant while the parts are under pressure. Then the current flow stops and the parts are kept under pressure while the weld cools, and the implant remains at the joint interface. 

The most widely used austenitic stainless steel is Type 304, known as 18—8. It has excellent corrosion resistance and, because of its austenitic stmcture, excellent ductihty. It may be deep-drawn or stretch formed. It can be readily welded, but carbide precipitation must be avoided in and near the weld by cooling rapidly enough after welding. Where carbide precipitation presents problems. Types 321, 347, or 304L may be used. The appHcations of Types 304 are wide and varied, including kitchen equipment and utensils, dairy installations, transportation equipment, and oil-, chemical-, paper- (qv), and food-processing (qv) machinery. 

Plate and frame coolers using HasteUoy C-276 plates have been used successfuUy. Anodically protected plate coolers are available as weU as plate coolers with plates welded together to minimize gasketing. Another promising development is the introduction of plate coolers made of HasteUoy D205 (105). This aUoy has considerably better corrosion resistance to concentrated sulfuric acid at higher temperatures than does C-276. Because of the close clearance between plates, cooling water for plate coolers must be relatively clean. 

Note that low carbon or stabilized grades of stainless steel do not possess intrinsically greater corrosion resistance than their unadjusted counterparts. Their sole value in typical cooling water systems results from their resistance to sensitization and potential weld decay that can result when the metals are welded. It is therefore not economically justifiable to specify low carbon or stabilized grades of stainless steel for typical cooling water system components that are not to be welded. 

The peritectic transformation generally has little effect on the structure, properties or corrosion resistance of steels at room temperature an exception to this occurs in the welding of certain steels, when 6-ferrite can be retained at room temperature and can affect corrosion resistance. Furthermore, since most steels contain less than about 1 -0 oC (and by far the greatest tonnage contains less than about 0-3%C) the eutectic reaction is of relevance only in relation to the structure and properties of cast irons, which generally contain 2-4%C. This discussion, therefore, will be limited to the eutectoid reaction that occurs when homogeneous austenite is cooled. 

A heliarc-welded, all-nickel can of 850-mL volume was filled with an intimate mixture of NiF, (290 g, 3 mol) and anhyd KF (52 g, 9 mol). The can was valved to a tank of F2 gas and a vacuum pump and was heated by an electric-resistance furnace. The can was heated slowly to 500 C under 10 atm of F2 and then cooled to 250 C, while still under several atm of F2. Several such cycles were carried out before using the device for the regeneration of F2. For the regeneration of F2, the salt was fluorinated at 250 C until no more F2 was taken up. The can was then cooled to 225 C and evacuated to remove the excess F2 and any volatile impurities. The temperature was then raised until the desired F, pressure (at 400 C, 25 atm) was achieved. 

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