TO SHIP CONSTRUCTION. 201 



METHODS OF WELDING. 



The methods of electric welding applicable to ship construction can be divided 

 under two main headings, resistance welding and arc welding. 



Resistance Welding. 



As the term implies, it is the resistance offered to the flow of electric current 

 through the material and contact surfaces to be united which causes the metal in 

 the path of the current to heat up to the necessary temperature. Welding is then 

 accomplished by the application of pressure. The heat is largely localized in the 

 weld by the use of clamping electrodes of low resistance and by the additional re- 

 sistance in the contact surfaces themselves as compared with the solid metal. Al- 

 ternating current of relatively high amperage and low voltage is used and the pres- 

 sure applied either hydrauHcally, pneumatically or by hand, depending upon the size 

 of the job. 



There are two distinct kinds of resistance welding — spot welding and butt 

 welding. 



(a) Spot Welding. — Spot welding has been extensively used for jointures of 

 thin sheet metal such as ventilators, lifeboats and railway cars with considerable 

 success, but it is only recently that its application to plates and bars of the thick- 

 ness required in ship construction has been investigated. 



The two or more pieces to be welded are overlapped or superposed in way of the 

 joint and clamped between two copper electrodes ; the current is passed through and 

 pressure applied. In order to ensure a successful weld the pressure should be suffi- 

 cient to cause the metal to "flow" at the weld and to extrude all oxides, slag, etc., 

 which may form. The pressure is maintained for a short time after the current is 

 cut off, and the operation is then repeated at the next spot, and so on. 



Small buttons or discs of metal are sometimes placed under one or both of the 

 electrodes and when of proper thickness are completely submerged in the plate metal 

 during the operation of forming the weld. 



As in smith welding, what is going on at the welding surfaces cannot be seen, 

 and although the material may have been properly prepared, the success of the oper- 

 ation obviously depends upon : — 



( 1 ) The amount of current applied to produce the proper heat throughout the 

 various stages of the operation. 



(2) A sufficient pressure to weld the surfaces and extrude oxides, gases, etc., 

 without undue distortion of the material. 



(3) Sufficient time being allowed. 



(4) The size and type of electrodes used. 



In light work these variables have to a great extent been determined by exten- 

 sive practice and experience, but in the heavier material used in ship construction 

 there has, until recently, been little or no experience available. 



However, the advantage of successful application of this method of uniting ma- 



