Figure 7-45.—Angle iron jig. Align the piping before welding and maintain it in alignment  during  the  welding  operation.  The  maximum alignment tolerance is 20 percent of the pipe thickness. To ensure proper initial alignment, you should use clamps or jigs as holding devices. Apiece of angle iron makes a good jig for a small-diameter pipe (fig. 7-45), while a section of channel or I-beam is more suitable for larger  diameter  pipe. TACK WELDING When  welding  material  solidly,  you  may  use  tack welds  to  hold  it  in  place  temporarily.  Tack  welding  is one of the most important steps in pipe welding or any other type of welding. The number of tack welds re- quired depends upon the diameter of the pipe. For 1/2-inch pipe, you need two tacks; place them directly opposite each other. As a rule, four tacks are adequate for standard size of pipe. The size of a tack weld is determined  by  the  wall  thickness  of  the  pipe.  Be  sure that a tack weld is not more than twice the pipe thickness in length or two thirds of the pipe thickness in depth. Tack welds should be the same quality as the final weld. Ensure that the tack welds have good fusion and are thoroughly cleaned before proceeding with the weld. SPACERS In addition to tack welds, spacers sometimes are required to maintain proper joint alignment. Spacers are accurately machined pieces of metal that conform to the dimensions of the joint design used. Spacers are some- times referred to as chill rings or backing rings, and they serve a number of purposes. They provide a means for maintaining  the  specified  root  opening,  provide  a  con- venient location for tack welds, and aid in the pipe alignment. In addition, spacers can prevent weld spatter and the formation of slag or icicles inside the pipe. ELECTRODE  SELECTION Select the electrode that is best suited for the posi- tion and type of welding to be done. For the root pass of a multilayer weld, you need an electrode large enough, yet  not  exceeding  3/16  inch,  that  ensures  complete fusion  and  penetration  without  undercutting  and  slag inclusions. Make certain the welding current is within the range recommended by the manufacturers of the welding machines  and  electrodes. WEATHER  CONDITIONS Do not assign a welder to a job under any of the following  conditions  listed  below  unless  the  welder  and the work area are properly protected: c  When  the  atmospheric  temperature  is  less  than 0°F l When the surfaces are wet l  When  rain  or  snow  is  falling,  or  moisture  is condensing on the weld surfaces l During periods of high wind At  temperatures  between  0°F  and  32°F,  heat  the weld area within 3 inches of the joint with a torch to a temperature warm to the hand before beginning to weld. WEARFACING The Seabee welder can greatly extend the life of construction  equipment  by  the  use  of  wearfacing  proce- dures. Wearfacing is the process of applying a layer of special  composition  metal  onto  the  surface  of  another type of metal for the purpose of reducing wear. The selection of a wearfacing alloy for application is based on the ability of the alloy to withstand impact or abra- sion. Impact refers to a blow or series of blows to a surface that results in fracture or gradual deterioration. Abrasion is the grinding action that results when one surface slides, rolls, or rubs against another. Under high-compressive loads, this action can result in goug- ing. Alloys that are abrasion resistant are poor in with- standing impact. Conversely, those that withstand im- pact  well  are  poor  in  resisting  abrasion;  however,  there are  many  alloys  whose  wearfacing  properties  fall  be- tween  the  two  extremes.  These  alloys  offer  some  pro- tection against abrasion and withstand impact well. WORKPIECE  PREPARATION Before you wear-face a workpiece, all dirt, oil, rust, grease, and other foreign matter must be removed. If you do not, your finished product will be porous and subject to  spalling.  You  also  need  a  solid  foundation;  therefore, repair all cracks and remove any metal that is fatigued or rolled over. 7-27