slight circular motion make the second, third, and fourth passes. This motion of the electrode permits greater control  and  better  distribution  of  the  weld  metal. Remove all slag and oxides from the surface of each pass by chipping or wire brushing before applying additional beads  to  the  joint. WELDING  DIFFICULTIES Many of the welding difficulties in metal-arc weld- ing  are  the  same  as  in  oxygas  welding.  A  few  such problems include undercut, cracked welds, poor fusion, and  incomplete  penetration. Table 7-3 provides an illustration of the most com- mon  welding  problems  encountered  during  the  arc- welding process and methods to correct them. Every welder has the responsibility of making each weld the best one possible. You can produce quality welds by adhering to the rules that follow. 1.   Use   only   high-quality   welding   machines, electrodes,  and  welding  accessories. 2. Know the base material that you are working on. 3. Select the proper welding process that gives the highest quality welds for the base material used. 4. Select the proper welding procedure that meets the service requirement of the finished weldment. 5.  Select  the  correct  electrode  for  the  job  in question. 6. When preheating is specified or required make sure you meet the temperature requirements. In any case, do not weld on material that is below 32°F without first  preheating. 7. Clean the base metal of all slag, paint, grease, oil, moisture, or any other foreign materials. 8. Remove weld slag and thoroughly clean each bead before making the next bead or pass. 9. Do not weld over cracks or porous tack welds. Remove  defective  tack  welds  before  welding. 10. Be particularly alert to obtain root fusion on the first pass of fillet and groove welds. 11.  When  groove  weld  root  gaps  are  excessive, build up one side of the joint before welding the pieces together. 12. When fillet weld root gaps are excessive, be sure you increase the size of the fillet weld to the size of the  root  gap  to  maintain  the  strength  requirement.  In some  cases,  it  is  advantageous  to  make  a  groove  weld  l to avoid extremely large fillet welds. 13.  Inspect  your  work  after  completion  and immediately  remove  and  replace  any  defective  weld. 14. Observe the size requirement for each weld and make sure that you meet or slightly exceed the specified size. 15. Make sure that the finished appearance of the weld  is  smooth  and  that  overlaps  and  undercuts  have been  repaired. PIPE  WELDING Welding is the simplest and easiest way to join sections of pipe. The need for complicated joint designs and special threading equipment is eliminated. Welded pipe  has  reduced  flow  restrictions  compared  to  me- chanical  connections  and  the  overall  installation  costs are  less.  The  most  popular  method  for  welding  pipe  is the  shielded  metal-arc  process;  however,  gas  shielded arc methods have made big inroads as a result of new advances  in  welding  technology. Pipe welding has become recognized as a profes- sion in itself. Even though many of the skills are com- parable to other types of welding, pipe welders develop skills that are unique only to pipe welding. Because of the hazardous materials that most pipelines carry, pipe welders  are  required  to  pass  specific  tests  before  they can be certified In  the  following  paragraphs,  pipe  welding  positions, pipe  welding  procedures,  definitions,  and  related  infor- mation  are  discussed. PIPE WELDING POSITIONS You may recall from chapter 3 of this manual that there are four positions used in pipe welding (fig. 3-30). They are known as the  horizontal  rolled  position  (1G), the horizontal fixed position (5G), pipe inclined fixed (6G), and the vertical position (2G). Remember: these terms refer to the position of the pipe and not to the weld PIPE  WELDING  PROCEDURES Welds that you cannot make in a single pass should be made in interlocked multiple layers, not less than one layer for each 1/8 inch of pipe thickness. Deposit each layer  with  a  weaving  or  oscillating  motion.  To  prevent entrapping slag in the weld metal, you should clean each layer thoroughly before depositing the next layer. 7-25