Figure7-46.—Wearfacing techniques. PREHEATING Depending on the type of metal, sometimes it is necessary to preheat the base metal to lessen distortion, to prevent spalling or cracking, and to avoid thermal shock The preheating temperature depends on the car- bon and alloy content of the base metal. In general, as carbon  content  increases  so  does  the  preheating  tem- perature. Improper heating can adversely affect a metal by reducing its resistance to wear, by making it hard and brittle, or by making it more prone to oxidation and scaling. To  preheat  properly,  you  must  know  the  composi- tion of the base metal. A magnet can be used to deter- mine if you are working with carbon steel or austenitic manganese steel. Carbon steel is magnetic, but be care- ful because work-hardened austenitic manganese steel is also magnetic. Make sure that you check for magnet- ism  in  a  nonworked  part  of  the  austenitic  manganese steel. There are other ways to tell the difference between metals, such as cast iron and cast steel. Cast iron chips or cracks, while cast steel shaves. Also, some metals give off telltale sparks when struck by a chisel. In preheating, you should raise the surface tempera- ture of the workpiece to the desired point and then soak it until the heat reaches its core. After wearfacing, cool the  work  places  slowly. TECHNIQUES Where  possible,  position  the  workpiece  for  down- hand welding. This allows you to finish the job quicker and at less cost. The building up and wearfacing of cast iron is not generally  recommended  because  cast  iron  tends  to crack. However, some cast-iron parts that are subject to straight abrasion can be wearfaced successfully. You must preheat these parts to temperatures of 1000°F to 1200°F and then allow them to cool slowly after wear- facing. Peening deposits on cast iron helps to relieve stresses  after  welding. Welding  materials  for  building  up  worn  parts  differ from  those  used  in  wearfacing  the  same  parts.  Before wearfacing a badly worn part, you must first build it up to 3/16 to 3/8 of an inch of its finished size. The buildup material must be compatible with both the base metal and the wearfacing overlay as well as being strong enough to meet the structural requirements. Also, they must have the properties that enable them to resist cold flowing, mushing under high-compressive loads, and plastic  deformation  under  heavy  impact.  Without  these properties, the buildup materials cannot support the wearfacing  overlay.  When  the  overlay  is  not  properly supported,  it  will  span. Many times high-alloy wearfacing materials are deposited on the parts before they are placed in service. The maximum allowable wear is usually no more than two layers deep (1/4 inch) before wearfacing. Try to deposit the wearfacing alloy in layers that are not too thick. Thick layers creates more problems than no over- lay at all. Usually you only need two layers. The frost layer produces an admixture with the base metal; the second  forms  a  wear-resistant  surface. In  wearfacing  built-up  carbon-steel  parts,  maintain high interpass temperatures and use a weaving bead, rather than a stringer bead. (See fig. 7-46.) Limit the thick- ness of a single pass bead to 3/16 inch. Use the same technique for each layer and avoid severe quenching. Deposits  made  with check  on  the  surface. high-alloy Checking electrodes   should reduces  residual Figure  7-47.—Comparison  between  cross-checking  and  cracking. 7-28