•  When the weld is complete and still in the red heat,  circle the outer envelope of the torch around the entire  weldment to bring it evenly to a dull red. Slowly back  the torch away from the weldment to ensure a slow  cooling rate. •  Keep the hot end of the welding rod in the weld pool  or within the flame envelope. •  Maintain a flame sufficient to penetrate the metal and  manipulate it so that the molten metal is protected from  the air by the outer envelope of flame. structural components is cold worked or cold rolled and, if  heated, loses some of its strength. Nonstructural stainless  steel is obtained in sheet and tubing form and is often used for  exhaust collectors, stacks, or manifolds. Oxygen combines  very readily with this metal in the molten state, and you must  take extreme care to prevent this from occurring. A slightly carburizing flame is recommended for welding  stainless steel. The flame should be adjusted so that a feather  of excess acetylene, about  1 ⁄ 16 -inch long, forms around the  inner cone. Too much acetylene, however, adds carbon to  the metal and causes it to lose its resistance to corrosion.  The torch tip size should be one or two sizes smaller than  that prescribed for a similar gauge of low-carbon steel. The  smaller tip lessens the chances of overheating and subsequent  loss of the corrosion-resistant qualities of the metal. The welding technique for chrome molybdenum (chrome- moly) is practically the same as that for carbon steels, except  for sections over  3 ⁄ 16 -inch thick. The surrounding area must be  preheated to a temperature between 300 °F and 400 °F before  beginning to weld. If this is not done, the sudden quenching  of the weld area after the weld is complete may cause a  brittle grain structure of untempered martensite that must  be eliminated with post-weld heat treatments. Untempered  martensite is a glass-like structure that takes the place of the  normally ductile steel structure and makes the steel prone to  cracking, usually near the edge of the weld. This preheating  also helps to alleviate some of the distortion caused by  welding along with using proper practices found in other  sections of this chapter.  A soft neutral flame should be used for welding and must be  maintained during the process. If the flame is not kept neutral,  an oxidizing flame may cause oxide inclusions and fissures.  A carburizing flame makes the metal more hardenable by  raising the carbon content. The volume of the flame must  be sufficient to melt the base metal, but not hot enough to  overheat the base metal and cause oxide inclusions or a loss  of metal thickness. The filler rod should be compatible with  the base metal. If the weld requires high strength, special low- alloy filler is used, and the piece is heat treated after welding. It may be advantageous to TIG weld 4130 chrome-moly  sections over 0.093-inch thickness followed by a proper  post-weld heat treatment as this can result in less overall  distortion. However, do not eliminate the post-weld heat  treatment as doing so could severely limit the fatigue life of  the weldment due to the formed martensitic grain structure. The procedure for welding stainless steel is basically the  same as that for carbon steels. There are, however, some  special precautions you must take to obtain the best results. Only stainless steel used for nonstructural members of aircraft  can be welded satisfactorily. The stainless steel used for  Nonferrous metals are those that contain no iron. Examples  of nonferrous metals are lead, copper, silver, magnesium,  and the most important in aircraft construction, aluminum.  Some of these metals are lighter than the ferrous metals,  but in most cases, they are not as strong. Aluminum  manufacturers have compensated for the lack of strength of  pure aluminum by alloying it with other metals or by cold  working it. For still greater strength, some aluminum alloys  are also heat treated. 5-17 Another method used to keep oxygen from reaching the  metal is to surround the weld with a blanket of inert gas.  This is done by using a TIG welder to perform welding of  stainless steel. It is a recommended method for excellent  weld results and does not require the application of flux and  its subsequent cleanup.  When welding, hold the filler rod within the envelope of the  torch flame so that the rod is melted in place or melted at the  same time as the base metal. Add the filler rod by allowing  it to flow into the molten pool. Do not stir the weld pool,  because air enters the weld and increases oxidation. Avoid  rewelding any portion or welding on the reverse side of the  weld, which results in warping and overheating of the metal. To prevent the formation of chromium oxide, a specially  compounded flux for stainless steel, should be used. The flux,  when mixed with water, can be spread on the underside of the  joint and on the filler rod. Since oxidation must be avoided  as much as possible, use sufficient flux. The filler rod used  should be of the same composition as the base metal. Chrome Molybdenum Stainless Steel Oxy-Acetylene Welding of Nonferrous  Metals