The rod is added to the puddle in the amount that provides  for the completed fillet to be built up about one-fourth the  thickness of the base metal. The filler rod selected should be  compatible with the base metal being welded.  Low-carbon steel, low-alloy steel (e.g., 4130), cast steel, and  wrought iron are easily welded with the oxy-acetylene flame.  Low-carbon and low-alloy steels are the ferrous materials  that are gas welded most frequently. As the carbon content of  steel increases, it may be repaired by welding using specific  procedures for various alloy types. Factors involved are the  carbon content and hardenability. For corrosion-resistant  and heat-resistant nickel chromium steels, the allowed  weldability depends upon their stability, carbon content, and  reheat treatment. The form of the weld metal has considerable bearing upon  the strength and fatigue resistance of a joint. The strength  of an improperly made weld is usually less than the strength  for which the joint was designed. Low-strength welds are  generally the result of insufficient penetration; undercutting  of the base metal at the toe of the weld; poor fusion of the  weld metal with the base metal; trapped oxides, slag, or gas  pockets in the weld; overlap of the weld metal on the base  metal; too much or too little reinforcement; or overheating  of the weld. A completed weld should have the following characteristics: The Society of Automotive Engineers (SAE) and the  American Iron and Steel Institute (AISI) provide a  designation system that is an accepted standard for the  industry. SAE 4130 is an alloy steel that is an ideal material  for constructing fuselages and framework on small aircraft; it  is also used for motorcycle and high-end bicycle frames and  race car frames and roll cages. The tubing has high tensile  strength, malleability, and is easy to weld.  The number ‘4130’ is also an AISI 4-digit code that defines  the approximate chemical composition of the steel. The  ‘41’ indicates a low-alloy steel containing chromium and  molybdenum (chromoly) and the ‘30’ designates a carbon  content of 0.3 percent. 4130 steel also contains small amounts  of manganese, phosphorus, sulfur, and silicon, but like all  steels, it contains mostly iron. Although a clean, smooth weld is desirable, this characteristic  does not necessarily mean that the weld is a good one; it may  be dangerously weak inside. However, when a weld is rough,  uneven, and pitted, it is almost always unsatisfactory inside.  Welds should never be filed to give them a better appearance,  since filing deprives the weld of part of its strength. Welds  should never be filled with solder, brazing material, or filler  of any sort. In order to make a good weld, the carbon content of the  steel must not be altered to any appreciable degree, nor  can other atmospheric chemical constituents be added to or  subtracted from the base metal without seriously altering the  properties of the metal. However, many welding filler wires  do contain constituents different from the base material for  specific reasons, which is perfectly normal and acceptable if  approved materials are used. Molten steel has a great affinity  for carbon, oxygen, and nitrogen combining with the molten  puddle to form oxides and nitrates, both of which lower the  strength of steel. When welding with an oxy-acetylene flame,  the inclusion of impurities can be minimized by observing  the following precautions: When it is necessary to reweld a joint, all old weld material  must be removed before the operation is begun. It must be  remembered that reheating the area may cause the base metal  to lose some of its strength and become brittle. This should  not be confused with a post-weld heat treatment that does not  raise the metal to a high enough temperature to cause harm  to the base material. •  Maintain an exact neutral flame for most steels and  a slight excess of acetylene when welding alloys  with a high nickel or chromium content, such as  stainless steel. •  Maintain a soft flame and control the puddle. 1.  The seam should be smooth, the bead ripples evenly  spaced, and of a uniform thickness. 2.  The weld should be built up, slightly convex, thus  providing extra thickness at the joint. 3.  The weld should taper off smoothly into the  base metal. 4.  No oxide should be formed on the base metal close to  the weld. 5.  The weld should show no signs of blowholes, porosity,  or projecting globules. 6.  The base metal should show no signs of burns, pits,  cracks, or distortion.  Characteristics of a Good Weld Correct Forming of a Weld Oxy-Acetylene Welding of Ferrous Metals  Steel (Including SAE 4130) 5-16