TM 1-1500-204-23-8 3-18           Change 1 (4) Arc welding procedures.    The following pro- cedures will produce welds of good quality: (a) Grind tungsten electrode  to an approxi- mate 60-degree point (included angle). (b) Do not attempt to strike an arc in a mol- ten (weld) pool. (c) Regrind electrode if arc is accidentally struck in molten pool. (d) Use gas welding flux, MIL-F-7516, as applicable,  sparingly  on  underside  of  weld  area  when necessary to control oxidation. (e) Use only gas welding rods as applica- ble or as specified in paragraph (a). NOTE Maximum length of electrode protruding from gas cup should not exceed 1/4 inch. (f) Adhere  closely  to  amperage,  voltage, gas flow and electrode size as specified in table  3-8. (5) Inert-arc welding.     A comparatively recent development in arc welding, which is referred to as inet- arc, has earned wide recognition in the field of corrsion- resistant steel and nickel alloy welding, particularly as applied to thin sheet, 0.050-inch thickness and under. The  use  of  this  process  in  fabrication  of  thin sections produces welds having very good strength and ductility. This  type  of  welding  was  originally  applied  to  magne- sium, with  helium gas as the shielding medium. Although helium may also be used in welding of steels and nickel alloy in heavier sections, argon gas is preferred on thin sections due to difficulties encountered in establishing the arc with low current setting when using  helium. A lower arc voltage is required to establish and maintain the arc with use of argon. The arc in argon envelope has the appearance of a gas welding flame, whereas  helium produces a blunt or ball-like flame. Direct current with straight polarity is used in welding steels and nickel alloys by this process. The arc welder used with this special equipment should conform to MIL-W-80019. e.  Corrosion-Resistant  Steel  Spot  Welding.    Spot welding may be used on low carbon steels, austenitic steels, hardenable steels, and nickel alloys. The follow- ing paragraphs describe spot welding practices and pro- cedures. WARNING Drycleaning  solvent  is  flammable,  and  sol- vent vapors are toxic. Use P-D-680, Type II Solvent in a well-ventilated area. Keep away from  open  flames.  Avoid  prolonged  solvent contact with skin. (1) Cleaning.     Clean surfaces to be welded of objectionable films, such as oxides, scales, ink,  grease, or dust, using a clean  rag or bristle brush moistened with drycleaning solvent, Federal Specification P-D-680. (2) Spot spacing.    The edge distance will be so that no deformation or bulge will occur at edge of sheet. (3) Surface indentation.     Where aerodynamic consideration  is  a  requisite,  the  electrode  indentation shall not exceed 0.003 inch. In all other cases, indenta- tion is not acceptable when depth exceeds 10 percent of sheet thickness. (4) Defects.    Weld defects such as voids, blow holes, porosity, and cracks shall not exceed the following limits: 15  percent  of  area  of  the  weld  nugget  in  the plane in which the measurement is taken, or a linear dimension greater than 25 percent of the weld diameter. No internal defect shall extend to within 15 per- cent of weld diameter of the boundaries of the case weld structure. (a)    Welding    Stresses    and    Distortion.  In welding of corrosion-resistant steels or nickel chromium iron alloy, care must be taken to avoid welding stresses and  distortion  as  much  as  possible.  In  making  joints, sheets should be spread slightly apart and tack welded. The amount of spread will depend on whether arc or gas welding  is  employed,  less  spread  being  required  with arc than with gas welding. For this reason, and the fact that there is less tendency toward carbide precipitation in welding of corrosion-resistant steels with use of the former,   the  arc   process   is  recommended. The   arc process  is  particularly  recommended  on  applications where   it   is   impossible  to  allow  for   expansion and contraction  of  material.  The  distribution  of  contraction stresses  can  be  further  improved  by  using  back  step welding in which electrode is advance a short distance ahead of weld and joint is welded back toward original starting  point.–This  process  is  repeated  until  weld  is completed. When practicable, jigs shall be used to hold