Welding Pre-hardened 40 Rockwell Material

Key tips help ensure safe shop welding practices when using pre-hardened 40 Rockwell material.
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Welding pre-hardened 40 Rockwell material requires certain conditions and considerations. Here are some tips for employing safe shop welding practices.

Prior to making a welding repair, slowly heat the workpiece in a furnace or with a gas burner to between 600°F and 750°F. Heat the workpiece from the bottom if a gas burner is used. Maintain uniform temperatures within the recommended range during the entire repair. It is ideal to heat the mold in a furnace to achieve uniform temperatures. This is easy to do for small molds but may not be practical for large molds. 

Localized preheating is the most effective option for large molds. All the same, adhere to the following steps: 

  • Maintain the preheat temperature at least 2 inches away from the area that will be welded in all directions.
  • Use an oxygen-propane gas burner with a low-flame temperature.
  • Heat the mold carefully and gradually while keeping 18 inches between the flame and the mold surface.
  • Apply a temperature choke or a surface contact thermometer to accurately measure the preheating temperature.
  • Reheat as necessary during welding to maintain a temperature above 600°F.

The recommended welding parameters for pre-hardened 40 Rockwell material are as follows:

 Rod Diameter  Electrode Diameter  Current/Amps
  0.0470"  0.0470"    40~70 
  0.0630"  0.0630"  70~150
 0.0946"  0.0946"  150~250 

Only use pre-hardened 40 Rockwell copper-coated welding rods, which are copper-coated for either TIG or Heli-Arc welding.

When you are ready to begin your weld, ensure that the mold is free of oil, rust, scale residue or any other potential contaminants. Completely remove all cracks and surface treatments. Remove sufficient stock, and ensure only sound material remains to repair any cracks. Round all sharp or square corners to a minimum radius of 0.120 inch, and dress corners where stock was removed by rounding them to a minimum radius of 0.120 inch. Once welding is complete, it is time for post-weld heating. This requires the following procedure as it will ensure the welded section is completely restored to a uniform hardness: 

  • Heat the weld-repaired workpiece to between 860°F and 940°F.
  • Hold this range for a minimum of one hour to re-age the material. Conduct the re-aging process immediately after welding.
  • Heat with a furnace or a gas burner. If a gas burner is used, heat from the bottom, but keep the entire welded area and 2 inches surrounding the weld in the post-heat temperature range for a minimum of one hour.
  • Cool slowly to room temperature.
  • Perform post-weld heating after every three layers of weld buildup to alleviate welding stress and avoid over-aging of the adjacent parent metal.