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Guidelines for Applying Hardfacing
Remove rust, dirt, grease, oil and other contaminants from the surfaces to be welded.  Previous hardfacing deposits should be removed if:
    they are unknown
    they are incompatible with the new deposit
    they are porous
    they are badly cracked and deformed
    work-hardened surfaces should be removed by grinding and repaired
When a hardfaced edge is subjected to impact or shock preparation is critical.  The following designs are suggested.  Sharp corners, where stress cracks can initiate, must be avoided:

Limit deposit thickness.  Thick hardfacing deposits crack and break off rapidly in service.  If thick deposits are needed, use an appropriate cushion or build-up alloy before hardfacing.

Deposit Hardness

Maximum Layers

65 0r higher
50-64
40-50

1-2
2-3
3-4
Preheating:
Manganese Steel: Do not preheat manganese other than to bring it up to room temperature if it has been in the cold.
Cast Iron:  Cast iron requires high preheat temperatures for hardfacing applications.  A good rule of thumb is a dull red.
Carbon and Low Alloy Steels:  Preheating of some carbon and low alloy steels may be necessary to prevent spalling, underbead cracking and cracking in the base metal.  Consult the preheat chart or call Postle Industries for recommendations. 
Dilution and Penetration:
Avoid excessive penetration and dilution as it will reduce hardness, the performance of the hardfacing alloy, and perhaps cause embrittlement or cracking. Consideration must also be given to the dilution that will occur with the base metal.  A weld deposit is a mixture of filler metal and base metal, and the deposit chemistry will depend on how much each is present.  Wear resistance is reduced by too much dilution.  The following suggestions will help minimize dilution, resulting in greater wear resistance.
1. Do not use excessive welding currents.
2. Direct the arc on the pool of molten weld metal rather than on the base metal.
3. Use close overlap (50 to 75%) when placing weld beads side by side.
4. Use DC straight polarity if possible (electrode negative)
5. Do not use excessive preheat.  Preheat with recommended ranges.
6. Regardless of stringer or wide weave beads, the travel speed should be adjusted to direct the arc on the weld puddle.
7. When using wire processes, a longer stick-out will reduce penetration.
8. In order of decreasing penetration and dilution - vertical up (highest), horizontal, up hill, flat and down hill (lowest).