Preheat, Interpass, and Postheat
The temperatures for various types of surfacing
operations are dependent on many factors, including:
• Type of base material.
• Thickness of base material.
• Complexity of weldment design and type of weld
joint.
• Type of filler materials to be used.
• Mechanical and chemical values of the completed
weld.
• Final heat treatment of the surfaced part.
Other areas must be considered before welding the
part. Some metals cannot be heated beyond a certain
temperature—the heat will destroy the grain structure of
the base material and cause the weldment to fail. Other
metals must be postheated to prevent hardening of the
final welds and cracking as the weld metal cools.
The following conditions for heating operations
generally apply:
• Austenitic manganese steels are not heated.
• Chrome-nickel stainless steels are not heated.
• Low-carbon steels are not heated.
• Cast irons are heated.
• Extremely cold materials are heated. (Thermal
shock may cause cracking of the base material or
the weld.)
• High-chromium steels are heated.
• Medium- and high-carbon steels are heated.
• Very hard surfacing deposits are heated.
In general, if the part is to be heated, then the entire
heat-affected zone requires heat. Complex design parts
must be heated throughout to prevent cooling stresses
from forming. If preheating is required, interpass heating
and postheating are done also. Interpass temperature
should be maintained until the welding operation is
complete. The final heating should be started before the
part drops below the interpass temperature. Final cooling
should be done by covering the part with an insulating
material or a blanket, or placing it in a furnace to prevent
rapid cooling. Rapid cooling results in internal or
external stresses that may cause cracking of the weld or
the weldment.
Several companies have developed preheat and
interpass temperature calculators for determining
required temperatures when welding and surfacing
carbon and low-alloy steels. See Figure 12-6.
Welding Techniques
Many completed hardfacing welds on construction
equipment have patterns with no definite designs that
cover only part of the wearing surface. See Figure 12-7.
Chapter 12 Surfacing 105
Figure 12-6. A preheat calculator can be used to determine the
heating temperatures required for surfacing carbon and low-
alloy steels. (Lincoln Electric Co.)
Figure 12-7. Many patterns were used to hardface this dragline bucket. (Lincoln Electric Co.)