Depending on the type of metal, sometimes it is
necessary to preheat the base metal to lessen distortion,
to prevent spalling or cracking, and to avoid thermal
shock The preheating temperature depends on the car-
bon and alloy content of the base metal. In general, as
carbon content increases so does the preheating tem-
perature. Improper heating can adversely affect a metal
by reducing its resistance to wear, by making it hard and
brittle, or by making it more prone to oxidation and
To preheat properly, you must know the composi-
tion of the base metal. A magnet can be used to deter-
mine if you are working with carbon steel or austenitic
manganese steel. Carbon steel is magnetic, but be care-
ful because work-hardened austenitic manganese steel
is also magnetic. Make sure that you check for magnet-
ism in a nonworked part of the austenitic manganese
steel. There are other ways to tell the difference between
metals, such as cast iron and cast steel. Cast iron chips
or cracks, while cast steel shaves. Also, some metals
give off telltale sparks when struck by a chisel.
In preheating, you should raise the surface tempera-
ture of the workpiece to the desired point and then soak
it until the heat reaches its core. After wearfacing, cool
the work places slowly.
Where possible, position the workpiece for down-
hand welding. This allows you to finish the job quicker
and at less cost.
The building up and wearfacing of cast iron is not
generally recommended because cast iron tends to
crack. However, some cast-iron parts that are subject to
straight abrasion can be wearfaced successfully. You
must preheat these parts to temperatures of 1000°F to
1200°F and then allow them to cool slowly after wear-
facing. Peening deposits on cast iron helps to relieve
stresses after welding.
Welding materials for building up worn parts differ
from those used in wearfacing the same parts. Before
wearfacing a badly worn part, you must first build it up
to 3/16 to 3/8 of an inch of its finished size. The buildup
material must be compatible with both the base metal
and the wearfacing overlay as well as being strong
enough to meet the structural requirements. Also, they
must have the properties that enable them to resist cold
flowing, mushing under high-compressive loads, and
plastic deformation under heavy impact. Without these
properties, the buildup materials cannot support the
wearfacing overlay. When the overlay is not properly
supported, it will span.
Many times high-alloy wearfacing materials are
deposited on the parts before they are placed in service.
The maximum allowable wear is usually no more than
two layers deep (1/4 inch) before wearfacing. Try to
deposit the wearfacing alloy in layers that are not too
thick. Thick layers creates more problems than no over-
lay at all. Usually you only need two layers. The frost
layer produces an admixture with the base metal; the
second forms a wear-resistant surface.
In wearfacing built-up carbon-steel parts, maintain
high interpass temperatures and use a weaving bead,
rather than a stringer bead. (See fig. 7-46.) Limit the thick-
ness of a single pass bead to 3/16 inch. Use the same
technique for each layer and avoid severe quenching.
Deposits made with
check on the surface.
Figure 7-47.Comparison between cross-checking and cracking.