©
COPYRIGHT 2000 THE ESAB GROUP, INC. LESSON
VIII 8.1.2
Buildup
Alloys -
Buildup alloys are used for two purposes. They may serve as a
supporting base for a more wear resistant
overlay, or they may serve as a moderately wear resistant
alloy surface when it is necessary to machine the part to size. In some
cases, the buildup
alloys may also be used for high strength attachment welds. The selection
of the proper
buildup alloy for parts worn beyond practical limits of hardfacing deposits is
most important,
since the success of the hardfacing overlay depends upon the rigidity and defor-
mation resistance of the base metal.
Experience has proven that in many cases where hardsurfacing
overlays have failed, deformation of the base metal or buildup alloy took
place, causing spalling of the overlay
alloy. 8.1.2.1
Badly worn parts should be restored
to within 3/16” to ¼” of their original size with
a buildup alloy, which is compatible
for welding to the base material and the hardfacing alloy.
8.1.3
Hardfacing
Alloys -
Hardfacing alloys are designed to provide maximum wear
resistance to a specific wear factor
or a combination of wear factors. Performance of the
overlay is in direct relationship to
the amount of carbide forming elements - chromium, molybdenum,
tungsten, vanadium, and iron - in combination with carbon. Wear resisting
carbides are formed when one of these
elements is allowed to react with carbon, and as a result,
is completely saturated forming a carbide consisting only of carbon and the element.
The balance of the carbon remains in
solution to form a semi-austenitic matrix (bonding metal)
in which the hard, wear resistant carbides are evenly distributed. As the
ratio of wear
resistant carbides to alloy matrix increases, abrasion resistance increases. This
same increase reduces the toughness
of the overlay, thereby lowering its impact value. Figure
1 illustrates the effects of the carbide-to-matrix ratio.
VERY HARD CARBIDES
MATRIX BETTER
IMPACT RESISTANCE
BETTER ABRASION
RESISTANCE CARBIDES
- MATRIX RATIO FIGURE
1
