| The most significant attributes that are needed for dies used for die casting or squeeze casting are:
Resistance to thermal fatigue.
- Minimize softening of die.
- High fracture toughness.
- Removal of residual stress.
Other properties such as erosion/washout reduction and soldering resistance are also important,
but can be influenced more by die filling conditions and statistical process controls. We will
discuss in future bulletins additional benefits of MetaLL ifeÒ that also help in these areas.
In the previous March issue ofour newsletter, we began our discussion of the “K” value formula
and how the parameters presented affect die thermal fatigue. The higher the “K” value, the
more resistance to thermal fatigue.. Examining the formula we see that increased thermal
conductivity and yield strength are directly proportional to obtaining optimum K values while
thermal expansion and modulus of elasticity properties are in directly proportional to a high “K”
value.
Almost all premium grades ofH-13 steel have sufficient percentages of molybdenum ranging
anywhere from .90% to 1.85%. Martensitic steels are fairly good for both a high thermal
conductivity and low thermal expansion, while austenitic superalloys are not as good. The
modulus of elasticity is difficult to change. High yield strength should be maintained with
continued use to avoid die softening.
NADCA’s Die Materials Committee projects are currently evaluating new types of hot work
steels.
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Some of these show significant improvements in the reduction of heat stress cracking
along with higher charpy impact values. One of these is KDA1, a steel originally
One of these is KDA1, a steel originally produced and sold by Nippon Koshuha Steel Co., Ltd. in
Japan. The steel is now available and being sold in the US. One of the lab specimens of this steel
that was austenitized at 1925 degrees F and oil quenched showed almost no cracking after
20,000 dunks in the accepted Case Western Dip Tank Test. Charpy impact values at normal
elevated temperatures of 300 to 400 degrees F were in the range of 20-33ft.lbs. Also no
measurable die softening occurred after completing 20,000cycles in the dip tank. The rockwell
value on the specimen maintained its original 45Rc hardness. Bear in mind that these specimens,
as well as the other types of steel tested, were specially heat treated. Some die casters on the west
coast, however, are using the KDA1 material and receiving similar benefits without doing any
special heat treatment to the steel.
Could this be the steel of choice or will one of the other types also being tested show promise? It is
important to remember that even with these new steels, stresses will eventually build-up that
must be removed and countered with periodic heat stress tempering and MetaLL ifeÒ when the die
is NEW after sample approval and periodically through the life of the tool.
This concludes our tutorial on the modes and causes of die failure relating to Die Stress and
Thermal Fatigue. In subsequent issues, we will examine and discuss the remaining modes of
failure which include Mechanical Erosion or Washout, and Chemical/Mechanical Soldering.
Some encouraging work using coatings to reduce soldering and oppose erosion is being done by
Ohio State in conjunction with NADCA and the Die Materials Committee. Badger Metal is also
active with field tests using MetaLL ifeÒ as the substrate treatment for these state of the art coatings.
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