Nitriding of Extrusion Dies: Problems and Solutions
Tahar N. Tarfa and Andrzej Czelusniak, Nitrex Metal Inc.
Track: DIE DESIGN AND TECHNOLOGY - Surface Treatments and Materials
ABSTRACT --- Gas nitriding, based on a nitriding-potential control applied to enhance wear
resistance of aluminum extrusion dies offers several technological and economical
advantages in comparison with other recognized technologies. Experience shows that
inadequate or absence of process control is usually conducive to poor die performance.
Ineffective nitriding parameters may yield a soft nitride surface, which will tend to wear out
prematurely in service, whereas too aggressive nitriding will produce brittle nitrided layers, due
to excessive nitrogen concentration. The use of controlled gas nitriding ensures that the best
compromise between the metallurgical properties of the surface (hardness vs. toughness) is
obtained and consistently reproduced. The paper classifies and analyzes typical problems
caused by poor or inadequate nitriding. It also places emphasis on those operations
preceding the nitriding process (heat treatment, EDM
) as well as those carried out after its
completion (die preheating, die cleaning
). It is shown how good-quality nitriding must be
accompanied by correctly carried out die-handling operations. This becomes particularly
acute in the case of aluminum extrusion dies, where washout, flaking and corner chipping are
the typical causes of premature failure. A study of nitrided layer stability in working conditions
points to uncontrolled oxidation as conducive to premature failure. It is shown how excessive
time and temperature adversely affect the nitrided layer properties and how the use of
protective atmosphere may significantly retard detrimental effects.
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