Problems in Tool Design
Manufacturing defects, operating errors and unforeseen events all have an impact on tool operating life. Heat treatment naturally plays a wide role due to its significant influence on the tool's physical and mechanical properties, and indeed most defects appear after the heat treatment stage. Especially with some more recent heat treatment methods like annealing, remedial action is undoubtedly called for. To optimise the operating life of a given cutting tool, failures must be minimised in all manufacturing process going into its production and its proper use must be ensured. A register of tool failures covering the full range of failure sources can, therefore, contribute significantly to a tool operating life improvement and hence, to more efficient manufacturing.
Heat treatment problems are mostly attributable due to weak structural
toughness resulting in the premature failure in the form of tool
breakage/fracturing. This problem has several causes, one of which is the widespread
conversion of effectively all heat treatment operations to vacuum-hardening technology. The previously common brine hardening method has become rare as a
result of environmental effects. However, it had the advantage of permitting the
quenching process at variable rates in oil, air, or water. With the vacuum
process, the quench rate lies somewhere between air quenching and oil
quenching, i.e., it is slower than in the brine hardening process. This
essential parameter, which gives measurement, defines the quench rate in today’s
applications. In order to get more technical information about different tools
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Premature tool failures due to defects (including heat treatment
failures) are particularly critical in high-performance tools pushing the
limits of achievable material mechanical properties. Optimized heat treatment
can help greatly in these cases to make the performance capability of such high-performance
tools fully accessible.
Tool Failures and Their Significance:
Understanding and prevention of tool failures must be viewed from
different perspectives:
Prevention of economic loss to,
- Tool users
- Tool
manufacturers
- Steelmakers
- Safety issues, i.e.,
the need to ensure
- Operating safety
- Labour safety
- Environmental safety
- Compliance with
quality assurance requirements, e.g., QS 9000
- Performance and
service life improvement
- Increased
production reliability, since tool failures usually entail production
disruptions up to the point of a production shutdown.
Investigation of Tool
Failures:
The investigation of tool failures comprises several stages. Starting
with the macroscopic analysis, cracks and fracture paths, pores and fracture
surface features are evaluated. Chemical tests identify cases of incorrect
material identity. Hardness testing is conducted to check for an adequate
working hardness or, where appropriate, hardness distribution. The main
instrument in tool failure analysis is the metallographic investigation, which
determines material properties on the one hand (carbide distribution,
cleanliness, grain size) and material defects on the other (porosity, shrinkage
holes, excessive segregation, inclusions). Machining defects (grinding or erosion
faults) are likewise detected at this stage. The core element of any
metallographic investigation is the inspection of the microstructure imparted
by heat treatment (tempering condition, retained austenite, banding, coarseness,
and decarburisation).
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