Page 42 - MetalForming July 2015

Tooling by Design
By Peter Ulintz
Flawed Designs and Heattreatment Procedures Cause Die-Steel Failures
Two characteristics of metal-stamping dies:
• They manufacture other products.
• They are constructed of materials higher in hardness than most manufactured parts.
Because of their high hardness, die materials can some- times be brittle and susceptible to
fracture. Die materials must be hard
enough to resist wear yet tough
enough to resist breakage. As a result, hardness becomes a com- promise between these two require- ments.
Most tool steel is heattreated in a
multistep process that begins with
austenizing—heating to a tempera-
ture high enough to create a solid
solution of iron and carbon. Next
comes quenching to produce a hard
martensitic microstructure. Quenched martensite, strong but very brittle, must be tempered to become tough and slightly more ductile.
A survey of die failures conducted by the American Soci- ety for Metals (ASM) reveals that 70 percent of failures result- ed from “irregularities during heattreatment.” Of these, 40 percent were due to failure to control tool-steel surface composition; 20 percent were due to quenching at too low a temperature or ineffective tempering; and 10 percent due to quenching from excessively high austenizing temperatures. Other causes include bad component design, abusive grind- ing, service abuse and selecting an improper steel grade.
The results of this study suggest that any die-failure analy- sis must begin with a complete understanding and full inves- tigation of the heattreatment process. Unfortunately, for most die shops and contract manufacturing companies heat treatment is a “black-magic” process. The die shop sends soft die details out to the local heat treater who returns to the shop the hardened details. The shop may perform cursory Rock-
Peter Ulintz has worked in the metal stamping and tool and die industry since 1978. His background includes tool and die making, tool engineering, process design, engineering management and advanced product development. As an educator and technical presenter, Peter speaks at PMA national seminars, regional roundtables, international confer- ences, and college and university programs. He also provides onsite training and consultations to the met- alforming industry.
Peter Ulintz pete.ulintz@toolingbydesign.com www.toolingbydesign.com
well hardness tests when the parts return and, if they meet hardness requirements, they usually are accepted regardless of how they may have been processed.
Die failures often occur due to overheating damage. Most die steels, relatively high in carbon or alloy content, are sen- sitive to damage by overheating during austenizing. Overheating causes grain growth, the formation of coarse martensite and excessive amounts of retained austenite, all of which results in brittle dies susceptible to failure in service—even when surface hardness
is within specification.
Best Practices
Ideally, most dies should be hard- ened in a neutral atmosphere to pre- vent carbon migration into or out of
the surface. The base carbon content of the tool steel must be maintained in order to obtain the proper working hard- ness and service life. Accomplish this by heattreating in controlled, protective gas-atmosphere furnaces, vacuum furnaces and molten salt baths.
Tempering of tools and dies is a critical operation. Give careful consideration to the tool-steel grade, its hardenabil- ity, section thickness and the die design.
Tempering, which serves more than one purpose, first and foremost must relieve residual stresses. These stresses often are close to the tensile strength of the die material. Dies placed into service without proper tempering will likely fail quickly.
Dies built from the higher-alloy grades of tool steel (D, M or T grades) require double and sometimes triple tempering. These grades retain significant amounts of austenite, which transforms to martensite during cooling from the first tem- pering. At least one additional tempering operation is required to relieve the stresses caused by the transformation that takes place during the first tempering step.
As discussed in last month’s Tooling by Design column, design errors also contribute to die failure. Design errors can be classified into two basic groups:
• Those that fail from heattreatment
• Those that fail in service.
Design faults that cause failures from heattreatment
include the presence of thick sections adjacent to thin sec- tions; sharp corners; blind holes; stamp marks; fillets with insufficient radii; poorly located or designed grooves or notches; abrupt changes in cross section and the location of
40 MetalForming/July 2015
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