Page 50 - MetalForming March 2012

Tooling by Design
By Peter Ulintz
Sub-Zero Treatment of Tool Steels
Tooling Technology
Areader recently inquired about the benefits of sub-zero treatment of tool and die components after heat treat- ment. He also wondered if the sub-zero process was the same as a cryogenic treatment.
Cryogenics is not new. During World War II, scientists found that metals frozen to low temperatures exhibited increased wear resistance. And, NASA engineers observed the cryogenic effect on metal parts recovered from the extreme cold of space—in particular, aluminum parts, which exhib- ited increased dimensional stability after exposure to extreme cold.
Sub-zero treatment processes are grouped into three broad categories, each impacting the treated steel in different ways:
1) Shrink fitting—reduces the diameter of a steel shaft so that it can be readily assembled with other components;
2) Cold treatment—completes the metallurgical phase transformation of austenite into martensite during the hardening of steels, via quench and temper heat treatment; and
3) Cryogenic treatment—at liq- uid-nitrogen temperatures, creates conditions for precipitation of very fine carbides in higher alloy steels.
Shrink fitting is a convenient
method to assemble a pressed-fit
guide post to a die set. The die post is cooled to sub-zero tem- peratures (by packing in dry ice, for example) to decrease its diameter prior to assembly. The assembled post then is allowed to warm to room temperature, creating a very tight interference fit.
Peter Ulintz has worked in the metal stamping and tool and die industries since 1978. He has been employed with the Anchor Manufacturing Group in Cleveland, OH, since 1989. His background includes tool and die making, tool engineering, process engi- neering, engineering management and product devel- opment. Peter speaks regularly at PMA seminars and conferences and maintains the website, www.Tooling- byDesign.com. The site serves as a web-based source for the transfer of modern metalforming technology and the advancement of “Performance-Based Die Engineering Strategies.”
Peter Ulintz
pete.ulintz@toolingbydesign.com www.toolingbydesign.com
Cold treatments and cryogenic treatments are addition- al steps in the heat treatment and hardening process.
The heat treatment of tool steels includes heating the tool- ing component to austenitizing temperature, followed by quenching—or rapid cooling—to transform most, but not all, of the austenite into the higher-strength martensite structure. The newly formed untempered martensite is supersaturat- ed with carbon, which makes it unstable, brittle and likely to crack. Tempering allows the supersaturated carbon to form carbides that help relieve stresses in the martensite matrix
Sub-Zero Treatment Processes for Steel
Process
Description
Parameters
Objective
Shrink fitting
Overall contraction of metals when cooled allows tight assembly of parts
-70 to -120 C (-90 to -190 F) until metal
is cold throughout
Temporary change in size
Cold treatment of steels
Complete martensitic phase transformation
-70 to -120 C (-90 to -190 F) for 1 hr. per 3 cm of cross section
• Transformation of retained austenite to maretnsite
• Increase hardness
• Dimensional stability
Cryotreatment of steels
Cyrotreatment temperatures can create sites to nucleate fine carbides that improve wear resistance in tool steels
-135 C (-210 F) and below for 24 hr. or longer
Improved wear resistance through carbide precipitation
Source: Linde AG, Hollriegelskreuth, Germany
and prevent cracking. Most tool and die professionals are familiar with this heat-treatment sequence.
Quenching usually occurs at room temperature. Most medium-carbon and low-alloy steels will undergo complete transformation to martensite at room temperature. However, high-carbon and high-alloy steels will retain austenite at room temperature. To eliminate retained austenite, the tem- perature must be lowered.
Cold-treatment processes tend to operate in a tempera- turerangeof-90to-190F.Thisprocesscompletesthetrans- formation of retained austenite in the microstructure to the stronger and harder martensite structure. The resulting steel hardness correspondingly increases with the increased per- centage of martensite in the microstructure. This also increas- es the steel’s strength and wear resistance.
To summarize, the primary purpose for cold treating tool steel is to promote the further transformation of retained austenite to martensite, to increase the hardness, strength
48 MetalForming/March 2012
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