ASTM - Intensive Quenching Systems - Engineering and Design 2010 - N I Kobasko, M A Aronov, J A Powell, G E Totten

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MNL64-EB/NOV. 2010
Thermal and Metallurgical Basics of Design
of High-Strength Steels
N. I. Kobasko 1
1.1 INTRODUCTION
The objective of heat treatment of metals is the creation of
high-strength materials by heating and quenching. It is often
recommended that alloy and high-alloy steels should be
through-hardened in petroleum oils or high concentrations
of aqueous polymer solutions and plain carbon, and that
low-alloy steels should be quenched in water. Petroleum oils
are used to reduce quench cracking and distortion of steel
parts during the through-hardening process. For this reason,
slow cooling is used, and expensive alloy elements provide
through-hardening. Oil quenching is most often performed
at low to moderate temperatures with generally acceptable
thermal gradients in the cross-sections of steel parts.
To increase the strength of parts, engineers often utilize
high-temperature or low-temperature thermomechanical treatment.
Typically, the potential use of intensive steel quenching
methods for alloy and high-alloy steel grades is not considered,
because it is a widely accepted point of view that alloy
steels should be quenched very slowly within the martensite
range; this is commonly stated in various manuals and handbooks
on heat treatment of steels. In this book, the problem
of the creation of high-strength materials and minimizing
quench is addressed by the intensification of heat transfer
within the martensite range. So, what was previously discouraged
is now used to achieve high-strength materials while
minimizing distortion.
To obtain a fundamental understanding of the physics
of processes occurring during intensive quenching of alloy
and high-alloy steels, the regularities involved in the quenching
of high-alloy steels will now be discussed. One of the factors
exhibiting a significant effect on part distortion is the
formation of high thermal gradients in the cross-sections
during quenching.
This book describes a new approach in the quenching
technology of alloy and high-alloy steel grades [1], which
consists of the following:
• Intensive quenching is performed throughout the entire
quenching process, including the martensite temperature
range.
• Intensive quenching is interrupted when an optimal
thickness of the outer quenched layer is formed.
• Intensive quenching results in the creation of high compressive
stresses at the surface of parts during the throughhardening
process.
• Intensive quenching within the martensite temperature
range creates a high dislocation density, resulting in
improvement of material strength.
• During intensive quenching, dislocations are “frozen”
and are not accumulated at the grain boundary, which
improves the plastic properties of the material.
• The creation of high dislocation density and high compressive
stresses within the surface layers increases the
service life of steel parts.
Intensive quenching provides the following benefits:
• Uses less expensive steels instead of more expensive
alloy and high-alloy steel grades
• Increases the hardness of the quenched surface by HRC
2–5, which in some cases provides for the elimination
of carburizing or a reduction of carburizing time
• Minimizes quench distortion
• Maximizes labor productivity
• Reduces the number of manufacturing operations
• Replaces expensive and flammable quench oils
• Provides for an environmentally friendly quenching process
Factors affecting the strength and service life of steel
parts will be considered in this book.
Intensive quenching provides additional opportunities
for high- and low-temperature thermomechanical treatment.
The first opportunity is the potential use of intensive quenching
of forged parts immediately after forging (direct forgequenching).
The second is the delay of martensitic transformations
with further low-temperature thermomechanical
treatment; this is of particular importance since intensive
quenching within the martensitic range is equivalent to lowtemperature
thermomechanical treatment, which significantly
simplifies the manufacturing process. Detailed information
about high- and low-temperature thermomechanical
treatments is provided later in this chapter.
As stated above, material strengthening may be achieved
with intensive quenching and thermomechanical heat treatment
to achieve high strength and high plasticity. Both
approaches require process optimization to prevent quench
crack formation and to minimize distortion during rapid
quenching. This can be done by delaying transformation of
austenite into martensite during intensive quenching. Due to
the discovery of an unconventional phenomenon—that intensive
quenching prevents crack formation, decreases distortion,
and increases mechanical properties of the materials—
new opportunities are now available for heat treaters [1–3].
These problems are discussed in detail in many chapters
of this book. Chapter 2 contains a discussion of the so-called
self-regulated thermal process, which controls the temperature
field and microstructure formation when the heat transfer
coefficient approaches infinity. This is obvious since at
1 IQ Technologies, Inc., Akron, Ohio, and Intensive Technologies Ltd., Kyiv, Ukraine
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