One of the greatest concerns in the heat treating process is
the cracking of finished and semi-finished material during quenching. Many
times, the only source of cracking considered is the quenchant. However, there
are various sources of steel cracking other than the quench which must be
considered.
One of the greatest phenomena of most ferrous materials is
their ability to be heated and cooled to produce higher physical properties.
This procedure consists of heating the metal to a high temperature
(austenitizing) for a specified time to complete transformation to austenite
and diffusion of constituents and then cooling in a quenching medium that
produces the desired microstructure and as-quenched hardness.
This hardening treatment is most often followed by a lower
temperature heating process (tempering) for stress relieving and finalizing the
required microstructure to achieve the necessary physical properties. This
sequence is illustrated in the Figure below.
Figure - Diagram of
hardening and tempering cycle.
Unfortunately, when cracking is encountered, it is often
attributed to the severity of the quenching medium without microstructural
verification. Although excessive quench severity is often the cause of quench
cracking, there are many other sources that must also be considered. Here are
the various sources of steel cracking during heat treatment including:
A. Quench
Cracking Related to Severity of Quench
B. Quench
Cracking Related to NonUniform Heat Transfer
C. Prior Steel
Structure
D. Non-Uniform
Heating
E. Excessive
Heating Rate
F. Transformation
Temperature Range
G. Stress Risers
from Prior Machining, Laps and Seams
H. Chemical
Segregation – Banding
I. Porosity
J. Alloy
Depletion
K. Improper Steel
Chemistry
Although quench cracking of steel may arise from
insufficiently low quench severity, there are numerous other potential
contributors to this problem. They include: non-uniform quenching due to poor
system design, racking procedures that inhibit uniform quenchant flow around
the part during the quench or contaminated quenchants.
However, other potential sources of cracking are due to
mechanical or material flaws which include: non-metallic inclusions, laps or
seams, stress risers from prior machining, alloy non-uniformity and porosity.
These problems are not readily detectable without microstructural
characterization. Therefore, it is recommended that metallurgical analyses be
conducted to determine the root cause of steel cracking during heat treat
processing.
