Conditions leading to quench cracking other than severity of quench

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.

Quench Cracks

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.

A major concern of the heat treater is cracking of finished and semi-finished material during the quenching cycle. Besides the monetary expense of the material involved, there is a loss of production time which is in addition to the monetary loss.

 

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.

Source: Conditions leading to quench cracking other than severity of quench