Causes of Quench Cracking in Steel

27 July 2018

Quenching is an essential stage in heat treatment technology. The rapid cooling of a hot workpiece stops unacceptable low-temperature processes, including the microcrystalline alterations that would otherwise compromise the procedure. Hardness quotients drop while overall material toughness rises, all by applying a well-controlled quench. However, there are trade-offs to be made when steel parts are exposed to fluid-contact work. Among them, quench cracking can frustrate the finest heat treatment operations.

What is Quench Cracking?

By “trade-offs,” we’re referring to the hardness-to-toughness quotient that occurs when the steel part is rapidly cooled. Done properly, a desired amount of hardness is offset by an equally important measure of material ductility. Otherwise, the steel would fracture as soon as it was put to work. But this is a dynamic stage. The fluid sizzles, the metal cools right away, and there are energetic forces in play as the part returns swiftly to room temperature. Sometimes, even in a well-regulated operation, the conflicting energies tear at the metal so strongly that it fractures.

Material Expansion and Contraction

In this case, with steel as the workpiece, the part expands as it reaches its maximum transformative phase. By lowering it into a cooling medium, the metal instantly contracts. Quench cracking in steel parts can take place when those conflicting energies rupture the metal’s microcrystalline form. But wait a moment, the clashing thermal loads are dynamic, but are they the sole reason for this undesirable phenomenon? After all, steel is graded to handle the heat, plus the cooling medium that quenches the component, so why is a known contraction incident cracking the steel?

Eliminating Processing Errors

The steel part should be lowered promptly into the quenching pool. If it enters the water or oil too slowly, then cooling takes place in the submerged material, not the still super-heated metal. Furthermore, the quenching station must insert the workpiece at a planned angle of attack. If the part enters the medium at a poorly assigned approach angle, then additional stresses are likely. The steel cracks. Even the quenching medium influences an acceptable cooling curve. Oil and water are both popular cooling fluids, but then there’s brine water, polymers, and more. By selecting the correct fluid, we ensure a safely regulated heat dissipation factor.

In the processing line, equipment misalignments and timing woes cause quench cracking in steel components. Overheated steel also fractures, with the cooling medium pushing the alloy beyond its material limits. The quench fluid is next, then there’s the geometry of the component, its profile and sectional mass. Avoid this expensive mistake by planning for all of these crack-propagating factors.

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