Quench cracks result from stresses produced during the transition from Austenite to Martensite, which involves an increase in volume.
The martensitic transformation starts at the outermost surfaces of the part being quenched. As the transformation goes deeper into the softer austenite towards center of mass, its change in volume is restricted by the martensite already created  in the outer volumes of the part adjacent to the surface.
This creates internal stresses which place the surface into tension.
When enough martensite has formed to create internal stress greater than the ultimate strength (tensile strength) of the as quenched martensite at the surface, a crack results.
As-quenched Martensite is hard and brittle- it has virtually no ductility.
Here are 3 ways to recognize a quench crack:
1) The crack runs from the surface towards the center of mass in a fairly straight line. The crack will also tend to be open or spread at the OD surface.

Quench cracks open from the surface and travel relatively directly toward center of mass.
Quench cracks open from the surface and travel relatively directly toward center of mass.

2) Quench cracks do not have decarburization apparent, since the quenching occurs at relatively low temperatures. If there is decarb associated with a crack, that shows that the crack existed at the time the material was at temperatures hot enough to decarburize. In other words, the crack existed prior to austenitizing.
Quench cracks exhibit no decarburization.
Quench cracks exhibit no decarburization.

3) The fracture surfaces will exhibit a fine crystalline structure. I remember the first time I saw a quench crack, thinking, “it crystallized.” Well, the steel is already crystalline, but the fine martensitic structure revealed by the crack showed that there was absolutely no ductility in the material…
Scale within the quench crack tells us it was open before tempering.
Scale within the quench crack tells us it was open before tempering.

Bonus tip: if you see a build up of scale in the crack itself, that tells you that the crack was there after quenching but before tempering. During the tempering operation at tempering temperature, oxygen in the atmosphere created a scale where it could reach the iron in the crack.
For more information on Quench Cracks, look at our blog posts Here and Here.

Failures of steel parts in service or production occur very infrequently. However, when steel parts fail, the consequences are dire.

Quench crack- this is not good!

Here are 7 ways that steel can fail as a result of Quench Cracking from heat treatment.

  1. Overheating during the austenitizing portion of the heat treatment cycle can coarsen normally fine grained steels. coarse grained steels increase hardening depth and are more prone to quench cracking than fine grain steels. Avoid over heating and overly long dwell times while austenitizing.
  2. Improper quenchant. Yes, water, brine, or caustic will get the steel “harder.” If the steel is an oil hardening steel, the use of these overly aggressive quenchants will lead to cracking.
  3. Improper selection of steel for the process.
  4. Too much time between the quenching and the tempering of the heat treated parts.  A common misconception is that quench cracks can occur only while the piece is being quenched. This is not true. If the work is not tempered right away, quench cracks can (and will) occur.
  5. Improper design– Sharp changes of section, lack of radii, holes, sharp keyways, unbalanced sectional mass, and other stress risers.
  6. Improper entry of the part/ delivery of the quenchant to the part. Differences in cooling rates can be created, for example, if parts are massed together in a basket resulting in  the parts along the edges cooling faster than those in the mass  in the center. Part geometry can also interfere with quenchant delivery and effectiveness, especially on induction lines.
  7. Failure to take sufficient stock removal from the original part during machining. This can leave remnants of seams or other surface imperfections which can act as a nucleation site for a quench crack.

Finally, materials that are heat treated to very high strength levels, even though they did not quench crack, may contain localized concentrations of high residual stresses. If these stresses are acting in the same direction as the load applied in service, an instantaneous failure can occur. This will be virtually indistinguishable from a quench crack during an examination, due to its brittle failure mode, lack of decarburization on surface of the fracture, or other forensic evidence of a process failure.
When looking at quench cracking failures under the microscope, cracks and crack tributaries that follow the prior austenitic grain boundaries are a pretty good clue that grain coarsening and or its causes-  overheating or too long time at temperature- occurred. Temper scale on the fracture surface helps the metallurgist know that the crack was present before tempering. Decarburization may show that the crack was open prior to quenching.
Photo1 Thanks to WIP SAMI over at British Blades for the photo.