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.

Hardness of Quench and Tempered alloy steels is a function of the tempering temperature. The higher the tempering temperature, the lower the hardness.
This is called an inverse relationship.
And it’s why some people call tempering “drawing.”
The temper “draws the hardness out of the steel.”

Normalized at 1600F, Quenched in oil 1550 F, Tempered 2 hours

 These curves are a rough approximation of the as tempered brinell hardness for the grades shown. For example, I have other data for 4340 that shows 440 BHN at 800F; 410 at 900F; 380 at 1000F; 340 at 1100F, and 310 at 1200F temper temperature.
Fire! Can't do a blog on heat treat without a picture of fire.

Your mileage may vary, in other words, but this graph is close enough for ‘considered judgement.’
Additional 4140 data that I have from my notes suggests 397 BHN at 800F; 367 at 900F; 335 at 1000F; 305 at 1100F and 256 at 1200F.
If you have better data from your process – USE IT.
Better yet, if you have time, send a sample to your heat treater for a pilot study.
In the absence of data from your process, the above figure and data will give you “a place to stand” in understanding what is possible when heat treating .40 carbon alloy steels- the steels most commonly encountered in our precision machining shops for Automotive, Aerospace, Agricultural and general applications.
Here is a video from PMPA member company Nevada Heat Treating to give you an inside look at what goes on at a heat treat service provider.

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.

Temper colors of 0.95% carbon steel at temperatures indicated.

In steels, tempering is reheating hardened steel to some temperature below the lower critical temperature for the purpose of decreasing hardness and increasing toughness.
(The lower critical temperature is the temperature of the austenite-to-pearlite eutectoid transformation in steels- below this temperature austenite does not exist.)
Tempering is also sometimes applied to normalized steels. For the same reasons- decrease hardness and improve toughness.
The chart above shows the colors that are elicited by tempering a 0.95% carbon content steel at the temperatures shown. (Think drill rod.)
I saved this chart back in my youth from a Bethlehem Steel Handbook.
This is what we here at PMPA call “Knowledge Retention” and “Tools You Can Use.”