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.

And how to minimize them.
Upon heating, steel parts change volume as they change crystal structure (I’ll spare you the technical terms and details). When these heated parts are quenched, their internal crystal structure changes again, and that volume change is not necessarily sufficient to offset the change upon heating. This change of volume can cause  dimensional distortion.  The rule of thumb that I have used for medium carbon alloy steels is  to expect a change in linear dimensions of about 0.125% maximum. That is, one eighth of a percent of the linear dimensions could be the change encountered  from heat treatment and quench. It generally is less, but 0.125% gives me a rule of thumb to evaluate capability to hold dimensions after heat treat. What rule of thumb do you use to estimate part growth as a result of heat treat?
Warpage or shape distortion  as a result of heat treat is different because it is usually a result of process and design issues rather than the expected phase changes of the material.

Salvador Dali understood!
Salvador Dali understood!

Here are 8 reasons steel parts can warp upon quench and tempering:

  1. Rapid heating.
  2. Overheating.
  3. Non-uniform heating.
  4. Non-uniform cooling.
  5. Non-uniform agitation.
  6. Water contamination in oil. 
  7. Large changes of mass and section.
  8. Asymmetric features.

Rapid heating can cause stresses to develop in parts due to excessive temperature gradients. Overheating similarly lowers mechanical properties,  potentially leading to parts sagging or creeping depending on orientation in the furnace. Non-uniform heating also creates differences in properties within the parts as well as leading to incomplete transformation products or hybrid structures upon quenching. Non-uniform cooling allows unbalanced stresses to develop during the quench, as does non-uniform agitation of quench medium.  Often non-uniform heating or cooling result from the way parts are stacked or piled in the basket or on the belt such that gradients of temperture are created. Water contamination in oil. This is difficult to figure out, but in addition to warped parts, inconsistent hardness readings between parts or on the same part are a sign of this. Parts with large section changes or that have asymmetric features are also more likely to warp than parts with balanced and uniformly distributed mass, regardless of process control.
Choosing steels with higher hardenability (alloys rather than plain carbon steels), finer grain size, and paying attention to the details of loading, time at temperature, and quenchant delivery are all steps that can minimize warpage distortion, even when part design is less than optimum.