There are many aspects to the troubleshooting of heat-treatment problems. In fact, that may not be quite a true statement as there are many factors that contribute to heat-treatment problems. The problems are not necessarily created by the heat-treatment procedure; the problems can be created prior to heat treatment.

Given below are some of the contributing factors that can cause a problem to arise at the heat-treatment operation:
  • Steel chemistry
  • Steel quality (grain size, nonmetallic inclusions, oxides)
  • Hot rolling or forging practice
  • Temperature control at forging
  • Mechanical design (sharp corners, sectional thickness changes, keyways, threads, holes and many other design features)
  • Normalizing practice
  • Lack of intermediate stress relieving
  • Machining practice
  • Heat-treatment practice (lack of preheating, overheating, under tempering, not tempering quickly enough)
Focusing on the potential for heat-treatment problems, remember that as a heat treater you cannot stop the phase changes from occurring, which will result in a volumetric size change of the workpiece!

When heating to an austenitizing temperature for hardening, if preheating is not considered (especially for a tool steel), there will be a great potential for cracking and certainly for aggressive distortion to occur.

There is no magic formula for the temperature ramp up to the austenitizing temperature. One would need to look at the part complexity to determine how to ramp up to the austenitizing temperature. The more complex the part geometry, the slower the ramp up to temperature should be.

Overheating is another problem often encountered with austenitizing for hardening. High austenitizing temperatures can often be chosen by the heat treater to “be sure that the steel is truly in the austenitizing temperature range.” There are very significant problems with overheating, including:
  • Grain growth
  • Potential for retained austenite
  • Dissolution of alloy carbides at high temperature
  • Potential for quench cracking along grain boundaries
  • Potential for service failure
Problems at heat treatment can also be caused by non-metallic inclusions. These have the potential to lead to phosphorous segregation at the austenite grain boundaries as well as cementite particle precipitation also on grain boundaries.

Non-metallic phases such as sulfides, alumina and silicates can influence the fracture toughness of the steel in question.

Furnace temperature uniformity (particularly on tempering) will strongly influence hardness profiles. So, it is necessary to ensure that the furnace temperature control is both uniform and accurate. Temperature uniformity is extremely important during the tempering procedure. Remember that the thermocouple measures temperature only at the tip of the thermocouple. We assume that the thermocouple reading is that of the furnace. We can only safely make that assumption if there is an air circulation fan in the furnace.

Heat-treatment troubleshooting is a progressive “process of elimination.” It is necessary to start at the beginning of the life of the steel (in other words, the steel as it arrived from the mill) by checking the test certificate. The test certificate contains significant information that should be checked, including:
  • Analysis if possible (if not, proceed with the following information)
  • Mill hardness
  • Micro-cleanliness test for sulfides, etc. (ASTM E45 Method A)
  • Mill scale thickness and decarburization depth
  • Grain size
  • Hardenability (Jominy end-quench test)
It is not always necessary to check the analysis. If the steel does not conform to above reported values on the test certificate, there is cause for concern about that steel.