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)
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
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)