While large heat-treated components will always contain some stress, these stresses can be minimized in order to reduce the likelihood of catastrophic failure while still obtaining mechanical properties. An added benefit to this approach is increased throughput, lowered asset constraints, and increased safety. I’d like to devote this first column to defining some of the terms that I’ll be using: “quench cracking” and “large.”
Quench cracking is typically seen as a result of several factors including: cooling too fast; uneven cooling; pre-existing stress risers; and large percentages of martensite formation. However, consider the case of a simple bar that has few stress risers, cools evenly, and contains little to no martensite, yet still cracks. With large cross-section components, quench cracks can be catastrophic, resulting in the component splitting entirely in half.
“Large” must also be defined. Large components have an increased risk of cracking due to a gradient of stresses through their thickness. These stresses develop during phase transformations and temperature differences from surface to center. Inherently, thicker sections will have increased differences in temperature upon heating and cooling. In addition, the demand for higher mechanical properties requires faster cooling, which will increase thermal and transformational stresses.
Large is relative to part geometry, grade chemistry, and heat treatment, and size ranges will be broken down into small, medium, and large. Small forgings are composed mostly of martensite. Medium forgings have a mix of martensite, lower bainite, and Upper Transformation Products (UTP). Large forgings are those, which consist of mostly UTP. Small forgings typically crack from the outside toward the center. This occurs because the DI, quench rate, and size are sufficient to produce a mostly martensitic microstructure. Martensite has a volumetric expansion upon formation. This expansion will put the surface in a tensile residual stress and the center in compression. If the stress is large enough, the forging will crack at the surface and propagate toward the center.
Medium-sized forgings have a mixture of microstructures. This mix gives the bar a better balance of temperature and transformational stresses resulting in a lower observed stress. No efforts have been put forth into investigating yet, though it is possible there are size ranges for a given material that can be quenched with Grossman quench factors approaching infinity with low risk of cracking!
Large forgings have a size and chemistry range, which restricts them to only obtaining microstructures with a majority of Upper Transformation Products. Their size and microstructure lends itself to residual tensile stresses in the center. Typical cracking occurs from the center of the thickness propagating to the surface. The crack origin location is often about half the thickness in from one of the ends (see images above). Figure 1, Figure 2
Difficulties arise with the processing of larger cross section components due to non-equilibrium conditions. Some examples are: variable cooling rates during solidification resulting in segregation; differing cooling rates during thermal processing; and variances in microstructure. Each of these variables is inherent in the processing of thick sections. These non-equilibrium conditions will create stresses in the product during processing and can lead to catastrophic failures such as quench cracking. Quench cracking is an issue that heat treaters have seen since the beginning of the art. There are multiple reasons a part can crack due to quenching that include:
• Improper steel selection
• Part design— stress concentrators
• Austenitization temperature—increases temperature differentials and possibly increases grain size thereby lowering toughness of the material
• Improper quenchant— wetting, non-uniform cooling, and quench severity
• Improper fixturing or entry into the quenchant
• Lag time from quench to temper
If we were to assume that all factors listed above are seemingly for the quench, how can we explain when cracking still occurs? Additionally, if properties are difficult to meet and a more aggressive quench is needed, how can one reduce the risk of cracking?
In my next column, I’ll discuss the reduction of transformational and thermal stresses.