IHTS Case Studies
Shorter Carburizing Cycles For Case Hardened Steel Parts
Reduce or eliminate long-batch carburizing cycles. The higher the residual compressive surface stresses, the longer the part cyclic fatigue life that can be expected. This is because the surface compression holds the part like a die and this compressive force must be overcome before the part will begin to bend and then fatigue.
The reason that case hardening of a martensitic steel part imparts surface compressive stresses is the crystalline structure in the martensite shell is Body Centered Tetragonal Iron (BCT). BCT grains have a larger volume than the Face Centered Cubic structure in the austenitized surface layer. The larger volume BCT grains literally press against each other and the contiguous grains in the “case” creating hardness and strength under compression. The martensite not only imparts hardness (strength), but the compressive stresses will also help resist bending fatigue in the hardened layer.
One way to increase surface compression is to case harden the part by heating only the shell of the part (by selective induction or flame heating) to the austenitizing temperature, keeping the core cold, then quenching to produce a martensite shell. Since the core remains soft, there is no martensite phase change swelling to BCT to reduce the compression in the martensite surface layer.
Beneficial surface compressive stresses can also be induced into the surface of the part by mechanical shot peening after hardening.
A third way to impart beneficial compressive surface stress is through carburizing the surface, adding carbon atoms to the steel part surface layer then hardening that layer by quenching. During the high temperature carburization process a carbon rich gas atmosphere diffuses carbon atoms in the part surface layer raising the percentage of carbon from the base carbon steel (usually .10% C to .20% C) to .70% C to 1.10% C.
The higher the carbon percentage in the case gradient, the greater the hardenability of the austenite will be when quenched from the austenitizing temperature (~ 1550○F / 843○C). When a case carburized part has between .65% C to 1.10% C diffused in its surface layer and the carbon is diffused into the shell to required case depth, it is ready to be quenched. The additional atoms of carbon also adds volume to the grains creating compression in addition to the higher volume martensite BCT grains.
As shown below, the higher the percentage of carbon, the higher the hardness in the steel as quenched. In addition, the faster the hot austenite is quenched for a given level of carbon diffused into the case, the higher the as quenched hardness in the case at that depth of diffusion (a minimum 50 HRC is usually considered to be the measure of the “Effective Case Depth” (ECD). The oil quench needs approximately .30% C to achieve a minimum level of 50 HRC in the case.
The intensive water quench only needs .02% C in the case to achieve 50 HRC minimum for the same ECD. This means that an intensive water quench cooling rate when applied to a carburized part significantly reduces the carburizing cycle times by 30% to 50% compared to a traditional oil quench to achieve a minimum hardness of 50 HRC ECD.
IQ Process Reduces Carburization Cycle
Correlation between Steel Hardness and Carbon Content
For a given carbon content, the IQ process provides greater hardness compared to conventional oil or water quenching. Effective Case Depth (ECD = 50 HRC) achieved with lover % carbon in the case gradient.
The physical mass of the carbon atoms diffused into the case also increases the compressive residual surface stresses in the hardened martensitic case. Combined with the austenite to martensite phase change expansion, the added mass of the carbon atoms increases the volume of the crystalline structure that puts the case under higher compression than it would be for a “through-hardened” part of similar carbon content. As the surface shell quenches and martensite phase change volume expansion occurs, and like layers of an onion, the layers below will also expand. Depending on the thermal gradient and the timing of the formation of layers of martensite on the surface shell, the swelling below the shell will partially or fully cancel the compression formed in the layers above. This phase change expansion below the surface and cancellation of compressive stress in the layers above can be so complete that depending on the timing of the expansion relative to when the shell was formed, in high hardenability
Steels the core swelling can blow off the hardened surface layer and crack the part. Therefore, if a part is through-hardened, the initial surface layer of compression (formed as the surface shell cooled to higher volume martensite) can be cancelled as the part layers below cool to the martensite start temperature and begin to swell. So a higher hardenability “better alloy” steel used for either a case hardened part or a through-hardened part can be detrimental to the formation and retention of beneficial compressive surface stresses and reduce cyclic fatigue life. This lower cyclic fatigue life is due to a combination of the higher core hardness that has less ductility and also the reduction of residual compressive stresses that resist the bending fatigue at the surface.
The way to overcome the cancellation of the beneficial surface compressive stresses in the surface shell from the core swelling in high hardenability steels (e.g., 52100) is to form high “current” compressive surface stresses in the martensite surface shell as fast as possible over the hot core while it is still thermally swollen austenite. To create this very high thermal gradient between the cold martensite surface shell and the thermally swollen austenitic core is to uniformly and intensively cool the shell. A uniform and intensive cooling rate at the surface very quickly forms the martensite shell and creates high “current” compressive stresses in the surface shell. As the thermally swollen core shrinks, it will draw down the surface shell under even higher compression.
If the intensive water quenching surface cooling rate is then interrupted when the core is still above the martensite start temperature, and the part allowed to finish cooling in the air, the transformation in the core is slowed, and the associated core swelling at martensite start is likewise slowed. If the intensive water quenching surface cooling rate is then interrupted when the core is still above the martensite start temperature, and the part allowed to finish cooling in the air, the transformation in the core is slowed, and the associated core swelling at martensite start is likewise slowed.
The current compressive stresses in surface shell also hold the part like a die over the hot, still plastic, core. Once this hard and uniform surface shell is formed, it holds the core as it cools by uniform conduction. The uniform cooling of shell and core make the size change more predictable and consistent. Uniformly and intensively quenched gears can actually be machined before heat treat so that they will “distort to fit” after quenching into a near net shape that needs less hard machining or grinding.
The combination of high residual compressive surface stresses the finer grain from a given alloy of steel and low predictable distortion are perfect for longer fatigue life as well as lighter higher power density parts that need less post- hardening processing (e g hard machining or grinding).
The advent of high quality “limited hardenability” (LH) steels yields this same combination of benefits cited above, but also eliminates the need for the long batch carburization cycle. Ultra-low alloy plain carbon steels with between .60% C to 1.00% C, when through-heated and then uniformly and intensively water quenched, can produce a case hardened surface layer with extremely high residual compressive surface stresses, and a properly toughened core. The elimination of the batch carburization cycle also makes in-line, single-part flow a reality for “case hardened + core toughened” parts. With single-part induction through- heating, automated part handling and uniform and intensive water quenching, the complicated atmosphere generators and controls for the carburization process are also eliminated; further leaning out the manufacture of case hardened parts.