The following question caught our eye because it fits well with January’s IH Prescription Podcast.
Question: We have been facing certain problems during case carburizing of rollers used in chains. While referring to some articles on the Internet, we came across your articles. We thought of seeking your expert advice to overcome the same.
We have a facility that carburizes chain rollers of SAE 1018 steel with dimensions of 0.39 inch (10 mm) with a wall thickness of 0.06 inch (1.5 mm). We case carburize the rollers at 1616°F (880°C) to a depth of 0.006-0.008 inches (0.15-0.20 mm) using low-pressure gas in a rotary-retort furnace. The rollers are allowed to soak at temperature for 40 minutes, plus four minutes of diffusion time. The parts are quenched in oil and then tempered 572°F (300°C) for 40 minutes.
In an as-quenched condition, 50% of the samples have revealed retained austenite with varied hardness values and case depths. These samples have hardness values of 74-76 HRA. After tempering, however, these same pieces exhibit a hardness of 72-75 HRA. The remaining 50% of samples exhibit no retained austenite and have hardness ranges of 78-82 HRA before tempering and 72-75 HRA after tempering. The desired hardness values are 78-83 HRC as quenched and 72-75 HRC after tempering. How can we avoid the retained austenite in order to achieve the desired hardness values?”
The quench oil volume is 800 gallons (3,000 liters). However, the rollers are dropped into a 20-inch (500-mm) diameter perforated conical basket. Each charge is of 220 pounds (100 kg). The oil is well agitated and cooled by a plate-type heat exchanger. Generally, the rise in temperature is 20-22°F (12-13°C), which is cooled by 24°F (10˚C) in 45 minutes. During carburizing, LPG is let in at a rate of approximately 2 cfh (0.36-0.38 lpm), and the retort is maintained at 14 rpm. During weekends, we switch off the furnace and restart on Monday morning. We purge the furnace retort with LPG for 30-40 minutes after attaining the set temperature. However, we find inconsistency in the results (hardness) in the first 3-4 batches even though the process parameters are strictly followed.
Tempering is carried out in a continuous-type mesh-belt furnace maintained at a temperature around 572°F (300°C), and soaking time in the furnace will be approximately 40 minutes.
Answer: Austenite that does not transform to martensite upon quenching is called retained austenite (RA). Thus, RAoccurs when steel is either not fully quenched or does not reach the Mf temperature (i.e. martensite finish – low enough to form 100% martensite). Because the Mf is below room temperature in alloys containing more than 0.30% carbon (including carburized alloys), significant amounts of untransformed, or retained austenite, may be present, intermingled with martensite at room temperature.
Retained austenite will transform to martensite if the temperature drops significantly below the lowest temperature to which it was quenched or if the room temperature austenite is subjected to high levels of mechanical stress. Martensite, a body-centered tetragonal (BCT) crystal structure has a larger volume than the face-centered cubic (FCC) austenite that it replaces. Where transformation occurs, there will be a localized 4-5% increase in the volume of the microstructure at room temperature and a resulting dimensional change in the geometry of the component. If great enough, this dimensional change could lead to growth and, in severe instances, crack initiation.
Performing a deep freeze on the parts at temperatures in the neighborhood of -120°F (-84°C) will ensure transformation of retained austenite to untempered martensite. A subsequent tempering operation is then performed to transform untempered martensite to tempered martensite. In those instances where exceptionally low levels of retained austenite are desired, multiple deep-freeze and tempering operations can be performed. In addition, a snap temper at approximately 300°F (150°C) is recommended to avoid inducing additional stress into the material.
It is important to note that oil capacity alone is not always an assurance of success. For example, parts run in continuous furnaces, which discharge parts into quench chutes, may see problems with low hardness or staining due to breakdown (fractionation) of the oil in a small localized area, lack of proper heat extraction or poor oil circulation in the quench chute. Cooling systems should be sized to handle the heat extraction and should be free of copper and other materials known to be catalysts for oxidation of oil products.