Martempering and marquenching are terms often associated with hot oil quenching. Formally, martempering (full martempering, true martempering) is a term applied when an austenitized work piece is quenched into a medium whose temperature is essentially maintained in a bath just above the martensite start (Ms) temperature of the steel and held in that medium until its temperature is uniform throughout - but not long enough to permit bainite (or pearlite) formation - and then allowed to cool in air (Fig 1a). When the martempering process is applied to carburized material, the controlling martensite start temperature is that of the case and as such this process variation is called marquenching.
.....A modified form of these processes is called hot oil quenching (modified marquenching) and takes place just below the martensite start temperature (Fig. 1b). In many steels the required critical cooling rate is such that faster quenching than that possible by marquenching or martempering is necessary to obtain full hardness. It has also been found effective in reducing quench stresses and improving part dimensional stability. But how is hot oil quenching best accomplished in the real world? Let's learn more.
What is Hot Oil?Quenching into oil above 212°F (100°C) has traditionally been referred to as "hot oil" quenching. Oil temperatures in the 195°F - 450°F (90°C - 230°C) range have been used with both ends of the spectrum normally reserved for special applications. Hot oil temperatures in the 220°F - 375°F (105°C - 190°C) range are common in the heat-treating industry and take place in either batch or continuous atmosphere or vacuum-sealed quench furnaces.
Part DistortionDistortion arising from heat treatment can be classified as two types: size distortion due to volume changes resulting from phase transformations (which are generally predictable) and shape change or warpage (which is generally unpredictable). The latter takes many forms including out-of-round, out-of-flat, bending, bowing, bucking, taper, dishing, closing-in of bores and can arise either during heating or austenitization through the relief of internal stresses from prior operations; by sagging or creep due to inadequate part support during heating; mechanical damage; non-uniform heating; or during quenching, the result of an imbalance of the internal residual stresses generated. A number of key factors influence these changes including:
In the real world we find that cooling rates vary widely based on the type, temperature, condition, and circulation of the quenchant, which changes daily. Due to the fact that cooling rarely (if ever) occurs uniformly on the component part, we find that different quenchants produce different time durations throughout the three stages of liquid quenching (vapor phase, boiling phase, convective phase) and have a direct bearing on distortion.
Operating temperature can have a dramatic influence on distortion; hence oils have been developed for martempering and marquenching applications to minimize residual stresses by promoting uniform transformation. Hot oils are generally applied to high precision engineering components such as thin-section bearing races, and transmission gears and shafts requiring critical dimensional control. The condition of the quenchant, which for hot oils involves oxidation, contamination, and degradation, dramatically influences results since viscosity increases. It is not uncommon to filter or centrifuge hot oil baths daily and to replace the oil every 12 to 18 months. With hot oil, the cooling rate in the convective phase is increased significantly with agitation, so excessive agitation is to be avoided to counter distortion. The speed and direction of oil flow over the workpiece can also determine the nature of the distortion.
Oil SelectionThe choice of oil depends on the need and is dependent on such factors as:
For example, leaner alloyed parts usually require lower viscosity martempering oil often with speed enhancers added. Higher alloyed parts usually require higher viscosity oils and certain additives allow higher oil temperatures where distortion control is maximized. Thin distortion prone components are best martempered or quenched into the slowest speed martempering oil, which will produce the required metallurgical properties.
Hints for Extending Oil LifeThe hotter the oil's temperature, the better the distortion control but the faster the oil's degradation. Also, the higher the austenitizing temperature from which a part is quenched, the more damage to the oil and the faster the oil will deteriorate. Other common concerns are oxidation and slug/contamination buildup. These can be minimized to a degree by the addition of antioxidants and with the use of a protective atmosphere cover (such as a nitrogen blanket) over the oil during heatup and operation. Oil without antioxidant additives will give the brightest and most consistent part surface appearance but will oxidize rapidly, then discolor the work. Antioxidant additives will normally produce a consistent surface finish while extending the oil's useful life. Fresh or make-up oil can be added to further reduce oil degradation. A hidden danger is heating marquench oil up too rapidly, which can also degrade the oil (low velocity burners or low watt release resistance heaters should be used).
It is important to note that oil capacity is not always an assurance of success. For example, parts run in continuous furnaces discharging 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.
Effect of Oil Changes Over TimeAs the oil ages, its heat extraction capabilities will change, influencing the end product gradually over time. In one study, during the life of a hot oil, the cooling characteristics changed progressively (Fig. 2). As seen, the vapor blanket phase disappeared and the maximum quench rate increased (and occurred at higher temperature) resulting in a slowing down of the quench rate at lower temperature.
Finally, when changing to marquenching oils, it is important to recognize that the design of the quench tank may have to be changed or optimized for their use. Previous quenchants, sludge and water should be removed from the tank prior to introduction of the hot oil. Remember that the oil will expand and its viscosity change during heating. Protective gas atmosphere over the oil should always be used during initial heatup, and exposure to air must be minimized at all times.
Final ThoughtsMartempering oils, when applied properly, can improve part distortion control and minimize rejects due to such issues as part cracking. Martempering oils can replace other quenchants, but testing should be performed to balance the various factors discussed and to optimize their performance and life. Reduction of part production cost should be possible where the use of oil has advantages over other quenchants. Safety issues are little different than that with any other oil. IH
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