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Retained Austenite rial H THE HEAT TREAT DOCTOR • • T Daniel H. Herring | 630.834.3017 | [email protected] H A Discussion of Retained Austenite hat is retained austenite temperature; that is, low enough to form How does RA behave? and how does it affect 100% martensite. Because the Mf is Martensite is hard, strong and brittle the properties of a com- below room temperature in alloys con- while austenite is soft and tough. In ponent? How much, if taining more than 0.30% carbon, signifi- some instances, when combined, the any, retained austenite should be present cant amounts of untransformed, or mixture of austenite and martensite cre- in a particular component microstruc- retained austenite, may be present, inter- ates a composite material that has some ture? Is the presence of retained austenite mingled with martensite at room temper- of the benefits of each, while compensat- in a microstructure a good thing or a con- ature (Fig. 1). Retained austenite is a spe- ing for the shortcomings of both. cern? These are questions that metallur- cific crystalline form of iron and steel. The For any given application, mechanical gists have spent countless dark-colored needles shown are tempered properties are affected by a high percent- hours debating. What do we martensite crystals and the light-colored age of retained austenite content. For as heat treaters need to areas are retained austenite crystals. The example, retained austenite affects the know about retained austen- amount of retained austenite is a function following properties of bearing steels: ite and how is retained of the carbon content, alloy content austenite viewed by various (especially nickel and manganese), quen- • Dimensional stability: Retained industries? Let's learn more. chant temperature and subsequent ther- austenite will transform to marten- mal and/or mechanical treatments. site if the temperature drops signifi- What is retained Depending on the steel chemistry and cantly below the lowest temperature austenite? specific heat treatment, the retained to which it was quenched, or if the Austenite that does not transform to austenite level in the case can vary from room temperature austenite is sub- martensite upon quenching is called over 50% of the structure to nearly zero. jected to high levels of mechanical retained austenite (RA). Thus, retained While large amounts of retained austen- stress. Martensite, a body centered austenite occurs when steel is not ite (>15%) can be detected and estimat- tetragonal crystal structure, has a quenched to the Mf, or martensite finish, ed by optical microscopy, specialized larger volume than the face centered equipment and techniques, such as x-ray cubic austenite that it replaces. diffraction methods, are required to Where transformation occurs, there accurately measure the amount of will be a localized 4-5% increase in retained austenite to as low as 0.5%. the volume of the microstructure at room temperature and a resulting Why is retained austenite dimensional change in the geometry problematic? of the component. If great enough, The very characteristics that give re- this dimensional change could lead tained austenite many of its unique prop- to growth and in severe instances, erties are those responsible for significant crack initiation. problems in most applications. We know • Fatigue: Low retained austenite con- that austenite is the normal phase of steel tent and fine austenitic grain sizes, at high temperatures, but not at room which create a microstructure of fine- temperature. Because retained austenite ly dispersed retained austenite and exists outside of its normal temperature tempered martensite, prevent nucle- range, it is metastable. This means that ation of fatigue cracks, or retard when given the opportunity, it will fatigue crack initiation until very change or transform from austenite into high stress levels are reached. In con- martensite. In addition, a volume change trast, low-stress applications that frac- (increase) accompanies this transforma- ture at low cycles are related to high Fig. 1. RA present in a case carburized component [1]. Photomicrograph courtesy of Alan Stone, tion and induces a great deal of internal retained austenite levels and coarse Aston Metallurgical Services (www.astonmet.com). stress in a component, often manifesting austenite grain sizes. For example, Etchant: 2% nital. 1,000× itself as cracks. one type of fatigue strength of interest 14 March 2005 – IndustrialHeating.com rial H • • T H Dimensional Stability HigherB Lower A L Fatigue LowerA Higher strength N C Impact E Lower Higher strength 0% 20% 40% Fig. 2. (left) Balancing properties and RA content [2] Increasing retained austenite content Fig. 3. (above) Gear tooth failure due to spalling (macropitting) to many users is rolling contact fatigue. ness to resist wear. Conclusion Two aspects of retained austenite can The bearing and gear industries have a By controlling the level of retained austen- improve rolling contact fatigue life. First, more favorable view toward having some ite, its beneficial effects can be realized the inherent ductility of retained austen- retained austenite (5 to 30% determined without suffering from its negative influ- ite helps to delay crack growth by blunt- by optical metallography, usually by com- ences, such as excessive dimensional ing the tips of cracks as they form. parison to known standards). While some growth. Many industries have found a Second, as retained austenite transforms of the same mechanisms that affect tooling “sweet spot” exists for retained austenite during service, compressive residual applications also affect gears, there are content to achieve a balance of stresses increase in the case. These com- some major differences. Gears are typically fatigue/impact strength and dimensional pressive stresses delay crack growth by made of case-hardened steel, which has stability. functioning like a vise and clamping high impact strength. While most tools To obtain the optimum level of retained cracks closed. These benefits are not fail by wear or fracture, many gear failures austenite requires a delicate balance of con- present in a part with insufficient are the result of spalling in the tooth area. trols and must take into account such items retained austenite. Spalling is progressive macropitting that as material chemistry and heat treatment • Impact: Impact strength is the measure of occurs when pits coalesce and form irregu- process variables. These variables include the ability of a steel to resist fracture when lar craters, which cover a significant area steel chemistry, carbon content, austenitiz- subjected to a sharp blow. Austenite is not of the tooth surface (Fig. 3). Spalling ing temperature, quenching rate and tem- only very tough, but also it has higher occurs when the surface of a metal compo- pering temperature. IH impact strength than martensite. The nent is subjected to repeated cyclic loads. steel's impact strength increases with A crack forms and grows until a small por- References increasing austenite content. Higher tion of the surface breaks loose, damaging • Private correspondence,Technical Forum impact strength can provide extra protec- the surface and adding debris to the sys- 00-1 Retained Austenite: It's Impact on tion against cracking, which, in turn, tem. The gear industry balances the Bearing Performance, BRENCO Inc. helps prevent such problems as spalling. amount of retained austenite in a gear (www.brencoqbs.com) tooth to delay the onset of spalling by sup- • Krauss, G., Steels: Heat Treatment and It is important to recognize that a bal- pressing crack growth. Processing Principles, ASM International, 1990 ance must be created between the mechan- • Private correspondence, Dr. Jeff Levine, ical properties of a component and the opti- How is the percentage of RA reduced? Applied Cryogenics (781-270-1180) mum percentage of retained austenite for a Tempering is one method used to transform given application (Fig. 2). retained austenite. A key is to hold for an adequate amount of time at temperature. Additional related information may be found by searching for these (and other) key How some industries view retained Multiple tempers are often performed to words/terms via BNP Media LINX at austenite ensure the maximum amount of retained www.industrialheating.com: retained austen- Retained austenite is highly undesirable in austenite has been transformed. Other pop- ite, metastable, untransformed austenite, components for the tool and die industry. ular methods include cold treatment at - martensite finish temperature, dimensional RA is recognized as a major cause for pre- 120ºF (-85ºC) or cryogenic treatment to - stability, rolling contact fatigue, impact mature failure. The low hardness of RA is 320ºF (-195ºC). It is well documented that strength, spalling, macropitting, tempering, also incompatible with most applications as the temperature is lowered the degree of cryogenic treatment. that require the maximum attainable hard- transformation increases. 16 March 2005 – IndustrialHeating.com.
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