Austempering is a unique process which both heat treaters and parts designers should understand; especially the factors that influence both process parameters and part properties. Let’s learn more.
What is Austempering?
Austempering is essentially an arrested quench process designed to produce a bainitic microstructure (Fig. 1) having properties that combine high hardness with toughness, resulting in a resistance to brittle fatigue. Austempering involves an isothermal transformation at a temperature below that of pearlite formation and above that of martensite formation. Advantages include higher ductility at high hardness, increased strength and ductility at a given hardness, increased toughness (over 40 HRC), greater fatigue life and less distortion and cracking (especially in higher carbon steels). Limitations include section size and carbon level (for a given steel).
Material and Process Parameters
Several factors must be considered when specifying a steel for austempering. Figure 2 shows the influence of carbon and manganese content on maximum section size.
For example, a properly austempered 1062 steel (0.63% C, 1.08% Mn) achieves a typical bainitic structure with hardness of 52.5 HRC. By comparison, a 1060 steel (0.62%C, 0.87% Mn) can have a mixed microstructure (martensite, bainite and intermediate transformation products) with the same hardness value.
Other factors to consider include part design parameters; post heat treatment operations; heat treat equipment; and process parameters such as time, temperature, furnace atmosphere and quench variables (bath chemistry, temperature, immersion time, and rate of heat transfer to the parts from the quench media).
Part thickness is an important variable in austempering. As hardenability of the material increases, section size can also be increased. High hardness and high ductility levels are limited by the carbon level in the steel as shown in Figure 3.
Austempering can produce better mechanical properties than conventional quench and temper if the hardness is greater than 40 HRC as shown in Figure 4.
For a hardness lower than 40 HRC, distortion and other factors become the prime consideration. For example, impact strength can be increased by as much as 300%. Some materials and part shapes are more adaptable to one type of equipment than another (salt-to-salt or atmosphere-to-salt). The factors mentioned above usually are specified by the part designer.
Austenitizing conditions that influence final properties or part functionality are heating method, temperature and time at temperature. Generally, published austenitizing temperature ranges are used. Where high hardness and ductility are required, it is desirable to get all carbides into solution during austenitizing. Where wear resistance or minimum distortion are required, the goal is to dissolve only enough carbides to meet the hardness and obtain a bainitic microstructure, and to austenitize at the lowest possible temperature and quench into the highest possible bath temperature.
Heating parts in a salt bath results in some decarburization. In this case, carbon correction should be made prior to austenitizing. Salt-bath austenitizing is best suited for minimum distortion, especially of long, slender parts. Austenitizing in a furnace under protective atmosphere (e.g., endothermic) allows controlling carbon content in the part using conventional techniques (oxygen probe, infrared, dew point), and is better suited for small, symmetrical parts that can be hopper fed, and which if treated in a salt bath are prone to carrying austenitizing salts into the quench. Austenitizing in a controlled atmosphere typically allows using lower hardenability steels having 5-10% thicker sections than salt bath processing.
The desired strength (hardness) level governs the salt quench-bath temperature. Immersion time in the bath is a function of the desired part hardness, material chemistry and section thickness (Table 1). In general, immersion time decreases as the transformation temperature increases, and transformation time at the same transformation temperature increases with increasing steel carbon content.
Factors that favorably influence heat transfer from parts in the salt quench bath include a long free fall through the quench medium and/or agitation of quench medium or parts and water additions to the quench medium. Water additions to the salt bath improve heat transfer from the parts at times when steel of marginal hardenability or large sections are used. Unfavorable factors are contamination of the quench bath (soot, scale, high heat salt carryover) and loss of water.
Rules of thumb
The following rules are recommended for austempering to a given hardness value.
Rule 1: Achieve a 100% bainitic microstructure.
• The section size that will through harden is a function of both the material hardenability and the equipment being used (e.g., quench agitation, water addition, etc.).
• Critical diameter calculations are required (critical diameter is the diameter of a cylindrical bar that just quenches to the desired microstructure at the center).
• Material and process factors should be empirically determined.
• Raise the austenitizing temperature toward the upper end for a steel having borderline hardenability.
Rule 2: Operate the quench bath temperature at Ms +25°F (+ 14°C).
• Calculate the martensite start (Ms) temperature using commercially available methods.
• A higher quench bath temperature produces lower strength and hardness; a practical salt-temperature equipment limitation is ~750°F (400°C).
Rule 3: Maintain a minimum time of 20 minutes in the quench (under the salt).
• Carbon content is the single greatest influence on time (e.g., a 0.50 C steel part has a 2-4 HRC difference in surface hardness compared with an 0.80 C part).
Rule 4: Use a slightly carburizing atmosphere (i.e., high carbon bainite is desirable)
• Decarburized areas require a higher Ms temperature.
• It is better to avoid pearlite than martensite.
Rule 5: Check your results.
• Use 10% sodium metabisulfite etchant to selectively color etch to differentiate between martensite and bainite; martensite is tinted brown and bainite tinted blue.
• Use Villela’s reagent to resolve bainite.
• Use a 2% nital (fresh solution) etchant to determine microstructural constituents; bainite has straw color while
pearlite appears as little specs of blue, black or gray (due to different etching response).
Austempering is an oft times overlooked process which offers great value where parts require a combination of high hardness and high ductility. Austempering is also of great benefit for parts requiring less heat treat distortion or dimensional variation and when breakage must be kept to a minimum.
1. Suffredini, R.L., Factors Influencing Austempering, Heat Treating, Jan., 1980
2. Austempering of Steel, ASM Handbook,Vol. 4: Heat Treating, p 152-163, 1991
3. Dr. Kathy Hayrynen, Applied Process (www.appliedprocess,com), private correspondence