The Doctor is often asked questions about tempering, from something as fundamental as “Is it necessary?” to something a bit more complex like “What are the rules when re-tempering parts?” Other frequent questions include, “How long do I have after quenching before I temper?” or “How many tempers should I perform?” These are all important questions, and every heat treater must know the answers. Let’s learn more.
What is tempering?
While the end-use application of a component determines its heat treatment, as heat treaters we are constantly striving to achieve a delicate balance between the properties of strength and ductility. Nowhere is this fine line more evident than in the tempering (aka drawing) process for a given steel, where precise control of time, temperature and (in some instances) cooling rate are critical.
Essentially, tempering (Fig. 1) is the modification of the newly formed hardened microstructure toward equilibrium. For this reason (if no other), tempering should always be performed. Almost all steels that are subjected to any type of hardening process are tempered – subjected to a subcritical heat treatment that alters their microstructure and properties (Fig. 2). In general, tempering lowers strength and hardness while improving ductility and toughness of the as-quenched martensite.
Precipitation-hardened alloys, including many grades of aluminum and superalloys, are tempered to precipitate intermetallic particles, which strengthen the metal. Tool and high-speed steels are often tempered multiple times to achieve proper hardness while transforming retained austenite first to untempered martensite and on subsequent tempers to tempered martensite.
Snap (aka Safety) Temper
Low-temperature tempering in the range of 135-150°C (275-300°F) prior to conducting a standard temper operation is often referred to in the heat-treatment industry as a “snap” or “safety” temper. It is also known out on the shop floor as “taking the curse off” the material. It is a step added to the process recipe when the time between the quench and temper operations will be longer than 1-2 hours or no longer than 15-30 minutes after quenching for high-hardenability steels. A snap temper should always be performed at a temperature lower than the final tempering temperature. Snap tempering is also commonly performed prior to an initial deep-freeze operation to minimize stress, negate geometry effects and avoid cracking.
Retempering is an operation in heat-treat shops that is often added to a process sequence for one of several reasons. One of the more common reasons for retempering is the as-hardened and tempered product exceeds the hardness aim (or range) and is concomitantly low in ductility and/or toughness. This can be a somewhat common occurrence, especially in the early history of a component’s manufacture, until an empirical dataset can be established over many cycle runs and heats of material. Most manufacturers would “rather retemper than requench.” No one wants to repeat multiple operations if a simple retemper will resolve the issue. If the initial results are out of range or too far from desired target values, the parameters for a retempering operation can be readily determined.
Retempering is performed over a wide variety of temperatures and times relative to the original tempering operation, depending on a host of material and process factors. As previously noted, tempering is a time-temperature-dependent process, and an added temper (or multiple tempers) enhances the effects of the original tempering operation.
The Hollomon-Jaffe and/or Larson-Miller time-temperature tempering-parameter equations are widely used and have been refined over the years. These equations are extremely useful and can aid the heat treater in making estimates of the effects of the original temper and the cumulative effects of any subsequent tempers on many different material properties (e.g., strength, hardness, ductility, toughness, microconstituent details). This is crucial to ensure the retempering operation will not only provide the desired corrective effects on the property of interest but will also not create any foreseen or unforeseen detrimental effects.
Caution must be exercised that a retemper does not raise the tempering temperature into the tempered-martensite-embrittlement (TME) zone. This would not only embrittle the steel, but – since TME is an irreversible phenomenon – the components would have to be re-hardened. This has the potential to create many additional problems, and it is a very easy processing trap to fall into. While the hardness and strength could be readily adjusted to a desirable level, the toughness could be significantly compromised.
A second common reason for retempering is to recover tensile yield strength when a product is cold deformed in a compressive manner during its manufacture. The working in compression during a cold-forming, cold-drawing or cold-straightening operation can trigger the Bauschinger Effect, which results from strain-hardening effects. The result is that when quenched-and-tempered material is strained in compression, the yield strength in tension decreases. Fortunately, the phenomenon is reversible and can generally be resolved via a retempering operation. Such a retemper is usually conducted at a temperature 28-56°C (50-100°F), or more, lower than the initial temper. This is done to preserve the ultimate strength/hardness while recovering the tensile yield strength by eliminating the strain-hardening directionality effects.
Caution must also be exercised when processing quenched-and-tempered steels that are susceptible to temper embrittlement (TE). A susceptible steel initially tempered at 590°C (1100°F), cold worked and then stress-relief tempered at 540°C (1000°F) could be detrimentally impacted.
Similarly, one must be aware when retempering is conducted in the range where secondary hardening can occur in the more highly alloyed steels or in precipitation-hardening materials in various alloy systems. An engineering review of the tempering characteristics of any workpiece material should always be conducted and appropriate heat-treating plans designed. This is especially true for instances where the alloy content can result in the “fourth stage of tempering” and provide significant effects to the resulting properties.
The question naturally arises as to the acceptability of retempering in a given operation. One must understand the extent to which retempering is to be considered a rework/reprocess operation. It is important to establish the proper protocols for advising specific parties of a retemper when one is conducted. One must be aware of the governing specification(s) for the product and operations at hand, those of the processor and those of the customer. It is good practice to have these protocols predetermined.
An excellent example is provided by AIAG in the current CQI-9 specification, which states the following relative to heat-treat processing and retempering. “Question: Is the OEM customer to be notified when parts are reprocessed?” Requirements and Guidance (partial list, paraphrased):
• The OEM is to be notified when parts are reprocessed
0 Notification on a case-by-case basis is preferred
• Some reprocessing (such as, but not limited to, retempering operations) may be preapproved
0 To be preapproved, the heat treater:
• Shall submit a reprocessing procedure for approval by the OEM
• The procedure shall describe process characteristics for which reprocessing is permissible and those for which it is not
• Any reprocessing activity requires a new processing control sheet, which denotes the necessary heat-treat modifications
• Records shall show when and how the material was reprocessed
Retempering can be, and often is, a commonly employed process in heat-treatment operations for intermediate and final products for a large number of industries and various alloy systems. The heat treater must know and understand the metallurgical properties and thermal-processing characteristics of their workpiece materials if retempering is employed.
When it comes to tempering, there seems to be resistance to performing multiple tempers simply due to the delay involved in releasing the final product back to manufacturing. In this writer’s eyes, multiple tempers are always advantageous, unless very long tempering times are involved.
References available online
1. Herring, Daniel H., Atmosphere Heat Treatment, Volume I, BNP Media Group, 2014.
2. Herring, Daniel H., Atmosphere Heat Treatment, Volume II, BNP Media Group, 2015 (in preparation).
3. Krause, G., Steels: Heat Treatment and Processing Principles, ASM International, 1990.
4. Herring, Daniel H., “What Do We Really Know About Tempering?,” Industrial Heating, July 2007.
5. Brandt, Daniel A., Metallurgy Fundamentals, 4th Edition, The Goodheart-Wilcox Company, Inc., 1984, pg. 216.
6. Canale, L. C .F., Yao, X., Gu, J. and Totten, G .E. (2008) “A historical overview of steel tempering parameters,” Int. J. Microstructure and Material Properties, Vol. 3, Nos. 4-5.
7. CQI-9, 3rd Edition, AIAG (Automotive Industry Action Group), issued January 2012, last update November 2013.