What Do We Really Know About Tempering?
Tempering, also known as “drawing,” is one of the most common heat-treatment processes – and one that is all too often taken for granted. We all know it’s important, yet we spend little time focused either on the process or on the equipment in which it is performed. Let’s learn more.
What is Tempering?When we temper a part, we tend to focus on a single parameter – hardness as a determination of the success or failure of the tempering operation. We must broaden our perspective and understand that tempering is done to “toughen” the steel that has been previously hardened or normalized. The as-quenched microstructure of steel being primarily martensite is highly unstable and in a strain-induced state. The resulting change of martensite during tempering into a mixture of cementite (Fe3C) and ferrite typically results in an increase in grain size and a decrease in volume as a function of increasing tempering temperature.
Precipitation-hardened alloys – including many grades of aluminum and superalloys – are tempered so as to precipitate intermetallic particles that 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 runs to tempered martensite. What is important to remember in all of this is that tempering is the modification of the hardened microstructure toward an equilibrium state.
Tempering temperature, time at temperature, cooling rate from tempering temperature and steel chemistry are the variables associated with tempering that affect the mechanical properties and microstructure of the finished part. Changes to the microstructure by tempering typically decrease hardness and strength (tensile and yield) while increasing ductility and toughness. Tempering results in an increase in softness, malleability, impact resistance and improved dimensional stability.
A good rule to remember is that all steel should be tempered soon after being removed from the quench and before it is completely cold. Failure to temper correctly may lead to a myriad of performance problems such as premature failure or shorter than normal service life. A common call-out in various specifications is to “temper immediately,” which is subject to broad interpretation throughout the industry. For materials such as 4340, tempering must take place within 15 minutes of removal from the quench to avoid quench cracking. For high-speed steels, the tools should normally be allowed to cool to 120°-150°F (50°-65°C) and then tempered, but it is all too common to find loads sitting and waiting for a temper furnace for two or more hours.
Tempering is always performed be-low the lower critical temperature (A1) of the steel, which differentiates tempering from such processes as annealing, normalizing and hardening. When hardened steel is reheated, temper-ing effects start to occur as low as 212°F (100°C) and accelerate as the temperature increases. By selecting a definite tempering temperature, you can predetermine the resulting hardness and strength. One caution is the avoidance of issues such as temper embrittlement (for more detail, see “The Embrittlement Phenomena in Hardened & Tempered Steel,” Industrial Heating, October 2006).
The minimum tem-perature time for tempering should be one hour. A good “rule of thumb” for furnace or oven tempering is that if the part is more than 1 inch (25 mm) thick, increase the time by one hour for each additional inch (25mm) of thickness.
Stages of TemperingTempering is said to occur in three distinct stages in overlapping temperature ranges (Table 1). The precipitation of finely dispersed alloy carbides responsible for secondary hardening (in highly alloyed steels) is sometimes referred to as the fourth stage of tempering. It is important to know what is happening to the part microstructure at the tempering temperature you select.
Considerations in Tempering EquipmentWhen designing a tempering process, consideration must be given to the type and condition of the tempering equipment. In particular, airflow and temperature uniformity play a critical role. Tight temperature uniformity – typically ± 10°F (± 5.5°C) – is required throughout the load with ± 5°F (± 2.75°C) preferred, especially for high-speed and precipitation-hardening steels. The ability to have a rapid heating rate will shorten overall cycle time.
Various forms of self-tempering and accelerated tempering via induction or ultrahigh air convection ovens and furnaces have shown promise in a number of applications [2,3]. Although soak times and temperatures are typically fixed by steel chemistry, substantial reductions in processing times can be achieved by accelerating the heat-up time. This can be accomplished by designing more efficient heat transfer between the heated atmosphere and the load using high-speed convective, turbulent flow patterns.
For example, it was found that effective tempering of axle shafts after induction hardening was dependent on actual heat-transfer rates and compo-nent temperature rather than on time at a specified tem-perature. This study achieved the following results with a savings of over 80% in cycle time (4 hours to 36 minutes): equivalent post-temper hard-ness at the bearing journal surfaces; equivalent hardness and residual stress profiles within the shaft; equivalent yield and rupture strengths; and equivalent reverse torsional and rolling bending-fatigue life.
Temper ColorsThe use of temper color is one method of not only visually determining if a part has been exposed to the proper tempering temperature but to check if all parts in a given load reached a uniform temperature. When steel is heated and exposed to air (or an oxidizing atmosphere) for a short period of time, it will change color due to the presence of a thin, tightly adhering oxide. The temper color and thickness of the oxide layer varies with both time and temperature (Table 2). Different steel chemistries also result in slight color variations. The colors produced are typically not uniform because of surface condition and fluctuation of temperature. To see the colors clearly, you must turn the part from side to side and have good lighting. Natural lighting is always best.
Summing UpTempering – because it is often the last heat-treat operation – is considered by most to be relatively simple and straightforward. The heat treater, however, must remember that it is a complex process in which all of the process and equipment variability must be carefully controlled. IH
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: secondary hardening, retained austenite, toughness, martensite, temper color (heat tint)