The word ‘tempering’ generally means acting as a counterbalancing force, moving an object (or a situation) away from the extremes, toward a more balanced state. The same principle applies to heat treatment: when we temper a metal part, we are adjusting the proportion of one mechanical property against another. Specifically, we are increasing ductility while simultaneously decreasing strength and hardness.
Tempering is most often performed after a metal piece has been seriously stressed through hardening or after normalizing. When a material is hardened (especially quench hardened, the most common method of hardening), it is in an unbalanced state. In metallurgical terms, we say the metal is unstable: its molecules would like to rearrange into a more stable structure, but the quench hardening process freezes them before they get the chance. Likewise, the process of normalizing takes the steel molecules through a phase transformation, leaving stresses in its wake (for a longer discussion of phase transformations, check out our Introduction to Heat Treatment for Cast Parts).
In this suspended state, the material is looking for a way to achieve a more sustainable equilibrium. Without further processing, it will do this simply by cracking, which is a natural form of stress relief. To avoid this negative outcome, we apply the final step of tempering to restore some ductility to the steel workpiece. As such, tempering is often the final step in a series of thermal processes and can be seen as a type of process or finishing heat treatment.
There are a host of practical benefits to hardening and normalizing steel parts through heat treatment. The goal of tempering is not to reverse these benefits entirely; it is to maintain the benefits of the hardening and normalizing processes while stabilizing some of the stressed structure. In this article, we’ll explore just how steel casting foundries like Eagle Alloy and Eagle Precision use tempering to finalize a workpiece for delivery.
When and Why We Use Tempering Heat Treatment
When Is Tempering Required?
Tempering is often the next step after quench hardening. Quench hardening can be performed in a number of ways, usually dictated by the kind of quench medium selected for the job. Each quench medium carries a different quench severity, which is the speed at which the medium removes heat from the steel (check out our quenching blog for the full scoop on quenching mediums). As a comparison, think about the difference between dropping a burning match in a glass of water versus a bowl of olive oil – the hissing sound of the water is the sonic signal telling us just how quickly water extinguishes heat relative to other mediums, like olive oil in this case. We say that water has a very high quench severity (in fact, H2O is one of the severest quench mediums out there), while oils have a lower quench severity relative to water.
The state of the workpiece after hardening is thus influenced by how it was quenched, i.e. how much quench severity it was subjected to. In many cases, the end result is a material that is very hard, but also very brittle and prone to cracking due to internal stress.
Common Tempered Steel Grades
Alloy grades known as quenched and tempered steels are many in number. A handful of common ASTM grades for structural applications that are specified as “quenched and tempered” in are:
The Eagle Group is a leading manufacturer of pressure-containing castings and, as such, some of the most common grades we work with are:
The level of specification with regards to the heat treatment requirements varies. Some grades include heat treatment tables that outline minimum quenching and tempering temperatures separately, as well as specifying what type of quench medium should be used (liquid or gas). Others include a more guideline-like section that discusses general heat treatment procedures that should be applied to attain the desired mechanical properties.
As you can see, quench and tempered steels are used in a variety of applications: from pressure-containing vessels to steel plates for structural and high-wear machine applications, to small parts like fasteners, bolts and studs. The uses for tempered steel run the gamut of industrial applications. Indeed, quench and tempered steel is all around us: the piping in our walls, the storage tanks in our basements, as well as the steel beams holding up manufacturing facilities and residential high-rises.
Where Does Tempering Fit in a Heat Treatment Plan?
Often, tempering is the final step in a workpiece’s manufacturing lifecycle.
Tempering is used to render hardened steels more malleable and ductile, thereby increasing fracture resistance and toughness, and decreasing strength and hardness. As such, tempering is the natural subsequent thermal process after hardening. This step-wise process must be performed quickly: hardened steel is in a state of high stress, and if the stresses aren’t relieved quickly enough, the metal is at risk of cracking. In other words, when you think hardening, you usually also think tempering, and the recommended time between them is often hours or even minutes. In fact, one of the considerations we take into account when organizing the heat treatment facilities at Eagle Alloy is the optimal setup to transition between quenching and tempering.
How Does Tempering Improve Performance of Cast Parts?
Tempering increases a part’s ductility, making it more able to absorb unexpected stresses.
Ductility is a critical mechanical property and has an inverse relationship to strength and hardness: the more you get of one, the less you have of the other. Whereas ductility describes a metal’s amenability to being drawn, or resisting fracture even when it is bent, strength describes a metal’s ability to resist bending at all in the first place (to resist deformation, in metallurgical terms). Ductility is informed by a principle known as the Hall-Petch relationship, which states that the larger the grain size, the more ductile the material. Grains are the individual clusters of crystals that make up a steel’s structure. It will come as no surprise that tempering results in larger grain sizes.
And by increasing grain size, tempering aims to strike the optimal balance between these two properties, ensuring parts out in the field carry enough strength to resist deformation, while still possessing sufficient ductility to deal with excessive stresses and deformations if they arise.
Tempering and Welding
Welds are a form of localized heat treatment. The heat-affected zone (HAZ) is the area on the base metal where heat has been applied and the weld has been placed. Due to the intense isolated heating in this area, it will naturally be in a state of stress. Tempering is therefore also an important finishing process for welded materials, in order to erase unwanted residual stresses.
The Science of Tempering
The type of steel that comes about from hardening a medium- to high-carbon steel is known as martensite. Martensite is formed when an austenitic carbon steel (a carbon steel heated above its upper critical temperature) is quenched, causing it to undergo a microstructural change without giving it enough time to expel the surplus carbon, which it naturally would if cooled at a slower pace. The end result is a strong, hard, and fairly brittle steel.
If we take this steel and temper it, we get tempered martensite, a very tough steel with a huge number of industrial applications, and that’s also due to the Hall-Petch relationship described above: larger grain size, and a therefore more practical balance of mechanical properties.
Martensite vs Tempered Martensite
Martensite (before tempering) has what is known as a metastable structure, meaning the material is frozen in an intermediate energetic state. In other words, the atoms are somewhat stable, but would still like to be able to rearrange into an even more stable structure. In the words of the inimitable engineering textbook, DeGarmo’s Materials and Processes in Manufacturing:
“Despite its great strength, medium- or high-carbon martensite, in its as-quenched form, lacks sufficient toughness and ductility to be a useful engineering structure...[Tempering, is usually required to impart the necessary ductility and fracture resistance and relax undesirable residual stresses. A with most property-changing processes, however, there is a concurrent drop in other features, most notably strength and hardness.” (1)
When martensite exits the quench tanks, it is too brittle. In technical terms, it has retained too much austenite, the microstructure carrying more carbon and iron bonds than is ideal for practical purposes. The reason it’s so brittle is because quenching actually changes the shape of the structure the atoms form when coming together, and this structure is highly unstable.
Tempered martensite is just what it sounds like; after tempering martensite, the retained austenite is either transformed or stabilized, giving the workpiece better ductility and fracture resistance at the expense of some strength and hardness. It is this tempered version of martensite that has so many practical applications, and not its non-tempered predecessor, plain old martensite.
The tempering of martensite must happen very quickly after the martensite is lifted out of the quench tanks. The stresses from quenching could become permanent and fatally damage the workpiece if left too long in the quenched state. It’s therefore vital for foundries to have a well-organized heat treatment operation, with quench tanks within reach of the ovens to minimize downtime between quench and temper. At the Eagle Group, our three quench tanks are located right next to our ovens, ensuring each process run is optimized and guarantees the highest possible number of performing parts thanks to these process efficiencies. For a closer look at our in-house heat treatment facilities, check out our dedicated blog.
Thermal Recipe for Tempering
Like all heat treatments, tempering has three steps: heating, soaking, and cooling.
Heating:
Tempering requires ramping up to a temperature below its lower critical temperature (for a discussion of lower vs. upper critical temperatures, head to our Introduction to Heat Treatment blog). In short, the lower critical temperature is the point after which meaningful microstructural changes begin to happen. In the case of tempering, any changes above that temperature would result in undoing the benefits of previous hardening, so heating above this temperature must be avoided.
Equally important in this phase is the heating rate, i.e. the speed at which the material is heated to its desired temperature. Heating it too quickly can cause even more stress, whereas heating it too slowly is inefficient and results in lost time and money.
Soaking:
The metal must then remain, or soak, at the prescribed temperature for the prescribed period of time. Both the time and temperature variables are determined based on the requirements of the workpiece in question; namely, its chemistry and geometry. Additionally, the soaking temperature needs to remain constant to avoid distortion, such as unwanted brittleness unevenly distributed across the piece, a common defect caused by mismanaging soaking temperatures.
Cooling:
Proper cooling calls for bringing the temperature down to room temperature at a controlled rate and in a controlled environment. In the case of tempering, the workpieces are usually removed from the oven and allowed to cool at or around room temperature.
Precise knowledge of the appropriate temperature range is vital, and working with experienced metallurgists is just as important, as they can calibrate the conditions to meet the needs of each particular part and geometry. Standard setting organizations like the ASTM sometimes provide specifications, but sometimes only guidelines. It’s therefore the foundry’s job to interpret and implement the information to produce the desired grades.
Tempering at the Eagle Group
At the Eagle Group, we know heat treatment. With a fully equipped in-house heat treatment facility, we do almost all of our heat treatment at home. Tempering, especially, is a process we have the capabilities to handle for nearly every part we cast. Our car-bottom furnaces, induction ovens and range of quench tanks work together to give finished parts the exact balance of mechanical properties they require out in the field.
Combining casting expertise with thermal processing knowhow is part of our concept-to-completion ethos. Our heat treatment facilities have improved and expanded as demand for our cast products has grown. Today, we pour hundreds of alloy grades and cast thousands of unique parts, and this type of versatility requires striving for process perfection. For us, that translates to a continuing endeavor to match our casting capacity with our ability to finish parts with the perfect thermal treatment.
Learn More About Heat Treatment in Our Dedicated Series
(1) Black, JT., & Kohser, Ronald A. (2019). DeGarmo’s Materials and Processes in Manufacturing (13th Edition). John Wiley & Sons, Inc (pg. 75)
Tags: Processes, Metallurgy, Heat Treatment
Written by Jason Bergman
Jason Bergman is Senior Quality Engineer and Metallurgist at Eagle Alloy. He has been with the company since 2013.