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).

Toughness is a mechanical property that measures the amount of energy a material can withstand, or absorb, before fracturing. In the context of other mechanical properties, toughness for steel products represents a balance of strength and ductility.

As a material term, toughness can sometimes cause confusion: two very different materials can both be considered tough. For example, an iron anvil is tough because its strength is very high, and it resists fracture by maintaining its material integrity thanks to its strength. On the other end of the spectrum, silly putty is also a tough material, not because it’s strong but because it’s very ductile and will resist fracture until you pull it far enough apart that the two ends separate. Anvils and silly putty are very different, but they’re similar in that they are both tough, and they both absorb a good deal of energy before fracturing.

The heat treatment known as annealing encompasses a group of processing heat treatments. Unlike thermal processes that are used to improve the strength and wear resistance of a part (such as hardening heat treatments), processing heat treatments are used to prepare a workpiece for further manufacturing processes. Chiefly, this means improving a material’s responsiveness to machining or cold-working by restoring (and even enhancing) ductility.

After an annealing process, the part can be expected to go to the machine shop for additional subtractive manufacturing, or to a mill to be cold worked. In fact, annealing makes machining and cold working easier (even possible) by relieving internal stresses, reducing hardness, increasing ductility and toughness, and softening the material to make it more machinable and workable.

Imagine a medieval blacksmith hammering a sword fresh out of the fire. With the weapon still red-hot, he dunks it in water. The water hisses and boils around the submerged steel as it turns color from glowing orange to grey. This iconic scene of rapidly cooling a workpiece is called quenching, or quench hardening, and is one of the oldest and most common heat treatment processes used to harden and strengthen common carbon steels. In modern metallurgy, superior corrosion resistance for most stainless steels is also an important property achieved through quenching.

Internal stresses form naturally during many manufacturing processes. Machining, stamping and welding, for example, are common manufacturing methods that augment the amount of stress in a material’s fabric. And although stress build-up is a natural byproduct of these processes, too much of it leads very commonly to part distortion. Most of these stresses must therefore be removed before a part can be considered ready and safe to use. Thanks to heat treatment, these stresses can be relieved (without changing a part’s mechanical properties) through a thermal process known as stress relieving.

Joining multiple metal parts together into a single, cohesive unit is an indispensable process to manufacturing. Bolts, screws, and adhesives are often used to unite separate parts, but many applications call for a more solid connection. Thanks to the craft of welding, manufacturers are able to effectively create a single part from multiple components. Casting facilities are also able to use welding to repair minor casting defects, improving overall process efficiency and reducing part cost.

Understanding the weldability of various alloys is key to creating welds that last and parts that perform in the field. As a mechanical property, weldability is also an important consideration in designing and producing cast parts.

Hardening a metal achieves much more than simply making it mechanically harder. It also improves strength, making the alloy more resistant to plastic deformation, and can aid with corrosion resistance. However, a harder metal is also often a more brittle metal. As such, hardening is usually just one part of a properly designed heat treatment plan.

Because hardening can be accomplished through numerous heat treatment strategies, it’s important to know what options are available to a casting customer, how the methods differ, and where hardening fits into the master plan.

In this article, we’ll explore the science and practice of hardening, and explain when conditions are appropriate for a particular hardening strategy. Whether you’re working with the Eagle Group or another metal manufacturer, hardening is an important step in manufacturing, so it’s good to know the basics.

Normalizing is one of the most common heat treatments used in manufacturing carbon steel. It is a vital process to ensure that the mechanical and physical properties of a carbon steel workpiece are integrated and distributed uniformly across the material.

In steelmaking, material uniformity is achieved by carefully controlling a casting’s microstructure – specifically, its grains. Grains are distinct areas of crystal structures oriented in the same direction. Multiple grains together make up a metal’s structure. The goal of normalizing is to target these grains in order to even out the differences between them, resulting in a more mechanically stable product. It’s for this reason that the process is named as such: it ‘normalizes’, or homogenizes, grain size, shape, and orientation.

“Hardness” is a concept we use every day to compare objects: fresh bread vs. stale bread, a baseball vs. a softball. In manufacturing the definition is a bit more specific: hardness is a measure of a material’s ability to resist localized scratching or deformation by indentation at a specific loading location.

Our understanding of hardness and relative hardness between materials is key to manufacturing metal products. The correlation of hardness with other mechanical properties also makes it possible for us to predict, with a high degree of accuracy, the strength and ductility of a material without submitting it to destructive testing.

Past civilizations had practical reasons for caring about metal properties. They asked questions like “Can I break my enemy’s sword before he breaks mine?” or “How thick does my armor need to be to provide enough protection?” These warriors had a vested interest in material strength.