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Why Does Quenching Make Metal Brittle?


Metalworkers have to be multitalented.  Not only do they need to have experience with many different techniques for working metal to the desired specifications, but they also must be familiar with how various types of metal respond to those techniques. Even with regards to a single type of metal, such as aluminum, each distinct alloy has its own unique properties that fundamentally alter its chemical attributes and affect the way it reacts to processes.

Quenching is one of the methods available to metallurgists when processing metal.  It is essential that a manufacturer understand how a specific alloy will react in order to select the best material for an application.  Of course, quenching is known to make the metal in question brittle, and so it is often necessary to apply other processes before or after quenching to ensure success, or to avoid quenching all together.

Today, we’ll look at how quenching affects both stainless steel and aluminum.

What is quenching?

Quenching is defined as a rapid cooling of a work piece in a particular medium, such as water or air.  This process, which is a kind of heat treatment, lends certain material properties to the work piece and precludes unwanted processes that can occur at low temperatures, such as phase transformations.  The process works because the window of time when the unwanted reactions can occur is shortened. As an example, quenching helps to lower the size of crystal grains in metals, which increases their hardness.

It seems that quenching originated as far back as the second millennium BC, when early blacksmiths knew that by varying the cooling process they could materially affect the properties of steel and iron.  However, it wasn’t until the 15th century that we have evidence of separate processes for quenching and tempering.  Blacksmiths eventually developed techniques for performing full quenching, slack quenching, and other operations that allowed for greater control over the strength and brittleness of the final product.

Today, the process of quenching involves first uniformly heating the sample between 815 and 900 °C.  It’s important to avoid uneven heating or overheating to best ensure the desired properties in the finished piece.  Next, the work piece is soaked in an air furnace, a liquid bath, or a vacuum.

After soaking, the metal is submerged into the quenching fluid.  The nature of the fluid further affects the finished properties of the metal, with water generally considered one of the most efficient quenching liquids in order to get the maximum hardness. Mineral oils are also an option.  Since the cooling rate when using oil is lower than with water it is less efficient, but it also can help to avoid distortion or cracking.

How to use quenching with stainless steel

When quenching steel, the most common scenario for hardening the metal is to introduce martensite, which occurs when rapidly cooling the steel through its eutectoid point.  The eutectoid point is probably familiar to most metallurgists and is defined as the temperature at which austenite becomes unstable.  In alloys that have a significant amount of nickel and manganese, the eutectoid point is lower yet the kinetic barriers to phase transformation remain the same.  Therefore, quenching can take place at a cooler temperature.

It is important to realize that while stainless steel that has been properly quenched and has entered the martensitic stage is harder than before, it also tends to be more brittle.  This has to do with the crystalline structure of the steel and the differences between martensitic and austenitic stainless steels.  What you need to know is that because the quenched steel is more stressed, you’ll need to temper the steel after it has been quenched.

The tempering process will lower the hardness while increasing ductility.  It generally involves heating the metal again for a set period of time and then allowing it to cool in still air rather than quenching it again.  Depending on the required properties, the temperature and other variables of both the quenching and tempering processes can be fine tuned for the desired outcome.

How to use quenching with aluminum

While the tempering of steel has been developed over thousands of years, aluminum is a much newer metal and the quenching techniques are more recent.  This may be why people are more likely to associate quenching with stainless steel, even though it is certainly possible to quench aluminum if desired.

As with stainless steel, there are a number of possible liquids that can be used with aluminum, with water being the most common.  Water is the most widely used choice for a number of reasons, starting with the fact that water is readily available and usually the cheapest option.  Another advantage is that water offers the rapid quenching speeds that are required for certain properties in various alloys.  Water also allows for flexibility by altering its temperature as needed.

Water can be used to quench aluminum with a number of techniques, including cold water quenching, hot water quenching, and water spray.  Other quenching materials include still air, air blasting, glycols, polymers, liquid nitrogen, oils, brine solutions, and more.  To fine tune the quenching process and draw out very specific and targeted properties, you can do some combination of the above.

The major drawback to cold water quenching is that as with stainless steel, it can lead to undesirable warping or distortion in the aluminum.  It’s also true that quenching especially thick pieces may produce a higher level of residual stress.  That’s one of the major reasons to try one of the other quenching substances.

Your technical services provider

At Clinton Aluminum, we pride ourselves on our ability to help our clients through every step of the material procurement process, ensuring that not only do they get the right product fitted to their needs, but that they maximize their investment and get exactly what they need in a timely and friendly manner.

We’re able to do this because our sales and warehouse team members have an average of thirteen years with the company, so not only do they know the industry, but they are personally invested in making sure our customers succeed.  Contact us today to learn more about which quenching options make the most sense for your application.

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