Austenitic – What Does The Mean?
“Austenitic” is probably a new term for most people. The word qualifies a particular class of steels and is the subject of this blog post. As the title states, why should we care that stainless steel is austenitic? The vast majority of stainless steel used in the manufacturing of watches is austenitic. But the reason you may not have heard before is that this technical demarcation is vital for those involved in the design and manufacture of quality watch cases. And here is something that may bring a new perspective to a watch enthusiast about their watch case: a little-known fact about austenitic stainless steel is that its properties can all be traced to how the atoms are arranged the steel.
Our Journey to Austenitic Stainless Steel
We had set off on this journey through the steels of watchmaking with an introduction to steel. It was there that the term “austenitic” was introduced. This is a technical term from the world of steel. For our purposes, we will consider austenitic stainless steel as one that has a consistent chemical composition throughout its atomic structure. Or, to use the more technical descriptor, it is a solid solution. The solid solution concept was introduced in my first blog on steels, and this is a useful analogy that we will visit again a little later.
Making a steel stainless was the focus of the second installment in our journey through steels. Here we looked at the impact of Chromium on the corrosion resistance of stainless steel. This is the key alloying element pivotal in creating the oxide coating on the steel that renders it stainless. But it also has a material impact on the atomic structure of the stainless steel, making the austenitic quality much more significant.
Who Cares About Austenite
In what way is the austenitic attribute so significant? And why should we care? The easiest way to answer this is by looking at what happens when the steel is not austenitic. There are various atomic structures of steel, depending upon its carbon concentration. Generally speaking, the higher the carbon concentration (particularly if it is higher than 0.2% carbon), the steel will become less ductile, harder, and more brittle. When the carbon content is greater than 0.2%, the steel will not remain a solid solution but dissociate into two specific chemical compounds: cementite (Fe3C) and ferrite (Fe). These steels are generally referred to as martensitic steels and are both hard and brittle. These physical characteristics are not desirable when we consider the manufacturing process for a piece of precision engineering.
How A Watch Case Is Made
This is where the importance of austenitic steel becomes obvious. The manufacturing of a watch case starts with a sheet of stainless steel being formed into the desired dimensions. When they are lined up, a center hole (where the moving parts will sit) is drilled. A number of the watch case blanks are accumulated onto a milling rod where the rough exterior shape is then milled.
Once the rough size and shape have been achieved (which is important to the volume of material), the next stage on the journey to a precision piece begins. To achieve the required shape, the watch
case workpiece is put through a series of cold-work presses, shaping each blank with extreme force between two molds. When the steel is put under high pressure, the steel workpiece takes on the mold’s shape.
As you can imagine, steel does not mold very easily, and if it is forced to transform its shape too far too fast, areas of distortion will appear on the surface of the workpiece. The movement of the material, particularly in a cold process, significantly stresses and strains the steel’s atomic structure. As the steel is worked and shaped at room temperature, strains are created on the steel surface. These strains in the microstructure of the material change the steel’s physical properties, making it harder and more brittle. In turn, hard, brittle materials are more difficult to shape and machine accurately. So the concern is how to remove these micro strains before the next process.
Back to Austenitic
This is where the necessity of austenitic stainless steel becomes obvious. The micro-strains in the workpiece need to be relieved. If they remain, the steel will not take on the form of the new mold and show surface defects such as cracking or dimpling on the workpiece’s surface. This is done by heating the workpiece to approximately 800oC for about 10 ~ 15 minutes and then air-cooled back to room temperature. During this process, there must be no chemical reactions that will change the stainless steel’s performance. This
heating and cooling cycle, called annealing, occurs in such a way that the workpiece maintains its austenitic structure throughout the process. Of equal importance is that this heating and cooling will remove all the microstrains within the structure, and the workpiece will be ready once again to be cold-worked.
What If It Were Not Austenitic
If the stainless steel were not austenitic (say, for instance, it were martensitic)? When the workpiece was heated, its composition at an atomic structure would be completely changed. There would be atomic reactions in the microstructure resulting in a transformation in the workpiece’s physical properties while it went through the shaping and machining processes. This would make it unsuitable material to undergo multiple rounds of shaping and machining. It would react unpredictably to the machining and shaping, which is undesirable for a piece that needs to be manufactured to a precision of fractions of millimeters.
It Is All In The Details
Having a very consistent material at the microscopic level is very important for the piece’s machining and finishing processes. This allows the machining and finishing to happen in a very predictable way. Martensitic steel is effectively a composite material, a mixture of two materials distinctly different at the microscopic level. Ferrite, which is soft and pliable; and Cementite is very hard, strong, and brittle. To achieve accurate machining, the material must not fracture with the impact of the cutting tool. With high carbon steel, machining becomes more and more difficult with increasing carbon content. The final consideration is the surface finishes. If the material is not consistent in its physical properties at a micro level, it becomes almost impossible to achieve a consistent and high-quality finish across its whole surface,
One Last, But Important Benefit of Austenitic Stainless Steel
One often forgotten benefit of austenitic stainless steel is that it is not magnetic. Mechanical watches need to be shielded as much as possible from magnetic fields. Once again, martensitic steel is capable of holding small amounts of magnetism. This is due to the presence of ferrite in the microstructure. This is just one further reason why austenitic stainless steel is used almost exclusively for watch cases.
As we look back at the original description of the 316L stainless steel from our first blog post on steel, we can see that we are slowly building up a picture of the physical properties and why this specific steel is selected. Let’s go through them briefly. Steel is chosen because it is one of the most cost-effective materials to use. The stainless aspect is introduced to ensure that your watch is fit for its purpose and does not decay over time. Finally, austenitic stainless steel is used to allow for exact machining and allow for wonderful and consistent finishes to be applied to the final piece.
Next time we are going to look into the corrosion resistance of stainless steel. In particular, we will look at the most commonly used steels’ details and compare them and their advantages and disadvantages. That is where we will understand the key differences between 316L, 304, and 904L steels and really unravel the mysteries of all these steel references and what they really mean for the watch enthusiast.