Factors to consider in stainless steel machining
The concept of EXTático emerges from the design of shapes for industrial objects, the use of different processes and the use of different materials for the same design of the watch. One of these materials is stainless steel for our watch cases, which is chosen for its resistance to corrosion and the ease of achieving it. The problem is that it contains nickel is that the more quantity in the alloy much more possibility of contact allergies.
Steels are alloys of iron-carbon wherein carbon is 2%. Stainless steels are steel alloys containing chromium with a minimum of 10.5%, which allows to increase the resistance to corrosion. Chromium combined with oxygen produces a thin layer of oxide that allows its passivation. The corrosion resistance can be improved with the addition of nickel, which in turn increases the austenitic phase or molybdenum.
Stainless steels are classified into martensitic, ferritic, austenitic, precipitation hardened stainless and duplex. Martensitic steels are discarded for watch cases because of their difficulty in machining and their fragility. Ferritic steels are paramagnetic and can interfere with the accuracy of the watch mechanism. Despite the low nickel content in the steels by precipitation as 420 or the F51 duplex we also reject them due to their magnetic affinity.
Austenitic steels are the most interesting for watch cases that we can use by EXTático, whose main component is austenite. Austenite, also known as gamma steel (γ) is a particular ordering mode of iron and carbon atoms. Gamma or austenitic steel is the stable form of pure iron at temperatures ranging from 900 ° C to 1400 ° C. It is made by a solid solution of carbon in iron with a crystalline structure, the austenite which is of the cubic type of centered faces (CCC). Austenite is not stable at room temperature except in some stainless steels with high manganese content (12%) and in austenitic stainless steels with nickel contents around 8%. Nickel has the effect of enlarging the austenitic region in the phase diagram of iron to carbon, which makes it stable at room temperature and tough because of its crystalline structure which is the face-centered cubic form (CCC) of iron.
Unlike ferrite, austenite is not ferromagnetic at any temperature. Austenitic stainless steels are ternary iron-chromium-nickel alloys containing between 16 and 25 percent Cr and 7 to 20 percent Ni. The presence of nickel, which has a cubic crystalline structure at the center face CCC, allows the structure to be maintained at room temperature. The high malleability of austenitic stainless steels is due to their crystalline CCC structure. These steels usually have higher corrosion resistance than the ferritic and martensitic type because the carbides can be preserved in solid solution by rapid cooling from high temperatures. However, if these alloys are welded or cooled slowly from high temperatures in the range of 500 ° -1100 °, they may become susceptible to intergranular corrosion because chromium-containing carbides precipitate at the grain boundaries. This phenomenon is called sensitization.
Austenitic stainless steels can be hardened only by cold working. The heat treatment is only used to soften them. Stainless steels in the 200 and 300 series are characterized by excellent corrosion resistance, unusually good plasticity and the ability to develop excellent resistance characteristics by cold machining. Annealed that is produced by the manufacturer prior to its cold work gives them maximum corrosion resistance, ductility, good performance and tensile strength, impact and freedom of shear. Typical of this group is type 304, also known as 18-8 stainless steel (which refers to 18 percent chromium, 8 percent nickel).
Steels most used for the machining of watches are 303, 304 / 304L, 316 / 316L and 904L. 303 stainless steel is the typical used by Chinese watches for the alloy watch case back. This type of steel gives very good outcome because of its good machinability, its resistance to corrosion but over time can have pitting or corrosive stings caused by sweat. This can be avoided with a very good polishing of the case back. 303 also have no problem with sea water since it is not usually used for the bathroom because this type of watches does not usually have more than 3 ATM.
304 increases its chromium content by increasing its resistance to corrosion, sweat and acids. The difference with 304 L is the carbon content because the 304 can reach a 0.08 carbon which can increase its sensitization to the heat and form interstitial carbides in the welds. L of 304L means Low Carbon which improves corrosion to the welding temperature, a maximum of 0.03% in this type of steel. For watches it does not matter because they are rarely welded and are cold worked by mechanization, which does not influence unless they get very hot in cold work and change their austenitic structure by a martensitic. This steel is difficult to machine but with the correct speeds and advances there can be no serious difficulties in the manufacture of a watch case by milling.
316 L steel increases molybdenum content by 3-4% over 304/304 L, which improves corrosion and the risk of pitting especially in environments with sodium chlorides or similar. The addition of molybdenum improves the action of chromium for salt corrosion. As molybdenum changes the crystal structure increases the content of nickel to make the steel more austenitic.
As explained above, one problem with these austenitic steels is the carbon content with respect to sensitization. If the steel is welded or heat treated at high temperatures, between 500⁰-1100⁰ is converted to carbide. This carbide is deposited in the boundary grains producing corrosion. The problem with watch cases is the coating or PVD process (physical vapor deposition) that can reach 500⁰. This can be observed in 316L steel watches with PVD coating that in parts of the engraving or areas where the coating can corrode by this point. The choice to coat this type of steels is bluing steel. It achieves a great hardness of surface, besides offering a black color that maintains the finish of the base of the metal, it is sanded, polished, satin etc.
The superaustenitic steel 904 L. It is called superaustenitic because it contains up to 25% of nickel which increases the austenitic phase. In addition the maximum carbon content is 0.02%, less than 304 L and 316 L, which increases its welding capacity and resistance to corrosion. For cold machining it is like 316 L and therefore a bit abrasive. The 304 L is not as abrasive for tools as 316L or 904L. The problem with 904 L is the high nickel content of more than 25% that causes contact allergies in the wrist. Nickel allergies occur when a level in the blood of nickel that produces a sensitization. Not only is nickel absorbed by the skin but all the nickel that penetrates the individual. Thus the possibilities that this sensitization takes place are superior to the other alloys like 304 L or 316 L.
In summary, I believe 904 L alloys will only be used for prototypes to know the cutting speeds or special orders of customers who know the possibility of contact allergies due to the high nickel content. 304 L and 316 L will be used for both watch and watch parts. 303 will hardly use it.