Q: What types of stainless steel are used in the watch industry?
A: Currently, stainless steel (SS) used in the watch industry is broadly divided into three types: 304, 316, and 904. There are several standards, but the AISI standard established by the American Iron and Steel Institute is commonly and widely used in the watch industry, and the names 304 and 316 follow this AISI standard. AISI is nearly identical to the SUS standard established by JIS, so AISI 304 can be considered the same as SUS 304.
There are also a limited number of material manufacturers that supply case manufacturers. Why are stainless steel watch cases limited to these three types, even though other materials are increasingly being adopted as case materials?
One reason is that watch cases are often polished. Industrial stainless steel has a lot of inclusions (dust), which can become visible on the surface when polished. For this reason, watch manufacturers generally use special stainless steel with few inclusions. Because they cannot guarantee the quality of the polished steel, high-performance stainless steel is difficult to adopt in the watch industry.
304 is widely used in industrial products, not just in the watch industry. Its production volume exceeds 60% of all stainless steel production. 316 is the result of increasing the nickel content of this material and adding molybdenum. Its distinctive feature is improved corrosion resistance (rust resistance) compared to 304. A further improvement on this material is 316L (the L stands for low carbon), which is often used in luxury watches. It has the same basic characteristics, but is said to be less prone to surface irregularities over long periods of use.
904L is a stainless steel that further improves the performance of 316L. The main difference between 904L and 316L is that it contains a higher amount of chromium, molybdenum, and nickel. While it is highly corrosion-resistant, it is considered difficult to process. Currently, 904L is used in all stainless steel models of Rolex and NH watches, as well as some models from Sinn, Ball Watch, and Girard-Perregaux.
In recent years, there have also been attempts to use stainless steel other than these. Representative examples include Capek's "XO Steel," Chopard's "Lucent Steel," Seiko's "Ever Brilliant Steel," and Sinn's "U-Boat Steel." Only Sinn's steel is known to be made of this material, which is classified as XM 19 stainless steel according to the AISI standard.
