Stainless Steel Types in Medical Instruments

Iron-based alloys, known for exceptional rust resistance due to Chromium, were initially strengthened with small amounts of Chromium since the construction of the Eads Bridge in 1872. The term “Stainless” originated in 1916 in Sheffield, England. In 1913, 316L stainless steel was patented, and in 1926, an 18% Chromium and 8% Nickel alloy was developed, later enhanced with 2-4% Molybdenum for improved corrosion resistance.

Stainless steels are categorized into three types:

1. Ferritic alloys, adding Chromium to resist corrosion without hardening, used in construction and automobiles.
2. Martensitic alloys, formed by adding Chromium to steels with specific Carbon content, ranging from 410 to 440-C grades for various applications.
3. Austenitic alloys, containing Nickel for stability, known for unique properties like workability, toughness, and corrosion resistance.

For “Surgical” Steel, 316L is commonly used, recognized for its strength, workability, and corrosion resistance. Standards such as SAE, AISI, and ASTM classify these steels, with 316L identified as 30316L or S31603.

Composition guidelines, such as ASTM F138, define the production of “Surgical” Steel. Despite being termed “Stainless,” these steels can stain and corrode at lower rates. Chromium content exceeding 10.5% forms a thin, self-repairing Chromium Di-Oxide film, providing remarkable corrosion resistance, especially in the human body. Molybdenum adds resistance to pitting corrosion.

The exact composition of 316L includes:
– Chromium: 17.00%
– Nickel: 12.00%
– Molybdenum: 2.25% Max
– Other elements in specific proportions, ensuring its desired properties.