TYPES OF STAINLESS STEEL USED IN MEDICAL INSTRUMENTS

Stainless Steel is a generic term commonly used to describe a group of Iron-based alloys which exhibit a phenomenal resistance to rusting or corrosion because of it’s Chromium content. Chromium has been added in small amounts to strengthen Steel since the Famous Eads Bridge spanned the Mississippi River at St. Louis, Missouri, in 1872. But it was only discovered in the present century that when Iron was alloyed with Chromium in excess of 10%, and Carbon held suitably low, it was effectively rustproof. The term “Stainless” was
first used to describe steel cutlery that was produced in Sheffield England in C.1916. In 1913 316L stainless Steel was first patented, In 1926 a Stainless Steel composed of 18% Chromium and 8% Nickel was developed, however, this steel still had problems with corrosion resistance. This problem was solved by the addition of 2-4% Molybdenum. Stainless Steels can be broken down into three main types:

Ferritic:

Ferritic alloys are obtained when Chromium is added to Iron to develop corrosion resistance without steel-type hardening. ferritic alloys are mainly used for construction, and for use in automobiles.

Martensitic:

Martensitic alloys result when Chromium is added to steels with carefully chosen Carbon contents, they range from the least expensive and most widely used 410 grade, to 440-C grade which is used for knives and surgical tools due to its great hardness.

Austenitic:

Nickel is the next most important alloying element, the most famous alloy being “18-8″ containing 18% Chromium and 8% Nickel. Unlike the atomic structure of ferritic or Martensitic Steels, Austenitic Steel has a face-centered cubic form made stable by the presence of Nickel. This gives the Austenitic Steels unique properties of workability, toughness, and corrosion resistance. The Austenitic grades of Steel are the most highly prised for these properties, and also the most expensive.

STANDARDS FOR “SURGICAL” STEEL

Some reports in Body Piercing literature state that there is no such thing as “Surgical” Steel, However, there is a certain grade of steel which is used for implantation into the body or “Surgical” purposes. So we can, therefore, use the term “Surgical Steel” even though that’s not the exact scientific terminology, it’s easier to use that term than to try to get the public used to all the other more complex classifications used for it.

316L is the most useful for body piercing jewelry, as it is fairly strong, easy to work with, and will not lose its corrosion resistance during manufacture Type 316L (L = Low Carbon at 0.03%) 316LVM (Low carbon Vacuum Manufacture), and 317 are the only steels classified for use for surgical implantation.

MANUFACTURER’S CLASSIFICATIONS:

Because there are so many different alloys of steel available on the market, industry groups i.e. SAE (Society of Automotive Engineers) and AISI (American Iron & Steel Institute) have devised a numbering system, 316L is the old version, the new version is 30316L, and this has been superseded by the Unified Numbering System (UNI) which is S31603. But the old version has become almost a generic term and is still recognized by most manufacturers and body piercers, and I will, therefore, use it here. There are various standards applying to Stainless Steels throughout the world, and several different classifications are used to designate “Surgical” Steel. The most common are listed here.

Manufacturer’s Codes
S.A.E. ASSAB SANDVIK
30316L 926L 3R60
National Codes
British German Japanese United States
new type W.No DIN JIS UNS.No ASTM/AISI type
316S11 1.4404 X2CrNiMo17 13 2 SU316L S31603 316L

COMPOSITION: 
There are very specific guidelines for the production of “Surgical” Steel, the one used for the purposes of this article are the American Society for Testing & Materials (ASTM) standards, specifically ASTM F138 (1982) Standard Specification, Stainless Steel Bar and Wire for Surgical Implants (Special Quality).

The term Stainless Steel is a misnomer, these steels are subject to staining and corrosion, although at much lower rates than standard steel. Chromium content in excess of 10.5% is what makes steel so-called “Stainless”. The oxidization of this Chromium content upon exposure to air causes a very thin film of Chromium Di-Oxide to form on the surface of the steel. The Film of Chromium Di-Oxide is extremely thin, passive, continuous, tenacious, stable and self-repairable. It renders the surface inert to many chemical reactions and
therefore passive, thereby giving stainless steel it’s phenomenal built-in corrosion resistance, especially in organic environments like the internal Human body. The Molybdenum confers a special resistance to pitting corrosion.

The exact Metallic composition of 316L by Heat analysis is:

Chromium 17.00%
Nickel 12.00%
Molybdenum 2.25% Maximum
Manganese 2.00% Maximum
Silicon 0.75% Maximum
Copper 0.50% Maximum
Nitrogen 0.10% Maximum
Carbon 0.03% Maximum
Phosphorus 0.025% Maximum
Sulfur 0.010% Maximum
Iron 65.345%*

*Approximately the remaining balance