Types of Stainless Steel used in Medical Instruments
Stainless Steel used in Instruments and wearable metal objects.
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 it’s 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, Austenic Steel has a face-centred
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 jewellery, as it is fairly strong,
easy to work with, and will not lose it’s 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 superceded 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. A 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 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
Sulphur 0.010% Maximum
Iron 65.345%*
*Approximately the remaining balance
![Reblog this post [with Zemanta]](http://img.zemanta.com/reblog_e.png?x-id=9c64c51c-4b20-44ad-a51c-20f6ce3c84da)