Prowa Medical Instruments | Stainless Steel

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.

Steel alloys composed with a minimum of 10.5% chromium fall into the stainless category. 

Is Stainless Steel Magnetic? Most people are unsure whether stainless steel is a magnetic or a non-magnetic metal. Some believe it could be magnetic because it has traces of iron, but in a real sense, the answer is neither a yes or a no. Some stainless steel varieties are magnetic while others aren’t.

What Makes Stainless Steel Magnetic? Steel alloys composed with a minimum of 10.5% chromium fall into the stainless category. 

For stainless steel to be magnetic, it has to meet certain requirements:

  • The alloy must have iron in it.
  • The crystal structure of the alloy must be arranged in a martensitic or ferritic structure.

All stainless steel metals are a type of steel. That means their chemical composition contains iron. In most cases, stainless steel varieties with iron in their composition are magnetic. If the alloy has an austenitic crystal structure, then it’s not magnetic

These days surgical Instruments are made from a variety of materials including composites, plastics, titanium, and (the more traditional) stainless steel. Typically two types of stainless steel series are most commonly used for instruments manufacturing.

  • 300 series (Austenitic) stainless steel
  • 400 series (Martensitic) stainless steel

The Austenitic Steel, sometimes referred to as 18/8 because of its 18% chromium and 8% nickel, cannot be hardened through heat treatment. Most stainless steels falling under this category are non-magnetic because they contain high amounts of austenite. Even though some of the metals like grade 304 and 316 have iron in their chemical composition, they are austenite, meaning they are non-ferromagnetic. The 304 grade is the typical grade used for surgical instruments and is suited for items which require some malleability, that is, they can be pressed or formed into another shape without breaking the metal. Cannulas, retractors, probes, mallets, and spreaders are amongst some of the instruments that are traditionally manufactured through this series of steel.

The Martensitic Steel are low carbon steel that can be hardened and tempered through heat treatment process. Most of the stainless steels in this category are magnetic. If iron is present, the crystal structure of martensitic stainless steel can be ferromagnetic. Because iron is the primary material in stainless steel, martensitic steels have magnetic properties. Due to the strength achievable through hardening process, this series of steel is more suitable for instruments that require strength and tough cutting edges. Bone cutters, clamps, forceps, needle holders, and chisels are some of the instruments that are traditionally manufactured through the 410/420 grade of this series of steel.

As a general rule, the more a harder edge is required to be on the instrument for cutting, the more carbon the instrument will contain. The carbon allows the steel to be hardened, so it doesn’t bend during use and also can be sharpened so it retains its sharpness for cutting. However, during maintenance this is very relevant, as you must ensure you maintain this hardness.

Chromium is the part of the composition of the steel that gives it the shiny chrome presentation. This also helps with rust resistance, along with nickel and the more chrome, the more likely it will be to resist corrosion.

A crucial part of instrument finishing is the Passivation Process, this is a Nitric Acid surface treatment to remove foreign bodies and oxidise the surface of the material to increase corrosion resistance. Passivation should only be performed by trained technicians, who have suitable protective equipment and knowhow.

The ability of an instrument to re-process through sterile services, without any problems i.e. rusting and corrosion, is a very good sign of its quality, however there are many factors such as AWD used and chemicals used for cleaning. This is particularly important, as most instruments are made from Martensitic Steels, and therefore have more chance of corrosion if for example, they are left wet. As an analogy, think of the staining on your cutlery in the dishwasher if left wet.

A copper sulphate test may be used to check for improper material selection and we may do this, if passivation indicates poor materials. This is also very useful, when we are processing repairs and refurbishing instrument trays, as it aids identification of poor quality steels, which may contaminate whole trays with rust. The test can also be used to check passivation has been carried out correctly, as it identified any free iron, remaining on the surface of the stainless steel, which would be a corrosion risk.

Even though the stainless steel is generally corrosion resistant, it does not mean that rust and corrosion cannot occur on it. Perhaps a better term that could reduce this misconception is using ‘stain-resistant’ not ‘stainless or stain-free’. This is because there are many things that may cause corrosion on stainless steel. These things include high acid, or alkaline concentrated detergent, friction with other non-corrosion resistant metals (during manufacturing or afterwards), defects in steel, lubrication electrolytes, conditions involving high chlorides and other factors. Having said this, corrosion can be prevented through proper material and tools selection, manufacturing care and processes like passivation, and proper care and handling in the hospitals before and after each use.

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