Surgical Grade Stainless Steel Types Used in the Manufacturing of Surgical Instruments
Steel is the most widely used metal in surgical instrument manufacturing and various grades of stainless steel are currently in use. Stainless steel can resist temperatures as high as 400 degrees Celsius and for this reason surgical instruments made of stainless steel can be sterilized in autoclave, which has a temperature around 180 degrees Celsius.
A number of different metals are used for making different varieties of steel alloys. Different metals in varying combinations confer characteristic properties to different steel types. Commonly used alloying elements include Carbon, Chromium, Nickel, Molybdenum and Tungsten.
In surgical instrument manufacturing, the commonly used stainless steel types/grades include AISI-301, 302, 303, 304, 316L, 410, 420 and 440. Some prominent features of these steel types are discussed below:
It contains 18% Chromium/8% nickel combination and is heat resistant. This steel type is frequently utilized for manufacturing ‘springs’ which play a vital role in the functioning of some surgical instruments. Cold-working is required to enhance the strength of 301 type steel, but it loses is corrosion-resistance during this process. This steel type has superior strength, excellent corrosion-resistance and ductility.
302 stainless steel is a chrome-nickel heat-resisting steel, which is more corrosion resistant to the 301 steel type, since it contains higher nickel content. This steel type has a unique characteristic; it is light-weight yet has superior strength. It resists oxidation, can be easily fabricated and is easy to clean. Hence, stainless flatware, like surgical instruments or cookware can easily be manufactured by using this steel type.
This steel type is also termed ‘A1’. In order to improve machining characteristics, sulfur and phosphorus are added to this steel type. Its corrosion-resistance is inferior to both 302 and 304 steel types, since it contains sulfur.
304 (also termed 18/8 or A2)
This steel type consists of a minimum of 18% chromium + 8% nickel, combined with a maximum of 0.08% carbon. This steel type is rust proof, resists staining and has antibacterial properties. Since it cannot be hardened by heat-treatment, higher tensile strengths are achieved through cold-working. Resistance to oxidation and corrosion is due to the presence of chromium. This steel type can resist ordinary rusting and can also withstand oxidizing acids. It can resist the action of most dyes and sterilizing solutions. However, with the passage of time and with repeated use, it is likely to tarnish.
This steel is considered one of the best materials for use in medical device manufacturing. Since steel 304 is extremely workable, it can easily be molded into different shapes to manufacture products like hollow ware, pans, bowls and sinks.
Another variety of 304 type steel is the 304L stainless steel. This variety is considered a ‘cryogenic’ steel type since it has the ability to withstand extremely low temperatures without being affected. An additional feature of 304L is its extra resistance to corrosion.
316 & 316L
In comparison to the 304 stainless steel variety, the nickel content in the 316 steel type is higher. Additionally, it contains about 3% molybdenum. Molybdenum bestows supreme anti-corrosive properties to this alloy. Hence, the 316 steel type is capable of resisting the corrosive action of chemicals like phosphoric acid, sulfurous acids, hypochlorite solutions, brines of calcium and sodium etc.
The low carbon variety of 316 steel, termed 316L, is more resistant to corrosion and pitting. The L-grades of steels have no more than 0.03% carbon content. The 316L steel type has a long-standing history for use in the dental and orthopedic implant devices. This is due to the fact that in-vivo breakdown of this alloy is minimal and implants retrieved from patients’ bodies after extended periods of time show minimal corrosion or pitting.
This steel type is heat and corrosion resistant chromium stainless steel. It resists oxidation and scaling. It can be welded easily and has a better impact strength. It is always magnetic and heat-treatment can contribute to its hardening. The process of annealing, (in which heating and then cooling slowly softens an alloy), can be used to achieve cold drawing and forming of this steel type.
This is a chromium stainless steel, which can be hardened to 500 Brinell (500 BHN) by applying thermal treatment. It is always magnetic, very durable and has supreme corrosion resistance. Its corrosion resistance is maximized only when it is fully hardened. It is widely used in dental and surgical instrument manufacturing.
This steel type is commonly termed ‘razor blade steel’. It is moderately corrosion resistant, yet it possesses high strength. Thermal treatment allows its annealing and hardening; weldability is another feature of this steel variety.
Since it has a high carbon content, it is very hard – a feature which makes it difficult to work with; although its machinability is considered better. By varying the carbon content in 440 stainless steel, different varieties of this steel are produced. These include, 440A, 440B, 440C and 440F.