Corrosion Resistance Test

For stainless steel surgical instruments, there are many tests which can be done,but the most important are the following:-

i- Copper Sulphate Test:
   During the process of manufacturing, it is very likely that certain free iron particles are left on the surface, which  are not visible ordinarily. This test is carried out to confirm the presence or otherwise of such particles. This test is done in a solution of CUSO and HSO at 15 C for 6 minutes. The instruments are thoroughly cleaned with soap and water and then rinsed in hot water. The instruments are then immersed in CUSO solution which   reacts with any Free Iron  on the surface of the instruments, and copper deposits on them. This Cu.deposit is removed by wet piece of cotton cloth. There shall be no plating at the periphery or drops of solution or at soldered or brazed junctions or dulling of polished surface caused by coppersulphate solution shall be disregarded. A slight plating of copper in small part of looks, ratchets and serrations of jaws shall also be discarded.

ii- Boil Test:

 The boil test requires immersing the instruments, in boiling distilled water. The instruments are cleaned with soapier water and thoroughly rinsed in hot water before putting in boiling water. The instruments are heated in boiling water for 30 minutes and let to stay in the water for 2 hours. These are removed and kept for 2 hours in open air prior to examination. The instruments are wiped with a dry cloth and inspected for visible sign of corrosion. Any blemish, not removed by vigorous hand rubbing with a cloth and shall be considered evidence of corrosion.

Selection Of Stainless Steel For Surgical Instruments

Introduction

Stainless steel for surgical & dental instruments which are commonly used have been given under in grade tables related Austentic, Ferritic and Martensitic types of steels.You will find useful information in comparison of different grades.
These table are intended to relate former BS, En, German and Swedish grade designations to the current EN steel numbers, AISI grades (‘grades’ in (brackets) are not a true AISI grades) and UNS (Unified Numbering System) numbers. The table is based on the ‘Wrought’ ie long products (bars etc), flat products (plates etc) steel numbers published in En 10088 and related standards. Castings products use different compositions and so have their own steel numbers in EN 10283. The related castings grades in both EN 10283 and BS 3100 are included in the table.

Applications

Grades AISI 410 / 420 Types and are used extensively for dental and surgical instruments. They offer moderate corrosion resistance in comparison to other types of stainless steel (eg austenitic and duplex grades).They is used for applications where cutting edges, wear resistance and strength are required. A good combination of corrosion resistance and a range of mechanical strength via heat treatment can be expected from these grades.
Long  service lives should be expected from martensitic  stainless steel dental and surgical instruments, properly manufactured and subjected to appropriate cleaning procedures. For example, dental extraction forceps usually have an average service life of 15 years. There are some examples where such instruments have given 30 years service life. Other more delicate instruments and those with cutting edges may be expected to have a much shorter service lives, but they should not be expected to suffer corrosion damage.

Application Of Stainless-Steel Grades In different Types Of Surgical Instruments

Knives               302, 303, 420, 440

Osteotomes       410, 440

Reamers            410, 630

Rongeurs           410, 420

Scalpels             420, 440

Scissors             410, 420

Dissectors          410, 416, 420

Noncutting

Cannulae, Needle Vents                302, 303, 304

Forceps                                        302, 303, 304, 410

Retractors                                     302, 303, 304, 410, 416, 420, 431, 440

Specula                                         302, 303, 304, 316

Spreaders                                     302, 303, 304, 410, 416, 440

Clamps                                         303, 304, 410, 416, 420

Drills                                             303, 440

Handles                                        303, 304

Hammers, Mallets, Rulers,
Screws, Tunnelers                        303

Punches                                       303, 410, 416, 420

Skin Hooks                                 303, 410, 416, 420

Suction Tubes                              303, 304

Probes, Tongs                             303, 440

Holders                                       304, 410

Clip Applicators, Dilators            410

Elevators                                    410, 420

Burrs                                         420F

Orthopedic Instruments              430

Needles                                     420

Table 1, Types Of Stainless Steel Used For Medical Instruments

Family of Stainless Steel.

There are several types of stainless steel-FERRITIC, MARTENSITIC, AUSTENITIC and DUPLEX.

Ferritic Steel
The ferritic steel are magnetic, have a low carbon contain chromium as the main alloying element, typically at the 13% and 17% levels.

Martensitic Steel
The martensitic steel are magnetic, contaning typically 12% chromium and a moderate carbon content.They are hardenable by quenching and tempering like plain carbon steels and find their main application in cutlery manufacture, aerospace and general engineering.

Austenitic Steel
The austenitic steels are non-magnetic and, in addition to chromium typically at the 18% level, contain nickel, which increases their corrosion resistance.They are the most widely used group of stainless steel.

Duplex Steel
Duplex steels are used where combinations of higher strength and corrosion resistance are needed.

Super Austenitic
Super austenitic grades and precipitation hardened grades of steel are also available.

Useful information concerning the steel alloy elements

Carbon (C):

Carbon is the foremost alloy element of steel and it has the farthest reaching influence on it. In addition to carbon every unload steel contains silicon, manganese, phosphorous and sulphur which are introduced unintentionally during the manufacture. The addition of further alloy elements to produce specific, desired effects and the intentional increase of the contents of manganese and silicon gives rise to alloy steel. As the carbon content rises, the mechanical strength and the hardening properties of the steel improve, but its elasticity, forging, welding, and cutting properties suffer. The carbon content has substantially no influence at all on the corrosion resistance to water, acids and hot gases.

Cobalt (Co) Melting Point 1492 °C:

Cobalt doesn’t form carbide. It hinders the grain growth at higher temperatures and greatly improves the resistance to tempering and the hot tensile strength; it is therefore, often an alloy element of high speed steel, hot work steels, and heat resisting raw materials. It acts favorably on the graphitic formation, and greatly increases residual magnetism, coercive force, and thermal conductivity, therefore the alloy basis for high grade permanent magnet steels and alloys. If subjected to neutron rays it forms a strong radio-active isotope Cobalt 60 for which reason it is undesirable in steels for atom reactors.

Chromium (Cr) Melting Point 1920 °C:

Chromium increases the hardness and strength and only minimally reduces the elasticity. It improves the resistance to heat and non scaling properties. With higher Chrome content the steels become corrosion resistant and with Carbon form a high wear resisting Carbide. The welding properties deteriorate in pure Chromium steels with increasing Chromium content. Chromium is a strong Carbide former. The tensile strength of the steel rises by 8----10 Kg/mm² per 1% Chromium. The yield point is likewise increased, however not at the same rate, but the notch impact value is lowered.

Manganese (Mn) Melting Point 1244 °C:

Manganese improves the strength properties of steel, while only slightly impairing its elasticity: furthermore, manganese has a favorable influence on the forging and welding properties. Higher contents of manganese in the presence of carbon increase the wear resistance very substantially. With up to 3% Mn the tensile strength of the steels is increased by about 10 Kg/mm²  for every percent of Mn with Mn contents above 3 to 8% the increase rises more slowly and at more than 8% of Mn it drops off again. The yield point behaves in a similar manner. Manganese increases substantially the depth of hardening.

Molybdenum (Mo) Melting Point 2610 °C:

Molybdenum improves the tensile strength and especially the heat resistance and also a favorable influence on the welding properties. Steel with a higher Mo content tends to be difficult to forge. Molybdenum is frequently used in conjunction with chromium. The behavior of Molybdenum resembles that of Tungsten. When used in alloy steels in combination with Chromium and Nickel, Molybdenum may produce high yield point and tensile strength values. Molybdenum has a strong tendency to form Carbide and is the alloy element of choice in high speed and hot working steels; in austenitic corrosion- resistant steels, case hardening and heat-treating steels as well as in heat resistant steels, also in view of the diminution to over-drawing brittleness.

Nickel (Ni) Melting Point 1453 °C:

Nickel raises the strength of steel less than does Silicon or Manganese with elasticity dropping only insignificantly. Ni ensures good through hardening, especially so when the steel contains also Chromium. Chrome nickel steels are stainless and resistant to scaling and also heat resistant. Nickel doesn’t impair the welding properties. Nickel increases the notch impact value of structural steels considerably, especially at low temperatures. In the sphere of steel alloying Nickel is especially suitable for use in austenitic steels, steels resistant to corrosion and scaling and in casehardening and heat treating steels to improve their toughness.

Phosphorus (P) Melting Point 44 °C:

There are various kinds of Phosphorus, viz. white (yellow0, red (purple), Black Phosphorus and others. Quite generally, Phosphorus is considered to be detrimental to steel so that it is endeavored to keep the P content in high grade steels at a maximum level of 0.03 to 0.05%.

Sulphur (S) Melting Point 118 °C:

Sulphur produces “red shortness”, makes steel brittle and is therefore harmful-Contents of 0.025% or 0.030% are permitted. Exceptions are the free-machining steels to which is added up to 0.30% so that the small distributed Sulphide inclusions disturb the metallic cohesion and therefore contribute to the formation of short turnings.   

Silicon (Si) Melting Point 1410 °C:

Like Manganese, Silicon is present in all steels since the iron ores used in their manufacture contain a varying amount of it. Further Silicon stemming from the refractory lining of the furnace is introduced into the melt during the manufacturing of steel. The term “Silicon steels” however, includes only steels having Silicon content above 0.40%. Si is not a metal but rather a so-called metalloid like, for example, Phosphorus and Sulphur. Silicon increases the mechanical strength, the resistance to scaling and the density; especially of cast steel. The elasticity is only insignificantly affected; while the tensile strength is increased by about 10 Kg/mm²  for each percent of Si and the yield point is raised to a similar degree. Steel having a higher content of Silicon turns coarsely granular. A high Silicon content (about 14%) enables steel to resist chemical attacks but it can no longer be forged.

Introduction-AISI ASTM ASME And UNS,Chemical Compositions of AISI (ASTM/ASME) and UNS austenitic stainless Steel grades,Martensitic and Precipitation hardening steel grades

Most ASTM and ASME standards list the steel grades by their UNS (Unified Numbering System) numbers but also make reference, where appropriate to the more general AISI (American Iron And Steel Institute) grade designations.These grade numbering systems are widely used in the USA,where they originated and are recognized by most stainless steel specifiers and users.

The American Iron And Steel Institute (AISI) developed designations such as 304, 430 etc and published compositions for these in their "Steel Products Manual" (1974)

These are NOT specifications,but steel grade composition ranges only.

These grades were used by the American Society Testing And Materials (ASTM) to identify grades in a wide range of standards they published for stainless products, such as sheet and plates (ASTM A240), bars (ASTM A276) and tubes (ASTM A269).The compositions of the AISI grades were made more specific with the introduction of the "Unified Numbering System",jointly established by ASTM and SAE (Society Of Automotive Engineers).

This five digit number, preceded by the letter 'S' for most stainless steels,identified the specific variant of the grade.

e.g. 420 being UNS S42000, 630 or 17/4PH being UNS S17400

e.g. 304 being UNS S30400, 304L being UNS S30403

American Society Of Mechanical Engineers (ASME) publish design codes and standards for pressure applications.ASME standards are generally based on the appropriate ASTM standards,but the standard numbers are preceded with the letter 'SA'; rather than just the 'A' of the ASTM standards.

Where UNS grade numbers are used, these are universal, regardless of the product type or the standard in which they appear.

The AISI designations have been adopted in Japan.The aisi number is preceded with the letters SUS e.g. SUS 304.The Japanese have developed variants on some grades for certain products forms e.g. SUS 304J1 for sheet productss.

The composition ranges should always be checked in the appropriate standard.

Metallurgical And Chemicl Aspects Ot The Surgical Instruments Made From Matensitic Stainless Steel.

The main necessity of all types of surgical instruments from the surgeon's point of view is Resistance To Corrosion.Hence, while evaluating alloy for an instrument, it is extremely essential to consider this property.However,the other factors,pertaining to manufacturing such as Forge-ability ,Weld-ability,Machine-ability,Cost etc; are also important.

Both martensitic and austenitic stainless steel are used for the manufacturing of surgical instruments.The typical applications include:-

i- AISI-304 (For Sheet Metal Item)

ii-AISI-410 (For Homeostatic Forceps)

iii-AISI-420 (For Scissors And Ronguers)

iv-AISI-440 (For Knives And Dental Pliers)

The carbon content between 0.12 to 0.15 is sufficient for Forceps as the requirement of hardness is up to 40 HRC,but for cutting instruments like Scissors and Knives, the carbon content, must be at least between 0.30 to 0.35%.The Chromium content up to 13% is acceptable, as up to this range, corrosion can be controlled,provided  proper fabrication polishing and cleaning methods are followed.

Introduction To Stainless Steel

Stainless steel are iron alloys with a minimum of 12% chromium.Other alloying elements are added to enhance their structure and properties such as formability,strength and toughness.

The main requirement for stainless steel is that they should be corrosion resistant for a specified application or environment.The selection of a particular "type" and "grade" of stainless steel must initially meet the corrosion resistance requirements.Additional mechanical or physical properties may also need to be considered to achieve the overall service performance requirements.