Selection of Stainless Steel for Surgical Implant
Stainless steel for surgical implants are specified in ISO 5832. Part 1 (1997) covers two steel types, compositions D and E. Part 9 (1992) covers a single high nitrogen grade.
Both these ISO standard are incorporated into British Standard as BS 7252, parts 1:1997 and 9:1993 respectively. The other parts of these standard cover other alloys for implants, including titanium, nickel and cobalt alloys.
Stainless steel grades for surgical implants
The three grades listed in these standard are austenitic types with specific chemical composition for these special applications. The grades in Part 1 are sometimes wrongly referred to as the '316 implant grades', although the chromium, nickel and molybdenum contents overlap some of the commercially available 316 steel.
There Are No Equivalent Steel Grades In EN 10088 OR The Obsolete BS970 Standard.
The compositions, in percent, are shown below:
| Elements |
ISO 5832-1 'D' |
ISO 5832-1 'E' |
ISO 5832-9 |
| Carbon - C |
0.030 |
0.030 |
0.08 |
| Silicon - Si |
1.0 |
1.0 |
0.75 |
| Manganese - Mn |
2.0 |
2.0 |
2-4.25 |
| Phosphorus - P |
0.025 |
0.025 |
0.025 |
| Sulphur - S |
0.010 |
0.010 |
0.01 |
| Nitrogen - N |
0.10 |
0.10-0.20 |
0.25-0.5 |
| Chromium - Cr |
17.0-19.0 |
17.0-19.0 |
19.5-22 |
| Molybdenum - Mo |
2.25-3.5 |
2.35-4.2 |
2-3 |
| Nickel -Ni |
13.0-15.0 |
14.0-16.0 |
9-11 |
| Copper - Cu |
0.50 |
0.50 |
0.25 |
| Niobium - Nb |
- |
- |
0.25-0.8 |
Figures are maxima, unless a range is shown.
Additional requirements for implant stainless steel
Chemical Composition
ISO 5832-1 also requires that the basic pitting index, expressed as C=3.3 Mo + Cr, is 26 minimum for steel compositions D and E.
In addition to the special composition limits, there are additional requirements that preclude the use of standard '316' grades, produced by normal, commercial, steelmaking method.
All steels must have an ASTM grain size finer (ie a higher number) than No4 and be free from detectable ferrite
Non-metallic inclusion levels (steel cleanness)
Both standards require that the non-metallic inclusion levels are measured when the steel is in the billet form and set limits for sulhpide, aluminate, silicate and oxide inclusion types. Although not made mandatory by the standards, vacuum or electroslag re-melt steelmaking methods are suggested to meet the specified steel cleanness levels.
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