Selection of stainless steels for surgical implants

Introduction

Stainless steels 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 standards are incorporated into British Standards as BS 7252, parts 1:1997 and 9:1993 respectively. The other parts of these standards cover other alloys for implants, including titanium, nickel and cobalt alloys.

Stainless steel grades for surgical implants

The three grades listed in these standards are austenitic types with specific compositions 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 steels.

THERE ARE NO EQUIVALENT STEEL GRADES IN BSEN 10088 OR THE OBSOLETE BS 970 STANDARDS.

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 steels

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 methods.

Grain size and ferrite level

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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