F138 stainless steel (316LVM) – UNS S31673
18Cr-14Ni-2.5Mo austenitic stainless steel for surgical implants
Variant of 316L with higher Ni and Mo, rigorous inclusion cleanliness and a fully austenitic microstructure (no delta ferrite), specified by ASTM F138 for implant bar and wire. Normally vacuum remelted (VAR) or ESR, hence the name 316LVM.
Designations and equivalents
| System | Designation |
|---|---|
| UNS | S31673 |
| W.Nr./EN | 1.4441 (X2CrNiMo18-15-3) |
| DIN | X2CrNiMo18-15-3 |
| Trade names | 316LVM |
| Others | ISO 5832-1 (composition D); ASTM F139 (sheet) |
Standards by product form
| Form | Standard |
|---|---|
| Bar and wire | ASTM F138 |
| Sheet and strip | ASTM F139 |
| Forging | ASTM F621 |
| International | ISO 5832-1 |
Tolerances and HS codes
- Forged bars (round and square): machining allowance from 2 mm (Ø 6–16 mm) to 24 mm (Ø 400–500 mm) on the diameter. E.g. Ø 100–125 mm finished: +7 mm, tolerance +6 / −0 (length up to 3,500 mm).
- Forged discs: allowance of 4 to 9 mm on the diameter and 3 to 11 mm on the height, plus-only tolerance.
- Cutting and machining: band saw −0 / +5 to +15 mm on length; waterjet +10 % of thickness on the diameter (min. 2 mm); machined parts −0 / +2 to +5 mm on the diameter. See the full tables (INF 002 and INF 003, the same for every alloy).
| Form | NCM (Brazil, 8 digits) | HS (international, 6 digits) | HTS (USA, 10 digits)* | CN (European Union, 8 digits)* |
|---|---|---|---|---|
| Forged bar | 7222.30.00 | 7222.30 | 7222.30.0024 | 7222 30 51 |
| Hot-rolled bar | 7222.11.00 | 7222.11 | 7222.11.0059 | 7222 11 11 |
| Plate (> 10 mm, width ≥ 600 mm) | 7219.21.00 | 7219.21 | 7219.21.0020 | 7219 21 10 |
| Flange and fitting | 7307.21.00 | 7307.21 | 7307.21.5000 | 7307 21 00 |
Tariff class: stainless steel (by chemical composition, under the Chapter 72 Notes and Section XV Note 5).
*: US and EU codes for reference. NCM per current legislation (General Rules of Interpretation, Notes to Chapters 72, 74 and 75 and Section XV Note 5). Confirm the final code with your customs broker according to the item size and finish. HS = first 6 digits of the NCM.
Chemical composition (%)
ASTM F138
| Element | Range (%) |
|---|---|
| C | ≤ 0.030 |
| Mn | ≤ 2.00 |
| P | ≤ 0.025 |
| S | ≤ 0.010 |
| Si | ≤ 0.75 |
| Cr | 17.00 – 19.00 |
| Ni | 13.00 – 15.00 |
| Mo | 2.25 – 3.00 |
| N | ≤ 0.10 |
| Cu | ≤ 0.50 |
| Fe | balance |
| Cr + 3.3×Mo | ≥ 26.0 |
Mechanical properties
Annealed bar (ASTM F138); cold worked in parentheses
| Property | Value |
|---|---|
| Yield strength (0.2%) | ≥ 190 MPa (cold worked: ≥ 690) |
| Tensile strength | ≥ 490 MPa (cold worked: ≥ 860) |
| Elongation (4D) | ≥ 40% (cold worked: ≥ 12%) |
| Extra hard (Ø 1.6 – 6.4 mm) | Rm ≥ 1,350 MPa |
| Grain size | ASTM 5 or finer |
| Inclusions (E45 A) | ≤ 1.5 thin / ≤ 1.0 heavy |
Typical physical properties
| Property | Value |
|---|---|
| Density | 8.0 g/cm³ |
| Melting range | ≈ 1,375 – 1,400 °C |
| Modulus of elasticity | ≈ 193 – 200 GPa |
| Thermal conductivity (20 °C) | ≈ 15 W/m·K (typical) |
| Thermal expansion (20–100 °C) | ≈ 16 µm/m·K (typical) |
| Magnetic permeability | non-magnetic (no delta ferrite) |
Heat treatment, welding and machining
| Item | Usual practice |
|---|---|
| Service temperature | Use at body temperature; not applicable to thermal service. |
| Heat treatment | Supplied annealed (solution annealing ≈ 1,040 – 1,100 °C, rapid cooling) or cold worked according to the condition ordered. No delta ferrite, chi or sigma (ASTM E407, 100×). |
| Welding | Normally not welded in implants. In instruments, TIG with high-purity ER316L (typical). |
| Machining | Machinability lower than carbon steel: it work hardens quickly; use sharp tools, positive rake, constant feed without 'rubbing' and abundant coolant. Inclusion cleanliness reduces machinability compared with ordinary 316L. |
Filler metal for welding
The wire composition is usually not identical to the material: each page explains why.
Typical applications
- Osteosynthesis plates and screws
- Intramedullary nails and pins
- Kirschner and cerclage wires
- Steinmann pins and external fixators
- Temporary trauma implants
Alternatives: Titanium Ti-6Al-4V ELI (ASTM F136) for permanent implants and better biocompatibility; high-N steel (ASTM F1586) when more strength is required; CoCrMo (F1537) in joint prostheses.
F138 stainless steel (316LVM): complete guide
What ASTM F138 steel is
F138 is the number of the ASTM standard that specifies stainless steel bar and wire for surgical implants; the material itself has the designation UNS S31673 and is known as 316LVM (vacuum melted). It belongs to the family of austenitic stainless steels and derives from 316L, but with purity and microstructure requirements that industrial 316L does not meet.
Chemical composition and additional requirements
Chromium between 17 and 19%, nickel between 13 and 15% and molybdenum between 2.25 and 3.00%, with carbon up to 0.030%, sulfur up to 0.010% and phosphorus up to 0.025%. Besides the composition, the standard requires Cr + 3.3 × Mo of at least 26, ASTM grain size No. 5 or finer, inclusion limits per ASTM E45 and the absence of delta ferrite. Manganese is limited to 2%.
Mechanical properties by condition
Annealed, F138 has a minimum yield strength of 190 MPa, minimum tensile strength of 490 MPa and minimum elongation of 40%, and that 40% is a minimum, not a maximum. In the cold-worked condition, used in bone plates and screws, the minimums rise to 690 MPa yield and 860 MPa tensile, with a minimum elongation of 12%. In the extra-hard condition, for wires and thin rods, the minimum tensile strength is 1,350 MPa.
Corrosion and biocompatibility
The requirement Cr + 3.3 Mo ≥ 26 ensures pitting resistance in chloride-containing physiological fluid at 37 °C, and vacuum remelting reduces the inclusions that initiate pitting and fatigue cracks. F138 is indicated for temporary implants, such as osteosynthesis plates, screws and nails, and for instruments; permanent joint implants use titanium and cobalt-chromium alloys.
Applications
Osteosynthesis plates, screws, intramedullary nails and wires, external fixators, surgical and dental instruments, medical equipment components in contact with the body. For generic industrial use, ordinary 316L is the economical choice.
Supply forms and Dustre services
Round bars with an inspection certificate per ASTM F138 and heat traceability, band saw cutting to size and rough or finish machining.
Frequently asked questions
What is the chemical composition of F138 stainless steel (316LVM)?
Per ASTM F138: C ≤ 0.030; Mn ≤ 2.00; P ≤ 0.025; S ≤ 0.010; Si ≤ 0.75; Cr 17.00 – 19.00.
Which standards cover F138 stainless steel (316LVM)?
Bar and wire: ASTM F138. Sheet and strip: ASTM F139. Forging: ASTM F621. International: ISO 5832-1.
Up to what temperature can F138 stainless steel (316LVM) be used?
Use at body temperature; not applicable to thermal service.
Which materials can replace F138 stainless steel (316LVM)?
Titanium Ti-6Al-4V ELI (ASTM F136) for permanent implants and better biocompatibility; high-N steel (ASTM F1586) when more strength is required; CoCrMo (F1537) in joint prostheses.
