High-strength, cold-forged type 316L stainless steel for orthopedic implant

• US 4,718,908 A
• Filed: 06/04/1984
• Issued: 01/12/1988
• Est. Priority Date: 06/04/1984
• Status: Expired due to Term

• Current Assignee: Smith & Nephew Richards Incorporated (Smith & Nephew PLC)
• Original Assignee: Richards Medical Company
• Inventors: Wigginton, Robert E., Gordon, Barry, Baswell, Imogene, Garner, Steve
• Primary Examiner(s): Larson, Lowell A.
• Application Number:
  US06/616,659
• Time in Patent Office: 1,317 Days
• Field of Search: 148/12 E, 148/38, 72/377, 72/700, 128/92 B, 128/92 BA, 128/92 BB, 128/92 BC, 128/92 D, 3/1.9, 75/128 R, 623/16, 623/23
• US Class Current: 623/23.53
• CPC Class Codes: A61F 2/36 Femoral heads ; Femoral endoprostheses
• CPC Class Codes: A61F 2310/00017 Iron- or Fe-based alloys, e.g. stainless steel
• CPC Class Codes: A61L 27/042 Iron or iron alloys
• CPC Class Codes: C21D 7/10 of the whole cross-section, e.g. of concrete reinforcing bars
• CPC Class Codes: C21D 8/005 of ferrous alloys C21D8/02 - C21D8/12 take precedence
• CPC Class Codes: Y10S 72/70 Deforming specified alloys or uncommon metal or bimetallic work

Abstract

A surgical implant manufactured from austenitic stainless steel by cold-forging has improved mechanical properties and corrosion fatigue resistance. The grain-size index for the cold-forged 316L stainless steel is increased from an ASTM grain-size index of about 8 to an ASTM grain-size index of about 14 by cold-forging the steel into the desired shape. Further improvement in mechanical properties and corrosion fatigue resistance is obtained by stress relieving the material after cold-forging into the desired configuration. The microstructure of the cold-forged material is characterized by the absence of visible slip planes at 600× magnification, in contrast with visible slip planes present in the microstructure of the austenitic stainless steel starting material.

Claims

1. A corrosion resistant, austenitic, stainless steel prosthetic device, adapted to be implanted in animals, and formed as a result of a method comprising the step of cold forging a starting material consisting essentially of biocompatible 316L stainless steel with a maximum carbon content of about 0.03% until it has a microstructure comprising an ASTM grain-size index above about 14 and having an absence of discernible slip planes at a magnification of about 600×

.

2. The method of claim 1 wherein the cold-forging step is practiced to alter the ASTM grain-size index of the material from about 8 for the starting material to an ASTM grain-size index of about 14 or higher for the forged material.

3. The method of claim 2 wherein the starting material is a length of bar stock and the cold forging step includes:

initially forging opposite end portions of the bar stock;

bending the initially forged bar stock about an intermediate portion thereof; and

further forging one end portion of the initially forged, bent stock to a predetermined shape.

4. The method of claim 3 wherein the initially forging step of the cold forging process includes a rapid reduction in diameter of the starting bar.

5. The method of claim 3 wherein the further forging step of the cold forging step includes pressing the one end portion of the stock with a shaped die.

6. The method of claim 5, further comprising the step of:

stress relieving the forged material after the rapid reduction step, wherein the stress relieving step includes

heating the material to the temperature of about 750°

F. (399°

C.); and

maintaining the material at the temperature for about two hours, followed by ambient air cooling.

7. The method of claim 6 wherein the cold forging step is practiced to form a surgical implant.

8. The method of claim 7 wherein the surgical implant is a hip plate.

9. The method of claim 8 wherein the starting material is formed of Type 316L stainless steel.

10. The method of claim 9 wherein the pressing step includes applying a force greater than about 700 U.S. tons.

11. A method of manufacturing a surgical implant device from bar stock Type 316L stainless steel starting material below its recrystallization temperature, comprising the steps of:

rapidly reducing opposite end portions of the starting material, leaving an intermediate portion unreduced;

bending the reduced bar about the intermediate portion to form a predetermined angle between the opposite end portions;

pressing one of the end portions into a suitable configuration; and

wherein the rapid reduction and pressing steps result in the device having an ASTM grain-size index above about 14 and having an absence of discernable slip planes at a magnification of about 600×

.

12. The method of claim 11, wherein the device is stress relieved after the cold forging steps.