| |
|
|
Stainless Steels for Tubular Components in Surgical Instruments
By: Robert S. Brown
RSB Alloy Applications
Leesport, PA (USA)
William Fender
VERIDIAM
San Diego, CA (USA)
New Demands on Surgical Instruments
Biocompatibility
Continuing advances in surgical techniques are requiring more instruments that use metal tubular components. The design requirements for these instruments are becoming ever more demanding as the procedures become more complex and confined. Trends in minimally invasive surgical techniques lead to devices that pack more energetic functions into smaller working volumes. Those preferences, in turn, increase demands on the components and their materials of construction. As a result, the designer's material selection process is more complex.
This material selection is critical to insure the most cost effective instrument, one that not only performs as intended, but one that can be fabricated in an economical manner. The optimum material is rarely the least expensive.
The introduction of BioDur® Custom 465® stainless instrument tubing offers the instrument designer a new alloy with superior properties for the tubular components used in many surgical instruments. Custom 465 stainless is a precipitation hardening stainless steel that offers the best combination of strength, fracture toughness, and corrosion resistance commercially available. Its corrosion resistance is similar to Type 304 stainless, while its hardness is equal to Custom 455® stainless, BioDur® TrimRite® stainless and Type 420 stainless.
Selection of the proper material starts with an understanding of the intended use of the instrument. The designer also needs to anticipate how the surgeon will actually use the instrument. In general, the mechanical and wear conditions experienced by hand operated instruments are not as harsh as those seen in power–operated instruments.
Corrosion resistance, bio–compatibility, strength, toughness, fatigue strength, edge retention, wear resistance, galling resistance and fabricability must all be considered in selecting a material.
The following review of Custom 465 attributes illustrates why it is an ideal alloy for the tubular components of today's surgical instruments.
Corrosion Resistance In Body Fluids and Cleaning Agents
The first attribute of a material used for a surgical instrument is its corrosion resistance. Historically, the inherent corrosion resistance of "stainless steel" has been deemed adequate for surgical instruments. In today's environment, improved corrosion resistance is considered a must. It is crucial to consider not only the body fluids, but the post–surgical instrument cleaning techniques.
When the components of an instrument are "passive" in the design (i.e., they don't transmit electricity, ultrasonic vibration, bear a load, rotate at high frequency, etc.), For "active" components, premature failure from corrosion moves up the designer's list of concerns.
Improved Corrosion Resistance (Increasing resistance from left to right)
| Type 420 |
Custom 455®, BioDur®, TrimRite® |
Custom 465®, Custom 450® |
Type 316 /L |
|
|
Custom 630® (17-4PH) |
|
|
|
Type 304/L |
|
Strength and Toughness of Stainless Steels
Mechanical properties (strength, ductility, hardness, fatigue strength and toughness) of metals are interrelated. Generally, as the strength and hardness increase, the ductility and toughness decrease. The degree to which this occurs differs between the alloy families and to some degree within a family. Stainless steels are strengthened by one or a combination of the following three methods – precipitation hardening; hardening and tempering; and cold working.
Custom 465 stainless offers the best combination of strength and toughness of any stainless steel commercially available.
Strength and Toughness
| Alloy Family |
Method of Strengthening |
Tensile Strength |
Toughness
& Fatigue Strength
|
| Custom 465® stainless |
Precipitation Hardening –
Age 900ºF (482ºC) to 1150ºF (621ºC)
|
to 260 Ksi
(1794 MPa) |
Excellent |
| Precipitation Hardening Custom 455® |
Precipitation Hardening ®
Age 900ºF (482ºC) to 1150ºF (621ºC) |
to 245 Ksi
(1689 MPa) |
Good |
| Precipitation Hardening Custom 450®, Custom 630® (17-4 PH) |
Precipitation Hardening –
Age 900ºF (482ºC) to 1150ºF (621ºC) |
to 196Ksi
(1351 MPa) |
Good |
| Martensitic
BioDur®, TrimRite®,
Types 420 |
Harden at 1850/1900 ºF (1010/1066ºC), quench and temper in the range of 350ºF to 700ºF (177ºC to 371ºC) |
to 235 Ksi (1620 MPa) |
Poor to Fair |
| Austenitic
Types 304/L, 316/L |
Cold deformation, cold rolling or cold drawing. Not hardenable by heat treatment |
to 200 Ksi
(1385Mpa)
|
Good |
Edge Retention and Wear Resistance of Stainless Steels.
The edge retention and wear resistance of a metal is determined by the material's hardness, the corrosive environment, and the mating material (if any). Generally, as the hardness of a metal increases, so does edge retention and wear resistance.
However, the method by which the hardness is obtained is also critical. The relationship between alloy families, how they are hardened and the effect on edge retention is shown in the following table.
Edge Retention
| Alloy Family |
Method of Hardening |
Relative Edge Retention |
| Custom 465® stainless |
Heat treatment causes the formation of fine intragranular precipitates which strains the molecular structure and hardens the material. The precipitates are not particularly hard. |
Very Good |
| Precipitation Hardening Custom 455® |
Heat treatment causes the formation of fine intragranular precipitates which strains the molecular structure and hardens the material. The precipitates are not particularly hard. |
Very Good |
| Precipitation Hardening Custom 450®, Custom 630® (17-4PH) |
Heat treatment causes the formation of fine intragranular precipitates which strains the molecular structure and hardens the material. The precipitates are not particularly hard. |
Good |
| Martensitic BioDur® TrimRite® Type 420 |
Formation of hard, carbon rich particles (carbides) through heat treatment (harden and temper). |
Excellent |
| Austenitic Types 304/L, 316/L |
Cold working causes deformation of the metal's structure, which results in an increase in hardness. Heat treating will generally not cause an increase in hardness. |
Fair to Poor |
The edge retention of a martensitic stainless will be better than that of a precipitation hardening stainless or austenitic stainless at the same hardness due to the wear resistance of the hard carbides in the martensitic stainless.
Effect of Welding
The heat generated when a metal is welded causes metallurgical changes which differ with each alloy family. These changes range from softening the metal to making it very hard and brittle.
While the welding method can influence these changes, all fusion (Metal Inert Gas – MIG, Tungsten Inert Gas –TIG, Laser, and Electron Beam - EB) welding processes cause them to occur. These effects tend to be less severe with Laser and EB welding than with MIG and TIG welding. Resistance and inertia welding minimize these changes. The following table summarizes these changes and corrective heat treatments.
Welding
| Alloy Family |
Weld Area Metallurgical Changes |
Post Weld Heat Treatment |
| Custom 465® stainless |
Base metal exhibits both aged andannealed properties. Grain growth will occur. Toughness may be reduced. |
Requires solution anneal and age to recover maximum properties |
Precipitation Hardening Custom 455®,
Custom 450®,
Custom 630® (17-4PH) |
Base metal exhibits both aged and annealed properties. Grain growth will occur. Toughness may be reduced. |
Most PH alloys require solution anneal and age to recover properties. Custom 450® may be aged directly after welding. |
| Martensitic BioDur® TrimRite® Type 420, Types 440A, B, C |
Base metal in high temperature heat affected zone and weld deposit becomes hard and brittle. Rapid grain growth occurs in these areas. Severity increases as the alloy's carbon content increases. |
Material must be properly cooled from the welding temperature, annealed, hardened and tempered. Lower carbon grades such as 420 & TrimRite® may usually be hardened and tempered without solution annealing. |
| Austenitic Types 304, 316 |
Base metal is annealed, softened, in the high temperature heat affected zone. Potential significant loss of corrosion resistance in heat affected zone. |
Cannot regain original strength if material had been cold worked. Corrosion resistance regained by annealing. |
The post weld heat treatments shown above are the technically correct treatments. Many companies do not follow these procedures. In some cases they develop short cuts or do no heat treatment after welding, because the product is "good enough" for the application.
Custom 465 – The Ideal Solution
Custom 465 stainless offers the unique combination of mechanical and physical properties that makes it the ideal tubular material for surgical instruments. It is a very tough, high strength stainless with the corrosion resistance of Type 304. The alloy's high hardness and resulting edge retention is superior to cold–worked type 304 stainless, and almost on par with some martensitic alloys.
Custom 465's consistent response to heat treatment and good fabricability ensure cost effective manufacturing and consistent instrument quality.
When used for medical tubular components, this PH stainless steel (1) allows the design of thinner tube walls (2) provides more working volume in limited space (3) offers more wear resistance for rotating instruments (4) combines the edge retention and corrosion resistance that are needed for soft tissue cutting applications and (5) has a high strength–to–weight ratio for mechanically demanding surgical techniques.
Custom 465, Custom 455, BioDur and TrimRite are registered trademarks of Carpenter Technology Corporation.
Reprinted with permission of the authors.
|
