How is surgical stainless steel different from ordinary stainless steel?

I knew keeping my old metallurgy textbook would come in handy.

To begin with, there is no such this as "ordinary stainless steel". There are some 200 alloys on the market that are classified as stainless steels (SS), although there are, perhaps, only 15 or 20 in common use. A stainless steel is a high-alloy steel with a minimum Chromium content of 10.5%.

Surgical-grade SS products have to meet certain criteria. They need to be very resistant to corrosion, in particular to those chemicals found within the human body. They cannot be toxic or provoke a reaction from the immune system. As with any material intended for manufacture, SS alloys go through a selection process to match the material properties to the specific application. For example, surgical SS parts do not need to be welded, so a designer could ignore alloys designed for this process. They are usually not machined, so a free-machining SS may not be required. A designer would want to select a high-strength alloy, with low brittleness, because of the difficulty in replacing broken parts.

A designer who selects the wrong material may be faced with premature component failures or unacceptably high manufacturing costs. Selection criteria may include the following.

- Pins: manufactured from wire, usually with little additional forming.

- Screws: also manufactured from wire, but with forged heads and rolled threads. The designer would need to select an alloy that forms easily.

- Plates: attached to bones and manufactuired from plate or sheet. Plates are cut to size, with holes punched and countersunk, probably by forming rather than machining.

- Joints: typically manufactured by forging bar stock, but could be cast. In the former case, a high-strength forgeable alloy is required. A hip joint, for example, is a major load-bearing joint and could not tolerate an alloy that is brittle or has low fatigue strength. Castings can be difficult to produce and care needs to be taken to avoid producing a brittle casting or inclusions (entrapped gas bubbles).

I have a few such parts kicking around the place - I have the pins, plate, and screws that spent the best part of a year in one of my legs.

Alloys intended for the food industry follow a similar design process. The tank on a milk truck, for example, is fabricated by welding. SS alloys can be difficult to weld and the welding process can significantly alter the properties of the material at and adjacent to the weld. Therefore, a SS alloy designed for forming and welding must be used.