n order to understand stainless steel particulate it is first necessary to have a basic understanding of stainless steel as well as forming and finishing operations.
ugh there are five different categories of stainless steel and hundreds of various alloys therein, bio-pharmaceutical needs center on the AUSTENITIC alloys. This category of alloys, and in particular type 316L, is a best choice for machining, forming, and welding for use in depyrogenation, autoclaving and lyophilization operations.
In 1913 Harry Brearly of Sheffield, England accidentally discovered that adding chromium (CR) to iron rifle barrels gave the barrels stain (oxidation) resistance, hence, STAINLESS steel. Subsequent scientific investigation over the next 40 years showed that chromium atoms and their oxides are approximately the same size and fit together tightly on a metal surface to form a protective stable shield just a few atoms thick. Of all the ingredients in stainless steel, chromium has the least corrosion resistance and quickly forms a protective chromium oxide layer in the presence of oxygen.
In addition to chromium, it is also important to understand the 316L alloy element molybdenum (MO) with the sixth highest melting point of any element. First isolated in 1781, molybdenum was not used for over 100 years, when it was found to enhance structural iron protection from oxidation by chlorides common in seaside air environments. By the 1950s boat builders realized that 316L stainless steel protected boat fittings from salt air and saltwater making it a best choice for quality nautical hardware. Molybdenum readily forms carbides in stainless steel, adding strength and durability.
316L stainless steel alloy with alloy enhancing molybdenum features include:
- Additional hardness over common type 304 stainless steel (#316L: 217 Brinnel hardness vs. #304: 123 Brinnel hardness)
- Low carbon (C) content for optimal welding (hence the “L” in 316 L)
- Best surface protection in a low oxygen environment
- Additional high heat chromium oxide protection. (Note: all stainless alloys with chromium content over 18% have a high heat capability to withstand 870 degrees Celsius. 316L resists oxidation at higher heats than stainless alloys without molybdenum.)
Converting the 316L Brinnel scale hardness of 217 to the Rockwell ‘C’ scale rates 316L stainless steel at a relatively soft 17Rc. In comparison, boron silicate glassware common to the industry is rated at 68 to 72Rc. This easily explains rapid wear and tear of stainless steel trays used in conjunction with vials and bottles. Other abrading factors include glass finish (extruded or molded), weight, size and traffic.
Boron silicate glassware is the hardest material to normally contact stainless steel vial trays. The second hardest material is stainless steel itself as stacked upon itself or stored on stainless steel rail supports in cabinets and carts.
Stainless steel particulate is generated by breaking off ultra thin ridges formed by scratching or roughening stainless steel. Ultra-thin ridges may also be formed by shearing and punch perforating.
Considering the obvious softness of stainless steel and the hardness of boron silicate glass, there is no way around generating some stainless particulate in use. This particulate, however, may be minimized in several ways.
OPTIMAL STAINLESS STEEL VIAL TRAY MANUFACTURING