What’s In a Fastener?

The Fasteners category features a superabundance of different metals and alloys with varying properties and applications. John Power talks to Rob Hodgson, Technical & Quality Manager at Fuji Fasteners in Melbourne, to make sense of fastener materials…

It is hardly surprising that many hardware retail staff find the Fasteners category extremely difficult to handle. With so many references to different steel carbon contents, varying alloy percentages and mysterious plating materials, the task of defining modern fasteners might seem akin to categorising all the plants in the Amazon jungle…where do you start?

A threading machine, used to imprint threads into screws and bolts

Rob Hodgson, with 40 years’ experience in the Fasteners category, is used to overseeing the production of bolts and screws for specialist industrial clients, the majority of whom are from the automotive industry.

Rob says it pays to think of the category in terms of its basic principles, and to explore subsets of certain categories afterwards. So what are the basics? Any analysis of the fasteners sector must begin with steel.

Steel is the foundation of the category, however there are many different types of steel and steel alloys that have clear specialist uses.

“There are basically three types of steel used in typical fasteners,” says Rob. “They are Low Carbon Steel, Medium Carbon Steel and Medium Carbon Alloy Steel.”

 

  • Low Carbon Steels are below 0.25% carbon and are relatively soft and ductile. They get their strength from cold working of the wire and during the cold forming operations used to form the head and thread. Such steels are commonly used for items such as nails and low-strength bolts to ISO property Class 4.6.
  • Medium Carbon Steels contain carbon contents in the range of 0.25 to 0.55%. Fasteners made from these grades are capable of higher strength due to their ability to be hardened by heat treatment. So-called “High Tensile” fasteners to ISO Class 8.8 are typically made from this type of steel.
  • Medium Carbon Alloy Steels have similar carbon contents to the Medium Carbon Steels but have additions of one or more alloying elements such as Boron, Chromium, Manganese and Molybdenum. After heat treatment these alloy steels can attain mechanical properties to ISO 10.9 and 12.9. Fasteners made from these alloy steels usually suit specific applications which may include the requirement of a high-tightening torque or an ability to withstand high temperatures or weights (smaller high strength fastener versus larger low strength fastener).

 

While steel fasteners generally fall nicely into the above categories there can be applications where overlapping will occur. For example, Self Drilling and Self Tapping screws fall within the Low Carbon Steel category but are case hardened by heat treatment to provide a hard surface for the drilling and thread forming required in their application.

Stainless Steel
One of the great misapprehensions about stainless steel is that it is simply a form of High Carbon Steel. This is untrue. According to Rob, stainless steel is iron mixed with varying levels of chromium and nickel. When people talk of stainless steel fasteners they are usually referring to the 300 Series austenitic alloy, which contains around 18% chromium and 8% nickel. The balance is made up of iron with a carbon content less than 0.1%.

Rob Hodgson, from Fuji Fasteners in Melbourne

Another misapprehension about stainless steel is that it is the hardest steel. Again, this is untrue. Rob says common 300 Series stainless steel is harder than Low Carbon Steel, but does not respond to heat treatment. Its major advantage, as the name suggests, is that it resists corrosion well. Standard 300 Series stainless steel, as might be seen in household cutlery, is extremely long-lasting and unlikely to corrode. On the other hand, says Rob, in a chlorine containing atmosphere (as might be found near seawater), the standard 300 Series will corrode. A variation of the 300 Series, designated “316”, has been developed to minimise the corrosion effects of chlorine.

Yet another variation of stainless steel is the heat-treatable 400 Series, occasionally used for high-strength stainless steel fasteners. However, the high-strength 400 Series does not provide as much corrosion resistance as the 300 Series. Good-quality knife blades made from a 400 Series will hold a sharp edge longer. Rob says “it’s always a matter of compromising between hardness and anti-corrosion.”

The compromise is equally subject to cost-efficiency, which ultimately explains why the hardware industry has not adopted High Carbon, High Grade stainless steel for all fasteners: Cost dictates the broad use of lesser-quality materials which may not be subjected to long-life, heavy-load applications.

You can read the rest of this feature on fasteners in the April issue of the Australian Hardware Journal.