What is the difference between ASTM A320 L7 and ASTM A320 L43?
As far as the mechanical and chemical makeup of both A320 Grades L7 and L43, there isn’t much of a difference at all. The main difference is that the raw material used to make A320 L7 fasteners is typically 4140, whereas the raw material used to make A320 L43 is 4340. Below is information found in the ASTM A320 specification showing the mechanical and chemical differences between the two grades. As you can see the two grades are very similar in all aspects, including the recommended nuts and washers.
Strain hardened nuts available as a supplementary requirement
So why would engineers recommend or specify one grade over the other? A320 L7 fasteners are common or can be manufactured in diameters 2-1/2” and below with relative ease. However, once you get into diameters above 2-1/2”, you will often times see A320 L43 specified. The reason for this is mainly due to material availability, but it is also due to the raw material 4340 being more successful in the heat treating process. It is important to note that even though A320 L7 is not limited to 2-1/2” diameter and below, the raw material will likely need to be rolled and will result in extended lead times and higher costs. Also, the A320 L7 and L43 specifications requires a Charpy Impact test, which becomes more difficult to pass for A320 L7, but is not as difficult with A320 L43 material. This is another reason why engineers will often times specify A320 L43 over A320 L7.