Nuts and bolts fasten together with their threads. The weakest shear plane in the thread profile is where failure can start. That is, the weakest material determines the strength of the connection. The point is, nuts and bolts should be made of the same alloys. Not only that, but your nut should meet or exceed the maximum tensile strength of the bolt, which is the amount of pull the bolt can withstand.
If safety is a key consideration in your application, then you want your nut to be stronger than your bolt. In these instances, the Industrial Fastener Institute (IFI) recommends that your nut should exceed your bolt’s tensile strength by 20%.
Bolt grades indicate the strength of your fastener. Understanding bolt grades is critical to choosing the right fastener. Typically, identification markings on bolt heads include the grade and the manufacturer’s mark. The grades are indicated by raised dashes or numbers.