Drawbar extension springs use 316 stainless steel or Porch swing springs are assemblies in which the spring will compress as the drawbar arms extend under an applied load. They are often capable of withstanding loads far in excess of the compression spring closing force and should be considered in applications where a positive stop or overload protection is required.
Drawbar extension spring Material
Low Carbon Steel (Drawbar)
5.75 mm (Compression Spring)
5.00 mm (Drawbar)
183.8 mm (7 1/4 inch)
300 - 350 lbs
If you need the spring not easy to rust, then stainless steel will be the right