Geoff,
Just info.
17-4pH Stainless is made by precipitation-hardening stainless steel. Offers a good balance between corrosion resistance and strength. Magnetic. Hardenable to various strength levels and hardness by varying the aging heat treat temperature. Typically provides better corrosion resistance than the 400 series stainless steels, and better strength than the 300 series stainless steels. Often used for aircraft, dental, marine, medical, surgical, and applications where high levels of strength and hardness, and good corrosion resistance is required.
15-5 stainless is a 15.0% chromium stainless steel that provides the corrosion resistance of 440 C with the added benefit of improved carbide size and distribution for noiseless miniature precision bearing operation. The alloy can be through-hardened to achieve a minimum hardness of Rockwell C 60. The optimized alloy additions of C, N, Cr, and Mo provide a combination of the best carbide structure, corrosion resistance and heat treat response. The high hardness and corrosion resistance make it a suitable candidate for use in miniature bearing, cutlery, medical implement, and other special applications.
Tensile properties are for Austenitized at 1975°F, warm oil quenched, subzero treated to -120°F, and tempered at 350°F
Both are well suited for the applicaiton.
Aluminum displays very different properties depending upon how it is processed. Billet or forged aluminum is not processed quite the same as steel but is still considerably stronger than cast aluminum. Casting does not provide as dense a material and does not align the grain structure of the material - when it fails, it has a rough gritty looking surface where it failed. Forged or billet aluminum will srtetch as it fails and has a smoother torn look. A well made aluminum rocker is adequate for 90% of the applications out there. It will take repeated stress cycles almost indefinitely as long as the load is equal to or below 70% of it's yeild.
Sure some say that aluminum is not a suitable material for rockers but they also used to say that it was not suitable for connecting rods, blocks, etc.
If you have an aversion to Al that is fine, stick with what you are comfortable with by all means. Just wanted to let you know that all the Al rocker manufacturers out there are not selling junk.
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