Re: improper heat treat vs improper alloys
Take 4150 and harden and draw it to an Rc58 (easy to do).
Take O6 tool steel and H/D it to same spec hardness. Again easy to do.
When you compare the scuff resistance of O6 to 4150 when subjected to high loads and marginal lubrication, HUGE difference and that occurs even though the parts are of the same hardness.
Hardness and tribology are NOT interchangeable properties. Neither is harness and the tensile and fatigue properties.
Although 6061 and 2018 have comparable properties (after all, to some folks they are both heat treatable aluminum), they are NOT interchangeable should you be in the market for an alloy for forged pistons.
In some cases, metal that "rusts, sinks and is magnetic" is quite adequate for an application. However, for other situations, you better be more specifice with the alloy and the H/T specs.