4140 LA Steel seem to be a poor choice for HT205-KSI min.... especially solid bar.
Per MMPDS... a 1.00-inch [2.54-cm] Dia bar is thru hardenable to 200-KSI... this is where the 'equivalent-round' table stops for 4140 aircraft quality steel.
For LA Steel, diameters up-to 1.0-inch equivalent-round require tempering, tempering above 200-KSI, the tempering-temperatures have to be 'lower'... and will enter the ~500-to-900F [285-to-755C] 'blue-brittle' tempering temperature range... which has disastrous effect on toughness due to drastic metallurgical changes specific to being in this 'tempering-temperature' range.
For diameters larger-than 1.0, thru hardening decreases dramatically due to a host of quench/temper issues... again related-to the metallurgy of 4140.
NOTE1. 4330, 4340 LA steels are far better choices for combined strength, toughness, machinability, up ~2.5-inches [6.35-cm] Equivalent round.
NOTE2 Also a serious problem with HT/tempering of any LA steel is preventing oxidation-scaling of the surface. This can be done in ~3-ways.
1. Vacuum or inert atmospheres.
2. Coatings that protect the surfaces... but will-not affect the surfaces... such as copper-plating, ceramic-like coatings, etc].
3. Heat treat unprotected... then simply machine-off all surfaces roughly 0.05-to-0.15-inch [depends]… to physically REMOVE the deep-scale/oxidation, exposing bare/pristine metal.
CAUTION1. High strength LA steels have an annoying way of rusting without protective finishes [inorganic or organic]. This problem is much more serious at higher strength [lower ductility] levels... rusting produces surface defects that are stress concentrations... which can progress to being cracks... rusted = busted.
NOTE2.
A stainless-steel bar, such as PH13-8Mo, precipitation hardened to H1000 would be a very good/close fit with few metallurgical issues to contend with. Although initially more expensive... ultimately it would be far simpler to obtain strength/toughness with fewer issues to worry about... and rusting should NEVER be an issue!
Regards, Wil Taylor
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