increasing tensile strength of 4140 shaft 7ft?

To the OP in Pakistan.
Surface hardening will somewhat increase the strength overall.
It is likely, imo, to be sufficient for barbells, but I suspect it will not be perfectly steady and reliable to be industrially rated above 205 ksi.
IMO:
Yes it works, No we cannot certify it -- says the insurance (lab).

Someone has a bigger quenching setup.
Industrial (auto, truck, machinery) and artisanal quenching goes on all the time in pakistan.
And quenching goes on in every big economy that makes modern pieces.

Imho:
Ask the tooling companies like kyocera, iscar, sumitomo and mitsubishi for who does quenching.
The tooling reps always know who does what.

And this is one of the few questions they can answer.
Most of the time the tooling reps cannot answer anything to anyone, to keep their trust.
(Like rigging reps).

The MD of any of the tooling reps is actually likely to be happy to talk to You.
(I was in that position several times.)
He is likelt to help You.

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