Well the temper temperature will be down around 800-850F since you are looking for more than 180ksi UTS.
Often small diameter stuff is done hanging from a rack in order to try and keep it straighter.
We are looking for a flexible 1/4" rod steel but hard enough not to bend. We have been told that 4140 42HRC it is what we need but can't find a company to heat treat the steel for us.
"Hard enough not to bend". This is going to need clarification. Heat treating this material will not change the modulus of elasticity, so the stiffness will not change. Heat treating it will however increase the yield strength. So if you are expecting the material to deflect less under the same load after heat treatment, you are going to be disappointed.
At 1/4" diameter you do not need alloy steel to achieve full through hardening. Something like 1040 will do.
I assume what you mean by 'not bending' is 'not permanently deforming'.
"Everyone is entitled to their own opinions, but they are not entitled to their own facts."
