It broke before, so an upgrade is on order eh ?
Also, 4140-ht is pretty darn common, and easily available, the others listed
above, maybe not so much.
2 Things.. First is that the shaft didn't break, they ran it until the bearings ate into the shaft..
That is not the shafts fault..
2nd.. Any time you upgrade the strength of one component, you run the risk of pushing the failure down the
line.. And if the "system" was designed correctly (for easy repair), the thing that does break(or wear out) should be
the cheapest and easiest part to replace... No point in "up grading" a $5 shear pin if it costs you a $1000 gear
box the next time you hit a rock.
In this case.. Having dealt with some AG stuff, its almost all 1018, I figure its because its cheaper to
add $2 of material to get the strength instead of up grading material ($$$) heat treating the material(time and $$$),
and then having more difficult welding, and possibly machining, and possibly warping..
Its not like a tractor needs to be light enough to fly..
And extra strength isn't going to save you from neglected maintenance and trashed bearings.