[AT] Heat treating tractor parts

[ROBBRUT at aol.com]

Dean-

Since I am in the aerospace industry, I work with these materials constantly.

I also have a small capacity programmable heat treating furnace at home for 
my own small parts.

The material you have chosen (4340) is a good general purpose heat treatable 
alloy.

I would suggest that 40 Rockwell C is too hard/brittle for your application, 
and would shoot for 32-34Rc for a better combination of toughness/tensile 
strength.

As to the mode of failure you mentioned, it is exactly what is seen in many 
other applications where a load bearing diameter first reduces to a smaller 
diameter (by the cut thread).

The design solution is to neck the entire load bearing portion down to a 
little under the minor diameter of the threads, that way the entire length of the 
rod can flex and twist under load, instead of concentrating all of the those 
loads at one location, which quickly fatigues that very localized area with the 
resulting quick failure.

Heat treatment of these alloys is done in two stages
The first stage hardens the material to its maximum, but at this 
(martensitic) stage it is very brittle.
The second stage (tempering) re-heats the material so that a transformation 
to the structure takes place.
There are temperature/time charts used to establish the combination of 
hardness/toughness properties you want for most materials.
For 4340, the higher the tempering temperature, the softer the material 
becomes.

However at the hardness range I recommended for the material/purpose you 
describe I would advise pre-heat treating, since 4340 is very machinable up to RC 
38, and not too bad above that up to about RC 42.

-Bruce Thompson