With recent conversations about this, I decided it would be a good idea to have a thread dedicated to this, so that TJ owners who are looking to potentially upgrade their Dana 35 axle shafts can make an informed decision based off good, solid information. The following is a quote from @mrblaine in another thread regarding upgrading the Dana 35 axle shafts.
There's a whole bunch of confusion and one side of the equation going on so far. First, let's get apples to apples in perspective. 1541H is a case hardening alloy. 4340 is a through hardening alloy. They are not readily interchangeable due to the 35 axle having a bearing journal that the bearing rides on directly without adding a hardened race. For that application, the superior axle material is 1541H because it is plenty strong and because it is case hardening, you can bring the surface hardness up high enough to withstand bearing rollers riding on it. "IF" you try to bring 4340 up to the same hardness, it will be brittle because there is no ductility left at those levels of hardness. The 1541H will have a very high surface hardness with a softer core for a good level of ductility.
This is typical of what 4340 does when you try to use it for a bearing shaft and don't nail the heat treat.
The very same thing that makes 4340 difficult to use for bearing shafts is what makes it a great material for non bearing shafts like the Dana 44 with Set 10 bearings. Since the hardness for a bearing journal isn't an issue, the heat treat can be brought up high enough that there is a lot of ductility left and a fairly high level of hardness throughout without it being brittle.
1541H Dana 35 shafts don't scare me, 4340 Dana 35 shafts scare me a lot.
There's a whole bunch of confusion and one side of the equation going on so far. First, let's get apples to apples in perspective. 1541H is a case hardening alloy. 4340 is a through hardening alloy. They are not readily interchangeable due to the 35 axle having a bearing journal that the bearing rides on directly without adding a hardened race. For that application, the superior axle material is 1541H because it is plenty strong and because it is case hardening, you can bring the surface hardness up high enough to withstand bearing rollers riding on it. "IF" you try to bring 4340 up to the same hardness, it will be brittle because there is no ductility left at those levels of hardness. The 1541H will have a very high surface hardness with a softer core for a good level of ductility.
This is typical of what 4340 does when you try to use it for a bearing shaft and don't nail the heat treat.
The very same thing that makes 4340 difficult to use for bearing shafts is what makes it a great material for non bearing shafts like the Dana 44 with Set 10 bearings. Since the hardness for a bearing journal isn't an issue, the heat treat can be brought up high enough that there is a lot of ductility left and a fairly high level of hardness throughout without it being brittle.
1541H Dana 35 shafts don't scare me, 4340 Dana 35 shafts scare me a lot.
