Wrong type of steel?

[Triggerscan]

I got a hold of a carriage stop for my LMS 3595 (SC4 clone) and am new to metal work. I needed to make a new clamping screw for the stop as it was only engaging with about 2-3 threads. I purchased some hexangonal stock and was using a 3/8" piece to duplicate the clamping screw. While putting a chamfer on the end after turning the M6 major diameter, I barely touched it and it snapped off. I noticed that the metal at the break has a dark gray powdery look to it. I know it wasn't tool steel, but the seller indicated that it was easy to machine. I'm assuming I am using the wrong alloy. Any suggestions on what I should be using? I've attached pics of what I was trying to duplicate (but longer external treads) and the end where it broke off.

 

[JimDawson]

Looks like too much unsupported material for the diameter. Maybe using a center would help.

 

[swiftab]

The steel lol ok. When you clear the threads at the hex, add a large radius. It will make a much stronger unit.

 

[Triggerscan]

I think I was also turning too slow. I assuming it was 4140 and was turning at 500 rpm. BTW, it broke at the base of the threads which were made with a die, not a threading cutter prior to rounding off the end. I'm redoing the piece again, but this time at 1000 rpm and the CB tool seems to be cutting better. Planning to do all shaping first and leave the threading to the end. Thanks for the comments.

 

[MrWhoopee]

Hex bar is frequently 12L14. Really nice to machine. Not tool steel but not crap either. Too much overhang was the problem, the chamfer should have been done before turning the entire 6mm diameter. Some lessons come cheaper than others, this one was pretty cheap.