Why do you think this failed?

[Donal Harris]

This is the Hardy stem on my guillotine tool. The base is mild steel. This stem is just a piece of mystery steel I had in the pile which was pretty close to the right size for the hole. It is carbon steel of some sort. 
 

I used 6011 and did not preheat the carbon steel first, which I learned later is something you should do prior to welding carbon steel. The weld held for about two years or so, but I can see from the rust around where it broke, it had been cracked for quite some time. 
 

Not knowing exactly the type of steel means no one can do more than just guess, but what things could I have done to give it a better chance of not failing?  The most obvious to me would have been mild square bar, and that is what I will go with when adding a stem back. I have several sticks of various sizes of mild square bar now. Would 7018 have been a better rod to use? Dad has both 7018 and 6011, but I’ve only ever used the 6011. It seems to be the welding rod equivalent of duct tape. 

[Daswulf]

Preheat, post heat, and mainly V it out for better penetration and more weld area. Even with mild steel it seems to start welding better preheated on larger stock. I couldn't say exactly on the rod. I mainly mig weld. And as for the mystery steel you had used, did you spark test it? 

[BIGGUNDOCTOR]

7018 on DC reverse is a good choice for dissimilar alloys.

Correct on 6011, we called it farmer rod because it would burn through rust, dirt, manure, paint, and hold. AC only and deep penetrating.

As Das said, a good Vee prep would have done a lot no matter the rod used.  It just allows that much more area to grip.

[Frosty]

D: All steel is "carbon" steel, perhaps you meant HIGH carbon steel?

Looks like you didn't scarf the joint, "angle grind" the edges. This allows the arc to melt a greater volume of base metals so the filler rod mixes with more of the base metals for a stronger joint. In lay terms greater penetration. 

Grind 3/8" or so off the edges then weld it up. Grind it so the V made when they're together is a little wider than 45*.

Either rod is fine the 6011 won't be as smooth but it's a LOT less fussy about getting the prep right.

Frosty The Lucky.

[BsnNFrnt]

Plus 1 on Frosty’s recommendation. Chamfer the edges on both sides of the stock, especially if you are planning to use 7018. In my experience the 7018 rod will work a whole lot better with the extra cleaning the chamfer will provide. That stock looked thick enough where you will need some significant grinding to get the space necessary. 

[ThomasPowers]

If it's a hardy tool you will not want the fillet extending out from the step piece much so definitely grind the chamfer on the stem fairly deep so you get a good amount of weld and not have it ground off to fit the hardy hole.  Preheat and slow cool is pretty much mandatory when welding HC; *especially* in cold weather where the cold mass can "contact quench" the steel at the critical temp to create HAZ cracking.   Spark testing to avoid using HC is a good idea.