looking to make a little anvil.  6 inch diameter by 5 inch thick 4140 or 1045 steel.  planning on concreting a 6x6 post to mount the "anvil" on. 

my question is should I try and harden and temper the steel, or just use as is? 

I'd say that if you have the means to accurately harden and temper a chunk of steel that big then go for it.  It will help you work a lot faster having a harder anvil.  I'm not sure about tempering color, subsequent tempering caused by hot metal or work hardening.  I do know that you could use it as is and still do great things. 

Also, I've seen some of the curmudgeons on here try to dissuade people from using a concrete post for supporting and anvil.  It apparently degrades quickly with use.  An upended I-beam would be perfect, though.  You may even find a mild steel post at a scrap yard for cheap.  I imagine that, if your 6x6 post is steel, it will be fine with the concrete inside.  I just wasn't certain if that is what you meant.

 

Either way, have fun building it.  Nothing like your first whack at hot iron on the anvil and post you built.  Way better than buying one someone else made.

Do you have the means to harden and temper a piece of steel that weighs 40lbs?

You could use one face for a while to see if it marks up badly and then decide if you want to harden it, by then you may come across something else or if not, flip it over and harden the other side. I had something similar (5" diameter by 6" tall mild steel) for a good few months and just redressed the face if required - I now have a London pattern but the little post still gets some use. I had mine on a 6x6 piece of wood with another taller piece behind that the anvil was strapped back to - the concrete option you are suggesting will likely move less but be a lot less mobile too.

If you have the equipment and SKILLS to heat treat it then heat treating will improve it.  If not then you are more likely to damage it, your shop or yourself.

If you have to ask we will probably lean in on the side of "not"...

Thanks for the feed back.  the logistics of heating and quenching something that big I was trying to work out, but was more concerned about damaging it.  I know I don't have enough experience quenching in general.  I just wasn't sure how well it will hold up as is.  But I do like the idea of use it and see.  I'm sure it will be better than the free cast iron anvil I was given.  

Reading what I wrote originally, realized I was a little vague on the anvil post.  I have a 4 foot 6x6 wood post I was planning on digging and concreting about a foot of it in the ground.   I don't have a shop so to speak of.  Unless the backyard is considered a shop.  

Full disclosure:  I've been learning as I go for the past few months.  Trial and error.  Just found this site a few weeks ago.  Have picked a good bit so far by reading.

 

As most of us work at 30" give or take an inch or two, i would plant more of the post in the ground, cement tends to hold moisture next to the wood, gravel is better.

So minus the chunk of steel you are using for an anvil from your post hight.

don't over think the anvil, for a few thousand years they were just wraught Iron lumps not much larger than a fist (two if you were rich). So a sledge hammer head, 4x4 drop, foot of rail or some such chunk just a bit bigger than your hammer will be good to start. Sure, if your using a 2 1/2" hammer, a 25-50 pound drop would be great, but a 8-10 pound sledge will work fine.