I used an old coil spring to make a cottonwood leaf.  I made a point and back about an inch I made the stem.  I rounded the stem and then went to flatten the leaf area.  I used my 50 lb Little Giant to flatten the leaf and I could see a crack near the side of the flatten area about half way back.  When I turned it over, I fond the crack traveled down towards the stem.  I assume this was a flaw in the coil spring.  It looked like a forge weld that didn't weld completely thru.  I didn't forge weld it at any time.  Any ideas as to why it would appear.  Clearly it was a weak spot in the coil, but I thought it was a solid one piece of steel. I used my gas forge to heat it.  I've used coil springs before and never ran into anything like this.      

Could be a stress crack that had been hidden and was just waiting for the right moment to appear. 

That would be my guess as well. Unless it was quenched at any point in the process or possibly worked too cold. 

Coil springs can break on cars let alone get stress fractures in them. Used steel can always be a gamble. 

I have made several pritchels from coil springs and a few knives.  After a couple broke from hidden fractures, I've steered away from coil springs.  Not fun getting half way through a project only to have a flaw in the steel.

Also to point out, "as a personal preference from my experiences", that I try to avoid higher carbon/ spring steels when making items that do not require them due to the fudge factor in working those steels. 

What are the different steels that you use in place of coil springs?  I do have access to sucker rod and the cost is easier on the wallet.  

For decorative work, mild steel is perfectly acceptable; most steel supply places will carry it (or A36 low-carbon structural steel). Steels with higher carbon content (sucker rod, coil spring, etc) are good for uses that require greater toughness or hardness.

If you are looking for "inexpensive" (which mild steel usually is new) alternative places to buy mild steel round stock check fabrication shops and such. They usually have a bunch and willing to sell at scrap price, since it was pretty much already paid for with the job. honestly it's just sometimes better in the long run to buy new mild steel stock from a supplier when needed. I love upcycling used stuff, but "right steel for the right job." Not saying you cant use spring or sucker rod for the job at all but with that steel comes the unknown. if time isnt an issue then cut off around that chunk and keep going.

Many car custom shops upgrade new cars with perfomance components.  (e.g. heavy duty springs, lift kits etc.) The parts that are switched out are new. They have never been used but  usually end up in their scrap bin.

I have bought such items for a little bit more than their scrap value. I pay cash and sometimes drop off some small smith items as "gifts'. (bottle openers etc.)

The mechanic are happy. And I get to smith steel that is unused and therefor has no stress fractures etc.

SLAG.

The other side of things is technique.  Working coil spring as if it was mild can cause cracking too. Thin high carbon being worked on a cold anvil can quench and crack if you are not careful.  Working too cold can result in cracking as can over working it for a set temperature.

In general using higher C for a project that does not require it is a waste of money due to the extra time needed to form it, the extra fuel used, the greater possibility of it failing in process, etc.  Free stock isn't free if it takes you longer to work it!

I used to get mild steel from a local ornamental iron place; they had to pay to have their scrap bin hauled to the dump and when I asked they were happy to let me go through it and remove hundreds of pounds of stuff each visit.  I asked, always wore PPE and left the bin neater and ready to hold more scrap than when I arrived. A few hand forged trinkets for the office staff and I was *golden*. This is what I would call a "medium" sized business: big ones won't let you past the office and small ones tend to save stuff they might be able to use.