How come mild steel doesn’t get a grain like wrought iron in forge welding

[Supaflupa]

My theory is that it has something to do with the forge welding process. Like using silica instead of borax, letting it oxidize just enough to replace scale from the bloom. But I have no experience with wrought iron and very minimal experience forge welding so I figure I’m wrong.

[Steve Sells]

modern steel is homogeneous from the crucible and wrought isnt being from a bloomery, and the layers of slag give the pattern, has nothing to do with welding

[Supaflupa]

So the the refining process isn’t what gives wrought iron its grain?

[Frosty]

NO.

Frosty The Lucky.

[SLAG]

Mr. Spflp*,

You can get a good overview concerning wrought iron making by checking the wrought iron article in wikipedia.

It is only about 2 and half pages and it's a good read.

Tell them SLAG sent you.  They'll get a good laugh.

SLAG.

* Supaflupa  is a cool name, I say.

[Supaflupa]

I’m not looking to make my own wrought iron through smelting or anything. I’m wondering why if were to forge weld a bar of mild on itself over and over it wouldn’t get a grain.

[TechnicusJoe]

Forge welding bars of mild steel together or welding it over on itself will produce a comparable internal "grain" to that of wrought iron. 
Pattern welded steel also has that "grain". 

Though, mild steels and other modern steels (for pattern welding steel - "damascus") are homogenous materials. 
The grain will come from the welds between them and that isn't very visible, because the material is homogenous. 

Wrought iron gets its pronounced grain from not just only being made of blooms and forge welded together. 
The very blooms are not homogenous. Then by forge welding blooms and turning them into bars, you get strongly pronounced different materials
that are now next to each other. Stacking new bars together and drawing them out, again and again will make that grain smaller and smaller, finer and finer. 

Mild steel lacks this, it's "too clean". 
Again, you can create a "grain" pattern that is hardly visible if you want to make that. But it's never the same, because mild steel doesn't contain bits of bloom with more, or with less bits of inclusions that have been drawn out to very long lengths. 

The alloy, composition of mild steel is not similar enough.  It's the combination of the alloy of wrought iron and how it was processed. - different from mild steel. 

[ThomasPowers]

Think of a piece of fiberglass and a piece of plastic.  Why doesn't the plastic piece have fibers in it?.

Wrought iron is a COMPOSITE material composed of generally a cleanish iron and ferrous silicate spicles. (in the finger grades there can be 100000 per sq inch!.

It is the presence of the ferrous silicates spicules that give it it's unique texture. (I've had folks wonder why adding silicon to steel as an alloying element didn't result in a wrought iron texture.)

May I commend to your attention: "Wrought Iron, It's Manufacture, Characteristics and Applications"  James Aston and Edward Story

When you do pattern welding of the same alloy you can get differential etching due to things like modifying the surface through adding/subtracting elements at welding temp with the interaction of the fuel, atmosphere and the metal---but it still won't look like the fibrous wrought iron!