Rolled vs Forged Bar 1

[metmike]

Hello,
A customer is asking for "forged bar" as the starting material for a certain finished part. We have in stock what the mill refers to as "rolled bar", which the customer will not accept. I can't seem to find a definitive difference between "forged" and "rolled" bar. Depending on where you look in ASTM specifications, rolled is sometimes listed as a type of forging process, elsewhere as a separate process.

If anyone can please explain the difference, especially within the context of an ASTM spec, I would greatly appreciate it.

 

[dbooker630]

At our shop something that is subsequently forged must be made from bar with a minimum 5:1 reduction ratio from the original ingot, bloom, or billet. If the product is to be machined directly from bar with no subsequent working we require 10:1. Check the data on the mill cert and report the reduction ratio to your customer. If your rolled bar has a high enough RR that might be acceptable.

 

[redpicker]

A forged bar is produced by taking an ingot or bloom and forging it down to size by, generally, two opposing flat dies. With a rolled bar, the ingot or bloom is brought to size by passing it through two rolls, often several sets of rolls. In general, forging produces more hot work through the thickness of the bar that rolling will. At least, many will believe it does.

Depending on the size of the bar, there are some rolled bars produced by continuous casting mills that may have very little rolling/forging reduction, which can have a negative effect on the mechanical properties. Generally, this is controlled by specifying the amount of reduction required, but perhaps your customer has other concerns as well.

The important thing is that your customer will not accept the rolled bar and wants a forged bar. Why is really not the issue. Most steel mills will be specific that their product is a "rolled bar". You will probably need to find an open die forging operation to obtain a forged bar. You can google Scot Forge for a start.

 

[metengr]

It boils down to reduction ratio for end product use by your client.

 

[Maui]

It may also depend upon the grade of steel in question. For bars that contain a significant amount of primary carbides, the forging process (if performed properly) effectively breaks up any inherent segregation from the original casting so that the carbide distribution becomes more uniform throughout the material. And it does this in three dimensions. A rolling operation on the other hand tends to deform the material only in the rolling direction. Any segregation that is present follows these flow lines in the material. And this can create what is referred to as "streak", or heavy banding of the carbide particles. This is an undesirable condition for some applications because it can lead to lower material toughness, duplex grain size, machining and grinding difficulties, etc.

In some mills, the forging of the original castings into intermediate products known as blooms is then followed by rolling to the final size and geometry. I would check to see if this is the case here. If it is, then your customer should be happy. If not, then you need to address their quality concerns.

Maui

http://www.EngineeringMetallurgy.com

 

[TVP]

Just in case anyone would like a more visual understanding of the various bar forging and rolling processes, here are some good images that I found:

Conventional open die forging at Akmetal in Turkey

http://www.akmetal.com/images/p1b.JPG

State-of-the-art open die forging at Timken Faircrest

http://www.france-metallurgie.com/wp-content/Timken.jpg

Radial forging at ATI Allvac

http://businessclimate.com/sites/de..._gallery_750x495_scale_crop/77802SUB011_0.jpg

another example of radial forging

http://www.samkimc.co.kr/bbs/data/file/01/3547592506_Cy1LTMlh_95B15D.JPG

State-of-the-art bar hot rolling mill (Daniel Morgårdshammar)

http://www.danieli.com/var/danieli/...-eng-GB/Special-Bar-Quality-Rolling-Mills.jpg

close-up of roll

http://www.danieli.com/var/danieli/...228-1-eng-GB/Morgaardshammar-Guide-System.jpg

 

[metmike]

Hi everyone,

Thanks for your input, it means a lot...

dbooker, metengr - the "rolled" bar that we planned on using has a 7:1 reduction ratio, 5.5" OD round bar.
We spoke to the mill who provided the bar. They said that they only make forged bar down to about a 6.5" OD. They said that as you reduce bar size, they roll their bar, as it is a much easier method. Forging may damage the bar and make it difficult to hold the bar size tolerances ASTM specs demand.

Maui - It is 316/L bar, so I don't think there is too much to worry about in terms of carbides, but that is some insightful input. Yeah it seems the providing mill (along with many others I understand) forges the ingots/billets and rolls them to final size.

We joke that if we heated the bar up to temperature and whacked it on an open die hammer a couple times, it qualifies as "forged" bar, even though it would be probably be making the bar worse.

 

[Maui]

TVP, that first image you posted from AK Steel appears on page 123 in my book where I talk about ingot breakdown during forging.

Maui

http://www.EngineeringMetallurgy.com

 

[metmike]

Hi everyone,

Does anyone know of any good research that has been done to support rolled bar over forged bar, or that they are at least both similar and acceptable practices for quality product?

 

[metengr]

There is one item which has not been discussed and that is the finished product form. In some cases, final product application may require a forging over rolled bar to reduce machining costs and to provide certain mechanical properties. Again, for proper comparison you need to know the end use to determine which would be the preferred hot working method.

You should discuss this with your client.

 

[tomwalz]

Rolled material can maintain a tendency to curve or bend in futher processing such as any form of heat treating.

Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.