Grain in steel? What's that?

[Warren Nakkela]

Hello all you Metallurgists:
I have heard of grain in steel (not wrought iron) so what does that mean? Like grain in wood? Or like grain in sand? Does it matter what direction steel is forged? Say I have a lump of tool steel and wish to form a cutting edge. Can it be forged in any direction with equal success? What about cutting or forming parts from, say, hot rolled A36? Will cutting parts be different than when steel is worked and compressed by forging?:confused: Supposed the steel is 4140?:confused:
Warren

[Dodge]

Its really hard to tell which way the grain run in steel unless its forged. Once its forged, it changes the grain anyway. Blade makers will be able to explain this better but the grain in milled steel (like coiled steel off the mill) runs with the linear length. In some raw coil steel that still has the mill scale, you can see the direction if you know what to look for and you can tell a piece the has been sheared from a coil if the rounded raw edge it sill there. the grain will be the same direction as that edge. When we cut a piece of steel from a coil, especially if it was to be bent we had to mark the grain so the bend line would not run with the grain. It could (and would sometimes) crack. I believe if you draw out a piece of steel, the grain wil run in the direction the steel is drawn.

Let the thrashing begin!! :D

[Gobae]

You need to know the context of the term where it's being used because it has changed.

Wrought iron has a "grain like wood" caused by linear slag inclusions. Modern steels have "grain like sand" caused from their crystalline structure.

I'm not going to answer the whole "compressed by forging" and "directional forging" questions. The compression issue (aka "edge packing") is a huge debate and I don't know enough metallurgy to give an definitive answer on the other.

[Steve Sells]

if you wont , then I will, There is no edge packing, no compression period. No room for debate when there are facts to back this up.

The process, of what people falsely call edge packing does help the finish of a blade, and I ausforge as well, but it does not pack the steel, there are no voids in the structure of steel, unless you have a cold shunt in a weld. All irt can do is smooth the surface, and fracture large grains into smaller ones, and with some steel, if forged too cold it will crack it.

[Warren Nakkela]

Steve, That's what logic tells me. But I understand that there is grain (iron crystal) deformation in the direction that steel is worked, i.e. rolled or forged. So does it matter which direction that a hook, for example, is flame cut from a hot rolled plate assuming there is no subsequent forging?
Warren

[Dodge]

Warren,
We would flame cut hooks all the time out of hot rolled plate steel. I asked about the logic and was told that as long as we didn't put a bend in them it was fine, Bend with the grain and it could split. I've seen that happen numerous times, which is way it was common shop practice to re-mark the grain if you cut the markings off a stock piece.

[Warren Nakkela]

Dodge,
Bending with the grain or rolling direction? You mean like forming angles/channels on a press brake? And forming the long way?
Warren

[Dodge]

Long or short is all relative to the original stock. The grain will always be in the direction it was rolled at the mill. in other words if the coil was 100 feet long when unwound and flattened, the grain would run with the 100 foot length. If the coil was 6 feet wide, and we wanted to cut, say a 4" piece to bend in a break press into a 2" x 2" angle iron, the longest piece we could cut would be 6 feet because we would want to cut across the grain and then bend across the grain. If you wanted to cut a piece 4" wide and 20 feet long out of that same 6' x 100' coil (now a sheet), you would have to cut it with the grain because the width of the stock sheet is only 6' and then if you bent it into a 2" x 2" angle in the break press, it could could split along the grain. Hope that makes sense.

[Warren Nakkela]

Sure it makes sense, have done it. But how about tensile strength? Will samples taken from hot rolled plate in different directions test the same? How about compression?
Warren

[Dodge]

Theoretically tensil should be the same throughout but I would wager that its not as good on the edges of a coil as the inner, say 95%. I've never worked in the testing labs so I couldn't say for sure. Compression?? Somebody with more knowledge that I will have to field that one :)

[Quenchcrack]

Steel does display different properties in different directions. Think of it as plywood. Pull with the grain and it is pretty strong. Pull across the grain and the strength is less. And Dodge is mostly correct when he says different places in the strip have different properties. The biggest difference is on the leading and trailing ends as opposed the the center of the strip. Now differences in properties in plate are less noticeable since the lead and trailing ends are closer together.

[Francis Trez Cole]

The way it was explained to me is that modren steel (not wrought iron) has a crystalline structure. The easist way to visulise it is like a cell in the human body you heat it it expands. You cool it, it contracts. If you heat it evenly it expands evenly. The same with cooling. In welding to high of a temp will expand some cells and not others leaving uneven growth. leading to cracking. hope this helps.

[Paul B]

I am not an expert on this by any means but I know when a tool maker grabs a piece of D2 or A2 or what ever, he does not examine it to decide what edge is good for what. Also I know that you can see the grain in cold rolled very easily after it is ground on a surface grinder.

[Steve Sells]

thats because A2 and D2 are usually heat treated after forming, otherwise why use those? the H/T resets the grain, unlike plate that most of the time is used as-is.

[nitewatchman]

Steel does indeed have a grain structure where the grain may be thoght of as a crystal that either grew as the material solidified, refined as the metal changed phases (Austenite to Martensite for example) or refined due to being worked.

Attached are two photos (5x and 40x)of 1080 steel that is mostly fine garined Pearlite with hardness of about Rc 40 and elongation of 12%. The white streak is Untempered Martensite with a hardness of Rc65 and elongation of 1%. The Martensite was formed when retained Austenite changed phase at room temperature rather than becoming Pearlite during quenching. The most likely cause of the quench failure is a chemical segragation in that area.

We knew to look for the Untempered Martensite here because the part broke when stressed with the crack coming out of the Martensite. The black spot is a just a booger.

The Untempered Martensite is characteized by the Asicular or needle like carbides in the white field. the Pearlite is recognized by the flat plates or lamela of Ferrite and Carbides. In this case we wanted all Pearlite since it wears better than Martensite at the same hardness and is tougher.

This would not be suitable for a blade since it is much to soft.

[Warren Nakkela]

The reason that I started this thread was that I believe that there is a lot of misunderstanding or no understanding at all about grain or grain growth in steel. The discussion here is worthwhile and I will continue to ask questions. Until I run out!!
Warren

So is a hook that is forged and bent stronger than one cut out of plate all other things being equal?
Warren

[Ed Tipton]

I may as well jump in and expose my ignorance as well. With my limited understanding, I have been led to believe that...all things being equal...the forged hook will be the stronger of the two. The simple act of forging, as usually understood, usually involves deliberate/controlled heating-working-and some form of quenching. The heating and working result in refining of the grain structure, and the quenching and tempering aid in the transformation of the various phases. Simply cutting a hook from a piece of plate is not going to yield the same quality of steel as what has been forged since there has probably not been sufficient work or phase transformation to cause any significant refinement of the grain structure, and there has been no quenching or tempering action taken. A properly forged hook would be much stronger.

[Dodge]

Ed I definitely would prefer the forged hook over the one cut from plate as long as its properly heat treated. However, the ones we cut from plate, as the engineer that gave us the work order for them explained, were over designed by about 12 times. In other words, even though he thought they would handle 12,000 pounds, the job he designed them for was only going to lift 1000 - 1500 pounds.

Edited March 28, 2009 by Dodge

[Warren Nakkela]

Controlling grain size?
Grain growth occurs when steel is heated and more so when overheated. Grain refinement occurs when steel is forged. What are the the temperatures when grain growth occurs and what are the forging temperatures to cause grain refinement? And are these temperatures different for different steels. Or in other words, "where on the chart are these points?"
Am I asking the right questions? Another thought, typically hoisting hooks are left in an annealed condition for safety sake.
Warren

Edited March 28, 2009 by Warren Nakkela
more thoughts

[mike-hr]

This is a very interesting topic. I find myself thinking about liability when things like this are dicussed. To follow up on what Dodge said, If you are engineering something close enough to the breaking point where you have to think about grain structure, as an open-die smith, you ought to build it bigger. My old welding teacher, Mr. Cone, used to preach, 'If in doubt, build her heck for stout'. Okay, I paraphrased his quote a bit. Family forum.

[Steve Sells]

normally this comes up in blade work. as grain size getting too large makes weak steel. It was asked 3where on the charts? We thermal cycle above/below AC3, the curie temp is where the steel turns into austenite, and theses grains tend to grow large if kept above critical for very long, to reduce large grains, cycle the temp from above AC3 to below a few times. Each time its changes from austenite it reforms, so this forces the grains to be smaller, and resolves the problem, of too large a grain.

if you want to know more, read about blade smithing. We have sticky's on heat treating too in the knife section. It might explain a lot more.

[BeaverDamForge]

BP0078 The Metallurgy of Heat Treating | Blueprints 000-100

[CBrann]

Grain I though was the crystaling form of steel/iron, in rolled steel it runthe length etc,

but I think the question is about crystal growth a temperature... like arc welding, the crystals are smaller in the weld and the stock, except near the weld they grow because of heat from the weld, that is why proper welds don't fail, the area around them does, and looks matte like cast iron, almost any good welding text will tell you more in depth about this..

and the metallurgy will also explain curie point, eutecnic point and solid solution hardening....

[forgemaster]

British and Australian standards regarding crane hooks both state "hook to forged cleanly from one billet of steel with the macroscopic grain flow lines to follow the profile of the hook, profile cutting of the hook from plate is not permitted, nor is welding. The hook may be after forging and bending be faired by means of machining or flame cutting or gouging".
Don't know about American standards as we have never been asked to produce to them.
When we forge shafts we are always required to forge with the grain, if we are forging rings or discs we are required to jump the billet up to force the grain to loop out into diametre of the ring or disc (rings and discs are normally forged for gear cutting applications).

[TASMITH]

The North American standard regarding crane hooks to be forged from a single billet are the same. At the plant I took my apprenticeship, we made all the hooks for the overhead cranes. The hooks we made ranged from 10 to 100 ton capacity and had a 25% safety factor. In other words a crane hook rated for a hundred tons could 'SAFELY' lift 125 tons although it is designated as a 100 ton capacity.

All the crane hooks were forged from a single billet on the 3,000# steam hammer then bent to shape. After bending they would then be heated and the taper section drawn out under the hammer to form the back side of the hook. After forging we had to grind all surfaces smooth to eliminate any hammer marks so the quality control people could check the hooks for cracks. After they passed the test for cracks they were sent to our in house foundry dept., were they were normalized in their large temp controlled furnaces then to the machine shop for machining the shank to fit the crane block.

The hundred ton hooks were made from a 14 inch square billet weighing approximately seven tons and required a week, and all the shop employees working as a team under the big hammer blacksmiths' control. This consisted of 14 men, 1 ten ton overhead crane, 3,000# steam hammer, and a 'LOT' of muscle power!

One thing I forgot to note in the previous post. They also made crane hooks from plate steel as Dodge has stated. However these hooks were for the hot metal cranes that moved the ladles of steel and iron in the steel making department.They were generally two to three HUNDRED TON capacity hooks. They flame cut the kooks from 1" thick plate, drilled holes through them and laminated up to twelve pieces together by riviting them together. These hooks were made in the boiler shop next to ours. they also had the large Hydraulic press we needed to bend our larger capacity crane hooks (40 ton and up).