Cast Steel, Cast Iron, Forging vs. Casting, and Understanding Grain

[Patrick Nowak]

[MOD NOTE: The following was originally part of the discussion on this thread, but its detail and clarity call for it to stand on its own.]

I'm going to jump in on this thread because there are several things that have been shared above which are not metallurgically correct. My apologies to the original poster as I know this material goes well beyond the question that was originally raised, but first I will address those questions.

The original poster asked how to tell the difference between ductile and grey cast iron. Ductile iron will ring much like steel when struck while grey cast iron tends not to do that. that is not an absolutely definitive method as you could still have a steel part. A spark test can also be useful in making this determination. None of the various forms of cast iron discussed below is typically  forged either hot or cold. The entire reason for casting the part to begin with to get the shape you want. Ductile iron could be forged, though that is quite rare. If you want to try it you need to heat the metal to a very dull red and use very light blows. Due to the high carbon content, the melting temperature is much lower than that of steel so it is easy to over heat. Additionally, the microstructures are not really suited to forging. If I had something I new was ductile iron I would probably try to adjust it without heat it.

Now to address the incorrect information:

1. Cast Steel Tools: Tools Stamped "CAST STEEL" were made from steel which was first cast into ingots and then forged or rolled to shape. This started well BEFORE the Bessemer process was introduced. Bessemer (and William Kelly in the US) both invented essentially the same process in the mid-1800s, but that was NOT the first time liquid steel as used (See The History of  Metals in America by Simcoe). The process was commercialized (but not invented) by  Benjimen Huntsman in about 1750. There are at least two documented references to the same process that predate 1700. See work by Cyril Stanly Smith. The use of steel made by the cast steel process for tools was common and these were stamped CAST STEEL to separate them from tools made from Blister or Shear steels. I am not aware that Bessmer steels were used for these applications. Note that I have in my own collection plane blades which are stamped "CAST STEEL" and which I have examined metallographically. These are wrought iron blades with very small amounts of steel forge welded to them. They were made by Ohio Tool which went out of business in 1920.

2. Forgings vs. Castings: Before dealing with this question please know that I am a degreed metallurgical Engineer with 20 years of experience, 18 of which have been in the open die forging industry. The terms that have been used so far in this thread are not metallurgically precise and therefore there are some misunderstandings. The "strength" of a metal in metallurgical terms is usually one of several specific types of strength: Ultimate tensile strength, yield strength or impact strength. Assuming we are comparing a casting and a forging of the same steel alloy, size and shape they could have the exact same ultimate tensile strength and yield strength because these properties are a function of heat treatment, not manufacturing method. Where forgings are superior to castings is in impact strength and fatigue properties. These properties are a function of several different variables including heat treatment, grains SIZE,  and grain flow. In carbon and low alloy steels the GRAIN SIZE is a function of heat treatment, so I can make a casting with the same grain size as that in  forging if I choose to do so. What I cannot do with a casting is create GRAIN FLOW around corners, shoulders or other features which I can do in a forging. The grain flow is due to small micro inclusions and non-metallic particles being elongated during rolling or forging and then being bent around features during final hot shaping. This gives forgings characteristics somewhat similar to wood. When you split a log you always put the wedges in the end and try to separate the wood at the boundary of one growth ring and another. When done correctly the wood splits easily. But if you drive a wedge into the side of the log such that the force is applied parallel to the length of the log it will not split. Likewise  if you are trying to split around a knot or series of knots you will have difficultly splitting the wood.  Forgings are just like that. loaded in one direction they will have greater toughness than loaded at 90 degrees to that direction. Castings tend to have uniform properties no matter how they are loaded.

3. Steel castings: Steel castings were not widely used prior to the implementation of the Bessemer process. Though liquid steel was available prior to that point, it was made is small batches, typically of less than 100 lbs and was generally used for tool steels and other specialty applications. There are records of large ingots, multiple thousands of pounds in fact, being cast in both Sheffield and Germany, but not until the mid-to later 1800s. (See Sheffield Steel by K.C. Barraclaugh and Steel, Iron and Cast Iron Before Bessemer by Buchwald). Wide spread use of steel castings (that is solidified in the near final shape) was not occurring until the early 1830s in the US and the 1850s in Germany and England (see History Cast in Metal). The reason for this is because the molding material used for steel has to withstand much higher temperatures that the green sand used for cast iron. It took quite a while for steel workers to figure this out and identify the correct mold material. The melting temperature of cast iron is about 2100 F while that of pure iron is about 2800 F. The cast steel being made by the Huntsman process was usually poured into cast iron ingot molds so there was no concern about reaction between the liquid metal and mold material as there is when making a near-net-shaped casting using sand molding techniques.

4. Cast Irons: There are 4 types of cast iron: White Cast iron, Grey Cast Iron, Malleable Iron and Ductile Iron. All cast irons have much higher carbon than steels, usually well in excess of 2%, often approaching 3-4%. In white cast iron the carbon is mostly in the form of iron carbide, making the metal extremely hard, brittle and wear resistant. this was the earliest form of cast iron, likely because the alloying elements needed to promote the formation of graphite flakes (primarily silicon) was not present in early cast irons. Grey cast iron is a much more recent development. In this form, there is much less iron carbide and much more graphite. The graphite is in a flake like form. Because graphite is so soft, this form of cast iron can be considered "pre cracked" since the flakes act just the same as if they were internal fractures. This is why gray cast iron cookware can be so fragile. Malleable cast iron is made from white cast iron which is subjected to very long heat treatments that force the iron carbide to transform to graphite having a spheroidal form. this eliminates the brittleness of white cast iron and grey cast iron giving  product which is actually quite tough. However, due to the very long thermal cycles needed to achieve these properties, this material has been replaced by a modern alternative-Ductile cast iron. this material was developed around 1950 and is dependent on the addition of magnesium to the liquid metal just prior to pouring it into the mold. When done correctly this forces the graphite to take on a spheroidal form which results in very tough, crack resistant material. When combined with the proper heat treatment, this material can have some properties similar to that of forgings.

5. Dislocations: A dislocation is not the same thing as a grain or a refined grain. A dislocation is a disruption in the crystal lattice at the atomic level. It is caused by any plastic deformation. During hot forging, dislocations form, but they go away quickly because they are not thermodynamically stable. They are removed by the nucleation of new grains (which are austentite grains by the way). Dislocations are only an issue (for good or bad) in parts which have been deformed at a low enough temperature that they remain in the structure. If you are not sure what a dislocation is, what it looks like or how it behaves, look up the Bubble demonstration by Bragg on you tube. It is from the 1950s and does an fantastic job of providing a visual demonstration of this topic.

6. Control of Grain size: When we talk about grain size we are really talking about the grain size of the austenite prior to cooling to room temperature. Actual grain size is a function of the temperature reached (it really doesn't have lot to do with forging). The hotter you heat the metal, the larger the grains will grow. In modern steel making, alloying elements are used to prevent grains from growing very large as long as the temperatures are below about 1750 F. This is true for castings and forgings. 

7. As cast structures: When dealing with large cross sections, such as the big ingots I deal with (some on the order of 5 feet in diameter and weighing 100,000#) the as cast structures will be very different from the forged structures because the solidification time is so long. In these cases, you can indeed have as-cast grains or crystals which are very large and which are broken down by the forging or rolling process. But if we are talking about the fairly small casting the original poster asked about, there is no reason to think the grains would have to be larger than those in an item made by forging. 

8. Info from the Milwaukee Forge website: While that information is factually correct it is not complete. they have intentionally selected pieces of data that promote forging over casting as a method of manufacture. they have not included anything about the grades, section sizes, types of forgings vs. casting etc. While I am 100% in favor of forgings (that's been my business for the last 18 years) those statements are extremely broad and are not qualified in any way. That makes them less than reliable in my book. Castings are often a fine way of making something. It really depends on the needs of the application. In general, forged or rolled metals will have better fatigue life, impact toughness (in certain directions) and ductility (in certain directions) than castings but all those things are highly dependent on a wide array of variables.

Patrick

 

[Frosty]

Thank you Patrick. 

Frosty The Lucky.

[George N. M.]

Patrick, thank you for the long and informative posting.  This should clear up a lot of misperceptions.

Much of what you describe is similar to mineral growth and orientation in igneous and, particularly, metamorphic rocks which are controlled by heat, pressure, cooling, etc. as well as rock chemistry.  In a sense, metallurgy is a sub-discipline of mineralogy.

"By hammer and hand all arts do stand."

[JHCC]

That’s a great comment, Patrick. Thank you so much for expanding on and clarifying what had been discussed previously. 

[dian]

patric,  if you forge a part, you would probably normalise it, austenize, quench and temper it.  or you choose some sort of "intercritical" treatment to refine grain and carbide in a perlitic structure. (or, or ...). so how much of the "grain flow" you mention will survive heat treatment? to what extend and in which case?

interestingly high end cranks are made from "billet", which i assume is forged and heat treated (normalised?) bar.

(it might be worth mentioning, that ci categories are not "god given". there is a number of other types and an even larger number of intermediate types. e.g. usually you dont get pure lamellar graphite, there will be some globular fraction. the smaller it is the higher quality the gray iron.)

roman

[anvil]

Thanks, Patrick, 

 

 

[wrenchguy]

Great stuff, but how does 1 know the material used and process of its manufacture without destructive test?  Visual id'ing i imagine a person would have to be extremely experienced in the forging and casting trades.

double click 2 enlarge

[dian]

imo the most important thing in any kind of metal working is to know exactly what you are dealing with. it starts with melting process (e.g. if vim/var you can assume a certain cleanliness), grain size, impurity level, heat treat (normalized, annealed, spheroidized, q+t) and exact composition (e.g. L2 too broad to be usefull). there is more. you have to find a supplier that is willing to give out as much info as possible. even an analysis on the optical microscope level doesnt tell much.

[Chelonian]

Thank you for the excellent post, Patrick! I've always been a bit unclear about many of the details you explained.

[BIGGUNDOCTOR]

Some Ford 9" rear carriers have a large N cast into the side denoting Nodular iron was used. These are the ones that drag racers prefer.

I had read somewhere while researching cast iron welding repairs that White Cast Iron was non-weldable.

 

[ThomasPowers]

White cast iron was often referred to as chilled cast iron in 19th century descriptions.

[SLAG]

Mr. Patrick,

I just into this thread.

Your description of various cast irons is a "tour de force".

I have "bookmarked" it for further reference.

thank you very much for posting it.

SLAG.

[Patrick Nowak]

On 6/12/2021 at 8:57 AM, dian said:

patric,  if you forge a part, you would probably normalise it, austenize, quench and temper it.  or you choose some sort of "intercritical" treatment to refine grain and carbide in a perlitic structure. (or, or ...). so how much of the "grain flow" you mention will survive heat treatment? to what extend and in which case?

The question regarding heat treatment vs. grain flow: Grain flow is completely independent of heat treatment. The grain flow is set by forging and is not undone or altered by heat treat treatment. This is because the grain flow is largely a function of small non-metallic inclusions such as manganese sulfides as well as bands of varying chemistry which follow the contour of the forging. (Note that is not unusual at all to find steel that, at the micro level, has layers of alternating chemistry somewhat like a pattern welded bar). Steel forgings are usually subjected to some type of heat treatment as you noted, but those treatments don't affect the grain flow. They can effect grain size and the specific microstructures present which have a direct effect on properties like hardness, the various strength properties I discussed above, fatigue life, wear resistance and fracture toughness.

Billet is just an intermediate shape between the raw as-cast product and the final forged shape. The billet may or may not be subjected to heat treatment depending on how it will be cut later and what grade of material it is. Quite often, there is no heat treatment but if there is it is usually subjected to a high temperature tempering cycle or maybe an anneal. Most other heat treatment cycles would be a waste when further forging is going to be done.

The shape of graphite, or even whether it is present or not, is a function of both composition and processing. It is not unusual to see a range or spectrum of features within a single piece. Also, the cast iron can be alloyed to develop other properties besides just control of graphite shape.

On 6/12/2021 at 10:37 AM, wrenchguy said:

Great stuff, but how does 1 know the material used and process of its manufacture without destructive test?  Visual id'ing i imagine a person would have to be extremely experienced in the forging and casting trades.

The simple way to tell a casting from a forging is that with a casting the various marks tend to be raised from the background while in a forging then tend to be stamped in. That is not definitive but is generally true. With respect to telling the difference between materials, a spark test is helpful. Cast iron in its various forms will have a different spark pattern than the steels. Also, steels will generally ring if tapped with hammer while gray cast iron will not. These are all just ball part trials but they can get you fairly close. They are not all non-destructive and telling the differnece between cast ductile iron and cast steel by site or the ring test could be pretty tough.

Edited June 15, 2021 by Mod30
Trim quote.

[dian]

sorry, i was not being clear, when i said: "interestingly high end cranks are made from "billet", which i assume is forged and heat treated (normalised?) bar.",  i meant to say they are machined. so not forged.

[anvil]

I believe cranks from a billet are drop forged, them machined as needed. Heat treat is a multi step process of which normalizing is one of the steps

[JHCC]

First forge, then machine. Milling from a solid billet takes more time, wastes more material, and causes more tool wear than milling from a drop forging that’s already pretty close. 

[jlpservicesinc]

2 hours ago, anvil said:

I believe cranks from a billet are drop forged, them machined as needed. Heat treat is a multi step process of which normalizing is one of the steps

Billet cranks were cut from solid stock.. Some are machined without offset and then twisted once machined and then finished.. 

A forged crank can also have the same process of twisting to align the crank and then finished.. 

Today anything that is machined is called Billet, but many companies latched onto the "billet" as they usually sell well and because they are machined are usually clean and well finished..  so, lots of eye bling.. 

Anybody who is anybody old school is still looking for forged cranks, piston rods and pistons..   

What has changed quite a bit over the last 20 years is the design and materials used..   aluminum, titanium for some parts are standard now. 

 

[jlpservicesinc]

21 hours ago, Patrick Nowak said:

The question regarding heat treatment vs. grain flow: Grain flow is completely independent of heat treatment. The grain flow is set by forging and is not undone or altered by heat treat treatment. This is because the grain flow is largely a function of small non-metallic inclusions such as manganese sulfides as well as bands of varying chemistry which follow the contour of the forging. (Note that is not unusual at all to find steel that, at the micro level, has layers of alternating chemistry somewhat like a pattern welded bar). Steel forgings are usually subjected to some type of heat treatment as you noted, but those treatments don't affect the grain flow. They can effect grain size and the specific microstructures present which have a direct effect on properties like hardness, the various strength properties I discussed above, fatigue life, wear resistance and fracture toughness.

Great read Patrick.. 

So about a year or so ago, I ended up doing a bunch of research into the  "Forged is better" vs "billet or machined" vs Cast steels.. 

My research was very interesting and the findings was such that a cast steel item can indeed have a grain flow much like forged.  (wish I saved the information as it was over a dozen or more websites).. 

The forging sequence of absolutely pure materials does not create the laminar flow boundaries that dirtier steels or irons produce.. (as you mentioned)

This was a very interesting concept..  "Pure materials do not form the laminar grain boundaries as easily" 

Steels or irons of lesser quality with inclusions and such show the largest grain structure laminar boundaries..  

I then looked into rolled steels and they indeed have a huge layered effect that can easily seen on all hot rolled bars..  Rolling is a forging process.. 

I then looked at cast steels and what was really interesting is Hofi iirc liked the cast hammers better than forged hammers ..   Between the casting and uneven cooling there was produced a grain boundary much like a forged item. 

The largest take away was that a forged item if forged well and at a differential temperature will create larger grain laminar flow structures.. 

I then did several experiments where I took a bar of 1" square 1020 HR and heated the bar as fast as possible so the center of the bar was cooler then the outside and forged the item as fast as I could with a good reduction in size by about 1/4"..  This created a huge fish mouth at the end of the bar and it was pretty easy to see how the layers were indeed slid by each other.. 

In the end I walked away with what I started out with..  A properly forged item will have a grain pattern depending one how much difference in temperature there is between layers as well as the impurities in the steel and also the alloys..   That slip plain between layers is the key. 

[JHCC]

Is that a case of slippage between layers, or simply due to the fact that the hotter outside is more plastic than the cooler inside and therefore will deform more?

[jlpservicesinc]

From what I understand it's a smearing effect. 

Kinda like when kneading cheeses together for ravioli. 

When the pressure is applied there is a slip plane. That slip plane happens at the grain boundary lines with some slipping more than others.  Impurities, alloys, etc, etc.

 

I had always assumed it was the alignment of the grain structure, but it's the differences that create the layering or boundaries.  

 

It made sense after reading it all.  And then it made even more sense looking at different levels of worked wrought iron.

[ThomasPowers]

Big discussion in knifemaking about which is better;  some folk don't seem to understand that the 1/4" steel the stock removal guy was using has already been forged down from the original huge billet with reductions from feet to a fraction of an inch.

Also that poor forging practices can make a weaker blade than good stock removal practices.  I've seen a lot of badly done pattern welding claiming superiority due to the name not the actuality!

But back when I was looking to be in the knife world; I learned that a lot of makers depend on "Hype" to sell their product and would latch onto almost anything to try to claim it's superiority.  (I worked for a professional swordmaker whose Father was a research Metallurgist; made for interesting viewpoints on the "biz"!)

[Frosty]

I've had the very same conversation with a bladesmith friend who produced documentation to prove his case that consisted of opinion rather than any sort of scientific analysis. Marketing, just like the time honored Japanese bladesmiths who have the ONLY source of iron sand, wood for charcoal, traditional technique, clay for hardening, even water for the quench to make THE best blades. 

Marketing is the tool of the sale.

Frosty The Lucky.

[ThomasPowers]

Of course I have to balance this with the fact that most people don't need the finest blade possible; a good one will meet their use specs (and a great one will exceed them!)

[Frosty]

We only got into the discussion because he was speaking in absolutes and waving smoke for evidence. 

Most people should buy the knife they need: size, profile and quality are available from commercial companies. I carry an Old Timer and my hunting knife is a Buck.

While I might play with pattern welding and developing patterns I'm not into making blades. Maybe fancy stove shovels and pokers. I only made the hawk to demonstrate a technique, it's just sat on the mantle for nearly 20 years. 

Frosty The Lucky.

[anvil]

17 hours ago, jlpservicesinc said:

bar of 1" square 1020 HR

Billits and cranks aren't my area of expertise, so thanks for the info.

However, if you slowly heat a piece of 1020 or a36 to a nice yellow throughout, and forge it rapidly with a 25# lil giant, you get the same fishmouth and you will see the same thing. Meaning it's not just a temp differential that causes shear. Basically no matter if it's a temp difference, type of steel, or too heavy of blows, the outside gets drawn out quicker than that towards the center, and something has to give.

I'm curious, did you do your experiment with your hand hammer, or did you have a power hammer?  

I agree about knife makers. There's is a very competitive market. Far more than architectural iron. I look at their business as being similar to photography. If you take 100 pics, you are likely to get one good one. Think monkeys and a typewriter. However, a good photographer will take that same 100 pics and have 99 excellent salable photos, and only chooses one to sell. That's one of the reasons I chose to not be a photographer