What is Semi-steel?

From:

Cast iron (not steel) of increased strength, obtained by using a large percentage of steel scrap with the pig iron. The quality varies, but if a known amount of good steel is added, like rail croppings, the resulting product will have more ductility and strength than cast iron alone. Used for casters, machine frames and other items where toughness is needed in a cast product.

------------------------------------------------ Lane

In most cases, it refers to a material, made in a cupola furnace, that's melted from scrap cast iron and a large percentage of scrap steel. It's considered a high-quality version of cast iron: somewhat freer of impurities, and with lower overall carbon, and with very little *free* carbon. It contains a lot of cementite (iron carbide), but it's almost as ductile and malleable as cast steel. How hard it is depends on how, or whether, it's been heat-treated. The object in making most of it is to achieve higher ductility than gray cast iron, so it's generally left soft and not heat-treated at all.

It was commercialized in high volumes during WWI and has been used in making machinery parts, mostly, up until recent years. I believe that ductile cast iron has replaced a lot of it, but I haven't checked in a couple of decades.

If you have a historical interest, be aware that it hasn't always meant exactly the same thing. In Bessemer's day he called steel with *lower* than normal carbon content "semi steel." This was a very soft product intermediate between steel (minimum around 0.08% carbon) and "soft iron," which had less carbon still, down close to zero percent. It also has virtually no sulfur and most other impurities have been oxidized out along with the sulfur. Basically, it's steel that's been oxidized to a very high degree, producing carbon levels that are below those normal for steel.

How much of that latter product was ever commercialized, I don't know. But what you have is almost certainly the first version of "semi steel" that I described above: cast iron made with a high percentage of steel scrap. It has *more* total carbon than steel does, but less than typical cast irons.

Ed Huntress

Rich, As the previous poster have said, "semi-steel" refers to a type of gray iron with a large amount of the charge composed of steel scrap. It is an obsolete term, although you will still find it on the material specs on blueprints.

These days, most foundries use at least a portion of their charge makeup in steel scrap and the term is made even more meaningless by the way people specify castings by tensile class (Class 20, 30, 40 ,50). That being said, when I get a request for quote from a customer with semi-steel called out on the print, I generally quote in Class 35 or 40.

Regards, Mike

Ductile iron.

Harold

Not the same thing, Harold, although you're on the right track in terms of properties.

Ductile iron is an alloy (iron plus magnesium or some other metal, which I forget). Semi-steel is plain iron/carbon. In semi-steel, the ductility is the result of the carbon percentage being reduced by adding scrap steel to the melt, and by much of the remaining carbon being tied up in the form of iron carbide -- few or no free flakes, unlike the case of gray iron. In ductile iron, on the other hand, the ductility is the result of the graphite being converted from flakes to nodules.

I think that most semi-steel applications have been taken over by ductile iron. Maybe someone can confirm that.

Ed Huntress

Hmm.

Cast iron Ductile iron Nodular iron Maleable iron

All terms seen in use.

There does seem to be *some* overlap here.

Jim

Yeah, in some cases, but not in others. "Cast iron" is generic. Gray iron is what we usually mean by "cast iron," but not necessarily.

Nodular iron is a name sometimes used for ductile iron. But the automobile industry, in particular, has drawn a distinction between nodular and ductile for its own purposes. What the distinction is, I don't know. Maybe just a slightly different alloy. We ran an article on it in _Machining_ a couple of years ago but I didn't read it. (Sometimes I don't read everything in the magazine.)

Maleable is a specific material. It's a form of cast iron (white iron, I think) which is heat treated for an exended time: something like 48 or 72 hours, IIRC, to nodularize the graphite flakes.

There are stainless alloys of ductile iron, and chilled iron (sometimes used as an alternate name for white iron), and wrought iron, and soft iron (extremely low carbon, used in the past for transformer and motor armature stampings; high-silicon, low-carbon iron is used for that today). And there are more.

Ferrous alloys fill entire books.

BTW, Meehanite is not a grade of iron. It's a process. The Meehanite company uses specific, proprietary iron alloys to make its products, but the iron itself isn't "Meehanite."

Ed Huntress

Semi-steel is basically a high grade of gray iron. Gray irons, no mattern how they are made, have virtually no ductility. The improvement one sees in a semi-steel gray cast iron is the result of low %'s of phos and sulfur. In a standard cupola iron made in the 1900's- 1950, tensile strengths were often quite low, as the charge materials were typically all cast iron and the resulting iron had phos contents ~.15-.25% and sulfur content >.08. Using large amounts of steel in the charge reduced these levels by dilution dramatically and greatly improved the quality of the iron. Another concept that aided gray iron metallurgy was the concept of carbon equivalent (CE), which is defined as CE= %C+ ((%Si +%P)/3). Tensile strength and hardness are generally inversely proportional to CE.

In some cases yes, that is true, but in general, casting buyers generally rely on the ASTM A48 specification when calling out material specs on gray iron castings. Some companies get even more specific when specifying gray iron requirements, calling out graphite flake morphology, flake size and the percentage of ferrite vs. pearlite. One must keep in mind that ductile irons have greater tensile strengths and the property of ductility, they are not a good choice on applications that require vibration-damping and certain thermal properties.

Mike Malone

Huntress

Good point Ed! I get lots of calls from buyers asking me if we produce Meehanite, as that is the material spec on their prints. I have to tell them that this is a trademarked name and patented process and that I can't offer them Meehanite, as we do not have a Meehanite license ( very expensive I might add!) IMHO, the whole Meehanite marketing process is one of the bigger scams I've seen.

Meehanite's original research in the need for inoculation and control of undercooling in the 1930's, laid the foundation for the consistent production of quality gray irons in the present day. But the Meehanite process is basic knowledge to most foundrymen today!

Mike Malone

Note: Meehanite is the registered trademark of Meehanite Metal Corp. All references below to Meehanite refer to the one and the same company.

------- Hmm....there are Meehanite licenced foundries, but as far as I know, there are no "Meehanite-owned" foundries. Basically any foundry is eligible to become a Meehanite-licensed foundry, if they are willing to pay the licensing fees and abide by the "rules" put out by Meehanite. These not only include the processing techniques but the foundry is subject to audit of their processes, and because the license may sometimes involves a fee on the tonnage produced then they have to open their shipment figures to Meehanite. There are marketing advantages as well.

At one point in time Meehanite also had company in the U.S. that made the inoculants (i.e. the iron alloys mentioned above) to Meehanite specifications, but that company does not exist any more. Meehanite is world-wide, so there may be licensed companies to make the inoculants (iron alloys).

The Meehanite Process was conceived in the USA by Meehan and Smalley in the early 1920's.

Wedge testing of iron is one important part of the process measurement techniques that Meehanite foundries would use. There are also A.S.T.M. specifications (I think A367) that describe wedge testing of iron. The main point of Meehanite is that the "process" of making the iron is controlled. One should never attempt to control the making of iron by chemistry alone (as with steel, aluminum, bronze, brass, etc.) Graphite structure control is the single most important thing about producing quality iron.

The basic difference in all major families or cast iron: gray, ductile (aka nodular, or spheroidal iron), malleable, and white irons is all about the basic structure of the carbon. Gray and Ductile irons can also be heavily alloyed, with Nickel, Copper, and Chrome and Molybdenum but the graphite will still be either "Flake-like" for gray iron or "ball-shaped" for ductile iron. The alloys mentioned above substitute for the element Fe to increase corrosion resistance, or increase useable working temp, etc. So there are 5 basic families if you classify the last one as Alloyed irons.

With Meehanite's very good marketing campaigns of the past, they were able to convince designers that the key to good casting design was to specify "Meehanite" on their blueprints. This also meant that legally if you are making a part with the "Meehanite" description on the blueprint, you had to use a Meenanite-licensed foundry.

Once designers' designs worked well with "Meehanite" castings, it is hard to convince them that an "non" Meehanite-licensed foundry can produce the same or better quality level castings. But they can.

Not taking a position one way or the other on Meehanite....I have worked for Meehanite and non-Meehanite foundries.

Here's the UK website:

Mark

So why isn't it white iron? Malleable?

Tim

-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @

Some white iron is malleable but not all of it. 1 to 1.5% Si seems to be the trick as well as 2.0 + < 3.0 in carbon.

Martin, White iron production can take several different approaches. The compositional limits you give will give a white iron suitable for malleablizing. However, many white irons are produced for non-malleable usages by adding 5 to 28% Cr, depending on specification and section size. Also, castings can be produced with a white iron outer layer and a traditional gray iron core by using a iron chill to form the outer surface of the casting. The chill promotes extremely rapid solidification in the outer sections, while providing a normal gray iron microstructure in the core of the casting. Very useful for as-cast gears and sprockets.

Mike

Thanks, all. That's all very interesting and informative, but what about the forest? Can someone guess, based on the on-going discussion, how the properties of semi-steel make it a good material for a silversmithing stake? What would make semi-steel preferable to, say, all-steel or non-steel (iron)?

Thanks again

"Semi-steel" does not exist except as a marketing term, and at one time _might_ have described a slightly superior cast iron. It is only describing that the foundry melted some steel to make the cast iron. Once melted into a cast iron base melt the steel no longer exists as steel.

Good quality cast iron (or "semi-steel") would be better than "ordinary" cast iron (i.e. that which was made by just throwing a lot of pig iron into a furnace, melting and pouring) because the silversmithing stake would be less likely to break.

Good quality cast iron (or "semi-steel") would be preferred over steel for the reason is that it's less costly than steel for many reasons, the raw materials themselves, because it's easier to cast gray iron than steel (leading to lower cost), etc. Good quality cast iron is easier to machine. Also the good quality cast iron will tend to absorb the vibrations of the hammer a little better than the steel, which will "ring".

The downside of the good quality cast iron is if you hit the silversmithing stake too hard it will fracture, and cast irons are harder to weld than steels. But usually this would mean the cracking is coming from abuse not ordinary use.

You already mentioned that the anvil will dent when hammered. If this is ordinary hammering and it's objectionable then you should buy a steel one, preferably hardened, and it will not dent as easily. It may cost a lot more. Also when a steel one does dent you could weld up the dents I suppose.

You probably already know from experience (I assume you've used the anvil) whether it works OK or not.

If you have a choice between buying one in any of the materials buy the lowest cost one that gets the job done. Don't worry about the semi-steel nomenclature cast into the base. It's cast iron. Think of it as somebody selling a cola. Do you want Pepsi-Cola, Coca-Cola, RC, or ??? They are all colas just different brands, quality levels, prices, etc. But if you have a choice between cast iron (i.e. regular, "semi-steel", "Meehanite", or ???) vs. steel, then that would be like a choice between a cola and a root beer.

Hope to not confuse it but there could be a dozen grades of steel that steel anvils are made of, then there's heat treating the casting, etc.

The gears on Logan lathes were made from semi steel per the manuals.

Like I said - a large box for white but a small box bounded as above inside the white that is malleable. See my prior post on the manual the subject has the www page. Martin