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White cast iron


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Hi Guys!

 

 

Lots of info about crucible steel´s and all most all say that white cast iron is better than others cast irons.

 

Other cast irons have a very high silica content.....is this only reason why its better. White has quite low Si cont.

 

So this white cast iron.... all C is bind fully in carbid ( cementite) but when all goes in crucible and under hi heat many h dosent this all chance? I know that they produce this white quite fast cooling rate. Also there is Cr and it strong carbide-greator.

 

Anyway why it´s "better" than other cast irons

 

 

Niko

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I think that gray cast irons have graphite embedded in the mixture. That helps to improve machinability. There will be some debate yet about whether a graphite molecule with a melting temperature in excess of what most of us could generate in the forge/smelter, can actually contribute carbon or not.

 

Therefore, a white cast iron without graphitic contamination is best. Calculate the carbon content and add irons to reduce it to the level you want when done. Simple, yes? The silicates will help some with forgeability.

There are three kinds of men. The one that learns by reading. The few who learn by observation. The rest of them have to pee on the electric fence for themselves. Will Rogers

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Hi Mike!

 

 

Thanks

 

Jep it´s easy ;)

 

In some white cast irons cont. Cr quite lot.... 2,0% to hi as 20% :blink:

I think this hi Cr cont gan greate so differns too...?

 

You maid by right on that...white cast contributes carbon "easyer".

....If this all whould not be this "easy" ...HOW about pig iron? How C is band in that one?

 

Maby im asking too much and things that dont make any differns in final product...?

 

Niko

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It depends on the purpose - if it needs to be machined o final shape then grey iron is better. If it needs to be hard - like for mining grinding balls - then white iron is better..

 

Standard metallurgist answer - "it depends...."

D. Scott MacKenzie, PhD

Heat Treating (Aluminum and Steel)

Quenching (Water, Polymer, Oil, Salt and Mar-Tempering)

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Hi!

 

 

Standard metallurgist answer??? Where ?

 

 

Everything deffenetly depends on the purpose <_<

 

 

My main point was in crucible steel, like i wrote!

So to clear out this more " wootz-type steel" --> Blades

 

Standard metallurgist question----> " Why" it´s better?

 

 

Niko Hynninen

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myself i've only used white cast iron.. and haven't used any of the grey... ( I think ) .... but i'll tell you that the clay graphite crucibles i used, donate carbon to the steel.... so graphite has contributed to the carb level...

-- as you can see the crucible getting thinner the more melts you do with it...

 

-- the problem maybe the rate at which this graphite dissolves... ? maybe it will take a slow time to go into solution ...... so you may need longer melt times.... or not... it would be a good experiment to do..

 

Greg

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.... but i'll tell you that the clay graphite crucibles i used, donate carbon to the steel.... so graphite has contributed to the carb level...

-- as you can see the crucible getting thinner the more melts you do with it...

 

-- the problem maybe the rate at which this graphite dissolves... ? maybe it will take a slow time to go into solution ...... so you may need longer melt times.... or not... it would be a good experiment to do..Greg

 

Given the melting point with possible sublimation (3562 Centigrade), boiling point (4827 Centigrade), I'd really like to see any hard data that proves this allegation. I'll allow possible carbon inclusions in the graphite matrix as a source, but really, let's see some measurements that support this.

 

Anyone playing with thermit may be able to approach the necessary temperatures, but most of us roaming around a normal welding or smelting temperatures are not going to come close.

 

Personally, I think the CLAY portion of the graphitic crucible is far more likely to erode. That appears to contribute something to the button, but I doubt it's carbon from graphite.

There are three kinds of men. The one that learns by reading. The few who learn by observation. The rest of them have to pee on the electric fence for themselves. Will Rogers

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well..... if you think about it...... how can you make grey cast iron if you can't form graphite in steel melting temperature.....

 

if you can make it at that temp....... why can you not dissolve it. ???

 

 

 

although this article is not what i'm looking for......it does say graphite precipitates from the melt.

http://www.ductile.org/magazine/2007_1/primarygraphite.htm

 

 

 

G

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well..... if you think about it...... how can you make grey cast iron if you can't form graphite in steel melting temperature.....

 

if you can make it at that temp....... why can you not dissolve it. ???

although this article is not what i'm looking for......it does say graphite precipitates from the melt.

http://www.ductile.org/magazine/2007_1/primarygraphite.htm

G

 

Because the graphite is trapped in between the iron/carbon formations, not necessarily a part of the molecular arrangement. It's more like an inclusion or a mixture riding in the spaces between, not an alloy. The graphitic stuff is basically non reactive. Even at smelted ladle temperatures...

 

Somewhere there is a metallurgist who can specify the residual carbon content of graphite and from that it may be possible to determine what could be contributed to a smelt load from the carbon/graphite mixture. But graphite by itself, no, I don't think so, not at the temperatures we're working at, or likely not even the temperatures in the steel mills.

There are three kinds of men. The one that learns by reading. The few who learn by observation. The rest of them have to pee on the electric fence for themselves. Will Rogers

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Got to disagree guys - graphite most definitely reacts with iron, whether the iron is molten or a solid.

 

Typical carbon content of grade 1651 graphite from Asbury Carbons is 96 to 97 % carbon, about .3 % moisture, and the balance is listed as ash, typically oxides such as MgO, SiO2, etc. that won't burn.

 

The reaction is a chemical one that doesn't involve melting of the graphite, but migration to even out a differential level of carbon content. It's what happens when thwey used to make blister steel, or what happens in modern Poder Metallurgy - graphite powder is added to plain iron powder pressed into a part and sintered at 2050 F, when the part is examined you have a consistent carbon content throughout (if you've done your sintering correctly). A solid state reaction - iron powder & graphite powder.

 

A similar situation occurs with aluminum and iron - aluminum with a melting point of 1221F will dissolve iron with a melting point of 2795 F, until iron reaches a saturation level.

 

Mundanely, think along the lines of sugar dissolving into water.

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Got to disagree guys - graphite most definitely reacts with iron, whether the iron is molten or a solid.

 

The reaction is a chemical one that doesn't involve melting of the graphite, but migration to even out a differential level of carbon content. It's what happens when thwey used to make blister steel, or what happens in modern Poder Metallurgy - graphite powder is added to plain iron powder pressed into a part and sintered at 2050 F, when the part is examined you have a consistent carbon content throughout (if you've done your sintering correctly). A solid state reaction - iron powder & graphite powder.

 

The graphitic inclusions in gray cast irons are precipitates from a hypersaturated solution of carbon. The graphite precipitates as the iron solidifies. It's not graphite giving up carbon to the iron to form carbides.

 

Here's one reference: http://members.lycos.nl/cvdv/carbidespart1.htm

 

This article implies that carbon-as-graphite remains a suspension in solution and precipitates on solidification and that if enough carbon nucleii are present (hypereutechtoid) graphite precipitates from the excess carbon on solidification.

 

But there still isn't any hint of chemical reactivity between the iron and the carbon/graphite, merely a solution state that ceases to exist on solidification.

There are three kinds of men. The one that learns by reading. The few who learn by observation. The rest of them have to pee on the electric fence for themselves. Will Rogers

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  • 2 weeks later...

OK - Check out the book "Foundry Engineering", by Flemings, Taylor and Wolf....

 

At a given chemistry, you can have either white iron or gray iron - the difference is the cooling rate. Cooled rapidly, you get white iron. Cooled more slowly you get gray iron. Refer to the diagram on page 98. I wish I had a scanner available now - I would include it.

 

If you take white iron and heat it up and hold it isothermally for a long time, then the graphite will precipitate out and form malleable iron...

 

Consider the same melt - if you add graphite, there is a solubility of graphite in liquid iron. Some of the graphite would dissolve in the melt, while some would remain as graphite - according to the equilibrium diagram. So it is possible for additions of graphite to increase the carbon content of a melt.

 

Hope this helps eliminate any confusion.

 

Scott

D. Scott MacKenzie, PhD

Heat Treating (Aluminum and Steel)

Quenching (Water, Polymer, Oil, Salt and Mar-Tempering)

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OK - Check out the book "Foundry Engineering", by Flemings, Taylor and Wolf....

 

At a given chemistry, you can have either white iron or gray iron - the difference is the cooling rate. Cooled rapidly, you get white iron. Cooled more slowly you get gray iron. Refer to the diagram on page 98. I wish I had a scanner available now - I would include it.

 

If you take white iron and heat it up and hold it isothermally for a long time, then the graphite will precipitate out and form malleable iron...

 

Consider the same melt - if you add graphite, there is a solubility of graphite in liquid iron. Some of the graphite would dissolve in the melt, while some would remain as graphite - according to the equilibrium diagram. So it is possible for additions of graphite to increase the carbon content of a melt.

 

Hope this helps eliminate any confusion.

 

Scott

 

 

I think this thought problem is a worthy exercise. But the steel mills add carbon to melts not graphite. The carbon in solution contributes to the irons, and graphite is the leftover precipitant. I'm still not convinced that graphite, the most stable form of carbon on the planet, will add anything at all. None of the articles I've found yet have helped clear any of the confusion.

Edited by Mike Blue

There are three kinds of men. The one that learns by reading. The few who learn by observation. The rest of them have to pee on the electric fence for themselves. Will Rogers

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Mike, I've worked in steel mills on and off for 30+ years a lot of the time as a metallurgist. We talk about adding carbon to a heat of steel, whether it's in a BOF or an EAF, but what we actually add is usually graphite - just a fancy name for carbon in a particular form. The supplier we use most for EAF adds right now is Asbury Graphite mills - same folks who supply us with powdered graphite for production of mixes used to make powdered metal parts.

 

The graphite is most definitely not a leftover precipitate - it's the source of increased carbon over what is in the original melt.

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Mike, I've worked in steel mills on and off for 30+ years a lot of the time as a metallurgist. We talk about adding carbon to a heat of steel, whether it's in a BOF or an EAF, but what we actually add is usually graphite - just a fancy name for carbon in a particular form. The supplier we use most for EAF adds right now is Asbury Graphite mills - same folks who supply us with powdered graphite for production of mixes used to make powdered metal parts.

 

The graphite is most definitely not a leftover precipitate - it's the source of increased carbon over what is in the original melt.

 

I can't possibly argue with what you did in the foundry. No one has yet explained the mechanism to satisfy the difference in the published chemical properties of the graphite and it's alleged performance in a molten steel bath. If it's not melting at the low temperatures used for smelting, then its the change due to some other physical property, or electrical? Graphite will corrode stainless steels at room temperature and is hell for oxidizing aluminium. But those have nothing to do with the performance expected from temperature alone.

 

If there is a chemical reaction then there should also be a balanced chemical equation that explains the consumption of the graphite. As a steel mill operator, you should be able to predict accurately that so much graphite will leave so much carbon behind in the melt, still accounting for precipitated percentage in the microstructure.

 

I can't balance a chemical equation that will produce an increase in carbon from a fixed quantity of graphite and a zero carbon iron. I think that violates some physical laws somewhere.

There are three kinds of men. The one that learns by reading. The few who learn by observation. The rest of them have to pee on the electric fence for themselves. Will Rogers

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Mike - it is straight forward. Look at the Fe-C phase diagram. There is a range of chemistries that have Fe+C and then there is pure graphite. The graphite will diffuse into the melt up to the equilibrium limit.

 

If you want a chemical equation, it is 3Fe +C(graphite) = Fe3C.

 

You can simply prove this yourself. Take a graphite crucible, and fill it with pure iron, or low carbon steel. Cover the crucible to prevent oxygen making CO and CO2. Heat slowly to different temperatures. After a period of time - you will carburize your steel. Heat up til melt and hold for a period of time, then cool. Measure the carbon content - it will have increased......

D. Scott MacKenzie, PhD

Heat Treating (Aluminum and Steel)

Quenching (Water, Polymer, Oil, Salt and Mar-Tempering)

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  • 2 weeks later...

When I have a problem to solve, I don't rest much until I can get my mind around the entire answer.

 

I was wrong about the direction I'd taken with the carbon-iron problem, despite the superficial logic behind my initial argument that graphite was stable at the temperatures of melting iron.

 

I pestered a couple more experts and the answer came back thus: "The element carbon is soluble in molten iron irrespective of what form the carbon might be in, graphite, carbon black, diamond, pyrolytic graphite, etc."

 

There is a mechanism to the relationship between carbon and iron that is not explained by temperature, or the chemical behaviors of either element alone.

 

I'm not that old a dog... B)

There are three kinds of men. The one that learns by reading. The few who learn by observation. The rest of them have to pee on the electric fence for themselves. Will Rogers

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this has been a great post.... i've learned alot...

 

thank you

 

its funny how you can observe something.... but when it comes down to backing it up and understanding whats going on...... man... that becomes a whole new ball game...

 

 

 

 

 

 

 

 

Greg

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