Cromium 2

[5123]

In 316 melting process, do you loose cromium? if so what percentage.

Thank you, Mario

 

[lgearhart]

That's one of the beauties of the Argon Oxygen Decarburization (AOD) process- the lowered partial pressure of the oxygen blowing through the molten steel affects the thermodynamics such that the carbon is oxidized in preference to the chromium. So yes, you'll still lose some chromium out of your ferrochrome during the standard melting process, but not a lot- and I don't think you lose any more if you do an additional VAR or ESR refining step.

Yet I'm not a melt shop metallurgist, so I'd suggest you wait until one of those all-knowing folks answers here, or call your local stainless melting facility.

Good luck!

Lee

 

[5123]

I work for a steel investment casting foundry. Lately we have been experiencing excessive shrink on parts that we never had a problem with. We seem to have a pretty tight process as far as mainting pre-heat and metal temperatures. We also seem to have a good handle on our chemistry, since we have our own lab we are able to test frequently. My question is a few months ago (before I started working here) the company started using a deoxidation process by adding some percentages of Ferrous Ti and alluminum pellets, Would this create the type of shrink we are seeing?

Thank you, Mario

 

[bruv]

Here are a few questions that you may or may not have considered while looking for your lost chromium. Forgive me if I ask some obvious ones, but I have to start somewhere.

What melting process are you using? How big is your melt, and how long does it take? Are you melting virgin materials only i.e. Fe, FeCr, Ni etc, or do you use a percentage of foundry returns/certified scrap plate? At what point in the melting process is the Cr added? Has the type of furnace lining changed recently? Have you got a new furnaceman?

I hope that gives you a few pointers where to look.

Regarding the other problem - will you get the opportunity to try a pair of controlled melts, identical in every way except for the deoxidation practice?

Best of luck

Chris

 

[arunmrao]

In air melting practice for 316 there is a loss of Chromium. This is compensated by adding Low Carbon Ferro Chrome. The extent of loss depend upon the type of scrap and the melting practice. It would be safe to use a figure of 5% as melting loss.

 

[mcguire]

Shrinkage of the metal as a function of the major alloying elements is not a major consideration if your chemistry is steady. Consult the correlation developed for welds; the DeLong diagram. More ferrite in the composition will result in less shrinkage. Less chromium would result in less ferrite.
Addind Ti and Al will remove nitrogen from effective participation in the alloy even though it will be there on analysis. Check the ferrite/austenite ratios on good versus bad heats/casts and see how that correlates with shrinkage.

 

[arunmrao]

Titanium addition is done to melt to combine with the carbon and improve stainlessness. Aluminium addition practice is coupled with the addition of calcium silicide to the ladle. I use CaSi along with Se for deoxidation. Before tapping the metal to reduce the carbon you may add mill scale or iron oxide. It is quite effective. However these additions have no role to play in the shrinkage phenomenon observed.
There are instances of isolated addition of aluminium causing pinholes. Are you referring to that. My best advise would be to cut off Al addition for one pour and observe the castings. This could nail the culprit.

 

[TVP]

mcguire may have made something of a leap in his attempt to answer the question from 5123. An important question for 5123: are you casting duplex alloys? Austenitic? Obviously mcguire's answer applies to the casting of duplex alloys, but not for austentic alloys. Also, what is the nature of the "shrink"? Is it solidification shrinkage or entrapped gas porosity/pinholes?

 

[5123]

You gentlemen are way too smart for me and that is complement. I don't know what you mean when you say duplex alloys, most of my background has been with titanium. All I know is that it is 316, some times we use ingot and some times we use our own gating scrap, these in different percentages. We most defenitely alloy it to bring it to spec., when the recycle is below. Can you please explain duplex, I'm confused. As you can see I'm no metallurgist but more of a process engineer. The shrink seems to be a little of every thing you mentioned above but accentuating more on thick section areas where is commonly found maybe related to solidification in this last case. We have tried some watering techniques to eliminate it at least on the thick sections, but no results.
Thank you for the help and time.

Mario

 

[mcguire]

Note to TVP. The first dendrites to solidify in a normal chemistry 316 are ferrite. These transform to austenite upon cooling. Very few stainless grades solidify austenitically. Those that do are nearly impossible to continuously cast and weld with great difficulty because they are hot-short; i.e. poor high temperature ductility.
316 should normally have about 5-10% delta ferrite as cast or as-welded before any subsequent annealing to dissolve the ferrite.
In the above problem I don't suspect gas so this leaves phase balance as the main suspect. I think it's possible the alloy is balanced to solidify austenitically and therefore not having the shrinking phase change in the first metal to solidify to reduce internal volume thus leading to shrinkage in the last volume to solidify.

 

[mcguire]

Addendum to above. In the allowed chemistry range of 316, it can solidify as either austenite or ferrite depending on the ratio of chromium,molybdenum and silicon to nickel,carbon and nitrogen.
Since austenite is about 4% more dense than ferrite, this can make a big difference on the total shrinkage the casting will undergo in freezing and cooling to room temp.
The freezing front will also reject solute according to the portion of the phase diagram the particular composition falls into.
I think in Mario's case he should try to have
C+N = 0.06%
Mn = 1.8%
Ni = 10.2
Cr = 16.5
Mo = 2.1
Si = 0.4
S = low as possible

 

[TVP]

5123,

Iron crystals/grains can exist in several forms, two of which are face-centered cubic (FCC) and body-centered cubic (BCC). In steels (stainless included) the FCC phase is called austenite and the BCC phase is called ferrite. Depending on the chemical composition and some other factors, one or both of the phases may be present.

In the case of stainless steels, most alloys are intended to be either fully austenitic (3xx series) or fully ferritic (most of the 4xx series) when produced by wrought processes. Some alloys have been developed to have a duplex stucture, that is, both austenite and ferrite are present. And as mcguire mentioned previously, normally austenitic alloys like 316 can have small amounts of ferrite (from 5-20%) in the as-cast condition.

FYI, I purposely avoided discussing martensitic (4xx alloys like 410 or 420) or precipitation hardening stainless steels in this discussion for clarity/brevity.

mcguire,

Thanks for the refresher on stainless solidification. I haven't really dealt much with 3xx series in the as-cast condition, since I have always been a user/specifier not a manufacturer. And my duplex experience is with seriously duplex alloys like 2205 and Nitronic 19D. Looking back at the original posts, I now see that I just assumed 5123 was investment casting 316 stainless steel, and that the intended microstructure was to be fully austenitic. It's amazing how quickly one can jump to conclusions based on what one thinks is happening...

 

[mcguire]

I think it's a case of there not always being all the critical information the first time around. Your answers keep me on my toes.

 

[5123]

Mcguire, I have looked at the chemistry you mentioned above and compared it to our 316 chemistry. I would like to try your input, unfortunately our 316 chemistry required by the customer is as follows per ASTM A 743 CF-8M:
C 0-.08
Cr 18-21
Cu -
Mn 0-1.5
Mo 2-3
Nb 0
Ni 9-12
P 0-.04
S 0-.04
Si 0-2.0
Fe remainder
Do you have any other suggestions based on this spec.
Thank you, Mario Estupinian

 

[arunmrao]

Yes there is a subtle difference in the chromium content. You may maintain all other elements as suggested by Mcguire,but increase Cr content to 18.5%. This is necessary for it to qualify as CF8M. Also Mn will have to be readjusted to meet the 1.5% max spec.
Despite all these controls, I still wonder if you can get your shrinkage porosity problem solved. You may neeed to rejig the mold and gating and risering.

 

[5123]

To arumarao, our cromium seem to be falling around max all the time, but the Mn is always at the low end of the spec. Here are some exmaples of 4 heats:
Heat 1 Heat2 Heat3 Heat 4
C .02 .01 .02 .03
Cr 18.55 18.42 18.64 18.43
Cu .10 .16 .07 .09
Mn .47 .78 .28 .42
MO 2.31 2.31 2.24 2.29
Nb 0 0 0 .18
Ni 9.52 9.54 9.92 9.78
P .02 .02 .02 .02
S .01 .01 .02 .02
Si 1.74 1.93 1.60 1.91
Fe 67.27 66.82 67.20 66.84

Do you think having the Mn this low may do it?

Thank you, Mario

 

[mcguire]

Back to one of the things you asked oroginally, can the ferro-titanium and aluminum additions affect the shrinkage.
They will form TiN and TiC and AlN in the liquid phase if the concentrations are high enough and this will cause propblems with fluidity. This is a more likely contributor to porosity, since it would affect how well the casting is fed.
It also has a powerful effect on ferrite/austenite balance.
Sorry to give you a chemistry for 316 when you wanted CF8M.
They are quite different animals.

 

[5123]

Mcguire, What would you consider High concentration. Our additions are as follows: Al pellets .05% and FeTi .25%, and at what point of the melt would you consider these additions to be safely deluted to avoid high concentrarions.

Thank you, Mario

 

[mcguire]

If the product of the titanium concentration and the nitrogen concentration exceeds a certain value the TiN precipitates in the liquid alloy.
The formula is [Al]x[N]>0.0025% then precipitation.
This formula is based on 409 stainless continuous casting. As Cr increase the solubility decreases, so it's very easy to get precipitation in your alloy. Furthermore these Al and Ti oxides and nitrides like to glom together. I suggest you re-examine your need to put the foo-foo dust in the melt unless it has a very well thought-out reason for being there. You have enough de-oxidation with the Mn/Si/Cr.

 

[mecengr]

In stainless steels, specially austenitic, problem of sesitization occurs if we cool slowly in the range of 400 to 700C. Therefore quenching of aust. SS is done commonly from higher temps. In sensitization, Cr combines with Iron which precipitates on the grain boundaries & can cause corrosion & failure of material. Also there is less free Cr left behind for the matel to defend against high temps & corrosive fluids. To avoid this, Titanium & Niobium is added. It combines with Carbon & further adds to metals properties. whereas Cr is left free to play its part.