How to produce Duplex ss by induction furnace and its improvements.

[AjSCCMelt]

thread330-428846

Dear all,
We just started casting duplex stainless steel castings (ASTM BPVC II A SA-995).
We have a 500kgs medium frequency induction furnace.

CASE 1:
We casted a ring like casting (impeller casing wearing ring) in ASTM BPVC II A SA-995 Grade 4A and all the mechanical and chemical properties where achieved but the casting developed pin holes even after taking precautions in moulding, melting and even after manual argon purging the pouring laddle with a lancing pipe (Although the amount of pin holes where hugely reduced as compared to standard manufacturing process).

CASE 2:
We casted a flange in ASTM BPVC II A SA-995 Grade 2A and achieved chemical composition as per standard but the casting cracked after heat treatment also the mechanical properties where not achieved by a huge margin.

Questions:
FOR CASE 1:
1. What should i do to get rid of pin holes.
2. Melting temp. at which metal is poured in laddle.
3. Improvement in argon purging that i could carry out.

FOR CASE 2:
1. Why does the casting cracks even after heat treatment according to standards?
2. Why does the mechanical properties are achieved in GRADE 4A but are far from standard in GRADE 2A?

Thanks and waiting for your valuable suggestions.

 

[EdStainless]

I'll start with #2.
Are there any internal defects? Do the crack locations align with any features? Gates?
You are annealing very soon after casting right?
And heating gently and cooling quickly?
If you aren't hitting 40-45% ferrite then double check chemistry (N also) and anneal temp.

Case #1 is tougher.
Are you pre-drying all charge materials?
Are you adding N? If so how?
Are you using Ar purge throughout the melt cycle?
When degassing with Ar you need fairly coarse bubbles.
And you also need to make sure that you don't cause too much turbulence.
It has been so long since I cast these grades that I don't recall the temperatures.
But I do recall real issues with going too hot.
And we used mold washes, and then force dried the molds.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[bobjustbob]

You should not need to argon purge CD3MN. We poured CD3MN regularly out of a 2000lb and 4000lb induction furnace.
What kind of ladle are you using? New or used and how is it treated before tapping?

What are you use for ladle additions?

I don't remember having any issues with cracking in CE8MN. How thick is the casting and what is your heat treat process?

Bob

 

[MagBen]

I would diagnose starting with micros examination.

first off composition is important. micros compose of about half-half ferrite and austenite? is there any carbides/intermetallic precipitates at interphase boundary? make sure solution temp is high enough to dissolve harmful intermetallic phases, cooling is fast enough to prevent precipitation.

is the crack hot or cold? ferrite can actually be beneficial to hot tearing so cracking i guess was caused by harmful phase(s), rather than thermal stress.

adding directly nitrogen is not an easy job if not using AOD/VOD. AOD helps to remove carbon and allows to add nitrogen directly and reduce sulfur, ideally to less than 50ppm, and so less MnS for cracking. low carbon and nitrogen addition could make induction furnace a more expensive choice.

before blaming N for pin holes, pay attention to slodification rate, heavy section can have N exceeded its solubility, forming gas holes. make sure mold have enough venting, pouring has not cause much turbulence. lower pouring temp, eg 2800F is also beneficial and doable since low carbon can gain extra fluidity.

 

[AjSCCMelt]

@EdStainless
Reply For #2 :
1. Are there any internal defects? Do the crack locations align with any features? Gates? : NO THERE ARE NO PIN HOLES, SAND FUSION ETC. NO THE CRACKS ARE GENERATED RANDOMLY NOT IN ANY SPECIFIC REGION SUCH AS NEAR GATES OR RUNNERS. (ASLO THE THICKNESS OF FLANGES ARE 40MM AND 50MM).
2.You are annealing very soon after casting right? : WE ARE ANNEALING AFTER 3-4 DAYS AFTER THE CASTING IS POURED.
3.And heating gently and cooling quickly?: YES AS THE STANDARD MENTIONS RAPID COOLING (WATER QUENCHING).
4.If you aren't hitting 40-45% ferrite then double check chemistry (N also) and anneal temp. : CAN YOU PLEASE ELABORATE AS OUR TAPPING TEMP. IS ABOUT 1530-1550[sup]0[/sup]C.

Reply for Case #1:
1.Are you pre-drying all charge materials? : NO, WE ARE NOT DRYING OUR SCRAPS BEFORE MELTING.
2.Are you adding N? If so how? : THE SCRAP WE USE ALREADY HAS N IN IT HENCE NO ADDITION OF N IS REQUIRED/DONE DURING MELTING.
3.Are you using Ar purge throughout the melt cycle? : NO, WE ARE MANUALLY PURGING AR WHILE THE METAL IS POURED INTO THE LADDLE FROM THE FURNACE. ARGON IS PURGED INTO THE LADDLE THROUGH A LANCING PIPE.
4.When degassing with Ar you need fairly coarse bubbles.And you also need to make sure that you don't cause too much turbulence.: IS IT POSSIBLE TO TELL THE APPROX FLOW RATE? OR ANY METHOD TO REGULATE SUCH EVENT?
5.It has been so long since I cast these grades that I don't recall the temperatures. : WE POUR THE METAL INTO THE LADDLE AT ABOUT 1550[sup]0[/sup]C.
6. But I do recall real issues with going too hot. : IT WOULD BE VERY HELPFUL IF YOU CAN RECALL IT.
7.And we used mold washes, and then force dried the molds.: YES WE DID MOULD WASHES AND FORCE DRIED THE MOULDS USING REGULATED WARM AIR FOR 7-8 HOURS.

THANKS FOR YOUR REPLY SIR HOPE YOU WILL GUIDE ME IN THIS CASE.

 

[AjSCCMelt]

@bobjustbob

WE USE BOTTON POURING LADDLE WESMAN MAKE. IT IS A USED LADDLE BUT IS CLEANED AFTER EVERY POURING.

THESE ARE CASTINGS OF FLANGE WITH THICKNESS OF 40 MM AND 50 MM. WE HEAT TREAT THE CASTINGS AS PER STANDARD: Heat to 2050°F [1120°C] minimum, hold for sufficient time to heat casting uniformly to temperature, quench in water or rapid cool by other means.

THANKS FOR YOUR REPLY AND WAITING FOR YOUR VALUABLE RESPONSE.

 

[bobjustbob]

Do you add anything into the ladle for deoxidization? I am not talking about degassing with argon.

Tell me more about the argon purge in the ladle. It sounds like you have a lance pipe in the ladle as the heat is being tapped out of the furnace. Is this correct?

You state that your scrap has sufficient nitrogen content for the heat, what is your typical nitrogen content on your final chemistry check?

 

[EdStainless]

How do the micros look? Any secondary phases?
Is the Austenitie/Ferrite ratio correct?
These are what matter in duplex grades.
A minor variation in annealing temp will mess up the A:F ratio.
And variations in chemistry will require different annealing temps.
How much revert (scrap) is in your charge?
Do you have a strict limit?
What is the rest of the charge, master alloy or pure metals?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[AjSCCMelt]

@MagBen
Thanks for your reply.
We are tapping the liquid into the laddle at 1540[sup]0[/sup]C. And in heat treatment castings are quenched within 30 secs from the door opening of HT furnace.
Is there any carbides/intermetallic precipitates at interphase boundary? : Im not technically capable enough to answer this question hence it would be very helpful if it is possible to explain in laymen terms.

We have poured duplex ss 4a @1540[sup]0[/sup]C.

 

[AjSCCMelt]

@bobjustbob
Thanks for your reply.

We add aluminium scrap pieces in ladle just before tapping for deoxidisation.

Yes sir, we use lance pipe to purge argon as long as the molten metal is poured into the ladle with an argon cylinder.

In our final chemistry : Nitrogen is 0.15%.

%c : 0.028
%Si: 0.64
%Mn : 1.17
%S : 0.003
%P : 0.018
%Cr : 21.50
%Ni : 6.30
% Mo : 2.88
%Cu: 0.12
%N : 0.15

 

[bobjustbob]

Your chemistry looks fine. I would reduce the Mn by half for mechanical properties but that should not be causing a gas issue. Your nitrogen is fine, is that checked by combustion analysis (LECO)?

Have you produced any heats without the argon purge? For this material you should not need an argon degas. When I have used an argon rinse in the past it was through the ladle after tapping. With the turbulence in the melt during the tap I would think that the argon purge through a lance would cause more problems that it might solve.

Bob

 

[EdStainless]

I fully agree with Bob.
If your argon bubbles are too aggressive then you are actually introducing more oxygen and moisture from the air.
Back when I was buying a lot of 2205 I could glance at a chemistry and identify the mill just by the Mn number. One ran about 0.50, another about 0.80, and one was always 1.25. They all worked, but the optimal anneal temps were different as there were also other slight chemistry differences.
With any duplex the microstructure is what really matters.
You should be measuring the austenitite:ferrite ratio and some type of intermetallic control (low temp impact or very good micros or corrosion test).
For your cracking it really sounds like casting related defects.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed