Boron stablizing M23C6 2

[deadrange]

I have a question regarding Boron treated steel containing no titanium. This steel contains roughyl 2% Cr and 1.5% W. I have an article that states that Boron is added to this steel to stablize the M23C6 carbides.

1 What is the mechanism for this stablization?
2 For this mechanism to occur does the the boron remain in solution since it has a very high affinty for Nitrogen. The presence of V,Cb,W make this problem a little more interesting since they all like nitrogen, but I am not sure they have a high enough affinty for override Boron's desire to form Boron-nitride, which from my understanding is very stable and does nothing for mechanical property

 

[TVP]

Keep in mind that it is not just affinity for N that is important, but at what temperature the nitrides or carbonitrides form. I am at home today, so I don't have my references in front of me, but I am fairly sure that Nb(C,N) formation occurs before BN, so the B should safely remain in solid solution.

Regarding stabilization of M₂₃C₆, I am not familiar with this mechanism. Can you provide the reference that you mention in your original post?

 

[deadrange]

I'm having a hard time agreeing with your initial statement that Nb(C,N) formation occurs before BN. If I remember correctly only a few elements like Zr and Ti have a higher affinity for Nitrogen compared to Boron. I don't have my books with me as they are in my library at home. However, the golden rule in steelmaking developed by Caterpillar is 4:1 Ti:N. I think a similar rule must apply for Nb. In that regard I don't have enough Nb to protect the Boron from completely forming BN.

I also just recently learned from my old faculty advisor that BN can actually promote the nucleation of ferrite. The microstructure that I am trying to avoid in this steel is ferrite.

Unfortunately, the reference is basically a single statement that Boron is added to stablize the M23C6. They don't give any more detail than that. I am trying to understand this mechanism myself.

I appreciate the time you have already invested in this question. I look forward to reading any futher ideas you have.

 

[deadrange]

I wouldl ike to also add, that this material is T23. It appears as though this forum has a great many people well versed in the pipe grades.

 

[metengr]

In documented studies from Oak Ridge National Lab on Creep Strength Enhanced Ferritic Steels reviewed by ASME B&PV Code, Section II, it is an established fact that for 9-12% Cr containing steels, the addition of boron promotes increased hardenability AND it has been reported that it segregates to the surface of the M₂₃C₆ carbide and reduces the rate of carbide coarsening, thus stabilizing the microstructure. The report mentioned that boron segregates to austenite grain boundaries upon cooling from the austenization temperature and during the first few minutes of tempering it is incorporated into the M₂₃C₆.

I have some technical reports on T23, which is a bainitic microstructure that behaves much differently than the 9-12% Cr creep strength enhanced ferritic steels.

 

[metengr]

As a follow-up regarding the T23 tube material, the following phases were reported as part of the justification for Code use;

Austenitize and temper at 700 deg C

V(C,N)
M₂₃C₆
M₆C
M₂B

 

[arunmrao]

With just 2% Cr,it is most likely that M7C3 or M12C6 types of carbides will form. To form complex M23C6,much higher amounts of Cr will be needed.

I remember adding boron in high chromium irons as a grain refiner and improve hardenability

If you can get hold of any old timers from Climax Moly research group,they might be able to give more insight into M23C6 formation and its stability.

Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)

 

[CoryPad]

Old Timers from Climax Molybdenum research lab still around:

Richard Gundlach

http://www.storksmt.com/page.html?ch=DEF&id=15881

David Sponseller (see page 3 for contact info)

http://www.asm-detroit.org/pdfs/me0704.pdf

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[arunmrao]

John Dodd, B Kovacs,Janowak are the other names that readily come to my mind. Their publications and works were of the highest standards. As a student then (late 70's), I could get lot of help from them by snail mail.(No telephone,e mail,video conferencing etc).

Perhaps if deadrange is from US and can establish contact with any of them,he could find a meaningful reply.

Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)

 

[deadrange]

To metengr:

"The report mentioned that boron segregates to austenite grain boundaries upon cooling from the austenization temperature and during the first few minutes of tempering it is incorporated into the M23C6. "

I have to assume that the boron you mention above is effective boron. Boron Nitrides are stable and will have no effect on the stablization of the M23C6.

From what I remember Boron only resides at the grain boundaries when it is "effective/free boron".

To arunnmrao:

2% chrome in this steel grade is not high compared to some of the other P/T type grades, but it should ahve enough Chrome/Vanadium/Moly/Tungsten to form M23C6. Since it is not strictly Cr content driving the formation of the carbide.

To all:

I think the intent of my original question has been mis-communicated to everyone. So I will try and restate it and hope someone has my answer I am looking for:

Grade T23: Is titanium addition required to protect the boron in the steel to meet the bainitic microstructure in an "as-normalize" condition, or is there enough alloying elements present to allow this transformation to occur? I have material that has no titanium and when I normalize i have a significant amount of ferrite in my microstructure. Some of this ferrite might be poorly resolved bainite but clearly not all of it is. Also to note, comparing against some literature values for similar cooling rates my hardness in an as-normalize condition, is significately softer than what is reported in the literature. I have tried to air quench and water quench the sample, and while my microstructure improves the hardness is still significantly lower than literature.
Nowhere in the literature reviews I have seen have they mentioned the composition of titanium in the steel.

I guess a shortened version of that is "Is titanium required for this steel to meet microstructure and hardness in as-normalized condition?"

Thanks for your time.

 

[metengr]

deadrange;
Come to think of it, your post is rather interesting because normally Grade T23 is supplied under an ASTM Specification as SA 213. In accordance with Table I, Ti is not a specified element, it can be present as a tramp element; the "other" alloy elements are Cb, B, N and Al. Why are you asking this question regarding Ti and Grade T23? Are you looking to modify the Grade T23?

 

[metengr]

I guess a shortened version of that is "Is titanium required for this steel to meet microstructure and hardness in as-normalized condition?"

According to the ASME Code case information I have on Grade T23, Ti is not required nor specified. Ti is specified (0.05 to 0.10% by mass) for the proposed Grade T24, which has not be through Section II review.

 

[deadrange]

metengr:

I'm not looking to modify the chemistry for this material. I am just trying to understand why my microstructure is different than the literature results. The only element that I can see that might have a significant effect that is not listed is titanium. I have the V&M handbook for T23/T24, and have compared my approxiamte cooling rate to their CCT diagram. I cannot achieve the same hardness or microstructure utilizing a normalize anneal. However, they do not list Boron in the list of elements. I have another paper that has Titanium content listed at nearly 0.030 wt% (not a residual) and similar hardness/microstructure as the V&M book. My only assumption is that the titanium is being used to tie up any nitrogen in the system and allowing boron to remain "free" to do its magical work.

My interpretation of the ASMT A213 specification is that it doesn't mandate the use of titanium, and since there is not upper limit I am allowed to utilize it but do not have to report its composition.

In reference to grade T24, the titanium is specified.... I think this is used to eliminate the possibility of BN formation.

 

[metengr]

My interpretation of the ASMT A213 specification is that it doesn't mandate the use of titanium, and since there is not upper limit I am allowed to utilize it but do not have to report its composition.

Incorrect. You need to review all of the ASTM General Requirements for tubing, pipe etc.

 

[deadrange]

quote:"Incorrect. You need to review all of the ASTM General Requirements for tubing, pipe etc."

I understand what you are stating based on ASTM A1016.
However, I am referecing a paper "Fabrication and weldability of grade 23 tubing and piping" by A. poli, et.al. and they list the composition of a heat of T23 that contains titanium to a level of 0.033 wt%. I know titanium at this level is not a tramp since it would have oxidized during the steelmaking operation. I would think a standard tramp for Ti would be well less than 0.010 wt%.

Knowing this, I feel that the titanium is a required addition but not controlled as such.

What is your feeling on this matter?

 

[deadrange]

I have re-read the specification ASTM A1016. I believe titanium is an allowed addition to the steel as long as it doesn't exceed the minimum of another steel grade with the same composition. I think the intent of this is to disallow the dual certing of materials.

 

[metengr]

deadrange;
I did a search and found this and another paper before you posted above. I reviewed it an I can't put my finger as to why they have 0.03% by mass of Ti in the CCT diagram and yet for weldability, Ti is never even mentioned in their chemical analysis results.

Knowing this, I feel that the titanium is a required addition but not controlled as such.

What is your feeling on this matter?

We purchased SA 213 Grade T23 jumper tubes from B&W that was sourced through VM. I re-reviewed the test reports, and no Ti was reported. So, either VM is choosing to use Ti and not report it, or something else is going on with your Grade 23 tube material. I would suggest you look elsewhere for why the material you have does not meet the required properties for Grade 23. My position is that with the other carbonitride (CN) formers like V and Cb, you don't need to add Ti, and if this was the case this would have been part of the Code case submittal for use as a required element.

 

[deadrange]

metengr:

If you have access to that B&W material and a spectrometer I would be extremely curious if you found any Ti present in their sample. And while I do agree that Cb and to a lesser extent V are great nitride formers, they pale in comparison to Boron. Only Ti and Zr seems to have a higher affinity for Nitrogen. (The last statement I am going based solely on memory. I haven't had time to look at the Gibbs free energy requirements for any of the nitrides.)

If however I do agree with your statement completely, then during the steelmkaing operation it is important to add all of the nitride formers prior to the boron addition. While this is typical steelmaking operation without the use of Titanium in this grade, I would be extremely leery about the nitrides.

Going back to my previously mention article and I am sure you have access to the V&M Handbook on T23/T24. Look at the V&M CCT diagram along with the article CCT diagram. Looking at the "hardness" achieved they are almost exact copies of one another. This observation coupled with my laboratory experiments is what lead my thinking about Titanium being added as a necessary Nitride former instead of Boron.

I also greatly appreciate our frank discussion on this matter. In industry on a daily basis I get little exposure or discussion on matters that require a great amoutn of thought. This discussion has brought to light the fact that there are still lots I don't know about steel and makes me want to learn more everyday.

 

[metengr]

deadrange;
After some down time from traveling and reviewing some of my EPRI resource material on Grade T23, I offer the following based on your quote below;

I cannot achieve the same hardness or microstructure utilizing a normalize anneal. However, they do not list Boron in the list of elements.

It dawned on me that if you refer to the heat treatment as a normalize anneal in the quote above this is not the correct heat treatment for this material. I would hope this was an oversight on your part.

I can tell you from expierence that the minimum austenitization temperature must be 1900 deg F and cooling can be in air or assisted by liquid. Did you temper after normalizing?

Typically, the response to heat treatment for Grade T23 is very similar to T22, the microstructure that occurs for Grade T23 supplied under SA 213 is bainite and ferrite.

For seamless Grade T23 tube material produced to SA 213

UTS min 74 Ksi
YS min 58 Ksi
%Elong min in 2" gage 20%

Normalization Heat treatment 1900 deg F min
Temper 1350 deg F

Hardness Brinell </= 220
Rockwell B scale </= 97
Vickers </= 230

Remember that ASME material specifications only provide minimum tempering temperature that translates to a maximum hardness specified for use in service. If you fall below this maximum value you have met the specified hardness for use in service. You also did not provide any information as to what your nonconforming hardness values were - were they higher or lower than SA 213?

 

[deadrange]

metengr:

First let me say how I greatly appreciate the time you have invested in my problem. I realize it can be very time comsuming and time being our most precious commodity I appreciate that you would spend so much of it on my problem. That being said, to answer a few of your questions.

1) I referred to it as a normalize because that it how the specifications refer to it. I was able to heat treat samples in a laboratory furance at 1900F. I know this is the bare minimum and I might not be fully austenitic, but I was afraid of bruning out my heating elements. After holding for 1 hour on a roughly 0.30" thick sample, I did three things. I water quenched, air cooled in still air, and blew a small fan on the sample. I subsequently took hardness and microstructure on each of these "as-normalized" samples. The water quench was 99% bainite, with a few "ferrite" grains which may or may not be unresolved bainite. The fan cooled sample, was comprised majority of bainite with some ferrite/unresolved bainite. Finally, the still air cooled sample was about 50/50 bainite and ferrite/unresolved.

2)Isn't the intent of ASME/ASTM definition of "normalize" to prevent the use of other media to aid in the cooling of the material? Shouldn't it be really classified as a quench?


I then compared the hardness values of these three samples and what the literature provided me. When I finished the hardness conversions, I would roughly 50 BHN softer than where I should be based on a CCT diagram. In fact my water quenched hardness was equivilent to a 1000 second cooling.

3)The samples were all tempered and met the requirements of SA-213 as far as hardness/mechanical properties. But we are afraid about the creep properties since there is a significant amount of ferrite. I understand the desire is to have no ferrite in the material, and that high temperature application will ultimately cause the carbides to spheroidize, the ferrite does not air in the creep resistence of the material.

Since you seem to understand the mechanics of this grade. To better understand this grade, I pose this question to you. Boron is a required addition to this grade. What is its purpose? And how does it achieve its purpose?