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

 

[metengr]

deadrange;

I understand the desire is to have no ferrite in the material

. No, Grade T23 will contain ferrite as far as following the SA 213 specification for heat treatment. My understanding is that the boron was added to retard carbide coarsening.

I believe the original code case for Grade 23 (2199-1) to ASME was from Sumitomo Industries. The feature of this material is that it behaves like T22 in terms of fabrication and welding. However, it is a ferritic creep strength enhanced steel because Mo is substituted for by tungsten, Cb, V, B and N, which provide for precipitates that exhibit resistance to softening.

 

[deadrange]

metengr:

I don't comepltely agree with your statement that ferrite is going to be present. Looking at the micrographs from the Handbook and article I referenced before, it looks like on tubing it is baiscally 100% tempered bainite. There is not apparent presence of ferrite. If I understand your comments a couple of posts ago. You stated that T23 grade are air cooled or assisted by liquid. Looking at the CCT diagrams you should bypass the ferrite phase and be bainite/martensite. At least that is the impression I get from literature. I am trying to acquire additional steel samples to verify chemistry and microstructure using the same sort of test that I have conducted in my laboratory.

I have etched samples in both Nital and Viellea's Reagent and while the Viellea's reagent showed a little more carbide in the "ferrite" it is still predominately free ferrite.

Boron use to prevent grain coarsening would require the boron to be "free/effective". Formation of Boron-Nitrides would not do anything for grain growth prevention. Vanadium/Columbium I agree will help, but I don't think it is sufficient to prevent complete grain coarsening.

I do agree with your statement that T23 is to behave like T22 in terms of fabrication and welding, and T22 with a normalize/temper process will achieve a ferrite/pearlite structure. I am going to look more into the ltierature and see what kind of structures are present in T23. I have a sneeky suspicion that it is going to bainite/martenisite with no ferrite present. I will follow up when I find out more.

 

[metengr]

deadrange;
I was finally able to locate the entire code case submittal for T23 tubing (45 pages in length and was submitted in late 1994). Actually Sumitomo in conjunction with Mitsubishi Heavy Industries (MHI) co-authored a technical paper that was presented in 1994 (PVP-Vol 288, Service Experience and Reliability Improvement: Nuclear, Fossil and Petrochemcial Plants, ASME 1994). I know one of the authors of this paper Dr. F. Masuyama/ MHI. The title of this paper is "Development of Tungsten Strengthened Low Alloy Steel with Improved Weldability". This paper set the stage for the ASME B&PV Code Case submittal for use of Grade T23 material and is under CC 2199.

The technical paper would be of extreme interest to you because it contains information on specific creep data, microstructures, formability test results and welding.

This paper in addition to a comprehensive proprietary data package was submitted for the code case. The mechanical property data, microstructure evaluation and CCT diagram show that depending on cooling rate, one can generate either all tempered bainite or ferrite and bainite structures with corresponding hardness values. What I wanted to point out to you in the above posts of mine is that the key to creep strength is not the development of a fully tempered bainite structure, it is the development of necessary precipitates to improve long term creep strength. This is why the 1900 deg F austenization is critical for this alloy. So, I would not get to worried about lower hardness values whereas I would be more concerned with making sure your heats of Grade 23 contain the necessary control on nitrogen, aluminum, and boron levels.

I cannot provide any more specifics on this because of proprietary information that was part of this code case submittal. I will assure you that the numerous heats that were evaluated and submitted in the the code case package, not one chemical analysis mentioned Ti. So, for the submittal by Sumitomo and MHI for alloy HMC2S (which by the way was used in Japan since 1993) Ti is not added intentionally.

 

[unclesyd]

Here is some information on the interaction of Boron, Titanium, and Nitrogen in a 9 Cr steel. I would assume that similar reactions would be applicable for the 2 1/2 Cr steels.

http://www.ingentaconnect.com/conte...c74632148763568293c6c567e504f58762f46fe19b601

metengr,
Would you think that Dr. Jawad and his group at ORNL might have looked Grade T23 in respect to the physical metallurgy?

 

[metengr]

Yes, I do unclesyd.

 

[deadrange]

unclesyd:

I apologize I haven't had the time to read that entire paper, but just briefly reading the abstract, I take away the fact that the boron added to the 9% Cr steel is "effective boron" Grain coarsening is occurring when additional nitrogen is being added because it form BN. It appears in this case that the steel being considers has 0.010 wt% Boron and some Nb. The nitrogen level is 0.003 wt% which is extremely low. Since Nb is going to tie up some of the availible nitrogen it allow a significant amount of boron to remain "free" and not as a BN. The increase in nitrogen forms more BN and thus effective removes the ability for "effective boron" to keep the M23C6 carbides on the austenite grain boundaries from growing.
Using this logic for T23, it would seem that since my boron content is significantly less than my nitrogen (0.0016 vs 0.0080) with CB in the range of 0.030. All of my boron is effectively tied up with Nitrogen as BN thus preventing grain coarsening of the material.

Adding titanium even to a small degree would help my desire for boron to remain effective.

metengr:

I am in the process of getting that article you recommended. However, I still believe that titanium is being added to this steel in small quantities (< 0.040) to protect the boron from the nitrogen. The chemical composition of the submitted heats may not have listed Titanium as an addition, but I would guess that to some degree titanium is being added.