Use and weldability of high-chromium molybdenum steels

By ssbn727 (*****)

Date 03-19-2014 20:46

"At least a dozen chromium molybdenum (CrMo) steel combinations exist. One of the more recent alloys consists of chromium, molybdenum, vanadium, niobium (columbium), and nitrogen. Beginning sometime in the 1970s, this material became popular for use in high-temperature applications, such as gas and coal-fired power plants. However, it is not intended to replace other CrMo types. Each alloy has its own place in the high-strength, low-alloy group of materials..."

These articles I posted here and in other sections in the forum give the novice some excellent information regarding the various alloys used in the welding industry today and the nice part about them is that they are written without too much overwhelming emphasis on technicalities although the data is technically correct...

http://www.thefabricator.com/article/metalsmaterials/use-and-weldability-of-high-chromium-molybdenum-steels

Enjoy the read. :cool:

Respectfully,
Henry

By 3.2 Inspector (***)

Date 05-21-2014 12:07

....on technicalities although the data is technically correct...

Thanks, I will read it now

By js55

Date 05-21-2014 13:42

Since the comment reactivated this post I will comment myself. I think the article is a bit overdone pertaining to V/Nb/N alloys, i.e., the creep strength enhanced alloys. Some of what is stated is not necessary for base metals where the properties are actually very good. But it can be applied to filler metals. For example a concern for toughness. These materials have excellent toughness above the ductile to brittle temperature. The filler metals are a different story altogether.

By Eutectic (**)

Date 07-24-2014 13:31

Henry,
Thanks for the article, on the topic I have a lot of questions (some of which I have not thought of yet).
But to all the gents that have already replied to your original post please feel free to give me your advice.
I am on the EC side of fabricating Power stations in SA (between the Client and the contractors). So it is imperative to up my knowledge to fence on both sides.

Question:
After welding and prior to PWHT it is either prescribed or recommended to perform a H2 Bake out. at a temperature below (mf) martensite transformation depending on the joint configuration and thickness. Is it then required to go directly into PWHT or could you go down to RT and perform NDE prior to PWHT?

I am of the opinion that the bake out is a risk mitigation for delayed cracking and although it is good practice it might not be requried given that proper NDE is performed to cater for any delayed cracking. Handling ofcourse is critical on this new martensite structure prior to PWHT. Any other metallurgical or fracture concerns?

Regards
Hanre

By ssbn727 (*****)

Date 07-24-2014 16:07

Details? Resulting in:Maybe give suggestion/recommendation/opinion.....

No Details in query? No possibility of giving you or anyone else potentially intelligent suggestions/recommendations/opinion.....

Note of Caution... Any intelligent or not - suggestions/recommendations/opinions are just that...We're not liable for any of these in answering a query from anyone who seeks specific technical advice... So if you wish to continue, then give us the details and maybe we can steer you in the right direction okay?

Btw, Where are you in SA? Habla Espanol? Buenos Suerte!

Respectfully,
Henry

By Eutectic (**)

Date 07-25-2014 10:31

Henry,
Apologies, (SA) South Africa, so should actually have been (ZAR) not (SA) South America. The only thing we have in common is the Soccer world cup 2010(ZAR) 2014(SA)

Material P91 or x10CrMoVNb9-1.

Joint configurations anything from Butt, T butt, Fillet. thickness 3mm to 80mm
Welding Processes GTAW, SMAW, SAW.

A typical requirement would be in the form of:
Cooling down to room temperature after welding is allowed considering the following:
WT<=50mm, for Butt welds
WT<=14mm Nozzle welds
WT<=8mm Nipple.Nozzle to Header welds.

Anything outside of these dimensions shall be brought down to 80-100Deg C for 60 min followed by PWHT.

Subsequent to this post I went ahead and did a literature review and found that there was a fundamental flaw in my reasoning regarding this lowering of temperature as H2 Bake out. Instead this is purely ensuring that the complete weld thickness has adequate time to transform to Martensite Prior to PWHT.
However the wording in the Client specification leads to an interpretation that if a weld conforms to the restrictions above then it can be cooled to RT. If it does not conform you SHALL hold and then SHALL PWHT immediately.

My interpretation:
1. Is that cooling down to RT for high restraint joints can cause cracking.
2. The requirement of PWHT immediately is risk mitigation action. Saying that it would be poor practice to delay PWHT for too long as this hard microstructure would be susceptible to SCC if exposed to moisture (condensation etc).

So my details are still not very detailed but the advice I am after is basically Best practices for the fabrication of P91 material in general with regards to Welding-Martensite Transformation- NDE (prior to PWHT)-PWHT-NDE (after PWHT). ANd I will not hold you liable but I have read enough of your posts to know that the BS is limited and the value add is great.

Cheers
HF