Bainite P91 4

[rollagirl]

Would someone explain to me how P-91, welded to F-91 with 91 filler could turn into bainite. I have never seen any studies, or CCT diagrams saying it could be anything but martensite (possibly with retained austenite), even after PWHT. It was reported bainite microstructure. All I could think was possibly one piece of the equation was not 91, but that has been checked.

 

[davefitz]

Not sure. For bainite to form in P91, the weld has to cool VERY quickly. There are published TTT curves - see the vallourec Mannesman booklet " the P91 book" for a typical TTT curve that demosntrates cooling rate vs crystal structure.

The tungsten /wolfram modified alloys P92, T23, and P911 are deliberately formulated to form bainite, but they use a different formulation than P91- they substitute W wolfram for molubdenum. They also need to be cooled extra fast in order to have the code required creep properties associated with bainite.

 

[davefitz]

error: P922 not P911.

 

[metengr]

Grade 91 alloy steel is alloyed to only form martensite with anticipated cooling rates from either welding or a normalization heat treatment. My experienced guess is that someone could not tell the difference between tempered martensite and bainite. It does happen. Your best confirmation other than having someone experienced at metallographic examination is to obtain hardness data for the weld metal and base metal heat affected zone.

 

[metengr]

Forgot to mention that Oak Ridge National Lab had performed extensive work into Grade 91 alloy development and other advanced ferritic materials, and yes here is a CCT diagram for this material (Oak Ridge presentation titled “Understanding Damage Mechanisms in Ferritic/Martensitic Steels” .

The alloy composition will not permit bainite to form, instead it will be either ferrite and carbides (held or slow cooled for 2 hours between 650 deg C and 800 deg C)or martensite + ferrite + carbides (cooled between 450 deg C and 650 deg F over a 2 hour period).

 

[unclesyd]

metengr

Speaking of ORNL, do you know whatever happened to the alloy that Dr. Jawad and ORNL were developing to overcome a lot the problems with welding and PWHT for this this family of alloys?

 

[rollagirl]

Thanks! I suspect they don't know what they are looking at as metenge suggests. It is frustrating not to have access to this to see for myself. Looking at the PWHT charts (exceptionally slow cooling rate) and hardness reading (low), they probably have martensite + ferrite + carbides.

 

[stanweld]

rollagirl,
Was the reported bainite in the base metal, weld metal or HAZ; through thickness or OD surface? How was preheating and dehydrogenation performed; with resistance elements or oxy-acetylene? I have seen some highly unusual microstructures in P91 recently - associated with suspect normalizing and tempering and preheating/dehydrogenation.
The reported bainite may have been coarse martensite + ferrite.

 

[metengr]

unclesyd;

Speaking of ORNL, do you know whatever happened to the alloy that Dr. Jawad and ORNL were developing to overcome a lot the problems with welding and PWHT for this family of alloys?

I presume you are referring to the Fe-3Cr-W(V) bainitic alloys, called 315, 315T? Do you have a copy of the following Technical Report-"Development of a Fe-3Cr-W(V) Ferritic Steels for Industrial Applications" by Dr Jawad (Nooter) and Dr Sikka (ORNL)?

Information is still being reviewed thru our ASME B&PV Code Subgroup on Materials. The report mentioned above is one of the most comprehensive status reports on these alloys.

 

[rollagirl]

Was the reported bainite in the base metal, weld metal or HAZ; --- all

through thickness or OD surface? --- od

How was preheating and dehydrogenation performed; with resistance elements or oxy-acetylene? --- resistance

Thermal history : Preheat to 400F, Weld, Post heat @ 625F for 2 hr, cool rate @ 175/hr to 100F, PWHT 1300F 2 hr, cool rate 200/hr. According to charts provided

 

[stanweld]

I assume that micros were surface replicas; if so, they might have not fully removed the decarburized surface layer before making the replica. Having said that, low hardness of the weld metal could not ocurr with the low PWHT temperature employed, IF TRUE; and adequate weld material would have been removed to permit a replica thereof. I would expect weld hardness to be near to or > 300 BHN with the PWHT stated.

 

[rollagirl]

So would I, but the hardnesses reported to me have been:
Lowest - 127 BHN
Highest - 223 BHN
Most between 150 and 190
I can't figure it out. The PWHT chart they supplied looks reasonable - if it is the right one. The hardnesses are ridiculous for the material and PWHT described - if they took them correctly. The microstructure reported shouldn't be, although I have two reports on material ID - all 91.

 

[stanweld]

What type of hardness test did they use?