P91 steel, a Cr-Mo Alloy covered under A335/SA335, consists of two phases: ferrite and martensite. It is also known as creep strength enhanced ferritic (CSEF) steel; the main constituents of alloy are 9% Cr, 1% Mo,0.25 V and balanced Fe. Modified P91 is widely used in different kinds of applications demanding high temperature and good creep resistance strength such as in the steam header, steam pipe, super-heater, pressure vessels and nuclear power plants.
SS Type 347H (UNS S34700) stainless steel is the higher carbon (0.04 % – 0.10% wt.) version of the alloy. It was developed for enhanced creep resistance and for higher strength at temperatures above 537°C. Alloy 347H (UNS S34700) is Niobium (a.k.a Columbium) added stabilized austenitic stainless steel with good corrosion resistance and a better resistance in strong oxidizing conditions than Type 321 (UNS S32100)- Titanium stabilized. It has excellent resistance to intergranular corrosion (IGC) after exposure to temperatures in the chromium carbide precipitation range of 427 – 816°C. The alloy has good oxidation resistance and creep strength up to 816°C. It also possesses good low-temperature toughness.
Many fabrications of power generating application equipment are involving dissimilar metal welding (DMW) processes, for example, Tube-to-Header Connections, Nozzle to Shell etc.
These DMWs in components are experiencing a temperature generally >550°C at elevated temperature, which also falls in the creep regime for 9%Cr -1% Mo Steel.
Many referred open-source literature (Ref 1-5) suggests that these, DMW’s are often at high metallurgical risk, whenever, they contain relatively high impurity contents, such as As, B, Cu, S, Sb, Sn, etc. in the weld metal that may lead to a premature failure in the 9%Cr HAZ, despite, when 9%Cr steel- Stainless Steel joints – DMWs are operating well within the creep regime. Although there are also other factors like restraint conditions, stress concentration, etc are also playing a role.
Similarly, whenever, Austenitic SS are welded with 9% Cr –CSEF Steel to perform dissimilar metal welds using over-matching filler materials and subjected to high-restraint applications, can be susceptible to stress relaxation cracking ( SRC), damage as an end result of PWHT or during exposure to elevated temperature service. These issues are typically observed in the HAZ, in which, at a sufficiently high temperature during welding heat, in a few grains near fusion zone carbo-nitride particles are re-solutionized The typical nature and associated risk factors have been investigated by many researchers worldwide and they have considered the effect of following phenomena in susceptibility to stress relaxation cracking ( SRC)
-Coarse grain size in the parent material
-Presence of Nb, Ti, and V like precipitation-strengthening elements.
-Presence of As, B, Cu, S, Sb, Sn etc. Impurity content that can lower creep ductility
-Exposure (either service or PWHT) in a region of reduced creep ductility
For DMW’s between Cr-Mo Steels and austenitic steels or nickel-based alloys, it is highly recommended to use nickel base welding consumables. To avoid embrittlement effects during PWHT or elevated temperature service above 300 °C, it is not recommended to use austenitic welding consumables like 309 types, which are commonly used for DMW’s between ferritic to austenitic steels for ambient temperature applications.
Nickel base welding consumables generally suffer from hot cracking issues. They are also recommended to be welded with as low as heat input. However, Cr-Mo Steel, grade P/T 91 requires a minimum preheat practice of at least 150 °C. So, the manufacturing sequence of DMW’s is as follows.
It is common practice to apply pre-heating to the grade P/T 91 component only. For thick-walled components, it is recommended to apply a butter layer with the nickel base consumable like ERNiCr-3 (Alloy 82) AWS A5.14/A5.14M or ENiCrFe-3, AWS A5.11/A5.11M SMAW on the Cr-Mo Steel grade 91 side, followed by a normal PWHT. Then, the welding to the austenitic steel or nickel alloys can be performed by using the same nickel base welding consumables as used for the buffering. Thin-walled components can be directly welded using the same nickel alloy consumable as long as the austenitic stainless steel is not badly affected by the PWHT range.
voestalpine Böhler official website (Ref. 4) containing open-source literature provides some guidelines on the range of preheating, interpass & PWHT temperature range as under
Preheat temperature [°C] ≥ 180
Interpass temperature [°C]≤ 250
PWHT holding temperature [°C] 740-770
In general, all welding processes can be applied for dissimilar welding of Cr-Mo Steel grade 91 to austenitic steels or nickel alloys but a precaution must be taken in order to limit the heat input (HI ≤ 1.2 kJ/mm), method of deposition of weld metal e.g. stringer beads, welding parameters and use of small electrode diameter for SMAW process as far as possible.
As per the prevailing thumb rule in the industry, generally, a higher-alloy material should never be welded with a lower-alloy material, due to the result of dilution of the alloying elements from the higher alloy material into the lower alloy metal during welding.
It is also worth noting, one interesting finding in the literature(Ref. 5), i.e. a kind of exception reported in the literature, in which, thick sectioned welds between 9% Cr CSEF Steel has been successfully welded with SS 347H, using a filler material matching ER90S-B9 or ER80S-B8 to the 9%Cr CSEF steel – without any defect. But that joint could have been produced with very less controlled H. I and with precise control over all welding process parameters.
However, interested readers/research scholars can also try out DMW’s between 9 %-Cr CSEF steel and SS 347H using near matching consumables like ER90S-B9 or ER80S-B8 to ferritic steel, test results would be obviously interesting. Although I would not take any guarantee regarding the successful weld joint, one test is worth doing for sake of research work! For welding professionals procedure qualification run is highly recommended.
In a conclusion, Nickel-based alloy welding consumables are the best choice to weld between ferritic steel and austenitic stainless steels for elevated temperature applications.
We hope this information is useful to you!
Welding Consultant,
Weld Met Advisory Services
References:-
1. A. Dhooge. “Survey on Reheat Cracking in Austenitic Stainless Steels and Ni-base Alloys. ”Welding in the World 41, 1998. pp. 206 to 219.
2. Factors Affecting Performance of Dissimilar Metal Welds: Creep Performance of Screening Dissimilar Metal Welds Between Grade 91 Steel and Stainless Steel 347H. EPRI, Palo Alto, CA: 2016. 3002007216.
3. Factors Affecting Performance of Dissimilar Metal Welds: Fabrication and Metallurgical Assessment of Screening Dissimilar Metal Welds between Grade 91 Steel and Stainless Steel 347H. EPRI, Palo Alto, CA: 2016. 3002007218
4. https://www.voestalpine.com/welding/de/content/download/14491/296688/file/BW_Grade+91_EN_2021_GL_135_Preview+%283%29.pdf
5. O. R. Carpenter, N. C. Jessen, J. L. Oberg, and R. D. Wylie. “Some Considerations in the Joining of Dissimilar Metals for High-Temperature High-Pressure Service.” Proceedings of the ASTM, Volume 50, 1950. pp. 809 to 857.
