Dear Sir,
Generally, for joining CS/LAS to 300 type Stainless Steel, austenitic filler materials like E/ER 308, 309,316 and 347 are used but these filler metals are used when service application is below 425°C. For higher temperature service application as you mentioned, I would suggest you for using High-Nickel filler metals ERNiCrFe-2, and ENiCrFe-2, ENiCrFe-3.
From metallurgical point of view, another advantage of Ni-base weld metal is that it can effectively reduce the carbon migration from the ferritic steels to the weld metal. While, it is well known fact that carbon migration into Stainless Steel weld metal beyond certain level, causing solidification cracking due to less or nil presence of ferrite in the solidifying weld metal.
In the petrochemical industries, the problem of cracking of joints between creep resisting ferritic steel 2.25Cr-1Mo steel and austenitic stainless steels- 304H, SS 310 are common. most of the time, cracking seem to be observed in HAZs of the ferritic steel just after few millimeters away from fusion boundary towards ferritic steel side. This phenomena is occurring when welded joint is in service application at elevated temperature around 565 °C. Most of the time, this is occurring due to stress concentrations and/or thermal fatigue generated owing to differences in coefficient of thermal expansions (CTE) values between the either sides base metals and weld metal. So, the choice of filler metal and resultant weld metal CTE value should be matching with the CTE values of base metal(s) being welded.
Another aspect for selection of filler metal is its tolerance for dilution from the base metal. Generally filler metal choice for joining austenitic stainless Steels (Type 304L, 316L, 321, 347 etc) with creep resisting ferritic steel 2.25Cr-1Mo, our obvious choice is made for a filler metal which can tolerate dilution by both base metals without formation of defects-like solidification / liquation cracking, precipitation of embrittling phases etc. in the joint and at the same time, providing optimum mechanical strength and corrosion resistance is the first priority.
A higher alloy filler metal like type 309 with a Ferrite Number 8-10 approximately, or Type 312 with FN 20-25 approximately or ferritic-austenitic type Type 2209 with Ferrite number 35-65 approximately seem to be attractive choice for their high capacity to tolerate dilutions from base metals. But, the potential threat with using these high ferrite filler metal is embrittlement of welds (and severe loss of ductility) due to transformation of delta ferrite in to brittle sigma phase when the weldment is heat treated or exposed to service environment in temperature between 704-593 C. So if the weldment is requiring PWHT in this range, as much as possible low ferrite in weld metal composition is preferred.
Other way is to first “Buttering: (Weld Overlaying) of thickness 13 mm or higher using SS 309 on 2.25 Cr-1Mo Side and subsequently filling pass to complete the groove using E/ER 308 filler metal. This should improve the weld ability but again, this technique is not suitable for higher temperature application requirement.
Because the Coefficient of Thermal expansion(CTE) of high Nickel alloys are approximately in between the ferritic steel and austenitic stainless steels since One of the referred literatures suggests that mean CTE in the temperature range 0 to 600° C for the E/ERNiCrFe-2, E/ERNiCrFe-3 filler metal 15 µm m-1K-1 is in between the 14.0 µm m-1K-1 for ferritic steels 2.25Cr-1Mo and 18 µm m-1K-1 for SS of types 310
So, In summary, the present weld joint between austenitic stainless steel type 310 and 2.25Cr-1Mo for high temperature furnace parts applications, can be successfully produced with ERNiCrFe-2 or ERNiCrFe-3 as per AWS A5.14/A5.14M:2018, Specification for Nickel and Nickel-Alloy Bare Welding Electrodes and Rods. And for fill pass, ENiCrFe-2, ENiCrFe-3, as per AWS A 5.11 Specification for Nickel and Nickel-Alloy Welding Electrodes for Shielded Metal Arc Welding to obtain optimum mechanical strength and corrosion resistance.
However, the weld metal is likely to solidify in 100% austenitic mode, which may pose problem of weld solidification cracking so a weld procedure qualification test is highly recommended.
We hope this information is useful to you!
Welding Consultant,
Weld Met Advisory Services
References:-
-J.R.Davis, Hardfacing, eld Cladding, and Dissimilar Metal Joining, ASM Handbook, Vol.6 ASM International, Ohio, USA, pp. 789-829
-Article on Dissimilar Metal Welding and Cladding – A.K.Bhaduri, Welding Technology for Engineers, Narosa Publishing House, New Delhi
-Bailey, N, ed “Welding Dissimilar Metals,” The Welding Institute, Cambridge, UK (1986)
– Richard E Avery- “Pay attention to Dissimilar Metal Welding”, , Nickel Development Institute.
