Material Considerations when Welding Dissimilar Combinations September 13

Material Considerations when Welding Dissimilar Combinations September 13, 2011 Morgan Gallagher, Ph. D. Applications Engineer, Materials Group Email: mgallagher@ewi. org Phone: 614. 688. 5134

Objective ― Provide an understanding of the importance of materials science in dissimilar materials joining ― Focuses: u Metallurgy u Fusion Welding Processes u Structural Materials and Corrosion-Resistant Alloys – Carbon and Low-Alloy Steels – Stainless Steels – Ni-Based Alloys

Meet Joe … ― Joseph Schweissen – BSWE from Ohio State (2011)

Joe’s First Job … ― Joe goes to work for TLA Welding Consultants ― Joe’s first assignment: u Qualify a weld procedure for joining an F 22 forging (2. 25 Cr-1 Mo) to X 65 pipeline steel u F 22 has a maximum hardness restriction u X 65 must maintain the minimum strength level (YS >65 ksi) #1

Joe’s First Assignment … ― 1 st Iteration: u PWHT schedule produces acceptable hardness in the F 22 HAZ, but the crossweld tensile samples fail in the X 65 base-metal. ― 2 nd Iteration: u Joe backs off on the PWHT schedule, and the cross-weld tensile samples pass … but now the F 22 HAZ is too hard!

Butter Welds ― Welding u One dissimilar hardenable steels material must be PWHT (HAZ hardness limit) u One material cannot be PWHT (strength loss) ― Solution: Butter weld u “Butter weld” a layer onto the member that must be PWHT – Use filler metal that (1) is not hardenable, and (2) may be PWHT without deleterious effects u PWHT the butter welded component u Machine butter layer to weld groove

Butter Welds Butter Weld F 22 PWHT F 22 625 Machine F 22 Closure Weld F 22 X 65 625

Joe’s Second Assignment … ― Qualify a cladding procedure to deposit IN 625 onto the ID of X 65 pipe u Joe selects a set of welding parameters and a PWHT schedule that produce acceptable results! u However, Joe notices a number of peculiarities when he examines the metallurgical section from the qualification coupon. 2 #

Cladding Steel with CRA ― Type-II Boundaries Reference: Kou (2003) Reference: Lippold and Kotecki (2005)

Cladding Steel with CRA ― Other Issues: 1. 2. Martensitic transition layer Carbon migration HAZ softening Creep failure Reference: Lippold and Kotecki (2005)

Joe’s Third Assignment … ― Qualify a welding procedure for IN 625 u The shop is out of matching filler metal u Joe selects 25. 10. 4 L duplex SS filler metal instead u The weld procedure passes the qualification tests, and is subsequently used to fabricate production parts u Only months into service, welds fabricated using this weld procedure start to experience corrosion failures … #3

Galvanic Corrosion ― Chemical potential difference between dissimilar metals ― Galvanic “couple” between dissimilar metals u Anode - active metal has lower potential u Cathode - noble metal has higher potential ― Net current flow from Galvanic series in seawater. Ref. : Jones (1996)

Solution Potential vs. Location Anode Solution Potential, m. V Anode (Good) (OK) Cathode Weld Metal Cathode (Bad) HAZ Base Metal Distance

Joe’s Fourth Assignment … ― Characterize a diffusion bond between an aluminum alloy and stainless steel u Joe finds intermetallics forming along the bondline of some of the joints u Joe must now explain why intermetallics only form sometimes 4 #

Intermetallic Formation 430 SS Al-alloy 436 SS (Mo+Nb additions) Al-alloy

Joe’s Fifth Assignment … ― Design a stainless steel to structural steel joint for a high temperature water environment u Significant coefficient of thermal expansion (CTE) mismatch u Stresses develop at the interface on heating/cooling u Cyclical heating and cooling can be especially problematic (i. e. , thermal fatigue) #5

CTE Mismatch Courtesy of EPRI MRP-169 R. 1

Joe’s Sixth Assignment … ― Qualify a welding procedure to join carbon steel to a copper alloy u Joe selects a copper alloy as the weld consumable #6

Copper to Steel Joint ― Iron Courtesy of www. metallographic. com and copper are insoluble in liquid form ― Copper penetrates into the stainless steel HAZ ― More appropriate choice: u Ni-200 (commercially pure Cu)

The moral of the story … ― After having spent a short time in industry, Joe (begrudgingly) develops a respect for the importance of metallurgy … especially with dissimilar joints. ― In fact, his experiences make him a pretty good metallurgist himself!

Questions? Morgan Gallagher Applications Engineer, Materials Group Email: mgallagher@ewi. org Phone: 614. 688. 5134