September 13, 2011

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: • Metallurgy • Fusion Welding Processes • 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)

#1 Joe’s First Job … • Joe goes to work for TLA Welding Consultants • Joe’s first assignment: • Qualify a weld procedure for joining an F22 forging (2.25Cr-1Mo) to X65 pipeline steel • F22 has a maximum hardness restriction • X65 must maintain the minimum strength level (YS >65 ksi)

Joe’s First Assignment … • 1st Iteration: • PWHT schedule produces acceptable hardness in the F22 HAZ, but the cross-weld tensile samples fail in the X65 base-metal. • 2nd Iteration: • Joe backs off on the PWHT schedule, and the cross-weld tensile samples pass … but now the F22 HAZ is too hard! • Xteenth Iteration: • Joe cannot find a combination of welding parameters and PWHT schedule that will produce the desired result!

Butter Welds • Welding dissimilar hardenable steels • One material must be PWHT (HAZ hardness limit) • One material cannot be PWHT (strength loss) • Solution: Butter weld • “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 • PWHT the butter welded component • Machine butter layer to weld groove • Weld second member to butter layer • No PWHT necessary for “closure weld”

F22 F22 625 PWHT F22 625 F22 F22 Butter Welds Butter Weld Machine Closure Weld X65

#2 Joe’s Second Assignment … • Qualify a cladding procedure to deposit IN625 onto the ID of X65 pipe • Joe selects a set of welding parameters and a PWHT schedule that produce acceptable results! • However, Joe notices a number of peculiarities when he examines the metallurgical section from the qualification coupon.

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

Cladding Steel with CRA • Other Issues: • Martensitic transition layer • Carbon migration  HAZ softening  Creep failure Reference: Lippold and Kotecki (2005)

#3 Joe’s Third Assignment … • Qualify a welding procedure for IN625 • The shop is out of matching filler metal • Joe selects 25.10.4L duplex SS filler metal instead • The weld procedure passes the qualification tests, and is subsequently used to fabricate production parts • Only months into service, welds fabricated using this weld procedure start to experience corrosion failures …

Galvanic Corrosion • Chemical potential difference between dissimilar metals • Galvanic “couple” between dissimilar metals • Anode - active metal has lower potential • Cathode - noble metal has higher potential • Net current flow from anode to cathode • Effect of dissimilar base and/or weld metals Galvanic series in seawater. Ref.: Jones (1996)

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

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

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

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

CTE Mismatch Courtesy of EPRI MRP-169 R.1

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

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

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

September 13, 2011