Transcript Slide 1
Material Considerations when Welding Dissimilar Combinations September 13, 2011 Morgan Gallagher, Ph.D. Applications Engineer, Materials Group Email: [email protected] 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) 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” Butter Welds Butter Weld F22 F22 PWHT F22 625 Machine F22 Closure Weld F22 X65 625 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. Cladding Steel with CRA Type-II Boundaries Reference: Kou (2003) Reference: Lippold and Kotecki (2005) Reference: Lippold and Kotecki (2005) Cladding Steel with CRA Other Issues: 1. Martensitic transition layer 2. Carbon migration HAZ softening Creep failure Reference: Lippold and Kotecki (2005) 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) Solution Potential vs. Location Anode Anode (Good) (OK) Cathode Cathode (Bad) Weld Metal HAZ Base Metal Distance 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 430SS Al-alloy 436SS (Mo+Nb additions) Al-alloy 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 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: [email protected] Phone: 614.688.5134