September 13, 2011
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Transcript of September 13, 2011
September 13, 2011
Material Considerations when Welding Dissimilar Combinations
Morgan Gallagher, Ph.D.Applications Engineer, Materials GroupEmail: [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
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)
#1
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”
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.
#2
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 layer2. 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 …
#3
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. LocationS
olu
tio
n P
ote
nti
al, m
V
Anode
Anode
Cathode Cathode
Weld Metal HAZ Base Metal
(Good)
(OK)
(Bad)
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
#4
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)
#5
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
#6
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
copperCourtesy 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 GallagherApplications Engineer, Materials GroupEmail: [email protected] Phone: 614.688.5134