1. Material Considerations when Welding Dissimilar Combinations
September 13, 2011
Morgan Gallagher, Ph.D.
Applications Engineer, Materials Group
Phone:

2. 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

3. Meet Joe … Joseph Schweissen – BSWE from Ohio State (2011)
-Joe is a recent welding engineering graduate
-Joe is a red-meat type of guy
-When it comes to welding, he likes arcs and sparks!
-And of course, he is a Buckeyes football fan on weekends
-What Joe didn’t like during school was all the metallurgy classes that he had to take for his degree
-Joe thinks: “If I wanted to study materials science, I would have gone into MSE! Plus, all my metallurgy TA’s were really nerdy…”

4. #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) #1
-Joe’s first job is with TLA Welding Consultants
-TLA is a welding engineering firm that works with a wide variety of industries, and deals quite often with dissimilar metal combinations
-For his first assignment, Joe’s boss asks him to qualify a weld procedure for joining an F22 forging to X65 pipeline steel
-However, the F22 has a max hardness restriction, and the X65 must maintain its min YS

5. 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!
-On the 1st iteration, the F22 HAZ has acceptable hardness, but the cross-weld tensile samples fail in the X65 base-material … the PWHT has softened the X65 too much
-On the 2nd iteration, Joe backs off on the PWHT schedule and the cross-weld tensile samples pass … but now the F22 HAZ is too hard (the PWHT has not effectively tempered the F22 HAZ)
-After many iterations, Joe cannot find a combination of welding parameters and PWHT that results in acceptable properties on both sides of the joint, and the poor guy is pulling his hair out
-Joe tells his boss, who suggests that he go see one of the company’s metallurgists … Joe reluctantly agrees …

6. 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”
-The metallurgist that Joe goes to see gives him the idea of butter welding a non-hardenable material to the F22, PWHT the butter welded component, and then perform a closure weld where the final HAZ is in the non-hardenable butter material, and not in the F22 … therefore, no PWHT is necessary for the closure weld.

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

8. 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
-Joe’s third assignment is to qualify a cladding procedure to deposit IN625 onto 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 cross-section from the qualification coupon
-Joe finds the metallurgist in his company to discuss …

9. Cladding Steel with CRA
Reference: Kou (2003)
Type-II Boundaries
-Note how clad disbonding is possible along the Type-II boundaries
Reference: Lippold and Kotecki (2005)
Reference: Lippold and Kotecki (2005)

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

11. Joe’s Third Assignment …
Qualify a welding procedure for IN625
The shop is out of matching filler metal
Joe selects L 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
-Joe’s fourth assignment is to qualify a welding procedure for IN625
-The shop is out of matching filler metal, so Joe selects to use L filler metal instead. Joe says ‘hey, they’re both CRAs, and there are components in the equipment that we’re fabricating that use duplex … what could go wrong?’
-Only months into service, welds fabricated using this weld procedure start to experience corrosion failures

12. Galvanic series in seawater. Ref.: Jones (1996)
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
-Joe once again seeks the help of the company metallurgist, who immediately recognizes the problem as galvanic corrosion!
-A chemical potential difference exists between dissimilar metals when they are both in contact with a corrosive and/or conductive medium.  If these metals are, in turn, in electrical contact in the presence of this same medium, the potential difference results in a flow of current between the two.  As a result, the corrosion resistance of the less-resistant material is increased at the expense of the more resistant material to which it is connected in this galvanic cell.  The less-resistant metal represents the anode and the more-resistant metal the cathode in the cell.  
Galvanic series in seawater. Ref.: Jones (1996)

13. Solution Potential vs. Location
Anode
Cathode
Weld Metal
HAZ
Base Metal
(Good)
(OK)
(Bad)
Solution Potential, mV
Controlling the relative differences in solution potential among the base metal, HAZ, and weld metal is the key to avoiding galvanic attack. In the galvanic couple, the anode (active metal) will corrode preferentially to the cathode (noble metal). If the anode area is small relative to the cathode, this attack can be quite rapid. Thus, it is important to insure that the base metal is anodic to the weld metal and HAZ.
Distance

14. 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

15. Intermetallic Formation
436SS (Mo+Nb additions)
430SS
-One solution is using a different grade of SS … 436SS has Mo+Nb additions. In this case, these additions have acted to slow down diffusion on the SS side, retarding the formation of intermetallics
-Another solution, if possible, is to bond in a temperature range where (a) intermetallics do not form, or (b) form slowly enough that there is not an appreciable amount
-This is an issue with other welding processes, such as friction welding of Al-steel joints … in that case, rigorous process control is used to avoid the formation of intermetallics
Al-alloy
Al-alloy

16. 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

17. CTE Mismatch
Courtesy of EPRI MRP-169 R.1

18. 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

19. Courtesy of www.metallographic.com
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

20. 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!

21. Questions? Morgan Gallagher Applications Engineer, Materials Group
Phone: