September 13, 2011 Material Considerations when Welding Dissimilar Combinations Morgan Gallagher, Ph.D. Applications Engineer, Materials Group Email: [email protected]@ewi.org.

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

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)

#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”

Butter Welds

F22

X65

F22 625

F22 625

Butter Weld

Machine

PWHT

F22

F22

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

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)

#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

─ 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