Dissimilar metal welds (DMWs) between creep strength enhancedferritic and stainless steels with Ni-based filler metals are commonlyused in power generation applications. Premature and unpredictablefailures near the fusion line of such welds have been reported. Largegrains of retained δ ferrite have been found in the HAZ of Grade 91DMWs, which are not typically found in HAZ of matching filler metalwelds. HAZ δ ferrite is not directly involved in the failure mechanism,yet revealing the metallurgical phenomena related to δ ferriteformation may help in understanding service failures.

Objective: Determine the mechanism of δ ferrite retention in the HAZ of Grade 91 DMWs with nickel based filler metals.

Hypothesis: Retention of δ ferrite is caused by HAZ carbon depletion during welding as a result of the difference in chemical and thermo-physical properties of Grade 91 steel and nickel based filler metals.

• Retained δ ferrite is found in the HAZ of Grade 91 DMWs with nickel based filler metals, but not in autogenous or matching filler metal welds.• The mechanism of δ ferrite retention in HAZ of Grade 91 DMWs is based on carbon depletion during welding. This phenomenon is controlled by

the chemical composition and thermo-physical properties of Ni-based filler metals, which:• introduce a chemical potential gradient that creates a driving force for carbon depletion in the HAZ• provide longer dwell time of the HAZ at high temperatures as δ ferrite that facilitates carbon diffusion.

• The magnitude of HAZ carbon depletion during welding depends on the chemical potential gradient, which is temperature dependent and alloyspecific. Alloy 625 filler metals generated the highest level of carbon depletion / retained delta ferrite, followed by Alloys 617, 62, and P87.

• The measured amount of retained δ ferrite directly correlates to the predicted average carbon concentration in the depleted region of HAZ.Future Work

• Submit a proposal to perform nano secondary ion mass spectroscopy (nanoSIMS) to confirm the carbon concentrations by direct measurement

Approach: Manufacture bead on plate welds with matching filler metal B91 and with nickel based Alloys 82, 625, 617, and P87. Measure and compare HAZ thermal histories. Quantify the amount of retained HAZ δ ferrite. Perform thermodynamic and kinetic simulations of carbon diffusion behavior in the HAZ and partially mixed zone during welding. Validate results using the hardness distribution and chemical profiles.

Introduction and Approach

• Chemical profiles across and between δ ferrite grains were statistically indistinguishable at the 95% confidence limit, showing no difference in distribution of ferrite and austenite stabilizing elements (carbon not measured)

-48-42-36-30-24-18-12

-606

121824

0 500 1000 1500

Che

mic

al P

oten

tial (

kJ/m

ol)

Temperature (°C)

Chemical Potential Gradient with Grade 91 Steel

625 82 617 P87 B91

Alloy 625 Weld P87 Weld

Mean (HV0.025) 95% CI (HV) Mean (HV0.025) 95% CI (HV)

HAZ Martensite 553 ± 3.2 548 ± 4.4

Ferrite Band - Martensite 445 ± 7.2 500 ± 12.3

Ferrite Band - Ferrite 223 ± 9.4 226 ± 12.0Fusion Zone 253 ± 3.1 225 ± 1.9

0.00

0.05

0.10

0.15

0.20

-100 -50 0 50 100

Car

bon

Con

cent

ratio

n (w

t%)

Distance from Fusion Boundary

P91 - Alloy 625 Carbon Profile During Welding

1450 1350 700

HAZ Fusion Zone

R² = 0.990610

15

20

25

30

0.07 0.08 0.09 0.1 0.11 0.12 0.13Nor

mal

ized

Fer

rite

Con

tent

(μ

m)

Average Carbon Concentration in the Depleted Region of the HAZ (wt%)

Carbon vs Ferrite Content in the HAZ

P87

617

625

82

Michael Kuper and Dr. Boian Alexandrov, The Ohio State University

Formation of Delta Ferrite in the HAZ of Grade 91 Dissimilar Metal Welds:

The Mechanism and Controlling Factors

CGHAZ

Fusion Zone

δ/

CGHAZ

Grade 91 / ER B91 (Matching Weld)

Effects of Using Nickel Based Filler Metals

• Higher heat capacity, lower thermal conductivity and solidification temperature range of nickel alloys extend HAZ dwell times at high temperatures (as δ ferrite ), allowing more time for diffusion to occur

• Nickel based filler metals create a temperature dependent chemical potential gradient for carbon that is specific to the alloy used

Carbon Diffusion Model

Characterization - Validation of Carbon Diffusion

Conclusions and Future Work

• Martensite was statistically softer in the ferrite band than the rest of the HAZ , indicating carbon depletion

• Very strong correlation indicates that the amount of δ ferrite present is related to the magnitude of carbon depletion during welding

5

7

1 2

Chemical Analysis - EPMA Alloy P87

P91

δ

• Diffusion model was performed in DICTRA using the measured weld thermal history and the composition of each material

• Predicted carbon depletion in the HAZ during welding, with magnitude variations controlled by filler metal composition

700800900

10001100120013001400150016001700

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

Tem

pera

ture

(°C

)

Carbon (wt%)

P91 Phase Diagram and Predicted HAZ Carbon Concentrations625 617 82 P87

L

δ + γ

δ

γ + α

α

γ

L + δ

Grade 91Predicted δ Threshold

• The predicted carbon concentration in all DMWs at high temperatures drops below the empirically predicted threshold for retained δ ferrite from [1]

Fusion Zone

Grade 91 / Alloy 625 (DMW)

[1] S. H. Ryu and J. Yu, "A new equation for the Cr equivalent in 9 to 12 pct Cr steels," Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, vol. 29, pp. 1573-1578, 1998.

Results and Discussion

HAZ Martensite

Fusion Zone

FerriteBand

Alloy 625

P91

Hardness Zones