Microstructural Characterization and Mechanical Property Evaluation of a Novel 10

wt% Ni Steel Welding ConsumableErin Barrick and John DuPont, Lehigh University

Background and Motivation

Results and Discussion

• Research Objective: Understand the microstructural features

influencing the mechanical behavior in the novel 10 wt% Ni

steel welding consumable

• Welds were fabricated using the GTAW and GMAW processes

with the 10 wt% Ni steel consumable

• To avoid unwanted cracking, the preheat and interpass

temperatures were maintained between 121 and 135°C for

both welds

• All other parameters for the welds are shown in Table 2.

• The mechanical properties were evaluated using Charpy

impact energy testing and microhardness mapping

Office of Naval Research Grant Number N00014-16-1-2633 & American Welding Society Glenn J Gibson Fellowship Grant

Results and Discussion Summary to Date & Future Work

References

Acknowledgements

Objectives and Experimental Approach• A newly developed 10 wt% Ni steel shows great

promise as a candidate naval steel because of its

high strength, toughness, and superior ballistic

resistance

• The continuous cooling transformation diagram

in Figure 1 demonstrates that 10 wt% Ni steel is

relatively cooling rate insensitive – this makes it

an ideal candidate for a welding consumable.

• A preliminary welding consumable has been

developed at the Naval Surface Warfare Center,

Carderock Division – the composition of the

consumable is shown in Table 1

• It is of interest to be able to weld using various

Element Fe C Ni Mo V Cr Mn Si O

GTAW Bal 0.022 9.83 0.63 0.14 0.01 0.73 0.47 <0.005

GMAW Bal 0.03 9.59 0.66 0.17 0.02 0.65 0.43 0.014

Table 1. As-deposited chemistry of the GTAW and GMAW (in wt%) of 10 Ni wt% Ni Steel

Table 2. Welding parameters used to fabricate the GTAWs and GMAWs

Mechanical Property Results: Microstructure:

Summary:

• Welds produced with the GTAW process exhibit superior toughness to those produced with

the GMAW process. However, the microhardness of the two welds is similar.

• To date, several microstructural influences on toughness in this system have been identified:

• Both the last weld pass and reheated passes within the GTAW have a finer microstructure

than the GMAW – this is one source of higher toughness

• Because of the 2% O2 used in the shielding gas for the GMAW, there are nonmetallic oxide

inclusions present in the GMAW. These oxides are known to be detrimental to toughness

Future Work:

• As has been identified, there are several microstructural influences on toughness in these

welds. Experiments will be performed to isolate the different influences to determine what is

causing the poor toughness of the GMAW.

• This will ultimately allow for full scale use of this novel consumable.

1. E.J. Barrick, D. Jain, J.N. DuPont, D.N. Seidman, Effects of heating and cooling rates on phase transformations in 10 wt pct Ni steel and their

application to gas tungsten arc welding, Metall. Mater. Trans. A, 2017, vol. 48a, pp. 5890-5910.

2. C. Wang, M. Wang, J. Shi, W. Hui, H. Dong, Effect of microstructural refinement on the toughness of low carbon martensitic steel, Scr. Mat.,

2008, vol. 58, pp. 492-495

3. S. Terashima, H.K.D.H. Bhadeshia, Size distribution of oxides and toughness of steel weld metals, Sci. Technol. Weld. Join., 2006, vol. 11, pp. 580-

582

0

200

400

600

800

1000

1200

1400

1 10 100 1000 10000

359

Ms

Mf

382 370 328 Hardness

(HV)

Time (s)

Tem

pe

ratu

re (°C

)

Figure 1. Continuous cooling transformation diagram experimentally determined for 10 wt %

Ni steel [1].

Before this consumable can

be considered for full-scale

use, the effects of

microstructural constituents

on the mechanical properties

must be established

Parameter GTAW GMAW

Shielding Gas 100% Ar98% Ar /

2% O2

Power (W) 2300 6480

Travel Speed (mm/s) 1.3 4.2

Heat Input (J/mm) 1811 1532

Wire feed rate (mm/s) 12.2 127

Total number of Weld Passes 43 16

Figure 2. Welding set-up

100 μm

100 μm

GTAW

GMAW

3 mm

3 mm

300 310 320 330 340 350 360HV

3 mm

Figure 5. EBSD maps for the last passes in both welds. Black boundaries are greater than 15°.

Figure 4. Microhardness maps for the GTAW and GMAW. The maps utilize the same hardness scale shown at the top from 300 to 360 HV.

GTAW* GMAW0

50

100

150

200

250

Ch

arp

y Im

pac

t En

erg

y at

-18

°C (

0°F

), [

J]

Figure 3. Charpy impact energy results at -18°C for the GTAW and GMAW.

50 μm

50 μm

5 μm

81 ± 2 J

5 μm

250 μm

Figure 7. Backscatter electron SEM image showing the presence of oxides (indicated with arrows) in

the GMAW.

Figure 8. (Top) Fracture surfaces of the GMAW showing the mixed

fracture mode. (Bottom) Higher magnification fractograph of only the microvoid coalescence region

the location of oxides.

Figure 6. EBSD maps of reheated regions for both welds. Prior austenite grain

boundaries are shown as thick black lines.

• The microstructure

of the welds was

characterized using

scanning electron

microscopy (SEM)

and electron

backscattered

diffraction (EBSD)

• The Charpy impact energy

results in Figure 3

demonstrate that the GTAW

has superior toughness to

the GMAW

• The microhardness maps in

Figure 4 show that despite

the dramatic differences in

toughness between the two

welds, the microhardness is

relatively consistent

between the GTAW and

GMAW

• This demonstrates that

whatever is negatively

influencing the toughness

of the GMAW is not having

a detrimental effect on the

microhardness

Nonmetallic Oxide Inclusions:

• Figures 5 and 6 show that in both

the last passes and reheated

regions of the weld, the

microstructure of the GTAW is finer

than the GMAW

• Microstructural refinement is

known to promote better

toughness [2], thus providing

evidence for the superior

toughness of the GTAW

• Figure 7 shows that there are oxide

inclusions in the GMAW. There are

none present in the GTAW.

• The fracture surface of the GMAW

shows mixed mode of fracture

• The inset in the bottom image

shows that the oxides sit within the

microvoids

• On the contrary, the GTAW (not shown) exhibits 100%

microvoid coalescence failure

• Oxides are known to be detrimental to toughness [3], thus

providing another influence on the toughness of the welds

• The 98% Ar / 2% O2 shielding

gas for the GMAW was

necessary for arc stability

processes, therefore the gas tungsten arc welding (GTAW) and

gas metal arc welding (GMAW) processes will be investigated