Simulating Welding Thermo-Mechanical Histories Leading to Ductility-Dip Cracking

Samuel Luther, Dr. Boian Alexandrov (Adviser), The Ohio State University

1. Problem Statement

3. Results

4. Discussion

5. Conclusions

2. Experimental Method

• Face-centered cubic (FCC) alloys are essential components to industry

due to their corrosion resistance and strength.

• Ductility-dip cracking (DDC) can occur when welding FCC alloys in

large, multipass welds as in Figure 1 and the precise micro-mechanistic

details of crack formation are unclear.

• Figure 2 illustrates the relationship between temperature and ductility of

a DDC-prone material.

• DDC is a reject-able defect in high-impact industries such as nuclear

power generation and oil & gas.

• A method of simulating welding conditions leading to DDC has been

developed to learn more about its mechanism.

• It is a Gleeble-based thermomechanical procedure referred to as fixed-

displacement thermal cycling (FDTC).

References: [1] S. McCracken and R. Smith, "Evaluation of Filler Metal 52M Hot Cracking When Welding on Cast Austenitic Stainless Steel Base Materials," in Proceedings ASME PVP Conference, Baltimore,

2011. [2] J. DuPont, et al., Welding Metallurgy and Weldability of Nickel-Base Alloys, Hoboken, NJ: Wiley, 2013. [3] S. McCracken and J. Tatman, "Comparison Of Ductility-Dip Cracking To Computer Modeling

With Sysweld™ In A Narrow Groove Multi-Pass Weld," in Pressure Vessels and Piping Conference, Vancouver, Canada, 2016. [4] B. Hemsworth, et al., "Classification and Definition of High Temperature Welding

Cracks in Alloys," Metal Construction & British Welding Journal, pp. 5-16, 1969.

• Weld metal microstructure throughout sample gauge section and high-temperature straining are

critical for the nucleation of DDC during FDTC testing. Future testing will incorporate these

findings.

• Multiple thermal cycling has a greater effect on mechanical energy imposed on samples than

preloading or tensile straining, however there is significant interaction between the factors.

• DDC generated with FDTC has a similar fracture surface to past research and actual DDC

failures.

Acknowledgements

Figure 2: Ductility vs. temperature graph

for a material prone to DDC [2].

Figure 4: FDTC Gleeble chamber setup.

Figure 1: Schematic of multipass weld used in nuclear industry with DDC-

susceptible region marked (52M Overlay) [1].

DDC Region

• Materials used in this study were 310 stainless steel

base metal, nickel 201 base metal, and nickel-based

filler metal alloy 52M.

• Each sample gauge section contained weld metal to

provide the solidification microstructure for testing.

• Samples machined to a geometry convenient for

Gleeble (Fig. 3).

• A thermocouple was used for temperature control and

a dilatometer was used for measuring strain (Fig. 4).

Figure 3: Example of machined sample.

310 Stainless Steel (austenitic base metal)

• FDTC proceeded with a welding

thermal cycle and high restraint

accumulated by fixing Gleeble jaws.

• Tensile straining was added on

heating in DDC range due to results

suggested by a Sysweld model from

McCracken and Tatman [3].

• Samples were free-cooled with fixed

displacement.

• A design of experiments (DOE) was

followed to evaluate test parameters:

preload, DDC temperature straining,

and number of cycles.

• 8 samples from each material were

tested. Nickel alloys evaluated preload

and 310 SS evaluated straining. All

three materials were subject to

varying number of cycles.

Nickel 201 (commercially pure base metal)

Alloy 52M (high-chromium, corrosion-resistant filler metal)

Figure 5: 310 SS as-welded condition.

Figure 6: 310 SS DDC fracture surface.

Table 1: 310 SS DOE results.

• No cracking found in as-welded condition weld

metal.

• DOE results show insignificant effect from straining

but significant effect from number of cycles and

significant interaction.

• Cracking present in high-cycle samples; Fig. 6

shows fracture surface from an over-strained

sample. Thermal faceting consistent with past

findings [4].

Table 2: Nickel 201 DOE results.

Figure 7: Nickel 201 as-welded condition.

• No cracking found in as-welded condition weld metal. However, no cracking found in samples

which underwent FDTC.

• DOE results show insignificant effect from preload but significant effect from number of cycles. A

significant interaction is again observed between preload and number of cycles.

Figure 8: Alloy 52M as-welded condition.

P-Value Coefficient R-sq

Preload 0.107 0.0285

99.15%Cycles <0.001 2.136

Interaction 0.038 -0.00679

Table 3: Alloy 52M DOE results.

Figure 9: Alloy 52M DDC fracture surface.

• No cracking found in as-welded condition weld

metal.

• DOE results show insignificant effect from preload

but significant effect from number of cycles and

another significant interaction.

• Cracking again present in high-cycle samples with

thermal faceting occurring on fracture surfaces as

with the 310 SS.

• Figure 9 shows a clear grain orientation

relationship to thermal faceting. Dark spots are

artifacts from SEM detector (shadowing).

• In general, number of cycles significantly impacts mechanical energy imposed (Fig. 10) to the

sample and loading/straining conditions do not by themselves. However, a statistically significant

interaction between the two appears and cracking is correlated with high imposed energy.

• Throughout testing, several important aspects of the procedure were deliberately altered to

improve the effectiveness of FDTC: elevated temperature straining, spot welding where needed,

elimination of preloading. The nickel 201 samples lacked spot welds whereas the 310 SS samples

had them.

• “Effectiveness” = ability to re-create severe welding conditions to the extent of causing DDC in

these materials.

• Based on these findings, future FDTC will: 1) have a sample containing spot welds for ideal

microstructure; 2) utilize elevated temperature tensile straining to nucleate DDC; 3) have a high

number of cycles for most severe conditions.

7.95 MJFigure 10: FDTC stress/strain plot with imposed

mechanical energy integral. Taken from a

single-cycle test with no preload or elevated

temperature straining. Integral calculated as a

scalar quantity; no “negative” area. This test

resulted in a 7.95 MJ energy input.

P-Value Coefficient R-sq

Preload 0.953 -0.0009

98.13%Cycles <0.001 1.660

Interaction 0.039 -0.00695

P-Value Coefficient R-sq

Strain 0.691 -0.69

94.62%Cycles 0.012 1.542

Interaction 0.038 0.772