Weld Hydrogen Cracking Susceptibility MAT-1-3

Project Overview Objective Complete experimental trials to further develop an understanding of the factors promoting hydrogen cracking in weld metals Develop hydrogen embrittlement curves for a range of dissimilar weld metals to define their material ductility and embrittlement indices. Use chemical, microstructural, strength and ductility properties of each weld metal to develop empirical formulae to predict the material ductility and embrittlement indices   Project will make use of previously developed slow bend test procedure 

Material Selection Materials selection proposed based upon strength, toughness, chemistry, manufacturer, type Manufacturer Electrode Name Type AWS Classifcation Phase 1 Lincoln Electric® Pipeliner® 8P+ Cellulosic E8010-P1 Lincoln Electric Shield-Arc® 70+ E8010-G Shield-Arc HYP+ E7010-P1 Böhler® FOX CEL 70P Excalibur® 8018-C1 MR Basic E8018-C1 H4R Pipeliner LH-D80 E8045-P2 H4R Excalibur 7018 MR E7018-1 H4R Böhler FOX BVD RP E8045-P2 Phase 2 Shield-Arc 70+ Pipeliner 6P+ E6010 FOX CEL+ Pipeliner 18P E8018-G H4 Fox EV Pipe E7016 SG 3-P GMAW ER70S-6 -Summary list of the welding consumables that were selected for both phases of testing -Some electrodes were tested in both phases (repeated) to provide additional test specimens and data points to characterize and understand the test results and trends Lincoln Electric, Pipeliner, Shield-Arc, Excalibur, MR, Jet-LH and Jetweld are registered trademarks of Lincoln Global, Inc. Böhler is a registered trademark of Voestalpine Edelstahl GmbH LIMITED LIABILITY COMPANY AUSTRIA Donau-City-Strasse 7 Vienna, AUSTRIA A-1220

Test Procedure Documentation Documented weld specimen preparation and testing procedure and apparatus Specimen dimensions including grooving Fixture for welding Welding parameters (electrode specific) to achieve filled groove based upon vendor recommended practice Base material tensile testing All weld metal tensile testing Report includes Appendix written in AWS format for consideration as a recommended practice guide -Test procedure previously developed by BMT. -Appendix to the final report has been written in a format that can be considered for future adoption by AWS for testing of SMAW welding consumables.

Material Characterization Basic Electrodes Base metal and weld metal stress strain curves captured Aged 3h at 150oC Chemical analysis Hardness testing Microstructure assessment -Tensile test results from the single pass, all-weld-metal specimens shown for each type (cellulosic & basic) of electrode. -Standard ASTM E8 tensile sample for base plate material Cellulosic Electrodes

Material Characterization Hydrogen effusion testing completed for all electrodes Simulate Hydrogen Effusion – Observe similar behaviour for class -Hydrogen effusion testing is completed using a modified AWS A4.3-93 diffusible hydrogen test. Specimen geometry and test procedure match A4.3 standard. -use X70 Skelp plate instead of A36 carbon steel to be consistent with the material used throughout the test program. -Specimens are allowed to effuse hydrogen at ambient shop temperature (~21°C) for 96 hrs. -Hydrogen readings are taken at time intervals (1, 2, 4, 8, 24, 48, 72, 96 hrs) to identify rate of hydrogen effusing from sample, instead of the single reading required by A4.3 to determine the total diffusible hydrogen.

Hydrogen Cracking Susceptibility Testing Preferred hydrogen cracking tests include notched 3 point bend specimens with known applied load Notched Specimen Slow Bend Load or Displacement -slow bends are completed using a 3 point bend test fixture with a 200 mm span. -specimens are notched with a Charpy notch in the weld to provide a stress concentration. -load rate has been previously demonstrated to be slow enough to allow hydrogen mobility in the specimen to support embrittlement of the material in stress concentrated location under the notch tip. Load Displ.

Hydrogen Cracking Susceptibility Testing Slow Bend Hydrogen Cracking Susceptibility Testing completed for all electrodes Define critical deflection curves -Example of a series of slow bend test specimens for a single welding consumable. -Longer aging times result in a lower diffusible hydrogen level in the weld and an improvement in the weld performance with an applied strain, shown by an increase in the load line displacement to failure. Pipeliner LH-D80 (Basic Electrode)

Critical Hydrogen Curve Development Specimen deflections are converted to applied stress or strain knowing the specimen and loading geometry Nonlinear ANSYS FEM with 20 noded brick elements Consider specimen geometry, materials and support contact Force [kN] Charpy notch -FE models are created for each welding consumable using the tensile properties from all weld metal specimens to model the weld, tensile properties of the base metal, and the geometry observed from cross sections of slow bend specimens. -critical strains are monitored below the Charpy V-notch as a function of the 3 point bend deflection to identify the critical material condition at the onset of load drop in the slow bend test. weld Deflection [mm] Base metal

Critical Hydrogen Curve Development Strains perpendicular to the plane of the notch are used to characterize the cracking state Reported from 0.18mm sub surface (Previously 0.23mm) Basic Pipeliner LH-D80

Observed Correlations Higher hydrogen cracking susceptibility related to: Lower elongation in the all weld metal tensile test Lower elongation correlates with lower ductility indices Carbon equivalent Higher CE correlates with greater embrittlement indices (cellulosic electrodes) Microstructures containing PF(I) intra-granular polygonal ferrite Presence of PF(I) correlates with high hydrogen embrittlement susceptibility for cellulosic electrodes Presence of PF(I) correlates with reduced hydrogen embrittlement susceptibility for basic electrodes Welding heat input t800-500 Increased cooling time through transition temperatures correlated with reduced susceptibility Cellulosic electrode type Showed stronger hydrogen susceptibility with increasing strength, hardness, and lower ductility than basic electrodes

Hydrogen Susceptibility Relationships Goal: Identify εcrit for different materials as a function of hydrogen concentration εcrit = A (Hconc)B -each set of slow bend test data assembled using the estimated hydrogen concentration at the time of slow bend testing, and the true critical material strain identified from FE models (corresponding to load line displacement at the initiation of failure) -curve fit completed using a power law to for each welding electrode. Curve fit parameter “A” is termed the ductility index and provides insight to the relative ductility behaviour between electrodes with diffusible hydrogen present in the weld, and curve fit parameter “B” is the embrittlement index which characterizes the relative transition from highly susceptible hydrogen concentrations to less susceptible hydrogen concentrations. Ductility Index A = f( ? ) Embrittlement Index B = f ( ? )

Hydrogen Susceptibility: Cellulosic Electrodes Ductility Index ‘A’ Strong Correlations: CEIIW (negative), E50-250 (positive) Good Correlations: Reported %el. (positive) UTS, AF%, Mn% (negative) A = 0.2 CEIIW -0.00055E50-250 + 24.72 For cellulosic electrodes, the CE-IIW and all weld metal modulus (measured between 50 and 250 Mpa) produced a good linear fit to the experimentally obtained ductility indexes -Ductility index also correlated to manufacturer reported tensile data, Mn content and fraction of Acicular Ferrite in the weld.

Hydrogen Susceptibility: Cellulosic Electrodes B = -0.0018*UTSTrue -0.0000035*E50-250 + 1.52 Embrittlement Index ‘B’ Strong Correlations: True UTS, E50-250 (negative) Good Correlations: Elongation (positive) YS, UTS, Resilience modulus (negative) The embrittlement index was found to be correlated to the all weld metal UTS and the modulus. Good linear predictive relationship was obtained. -YS, and resilience modulus (area under stress strain curve) also showed decent correlations. May be suitable to improve ability to predict Embrittlement index with additional data.

Basic Electrodes – Ductility Index ‘A’ −0.04 AF ∗ 2 + 7.56AF −13.57Si +83.74 Positive Correlations with: -Notch Hardness, Microstructure Constituents, Si% (positive) -Two relationships identified: -microstructure AF*(%) = AF(%) + 0.65*FS -hardness (HV10) -Si demonstrates positive influence on ductility 0.14HVnotch + 115 Si(%) - 62.2 Ductility index was found to be predicted with a reasonable fit (not as strong as the cellulosic electrode data) using two relationships: -Acicular ferrite area fraction and the Si content -HV hardness below the Charpy V-notch, and Si Content -The relationships are similar and the Acicular ferrite content corresponded closely with the weld metal hardness below the notch tip.

Basic Electrodes – Ductility Index ‘A’ Close correlation between microstructure and the HV10 hardness below the notch -Acicular ferrite (AF) + weighted fraction of FS, ferrite with second phases (aligned & non aligned) -Potential to develop a hardness and metallurgical assessment for weld procedure development Weld Hardness is closely was found to be closely linked to the acicular ferrite content in this study, demonstrating how the two relationships correspond.

Basic Electrodes – Embrittlement Index ‘B’ Correlates with Ferrite with Aligned Second Phase. -Weight different microstructures based on theoretical susceptibility to embrittlement i.e. FS(NA) lower susceptibility than FS(A), while PF(I) & PF(G) are ductile structures B = -0.0149 x {FS(A) + 0.45*FS(NA) – 0.46*PF(I + G)} - 0.202 One Outlier: Excalibur 7018 MR -Widmanstatten structure from prior austenite GB -not observed with other materials tested -embrittlement index for basic electrodes produced a strong predictive relationship using a weighted fraction of the microstructural constituents -metallurgically, aligned second phases in ferrite would be expected to have increased susceptibility than non-aligned second phases -ferrite structures in the absence of second phases (i.e. polygonal ferrite) are generally expected to have a reduced susceptibility -This theory corresponded to the observed data and a least squares fit was completed to obtain the weighting factor used to predict the embrittlement index. -One outlier – contained widmanstatten structure at prior austenite grain boundary locations. -predictive relationship only valid for weld structures without this structure -widmanstatten structure displayed high susceptibility to hydrogen.

GMAW Electrode Similar Microstructure to Basic Electrodes -Low C, promotes ferrite along prior austenite GB -Concentrated heat source produces microstructure similar to a higher heat input SMAW Basic electrode weld -A slight increase in GMAW heat input would be expected to enhance development of ferrite structures -May produce lower residual stresses and hence in practice reduce susceptibility. Further investigation required to support hypothesis -Embrittlement Index predicted by microstructure relationship identified for basic electrodes

Conclusions General… Cellulosic Electrodes 16 different consumables were tested, 13 unique Microstructural trends observed for both cellulosic and basic electrodes and correlated to fracture morphology of slow bend samples Cellulosic Electrodes Susceptibility curves as a function of weld metal hydrogen content were able to be predicted with the CE of the electrode and mechanical properties Increasing strength, hardness, modulus and CE contribute to increasing hydrogen cracking susceptibility Weld procedure influences on susceptibility were identified for repeat test electrodes Repeat electrode results suggest possibility of an embrittling threshold existing (combination of AF, UTS, HV10) beyond which material becomes highly susceptible to all hydrogen contents

Conclusions (cont’d) Basic Electrodes GMAW Wire Microstructure evolution showed correlation to the hydrogen susceptibility Relationships between weld microstructure and hardness were found to correlate to hydrogen embrittlement susceptibility and may be beneficial for assessing weld procedure qualification testing Welding procedure was found to influence the material properties and the susceptibility to hydrogen cracking GMAW Wire Insufficient data points to draw strong conclusions, however, microstructural characteristics and mechanical tests show similar performance to low hydrogen SMAW electrodes, but at lower attainable heat inputs

Recommended Future Investigation Improve the fit of the developed hydrogen cracking susceptibility relationships based upon ductility and embrittlement indices: Evaluate the number of slow bend samples required to achieve a repeatable fit of the susceptibility curve used to define a materials performance in the presence of hydrogen; Evaluate the influence of varying welding parameters within manufacturer recommended ranges on the fit of the ductility and embrittlement indices. Documentation and technical transfer of and testing procedures: Consider the transferability of the testing procedure to other test labs; and Consider assembly of a guidance note for the testing procedures. Extension of the defined relationships and improvement of metallurgical basis for the observed susceptibility relationships: Test the susceptibility of the weld HAZ to hydrogen embrittlement for material combinations that demonstrate low susceptibility in the weld; and

Recommended Future Investigation Characterize additional materials to increase the correlation basis for the hydrogen cracking susceptibility curve relationships: Perform additional tests using GMAW process to include commonly used filler materials; Perform additional GMAW welds to evaluate influence of common welding procedure variations (modified wave short circuit, spray transfer); and FCAW or metal-cored welding wires could be considered to consider their susceptibility. Long Term Objectives to relate hydrogen cracking susceptibility to weldment design: Consider the relationship between critical strain to the strain observed in typical weld anomalies (e.g. lack of fusion, slag inclusions); and Relate or define upper bound hydrogen concentrations as a function of welding parameters and total diffusible hydrogen for typical electrode types and weld parameters.