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All sourcesNASA NTRSDOE OSTI
NASA NTRSDate not supplied

Open-source Numerical Modeling of Solidification Cracking Susceptibility: Application to Refractory Alloy Systems

Introduction. Alloys such as aluminum, nickel-base, and austenitic stainless steels are susceptible to solidification cracking during welding and 3D printing. Compositional optimization is one method used to effectively mitigate solidification cracking of those alloy systems. With the surge in hypersonic and in-space propulsion activities, refractory metals (Nb, Mo, Ta, W, and Re) and their alloy derivatives are increasing in importance due to their extreme high melting point and retention of high-temperature strength; however, their chemistry was most typically optimized to promote ductility during mechanical operations such as drawing and forming. Welding of such alloys has been a challenge due to a number of issues including solidification cracking, atmospheric contamination (O, C, and N), as well as a shift in ductile-to-brittle transition to higher temperature following grain growth induced by welding. Compositional optimization of refractory alloys for solidification cracking resistance in particular is desirable as their usage increases with the advent of advanced manufacturing methods such as 3D printing. This work evaluates the effect of compositional variation in refractory metal systems on the solidification cracking susceptibility with the goals of optimizing existing alloys and joining process techniques, and formulating new alloys with increased solidification cracking resistance. Experimental Procedures. A python code was developed in a Jupyter notebook environment (Michael and Sowards, 2023) to facilitate the calculation of crack susceptibility index proposed by Kou (2015). Composition is entered as a single point, or as a 1-D or 2-D array. The notebook calls pycalphad (Otis and Liu, 2017 and Bocklund et al, 2020) to calculate the evolution of fraction solid as a function of temperature (under either Scheil or equilibrium assumptions) and then evaluates steepness of the fraction solid curve near the terminal stage of solidification to predict solidification cracking resistance. Open source thermodynamic databases available at online repositories are used (van de Walle). The process is setup in an automated fashion to generate plots that show variation in solidification cracking susceptibility according to composition on 1-D line plots or 2-D contour plots. The Jupyter notebook and crack susceptibility algorithm was also integrated with a widely used commercial CALPHAD code for validation and alloy exploration. Results and Discussion. The crack susceptibility model was first validated against a series of refractory alloy compositions evaluated in past work which utilized a specialized Varestraint test built inside a vacuum chamber environment (Lessman and Gold, 1971). The alloys tested in the Varestraint apparatus included T-111 (Ta-8W-2Hf), ASTAR-811C (Ta-8W-1Re-0.7Hf-0.025C), FS-85 (Nb-27Ta-10W-1Zr), T-222 (Ta-9.6W-2.4Hf-0.01C), Ta-10W, B-66 (Nb-5Mo-5V-1Zr), and SCb-291 (Nb-10W-10Ta). The initial test of the model showed a strong correlation with empirical Varestraint data, i.e., a Spearman rank correlation between model predictions and hot cracking measurements was observed to be greater than 0.8. Following the validation, a set of refractory metal binary mixtures was investigated to evaluate sensitivity of Nb, Mo, W, and Ta to C, N, and O content. A series of plots were produced that suggest ppmw ranges of C, N, and O where solidification cracking increases significantly and reaches a maximum. Also comparative ranking of each primary refractory metal to each interstitial was produced. For example C produces greater cracking response in Mo whereas O produces greater cracking response in Ta and Nb. Such compositional values have utility in setting limits on pickup of these interstitial elements during welding and printing rather than using a one-size-fits-all approach. Furthermore, the results have use in determining additive powder recycling requirements, which is especially pertinent for refractory metal powders due to their high cost compared to conventional alloys. Another application created thousands of hypothetical alloys within the nominal specified composition range of two widely used refractory alloys C103 (Nb-10Hf-1Ti) and TZM (Mo-0.5Ti-0.1Zr). The cracking index was calculated for the alloys and results were fed into machine learning regression techniques including Multiple Linear Regression, Ridge Regression, and Lasso Regression to determine relative potency each alloying element had on computed solidification cracking index. A series of linear equations were produced that relate composition of C103 and TZM to solidification cracking index. The crack susceptibility of C103 for example is described by an equation of the form: cracking index ~ O + 0.667*C + 0.635*N + 0.00037*Ta – 0.0008*Hf (in wt.%) From that equation, it is clear that O has strong propensity to induce solidification cracking. Interestingly, Hf is shown to reduce calculated cracking response. Finally, realizing the potential of this method to discover new refractory alloy formulations across the period table that have low solidification cracking sensitivity, the code was applied to new untested alloy systems including W-Zr-C, W-Ta-C, and others. Conclusions. In summary, an open source numerical method has been developed using Python code to calculate Kou’s crack susceptibility index. The method was applied to refractory metals which are inherently difficult to study from a weldability testing standpoint since inert shielding gas is not sufficient and welding is typically done in vacuum, especially in light of findings presented here where oxygen has profound influence on solidification cracking. This work revealed the effect of compositional variations on a series of refractory metals and showed the framework defined here will be useful in 1) the development of new alloys that have improved weldability and 3D printability, 2) placing compositional limits on existing alloys, and 3) ensuring adequate controls of manufacturing processes such as 3D printing where powder reuse is critical. Keywords. pycalphad; Python; refractory metals; solidification cracking. References. B. Bocklund et. al. (2020) http://doi.org/10.5281/zenodo.3630657. S. Kou. (2015) https://doi.org/10.1016/j.actamat.2015.01.034. G.G. Lessmann and R.E. Gold. Welding Journal, issue 1, pp. 1-s – 8-s (1971). F.N. Michael and J.W. Sowards. NASA/TM-20230002218 (2023). R. Otis and Z.-K. Liu. (2017) http://doi.org/10.5334/jors.140. A. Van de Wallle et. al. (2018) https://doi.org/10.1016/j.calphad.2018.04.003.

pycalphad↗
NASA NTRSDate not supplied

An Open-Source Numerical Model for Mitigating Refractory Alloy Hot Cracking Susceptibility

Refractory alloys are susceptible to solidification cracking during welding and 3D printing. Composition control is an effective method of controlling solidification cracking. This work evaluates the effect of compositional variation in refractory metal systems on a computed solidification cracking susceptibility. A numerical model has been developed using Python code and open-source CALPHAD software to calculate Kou’s crack susceptibility index. The model is validated against past weldability studies performed on several refractory alloy systems. The approach is extended towards the development of new alloys with improved 3D printability and weldability and is shown to have utility in defining compositional limits for existing alloys and feedstocks. Furthermore, the model will aid in determining process controls for powder reuse and recycling.

pycalphad↗
DOE OSTI2022

Determination of Location-Specific Solidification Cracking Susceptibility for a Mixed Dissimilar Alloy Processed by Wire-Arc Additive Manufacturing

Solidification cracking is a major obstacle when joining dissimilar alloys using additive manufacturing. In this work, location-specific solidification cracking susceptibility has been investigated using an integrated computational materials engineering (ICME) approach for a graded alloy formed by mixing P91 steel and Inconel 740H superalloy. An alloy mixture of 26 wt.% P91 and 74 wt.% Inconel 740H, with high configurational and total entropy, was fabricated using wire arc additive manufacturing. Microstructure characterization revealed intergranular solidification cracks in the FCC matrix, which increased in length along with the enrichment of Nb (~27 to 56 wt.%) and Cu (~87 wt.%) in the middle and top regions. DICTRA simulations to model location-specific solidification cracking susceptibility showed that the top region with the highest cooling rate (270 K/s) has the highest solidification cracking susceptibility in comparison with the middle and bottom regions. This is in good agreement with the experimentally observed varying crack length. From Scheil simulations, it was deduced that enrichment of Nb and Cu affected the solidification range as high as ~77%, in comparison with the matrix composition. The overall solidification cracking susceptibility and freezing range was highest for the 26 wt.% P91 alloy amongst the mixed compositions between P91 steel and 740H superalloy, proving that solidification characteristics play a major role in alloy design for additive manufacturing.

20 FOSSIL-FUELED POWER PLANTS↗
NASA NTRS2023

Numerical Study of Solidification Crack Susceptibility in Novel Refractory Alloy Systems

Calculation of Phase Diagrams (CALPHAD) -based solidification computa­tions, such as Scheil or equilibrium computations, have been utilized to propose crack solidification susceptibility indices (CSSIs) of cracking. These CSSIs have been proposed in order to predict the cracking susceptibility of an alloy in the solidification range, as a function of its solidification frac­tion and as dependent upon its wt.% elemental composition through the CALPHAD computation & via comparative methods between one composition & the next. Recently Kou at al. have proposed a novel CSSI where the gradient of the e.x. Scheil solidification curve is obtained in the critical solidification cracking region of greater than 0.95 fraction of solid. Therefore, a direct Temperature dependent metric is now available for the prediction, and presumably the control, of solidification cracking. In this TM, the researchers apply this Kou gradient method to refractory alloys for the first time & discuss justification for the approach via Spearman rank correlation with Varestraint cracking test data as well as by comparisons with Thermocalc based vulnerability time CSSI calculations. A Python Pycalphad module example of the approach is provided & can be utilized as an open-source resource that utilizes also open-source available thermodynamical databases, making the CSSI quantitative aspect of ICME more available and freely available to practitioners.

ICME integrated computational materials engineerin↗
DOE OSTI2017

Irradiation Assisted Stress Corrosion Cracking Susceptibility of Alloy X-750 exposed to BWR environments

The effect of irradiation on stress corrosion cracking susceptibility and fracture toughness of alloy X-750 has been investigated. The material has been irradiated at a target temperature of 288°C in the Advanced Test Reactor at Idaho National Laboratory to a fluence of approximately 1.93x1020 n/cm2 (E > 1 MeV). Stress corrosion cracking crack growth rates were determined in both unirradiated and irradiated materials in normal water chemistry and hydrogen water chemistry environments. Although the effect of irradiation on tensile properties and fracture toughness was observed, there was no significant effect of irradiation observed on the propagation rate of stress corrosion cracks.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of dissolved oxygen level on stress corrosion cracking susceptibility of structural steels

Stress corrosion cracking (SCC) is a potential risk in structural steels used for heat exchanger and circulation loop in LWR. Generally, SCC susceptibility of structural steels increases with increasing the level of dissolved oxygen (DO) in circulating water, but DO tolerance below which no SCC risk is present could differ by the type of steel. To investigate the DO effect on SCC susceptibility, T23, T24 and T91 steels under static load were exposed to stagnant and circulating water at 200°C with varying DO levels. The results indicated that among the tested steels, the SCC susceptibility was highest in T91 but lowest in T23 which did not exhibit crack initiation with 100 ppb DO. T24 showed no cracking with 50 ppb DO but cracked with 100 ppb DO, implying intermediate SCC susceptibility in between T23 and T91. Based on the cracking results and hardness data, SCC susceptibility in the tested steels is considered driven by corrosion which was initiated/affected by DO in the water.

Jun, Jiheon↗
DOE OSTI2015

Irradiation Programs and Test Plans to Assess High-Fluence Irradiation Assisted Stress Corrosion Cracking Susceptibility.

. Irradiation assisted stress corrosion cracking (IASCC) is a known issue in current reactors. In a 60 year lifetime, reactor core internals may experience fluence levels up to 15 dpa for boiling water reactors (BWR) and 100+ dpa for pressurized water reactors (PWR). To support a safe operation of our fleet of reactors and maintain their economic viability it is important to be able to predict any evolution of material behaviors as reactors age and therefore fluence accumulated by reactor core component increases. For PWR reactors, the difficulty to predict high fluence behavior comes from the fact that there is not a consensus of the mechanism of IASCC and that little data is available. It is however possible to use the current state of knowledge on the evolution of irradiated microstructure and on the processes that influences IASCC to emit hypotheses. This report identifies several potential changes in microstructure and proposes to identify their potential impact of IASCC. The susceptibility of a component to high fluence IASCC is considered to not only depends on the intrinsic IASCC susceptibility of the component due to radiation effects on the material but to also be related to the evolution of the loading history of the material and interaction with the environment as total fluence increases. Single variation type experiments are proposed to be performed with materials that are representative of PWR condition and with materials irradiated in other conditions. To address the lack of IASCC propagation and initiation data generated with material irradiated in PWR condition, it is proposed to investigate the effect of spectrum and flux rate on the evolution of microstructure. A long term irradiation, aimed to generate a well-controlled irradiation history on a set on selected materials is also proposed for consideration. For BWR, the study of available data permitted to identify an area of concern for long term performance of component. The efficiency of hydrogen water chemistry mitigation technology may decrease as fluence increases for high-stress intensity factors. This report describes a program plan to determine the efficiency of hydrogen water chemistry as a function of the stress intensity factor applied and fluence. The use of existing, available, materials and the generation of additional materials via irradiation in a research reactor are considered.

36 MATERIALS SCIENCE↗
NASA NTRS1935

Weldability of High-tensile Steels from Experience in Airplane Construction, with Special Reference to Welding Crack Susceptibility

The concept of welding crack tendency is explained and illustrated with practical examples. All pertinent causes are enumerated, and experimental measures are given through which the secondary effects can be removed and the principal causes analyzed: 1) welding stresses; and 2) material defects. The variations in length and stresses incident to welding a small bar as free weld, with restrained elongation and restrained elongation and contraction, are explored in three fundamental experiments.

Muller, J↗
DOE OSTI2019

Microstructure Evolution and Stress Corrosion Cracking Susceptibility of 12Cr Martensitic Steel Upon Long-Term Service in Power Plants

After 230,000-h long-term service at 550 °C/13.7 MPa, the microstructure of a kind of 12Cr martensitic steel was investigated using scanning electron microscopy, backscattering electron microscope and x-ray diffraction analysis. The results show that the precipitation and coarsening of carbides at grain/lath boundaries are the main cause of microstructure degradation. The static immersion tests and the slow strain rate test coupled with electrochemical impedance spectroscopy were conducted on the served steel in 1.0% NaCl solution, and it turns out that the pitting corrosion resistance and repassivation ability of the steel are significantly reduced as a result of microstructure degradation. The stress corrosion cracking susceptibility of the steel was also studied. Fracture morphology analysis shows that the secondary crack in conjunction with slip lines is a result of the coalescence of micro-cracks nucleated from the pits.

36 MATERIALS SCIENCE↗
NASA NTRS1982

The relative stress-corrosion-cracking susceptibility of candidate aluminum-lithium alloys for aerospace applications

Stress corrosion tests of Al-Li-Cu powder metallurgy alloys are described. Alloys investigated were Al-2.6% Li-1.4% and Al-2.6% Li-1.4% Cu-1.6% Mg. The base properties of the alloys were characterized. Process, heat treatment, and size/orientational effects on the tensile and fracture behavior were investigated. Metallurgical and electrochemical conditions are identified which provide reproducible and controlled parameters for stress corrosion evaluation. Preliminary stress corrosion test results are reported. Both Al-Li-Cu alloys appear more susceptible to stress corrosion crack initiation than 7075-T6 aluminum, with the magnesium bearing alloy being the most susceptible. Tests to determine the threshold stress intensity for the base and magnesium bearing alloys are underway. Twelve each, bolt loaded DCB type specimens are under test (120 days) and limited crack growth in these precracked specimens has been observed. General corrosion in the aqueous sodium chloride environment is thought to be obscuring results through crack tip blunting.

Pizzo, P. P.↗
NASA NTRS1974

Stress corrosion cracking susceptibility of 18 Ni maraging steel

The stress corrosion cracking (SCC) resistance of 18Ni maraging steel (grades 200, 250, 300, and 350) was determined in 3.5 percent salt (NaCl) solution, synthetic sea water, high humidity, and outside MSFC atmosphere. All grades of the maraging steel were found to be susceptible to SCC in varying degrees according to their strengths, with the lowest strength steel (grade 200) being the least susceptible and the highest strength steel (grade 350), the most susceptible to SCC. The SCC resistance of 250 grade maraging steel was also evaluated in salt and salt-chromate solutions using fracture mechanics techniques. The threshold value, K sub SCC, was found to be approximately 44 MN/sq m square root m, (40 ksi square root in.) or 40 percent of the K sub Q value.

Humphries, T. S.↗
NASA NTRS1973

Evaluation of Stress Corrosion Cracking Susceptibility Using Fracture Mechanics Techniques, Part 1

Stress corrosion cracking (SSC) tests were performed on 13 aluminum alloys, 13 precipitation hardening stainless steels, and two titanium 6Al-4V alloy forgings to compare fracture mechanics techniques with the conventional smooth specimen procedures. Commercially fabricated plate and rolled or forged bars 2 to 2.5-in. thick were tested. Exposures were conducted outdoors in a seacoast atmosphere and in an inland industrial atmosphere to relate the accelerated tests with service type environments. With the fracture mechanics technique tests were made chiefly on bolt loaded fatigue precracked compact tension specimens of the type used for plane-strain fracture toughness tests. Additional tests of the aluminum alloy were performed on ring loaded compact tension specimens and on bolt loaded double cantilever beams. For the smooth specimen procedure 0.125-in. dia. tensile specimens were loaded axially in constant deformation type frames. For both aluminum and steel alloys comparative SCC growth rates obtained from tests of precracked specimens provide an additional useful characterization of the SCC behavior of an alloy.

Sprowls, D. O.↗
DOE OSTI2024

Stress relaxation cracking susceptibility evaluation in 347H stainless steel welds

Abstract Stress relaxation cracking (SRC) has been reported as a dominant failure mechanism for 347H stainless steel welds at elevated service temperatures, occurring in either the heat-affected zone (HAZ) or fusion zone (FZ). In this study, SRC susceptibility of physically simulated HAZ and cross-welded E347-347H stainless steel welds was studied using a four-step accelerated SRC test with a thermomechanical physical simulator. The stress and temperature range studied represents weld-induced residual stress at 150–600 MPa and post-weld heat treatment temperatures between 800 and 1050 °C. For all temperature conditions, the cross-welded samples failed at a lower critical stress threshold than the simulated HAZ. A finite element model of a 50.8-mm-thick single-V groove weld was used to generate the residual stress maps induced by a 40-pass welding procedure, which in combination with the experimental threshold stress map predicted potential SRC failure locations. Postmortem microstructural evaluations were performed to identify contributing characteristics to SRC. It was found that HAZ samples exhibited a combination of creep void development, secondary cracks with intergranular fracture near grain boundary carbides, and eutectic phases. FZ samples showed brittle fracture with cracks propagating along straight, non-tortuous interdendritic grain boundaries.

42 ENGINEERING↗
DOE OSTI2013

US NRC-Sponsored Research on Stress Corrosion Cracking Susceptibility of Dry Storage Canister Materials in Marine Environments - 13344

At a number of locations in the U.S., spent nuclear fuel (SNF) is maintained at independent spent fuel storage installations (ISFSIs). These ISFSIs, which include operating and decommissioned reactor sites, Department of Energy facilities in Idaho, and others, are licensed by the U.S. Nuclear Regulatory Commission (NRC) under Title 10 of the Code of Federal Regulations, Part 72. The SNF is stored in dry cask storage systems, which most commonly consist of a welded austenitic stainless steel canister within a larger concrete vault or overpack vented to the external atmosphere to allow airflow for cooling. Some ISFSIs are located in marine environments where there may be high concentrations of airborne chloride salts. If salts were to deposit on the canisters via the external vents, a chloride-rich brine could form by deliquescence. Austenitic stainless steels are susceptible to chloride-induced stress corrosion cracking (SCC), particularly in the presence of residual tensile stresses from welding or other fabrication processes. SCC could allow helium to leak out of a canister if the wall is breached or otherwise compromise its structural integrity. There is currently limited understanding of the conditions that will affect the SCC susceptibility of austenitic stainless steel exposed to marine salts. NRC previously conducted a scoping study of this phenomenon, reported in NUREG/CR-7030 in 2010. Given apparent conservatisms and limitations in this study, NRC has sponsored a follow-on research program to more systematically investigate various factors that may affect SCC including temperature, humidity, salt concentration, and stress level. The activities within this research program include: (1) measurement of relative humidity (RH) for deliquescence of sea salt, (2) SCC testing within the range of natural absolute humidity, (3) SCC testing at elevated temperatures, (4) SCC testing at high humidity conditions, and (5) SCC testing with various applied stresses. Results to date indicate that the deliquescence RH for sea salt is close to that of MgCl{sub 2} pure salt. SCC is observed between 35 and 80 deg. C when the ambient (RH) is close to or higher than this level, even for a low surface salt concentration. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2018

Effect of Boron Content on Hot Ductility and Hot Cracking Susceptibility in 316L Austenitic Stainless Steel for Welding Components

Liquation cracking may occur in the heat-affected zone during welding. Two factors influence this phenomenon: the tensile stresses generated during welding and the potential loss of ductility due to the presence of a liquid film at grain boundaries depending on their chemical composition. Gleeble hot ductility tests have been used to study the combined effect of boron content and holding time on ductility drop in the liquation temperature range of a 316L type austenitic stainless steel. It is shown that high boron contents and short holding times promote the loss of ductility in this temperature range. Secondary ion mass spectrometry has been used to correlate mechanical results to boron distribution either at grain boundaries or in the bulk. Other welding tests have been performed to confirm the influence of boron content on hot cracking sensitivity of AISI 316L stainless steels. Results indicate that cracks appear on all specimens but at different strain levels. The higher the boron content is, the more the specimen exhibits tendency to hot cracking.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Influence of Thermomechanically Controlled Processing on Microstructure and Hydrogen Induced Cracking Susceptibility of API 5L X70 Pipeline Steel

The effect of different thermomechanical controlled processing routes on susceptibility of X70 pipeline steel to hydrogen induced cracking (HIC) have been studied. Two X70 pipeline steel specimens labelled WE and WD were investigated. These specimens have the same chemical composition, but they were processed with seperate thermomechanical treatments parameters. Microstructural examinations showed that WE consists of mainly acicular ferrite and polygonal ferrite, while WD consists of acicular ferrite and bainitic ferrite. After subjecting both specimens to hydrogen charging for 12 and 16 h in 0.2 M sulfuric acid and 3 g/L ammonium thiocyanate, early onset of HIC was observed in specimen WD. Post-hydrogen charging microstructural evaluation showed the nucleation of discontinuous cracks in WD after 12 h of charging. However, extended charging for up to 16 h resulted in HIC along the mid-thickness region of both specimens. Hydrogen diffusion across specimen WE was better than that of specimen WD. Therefore, hydrogen trapping at grain boundaries, banded deformed grains, inclusions and secondary phases such as martensite and cementite aided initiation and propagation of HIC in specimens. Nevertheless, the adverse effect of these features on HIC risks was more prominent in specimen WD compared to specimen WE. The Vickers microhardness values measured in WD (349.6 HV) and WE (307.4 HV) suggest that WD is harder than WE; and higher kernel average misorientation of 0.66° in WD than in WE (0.58°) shows higher dislocation density in WD. The results from slow strain rate tensile test confirmed that specimen WD was stronger and more susceptible to HIC than specimen WE. It was concluded that microstructural phases developed during thermomechanical processing improved strength in WD at the expense of its crack resistance, while WE with lower strength showed more ductility and higher resistance to HIC.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Measuring the residual stress and stress-corrosion cracking susceptibility of additively manufactured 316L by ASTM G36-94

Residual stress is a contributor to stress-corrosion cracking (SCC) and a common byproduct of additive manufacturing (AM). In this work, the relationship between residual stress and SCC susceptibility in laser powder bed fusion AM 316L stainless steel was studied through immersion in saturated boiling magnesium chloride per ASTM G36-94. The residual stress was varied by changing the sample height for the as-built condition and additionally by heat treatments at 600, 800, and 1200 °C to control, and in some cases reduce, residual stress. In general, all samples in the as-built condition showed susceptibility to SCC with the thinner, lower residual stress samples showing shallower cracks and crack propagation occurring perpendicular to melt tracks due to local residual stress fields. The heat-treated samples showed a reduction in residual stress for the 800 and 1200 °C samples. Both were free of cracks after >300 hours of immersion in MgCl 2 , while the 600 °C sample showed similar cracking to their as-built counterpart. Geometrically necessary dislocation (GND) density analysis indicates that the dislocation density may play a major role in the SCC susceptibility.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Hydrogen induced cracking susceptibility of API 5L X70 pipeline steel in relation to microstructure and crystallographic texture developed after different thermomechanical treatments

Highlights: • crack propagated mainly along deformed-banded microstructure. • Lower failure strength manifested at pipeline steel surface compared to the mid-thickness. • Greater fraction of martensite/austenite constituents featured in specimen WG relative to specimen WH. • Early onset of cracking in specimen WH showed no preference to specific crystallographic orientation. • Multi-component inclusion particle was revealed along crack propagation path. - Abstract: Thermomechanical treatments offer an innovative means of controlling microstructure development and improving resistance to hydrogen induced cracking (HIC) in pipeline steel. This study was conducted on two X70 pipeline steels (labelled WG and WH) of the same chemical composition, but processed with different thermomechanical parameters. Specimen WG was hot rolled at a higher temperature range of 880–820 {sup °}C, while specimen WH was hot rolled between 830 and 760 {sup °}C before cooling at a rate of 25 {sup °}C/s. Early onset of HIC was observed in specimen WH after electrochemical hydrogen charging for 12 h in a solution of 0.2 M H{sub 2}SO{sub 4} and 3 g/l NH{sub 4}SCN, without any applied stress. Also, test pieces from the surface and mid-thickness layers of each pipeline steel were subjected to slow strain rate tensile (SSRT) test with in-situ hydrogen charging. Characterization techniques such as Optical Microscopy, Scanning Electron Microscopy, Electron Backscattered Diffraction and X-ray Diffraction were used to investigate the structure pipeline steel specimens. The role of inclusions on HIC was studied by Energy Dispersive X-ray Spectroscopy. Although the microstructure of both specimens were predominantly ferritic, more martensite/retained austenite (M/A) constituents featured in WG. Further crystallographic texture analysis indicates that grain orientation at the surfaces of both test specimens were similar with deviations towards <111> on the inverse pole figure (IPF); whereas, pole intensity at the mid-thickness deviated towards the <001> direction. Considering the increased fraction of banded-deformed grains with high stored energy, the lower HIC resistance of specimen WH in comparison to specimen WG was justified.

36 MATERIALS SCIENCE↗