Friction stir welded steel pipelines were tested in high pressure hydrogen gas to examine the effects of hydrogen accelerated fatigue crack growth. Fatigue crack growth rate (da/dN) vs. stress-intensity factor range (K) relationships were measured for an X52 friction stir welded pipe tested in 21 MPa hydrogen gas at a frequency of 1 Hz and R = 0.5. Tests were performed on three regions: base metal (BM), center of friction stir weld (FSW), and 15 mm off-center of the weld. For all three material regions, tests in hydrogen exhibited accelerated fatigue crack growth rates that exceeded an order of magnitude compared to companion tests in air. Among tests in hydrogen, fatigue crack growth rates were modestly higher in the FSW than the BM and 15 mm off-center tests. Select regions of the fracture surfaces associated with specified K levels were examined which revealed intergranular fracture in the BM and 15 mm off-center specimens but an absence of intergranular features in the FSW specimens. The X52 friction stir weld and base metal tested in hydrogen exhibited fatigue crack growth rate relationships that are comparable to those for conventional arc welded steel pipeline of similar strength found in the literature.
Banded ferrite-pearlite X65 pipeline steel was tested in high pressure hydrogen gas to evaluate the effects of oriented pearlite on hydrogen assisted fatigue crack growth. Test specimens were oriented in the steel pipe such that cracks propagated either parallel or perpendicular to the banded pearlite. The ferrite-pearlite microstructure exhibited orientation dependent behavior in which fatigue crack growth rates were significantly lower for cracks oriented perpendicular to the banded pearlite compared to cracks oriented parallel to the bands. The reduction of hydrogen assisted fatigue crack growth across the banded pearlite is attributed to a combination of cracktip branching and impeded hydrogen diffusion across the banded pearlite.
Tensile specimens of 10B22 (22MnB5) sheet steels were austenitized, quenched to martensite, and tempered at temperatures between 150 and 520°C for various times. The heat treated specimens were charged with 1.7 ppm hydrogen and immediately tested. Fracture surfaces were examined by field emission scanning electron microscopy. As-quenched martensitic specimens exhibited the most severe embrittlement and failed by stress-controlled cleavage fracture at low stresses. The initiation of hydrogen-induced fracture in specimens tempered between 150 and 350°C was consistent with glide plane decohesion, and coarse inclusion particles served as sources of hydrogen for circular areas of hydrogen-induced cleavage. Specimens tempered at 460 and 520°C showed little sensitivity to hydrogen embrittlement. The progression of decreased sensitivity to hydrogen-induced fracture with increasing tempering temperature correlates with the reduction in dislocations, the principal hydrogen traps, and the formation of cementite particles, considered to be ineffectual traps, with increased tempering. Very small amounts of intergranular fracture were observed, only in as-quenched specimens, confirming that boron has little effect on hydrogen embrittlement of hardened low-carbon steels.KEY WORDS: hydrogen embrittlement; tempered martensite; low-carbon sheet steel; mechanical properties; SEM analysis; fracture mechanism.properties of quench and tempered specimens not exposed to hydrogen. 7,8) The specimens in the current study were tested immediately after hydrogen charging, and recent work by thermal desorption spectrometry on hydrogen trapping capability of quench and tempered low-carbon steels, 17,18) coupled with the results of mechanical testing and fractographic analysis of this investigation, provide useful information contributing to understanding hydrogen embrittlement of the heat treated 10B22 steel.
Experimental ProcedureThe chemical composition of the low-carbon steel is given in Table 1. Boron is effectively used in low carbon steels to provide hardenability, and several studies have shown that boron additions in steel have little effect on hydrogen embrittlement. 19,20) Standard ASTM E-8 longitudinal tensile samples were machined from 1.7 mm thick commercially produced cold-rolled sheet steel. The tensile samples were austenitized at 900°C for 10 min in a salt bath and quenched in water. The quenched samples were tempered at temperatures between 150°C and 520°C for times of 600 s, 3 600 s, and 36 000 s in either oil or salt baths and air cooled after tempering.The tensile samples were cathodically charged with hydrogen with a DC power supply in a solution of 1 N H 2 SO 4 with 1 mg/L As 2 O 3 as a hydrogen recombination poison.
21)Charged coupons, 7ϫ7ϫ1.5 mm 3 , were used to determine induced hydrogen content in a LECO RH-404 hydrogen analyzer, and parameters were established to induce a constant hydrogen content of 1.7 ppm in all samples. A current density of 5 mA/cm 2 for 30 min was used for the asquenched samples and 10 mA/cm 2...
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