Effect of Hydrogen on the Mechanical Properties of ASTM A182 F22 and ASTM A36 Steels Welded Joint Using Inconel 625 as Filler and Buttering Metal

Abstract

ASTM A 182 F22 steel, used in components subsea for oil extraction, are previously buttered with ductile metal such as Inconel 625, before being welded to steel pipes similar to ASTM A36 steels. The thermal buttering weld cycle provides the formation of high hardness micro-phases and carbides at the interface between F22 steel and the buttering with Inconel, which when in contact with hydrogen, originating from the cathodic protection applied to these equipment, can lead to the embrittlement of this region, causing fragile fractures. In this work, ASTM A 182 F22 steel, buttered with Inconel 625 and welded to A36 steel, submitted to post-weld heat treatment without hydrogenation and subjected to cathodic protection for hydrogen permeation were submitted to fracture toughness test. The welds and buttering were done using GMAW process with AWS ERNiCrMo-3 wire as filler and buttering metal and a mixture of Ar and He as shield gas. The results indicated a 56% of area reduction, and 15% in the elongation values in the tensile tests, in addition to a 13.3% reduction in the CTOD value, for welded joints subjected to hydrogen permeation, which showed a quasi-cleavage fracture mechanism.

Keywords:  ASTM A182 F22 steel; Inconel 625; Welded joints; Hydrogen embrittlement; Mechanical Properties

1. Introduction

The manifold is a control component that plays a crucial role in offshore oil exploration. This equipment made of forged steels, such as ASTM A182 F22 steel, arrives at sea in a pre-assembled state in order to facilitate field assembly and be welded to pipeline made of most malleable steel, such as ASTM A36 steel. Before welding, the part of this equipment, which will be joined to the pipelines, is previously subjected to a deposition of welding passes, named buttering, using Ni alloys such as Inconel 625, in order to reduce the effect caused by the welding thermal cycle on the heat affected zone (HAZ) in view of the dissimilarity of between steel and nickel alloy¹. Partially mixed zones (PMZs) and carbides can reach high hardness values, compromising the toughness of this region², involving carbon-rich microphases³ even after the application of post-weld heat treatment (PWHT)⁴.

After welding, this equipment undergoes a continuous cathodic protection process, thus guaranteeing a minimum of corrosion deterioration during its useful life⁵. In this protection, forced current flow is used to provide the potential for polarization through a galvanic electrochemical reaction⁶. However, this process provides hydrogen ion reduction reactions. This element is absorbed and diffuses into the metal due to its solubility in the metallic network, concentrating in voids and microstructural defects, and, in regions of high hardness such as PMZs, making the region of the interface between the buttering and the alloy of Ni and forged steel, very brittle and susceptible to fracture⁷.

This paper aims to evaluate the susceptibility to hydrogen embrittlement in the region of the interface between a buttering with the Inconel 625 alloy, and the ASTM A 182 F22 forged steel through tensile tests, with the useful part of the specimens containing the interface of the weld, and fracture toughness test using specimens for CTOD (crack tip opening displacement) with the notch locate 1 mm from the interface region. Both tested were done with (WH) and without (OH) the application of polarization potential for hydrogen generation.

2. Experimental Procedure

ASTM A182 F22 steel used in the present study was supplied in the form of a forged billet with dimensions 215 x 25.4 mm for diameter and height, respectively, which was sectioned into two sheets with dimensions of 180 x 76 mm x 25.4 mm. The chemical composition and mechanical properties of steel, were raised, respectively, through Optical Emission Spectroscopy (OES) and mechanical tests (Hardness and Traction), whose results are shown in Tables 1 and 2.

Table 1
Chemical composition (in weight percent), of ASTM A182 F22 steel.

C	Si	Mn	P	S	Ni	Cr	Mo	Cu	Al
0.17	0.14	0.35	0.01	0.01	0.04	2.11	0.93	0.04	0.30

Table 2
Average values of the mechanical properties of ASTM A182 F22 steel as received.

Hardness (HRC)	Yield Stress (MPa)	Tensile Strength Limit (MPa)	Elongation (%)
28.5	877	960	22

The other base metal used in the formation of the welded joint, representing the metal of the pipes, was ASTM A36 steel, obtained in the form of a rolled sheet, with a thickness of 25.4mm and cut in the same dimensions as the sheets of ASTM A182 F22 steel.

The welding process used was the Gas Metal Arc Welding (GMAW). This process was used both in the buttering of ASTM A182 F22 steel and in obtaining of welded joints. The shielding gas used was a mixture between Argon and Helium, in the proportions of 75% and 25% respectively. The filler metal used for buttering and weld process was the solid wire AWS ERNiCrMo-3 (Ni-based Alloy 625) with a diameter of 1.2 mm, whose chemical composition, according to the manufacturer, is specified in Table 3. The mechanical properties of the weld metal extracted from the weld joint, ASTM A182 F22 steel - Ni 625 alloy - ASTM A36 steel, are in accordance with the criteria AWS A5.14: ERNiCr Mo-3, and are elucidated in Table 4.

Table 3
Chemical composition (in weight percent), of the wire AWS ERNiCrMo-3 (manufacturer).

Ni	Cr		Mo		C		Fe		Al		Nb		Si		Mn
64.31	22.34		9.13		0.01		0.19		0.09		3.51		0.06		0.01
P		S		Co		Ti		Cu		Mg		Pb		Ta		N
0.003		0.001		0.03		0.21		<0.01		0.009		<0.001		0.02		0.02

Table 4
Mechanical properties of the weld metal AWS ERNiCrMo-3.

Yield Stress (MPa)	Ultimate Tensile Strength (MPa)	Elongation (%)
468	758	64

In Figure 1, there is a diagram showing part of the buttering weld, through which the arrangement of eight weld bead arranged in two layers is observed. In Figure 1a, the welding sequence adopted in this weld is verified, whose welds were arranged in parallel, along the thickness. Figure 1b shows the cross section of buttered steel. The welding parameters for executing the weld beads are shown in Table 5, both in the butter and in the welded joint used the GMAW conventional process with voltage adjustment.

Figure 1
Buttering weld. Procedure (a) and its transversal section (b).

Table 5
Buttering weld parameters.

Preheat temperature (ºC)	Current (A)	Voltage (V)	Wire feeding speed (m/min)	Welding speed (m/min)	Heat Input (kJ/mm)
≈300	199.87	25.85	7	0.21	1.48

This buttering consisted of a total of 16 weld beads, which were distributed in four layers composed of four beads. The total overlapping of the buttering layers allowed a thickness of 9.5 mm, which can be seen in Figure 2.

Figure 2
Dissimilar ASTM A183 F22 steel – Inconel 625 –ASTM A36 steel, welded joint showing the extraction direction of the tensile test specimens.

After applying the buttering, the set was subjected to post-weld heat treatment (PWHT). Time was used which, from a practical point of view, was feasible and at the same time sufficient to guarantee the reduction in the level of hardness that is specified by the NACE MR0175 standard for subsea applications. Oliveira⁸ found that, for a two hour PWHT, the hardness levels were practically unchanged. In turn, Dodge⁹ obtained interesting results using a ten hour PWHT. Then, in the interest of the present research, the time of six hours was defined at a plateau temperature of 677 ºC, was defined for relieve residual stresses from buttering weld composed of ASTM A182 F22 and Inconel 625 steel. The buttered material composed the dissimilar joint, ASTM A182 F22 steel - Inconel 625 - ASTM A36 steel, whose performance in the presence of hydrogen was evaluated. The PWHT parameters are shown in Table 6.

Table 6
PWHT parameters.

Heating rate	20 ºC/min
Temperature	677 ºC ± 10ºC
Stability time	6 h
Cooling	calm air

After rectification, the welded joint was performed, using nine welding passes with the parameters showed in Table 7.

Table 7
Welding procedure parameters.

Preheat temperature (ºC)	Current (A)	Voltage (V)	Wire feeding speed (m/min)	Welding speed (m/min)	Heat Input (kJ/mm)
≈300	174.71	24.95	7	0.21	1.25

Figure 2 shows an illustration of the buttered ASTM A182 F22 steel welded to ASTM A 36 steel. The Figure also shows the arrangement of the specimens for the tensile tests.

In order to evaluate the microstructure of the welded joint, samples from the interface region between ASTM F22 steel and Inconel 625 were subjected to analysis by optical microscopy (OM) and scanning electron microscopy (SEM). The specimen were polished with, 3 and 1/4μm diamond paste and etched by immersion in 2% nital solution, for 20s, in order to reveal the ASTM A182 F22 steel microstructure, afterwards an electrolytic etching was carried out with 10% chromic acid for a 5V voltage lasting 30s, which allowed the visualization of the microstructure of the Inconel 625. For analysis by electron microscopy, a LEO 1430 electron microscope with secondary electron detector (SE) was used.

Microhardness tests were performed using a Shimadzu equipment model HMV – 2, with a load of 100gf (HV_0.1) for 15s. In Figure 3, there is the demonstrative scheme of the test procedure, which corresponded to 09 columns, grouped 03 by 03, totaling 186 indentations per group. The distance between the impressions of the same column was 0.5 mm, as shown in Figure 3. From the five highlighted indentations, an average value was taken which corresponded to a point on the hardness profile curve.

Figure 3
Distribution of indentations in the microhardness test.

The tensile tests were performed on a 200kN Shimadzu Servo Pulser model. The dimensions of the specimen and other test parameters followed the recommendations of the ASTM standard¹⁰. The loading speed used was 0.6 mm/min. The ilustration of the specimen for the tensile test containing the interface and the F22 steel removed from the welded joint is showed in details in Figure 4.

Figure 4
Tension specimen of the welded joint¹¹.

For the performance of fracture toughness tests, a three-point bending test apparatus was used, with specimen of type SE (B), which were machined according to the standard¹² (See Figure 5). The position of the groove and the fatigue crack were approximately 1mm from the interface between ASTM A 182 F22 steel and the buttering with Inconel 625, as shown in Figure 6.

Figure 5
Dimension and tolerances of CTOD specimen, test¹².

Figure 6
Location of the fatigue pré-crack in CTOD welded joint specimens.

All conditions established by the standard for validation of CTOD were met, among which the following stand out:

• the a / W ratio is between 0.45 and 0.70;

• the difference between any of the seven central points and the length of crack a, does not exceed 0.1 * a;

• a ≥ 1.3 mm or 2.5% W, whichever is greater, the parameter CTOD (δ) was calculated using Equation 1 ¹².

  $\delta ={\left[\frac{FS}{B{W}^{\mathrm{1,5}}}xf\left(\frac{a_0}{W}\right)\right]}^2\frac{\left(1-{\nu }^2\right)}{2{\sigma }_{YS}E}+\frac{\mathrm{0,4}\left(W-a_0\right)V_P}{\mathrm{0,4}W+\mathrm{0,6}{a}_0+z}$(1)

Where Vp is the plastic component of the Force x Notch Opening Displacement (NOD) obtained in the test.

For the hydrogen permeation process in the specimens, simulating cathodic protection, using the recommendations of¹³ were followed. The NaCl solution, with a mass concentration of 35.00 g / L, was used to simulate sea water. In the cathodic protection apparatus of the printed current type, Figure 7, there is a reservoir, where the solution was disposed, together with the specimen to be protected (cathode - negative terminal) and the titanium plates (anode - positive terminal), as well as a voltage supply that comes from a direct current electrical source.

Figure 7
Cathodic protection apparatus.

In the hydrogen production process, pH 7 (neutral) and loading temperature of 0 °C was adopted for a period of 07 days, with a potential of -1.100 m V_SCE, as shown in Table 8.

Table 8
Cathodic protection parameters.

Electrolyte	Protection potential	Inert anode	Reference electrode	Continuous hydrogenation time
NaCl at 0ºC and pH 7	-1.100mVSCE	Pure titanium plate	Calomel type	168 hours

3. Results and Discussion

3.1. Macro and microstructural analyzes

Figure 8 shows the image of the interface between the buttering weld and the base metal, the region of which is homogeneous and free from discontinuities. It can also be observed that the heat affected zone (HAZ), the lightest region, outlined by the dashed line, wich presented an extension varying between approximately 4 and 5 mm.

Figure 8
Buttering weld interface as-welded.

In the images of Figures 9 and 10, there is the microstructure of the buttering weld interface, in which the partially mixed zones (PMZs) are observed, identified at the interface between the Ni layer and the steel. In this dissimilar interface, zones Δ, ϕ, Π, M and “swirls” and “islands” types are commonly found. The Δ zone, indicated by the dashed line in Figure 9a, contained in the coarse-grained region of the steel HAZ, is basically made up of depleted C ferrite due to the diffusion of this element to Ni¹⁴. Zone ϕ, shown in Figure 9b, is a high hardness Ni solid solution supersaturated of carbon diffused from steel, and, therefore, a region conducive to embrittlement, since the carbides present there, such as those from type Cr₇C₃ can act as hydrogen trapping sites¹⁵. The ϕ zone was uniform and approximately 10 µm in size.

Figure 9
Microstructure of the buttering weld interface. (a) 100x. (b) 500x.

Figure 10
Buttering weld. (a) OM from "islands " (500x). (b) OM from "swirl" (500x). (c) e (d) SEM of the region comprised by the red rectangle, respectively, (2000x) e (5000x).

Figure 9b also shows the cell dendritic zone (Π), located above zone Φ on the side of Inconel 625. Zone Π is formed due to a change in the solidification rate and temperature gradient¹⁶. The cellular dendritic structure tends to have carbides rich in Nb and Mo that decorate the dendritic boundaries¹⁷.

Figure 10a and b, shows images extracted by the LAMS technique¹⁸, showing macrosegregations of “island” and “swirl” type, with some of the microhardness values surveyed at the dissimilar interface, which is also shown in Table 9. Within the red rectangle of Figure 10b, there are dark regions, on the steel side, known as M zones. These zones are characterized by microstructural similarity with the base metal, having a very significant presence of martensite. Among the forms of presentation of zone M, Figures 9b and 10, a predominance of the types "beaches" and "swirl" was observed. The authors Dodge et al.¹⁵ mention that the amount of this type of formation is due to the welding parameters. Figure 10c and d correspond to SEM images of the M region, contained within the red rectangle, in which elongated microstructures are seen, suggestive of the martensitic microstructure. It can be also seen that the Vickers microhardness values in this microstructure (Figure 10b) and Table 9), were approximately 400 HV_0.1, which are compatible with this type of formation. The authors Frei et al.¹⁹ observed hardness values of the same order of magnitude in the ASTM A182 grade 22 steel HAZ, which were associated with virgin martensite. The authors Dai and Lippold²⁰ also observed the presence of those virgin martensite in the PMZs of 8630 steel samples buttered with Inconel 625 and treated above 600°C. This formation is justified by the occurrence of austenitic transformation during PWHT. They found that, even at very low cooling rates, this martensite is formed and exhibited levels of hardness around 330 to 450 VHN. Fenske et al.²¹, mention that the high Ni content can reduce the A_C1 temperature, making it lower than the PWHT temperature, favoring reaustenitize in discontinuous PMZs with microstructure of the base metal, and, consequently, generating virgin martensite after cooling. The high hardness of martensite makes it more susceptible to embrittlement by hydrogen. When the martensitic transformation occurs, the residual stress and the number of displacements increase. This will lead to a greater volume of capture sites, which will decrease the rate of diffusion and increase the concentration of hydrogen.

Table 9
Average values and standard deviation of microhardness tests (HV_0.1).

Zones	With PWHT
Δ	240.4 ± 21.6
ϕ	260.5 ± 31.6
Π	285.5 ± 42.3
M	404 ± 38.6

Figure 11 shows the average values of Vickers microhardness and the respective standard deviations for the regions of the weld metal (WM), which corresponds to the molten region, the HAZ and the base metal of the welded joint F22 - Inconel 625 - A36. In the abscissa axis there is the distance, in milimeters, from the region analyzed in relation to the fusion line. It is important to highlight that the hardness values presented previously, in Table 9, are not included in Figure 11. The materials applied in the production of oil and gas, with environments containing H₂S, must meet, among other criteria, the hardness levels established by the standard²², in order to reduce the risk of hydrogen embrittlement. This standard establishes that, after PWHT, the hardness in BM and HAZ must be less than 250HV. The BM and HAZ regions (Figure 11) present, in a global way (Table of Figure 11), average hardness values that meet the NACE standard.

Figure 11
Average Vickers microhardness profiles of the WM, HAZ and BM regions.

3.2. Tensile test

The average stress-strain curves of ASTM A182 F22 steel and the welded joint, in conditions without and with the presence of hydrogen, introduced by cathodic protection, are shown in Figures 12 and 13, respectively. Considering the base metal, it appears that the curves have similar behaviors, that is, the elongation, the area reduction and the flow limits and tensile strength, have not changed with the presence of hydrogen, as can be confirmed through Table 10 and Figure 12, as well as by calculating the fragility index (Fi), which is the relationship between deformations with (WH) and without (OH) the presence of hydrogen presented in Equation 2. It was shown that the value was close to 1 for the base metal.

Figure 12
Results of tensile test of ASTM A182 F22 steel extracted from the welded joint without and with hydrogenation.

Figure 13
Results of tensile test of welded joint without and with hydrogenation.

Table 10
Mechanical properties of the base metal (ASTM A182 F22 steel) without (OH) and with (WH) hydrogenation.

Condition	Specimen	Yield Limit Rp0.2 (MPa)	Tensile Strength Limit σrt (MPa)	Strain (%)	Area Reduction (%)
OH	1	629.15	718.57	23.80	60.00
	2	615.65	704.63	22.80	58.33
	3	616.92	707.75	22.80	59.38
	Average	620.57	710.32	23.13	59.24
WH	1	591.99	686.77	23.60	63.58
	2	621.15	702.87	24.80	62.54
	3	600.37	693.30	23.20	65.17
	Average	604.50	694.31	23.87	59.17

$$Fi={\epsilon }_H/\epsilon =23.87/23.13=1.03$$ (2)

Figure 13 shows that, for the welded joint specimens, there was a significant difference in the shape of the curves with regard to ductility. The hydrogenated specimens showed a significant reduction in ductility, as can be seen in Table 11 and by the F_I value which was equal to 0.85. In addition, all fractures occurred in the vicinity of the fusion line, region with presence of the PMZs that, as verified, have high hardness values, especially those regions close to zone M and ϕ, as shown in Figure 10 and Table 9. This result ensures that, even with the HAZ presenting a average hardness value of the order of 266HV_0.1, 1.5 mm from the fusion line (chart in Figure 11), the microphases present, close to this region, as the M zones (Figure 10 and Table 9) were responsible for susceptibility to hydrogen embrittlement.

Table 11
Mechanical properties of the welded joint, steel ASTM A182 F22 - Inconel 625 - steel ASTM A36, without (OH) and with (WH) hydrogenation.

Condition	Specimen	Yield Limit Rp0.2 (MPa)	Tensile Strength Limit σrt (MPa)	Strain (%)	Area Reduction (%)
OH	1	450.90	651.04	30.80	61.67
	2	462.76	655.86	31.60	58.33
	3	466.58	653.57	30.20	62.62
	Average	460.08	653.49	30.87	60.87
WH	1	461.82	657.68	26.40	28.50
	2	451.72	669.24	27.60	28.50
	3	458.80	668.95	25.00	23.50
	Average	457.44	665.29	26.33	26.83

On the other hand, it is observed that the values of the yield and tensile strength limits have practically not changed due to the presence of hydrogen. This result was already expected, as it has been observed that the phenomenon of hydrogen embrittlement acts more in the process of reducing ductility and fracture toughness²³.

The deformation results are consistent with the fractographs, which are presented in Figures 14 and 15, for the test specimens of the base metal and the welded joint, respectively. Through Figure 14, it can be seen that the samples of the base metal without and with hydrogenation presented fracture morphology typically ductile with microvoid coalescence (dimples).

Figure 14
Fractography of the tensile test of forged ASTM A182 F22 steel, extracted from the joint. (a) without. (b) with hydrogenation.

Figure 15
Fractography of the tensile test of the welded joint steel ASTM A182 F22 - Inconel 625 - steel ASTM A36. (a) without. (b) with hydrogenation.

The presence of a typically ductile fracture mechanism, with microvoid coalescence (dimples), was also observed for the welded joint without hydrogenation, Figure 15a. Regarding the fractured surface of the hydrogenated joint, a fragile fracture morphology was observed, with a predominance of the quasi-cleavage mechanism, Figure 15b.

Figure 16 shows the optical microscopy images of part of the cross section of the fracture surface of the hydrogenated welded joint. In the smallest scale image (500μm), the total width of the specimen is observed, after testing, and it is noted that, from the central region to the right end of this image, the rupture develops predominantly in the region of the HAZ, steel very close to the interface, which, as it was showed is the region where the PMZs occur, more specifically, the M and Δ zones. In the two images of greater magnification (100μm), it is possible to notice the occurrence of rupture predominantly close to the interface but on the steel side, that is, outside the ϕ zone. Thus, although some authors such as Dai and Lippold²⁰ attribute the fragile fracture mechanism mainly to the presence of the ϕ zone, the results obtained, with the fractures occurring in the steel region, suggests that the high hardness microfases of steel such as the M phase and the presence of carbides were more susceptible to hydrogen embrittlement. All specimens (SE(B)) of tested weld joints presented similar crack deviation behavior, regardless of the presence or absence of cathodic protection.

Figure 16
Optical microscopy of the cross section of the welded joint, steel ASTM A182 F22 - Inconel 625 - steel ASTM A36 with hydrogenation.

3.3. Fracture mechanics test

The average curves, Force vs. Notch Opening Displacement (NOD) for the fracture mechanics tests, of the base metal, in the conditions without (OH) and with (WH) hydrogenation are shown in Figure 17. The characteristics of the crack tests fatigue and CTOD are shown in Table 12, for both conditions tested.

Figure 17
Average force vs. Notch-Opening Displacement curves for base metal, ASTM A182 F22 steel, without and with hydrogenation.

Table 12
Results of CTOD tests of the base metal, ASTM A182 F22 steel, without (OH) and with (WH) hydrogenation.

Condition	Specimen	Fatigue crack			Stress intensity factor	CTOD
		Nº of cycles	a0 (mm)	a0/W	g (a0/W)	VP (mm)	Force (kN)	δm (mm)
OH	1	95985	10.13	0.507	2.72	0.63785	25.1880	0.23000
	2	71860	10.32	0.516	2.80	0.88848	25.1130	0.29619
	3	65005	10.18	0.509	2.74	0.95274	24.9870	0.31906
	Average					0.82636	25.0960	0.28175
WH	1	61812	10.04	0.502	2.68	0.94721	26.0295	0.32404
	2	61757	10.03	0.501	2.68	0.75913	25.7855	0.26636
	3	64960	10.02	0.501	2.67	0.85812	26.0600	0.29724
	Average					0.85482	25.9583	0.29588

The average Force vs. NOD for the base metal, subjected to cathodic protection, is in line with that of the non-hydrogenated base metal, suggesting that the hydrogen did not affect the plastic behavior of the base metal, since the average values of CTOD and the plastic component, shown in Table 12, had insignificant variations. It is observed that the δ_m values showed a slight increase, of only 5%, for the hydrogenated condition. This behavior of hydrogen, by which it favors the plasticity of the material, may be related to the model of localized plasticity, and to the increased mobility of dislocation caused by the presence of hydrogen. According to Birnbaum and Sofronis²⁴, the hydrogen dissolved in the crystalline network reduces the interactions between discordances, thus facilitating a greater flow of deformations.

In images of Figure 18a and b, we have the results of fractography of the base metal specimens without hydrogenation, whereas in images (c) and (d), of the same Figure, we have the results for the base metal with hydrogenation. The transversal surface of the analyzed specimens, being the regions of the fatigue crack (FC) and test crack (TC), highlighted by the dashed lines, are shown in Figure 18a and c.

Figure 18
Fractography of ASTM A182 F22 steel. (a) transverse surface and (b) SEM (1000x) without hydrogenation. (c) transverse surface and (d) SEM (1000x) with hydrogenation.

The test crack of the base metal without hydrogenation verified in the SEM image of Figure 18b, shows a predominance of a ductile type fracture mechanism, which involves nucleation, growth and the microvoid coalescence (dimples). In some studies, this ductile fracture mode is identified as a ductile rupture zone, or extension of stable cracks, and is associated with the rounding process that occurs in front of the fatigue crack tip during the growth of the test crack²⁵. In addition, other authors such as Fassina et al.²⁶, mention that second phase particles, such as precipitates and non-metallic inclusions and grain boundary, are sites of nucleation of these microvoids. In this respect, through Figure 18b, there are formations (dashed squares) suggestive of these inclusions located within the microvoids. Considering the chemical composition of ASTM A182 F22 steel, it can be associated, for example, that in these inclusions there are carbides, rich in Cr and Mo, which correspond to the alloying elements in greater proportions.

With respect to the hydrogenated base metal, SEM image of Figure 18d, a formation that resembles the "cellular" type (dashed line) is observed, which is mentioned by the authors Fassina et al.²⁶ when studying the ASTM A 182 F22 steel. This type of formation is characterized, in most cases, by the presence of inclusions in its interior and mixed fracture mechanism, characterized by a fragile mode in the central part of the cell and a ductile mode at the border. The "cellular" formation, seen in Figure 18d, has dimensions of approximately 70x110µm, very close to the results found by Fassina et al.²³.

Dai and Lippold²⁷ presented several failure morphologies of the welded joint ASTM A182 F22 and Inconel 625, among which the microvoid coalescence (MVC) and quasi-cleavage (QC), which are Hydrogen Assisted Cracking (HAC) morphologies, common in ASTM A182 F22 steel, according to Beachem's theory. The morphologies presented in Figure 18d suggest the presence of a mixed failure mechanism, involving dimples, MVC (white dashed line) and QC (blue dashed line), that is, a combination between the fragile and ductile mechanisms, with predominance of this last. Combining the results of CTOD and fractography, there is little role of hydrogen in the process of steel embrittlement, under the processing conditions that were used.

Figure 19 shows the average Force curves vs. NOD for ASTM A182 F22 dissimilar welded joint - Inconel 625 - ASTM A36 steel, without and with hydrogenation. Regarding the plastic behavior of these curves, it appears that this was also consistent with the failure mode in terms of δ_m, Table 13, similar to what was seen for the base metal.

Figure 19
Average force vs. Notch-Opening Displacement curves of the welded joint notch without and with hydrogenation.

Table 13
Results of CTOD tests of the welded joint, steel ASTM A182 F22 - Inconel 625 - steel ASTM A36, without (OH) and with (WH) hydrogenation.

Condition	Specimen	Fatigue crack			Stress intensity factor	CTOD
		Nº of cycles	a0 (mm)	a0/W	g (a0/W)	VP (mm)	Force (kN)	δm (mm)
OH	1	65406	10.17	0.508	2.73	1.10467	23.6880	0.36137
	2	55854	9.96	0.498	2.64	1.09830	24.1425	0.36826
	3	74537	10.07	0.503	2.69	1.10343	23.7390	0.36539
	Average					1.14099	23.8930	0.36501
WH	1	69578	10.19	0.509	2.74	0.96371	22.9415	0.31674
	2	53546	10.02	0.501	2.67	0.99787	23.9835	0.33468
	3	63645	10.07	0.503	2.69	0.88485	23.5900	0.29786
	Average					0.94881	23.5050	0.31643

When comparing the average values (δ_m) for the conditions without and with hydrogenation, it is observed that the welded joint ASTM A182 F22 steel - Inconel 625 - ASTM A36 steel had the fracture toughness reduced by approximately 16%, thus demonstrating susceptibility to hydrogen embrittlement. This embrittlement effect, induced by the hydrogen of the welded joint, was also evidenced by the failure mechanisms presented below. It is worth noting that the test in an assisted environment and at a low deformation rate is more effective in determining susceptibility to embrittlement²⁸; so, if we consider that the tests carried out were quasistatic and in air, it is believed that the susceptibility to embrittlement of the developed welded joint tends to be more pronounced.

In images of Figure 20a and b we have the results of fractography of the welded joint specimens without hydrogenation, whereas in images (c) and (d), of the same Figure, we have the results for the hydrogenated welded joint. The transversal surface of the analyzed specimens, whose regions of the fatigue crack (FC) and test crack (TC) are highlighted by the dashed lines, are shown in Figure 20a and c.

Figure 20
Fractography of the welded joint. (a) transverse surface and (b) SEM (1000x) without hydrogenation. (c) transverse surface and (d) SEM (1000x) with hydrogenation.

In the SEM image of Figure 20b, there are test crack regions of the welded joint without hydrogenation, which behaved similarly to the base metal (Figure 19b), the failure mechanism being predominantly ductile via dimples.

From Figure 20d, it appears that the fractured surface of the hydrogenated welded joint has a mixed failure mechanism, including MVC (white circles), QC (blue circle) and dimples. With the exception of the last mechanism, the others are associated with HAC. Considering the nomenclature adopted by Fassina et al.²⁶, the white circles are similar to the "cellular" formation.

Finally, it is evident that both the results of fractography and those of CTOD, of welded joints without and with hydrogenation, indicate the tendency of embrittlement in the welded joint, associated with the base metal HAZ. Additionally, even with this embrittlemnet indicator, the presence of "pop-in" was not observed, as well as no discontinuity in the F curves vs. NOD, which reinforces the fact that the presence of dissolved hydrogen in the crystalline network reduced the interactions between dislocations, promoting an increase in the plastic deformation in front of the crack, that is, thus facilitating a greater flow of deformations²⁴.

In Figure 21, there is an analysis of the deviation between the fatigue and the test crack, using optical microscopy. This analysis serves as a support to better locate the development region of the test crack. The specimens were analyzed under the conditions without (Figure 21a and b) and with (Figure 21c and d) cathodic protection. All specimens (SE (B)) of tested welded joints showed similar crack deviation behavior, regardless of the presence or absence of cathodic protection.

Figure 21
Fatigue cracks optical microscopy and testing of welded joint CTOD specimens, steel ASTM A182 F22 - Inconel 625 - steel ASTM A36(increase of 50x). (a) and (b) without hydrogenation. (c) e (d) with hydrogenation.

Figure 21a shows the fatigue crack (horizontal red line), which remained flat, and at 1.1 mm from the LF (vertical red line). Still in this Figure, it is possible to observe the beginning of the test crack, which, in relation to the fatigue crack, has a slight deviation. In Figure 21b, there is the continuity of this crack that was maintained in the HAZ region. However, it is possible to observe a more pronounced deviation when it approaches the fusion line, with strong indications that it does not reach or exceed the weld metal.

Images of Figure 21c and d, in turn, show the images of the crack deviation of the hydrogenated welded joint specimens. In Figure 21c, it is noted that the fatigue crack (horizontal red line) also remained flat, with a length close to approximately 1.2 mm (vertical red line), from the fusion line. Still with respect to Figure 21c, it appears that the test crack deviated slightly in relation to the fatigue crack. Such an occurrence is a strong indication that the hydrogenation process did not influence the direction of its development, that is, the result is similar to that of the material without hydrogenation.

The average values and standard deviation of the CTOD (δ_m), presented in Tables 12 and 13, for the base metal and welded joint, both in the conditions without (MF22S and MJSS) and with (MF22H and MJSH) hydrogenation, are also presented in the chart of Figure 22. Considering the standard deviation, it is evident that the reduction of fracture toughness of hydrogenated BM (yellow and green columns) has not been clearly proven. Regarding the welded joint (blue and gray columns), it is noted that, regardless of the standard deviation, the hydrogenated joint was susceptible to hydrogen embrittlement. This susceptibility to the embrittlement of the welded material suggests a direct relationship with the welding parameters and PWHT employed, responsible for the presence of the PMZs at the dissimilar interface. When we consider the fact that the fatigue crack is 1 mm from the LF, which has a hardness close to 246HV_0.1, while the point microhardness values of zone M (Figure 10 and Table 9) were higher than 400HV_0.1, it should be considered that: the use of a fatigue crack closer to the interface could provide a greater hydrogen trapping, in this interface, with greater hardness, and, thus, favor a greater reduction in the CTOD value of the welded joint.

Figure 22
Average and standard deviation of CTOD results (δ) for the four conditions tested.

Comparing the results of fracture mechanics with those of tensile test it was observed that the fatigue crack in the CTOD test was developed in the HAZ, but far from the PMZs, without including the M zones, and the hydrogen embrittlement was not as intense, as in the case of the tensile test, in which the entire weld interface, containing all regions of the PMZs and high hardness M zones, was subjected to tensile efforts.

4. Conclusions

In this study, it was researched the influence of hydrogen on the mechanical properties of ASTM A182 F22 steel and the dissimilar welded joint, consisting of the base metals (BMs) ASTM A182 F22 steel / ASTM A36 steel and weld metal (WM) Inconel 625, which was analyzed in the conditions without and with previous hydrogen loading. Therefore, based on the research developed and here within presented, the main conclusions are that:

1. ¹

   ASTM A182 F22 steel, subjected to previous hydrogenation, was not susceptible to hydrogen embrittlement, maintaining the same plasticity and ductile fracture characteristic of tensile tests without the presence of hydrogen.

2. ²

   The welded joints of ASTM A182 F22 steel with ASTM A 36 steel, using Inconel 625 as a filler and buttering metal, showed a significant sensitivity to hydrogen embrittlement, reducing elongation by 15% and reduction of area by 56%, in tensile tests, maintaining the levels of yield and tensile strength.

3. ³

   The toughness test results showed that the ASTM A182 F22 stell, had a low sensibility to hydrogen embrittlement while the ASTM A182 F22 steel welded joint - Inconel 625 - ASTM A36 steel, showed a reduction of 13.3% in the value of the CTOD for the samples submitted to hydrogenation.

4. ⁴

   All fractures of the welded joints in the tensile and CTOD tests occurred in the ASTM A182 F22 steel HAZ, which had partially mixed zones rich in martensite (M regions), with microhardness values around 400 HV.

5. ⁵

   The fracture morphology of the base metals specimens and welded joints, without hydrogenation, indicated the predominance of the ductile mechanism, both in the tensile tests and in the fracture mechanics. In hydrogenated specimens, a mixed mechanism was observed in the tensile tests, with the presence of mechanisms by dimples and transgranular cleavage, whereas, in the fracture mechanics tests, the presence of the MVC, QC and dimples mechanisms was evident.

5. Acknowledgements

The authors would like to thank Prof. Dr. Severino Jackson Guedes de Lima (in memorium) for all the legacy left to the Postgraduate Programs in Mechanical Engineering and in Materials Science and Engineering at the Federal University of Paraíba, whose present work is included.

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Publication Dates

• Publication in this collection
  11 Feb 2022
• Date of issue
  2022

History

• Received
  05 July 2021
• Reviewed
  29 Dec 2021
• Accepted
  13 Jan 2022