HYDROGEN EMBRITTLEMENT AND LOW TEMPERATURE EFFECTS ON CARBON STEELS

HYDROGEN EMBRITTLEMENT AND LOW TEMPERATURE EFFECTS ON CARBON STEELS LAURA VERGANI Politecnico di Milano, Department of Mechanical Engineering, Milano, Italy CHIARA COLOMBO Politecnico di Milano, Department of Mechanical Engineering, Milano, Italy AUGUSTO SCIUCCATI Politecnico di Milano, Department of Mechanical Engineering, Milano, Italy FABIO BOLZONI Politecnico di Milano, Department of Chemical Engineering, Milano, Italy ABSTRACT This paper presents an experimental study on the mechanical characteristics of carbon steels used for pipeline applications. The considered materials are: X65 micro-alloyed and F22 low-alloyed steel. C(T) specimens are extracted directly from the pipes, and fatigue crack growth tests are carried out considering three parameters affecting crack growth rates in mechanical components: absence or presence of hydrogen, low temperature and test frequency. This work is dedicated to the description of the setup used in the experimental tests and to the discussion of obtained results. Experimental results show an evident effect of the hydrogen presence on the fatigue crack growth. Fracture surface examination confirms the results of mechanical testing. INTRODUCTION In last years, energy supply question is becoming more and more urgent. As there is not one general solution, some energy sources and energy technologies have to be considered and reconsidered to ensure a continuous and safe energy source. Hydrogen as an energy carrier in applications, as fuel cells, are considered to play an important role in clean energy utilization. Metallic materials, such as carbon and low alloy steels, may suffer embrittlement and mechanical damage in presence of hydrogen at high pressure. The study of the mechanical characteristics of steels under the influence of hydrogen embrittlement is an essential topic due to the importance of these materials in different applications, like fuel cells, huge production and infrastructures for transportation, such as pipeline and vessels. In the literature, there are several studies dealing with the influence of hydrogen on fatigue behaviour of carbon and low alloy steels, using fracture mechanics approach, i.e., representing the data in terms of da/dn- K curves. These studies mainly aim to measure the fatigue properties of metals and welded joints in different environments, such as seawater, sweet (CO 2 ) or sour (H 2 S) condensates, boiling or pressurized water in nuclear plants, gaseous hydrogen at high pressure [1,2]. Authors of [3,4] carried out several experimental studies on austenitic stainless steels, and evidenced the effect of hydrogen by considering the microscopic fatigue mechanisms, i.e. slip bands and striations. [5] describes the 319

experimental results obtained by fatigue crack growth tests carried out on lowcarbon, Cr-Mo and stainless steels. They considered the coupled effect of hydrogen content, hydrogen diffusion coefficient, load frequency, slip bands and strain-induced martensite in austenitic stainless steels. In the present paper, the combined influence of hydrogen and low temperature on fatigue crack growth properties of two pipeline materials is investigated. This work is dedicated to the description of the setup of the experimental tests and to the discussion of their organization and obtained results. TESTED MATERIALS Tests are carried out on specimens extracted directly from two sections of seamless pipes in quenched and tempered condition. The considered steels, widely used in chemical and petrochemical plants, are: - 2 1/4 Cr-1Mo steel, namely ASME SA-182 F22 (outer diameter = 320 mm, thickness = 65 mm). From experimental tests carried out at room temperature, its mechanical tensile properties are: s y = 468±2.7 MPa, s UT = 592±2.1 MPa, E = 206,500±1500 MPa; - micro-alloyed C-Mn steel, API 5L X65 grade (outer diameter = 323 mm, thickness = 46 mm). From experimental tests carried out at room temperature, its mechanical tensile properties are: s y = 451.1±6.7 MPa, s UT = 609±5.7 MPa, E = 206,200±6050 MPa. F22 steel microstructure (Fig. 1.a) is typical of tempered lath martensite, i.e., elongated ferrite grains with finely dispersed carbides. Metallographic attacks also show prior-austenitic grains; no central segregation is present. Microhardness measurements of F22 steel give a mean value of 210.3±11.6 HV10; hardness is rather homogeneous through the thickness while there is a significant difference between longitudinal (HV = 217.4±10.75) and transverse (HV = 203.3±7.72) sections. X65 steel microstructure (Fig. 1.b) is equiaxed and acicular ferrite with finely dispersed carbides; microstructure is rather homogeneous. Material was Ca treated for desulfurization and inclusion shape control, therefore inclusions are manly composed by Al and Ca oxides and Ca sulfide, they are round and no elongated MnS inclusions are present. No central segregation is present; inclusion density is high: on the external surface slightly lower and with larger dimension. Microhardness measurements of X65 steel result in a mean value of 209.5±12.0 HV; hardness is rather homogeneous on both internal and external surfaces while it is lower on the center-line. a. b. Figure 1: Microstructures of F22 (a.) and X65 (b.) steels at optical microscopy. 320

EXPERIMENTAL SETUP A wide number of experimental tests are carried out to characterize the mechanical behaviour and fatigue crack growth rate of the steels in uncharged and hydrogen charged conditions [6]. C(T) specimens of F22 and X65 steels are cut from the pipes in accordance to ASTM E1823-11 in C-L orientation; specimens thickness is 20 mm and width 40 mm. Specimens are tested both in as received conditions and with hydrogen charge, obtained by the electrochemical method described in [6]. This procedure of hydrogen charging ensures a hydrogen presence in the range of 0.6-2 ppm. In order to avoid hydrogen release after charging operation and before mechanical tests, specimens are immersed in liquid nitrogen. The used testing machine is a 100kN MTS 810 servo-hydraulic loading frame. Fatigue tests are performed in load control following ASTM E647-08 standard, at constant stress ratio R = 0.1 and by using a thermal chamber. Before the test, specimens are brought at the test temperature by immersion in an ethanol-liquid nitrogen bath. Measurements of crack growth are made through the compliance method (crack opening displacement). Before the experimental tests, specimens are kept to the test temperature by immersion in a liquid bath obtained from a mixture of ethanol and liquind nitrogen. On both the steels, testing conditions are varied, considering three factors: - hydrogen absence or presence; - temperature (T = 23 C and T = -30 C); - test frequency (f =1Hz and f =10Hz). RESULTS OF FATIGUE TESTS F22 STEEL Fig. 2 shows results of the experimental tests for F22 steel. Fig. 2.a shows results for uncharged material, at constant test frequency of 20 Hz and considering three different temperatures. As widely shown in the literature, frequency is indeed not a parameter affecting crack growth rate, thus its influence in uncharged material was not investigated. From these experimental values, it can be observed that F22 steel presents a linear trend of the crack growth rate da/dn, which is very well reproducible by the Paris law in inert condition. No threshold value has been detected in the explored K range: this indicates that treshold for the stress intensity factor, K th, is lower than 14 MPa m. By the interpolation of the experimental data shown in Fig. 2.a, Paris coefficients can be evaluated at room temperature by considering da/dn in mm/cycle and K in MPa m as: da m -9 3.19 = C D K = ( 2.3 10 ) D K (1) dn Table 1: F22 steel: crack growth rate parameters for uncharged specimens. T [ C] f [Hz[] C [mm/cycle] m [-] 23 20 2.3 10-9 3.19-60 20 2.1 10-9 3.17-100 20 1.9 10-9 3.17 321

Other interpolations are shown in Tab. 1: estimated coefficients indicate that C coefficient decreases with the temperature, since yield strength is increased and consequently cyclic plastic zone is reduced. Considering the results in Fig. 2.b, different considerations can be proposed. The first one is that results of fatigue tests on hydrogen charged specimens are more scattered and less reproducible than the ones carried out on uncharged specimens. Indeed, percentage of charged hydrogen in the specimens is not a fixed value, but it is in the range of 0.6-2 ppm: specimens have a quite large thickness and hydrogen presence is not locally a constant. In any case, some well defined trends can be detected from Fig. 2.b. In hydrogen charged specimens tested at f = 10Hz and room temperature, a general increase in crack growth rate can be detected at similar K. In particular, hydrogen embrittling effect starts from K values lower than approximately 15 MPa m i. e. the lower limit of these experimental tests. 1.E-02 Crack growth rate, da/dn [mm/cycle] 1.E-03 1.E-04 1.E-05 y = Uncharged T=23 C T= -60 C T= -100 C a. 1.E-06 10 1.E-02 20 30 40 50 Stress intensity factor range, DK [MPa m 1/2 ] Crack growth rate, da/dn [mm/cycle] 1.E-03 1.E-04 1.E-05 1.E-06 10 Uncharged f=20hz Charged f=1hz T=23 C f=10hz T=23 C f=1hz T=-30 C f=10hz T=-30 C 20 30 40 50 Stress intensity factor range, DK [MPa m 1/2 ] b. Figure 2: Crack growth rate for F22 steel: a. uncharged specimens at different test temperatures; b. charged and uncharged specimens at different temperatures and frequencies. 322

In the lower range of explored K the difference in crack growth rates is very high, about one order of magnitude, then it reduces at higher values of K and finally the curves of hydrogen charged and uncharged specimens tend to merge. The curve of hydrogen charged specimens at f = 1Hz and room temperature shows a strong enhancement of the embrittling effect of almost two orders of magnitude when compared to the uncharged specimens. It can be observed that crack growth is somehow independent from load condition K since it propagates at constant rate. Table 2: F22 steel: crack growth rate parameters for charged specimens. T [ C] f [Hz] C [mm/cycle] m [-] 23 1 2.6 10-3 Plateau 23 10 3.3 10-6 1.33-30 1 7.4 10-4 Plateau -30 10 1.4 10-4 Plateau Considering the charged specimens tested at T = -30 C, da/dn trend flattens, nevertheless the embrittlement effect is reduced with respect to the same tests at 23 C. Especially at high K values, for tests carried out at T = -30 C and f = 10Hz on charged specimens, it was observed that the crack growth rate approaches the curve of the uncharged specimen at low K values (25-30 MPa m). Hydrogen influence on crack growth rate is therefore reduced, if compared to tests carried out at T = 23 C. Crack growth coefficients have been calculated also for these curves. In case of plateau trends, crack growth rates are approximately constant and independent on K. The calculated values are reported in Tab. 2. RESULTS OF FATIGUE TESTS X65 STEEL As for F22, in this paragraph results of the experimental tests on X65 steel are reported. Also for this steel, indeed, crack propagation tests were carried out for charged and uncharged specimens, by varying temperature and frequency. Since temperature effects on uncharged materials did not show considerable difference varying test temperature, only test at room temperature and f = 20Hz on uncharged X65 steel are performed. Fig. 3 presents a plot summarizing experimental results. For the uncharged specimens, crack propagation rate vs. K presents a double linear trend. Thus, experimental data can be interpolated by two straight lines with different slope. Tab. 3 reports the Paris coefficients for these two lines (I and II are respectively the first and second stage). Paris coefficient m is higher in the first part and equal to 4.4, when an expected value for steels is equal to 3, and lower in the second part where it is equal to 2.2. The transition between the two slopes occurs at K @ 25.5MPa m. Table 3: FX65 steel: crack growth rate parameters for the uncharged specimen. T [ C] f [Hz] C [mm/cycle] m (1.0 10-10 ) 23 20 I (4.38) I (1.0 10-7 ) II (2.22) II 323

1.E-02 Crack growth rate, da/dn [mm/cycle] 1.E-03 1.E-04 1.E-05 1.E-06 10 Uncharged f=20hz Charged f=1hz T=23 C f=10hz T=23 C f=1hz T=-30 C f=10hz T=-30 C 20 30 40 50 Stress intensity factor range, DK [MPa m 1/2 ] Figure 3: Crack growth rate for X65 steel: charged and uncharged specimens at different temperatures and frequencies. According to literature [7], variations in Paris exponents can be attributed to microstructural properties and in particular to its dimensions. Even if the observed phenomenon is typical of aluminium and titanium alloys, it can occur also in steels. It is controlled by the size of the cyclic plastic zone, indeed, when it reaches a definite length, dislocations are constrained and grain boundaries take part to the process. For this reason, there is a reduction in Paris exponent, which means an increased strength of the material and a higher constrain to dislocation movements. Considering hydrogen charged specimens, tested at room temperature and f = 1 and 10 Hz, results in Fig. 3 show embrittlement and crack growth acceleration, comparable to F22. Plateau regions are easily detectable for both test frequencies, crack growth rate, in fact, reaches a constant value independent from K. Finally, considering the specimens tested at T = -30 C and at the same frequencies, a smaller embrittling effect by the hydrogen presence can be observed. Especially at high K values, for tests carried out at T = -30 C and f = 10Hz on charged specimens, crack growth rate becomes very similar to the uncharged curve one. Therefore, hydrogen embrittlement is balanced by the increase in test frequency. Also, considering the curve at T = -30 C and f = 10Hz, hydrogen embrittlement effect has a maximum in an intermediate K range (20-40 MPa m), while, for lower and higher values material behaviour meets again the uncharged curve, that is the typical fatigue behaviour of the material without hydrogen. Crack propagation equations and coefficients for the X65 curves are reported in Tab. 4. For hydrogen charged specimens, calculation of Paris coefficient are done by considering the plateau region (II); as already discussed for F22, m coefficient approaches the zero at room temperature and low frequencies. A common characteristic emerging from experimental data is the presence of a transient and/or a large scatter of data at the beginning of the test until a certain threshold of K. 324

Table 4: X65 steel: crack growth rate parameters for charged specimens. T [ C] f [Hz] C [mm/cycle] m [-] 23 1 1.0 10-3 Plateau 23 10 3.4 10-4 Plateau -30 1 3.4 10-6 1.78-30 10 3.2 10-5 Plateau This value increases with frequency and decreases with temperature. In particular it seems lower for F22 steel (around K = 20 MPa m, as it can be noted from Fig. 2) and slightly higher for X65 steel (around K = 23 MPa m, see Fig. 3). After this transient, the behaviour of the material seems more stable. It is difficult to quantitatively assess this transient since accurate tests still need to be performed. FRACTOGRAPHIC ANALYSIS Fracture surfaces are analysed by means of a scanning microscope (SEM): in this paragraph some of the collected images will be shown and discussed. Considering F22 steel, the uncharged material presents a flat surface, with striations mainly perpendicular to the crack growth direction (Fig. 4.a). The other hydrogen charged specimens present, instead, two different fracture regions: a first one, smaller and mainly brittle with well developed striations and intergranular fracture, is near to the machined notch (Fig. 4.b). The second one, wider, extends up to the unstable crack propagation. This region is macroscopically rough, and fracture surface becomes cellular (Fig. 4.c), showing a mixed fracture mode: brittle in the central part of each cell ( quasicleavage ) and ductile at borders. Boudary between these two regions roughly corresponds to the fast increase in crack growth rate observed in Fig. 2.b. Reducing frequency from 10 to 1Hz, almost all the central part of the fracture surface is brittle and partially intergranular. Observations on specimens tested at lower temperature (T=-30 C) give similar results. a. 40 µm 6 µm ( b. 40 µm ( c. Figure 4: SEM observations of F22 specimens a. uncharged, T = 23 C and f = 20Hz: midthickness at a 0 + 9 mm ( K~40 MPa m); b. hydrogen charged, T = 23 C and f = 10Hz: ¼ thickness at a 0 + 2 mm ( K~20 MPa m); c. hydrogen charged, T = 23 C and f = 10Hz: midthickness at a 0 + 8 mm ( K~35 MPa m). Considering X65 steel, uncharged material shows a flat surface, similar to the one observed for F22. Fracture propagation is not influenced by the presence of inclusions. At very high K values, just before the unstable propagation, dimples are generated around each inclusion. 325

Fracture surfaces of hydrogen charged specimens are, on the contrary, very different from uncharged material. It is difficult to distinguish between pre-crack and test-crack surfaces. Hydrogen charged specimens tested at room temperature show flat surface in the center and coarser zone at the edges. Moreover, it is possible to distinguish two regions, corresponding to the two propagation stages observed in Fig. 3. In the stage corresponding to the scattered section of the fatigue curve, fracture shows small brittle facets ( brittle striations ), oriented in different directions irradiating from a point; small secondary cracks are also present (Fig. 5.a). In the second propagation stage, at a crack depth where growth rate is almost constant, fracture surface is still flat with a brittle appearance and shows fan-shaped facets larger than the ones observed in the first stage of fatigue propagation. At the end of propagation, fracture becomes partially cellular (Fig. 5.b). Similar observation are found for the specimen tested at f = 1Hz. Specimens tested at T=-30 C present two regions: the first one is almost flat and presents small cells regularly distributed (Fig. 5.c); the second one, wider, is very coarse and presents feather-like fractures, completely brittle. The boundary between these areas corresponds to an increase in crack growth velocity (see Fig. 3). 40 µm 40 µm ( b. 40 µm ( c. a. Figure 5: SEM observations of X65 specimens a. hydrogen charged, T = 23 C and f = 10Hz: midthickness at a 0 + 2 mm ( K~22 MPa m); b. hydrogen charged, T = 23 C and f = 10Hz: midthickness at a 0 + 7.5 mm ( K~40 MPa m); c. hydrogen charged, T = -30 C and f = 10Hz: midthickness at a 0 + 1 mm ( K~20 MPa m). CONCLUSIONS The effect of hydrogen on the fatigue crack growth rate of two steels, widely used in oil pipelines, has been investigated varying test frequency and temperature. The following conclusions can be drawn: - fatigue crack propagation tests on uncharged specimens at room temperature demonstrated that F22 shows typical fatigue behaviour of steels following the Paris law, while X65 shows two different slopes. Temperature had limited influence on fatigue behaviour, while frequency has no effect; - when specimens are charged with hydrogen the crack growth rate increases up to two orders of magnitude. At the same K values, da/dn increases when temperature increases or frequency decreases; - crack growth rate is less dependent (lower m value) or even independent on K: most of fatigue curves for hydrogen charged materials presented a well-defined horizontal plateau; - fractographic examination helped in understanding the fracture mechanisms occurring in hydrogen charged specimens. 326

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