LATTICE DILATION IN A HYDROGEN CHARGED STEEL

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1 Copyright JCPDS - International Centre for Diffraction Data 23, Advances in X-ray Analysis, Volume ISSN LATTICE DILATION IN A HYDROGEN CHARGED STEEL Guiru L. Nash 1, Hahn Choo 2, Philip Nash 3, Luc L. Daemen 4 and Mark A. M. Bourke 5 1. Electro-Motive Division, GM, 931 W. 55 th Street, LaGrange, IL Materials Science and Engineering, University of Tennessee, Knoxville, TN Thermal Processing Technology Center, Illinois Institute of Technology, Chicago, IL LANSCE-12, Los Alamos National Laboratory, Los Alamos, NM MST-8, Los Alamos National Laboratory, Los Alamos, NM ABSTRACT The effect of hydrogen charging on the lattice dilation in an ASTM A71-HSLA steel specimen was studied using neutron diffraction. First, spatially-resolved strain measurements were made on a pre-cracked steel double cantilever beam specimen under a constant crack opening displacement (COD). Measurements were made at the crack tip region for in-plane and throughthickness lattice strains. Then, the specimen, under the same applied COD, was electrochemically charged with hydrogen using dilute sulfuric acid and arsenic trioxide as a promoter to study the effect of hydrogen on the lattice dilation. A significant increase in the lattice strain was observed in the hydrogen charged specimen compared to the mechanical-loadonly specimen. INTRODUCTION Recent observations of a compliance drop during fatigue tests when the environment was changed from hydrogen charging to air in pre-cracked fatigue samples imply that a relaxation of the elastic modulus occurs [1]. Compliance is influenced by the effective modulus of material as observed during dynamic compliance measurements [2]. The effect has been successfully modeled by a finite element model (FEM) by assuming a modulus relaxation of about 25% in a region up to 3 mm ahead of the crack tip [3]. The modulus relaxation is postulated to occur due to the stress gradient assisted flux of hydrogen to the crack tip under loading. This phenomenon is usually referred to as the Gorsky effect [4]. The hydrogen concentration needed to produce the lattice dilation necessary for the large drop in modulus is several orders of magnitude larger than the equilibrium solubility typically achieved in charging experiments. The increase in hydrogen content at the crack tip causes an anelastic lattice dilation, which consequently results in a reduced elastic modulus. Such anelastic behavior is characterized by a relaxation time. In this case the relaxation time is of the order of.1 seconds and the phenomenon is therefore of importance in low frequency fatigue of structural materials used in ships, chemical plants and off-shore oil wells. This proposed explanation for the hydrogen embrittlement effect in steels provides an elegant basis for the prediction of fatigue crack growth rates in such structures. The objective of this work is to experimentally study the effect of hydrogen charging on the lattice dilation near and far from the crack tip in an ASTM A71-HSLA steel specimen using

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3 Copyright JCPDS - International Centre for Diffraction Data 23, Advances in X-ray Analysis, Volume ISSN neutron diffraction. In this paper, we present the measured residual strain associated with the fatigue pre-crack, the effect of increased crack opening displacement (COD) on the lattice strain near the crack tip, and the effect of hydrogen charging on the lattice strain. Specific questions regarding the effect of hydrogen charging include whether the increase in the lattice parameter could be measured, and also if the stress gradient near crack tip would affect the hydrogen concentration, and in turn, the lattice dilation. EXPERIMENTAL DETAILS Material The material used in this study was an ASTM A71 high strength low alloy (HSLA) steel. The sample was austenitized at 899 C for 3 minutes and then water-quenched. Then, the sample was precipitation hardened by aging at 598 C for 3 minutes followed by furnace cooling. The microstructure consists of the tempered martensite and polygonal-ferrite. Then, a contoured double cantilever beam (CDCB) specimen (approximately 152 mm wide x 165 mm tall x 12.7 mm thick) was machined from the steel plate. A crack guide groove was machined on both sides of the sample. More details on the geometry of the specimen and the material used in this study can be found in [3]. The CDCB specimen was pre-cracked by fatigue loading using an MTS machine. The CDCB specimen was pre-fatigued to a crack length of 25.7 mm (from the load line to the crack tip). The crack length was determined based on the relationship between the crack 6 length and compliance which is characterized as follows: C = a, where C is the compliance (mm/n), and a is the crack length (mm). Spatially resolved neutron diffraction measurements The elastic lattice strains in the CDCB specimen were measured using the Neutron Powder Diffractometer (NPD) at the Manuel Lujan Jr. Neutron Scattering Center. Neutron diffraction allows strain probing deep inside the material due to the typically large depth of penetration. Therefore, it is an effective, non-destructive technique for measuring internal strains [5]. Details of the spatially resolved lattice strain measurements using neutron diffraction are described in [5] and only brief description is given here. The internal lattice strains in the pre-cracked CDCB specimen were measured as a function of distance from the crack tip under three conditions: (1) as-received, (2) under applied constant COD, and (3) under hydrogen charged condition with applied constant COD. First, spatially resolved residual strain measurements were made on a fatigue pre-cracked steel CDCB specimen with the diffraction gauge volume of 15 mm 3 defined in the center of the specimen as shown in Figure 1 (a). The incident neutron beam was defined using boron nitride apertures with horizontal and vertical openings of 3 mm and 2 mm, respectively. The diffracted beam was defined using a radial collimator with opening of 2.5 mm. The specimen translation and alignment capabilities of the NPD instrument were used for accurate positioning of the sample at varying distances from the crack tip. Measurements were made at the crack tip region for in-plane (IP) and through-thickness (TT) strain components, as shown in Figure 1 (a), at about 1mm interval near the crack tip up to 5mm, and then at 5mm interval up to 3mm away from the crack tip. Figure 1 (b) shows the measurement positions near the crack tip. About 3

4 Copyright JCPDS - International Centre for Diffraction Data 23, Advances in X-ray Analysis, Volume ISSN Incident beam slits 3mm 2mm (a) TT IP 165 mm 152 mm 152 mm 12.7 mm Radial collimation 2.5mm of diffracted beam Gauge volume crack tip Figure 1. (a) Neutron diffraction strain measurement set up, and (b) Measurement positions. (b) notch minutes were taken for each measurement. The lattice parameters of bcc steel were obtained by Rietveld analysis [6] of the measured diffraction patterns. Then, lattice strains, ε, were determined at each measurement position using; ε = ( a i a ) / a, where a i and a are the lattice parameter of strained and unstrained conditions, respectively. Second, a constant COD was applied to the specimen using a specially designed fixture and the strain measurements were conducted using the same method described above. Finally, the specimen, under a constant COD, was electrochemically charged with hydrogen to study the effect of hydrogen on the lattice dilation, Figure 2. We used deuterated dilute sulfuric acid and arsenic trioxide as a promoter (5N H 2 SO mg/l As 2 O 3 ). A graphite bar was used as a counter electrode, and platinum was used as a reference electrode. A potentiostat was used (+) Potentiostat to apply a current to the sample. The specimen was charged under 12 ma/cm 2 (-) for 2 hours. After charging, the specimen was taken out of (ref) Specimen the solution, but still maintaining the constant Mechanical COD, and the strain measurements were Loading performed using the same method as above. RESULTS AND DISCUSSION Residual strains in a fatigue pre-cracked specimen Figure 3 shows the measured residual lattice strain in the as-received, fatigue pre-cracked steel specimen as a function of the distance Figure 2. Schematic diagram of the hydrogen charging set up. Neutron Diffraction

5 Copyright JCPDS - International Centre for Diffraction Data 23, Advances in X-ray Analysis, Volume ISSN from the crack tip. The specimen was 25 neither mechanically loaded nor 2 charged with hydrogen. The 15 1 measurements were carried out for inplane (IP) and through-thickness (TT) 5 strain component as shown in Figure 1. The IP strain is in tension with the -5-1 maximum strain of about 225 µε (i.e., x ) at approximately 2 mm from -2 the crack tip. The lattice strain seems In-Plane -25 Through-Thickness to decrease moving away from the -3 crack tip. The TT strain is in compression and the strain profile cannot be clearly defined. However, Figure 3. Measured residual strains in the as-received, fatigue pre-cracked specimen as a function of the the CDCB specimen is under plane distance from the crack tip. Both in-plane (IP) and strain condition with the thickness of through-thickness (TT) strains components are shown mm and yield stress of 613 MPa, which may explain why the strain profile is essentially constant in the TT direction. These measurements, however, provide a base line of lattice strain in the as-received condition. Lattice Strain (x1-6 ) Lattice strain under a constant COD Spatially resolved strain measurements were made on a pre-cracked CDCB specimen under a constant COD. Figure 4 shows the internal lattice strain in the specimen under two different CODs applied during the neutron diffraction measurements. The sample was not charged with hydrogen. Figure 4 (a) compares the IP strains in the specimen, when the COD was increased from, to.166mm (applied K of 5 MPa m, or load of 655 lbs), and.333mm (K of 1 MPa m, or load of 131 lbs). The IP strain is tensile reflecting the crack opening displacement applied in the in-plane direction. The results shown in Figure 4 are consistent with the results Lattice Strain (x1-6 ) As-received K=5 K=1 (a) IP Lattice Strain (x1-6 ) As-received K=5 K=1 (b) TT Figure 4. Lattice strains measured under various CODs as a function of the distance from the crack tip. (a) IP strains, and (b) TT strains.

6 Copyright JCPDS - International Centre for Diffraction Data 23, Advances in X-ray Analysis, Volume ISSN shown in the ref [7]. In the IP direction, increasing COD increased the lattice strain. However, the increase seems to be confined to the near crack tip region. At maximum, the increase is about 25 µε at around 2.5 mm away from the crack tip, when the COD is increased from to.333mm. In the TT direction, Figure 4 (b), the change is not substantial. The given increase in the COD does not significantly affect the through-thickness strains due to the plane strain condition of the CDCB specimen. Effect of hydrogen charging The lattice strain increase associated with the hydrogen charging of the sample was observed. The sample was under the same COD as the previous case and spatially resolved neutron diffraction measurements were made in the same way as for the previous sample. Figure 5 shows the changes in internal lattice strain near the crack tip when the sample was charged with hydrogen under the applied COD of.166 mm (K=5 MPa m). The IP lattice strain, shown in Figure 5 (a), increased significantly. It shows an overall increase of about 6 µε. The throughthickness lattice strain increased towards the tensile direction. It shows an overall increase of about 3~4 µε. A similar trend was observed with the COD of.333 mm, Figure 6. The hydrogen charging generated a large overall increase of about 6 µε in both in-plane and through-thickness directions. Therefore, in both cases (K=5 or 1) shown in Figures 5 and 6, the hydrogen charging resulted in a hydrostatic lattice dilation of about 6µε. The effect of the sign of pre-existing stress, i.e. whether tensile (IP) or compressive (TT), did not affect the dilation. This value of strain implies a substantial hydrogen concentration both at the crack tip and along the crack plane, in excess of the equilibrium amount expected in steel in the absence of a stress [8]. Since there is a stress gradient associated with both the crack tip and the crack guide grooves machined on both surfaces of the sample, some flux of hydrogen into this region is expected based on the Gorsky effect. The results, however, do not indicate that this flux results in a hydrogen concentration that is proportional to stress gradient as might be expected. One possible explanation for this is that there is saturation of hydrogen sites, resulting in the inability to accept further hydrogen into the stressed region. Additional work is being performed to understand this result. SUMMARY The as-received sample showed a small (< 25µε) tensile strain in the in-plane direction, and a small (< 15µε) compressive strain in the through-thickness direction. Increasing the COD from,.166, to.333mm (K=, K=5 to K=1 MPa m) did not change the lattice strain significantly. The largest change was observed in the IP direction very near the crack tip (~3mm) when a COD of.333mm (K=1) was applied. A significant (~6µε) increase in the lattice strain was observed in the hydrogen charged specimen. Both in-plane and through-thickness strains move towards the tensile direction indicating hydrostatic lattice volume increase. This increase is due to an enhanced hydrogen concentration in the crack plane resulting from a flux of hydrogen to the region. Although this flux results from the presence of a stress gradient, the enhancement in hydrogen concentration does not appear to be a function of the magnitude of the stress gradient.

7 Copyright JCPDS - International Centre for Diffraction Data 23, Advances in X-ray Analysis, Volume ISSN Lattice Strain (x1-6 ) no hydrogen hydrogen charged (a) IP Lattice Strain (x1-6 ) no hydrogen hydrogen charged (b) TT Figure 5. Lattice strains measured from the hydrogen charged specimen under the applied K of 5 MPa m. (a) IP strains, and (b) TT strains. Lattice Strain (x1-6 ) no hydrogen hydrogen charged (a) IP Lattice Strain (x1-6 ) no hydrogen hydrogen charged (b) TT Figure 6. Lattice strains measured from the hydrogen charged specimen under the applied K of 1 MPa m. (a) IP strains, and (b) TT strains. ACKOWLEDGEMENTS This work benefited from the use of the Los Alamos Neutron Science Center (LANSCE) at the Los Alamos National Laboratory. This facility is funded by the US Department of Energy under Contract W-745-ENG-36.

8 Copyright JCPDS - International Centre for Diffraction Data 23, Advances in X-ray Analysis, Volume ISSN REFERENCES [1] G.L. Nash and R.P. Foley, submitted to Fatigue Fract. Engng Mater. Struct. [2] F. Schlaet, Inter. J. Fracture, vol. 19, p. 37 (1982). [3] G.L. Nash, Ph.D. Thesis, Illinois Institute of Technology, May [4] J. Philibert, Les Editions de Physique, vol. 385, p.16 (1991). [5] D.W. Brown, R. Varma, M.A.M. Bourke, T. Ely, T.M. Holden, S. Spooner, Materials Science Forum, 44-4, pp (22). [6] H.M. Rietveld, J. Appl. Cryst., vol.2, pp (1969). [7] M. Ceretti, C.A. Hippsley, M.T. Hutchings, A. Lodini, C.G. Windsor, Physica B, vol , pp (1997). [8] V. M. Sidorenko and I. I. Sidorak, Fiz. Khim. Mekhan. Mater., vol. 9(1), pp (1973).