1M.A. Khattak, 2M.H. Maslan and 3M.N. Tamin
1,3Faculty of Mechanical Engineering Universiti Teknologi Malaysia
81310 UTM Skudai, Johor
3e-mail: [email protected]
2Faculty of Manufacturing Engineering Universiti Teknikal Malaysia Melaka
75450 UTeM Ayer Keroh, Melaka Abstract — Effects of absorbed hydrogen on
structure and properties of welded A516 Grade-70 steel are investigated. Emphasis is placed on ductility measure of the crack-tip plastic zone under Mode I loading. Specimens are cathodically charged in a cell with dilute sulphuric acid and corrosion inhibitor with uniform charging current density of 20 mA/cm2 and at different exposure time. Results indicate a change from coarse- to fine-grained microstructures in the weld region and heat affected zone (HAZ) of hydrogen-charged specimen. Well-defined ferrite-pearlite bands in the base metal are transformed into coarse-grain structure. Hardness variation along radial distance indicates higher values towards the center of the bar, possibly due to faster diffusion rate but limited solubility of hydrogen. Load-COD responses indicate that slow, stable crack propagation occurred in both base metal and HAZ. The measured provisional fracture toughness, KQ is higher for HAZ than that for the base metal. The toughness values decreases significantly for the initial three hours of hydrogen charging. The tensile fracture region in the immediate fatigue pre-crack tip forms a triangular (rough) zone due to limited constraint to free surface deformation in the thin specimen. Fracture surface of HAZ is dominated by intergranular fracture with localized cleavage facets.
1. Introduction
Chemical reactor vessels and pipelines are commonly constructed using welded steels and stainless steel liners. In oil refineries and chemical plants these steel vessels operate in corrosive environments where high concentration of hydrogen sulphide is present. The operating temperature typically ranges from -29 to 427 °C. Prolonged exposure of the steel to these conditions could lead to deleterious effects such as embrittlement, loss of toughness and creep rupture of
the steel [3]. Failures of pressure vessels and pressure piping related accidents are often fatal and involved loss of capital investment [eg. 1-2].
Previous research has identified possible mechanisms of hydrogen reaction embrittlement in steels, namely hydrogen blistering, internal embrittlement and environmental embrittlement. The presence of internal hydrogen produces a plastic loss in austenitic stainless steel [4]. At high temperature and pressure, this internal hydrogen facilitates de-carburization, promotes intergranular cracking and forms blistering in steels [5]. The pressure of molecular hydrogen causes the formation of brittle cracks in hydrogenated low carbon steel [6]. Constant extension-rate test on Type 216 steel samples pre-exposed to concentrated synthetic ground water at 80-150 oC showed evidence of environmental (hydrogen)-assisted cracking and moisture-induced ductility loss [7]. The susceptibility of steels with different heat treatment to hydrogen embrittlement was evaluated in terms of mechanical properties [8]. While tensile strength, yield and hardness increase with increasing hydrogenation time up to five hours, significant reduction in ductility was measured.
In a welded joint, the application of immense heat to fuse the base plate and weld metal (electrode) for a strong permanent joint resulted in a heterogeneous heat affected zone (HAZ). This zone is a common source for defects such as hard inclusions, blisters by trapped gas and microcracks that developed during fast cooling of the welded joint. Exposure to absorbed hydrogen can cause deleterious effects including the formation and propagation of brittle cracks. In controlled laboratory experiment hydrogen can be introduced in the specimen by gaseous hydrogen charging in an oxygen-free copper chamber with tantalum hydride at temperature in excess of 700 oC [9]. Others use high-pressure (20-35 MPa) hydrogen
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autoclave at 350 oC [10]. In the electrochemical method the metal specimen is soaked in dilute solution of sulphuric acid with a corrosion inhibitor. A controlled current is supplied to the cathodically-charged sample in the cell. The concentration of absorbed hydrogen is quantified in part-per-millon (ppm) or represented by hydrogen charging time duration.
The objective of this study is to establish effects of absorbed hydrogen on microstructure and properties of welded A516 Grade 70 steel. Emphasis is placed on ductility measure of the crack-tip plastic zone under Mode I loading.
2. Materials and Experimental Procedures
The material employed in this study is an ASTM A516-Grade 70 steel. The 16-mm thick pre-fabricated plates were butt welded to produce the curvature of a cylindrical vessel wall. The welded joint was produced by multiple-pass submerged arc welding (SAW) process to ANSI/AWS specifications. Post weld heat treatment (PWHT) was carried out at 620 oC. The different regions of the welded zone, namely weld metal, heat affected zone (HAZ) and the base metal are illustrated in Figure 1(a). The chemical composition of the base plate is shown in Table 1.
Tensile and yield strength of welded samples in the as-received condition is 480 and 360 MPa, respectively.
Table 1.
Chemical composition of A516 Grade 70 steel (wt. %)
Compact tension (CT) specimens were fabricated using wire cutting process to minimize local heating.
A sharp notch is machined to facilitate fatigue pre-cracking in the HAZ. Surface porosity and irregularities were removed by fine grinding. Fracture toughness test was conducted according to ASTM E399 procedures. The 10-mm thickness of the CT specimens is insufficient for establishing plane strain
fracture toughness of the material, thus provisional fracture toughness values, KQ were reported. Hardness measurements across the fusion zone of the welded joint were made using Vicker’s hardness tester with 10-kg indentation load. Microstructure study was performed on optical micrographs while scanning electron microscope (SEM) was employed in fractographic analysis.
Hydrogen charging of the polished CT specimens was carried out in an electrolytic cell with a stainless steel rod as anode and the A516 steel as cathode. The specimen was mounted in the cell filled with dilute sulphuric acid solution (0.5M H2SO4 solution) containing 1.97 gm of arsenic trioxide (As2O3) as corrosion inhibitor. A constant current density of 20 mA/cm2 was maintained for all the specimens. After hydrogen charging to preset time duration, the specimens were removed from the cell and rinsed with distilled water.
3. Results and Discussion
Effects of absorbed hydrogen in steel and welded steel joint are presented and discussed in terms of microsructural changes, measured hardness and toughness values and crack growth behavior under tensile load.
3.1 Microstructures
Microstructures of the various phases of the welded A516 steel in the as-received condition are shown in Figure 1(b). The base metal displays an aggregate of well-defined ferrite-pearlite bands. The HAZ is characterized by a network of equiaxed grain boundary ferrite with colonies of fine-grain acicular ferrite while the weld metal consists of coarse-grained Widmanstatten ferrite microstructure.
Microstructures of similar welded joint after a 3-hour hydrogen charging is compared in Figure 1(c). The base metal is transformed into a coarse-grained ferrite-pearlite structure. The HAZ showed coalescence of pearlitic and ferritic regions. Fine-grained Widmanstatten structure of the weld metal is noted.
C Mn Si Cr Cu Mo S P 0.263 1.12 0.46 0.02 1.41 0.01 0.01 0.004
Si Ni V Fe 0.015 0.02 0.005 Bal.
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Figure 1. (a) Cross-section of the fusion zone of the welded joint, (b) Microstructures of base metal (BM), HAZ and weld region (WM) of as-received sample and (c) microstructures for a 3-hour hydrogen charged sample. Mag. 10X.
3.2 Hardness Measurements
Hydrogen absorption characteristics of steel is represented by the variation of hardness in the cross-section of a 32-mm diameter steel rod (0.30 wt.%C) subjected to 6-hour hydrogen charging, as shown in
Figure 2. It is noted that the central portion of the cross-section is harder (HV = 250) than the exposed surface of the material (HV = 200). This is likely due to a faster diffusion rate of hydrogen into the material but hindered by limited solubility of the hydrogen atoms in the matrix. Excess hydrogen reacts to form molecular hydrogen that increases local pressure in the grain boundary region leading to increased hardness and brittleness of the material. In a welded joint, the residual tensile stress induced in the fusion zone could easily results in microcrack initiation. A gradual increase in hardness towards the center of the rod reflects a single diffusion mechanism for hydrogen absorption.
Figure 2. Hardness profile along radial locations of 0.30 wt.%C steel rod after 6-hour hydrogen charging.
Vicker’s hardness measurements across the welded joint for as-received and 3-hour hydrogen charged specimens are compared in Figure 3. The HAZ registered highest hardness number (HV = 136 ±5) compared with the base metal (HV = 104 ±5). Scatter of the measured hardness data is primarily due to in-homogeneity of the microstructure in the welding process zone.
Figure 3. Hardness profile across the fusion zone of welded A516 steel for as-received and 3-hour hydrogen charged sample.
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It is noted that the 3-hour hydrogen charging does not produce significant change in hardness of the welded joint measured on the exposed surface. It is believed that exposure of the welded joint to longer hydrogen charging time will cause an observable increase in hardness. Such effects of increased hardness in different heat-treated 0.31 wt. %C steels were reported for hydrogen charging beyond 3-hour period [8].
3.3 Tensile Crack Growth Behavior
The effect of absorbed hydrogen on fracture toughness of the HAZ is presented in Figure 4.
Results show that the apparent fracture toughness (in plane stress condition) for HAZ is higher than that for the base metal. Since hardness of both phases is similar (see Figure 3), the difference in measured toughness is likely due to significant difference in microstructures of HAZ compared with the base metal. Absorbed hydrogen decreases the toughness of the materials with prolonged exposure time. It is worth noting that the rate of decrease in toughness due to the presence of hydrogen is faster in the HAZ for the first 3-hour of charging, as reflected in the steep (negative) slope of the curve. The continuous decrease in toughness with prolonged exposure to hydrogen favors crack propagation leading to premature failure of the material.
Figure 4. Variation of fracture toughness (KQ) of HAZ and base metal with different hydrogen charging time.
Figure 5 compares fracture surfaces in the immediate region ahead of the fatigue pre-crack tip following crack propagation in HAZ under tensile load for the as-received (Figure 5(a)) and 3-hour hydrogen charged samples (Figure 5(b)).
Fatigue pre-crack plane appeared as a smooth surface on the left side of the fractographs. The tensile fracture region forms a triangular (rough) zone due to limited constraint to free surface deformation in the thin specimen under plane stress condition. Fracture surface of HAZ is dominated by intergranular fracture with localized cleavage facets. Deep secondary cracks between the grains are also observed. Fracture features are smaller for the hydrogenated HAZ due to finer grain size than that found in the as-received sample.
(a)
(b)
Figure 5. Morphology of fracture surfaces of HAZ in the immediate region of the fatigue pre-crack tip.
(a) as-received condition and (b) 3-hour hydrogen charged sample.
69 4. Conclusions
Effects of absorbed hydrogen on properties and tensile response of fatigue pre-crack tip in HAZ of welded A516 Grade 70 steel have been investigated.
Results show that:
• The network of equiaxed grain boundary ferrite with colonies of fine-grain acicular ferrite in the HAZ coalesce to form pearlitic and ferritic regions.
• Hardness variation along radial distance in a 0.30 wt.%C steel rod indicates higher values towards the center of the rod after 6-hour hydrogen charging.
• Three-hour hydrogen charging does not produce significant change in hardness of the welded joint measured on the exposed surface.
• The rate of decrease in toughness due to the presence of hydrogen is faster in the HAZ compared to that in the base metal.
• Fracture surface of HAZ is dominated by transgranular fracture with localized cleavage facets.
Acknowledgement
This project is supported by the Ministry of Science, Technology and Innovation (MOSTI), Government of Malaysia through e-Science Fund Project No. 79058.
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