Corrosion Fatigue of Type 304 Stainless Steel and Inconel™ 600 in a Boiling (140 °C) Concentrated Caustic Solution

Corrosion fatigue crack growth rate tests using compact tension specimens of Type 304 stainless steel and smooth bar corrosion fatigue tests of Inconel Alloy 600 were run in boiling (140 °C) 17.5M NaOH solution. Contrary to earlier results from smooth bar Type 304 specimens, the measured crack growth rates of Type 304 did not show a high accelerative environmental effect; there was also no significant effect of cyclic frequency (0.1 to 10 Hz),R ratio, or electrochemical potential. For Inconel 600 the boiling caustic solution increased fatigue life relative to life in air, and anodic passivation was beneficial. Sensitization, found beneficial in sodium hydroxide, was detrimental in air in spite of similar failure morphologies. A delay of crack initiation is proposed as primarily responsible for the effects of the solution and sensitization.     

A comparative evaluation of low-cycle fatigue behavior of type 316LN base metal, 316 weld metal,and 316LN/316 weld joint

A comparative evaluation of the low-cycle fatigue (LCF) behavior of type 316LN base metal, 316 weld metal, and 316LN/316 weld joints was carried out at 773 and 873 K. Total strain-controlled LCF tests were conducted at a constant strain rate of 3 × 10⁻³ s⁻¹ with strain amplitudes in the range ±0.20 to ±1.0 pct. Weld pads with single V and double V configuration were prepared by the shielded metal-arc welding (SMAW) process using 316 electrodes for weld-metal and weld-joint specimens. Optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) of the untested and tested samples were carried out to elucidate the deformation and the fracture behavior. The cyclic stress response of the base metal shows a very rapid hardening to a maximum stress followed by a saturated stress response. Weld metal undergoes a relatively short initial hardening followed by a gradual softening regime. Weld joints exhibit an initial hardening and a subsequent softening regime at all strain amplitudes, except at low strain amplitudes where a saturation regime is noticed. The initial hardening observed in base metal has been attributed to interaction between dislocations and solute atoms/complexes and cyclic saturation to saturation in the number density of slip bands. From TEM, the cyclic softening in weld metal was ascribed to the annihilation of dislocations during LCF. Type 316LN base metal exhibits better fatigue resistance than weld metal at 773 K, whereas the reverse holds true at 873 K. The weld joint shows the lowest life at both temperatures. The better fatigue resistance of weld metal is related to the brittle transformed delta ferrite structure and the high density of dislocations at the interface, which inhibits the growth rate of cracks by deflecting the crack path. The lower fatigue endurance of the weld joint was ascribed to the shortening of the crack initiation phase caused by surface intergranular crack initiation and to the poor crack propagation resistance of the coarse-grained region in the heat-affected zone.     

Fatigue crack growth behavior in inconel 706 at 297 K and 4.2 K

The near-threshold fatigue crack growth rate (FCGR) behavior of Inconel 706 was investigated at ambient (297K) and liquid helium (4.2 K) temperatures, respectively. Specimen orientation did not affect the FCGR properties of Inconel 706. At 297 K, a significant influence of R-ratio on the rates of crack propagation was observed while at 4.2 K, the R -ratio effect was minimal. The extent of oxide-induced crack closure was shown to be insignificant in influencing near-threshold crack growth kinetics of Inconel 706 at both temperatures of 297 and 4.2 K. Roughness-induced crack closure was believed to be the dominant mechanism responsible for the influence of R -ratio on the FCGR properties of Inconel 706; this was proved quantitatively by direct crack closure measurements conducted at 297 and 4.2 K. A greater degree of roughness-induced crack closure was observed at 297 K than at 4.2 K; this correlated with the more pronounced R-ratio effect at 297 K. Decreasing the temperature from 297 to 4.2 K decreased the growth rates of fatigue cracks in Inconel 706. The effect of temperature on crack propagation behavior increased with increasing R-ratio. Crack closure could not rationalize this temperature effect. Moreover, the increase in material strength or Young's modulus on cooling from 297 to 4.2 K could not totally account for the influence of temperature on the near-threshold FCGR properties. Dislocation dynamics appears to offer a qualitative explanation for this temperature effect.     

Residual stress measurements by x-ray diffraction in the ferritic-austenitic interface weldment

A mathematical model of calculating residual stresses at the weld interface of ferritic-austenitic steels has been developed. The Kurdjumov-Sachs orientation relationship (110)_bcc[111]// (11 l)_fcc[011] was determined at the interface of the transition joint. The resultant bcc-fcc lattice misfit gives rise to significant residual stresses. The performed X-ray analysis establishes macro- and microstress profiles extending up to 22 mm and 55 μm, respectively. The profiles indicate the development of compressive and tensile stresses on either side of the weld interface. With respect to this, tensile stresses in increasing sequence were computed from the parent metal toward the interface and compressive stresses were determined from the interface inward toward the weld bead in decreasing sequence. A remarkable stress discrepancy between the two profiles was observed, with the macrostresses falling in the range of +185 to −245 MPa and the microstress level ranging between +340 and −420 MPa. While the developed interfacial residual stresses are due to the difference in the bcc-fcc lattice parameters, the discrepancy observed in the determined stress level has its origin in the varying percentage of the two phases involved within a narrow mixing zone at the weld interface.     

Near-Neutral pH Stress Corrosion Cracking Susceptibility of Plastically Prestrained X70 Steel Weldment

The application of strain-based design for pipelines requires comprehensive understanding of the postyield mechanical behavior of materials. In this article, the impact of plastic prestrain on near-neutral pH stress corrosion cracking (SCC) susceptibility of welded X70 steel was investigated with a slow strain rate tensile (SSRT) test. Generally, plastic prestrain reduces the SCC resistance in various welded zones. The SCC susceptibility of the test materials can be put in the following order: heat-affected zone (HAZ) > weld metal (WM) > base metal (BM). Fractographic analysis indicates that there are two cracking modes, mode I and mode II, during SSRT tests. Mode I cracks propagate along the direction perpendicular to the maximum tensile stress, and mode II cracks lie in planes roughly parallel to the plane where the maximum shear exists. The SCC of the BM is governed by mode I cracking and fracture of the HAZ, and the WM is dominated by mode II cracking. Damage analysis shows that the detrimental impact of plastic prestrain on the residual SCC resistance cannot be evaluated with the linear superposition model. A plastic prestrain sensitivity, a material constant independent of plastic prestrain, is proposed to characterize the susceptibility of SCC resistance to plastic prestrain, and it increases with the SCC susceptibility of the steels. The enhanced SCC susceptibility caused by plastic prestrain may be related to an increase in yield strength. The correlation of the ratio of the reduction in area in NS4 solution to that in air (RA _SCC/RA _air) with the yield strength is microstructure dependent.     

Effect of strain rate on stress corrosion cracking in duplex type 304 stainless steel weld metal

The influence of strain-rate on the stress-corrosion cracking properties of wholly austenitic Type 304 base metal and duplex austeno-ferritic Type 304 weld metal in boiling MgCl₂ was investigated using constant extension rate tensile testing techniques. Transgranular SCC in both base and weld metals is preferred at low strain-rates, while intergranular cracking in the base metal and interphase cracking along the austenite-ferrite interface in the weld metal are preferred at higher strain-rates. Promotion of the intergranular stress-corrosion cracking in the base metal and “interphase-interface” stresscorrosion cracking in the weld metal with increases in strain-rate may be mechanistically analogous. Stress-induced alterations in the grain or interphase boundary defect structure may make these regions preferentially susceptible to dissolution. W. A. BAESLACK III, Lt., USAF, formerly with Rensselaer Polytechnic Institute, Troy, New York     

Fracture mechanics studies of thermal mismatch using a four-point bending specimen

Layered composites develop thermal residual stresses during cooling from processing temperature to room temperature. The thermal stresses reduce fracture toughness data measured in four-point bending tests. To obtain a material parameter characterizing the interface fracture toughness the measured data must be corrected for the influence of thermal stresses. Thermal stresses often lead to kinking of an interface crack out of its initial plane. This tendency can be quantified by two parameters: (i) the ratio G/G₀ of the energy release rate of the kinked crack G and the energy release rate of the interface crack G_O and (ii) the ratio of the local stress intensity factors at the tip of the interface crack, K_II/K_I. Both quantities have been computed for a variety of material combinations using the finite element method. They are found to be strongly affected by thermal and elastic mismatch. Fracture experiments have been performed using brittle glass/glass composites with different thermal mismatched. The obtained fracture toughness values and crack deflection angles have evaluated on the basis of the numerical results. Measured and calculated kinking angles are in excellent agreement. The contribution of residual thermal stresses to the interface fracture toughness K_c has been elaborated.     

Influence of residual stresses and loading frequencies on corrosion fatigue crack growth behavior of weldments

The effect of residual stresses and loading frequencies on corrosion fatigue crack growth behavior under synthetic seawater with a free corrosion potential was examined using center-cracked tension (CCT) and single edge-cracked tension (SECT) specimens machined from mild steel butt-welded joints and the parent material. A series of fatigue crack growth tests were carried out with a sinusoidal loading wave form at a stress ratio of 0.05 with a loading frequency of 0.017 to 6.7 Hz. The results show that the crack growth resistance of a weld metal in the SECT specimen is higher than that in the CCT specimen regardless of testing conditions. The discrepancy is attributed to the differences in residual stress distribution at the crack tip in the two specimen geometries. The crack growth rate of the weld metal in the CCT specimen in seawater increased with decreasing loading frequency. The acceleration of the crack growth rate may be related to the occurrence of brittle striation or cleavage due to hydrogen embrittlement. It was found that the corrosion fatigue crack growth rate of a welded joint with tensile residual stress can be predicted using the effective stress intensity factor range, which takes into account both the residual stress and the loading frequency effects.     

Environmental Crack Driving Force

The effect of environment on the crack driving force is considered, first by assuming quasistatic extension of a stationary crack and second, by use of stress corrosion cracking (SCC) crack growth rate models developed previously by this author and developed further here. A quasistatic thermodynamic energy balance approach, of the Griffith-Irwin type, is used to develop stationary crack threshold expressions, $ \tilde{J}_{\rm c} $ , which represent the conjoint mechanical and electrochemical conditions, below which stationary cracks are stable. Expressions for the electrochemical crack driving force (CDF) were derived using an analysis that is analogous to that used by Irwin to derive his “strain energy release rate,” G, which Rice showed as being equivalent to his mechanical CDF, J. The derivations show that electrochemical CDFs both for active path dissolution (APD) and hydrogen embrittlement (HE) mechanisms of SCC are simply proportional to Tafel’s electrochemical anodic and cathodic overpotentials, η _a and η _c, respectively. Phenomenological SCC models based on the kinetics of APD and HE crack growth are used to derive expressions for the kinetic threshold, J _scc, below which crack growth cannot be sustained. These models show how independent mechanical and environmental CDFs may act together to drive SCC crack advance. Development of a user-friendly computational tool for calculating Tafel’s overpotentials is advocated.     

Fatigue crack propagation through weld heat affected zones

Fatigue tests were performed on specimens containing weld heat affected zones at two orientations to the stress axis. Two heat affected zones were studied, one in Ducol W30 (a low alloy steel) and the other in mild steel. Under conditions of constant alternating and maximum stress intensity a fatigue crack only propagated at a uniform rate when it was remote from the heat affected zone. A heat affected zone which was harder than either the parent plate or weld metal was found to reduce crack propagation rates by a factor of up to 2 by restricting the plastic zone size around the crack tip. The changes in crack propagation rate could not be related uniquely to the conditions of the material immediately adjacent to the crack tip. Furthermore, the shape of the plastic zone was found to influence the direction of the propagation of a fatigue crack which always deviated toward regions of lower flow stress. A crack was never found to follow the interface between the weld metal and the parent metal heat affected zone because the flow stresses were not the same on either side of the interface. There was no difference in crack propagation mechanism between the parent plate and its heat affected zone for the stress conditions imposed. Formerly with Central Electricity Research Laboratories, Materials Division, Leatherhead, Surrey, England     

Dissolution Condensation Mechanism of Stress Corrosion Cracking in Liquid Metals: Driving Force and Crack Kinetics

Stress corrosion cracking (SCC) in aqueous solution is driven by exothermic reactions of metal oxidation. This stimulus, as well as classical mechanisms of SCC, does not apply to SCC in liquid metals (LMs). In the framework of the dissolution-condensation mechanism (DCM), we analyzed the driving force and crack kinetics for this nonelectrochemical mode of SCC that is loosely called “liquid metal embrittlement” (LME). According to DCM, a stress-induced increase in chemical potential at the crack tip acts as the driving force for out-of-the-tip diffusion mass transfer that is fast because diffusion in LMs is very fast and surface energy at the solid-liquid interface is small. In this article, we review two versions of DCM mechanism, discuss the major physics behind them, and develop DCM further. The refined mechanism is applied then to the experimental data on crack velocity V vs stress intensity factor, the activation energy of LME, and alloying effects. It is concluded that DCM provides a good conceptual framework for analysis of a unified kinetic mechanism of LME and may also contribute to SCC in aqueous solutions.     

Investigation on In Situ Tensile Behavior of Superalloy Bicrystals with Different GB Misorientations

The in situ tensile behavior of nickel-base superalloy bicrystals with different grain boundary (GB) misorientations of 2 deg, 10 deg, and 16 deg was preliminarily studied. Among three kinds of bicrystals, the bicrystal with 2 deg misorientation was characterized with continuous slow strain hardening as well as crack initiation and propagation along the SB-matrix interface due to the full development of slip bands (SBs). In contrast, GBs in 10 deg and 16 deg bicrystals could effectively impede the SBs. Thus, the crack initiation occurred along the carbide/matrix interface, and more specifically cracks in 16 deg bicrystal fully propagated along the GB. Irregular GB in superalloy bicrystals consists of three types of GB segments. Among them, parallel GB is distinctly beneficial to GB strengthening and improving the yield strength of bicrystal. The statistical results showed that the proportion of parallel GB in the 10 deg bicrystal is the highest. Meanwhile, the GB in 10 deg bicrystal possesses higher resistance to crack extending, which can be attributed to its highest carbide fraction at the GB. Thus, its fracture exhibited a mixed mode of both GB and SB cracking.     

Stress Corrosion Cracking Behavior of Multipass TIG-Welded AA2219 Aluminum Alloy in 3.5?wt?pct NaCl Solution

The stress corrosion cracking (SCC) behavior of the AA2219 aluminum alloy in the single-pass (SP) and multipass (MP) welded conditions was examined and compared with that of the base metal (BM) in 3.5?wt?pct NaCl solution using a slow-strain-rate technique (SSRT). The reduction in ductility was used as a parameter to evaluate the SCC susceptibility of both the BM and welded joints. The results showed that the ductility ratio (?? _NaCl/(?? _air) was 0.97 and 0.96, respectively, for the BM and MP welded joint, and the same was marginally reduced to 0.9 for the SP welded joint. The fractographic examination of the failed samples revealed a typical ductile cracking morphology for all the base and welded joints, indicating the good environmental cracking resistance of this alloy under all welded conditions. To understand the decrease in the ductility of the SP welded joint, preexposure SSRT followed by microstructural observations were made, which showed that the decrease in ductility ratio of the SP welded joint was caused by the electrochemical pitting that assisted the nucleation of cracks in the form of corrosion induced mechanical cracking rather than true SCC failure of the alloy. The microstructural examination and polarization tests demonstrated a clear grain boundary (GB) sensitization of the PMZ, resulting in severe galvanic corrosion of the SP weld joint, which initiated the necessary conditions for the localized corrosion and cracking along the PMZ. The absence of PMZ and a refined fusion zone (FZ) structure because of the lesser heat input and postweld heating effect improved the galvanic corrosion resistance of the MP welded joint greatly, and thus, failure occurred along the FZ.     

A critical evaluation of the stress-corrosion cracking mechanism in high-strength aluminum alloys

Attempts have been made to elucidate the mechanism of stress-corrosion cracking (SCC) in high-strength Al-Zn-Mg and Al-Li-Zr alloys exposed to aqueous environments by considering the temperature dependence of SCC susceptibility based upon the anodic dissolution and hydrogen embrittlement models. A quantitative correlation which involves the change of threshold stress intensity,K _ISCC, with temperature on the basis of anodic dissolution has been developed with the aid of linear elastic fracture mechanics. From the derived correlation, it is concluded that the threshold stress intensity decreases as the test temperature increases. This suggestion is inconsistent with that predicted on the basis of hydrogen embrittlement. It is experimentally observed from the Al-Zn-Mg and Al-Li-Zr alloys that the threshold stress intensity,K,_ISCC, decreases and the crack propagation rate,da/dt, over the stress intensity increases with increasing test temperature. From considering the change in SCC susceptibility with temperature, it is suggested that a gradual transition in the mechanism for the stress-corrosion crack propagation occurs from anodic dissolution in stage I, where the crack propagation rate increases sharply with stress intensity, to hydrogen embrittlement in stage II, where the crack propagation rate is independent of stress intensity.     

The effect of solution heat-treatment on grain boundary segregation and stress-corrosion cracking of Al-Zn-Mg alloys

The relationship between susceptibility to stress-corrosion cracking (SCC) and grain boundary (GB) chemistry was investigated to elucidate the SCC mechanism in two Al-Zn-Mg alloys (Al-6.92Zn-2.85Mg-0.13Zr, Al-4.40Zn-3.70Mg; wt pct). Grain boundary chemistry was measured by Auger electron spectroscopy (AES) fromin situ fractures on the actual GB surface. The fractures were produced by pre-exposing specimens to water-vapor-saturated air, which induced hydrogen embrittlement of the GB. Susceptibility to SCC was varied by changing either solution heat-treatment temperature (SHT) or aging time. The SCC susceptibility, as measured by plateau crack velocity or reciprocal time-to-failure in a chromate-inhibited brine solution, was shown to decrease with increasing SHT and in general, to decrease with increasing aging time. “Free Mg,”i.e., unbound in MgZn₂ precipitates and present in the region between two grains, was present on all boundaries, as shown by AES, but no correlation was observed between free Mg concentration and SCC susceptibility. Possible explanations for these results are discussed.     

Crystallographic fatigue crack growth in incompatible Aluminum Bicrystals: Its Dependence on Secondary Slip

The secondary slip behavior ahead of crystallographic fatigue cracks and its effect on the crack growth near the grain boundaries (GBs) in\([12\bar 1]\) tilt nonsymmetrical aluminum bicrystals under constant cyclic stress amplitude have been systematically examined. The displacement field ahead of short crack tips near the interfaces in two specimens has been measured by using a microfiducial grid technique. It has been observed that the critical persistent slip band (PSB) ahead of a short crack tip near the GB in a middle misoriented bicrystal was able to develop as long as the primary one and resulted in a temporary stage II growth. As a longer crystal- lographic crack grew into the grain boundary affected zone (GBAZ), activation of the critical slip ahead of the crack front and crack branching along the critical PSB occurred in all groups of the aluminum bicrystals, which reveals a crucial role of the critical slip in increasing the crack opening and triggering the slip in the adjacent grain. On the other hand, cross slip became the dominant slip mode ahead of the crystallographic crack front near the GB in a bicrystal of larger misfit angles and drove the crack along the cross PSB, a steep path with a remarkably high growth rate, until it propagated into the GBAZ. The resultant stress on the secondary slip system ahead of a crack front near the interface contributed by the internal stress due to both intergranular and intragranular incompatible strain, as well as the enhanced crack tip stress, has been evaluated and rationalizes the activation of the secondary slip systems.     

Time-Dependent Crack Growth Thresholds of Ni-Base Superalloys

A micromechanical model has been developed for predicting the time-dependent crack growth threshold and its variability by considering oxide formation or cavity formation ahead of an elastic crack subjected to a sustained load at a stress intensity factor, K, at elevated temperatures in air. It is demonstrated that stress relaxation associated with a volume-expansion process such as the formation of creep cavities or oxides with a positive transformation strain can induce residual stresses at the tip of the elastic crack. The near-tip residual stresses must be overcome by the external load, thereby instigating a growth threshold, K _th, for the onset of time-dependent crack growth. This micromechanical framework provides the basis for developing appropriate predictive models for the time-dependent crack growth thresholds associated with several damage processes, including (1) oxidation-assisted intergranular crack growth, (2) K-controlled creep crack growth along an intergranular path, and (3) stress corrosion cracking. The micromechanical threshold models have been utilized to predict the time-dependent crack growth thresholds of a variety of Ni-base superalloys. The material parameters that contribute to the variability of the time-dependent crack growth thresholds have been identified and related to variations of mixed oxides or creep cavities formed near the crack tip. A size scale effect is also predicted for the transformation toughening phenomenon, which is largest at or below K _th but diminishes at increasing K levels above the threshold. Finally, the micromechanical models are utilized to identify means for suppressing time-dependent crack growth in Ni-base alloys.     

Creep crack growth behavior of several structural alloys

Creep crack growth behavior of several high temperature alloys, Inconel 600, Inconel 625, Inconel X-750, Hastelloy X, Nimonic PE-16, Incoloy 800, and Haynes 25 (HS-25) was examined at 540, 650, 760, and 870 °C. Crack growth rates were analyzed in terms of both linear elastic stress intensity factor and J*-integral parameter. Among the alloys Inconel 600 and Hastelloy X did not show any observable crack growth. Instead, they deformed at a rapid rate resulting in severe blunting of the crack tip. The other alloys, Inconel 625, Inconel X-750, Incoloy 800, HS-25, and PE-16 showed crack growth at one or two temperatures and deformed continuously at other temperatures. Crack growth rates of the above alloys in terms ofJ* parameter were compared with the growth rates of other alloys published in the literature. Alloys such as Inconel X-750, Alloy 718, and IN-100 show very high growth rates as a result of their sensitivity to an air environment. Based on detailed fracture surface analysis, it is proposed that creep crack growth occurs by the nucleation and growth of wedge-type cracks at triple point junctions due to grain boundary sliding or by the formation and growth of cavities at the boundaries. Crack growth in the above alloys occurs only in some critical range of strain rates or temperatures. Since the service conditions for these alloys usually fall within this critical range, knowledge and understanding of creep crack growth behavior of the structural alloys are important.     

Microstructural Characterization and Fatigue Performance Evaluation of MIG‐Welded Ship Hull Steel

The effectiveness of MIG welding with Argo‐shield gas & ER70S‐6 electrode in joining LRS (Grade‐B) steel was investigated through structure–property correlation of the joint region. Microstructure, tensile and fatigue properties, and mode of fracture (SEM fractograph) were correlated. Fatigue behavior has been investigated in air and sea water with thin specimen at near‐endurance stress amplitude up to 10⁵ cycles. The crack growth rate (da/dN) maintained a non‐linear relationship with logarithm of stress intensity factor range (logΔK) for the near‐threshold values of ΔK. Considerable hardness and microstructural variation was observed across the weldment. Weld with more pearlite content was found to possess higher hardness and strength than the parent steel. Though, both in weld and in parent steel, either in air or in sea water, fatigue crack propagated at very slow rate with significant intermittent crack arrest, weld provided much higher resistance to crack growth in air. However, sea water accelerated the crack growth in weld and brought it closer to that in the parent steel. The morphologically complex microstructure of weld suffered much faster crack propagation in sea water than in air. While fatigue fracture in parent steel (both in air and sea water) and weld in air was found to occur through dimple rupture via microvoid coalescence, weld in sea water exhibited a mixed mode of failure.     

Three-Orthogonal-Direction Stress Mapping around a Fatigue-Crack Tip Using Neutron Diffraction

Quantitative determination of the stress fields around the crack tip is a challenging and important subject to understand the fatigue crack-growth mechanism. In the current study, we measured the distribution of residual stresses and the evolution of the stress fields around a fatigue crack tip subjected to the constant-amplitude cyclic loading in a 304L stainless steel compact-tension (CT) specimen. The three orthogonal stress components (i.e., crack growth, crack opening, and through thickness) of the CT specimen were determined as a function of distance from the crack tip with 1-mm spatial resolution along the crack-propagation direction. In-situ neutron-diffraction results show that the enlarged tensile stresses were developed during loading along the through-thickness direction at a localized volume close to the crack tip, resulting in the lattice expansion in all three orthogonal directions during P _max. The current study suggests that the atypical plane strainlike behavior observed at the midthickness position might be the reason for the mechanism of the faster crack-growth rate inside the interior than that near the surface.