A novel approach to the determination of the threshold for stress corrosion cracking (K ISCC) using round tensile specimens

This article discusses determination of threshold stress intensity for propagation of stress corrosion cracking (K _ISCC), using circumferential notch tensile (CNT) specimens. Use of round tensile specimens is a novel and cost-advantageous approach to determination of K _ISCC. However, compliance of this specimen geometry to the constraints for application of linear elastic fracture mechanics (LEFM) has traditionally been argued, and hence this aspect is addressed in detail. The LEFM suits best the materials that undergo brittle cracking, and hence a highly brittle material, cast iron, has been selected as the test material. However, susceptibility of this material to caustic embrittlement has been established employing another technique, viz. slow strain rate testing and fractography of the specimens. Using CNT specimens, K _ISCC has been determined for the cast iron in hot caustic solutions, and the features of intergranular caustic cracking and secondary cracking have been established using scanning electron microscopy.     

Effects of texture and microstructure on the propagation of iodine stress corrosion cracks in zircaloy

An investigation of stress corrosion crack propagation in Zircaloy is performed at 300 °C in four Pa flowing iodine environment. By varying the orientation of fracture mechanics specimens, the effect of crystallographic texture, heat treatment, and microstructure onK _ISCC is studied. Texture is found to have a strong effect on bothK _ISCC and the fracture path. As the resolved fraction of basal poles parallel to the direction of crack opening decreases,K _ISCC in stress-relieved material increases from 4 MPa√m atf = 0.70 to 17 MPa√m atf = 0.19. The same trend is observed in recrystallized material. However, theK _ISCC values are somewhat greater. Transgranular cleavage is the preferred mode of crack propagation. Several ductile modes of separation complement the cleavage process. At high crack velocity, tearing between facets is promoted. At lowK, nearK _ISCC, very little tearing is observed and cleavage zones larger than the grain size are common. Fluting is preferred in the low regime. In recrystallized material a transition to completely intergranular failure is observed nearK _ISCC.     

A Simple Approach to the Determination of Threshold Stress Intensity for Stress Corrosion Cracking (K ISCC) and Crack Growth of Sensitized Austenitic Stainless Steel

This article discusses the intricacies associated with the determination of threshold stress intensity for stress corrosion cracking (K _ISCC) of narrow regions such as the sensitized microstructure of austenitic stainless steel and presents a simple approach to the accurate determination of K _ISCC of a sensitized stainless steel. K _ISCC and crack growth rates of solution-annealed and sensitized AISI 304 stainless steel in the 42 wt pct MgCl₂ environment at 427 K (154 °C) were determined using the circumferential notch tensile (CNT) technique. The results presented here validate the ability of the CNT technique to overcome some of the fundamental difficulties in determination of the K _ISCC of narrow regions, using the traditional techniques. This article also discusses the mechanistic aspects of the difference in fractographic features of the sensitized and solution-annealed stainless steels.     

Effect of Crack-Tip Stresses on Stress Corrosion Cracking Behavior

It is well documented that the applied load for a given material/environment system has a significant effect on the stress corrosion cracking (SCC) behavior in terms of K _ISCC values as well as on the K-da/dt relation. Traditionally, the role of crack-tip stress has not received proper attention but rather the crack-tip strain that is supposedly critical in cracking the passive oxide exposing the fresh metal surface at the crack tip to the environment. This article discusses a different point of view pertinent to crack-tip stress calculations and their role on SCC behavior. We have examined different continuum mechanics solutions with respect to the role of blunting on crack-tip stress. Both solutions where free traction boundary conditions are satisfied on a sharp and blunt crack are analyzed and discussed. It was shown that the stress component perpendicular to the crack plane at the crack tip for the plateau region remains essentially the same due to blunting even with the increase in applied load. Constant crack-tip stress would also result in rather constant crack-tip strain or creep rate, and as such, the crack growth rate remains the same or constant with respect to applied stress. Presented stress analysis is based on available solutions taken from the literature and assumes that a chemical potential at the crack tip is fixed.     

Circumventing Practical Difficulties in Determination of Threshold Stress Intensity for Stress Corrosion Cracking of Narrow Regions of Welded Structures

Determination of the threshold stress intensity for stress corrosion cracking (K _Iscc) of narrow areas such as weld and heat-affected zone (HAZ) of a weldment is a nontrivial task because of the requirements of large specimens in testing by the traditional techniques and the difficulty of restricting crack propagation to narrow regions in such specimens. This article describes a successful application of the circumferential notch tensile (CNT) technique to determine the K _Iscc of narrow regions of the weld and HAZ. Also, the microstructure of the HAZ of the manual metal arc-welded steel was simulated over a relatively small length of specimens and its K _Iscc in a hot caustic solution was determined successfully. Intergranular stress corrosion cracking was confirmed with a scanning electron microscope.     

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.     

Effect of cyclic stress wave form on corrosion fatigue crack propagation in al-zn-mg alloys

Fatigue crack growth rates of a 7075 type aluminum alloy were measured as a function of environment, frequency, stress wave form, alloy chemistry, and thermomechanical treatment. At low ΔK values (belowK _ISCC ), the crack growth rates in a 3.5 pct sodium chloride solution were ten times greater than those in a reference argon environment. Comparison of the effects of a square wave, a negative-sawtooth wave, and a positivesawtooth wave at different frequencies indicates that the synergistic interaction with the environment occurs during the loading part of each cycle. Overaging the alloy and limiting the alloy impurity content results in a reduced corrosion fatigue crack growth rate, but a thermomechanical treatment leading to a grain size refinement increases it.     

Stress corrosion cracking of high strength HY-180M steel in 3.5 Pct NaCl

Stress corrosion cracking of HY-180M steel was studied at 22°C in an aqueous solution of 3.5 pct NaCl (pH = 6.5). The steel had a nominal weight percentage composition of 10Ni-14Co-2Cr-lMo-0.16C and was heat treated to yield a fracture toughness value ofK _Ic ≃ 160 MPa . m^1/2. The SCC velocity (v) was studied as a function of stress intensity (K _I) and electrochemical potential (E) using precracked compact tension specimens, a Ag/AgCl reference electrode and a 1000 h exposure test. Also, the polarization behavior, microstructure, fractography and corrosion products were studied. The results showed that SCC was markedly dependent uponE, and did not occur whenE =-0.52 V_SHE (-0.72 V_Ag/AgCl), which corresponded closely to the thermodynamically reversible potential of iron. However, SCC occurred at a more noble potential of-0.28 V_SHE (-0.48 V_Ag/AgCl ) and at a less noble potential of-0.80 V_SHE (-1.00 V_Ag/AgCl). The stress intensity below which SCC was not observed was K_ISCC ≃ 5.5 MPa . m^1/2 at -0.28 V_SHE and K_ISCC ≃ 60 MPa . m^1/2 at -0.80 V_SHE . Also, Region I behavior (v dependent uponK ₁) and Region II behavior (v independent ofK ₁) were observed. Cracking was considered to occur solely by hydrogen embrittlement at -0.80 Vshe, whereas anodic dissolution processes played a necessary role, either directly or indirectly, in SCC at -0.28 V_SHE . The indirect effects were discussed in relation to hydrolysis effects in the crack promoting hydrogen embrittlement and/or corrosion product wedging stresses.     

Crack propagation near cylindrical voids

Stress intensity factorsK _I andK _II are presented for a planar, sharp-ended crack subjected to nearby line forces and line force doublets. The resulting near crack tip stress field is used to predict the influence of such singularities upon the crack propagation direction. The concept of the criticality of the angle of crack departure from symmetric propagation is introduced and used to compare computer predictions with experiments performed on double cantilever beam (DCB) specimens of 7075 aluminum alloy. The form of the near crack tip elastic equations and the criticality are verified. The critical angle parameter is found to be a material and experimental constant, independent of the strength of the centers of stress.     

Stress corrosion cracking of high‐strength steels

The threshold stress intensity of stress corrosion cracking (SCC) in the NaCl solution, K_ISCC, has been measured for five low alloy steels. The effects of yield strength, alloy elements, microstructure and grain size on K_ISCC were studied. The results showed that K_ISCC decreased exponentially with increasing yield strength, σ_ys, i.e., K_ISCC = 1.38 · 10⁶exp(‐8.26 · 10^‐3σ_ys) for 40CrMoV steel and K_ISCC = 1.42 · 10⁶exp(‐4.66 · 10^‐3σ_ys) for 30CrMnSiNi steel. For low‐alloy high‐strength steels with σ_ys = 1400 MPa, the effect of alloy elements, microstructure and grain diameter larger than 7 μm on K_ISCC was little. The threshold stress intensity of hydrogen‐induced cracking during dynamical charging for 40CrMoTi steel decreased linearly with the logarithm of the concentration of diffusible hydrogen, C₀, i.e., K_IH = 31.3‐9.1lnC₀. This equation was also applicable to SCC of a high‐strength steel in aqueous solution, and in this case, C₀ is constant. The critical hydrogen enrichment concentration, C_th, necessary for SCC of high‐strength steel in water decreased exponentially with the increase in yield strength. It was possible to deduce the relationship between K_ISCC and σ_ys, i.e., K_ISCC = Ak₁exp(‐k₂σ_ys), where A = 3RT√πρ /2(1 + ν) , k₁ and k₂ are constants, which depend upon the compositions and microstructure of the steel as well as the test conditions.     

Stress Corrosion Cracking of HY-180 Steel in Aqueous 3.5 Pct NaCI

Stress corrosion cracking of HY-180 steel (Fe-10 Ni-2 Cr-1 Mo-8 Co-0.12 C) was studied in aqueous 3.5 pct NaCI (pH = 6.5) at 22 °C. The alloy was austenitized, water quenched and aged at 510 °C for 5 h. Specimens were of the precracked, double cantilever beam (DCB) variety and exposure times extended up to 1000 h. The crack propagation rates (v) were studied as a function of stress intensity(K,) under both freely corroding potentials(E ≈-0.36 V_SHE) and potentials produced by coupling to Zn(E ≈ -0.82 V_SHE. Crack fractography was studied by scanning electron microscopy and corrosion products were identified by electron diffraction analysis. The stress intensity, K_ISCC, below which SCC could not be detected was ~45 MPa m^1/2 for both freely corroding and Zn-coupled conditions. Analysis of the results showed that cracking was consistent with a hydrogen embrittlement mechanism, irrespective of potential. Furthermore, comparison of the data with previous studies on a similarly heat treated and closely related alloy (HY-180 M), containing 14 Co-0.16 C, showed no significant difference in SCC behavior, provided comparison was made at similar electrochemical potentials.     

Effect of testing frequency on the corrosion fatigue of a squeeze-cast aluminum alloy

The corrosion fatigue crack propagation behavior of a squeeze-cast Al-Si-Mg-Cu aluminum alloy (AC8A-T6), which had been precracked in air, was investigated at testing frequencies of 0.1, 1, 5, and 10 Hz under a stress ratio (R) of 0.1. Compact-toughness specimens were precracked about 6 mm in air prior to the corrosion fatigue test in a 3 pct saline solution. At some near-threshold conditions, these cracks propagated faster than would be predicted by the mechanical driving force. This anomalous corrosion fatigue crack growth was affected by the initial stress-intensity-factor range (ΔK _i), the precracking conditions, and the testing frequency. The initial crack propagation rate was as much as one order of magnitude higher than the rate for the same conditions in air. This rapid rate was associated with preferential propagation along the interphase interface in the eutectic structure. It is believed that a chemical reaction at the crack tip and/or hydrogen-assisted cracking produced the phenomenon. Eventual retardation and complete arrest of crack growth after this initial rapid growth occurred within a short period at low ΔK values, when the testing frequency was low (0.1 and 1 Hz). This retardation was accompanied by corrosion product-induced crack closure and could be better explained by the contributory stress-intensity-factor range (ΔK _cont) than by the effective stress-intensity-factor range (ΔK _eff).     

Hydrogen-induced delayed fracture under mode II loading

Hydrogen induced cracking (HIC) and stress corrosion cracking (SCC) of a high-strength steel 34CrNi₃Mo (T.S = 1700 MPa) under Mode II loading were investigated using notched specimens. The stress field around the notch tip was analyzed by means of finite element method. The result shows HIC and SCC under Mode II loading initiated at the back of the notch tip,i.e., θ = -110 deg, where hydrostatic stress has maximum value. However, cracking is oriented along the shear stress direction at the site, not normal to the direction of maximum principal stress component. On the contrary, if the specimens are loaded to fracture in air under Mode II loading, cracking at the maximum shear stress site around the notch tip and the cracking direction coincide with the direction of the maximum shear stress. The above facts indicate that hydrogen induced delayed plastic deformation is a necessary condition for HIC, and the nature of SCC for high-strength steel in 3.5 pct NaCl solution is HIC. The results show that HIC and SCC under Mode II loading can occur during dynamic charging with hydrogen and in 3.5 pct NaCl solution, respectively. The normalized threshold stress intensity factors under Mode II loading during dynamic charging in 1 N H₂SO₄ + 0.25 g As₂O₃/L solution and in 3.5 pct NaCl solution are K_IIH/K_IIX = 0.1 and K_IISCC/K_IIX = 0.45, respectively. The corresponding values under Mode I loading are K_IH/K_IX = 0.02 and K_ISCC/K_IX = 0.37, where K_IIX and K,_IX are critical values loaded to failure in air under Mode II and Mode I loading, respectively. Thus, (K_IIH/K_IIX)/ K_IH/K_IX) = 5 and (K_IISCC/K_IIX)/K,(_ISCC/K_IX) = 1.2. A typical intergranular fracture was observed during HIC and SCC under Modes II and I loading. But the fracture surfaces of specimens failed in air are composed of dimples for both kinds of loading. Formerly Student at Beijing University of Iron and Steel Technology     

Stress-corrosion cracking of Ti-8Al-lMo-lV in aqueous environments: 2. Plastic zones,crack morphology,and fractography

Optical and electron metallographic studies of stress-corrosion cracks in Ti-8Al-lMo-lV have verified that the principal crack extension mechanism is cleavage of theα grains. There are two distinct crack morphologies which correspond to the two regimes of subcritical crack velocity. At low stress intensities(a ∞ K _I) the microscopic crack front consists of small cleavage facets approximately 1 to 4α grain diameters in size, and ligaments of material which fracture by ductile rupture and corrosion. At high stress intensities (a ≅ constant), the crack front consists of large cleavage “fingers”, 20 to 50α grain diameters in length, separated by regions which fracture by a combination of cleavage (on a much smaller scale), ductile rupture, and corrosion. The transition from Stage I to Stage II crack propagation apparently occurs when the strain-energy release rate is sufficient to support two crack branches,i.e., K_I≥ √2K _Iscc. Thereafter, the diameter of the plastic zone at the crack tip remains constant, suggesting that the effective stress intensity at the tip of each branch is also invariant. The slip within the plastic zone is markedly nonhomogeneous, and trenches are often observed along the slip steps. Formerly with the Metal Science Group, Battelle Columbus Laboratories, Columbus, Ohio.     

Stress history effect on incubation time for stress corrosion crack growth in AISI 4340 steel

The effect of stress history on stress corrosion cracking of AISI 4340 steel in an aqueous environment has been studied with the use of double-cantilever beam specimens. The stress history effect was found to influence the incubation time period with changes in the stress intensity. When the stress intensity was decreased, the incubation time period was dependent on the △K and finalK _f during stress corrosion testing. When the stress intensity was increased, the incubation time period was independent of the applied stress intensity. However, the stress history effect did not influence the steady-state crack growth rates. In this report, the stress history effect is explained by using the hydrogen embrittlement mechanism.     

Stress corrosion cracking of stainless steels in NaCl solutions

The metallurgical influences on the stress corrosion resistance of many commercial stainless steels have been studied using the fracture mechanics approach. The straight-chromium ferritic stainless steels, two-phase ferritic-austenitic stainless steels and high-nickel solid solutions (like alloys 800 and 600) investigated are all fully resistant to stress corrosion cracking at stress intensity (K₁) levels ≤ MN • m-^3/2 in 22 pct NaCl solutions at 105 °C. Martensitic stainless steels, austenitic stainless steels and precipitation hardened superalloys, all with about 18 pct chromium, may be highly susceptible to stress corrosion cracking, depending on heat treatment and other alloying elements. Molybdenum additions improve the stress corrosion cracking resistance of austenitic stainless steels significantly. The fracture mechanics approach to stress corrosion testing of stainless steels yields results which are consistent with both the service experience and the results from testing with smooth specimens. In particular, the well known “Copson curve” is reproduced by plotting the stress corrosion threshold stress intensity (AT_ISCC) vs the nickel content of stainless steels with about 18 pct chromium. Formerly with the BBC Brown Boveri Company, Baden, Switzerland     

The role of crack tip strain rate in the stress corrosion cracking of high strength steels in water

Creep tests have been performed on fracture mechanics specimens of as-quenched 4340 and 3.5NiCrMoV rotor steel to confirm the importance of crack tip strain rate in causing stress corrosion cracking. By allowing creep in a noncracking environment, dry air for the high strength steels tested, cracking did not occur when water, the corrosive solution, was later added to the system. Thus, it is possible to inhibit stress corrosion in spite of conditions otherwise conducive to crack growth. Conditions necessary to restart cracking were also tested. The importance of this result in terms of the mechanism of stress corrosion and difficulties in measuring K_ISCC is discussed. I. O. Smith, formerly Associate Professoraf with the Department of Mining and Metallurgical Engineering, University of Queensland     

Carbide- matrix interface mechanism of stress corrosion cracking behavior of high-strength CrMo steels

The effects of tempering temperature and carbon content on the stress corrosion cracking (SCC) behavior of high-strength CrMo steels in 3.5 pct NaCl aqueous solution have been studied by means of Auger electron spectroscopy (AES) and scanning and transmission electron micros- copy (SEM and TEM). Experimental results show that the specimens with higher carbon content and tempered at lower temperatures have a higher tendency for intergranular fracture and lower threshold stress intensity K_ISCC The SCC behavior is significantly affected by the distribution of carbide particles, especially carbide coverage on prior austenitic grain boundaries, through a carbide-matrix interface mechanism as the interface is the preferential site for the nucleation and propagation of microcracks because of its strong ability to trap hydrogen atoms. In low- temperature tempered states, there is the serious segregation of carbon in the form of carbide particles at prior austenitic grain boundaries, causing low-stress intergranular fracture. After tempering at high temperatures (≥400 °C), both the coalescence of the carbide particles at the grain boundaries and the increase of carbide precipitation within grains cause the decrease of the tendency for intergranular fracture and the rise of K_ISCC. The higher the carbon content in steels, the more the carbide particles at the grain boundaries and, subsequently, the higher the tendency for low-stress intergranular fracture. The carbide effect on K_ISCC makes an important contribution to the phenomenon that K_ISCC decreases with the rise of yield strength of the steels.