CRACK NUCLEATION AND PROPAGATION IN BLADE STEEL MATERIAL

Stress corrosion cracking and corrosion fatigue of 12 Cr steel in sodium chloride solution has been investigated. Tests have been performed in air at room temperature and in aqueous solution with 22% NaCl at 80°C. The influence of corrosion pits on crack nucleation has been investigated. On fracture surfaces tested in environment (22% NaCl solution), crack initiation was observed in correspondence with corrosion pits; in this case fatigue life can be described using a fracture mechanics approach. The ΔK value for crack nucleation from a pit in rotating bending fatigue tests is very low in air (about 3 MPa√m). The results of slow strain rate tests on smooth specimens show that there is a threshold stress intensity, K_ISCC, of about 15 MPa√m and a plateau in stress corrosion crack growth rate of about 10^-5mm/s.     

Corrosion Fracture of Structural Metallic Materials: Effect of Electrochemical Conditions in Crack

Abstract: The work is a compressed review based on the summarised results and the original approach for study of corrosion crack growth, taking into account local electrochemical conditions in the crack tip, which was developed at the Karpenko Physico‐Mechanical Institute of NASU. The model scheme of the pre‐fracture zone in the corrosion crack tip, which can be defined by the local values of pH of solution, electrode potential of metal E and stress intensity factor K_I is proposed. For its realisation, the special method and testing equipment for corrosion crack growth study and local electrochemical measurements in the crack were developed. The variation of the electrochemical conditions in corrosion cracks was studied, and it has been found that some stabilised levels of the pH and E values can be achieved in the tip of a non‐propagating and a propagating crack under static and cyclic loading during of exposure time. On this ground, the method for forecasting of the threshold stress intensity factor K_ISCC under stress corrosion cracking was proposed using these characteristic values of pH and E. This method was also adopted for the determination of the threshold stress intensity factor K_th under corrosion fatigue. The special method for determining corrosion fatigue crack growth rate diagrams based on consideration of extreme electrochemical conditions in the crack tip was developed. It has been proven that such diagrams reflect the extreme influence of the environmental factor on corrosion fracture of material, and they may be recommended as the base for the remaining lifetime calculation of the structural elements exploited under environmental conditions.     

Effect of alloying additions onK ISCC of ultrahigh strength NiSiCr steel

The effect of alloying additions viz. cobalt, molybdenum, cerium and a combination of cobalt and molybdenum, on theK _ISCC of NiSiCr steel in 3·5% NaCl aqueous solution was studied. Addition of cobalt to NiSiCr steel resulted in an increase in theK _ISCC whereas molybdenum addition decreased theK _ISCC. Cerium addition did not affect theK _ISCC while the combination of cobalt and molybdenum resulted in an increase in theK _ISCC although not as much as in the case of cobalt addition. The effect of alloying elements onK _ISCC could be attributed to their effect on the critical fracture stress and yield strength.     

Experience with Rising Load/Rising Displacement Stress Corrosion Cracking Testing

New drafts of the ASTM standards propose to study stress corrosion cracking with an aim to determine K _ISCC and da/dt = f(K _I) for increasing loading (crack-opening displacements). The duration of the tests becomes smaller but the spread in data remains the same as in the case of constant loading and constant strains. The results of testing under increasing loading (opening displacement) strongly depend on the correct application of the method, especially on the proper choice of the rates of the process.     

Investigation of stress corrosion cracking of low-alloy steel in water

For four low-alloy steels with a wide range of tensile strength, the dynamical processes of the nucleation and propagation of stress corrosion cracking (SCC) in water with various polarization conditions and in a 0.1 N K₂Cr₂O₇ solution were traced with an optical microscope. The results show that if the tensile strength of the steel is higher than a critical value that is different in different polarization conditions and K_IK_ISCC, the plastic zone in front of a loaded crack tip is enlarged with time i.e. the delayed plastic deformation occurs in all the environments used. The nucleation and propagation of SCR will proceed when this delayed plastic deformation develops to a critical condition. Neither anodic and cathodic polarization nor the inhibitor can change the feature of the delayed plasticity and the nucleation and propagation of SCC in water.In all the environments used, K_ISCC is increased and $\text{da}\text{dt}$ is decreased with the decrease in the strength of the steel. K_ISCC is increased and da/dt is decreased with the anodic polarization and the addition of the inhibitor, but the cathodic polarization has the opposite effect.     

Effects of various environments on fatigue crack growth in Laser formed and IM Ti–6Al–4V alloys

The fatigue crack growth behaviors of Laser formed and ingot metallurgy (IM) Ti–6Al–4V alloys were studied in three environments: vacuum, air and 3.5% NaCl solution. Taking the Unified Fatigue Damage Approach, the fatigue crack growth data were analyzed with two intrinsic parameters, stress intensity amplitude ΔK and maximum stress intensity K_max, and their limiting values ΔK^* and . Fatigue crack growth rates da/dN were found increase with stress ratio R, highest in 3.5% NaCl solution, somewhat less in air and lowest in vacuum, and higher in IM alloy than in Laser formed one. In 3.5% NaCl solution, stress corrosion cracking (SCC) was superimposed on fatigue at R=0.9 for where K_max>K_ISCC, the threshold stress intensity for SCC. This and environment-assisted fatigue crack growth were evidenced by the deviation in fatigue crack growth trajectory (ΔK^* vs. curve) from the pure fatigue line where . Furthermore, the fractographic features, identified along the trajectory path, reflected the fatigue crack growth behaviors of both alloys in a given environment.     

The failure of an armour faced conveyor connector

The combined use of modern metallurgical techniques for fracture examination, laboratory test data and fracture mechanics calculations allows metallurgical failures to be examined in a quantitative manner. Complex load histories and environments can result in more than one sub-critical cracking mechanism occurring in a component. Quantitative understanding of the rate determining cracking process is a necessary prerequisite to rectifying the problem. The following case study describes a connector from an armour faced conveyor which failed in service. The failure investigation involved fractography, stress analysis, material property evaluation and fracture mechanics calculations. Fractographic evidence indicated a stress corrosion failure mechanism. Calculations of critical crack sizes showed that stress corrosion cracking alone could not account for the fracture. It was concluded that the failure was due to a sequence of three cracking processes which preceded unstable ductile fracture. Firstly, frictional heating caused rubbing or quench cracks typically 0.5–1 mm deep. Secondly, corrosion fatigue cracks grew several millimetres allowing the third fracture process, stress corrosion cracking, (SCC) to initiate and grow. In the situation described here, this process was much faster than corrosion fatigue. The influence of defect size due to rubbing cracks and the influence of K_ISCC have been compared with the corrosion fatigue life of the component. An increase in K_ISCC and hence critical defect size for SCC has been shown to increase the corrosion fatigue life of the component by a large factor. A change in design would also alleviate the problem of SCC by reducing the static stress, which is the driving force for SCC.     

POP-IN AND CRACK ARREST IN AN HY80 WELD

It is generally thought that, when a material is in its brittle to ductile transition, it is more difficult to design for crack arrest than to prevent crack initiation (cleavage). This report shows that this is not always true for weldments. Comparison is made between compact crack arrest (CCA), K_a, and crack tip opening displacement (CTOD), K_Jc, toughness for the same HY80 weld. The value of K_a is shown to be much higher than the minimum K_Jc for pop-in fracture initiation. It is considered that the results support the conclusion of Japanese research workers (Arimochi and Isaka) that small pop-ins (in the CTOD test) propagate and arrest without load drop. It follows that prediction of structural failure for weldments need not be based on minimum pop-in toughness from CTOD tests.     

Mechanisms for corrosion fatigue crack propagation

ABSTRACT The corrosion fatigue crack growth (FCG) behaviour, the effect of applied potential on corrosion FCG rates, and the fracture surfaces were studied for high‐strength low‐alloy steels, titanium alloys, and magnesium alloys. During investigation of the effect of applied potential on corrosion FCG rates, polarization was switched on for a time period in which it was possible to register the change in the crack growth rate corresponding to the open‐circuit potential and to measure the crack growth rate under polarization. Due to the higher resolution of the crack extension measurement technique, the time rarely exceeded 300 s. This approach made possible the observation of a non‐single mode effect of cathodic polarization on corrosion FCG rates. Cathodic polarization accelerated crack growth when the maximum stress intensity (K_max) exceeded a certain well‐defined critical value characteristic for a given material‐solution combination. When K_max was lower than the critical value, the same cathodic polarization, with all other conditions (specimen, solution, pH, loading frequency, stress ratio, temperature, etc.) being equal, retarded or had no influence on crack growth. The results and fractographic observations suggested that the acceleration in crack growth under cathodic polarization was due to hydrogen‐induced cracking (HIC). Therefore, critical values of K_max, as well as the stress intensity range (ΔK) were regarded as corresponding to the onset of corrosion FCG according to the HIC mechanism and designated as K_HIC and ΔK_HIC. HIC was the main mechanism of corrosion FCG at K_max > K_HIC (ΔK > ΔK_HIC). For most of the material‐solution combinations investigated, stress‐assisted dissolution played a dominant role in the corrosion fatigue crack propagation at K_max < K_HIC (ΔK < ΔK_HIC).     

Threshold stress intensity factor and crack growth rate for stress corrosion cracking of simulated heat affected zone in caustic solution

The threshold stress intensity factor for stress corrosion cracking (K_Iscc) of the simulated heat affected zone (HAZ) of mild steel in caustic solution has been determined using circumferential notch tensile (CNT) technique. The HAZ microstructure produced upon manual metal arc welding of grade 250 steel was simulated over a length of 35 mm of CNT specimens, using a thermo-mechanical simulator. Inter- and trans-granular stress corrosion cracking has been confirmed using a scanning electron microscope. The results presented here validate the ability of CNT technique for the determination of K_Iscc of HAZ and Base metal. Crack growth rates have also been determined using CNT technique. Further, the effect of microstructures on K_Iscc and crack growth rate is discussed in the present study. The determined K_Iscc of Base metal and simulated heat affected zone in 30% caustic solution is 24 and 45 MPa m^1/2, respectively.     

Effect of filler materials on the performance of gas tungsten arc welded AISI 304 and Monel 400

Dissimilar metal welding involving AISI 304 and Monel 400 finds widespread use in marine and offshore environments for the fabrication of heat exchangers, evaporators, piping and vessels in petrochemical and power generation industries. However studies on the performance of such weldments are scantly found in the literature even though the individual metals have higher corrosion resistance and strength. This paper reports the work carried out on welding of AISI 304 and Monel 400 using Gas Tungsten Arc Welding (GTAW) technique to examine the weldability, mechanical and metallurgical properties. Investigations have been carried out on the hot corrosion behavior of these joints subjected to cyclic air oxidation and K₂SO₄ + NaCl (60%) molten salt environment at 600 °C. A comparative analysis was carried out on these weldments for two different filler metals such as E309L and ENiCu-7. The oxide scales formed on the various zones of the weldment have been characterized systematically using surface analytical techniques. Weld zone was found to be more susceptible to degradation than base metals used. The effect of filler materials on the hot corrosion is discussed. The studies reported in this paper would be beneficial for fabricators embracing this type of dissimilar weldments in the petrochemical and power generation industries.     

Effect of BTA on electrochemical corrosion and stress corrosion cracking behaviour of type 304 stainless steel in 1 M HCl

Effect of benzotriazole (BTA) on polarization and stress corrosion cracking (SCC) behaviour of type 304 stainless steel in 1 M HCl was investigated. The anodic polarization curves showed that with BTA additions the anodic polarization kinetics in the active region was not affected, though a reduction in critical current density, i _crit, and passive current density, i _p, was observed. However, BTA was found to influence significantly the cathodic reaction kinetics. SCC results using smooth tensile test specimens showed an increase in time-to-failure, t _f, with BTA additions. Crack growth rate studies using single-edge notched (SEN) specimens showed an increase in threshold stress intensity for SCC, K _ISCC, and a decrease in crack growth rate, da/dt, with BTA additions. While the adsorption isotherms derived from weight loss data followed a Langmuir adsorption isotherm signifying a monolayer adsorption, the adsorption isotherms derived from SCC test data deviated from this at higher BTA concentrations. The paper discusses the possible reason for this deviation.     

Form und Mechanismen von Rissen und Brüchen unter gleichzeitiger korrosiver und dynamischer Beanspruchung mit unterschiedlichen Frequenzen

Form and Mechanisms of Cracks and Fractures Caused by Simultaneous Corrosive and Dynamic Stress under Various Frequencies Distinction of crack nucleation and crack propagation under variable frequencies and electrolytic corrosion. Relationship between crack nucleation and frequency due to differing sliding mechanisms. Crack propagation under decreasing frequencies causes transition from corrosion fatigue to cyclic stress corrosion cracking and finally stress corrosion cracking can be observed. Coordination of that transition with the increasing influence of the K_ISCC-value as determinant of the crack propagation. The influence of frequency on high temperature materals in hot gas atmosphere is mainly determined by creep mechanisms during top-load phase and the gas-metal-reactions. Accordingly the life time increases with growing load frequency.     

Effects of detergent concentration and ethylene oxide chain length of the detergent molecule on stress-cracking of low-density polyethylene

Stress-cracking behaviour of low-density polyethylene is investigated using a fracture mechanics approach. It is experimentally observed thatK _l independent crack speed is directly proportional to the concentration of Igepal CO-630 up to 25% concentration.K _ISCC is found to increase with detergent concentration; and the increase inK _ISCC is attributed to the larger amounts of detergent absorption in the higher concentrations, which decreases the strain localization at the crack tip. Micelle formation of detergents in water is thought to enhance the rate of cracking because of their ability to increase plasticization efficiency at the local area because the micelles contain larger amounts of aggressive molecules. In contrast to Williams' model, the cracking behaviour in the constant crack speed region is found to be not controlled by the flow properties of the environment.     

High temperature cross-weld characterisation of steel weldments by microtensile testing

Abstract

Study of local material properties and damage mechanisms are undertaken in order to characterise weldments that show significant variation of properties across weldments. One of the methods to characterise the local variation of properties is microtensile (MT) testing of specimens machined out of specific narrow zones of weldments. The literature data, though limited, on microtensile specimen testing are reported at their low temperature behaviour. On the other hand, systematic study of crossweld local material properties at high service temperatures have not yet been reported. In the present study, MT tests are conducted across similar welds of P22 and P91 steels at 550 and 600°C, respectively. In order to study deformation mechanisms and the role of surface condition on properties, specimens with different surface conditions (i.e. machined, polished and electropolished surfaces) are tested. Two different loading rates of 0.2mm/min and 0.5mm/min are used to study the effect of loading rate on deformation and mechanical properties. Variations of material properties yield strength (R_p0.2) and ultimate tensile strength (R_m), for the weldments are presented as a function of surface conditions of specimens and loading speeds. Higher loading rates yield higher values of R_p0.2 and R_m, and specimens with machined and polished surfaces show consistent and higher values of R_p0.2 and R_m compared to specimens with an electropolished surface finish. Deformation behaviour is studied on the side surfaces of tested microtensile specimens using an SEM. Deformation is correlated to microstructural constituent that is observed on specimen side surfaces. The metallographic information is used to interpret the variation of mechanical properties determined in tension at high temperatures. The MT data are compared with standard tensile data obtained on specimens with simulated microstructures. The prospects of using MT tests for characterising the material at high temperatures and feasibility of use of data for assessment of components under service loading conditions are reported.     

Crack significance evaluation with special reference to welded structures

Fracture mechanics parameters (stress intensity factorK _I and its critical valueK _Ic , crack opening displacement, and the contourJ integral) are originally defined for static and quasistatic loading conditions. On the basis of theoretical background, standard test methods for the experimental determination of their specific values were developed. Structural integrity analysis requires the extension of application of these parameters to other types of loading. We propose new parameters for stress corrosion (stress corrosion cracking thresholdK _1sco ), for cyclic loading (stress intensity factor range ΔK and fatigue threshold ΔK _th), and for creeping at elevated temperatures (C ^* andC _t integrals). The structural integrity of welded structures is mainly affected by cracks in welded joints. We demonstrate the practical application of fracture mechanics parameters to the evaluation of structural integrity under the above-mentioned loading conditions. Faculty of Technology and Metallurgy, University of Belgrade, Yugoslavia. Faculty of Mechanical Engineering, University of Belgrade, Yugoslavia. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 32, No. 2, pp. 107–118, March–April, 1996.     

THE EFFECTS OF CATHODIC POTENTIAL AND CALCAREOUS DEPOSITS ON CORROSION FATIGUE CRACK GROWTH RATE IN SEA WATER FOR TWO OFFSHORE STRUCTURAL STEELS

The effects of cathodic protection potential, corrosion products and stress ratio on corrosion fatigue crack growth rate have been studied on offshore structural steels. These materials were cathodically polarised in seawater and 3% sodium chloride solution at three potentials of -0.8, -1.0 and -1.1 V(SCE). The corrosion fatigue crack growth rate in seawater was greater than that in air and increased with more negative potentials. The maximum acceleration of crack growth rate in seawater was observed at the crack growth plateau which was independent of ΔK. Calcareous deposits precipitated within the cracks resulted in an increase of crack opening level and contributed to a reduction of the corrosion fatigue crack growth rate. Such a corrosion-product-wedging effect could be evaluated by using an effective stress intensity range, ΔK_eff. The estimation of corrosion fatigue crack growth rate in terms of ΔK_eff clarified the effect of hydrogen embrittlement under a cathodic potential. Thus the processes of cracking in seawater at cathodic potentials resulted from mechanical fatigue and hydrogen embrittlement with calcareous deposits reducing the crack growth rate. All these three mechanisms were mutually competitive.     

AgNO<Subscript>3</Subscript> spray tests: advantages,weaknesses, and various applications to quantify chloride ingress into concrete. Part 2: Non-steady-state migration tests and chloride diffusion coefficients

This paper forms the second part of a series devoted to the analysis of the capabilities of AgNO₃ spray tests, in view of using them for various issues, within the framework of the evaluation and the prediction of chloride-induced corrosion risks. More precisely, the Maultzsch procedure (spraying of 0.1-N AgNO₃ solution + K₂CrO₄) and the use of AgNO₃ alone have been investigated on a broad range of concretes. In this second part, the feasibility of colorimetric techniques on saturated specimens cast in laboratory, after non-steady-state migration tests carried out in various conditions is discussed. Colorimetric methods have been applied to the determination of the average penetration depth. In addition, the detection threshold of these techniques has been investigated. Moreover, colorimetric methods have been applied to the calculation of apparent and effective chloride diffusion coefficients from non-steady-state diffusion and migration tests. The results then obtained have been compared to the values obtained by other types of methods.     

An acoustic emission study of failure by stress corrosion in bundles of E-glass fibres

Fibre failures in bundles of parallel E-glass fibres loaded from their ends and immersed in aqueous HCl solutions or in water are detected using acoustic emission. In constant load and constant stress tests the times to failure of the fibres and the bundles are accounted for on the basis of a theory which assumes a Weibull distribution of initial strengths, and that the velocity (v) of subcritical stress-corrosion cracks in the fibres depends on the stress intensity (K _l) according to a power law (VK _l ⁿ ). The crack growth parameter (n) is measured by three methods, and its variation with acid strength is correlated with other corrosion studies. The stress corrosion measurements also allow a determination of the Weibull modulus of the fibres.     

Effect of prior uniform plastic strain on the KISCC of high-strength steels in sea water

A systematic study was conducted to determine the effects of prior uniform plastic strain, ?_p, on the mechanical and environmental properties of four high-strength steels ranging in strength from 80 to 200 ksi (550 to 1400 N/mm²): HY-80, HY-130, 12Ni-5Cr-3Mo maraging, and 10Ni-Cr-Mo-Co Q & T steels. Each steel was evaluated first in the unstrained condition, for reference purposes, and then after 1 and either 3 or 5 per cent plastic strain (?_p), depending on the specific steel and uniform strain limit (?_p,max). Standard tension and Charpy V-notch (CVN) specimens were used to characterize the mechanical properties. So-called precracked cantilever-beam specimens were used to evaluate both the relative fracture behavior in air (K_Ix) and the stress-corrosion-cracking (SCC) threshold behavior in synthetic sea water (K_ISCC); the K_ISCC value was determined under constant-load conditions (P = constant) using a nominal 1000-hr test duration.The results showed that, with increasing levels of plastic prestrain (?_p), the mechanical properties were altered in a generally beneficial manner, and the K_Ix value decreased in a consistent manner, depending on the initial yield strength (σ_ys) and the specific level of ?_p. The apparent K_ISCC value, however, did not follow any consistent pattern of behavior. The apparent K_ISCC for HY-80 steel (completely immune to SCC) was unaffected by ?_p, whereas the corresponding value for 12Ni-5Cr-3Mo maraging steel was increased and the values for the HY-130 and 10Ni-Cr-Mo-Co Q&T steels were decreased a moderate amount.It is emphasized that due to the limited specimen dimensions employed all K_Ix and apparent K_ISCC values must be interpreted on a relative, rather than an absolute, basis because they underestimate the corresponding intrinsic behavior K_IC and true K_ISCC). For both the HY-80 and HY-130 steels, the degree of underestimation is substantial. The systematic manner by which such K_Ix and apparent K_ISCC values underestimate the true behavior as a result of insufficient test-specimen geometry is described as the K_I-suppression effect. Supporting experimental evidence is presented to describe the general extent to which K_I suppression occurs in proportional-geometry test specimens.A clear perspective is thus provided for the proper interpretation of the present results and those of a similar nature on other materials. The necessity of recognizing and understanding the very conservative nature of results when K_I-suppression effects are operative is emphasized as being critical to the meaningful establishment of standard test methods and structural-design criteria.