Crack paths,microstructure, and fatigue crack growth in annealed and cold-rolled AISI 304 stainless steels

To assist in the understanding of micromechanisms for corrosion fatigue crack growth in metastable austenitic steels, the relationships between the crack paths and the underlying microstructure were investigated for annealed and cold-rolled (CR) 304 stainless steels that had been tested in a deaerated 3.5 pct NaCl solution, air, and vacuum. Corrosion fatigue in the deleterious environments (3.5 pct NaCl and air) was brittle and occurred primarily by {001}_γ and other unidentified, quasi-cleavage (QC), accompanied by preferential cracking along {111}_γ twin and grain boundaries. In contrast, fatigue cracking in vacuum was ductile, fully transgranular, and noncrystallographic. Transformation to alpha prime (α′-) martensite by fatigue was found to be essentially complete in the CR steel, which contained ε-martensite, and in the annealed steel tested in vacuum, but was substantially less in the annealed steel tested in air and 3.5 pct NaCl solution. These results, taken in conjunction with the crack growth and electrochemical reaction data, support hydrogen embrittlement (HE) as the mechanism for corrosion fatigue crack growth in 304 stainless steels in 3.5 pct NaCl solution. Martensitic transformation appears not to be the only responsible factor for embrittlement. Other microstructural components, such as twin and grain boundaries, slip bands, and cold work-induced lattice defects, may play more important roles in enhancing crack growth rates.     

Fatigue behavior of maraging steel 300

The cyclic stress-strain curves, the low cycle and high cycle fatigue lives and the fatigue crack growth rates of annealed (1 h 820°C) and aged (3 h 480°C) maraging steel 300 were determined. Incremental step testing and stable hysteresis loop tip measurements were used to determine the cyclic σ-ε curves. Both annealed and aged maraging steels were found to cyclically soften at room temperature over a plastic strain range from 0.1 to 20 pct. The S-N curves were determined from 10 to 10⁷ cycles to failure by plastic strain controlled low cycle fatigue tests performed in air and load controlled high cycle fatigue tests performed in dry argon. The test results compared very well with the theoretical lifetime predictions derived from Tomkins’ theory. Fatigue crack growth rates were measured in air and dry argon for the annealed and aged alloys. Crack growth rates of annealed maraging steel were found to be equal to those of aged maraging steel at rates between 10^-7 and 10^-5 in./cycle. A significant difference in crack growth rates in the two environments was found at low stress intensity factor ranges, indicating a high susceptibility to corrosion fatigue in the presence of water vapor. The mechanisms of cyclic softening in the two alloys are discussed in terms of dislocations rearrangement in the annealed alloy and dislocation-precipitate interactions in the aged alloy.     

Subcritical crack growth under sustained load

Compact tension specimens of annealed Ti-4 Al-3 Mo-l V were exposed under sustained load for periods of up to 8 days to determine the effects of initial stress intensity and test environment on subcritical crack growth. Crack growth occurred by a tunneling process with no surface crack extension until just prior to final rapid failure. Crack growth in vacuum or moist air environments occurred at stress intensities as low as 40 pct of the fracture toughness, and there was no evidence of a threshold stress intensity below which crack growth would not occur. Specimens tested in salt water behaved similarly at stress intensities of greater than about 60 pct of the fracture toughness, but showed crack arrest at lower stress intensities. At lower stress intensities, resistance to crack growth in a saltwater environment was superior to that in vacuum or moist air. Subcritical crack growth was readily identified on the fracture surface after exposure in all three environments through the presence of numerous cleavage-like facets. A critical strain concept, with crack growth occurring as a result of creep processes, can be used to explain the results.     

Effect of thermal aging upon the fatigue-crack propagation of austenitic stainless steels

The effect of thermal aging upon the elevated temperature fatigue-crack propagation behavior of several austenitic stainless steels was investigated using linear-elastic fracture mechanics. The steels studied were ABI Types 304, 304L, 316 (both annealed and cold-worked), and weldments in Type 304 using Type 308 filler. Aging temperatures of 1000°F and 1200°F were employed, and aging times ranged between 1500 and 6000 hours. In general, thermal aging produced beneficial results (i.e., lower crack growth rates) relative to unaged material in elevated temperature tests. It is suggested that the precipitation of the various carbide and intermetallic phases is responsible for the beneficial effect. One possible mechanism might be a blunting effect each time the crack tip encounters a second-phase particle, thereby requiring the crack to partially reinitiate itself before proceeding.     

The effect of environment on high-temperature hold time fatigue behavior of annealed 2.25 pct Cr 1 pct Mo steel

Total strain control fatigue tests with a 120-second hold period at either peak compressive or tensile strain were conducted on annealed 2.25 pct Cr 1 pct Mo steel. Tests were performed at the total strain range of 1.0 pct at 500 °C or 600 °C in air, 1.3 Pa (10⁻² torr) or 1.3 × 10⁻³ Pa (10⁻⁵ torr) vacuum. The nature of the hold and the environment affect fatigue life and surface crack patterns. A compressive hold is more deleterious than a tensile hold in high-temperature air, while the reverse is true in environments in which oxidation is limited. Observations of cracks at the surface and in cross section indicate that an oxidation-fatigue interaction accounts for the damaging effect of a compressive hold in air tests. In vacuum tests, creep damage has the opportunity to accumulate and causes the tension hold to exhibit the shortest fatigue lifetime.     

Fatigue crack propagation in martensitic and austenitic steels

Fatigue crack growth rates were measured in an annealed and in an aged maraging steel and in three different austenitic steels. Microhardness measurements were used to determine the plane strain plastic zone sizes as a function of ΔK and to evaluate the cyclic flow stress of the material near the crack tip. The presence of a reversed cyclic plastic zone within the monotonic plastic zone was confirmed. The two maraging steels work soften near the tip of the crack while the three austenitic steels work harden. The fatigue crack growth rates of the maraging steels are independent of the monotonic yield stress and are typical of the growth rates of steels with a bcc crystal structure. The crack growth rates in the stainless steels are an order of magnitude lower than for maraging steels for ΔK< 30 ksi √in. The excellent fatigue crack growth resistance of austenitic stainless steels is related to the de-formation induced phase transformations taking place in the plastic zone and to the low stacking fault energy of the alloys.     

Simple Predicting Method for Fatigue Crack Growth Rate Based on Tensile Strength of Carbon Steel

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An analysis of frequency and amplitude effects on corrosion fatigue crack propagation in Ti-8Al-1Mo-1V

Crack growth rates were measured in Ti-8Al-1Mo-1V, which exemplifies titanium alloys susceptible to transgranular stress-corrosion cracking (SCC). A wide stress intensity amplitude (δK) range, withR = 0, was employed at ½ to 30 cps in H₂O, 3 ½ pct NaCl solution, and methanol; at 10 to 30 cps in air; and at 20 to 30 cps in vacuum, the reference environment. Crack growth was augmented by two kinds of environmental effects. The first, Type A corrosion fatigue (cyclic SCC), was effective at low frequencies in the liquid environments when K_max≥ K_Iscc, intensified with increasing amplitude, but diminished with increasing frequency. In addition to fatigue striations, cleavage on a plane 15 deg from (0001)_α was observed. The second kind, Type B corrosion fatigue, was effective at low amplitudes and in all the environments, diminished at higher amplitudes, was unaffected by changes in cyclic frequency, and was not related to the SCC susceptibility of the alloy. Striated cleavage facets on (0001)_α, and on a plane 15 deg from (0001)_α occurred at low amplitudes, while ordinary ductile striations occurred at higher amplitudes. Fatigue in vacuum did not produce fractographic fatigue striations.     

Crack size effects on the chemical driving force for aqueous corrosion fatigue

Small crack size accelerates corrosion fatigue propagation through high strength 4130 steel in aqueous 3 pct NaCl. The size effect is attributed to crack geometry dependent mass transport and electrochemical reaction processes which govern embrittlement. For vacuum or moist air, growth rates are defined by stress intensity range independent of crack size (0.1 to 40 mm) and applied maximum stress (0.10 to 0.95 Φ_ys). In contrast small (0.1 to 2 mm) surface elliptical and edge cracks in saltwater grow up to 500 times faster than long (15 to 40 mm) cracks at constant δK. Small cracks grow along prior austenite grain boundaries, while long cracks propagate by a brittle transgranular mode associated with tempered martensite. The small crack acceleration is maximum at low δK levels and decreases with increasing crack length at constant stress, or with increasing stress at constant small crack size. Reductions in corrosion fatigue growth rate correlate with increased brittle transgranular cracking. Crack mouth opening, proportional to the crack solution volume to surface area ratio, determines the environmental enhancement of growth rate and the proportions of inter- and transgranular cracking. Small cracks grow at rapid rates because of enhanced hydrogen production, traceable to increased hydrolytic acidification and reduced oxygen inhibition within the occluded cell.     

Effect of cold work on stress corrosion cracking behavior of types 304 and 316 stainless steels

The influence of cold work (prestraining) in the range 2.3 to 56 pct on stress corrosion cracking (SCC) properties of types 304 and 316 stainless steels in boiling MgCl2 solution at 154 °C was investigated using a constant load method. In both materials, SCC initiation was in transgranular mode. Transition in stress corrosion cracking mode from transgranular to intergranular, as the crack proceeds, was observed at all cold work levels in 316 stainless steel and at cold work levels of 26 pct and 56 pct in 304 stainless steel. Both prestraining and increase in the initial applied stress facilitated the transition in crack morphology to intergranular mode. Increased tendency to intergranular SCC at high applied stresses and in cold worked specimens appears to be mechanistically analogous.     

The influence of gaseous environments on fatigue crack growth in a nickel-copper alloy

Cyclic crack propagation rates for a 65 pct nickel-33 pct copper alloy in low pressure, 0.013 MPa (100 torr), environments of hydrogen, oxygen, and nitrogen gas were compared to a reference crack propagation rate in a 1.3μPa vacuum. Crack propagation rates were determined over a range of temperatures for vacuum and hydrogen gas at a constant cyclic stress intensity. Crack propagation in the gaseous environment results in an increased crack propagation rate compared to growth rates in vacuum and a unique fracture mor-phology for each environment. Parallel investigations using transmission electron microscopy showed a unique dislocation structure adjacent to the fracture surface corre-sponding to each fracture morphology and environment. Fracture modes were transgran-ular in vacuum and nitrogen gas, transgranular with crystallographically-oriented features in oxygen gas, and intergranular over a range of temperature in hydrogen. A mechanism is suggested to explain gaseous environmental effects based on dislocation-gas atom inter-action.     

Effect of cold work on stress corrosion cracking behavior of types 304 and 316 stainless steels

The influence of cold work (prestraining) in the range 2.3 to 56 pct on stress corrosion cracking (SCC) properties of types 304 and 316 stainless steels in boiling MgCl2 solution at 154 °C was investigated using a constant load method. In both materials, SCC initiation was in transgranular mode. Transition in stress corrosion cracking mode from transgranular to intergranular, as the crack proceeds, was observed at all cold work levels in 316 stainless steel and at cold work levels of 26 pct and 56 pct in 304 stainless steel. Both prestraining and increase in the initial applied stress facilitated the transition in crack morphology to intergranular mode. Increased tendency to intergranular SCC at high applied stresses and in cold worked specimens appears to be mechanistically analogous.     

Crack propagation rates of austenitic stainless steels under high-temperature low-cycle fatigue conditions

Crack propagation rates of austenitic stainless steels under high temperature low-cycle fatigue conditions were obtained from the fracture surfaces. The crack propagation, rates were expressed as a function of the crack length, independent of the mode of cracks. The proportion of the cycles corresponding to stage II to the fatigue life could be expressed as a function of the fatigue life. The crack propagation rate for solution treated Type 321 steel at 700°C could be expressed as functions of the plastic strain range, the strain-rate, and the crack length.     

Effect of fatigue damage on the dynamic fracture toughness of En-8-grade steel

Machine components normally experience fatigue cycling during operation. Failure of these components is mostly due to fatigue. So, it is important to know the fatigue damage behavior and fatigue life of the material before selecting these steels for making different machine components. The En-8-grade (equivalent to SAE/AISI 1040) steel is generally used as a machine component in the annealed or hardened-and-tempered condition. The fatigue life (fatigue/endurance limit) is also dependent upon the tensile properties of any material. By suitable heat treatment, one can manipulate the tensile properties of any steel. The present work reports the effect of fatigue damage in En-8-grade heattreated steel (annealed and hardened and tempered), under different cyclic loading conditions at room temperature (25 °C), on the impact and dynamic fracture-toughness properties. The results indicate higher fracture toughness and impact toughness in hardened-and-tempered steel than in annealed steel. Cyclic hardening and softening occurs in both the hardened-and-tempered as well as the annealed steel. With the increase of peak stress and number of fatigue cycles, the K _ID and CVN values decrease in hardened-and-tempered steels. The results are discussed in terms of dislocations, slip bands, and their density, microstructure, and fracture morphology.     

Fatigue threshold and low-rate crack propagation properties for structural steels in 3 Pct sodium chloride aqueous solution

The fatigue threshold and low-rate crack propagation properties for a carbon steel, two high-strength steels, and two stainless steels were investigated in a 3 pct sodium chloride aqueous solution at frequencies between 0.03 and 30 Hz. Tests were conducted in a manner designed to avoid crack closure. Under freely corroding conditions, the effective values of the threshold stress intensity factor range, ΔK_th,eff, were lower than in air for all of the steels. In particular, the ΔK_th,eff values for the carbon and high-strength steels were almost equal to the theoretical ΔKth value of about 1 MPa m^1/2 calculated on the basis of the dislocation emission from the crack tip. At a given ΔK level higher than the threshold, the fatigue crack propagation rates accelerated with decreasing frequency for all of the steels. Under cathodic protection, the threshold and fatigue crack propagation properties were coincident with those in air regardless of material and frequency. The observed fatigue crack propagation behavior in a 3 pct NaCl solution was closely related to the corrosion reaction of the bare surface formed at the crack tip during each loading cycle.     

Use of the nanoindentation technique for studying microstructure/crack interactions in the fatigue of 4340 steel

The objectives of this research are to study the influence of microstructure on the fatigue crack growth behavior in 4340 steel and to explore the application of the nanoindentation technique for determining the plastic deformation zone at a fatigue crack tip. Two heat treatment conditions were chosen for the steel: annealed and quenched plus tempered. The annealed steel consists of coarse pearlite and proeutectoid ferrite, while the quenched and tempered steel consists of fine tempered martensite. Fatigue crack propagation tests were conducted on disklike compact (DCT) specimens. Subsequently, the nanoindentation technique was applied to quantitatively determine the plastic deformation zone at fatigue crack tips. The plastic deformation zone size determined by the nanoindentation test seems larger than the cyclic deformation zone calculated using the fracture mechanics equation, which involves many assumptions. The fatigue crack growth test results show that the annealed steel has a higher resistance to crack growth than the quenched and tempered steel. The fatigue crack in the annealed steel tends to grow along pearlite domain boundaries, or the cementite/ferrite interfaces within a pearlite domain. In contrast, the fatigue crack in the quenched and tempered steel tends to traverse the fine martensite laths. Consequently, the actual crack path in the annealed steel is rougher than in the quenched and tempered steel and more secondary cracks are observed in the annealed steel.     

Short fatigue crack growth behavior in a ferritic-bainitic steel

Short fatigue crack growth behavior was studied in a ferrite-bainite microstructure in C-Mn steel with respect to microstructural variations. Specimens were subjected to cyclic loading at three different stress levels: 559, 626, and 687 MPa. The crack propagation rates varied from 10^-4 to 10^-2 μm/cycle. Crack lengths were measured using a replication technique. The growth rates were systematically decreased at microstructural heterogeneities up to a length of 3 to 4 grain diameters. A two-stage short fatigue crack growth model previously developed by Hussain et al. was modified to predict the crack growth behavior. The calculated values were within 10 pct error of the experimentally determined results. The model was then used to present the effect of grain boundaries on cracks propagating at constant rates. It was shown that the mode of presenting of the fatigue data can help in understanding different practical problems in stage I. These include situations such as block loading and short-duration stress spikes in nonpropagating crack regimes.     

Effects of temperature and environment on the tensile and fatigue crack growth behavior of a Ni3AI-base alloy

The effects of temperature, frequency, and environment on the tensile and cyclic deformation behavior of a nickel aluminide alloy, Ni-9.0 wt pct Al-7.97 pct Cr-1.77 pct Zr (IC-221), have been determined. The tensile properties were obtained in vacuum at elevated temperatures and in air at room temperature. The alloy was not notch sensitive at room temperature or at 600 °C, unlike Cr-free Ni₃Al + B alloys. In general, crack growth rates of IC-221 increased with increasing temperature, decreasing frequency, exposure to air, or testing at higherR ratios. At 25 °C, crack growth rates were slightly higher than for a previously investigated Cr-free Ni₃Al alloy. However, at 600 °C, the crack growth rates for IC-221 were lower than for the Cr-free alloy. Substantial frequency effects were noted on crack growth of IC-221 at both 600 °C and 800 °C in both air and vacuum, especially at highK. The relative contributions of creep and environmental interactions to fatigue crack growth are discussed.     

On crack closure of precipitation hardened steels in aqueous solution

Fatigue crack propagation tests were carried out in air and in a 3.5 pct NaCl aqueous solution under cathodic potential of −0.85 V (Ag/AgCl) for aged-hardened high strength steel (Ni−Al−Cr−Mo−C steel). the emphasis in the study was placed on the crack closure behavior of age-hardened materials in air and in the NaCl aqueous solution. The degree of crack closure in air was dependent on the behavior of plastic deformation such as inhomogeneous or homogeneous slip under mixed modes I and II. The underaged material containing coherent precipitates with the matrix had a higher crack opening load in air, compared with the overaged steel containing incoherent precipitates with the matrix. The degrec of crack closure of the underaged material in the NaCl aqueous solution was lower than that in air and was similar to that of overaged materials in the NaCl aqueous solution. It was shown that the decreased crack closure level for the underaged material resulted from accelerated fatigue crack growth under mode I due to hydrogen embrittlement in the aqueous solution.     

Creep-fatigue interaction in austenitic stainless steels

Low cycle fatigue failures occur by the initiation and controlled growth of a surface crack. The development of crack propagation models, based on continuum mechanics, have enabled successful predictions of fatigue life at both room and elevated temperatures. This paper attempts to extend such models to cover the situations in which creep damage, introduced during periods of stress relaxation, influences the rate of growth of the surface fatigue crack. Equations predicting fatigue life as a function of hold period are in good agreement with experimental data, for Type 304 stainless steel, Type 316 stainless steel and Incoloy-800.