Influence of microstructure on fatigue crack initiation in fully pearlitic steels

The effect of microstructure on the fatigue crack initiation of fully pearlitic steels was studied through independent variation of the prior austenite grain size, pearlite colony size, and the pearlite interlamellar spacing. Increasing yield strength (controlled by decreasing the pearlite interlamellar spacing) was seen to increase the smooth and notched-bar crack initiation endurance limit. Grain and colony size variations, at constant yield strength, were seen to exhibit no effect on crack initiation. Scanning Electron Microscopy revealed smooth-bar cracks to have initiated at surface inclusions. The influence of the pearlite interlamellar spacing, reflecting a change in the effective slip length, and the differences between notched and smooth-bar fatigue specimens for studying the effects of microstructure on crack initiation are discussed. Formerly with Carnegie-Mellon University     

The role of microstructure on the strength and toughness of fully pearlitic steels

An experimental program was carried out to clarify the structure-property relationships in fully-pearlitic steels of moderately high strength levels, and to identify the critical microstructural features that control the deformation and fracture processes. Specifically, the yield strength was shown to be controlled primarily by the interlamellar pearlite spacing, which itself was a function of the isothermal transformation temperature and to a limited degree the prior-austenite grain size. Charpy tests on standard and fatigue precracked samples revealed that variations in the impact energy and dynamic fracture toughness were dependent primarily on the prior-austenite grain size, increasing with decreasing grain size, and to a lesser extent with decreasing pearlite colony size. These trends were substantiated by a statistical analysis of the data, that identified the relative contribution of each of the dependent variables on the value of the independent variable of interest. The results were examined in terms of the deformation behavior being controlled by the interaction of slip dislocations with the ferrite- cementite interface, and the fracture behavior being controlled by a structural subunit of constant ferrite orientation. Preliminary data suggests that the size of such units are controlled by, but are not identical to, the prior-austenite grain size. Possible origins of this fracture unit are considered.     

The role of microstructure on the strength and toughness of fully pearlitic steels

An experimental program was carried out to clarify the structure-property relationships in fully-pearlitic steels of moderately high strength levels, and to identify the critical microstructural features that control the deformation and fracture processes. Specifically, the yield strength was shown to be controlled primarily by the interlamellar pearlite spacing, which itself was a function of the isothermal transformation temperature and to a limited degree the prior-austenite grain size. Charpy tests on standard and fatigue precracked samples revealed that variations in the impact energy and dynamic fracture toughness were dependent primarily on the prior-austenite grain size, increasing with decreasing grain size, and to a lesser extent with decreasing pearlite colony size. These trends were substantiated by a statistical analysis of the data, that identified the relative contribution of each of the dependent variables on the value of the independent variable of interest. The results were examined in terms of the deformation behavior being controlled by the interaction of slip dislocations with the ferrite- cementite interface, and the fracture behavior being controlled by a structural subunit of constant ferrite orientation. Preliminary data suggests that the size of such units are controlled by, but are not identical to, the prior-austenite grain size. Possible origins of this fracture unit are considered.     

The effect of microstructure on the fatigue crack growth behavior of Age-Hardened high strength steels in a corrosive environment

Fatigue crack growth rates have been measured in laboratory air and in a 3.5 pct NaCl aqueous solution under cathodic polarization of -0.85 V(Ag/AgCl reference electrode) for various agehardened, high strength steels. At low ΔK values in air, the fatigue crack growth resistance of the underaged condition was improved compared to the overaged condition. This improvement can be explained with increased crack closure and slip reversibility. The fatigue crack growth resistance of material containing both coherent precipitates and incoherent precipitates with the matrix was similar to that of the material including only incoherent precipitates because of the restriction of inhomogeneous deformation by the incoherent precipitates. Corrosion fatigue crack growth resistance in the aqueous solution was dependent on aging conditions. The environmental sensitivity was greater in the underaged materials owing to hydrogen embrittlement. The environmental sensitivity of underaged materials containing coherent precipitates with the matrix was improved by the coexistence of incoherent precipitates. The decreased amount of strain to fracture due to hydrogen diffusing into plastic zone led to decreased crack closure.     

High-temperature fatigue crack growth behavior of 17-4 PH stainless steels

The fatigue crack growth (FCG) behavior was investigated for 17-4 PH stainless steels in three heat-treated conditions, i.e., unaged (condition A), peak-aged (condition H900), and overaged (condition H1150), at temperatures ranging from 300 °C to 500 °C. The high-temperature fatigue crack growth rates (FCGRs) of condition H1150 were increased with an increase in temperature. However, for conditions A and H900 tested at 500 °C, the FCGRs were lower than the lower temperature ones. At 300 °C and 400 °C, H1150 and H900 generally showed the lowest and highest FCGRs, respectively, with condition A demonstrating behavior between the two. At 500 °C, the FCGR curves for all material conditions merged together. The anomalous FCG behavior of 17-4 PH stainless steels at 500 °C was mainly caused by an in-situ precipitate-coarsening effect during test. This work was funded by the National Science Council of the Republic of China (Taiwan) under Contract Nos. NSC-90-2216-E-008-007 and NSC-91-2216-E-008-007.     

Influence of strain-induced martensitic transformations on fatigue crack growth rates in stainless steels

The fatigue crack growth rates (FCGR) of two unstable austenitic stainless steels (Fe-16 Cr-13Ni) and (Fe-18Cr-6.5Ni-0.19C) were determined in theM_s-M_d temperature range where a strain induced μ → α′ martensitic transformation occurs near the crack tip. These FCGR were compared to the rates measured in the stable austenitic phase of a Fe-31.5Ni and a Fe-34 Ni alloy and in the martensitic phase obtained by quenching the Fe-31.5 Ni alloy below M_s. In the Fe-31.5 Ni, the FCGR are an order of magnitude higher in the martensitic than in the austenitic structures for ΔK ≤ 40 ksi in. The FCGR of the stainless steels decrease markedly when the test temperature approachesM _s in theM _s - M_d range. The FCGR for the alloy Fe-18Cr-6.5 Ni-0.19 C in a warm-worked condition are consistently higher than for the same alloy in the annealed condition for ΔK ≤ 40 ksi √in.. The results are discussed in terms of the influence of phase structures, stacking fault energy and work hardening exponent on the FCGR.     

The influence of a duplex microstructure in steels on fatigue crack growth in the near-threshold region

The influence of a duplex microstructure on fatigue crack growth in the near threshold region has been studied for AISI types 1018, 1045, and 10B35, as well as 2.25 Cr-1 Mo steels. For a duplex microstructure which consists of a continuous martensitic phase encapsulating ferrite, an increase in threshold level and yield strength in both the AISI 1018 and 2.25 Cr-1 Mo steels was observed. However, with increase in carbon content and consequent decrease in the volume of ferrite, the threshold levels were not as significantly affected, although the yield strengths were higher. Y. MATSUO, formerly Graduate Student, University of Connecticut     

Role of microstructure and residual stresses on fatigue crack initiation of carbonitrided steels

In this paper the role of microstructure and residual stresses on fatigue crack initiation of gaseous carbonitrided steels is examined. Optimum for austenite content in the case is investigated for microstructures containing bainite and for those without bainite. Fractographic study suggests subsurface initiation sites for notched specimens when austenite content is high and initiation sites at the surface when austenite is low.     

Effect of environment on crack growth behavior in austenitic stainless steels under creep and fatigue conditions

Crack growth behavior at high temperatures under cyclic, static, and combined loads was studied in annealed and 20 pct cold-worked Type 316 and 20 pct cold-worked Type 304 austenitic stainless steels in air and vacuum. Under cyclic load, crack growth rates in annealed Type 316 steel are slightly lower in vacuum than in air, but this difference decreases with increase in crack growth rate. Most importantly, the effect of temperature on crack growth is present even in vacuum and arises mostly from the variation of elastic modulus with temperature. In the cold-worked Type 316 steel, the pronounced hold-time effects on fatigue crack growth in air reported in the literature persists even in vacuum. This implies that at high crack growth rates these hold-time effects arise mostly from creep-fatigue interaction rather than environment fatigue interaction. Environment has a negligible effect also on crack growth under static load. Thus, time dependent crack growth in these steels is due to creep processes. Crack growth behavior in annealed and cold-worked materials are compared and reasons for the enhanced time dependent crack growth in cold-worked material are discussed in detail.     

Fatigue crack growth behavior of 4340 steels

Fatigue crack growth behavior of 4340 steels was investigated in four gaseous environments; laboratory air, wet hydrogen, dry hydrogen and dry helium. Specimen orientation does not affect crack propagation rate results. The effects of R-ratio (load ratio) and environment on crack growth rate properties are interrelated. Increasing R -ratio increases the rates of near-threshold crack propagation. Nevertheless, the effect of R-ratio on crack growth rates in air is much more significant than that in the two dry environments. Interestingly, the R-ratio effect in wet hydrogen is comparable to that in dry environments. At an R-ratio of 0.1, the rates of crack propagation in air are slower than those in dry environments while crack growth rates are essentially identical in wet hydrogen and dry environments. Increasing R -ratio was found to decrease the environmental effect. Furthermore, increasing yield strength from 700 to 1040 MPa does not affect crack propagation behavior. While surface roughness-induced crack closure is thought to be minimal in affecting gaseous-environment near-threshold crack growth behavior of 4340 steels, oxide-induced crack closure governs crack propagation kinetics. It is suggested that in moisture-containing environments, thick oxide deposits measured on fracture surfaces may not result in high crack closure levels. Nevertheless, oxide-induced crack closure rationalized the effects of R-ratio and environment on near-threshold crack growth rate properties. Furthermore, hydrogen embrittlement is believed not to play an important role in influencing wet-hydrogen environment near-threshold crack propagation behavior. At higher ΔK levels (⩾ 12 MPa √m), an “intrinsic” dry hydrogen effect seems to be present, and crack closure, however, cannot account for the environmental effect.     

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.     

Near-threshold fatigue crack growth behavior in copper

Near-threshold fatigue crack growth rate data were developed in annealed, quarter-hard, and full-hard copper at various load ratios, (R = σ_min/σ_max). Increasing theR value decreases the resistance to threshold crack growth. At a fixed value ofR, annealed copper has the slowest near-threshold crack propagation rate while full-hard copper has the fastest crack growth rate. Waveform (sine and triangle) and specimen geometry (WOL, CT, and CCT) do not appear to affect the rates of near-threshold crack propagation. The influences of load ratio and material strength on threshold crack growth behavior can be rationalized by crack closure.     

Near-threshold fatigue crack growth behavior in metals

Near-threshold fatigue crack growth rate (FCGR) behavior in six alloy systems (iron, aluminum, copper, magnesium, nickel and titanium) is extensively reviewed and compared. It is suggested that a unique effective threshold stress intensity range, ΔK_th,eff, exists for each alloy system in room temperature air. The value of ΔK_th,eff was found to be directly proportional to Young's modulus, E Furthermore, Young's modulus normalizes the near-threshold fatigue crack propagation behavior in the various alloy systems investigated. The present findings are consistent with the existing threshold FCGR theories that relate ΔK_th,eff to E. Additional research is required, however, to incorporate crack closure phenomena in these near-threshold FCGR theories to assess the influence of microstructure, load ratio and environment.     

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.     

Influence of deformation-induced martensite on fatigue crack propagation in 304-type steels

This research reports an investigation into the influence of mechanically induced martensitic transformation on the rate of fatigue crack growth in 304-type austenitic stainless steels. Two steels of different composition, 304L and 304LN, were used to test the influence of composition; two test temperatures, 298 and 77 K, were used to study the influence of test temperature; and various load ratios were used to determine the influence of the mean stress. It was found thadecreasing the mechanical stability of the austenite by changing composition or lowering temperature reduces the fatigue crack growth rate and increases the threshold stress intensity for crack growth. However, this beneficial effect diminishes as the load ratio increases, even though increasing the load ratio increases the extent of the martensite transformation. Several mechanisms that may influence this behavior are discussed, including the perturbation of the crack tip stress field, crack deflection, work hardening, and the relative brittleness of the transformed material. The perturbation of the stress field seems to be the most important; by modifying previous models, we develop a quantitative analysis of the crack growth rate that provides a reasonable fit to the experimental results.     

Influence of deformation substructure on flow and fracture of fully pearlitic steel

Tensile stress-strain data over the whole strain range were obtained for a range of pearlites from very coarse to relatively fine (interlamellar spacings 0.53 and 0.13 μm, respectively). Transmission electron microscopy (TEM) for pearlite subjected to various amounts of strain was performed. Coupling mechanical data with TEM examination provided a detailed picture of how pearlite yields, deforms, work hardens, and fails under uniaxial tension. It is shown that yielding and work hardening of pearlite are largely controlled by processes occurring in ferrite. The role of a cementite plate at low stresses is mainly to limit the slip distance in ferrite. It is found that the tensile fracture is determined by processes in the colonies with lamellae parallel to the tensile axis and that the stress necessary to break a cementite plate corresponds to the true U.T.S. The influence of interlamellar spacing on the yield strength, flow stress, and the true U.T.S. is quantitatively explained.     

The effect of microstructure on the fatigue crack propagation behavior of an Al-Zn-Mg-Cu alloy

The effect of slip distribution on the fatigue crack propagation behavior in vacuum of a high purity Al-5.9Zn-2.6Mg-l.7Cu alloy in various age-hardened conditions has been investigated. The crack propagation resistance was observed to be significantly higher for underaged microstructures containing shearable precipitates in comparison to overaged conditions with nonshearable precipitates. The improved crack propagation resistance is attributed in part to an increased amount of reversed slip in the plastic zone at the crack tip due to a higher degree of planar slip for conditions with shearable precipitates. The observed increase in fatigue crack propagation resistance with decreasing precipitate size for microstructures containing a constant volume fraction of shearable precipitates cannot be explained on the basis of such slip reversibility alone. The variation in ductility for the different microstructures has also to be taken into account. It was found that the enhanced crack propagation resistance can be correlated to the increased ductility with decreasing precipitate size. This explanation was supported by the experimental observation that microstructures containing different volume fractions and sizes of shearable precipitates but exhibiting the same ductility showed approximately the same resistance against fatigue crack propagation. formerly with German Aerospace Research Establishment (DFVLR), Cologne, Germany. formerly with Ruhr-University, Bochum, Germany.     

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.     

A rationalisation of fatigue thresholds in pearlitic steels using a theoretical model

From a detailed re-examination of results in the literature, the effects of microstructure sizes, namely interlamellar spacing, pearlitic colony size and the prior austentitic grain size on the thresholds for fatigue crack growth (ΔK_th) and crack closure (K_{cl, th}) have been illustrated. It is shown that while interlamellar spacing explicitly controls yield strength, a similar effect on ΔK_th cannot be expected. On the other hand, the pearlitic colony size is shown to strongly influence ΔK_th and K_{cl, th} through the deflection and retardation of cracks at colony boundaries. Consequently, an increase in ΔK_th and K_{cl, th} with colony size has been found. The development of a theoretical model to illustrate the effects of colony size, shear flow stress in the slip band and macroscopic yield strength on K_{cl, th} and ΔK_th is presented. the model assumes colony boundaries as potential sites for slip band pile-up formation and subsequent crack deflection finally leading to zig-zag crack growth. Using the concepts of roughness induced crack closure, the magnitude of K_{cl, th} is quantified as a function of colony size. In deriving the model, the flow stress in the slip band has been considered to represent the work hardened state in pearlite. Comparison of the theoretically predicted trend with the experimental data demonstrates very good agreement. Further, the intrinsic or closure free component of the fatigue threshold, ΔK_{eff, th} is found to be insensitive to colony size and interlamellar spacing. Using a criterion for intrinsic fatigue threshold which considers the attainment of a critical fracture stress over a characteristic distance corresponding to interlamellar spacing, ΔK_th values at high R values can be estimated with reasonable accuracy. The magnitude of ΔK_th as a function of colony size is then obtained by summing up the average value of experimentally obtained ΔK_{eff, th} values and the predicted K_{cl, th} values as a function of colony size. Again, very good agreement of the theoretically predicted ΔK_th values with those experimentally obtained has been demonstrated.