Microstructural effects on the cleavage fracture stress of fully pearlitic eutectoid steel

The microstructural parameter(s) controlling the critical cleavage fracture stress, σ_F, of fully pearlitic eutectoid steel have been investigated. Independent variation of the pearlite interlamellar spacing,S _p, and the prior austenite grain size were accomplished through heat treatment. Critical cleavage fracture stresses were measured on bluntly-notched bend specimens tested over the temperature range -125 °C to 23 °C. The cleavage fracture stress increased with decreasingS _p, and was independent of prior austenite grain size. Fine pearlitic microstructures exhibited temperature, strain-rate, and notched-bar geometry independent values for σ_F, consistent with propagation-controlled cleavage fracture. Coarse pearlitic specimens exhibited temperature-dependent values for σ_F over a similar temperature range. Inclusion-initiated fractures were generally located at or beyond the location of the peak normal stress in the bend bar, while cracking associated with pearlite colonies was observed to be closer to the notch than the predicted peak stress location. The calculated values for σ_F were independent of both the type and location of initiation site(e. g., inclusion, pearlite colony). Thus, although inclusions may provide potent fracture initiation sites, their presence or absence does not necessarily change σ_F in fully pearlitic microstructures. formerly Graduate Student, Carnegie Mellon University     

Fracture of steels containing pearlite

The relative effects of pearlite and spherodite on ductile, cleavage, and fatigue failure are summarized. Neither the cleavage strength nor the fatigue endurance limit appear to depend directly on cementite contentper se. Spherodized steels cleave less readily than ferrite/pearlite steels. Ductile fracture resistance is lowered considerably by both types of cementite, pearlite being more deleterious. Ferrite/pearlite steels appear to exhibit slower fatigue crack growth rates at low stress intensity levels than high strength steels. At high stress intensity levels the behavior is reversed. Slip-incuded cracking of carbide lamellae appears easier than that of spherodized carbides. In ductile fracture situations the crack spreads progressively through a pearlite colony via preferential cracking of carbides and rupture of the intervening ferrite accompanied by large local shear strains. Fatigue fracture proceeds with formation of frequent branches, preferentially along the pearlite colony interface. This paper is based on a presentation made at a symposium on “The Cellular and the Pearlite Reactions,” held at the Detroit Meeting of The Metallurgical Society of AIME, October 20, 1971, under the sponsorship of the IMD Heat Treatment Committee.     

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     

Part I-the effects of pre-dissolved hydrogen on cleavage and grain boundary fracture initiation in metastable beta Ti-3Al-8V-6Cr-4Mo-4Zr

The effects of electrochemically pre-dissolved hydrogen on room-temperature fracture initiation in Beta-C titanium (Ti-3Al-8V-6Cr-4Mo-4Zr wt pct) have been investigated using circumferentially notched tensile specimens. Finite element-based analysis of notch stress fields was used to define relationships between the local threshold stress for crack initiation vs total internal hydrogen concentration. The as-received, solution heat treated (ST, σ_y.2 pct=865 MPa) and the ST + peak-aged conditions (STA, σ _y.2% pct=1260 MPa) were compared after defining the relationships between the fracture process zone hydrogen concentration, hydrogen-metal interactions (i.e., hydrostatic stress field occlusion, trapping, hydriding), and the resulting fracture initiation behavior of each. Solutionized + peak-aged (β+α) Beta-C fractured intergranularly above total hydrogen concentrations of ∼1000 wt ppm. (5.1 at. pct). A fracture mode consistent with cleavage occurred at ∼2100 wt ppm. (10.7 at. pct). Solutionized Beta-C resisted hydrogen-assisted cracking (e.g., did not crack intergranularly) but was not immune; cleavage cracking was provoked at ∼4000 wt ppm. (20.4 at. pct). Coldworked ST Beta-C (CW, σ _y.2 pct=1107 MPa) did not crack intergranularly; fracture initiation behavior was similar to the ST condition regardless of specimen orientation. This suggests that high yield strength alone does not account for the susceptibility to intergranular cracking observed in the STA β+α condition. Stroke-rate studies and X-ray diffraction investigation of H partitioning suggests that equilibrium hydriding and/or irreversible trapping does not singularly control intergranular fracture initiation of the STA condition. Fractographic evidence and finite element results show that a finite plastic zone exists prior to intergranular fracture of the STA condition. This suggests that a criterion for fracture that incorporates plastic strain and stress should be considered.     

Effect of Sulfides and Sulfide Morphology on Anisotropy of Tensile Ductility and Toughness of Hot-Rolled C-Mn Steels

The effects of sulfur content (0.004 or about 0.013 pct) and sulfide morphology (stringered or globular) on anisotropy of tensile ductility and Charpy V-notch (CVN) shelf energy were investigated in a series of 0.1 and 0.2 pct carbon, 1.0 pct manganese steels. The effect of sulfide inclusions on fracture strain or CVN shelf energy correlated with a single parameter,P, regardless of inclusion shape, stringered or globular, or test direction, longitudinal, transverse, or through-thickness. The parameterP was defined as the total projected length of inclusions per unit area on a plane parallel to the fracture plane. The fracture strain and CVN shelf energy decreased with an increase inP. The magnitude ofP was directly proportional to the volume fraction of inclusions and inversely proportional to the inclusion dimension perpendicular to the fracture plane. The lower tensile ductility and CVN shelf energy in the 0.2 as compared with the 0.1 pct carbon steels was a consequence of the greater pearlite content in the former steels. This greater pearlite content had no apparent effect on the work-hardening rate,H, but decreased the strain-rate sensitivity of the flow stress,M, and the strain-rate sensitivity of the work-hardening rate,B. The decrease inM andB with increasing pearlite content is in accord with a decrease in tensile ductility according to recent models of neck development in tension tests. It appears that the decrease in tensile ductility with increasing pearlite content is a result of enhanced localized shearing, which promotes the coalescence of voids nucleated at second phases.     

Simulation of transverse crack formation on continuously cast peritectic medium carbon steel slabs

Reliable data are limited to the critical strain for the formation of transverse cracks on the slabs, owing to experimental difficulty to simulate temperature gradient in solidified shell in continuous casting mold. The present study is to determine the critical strain, ?_c, for the formation of transverse cracks on continuously cast slabs. A convenient and simple hot tensile test using rectangular test pieces with either V-notch or semi-circle notch or oscillation marks has been developed by placing the specimen under similar temperature gradient to that in solidified shell in the mold. The ?_c has been determined at a better accuracy and reproducibility, and the ?_c at a strain rate of 5?10^?4s^?1 is found to be a high 35% for test pieces without notch. It sharply decreases, however, to 10% for those with V- and semi-circle-notches, slightly decreases with increasing notch depth, and further decreases for those with oscillation marks that accompany solute segregation. Reduction of the oscillation mark depth is shown to be important measure to prevent the occurrence of transverse cracking of continuously cast slabs.     

Hydrogen blistering and hydrogen‐induced fracture of rail steel

Hydrogen blistering, hydrogen‐induced plasticity loss (HIPL) under slow strain rate test and hydrogen‐induced cracking or fracture (HIC) under constant load for rail steel were evaluated. The threshold diffusible hydrogen concentrations for blistering, HIPL and HIC were 2.03 ppm, 0.26 ppm and 0.24 ppm, respectively. During charging, blistering formed first and fissure initiated at the wall of the blistering. HIPL (l_δ) was found to decrease linearly with the reciprocal of the diffusible hydrogen concentration (C₀), i.e. l_δ = 105 ‐ 27/C₀. The threshold stress for HIC (σ_c) in MPa decreases linearly with In C₀ in ppm i.e., σ_c = 642 ‐ 284 In C₀.     

Stress corrosion cracking in high strength steel under mode III loading

Stress corrosion cracking (SCC) of high-strength steel in aqueous environment and hydrogen induced cracking (HIC) during dynamic charging under Mode III loading were investigated. The threshold stress intensities for SCC and HIC under Modes III and I were measured and compared. It was found that both SCC and HIC under Mode III loading initiated and propagated on the planes inclined at 45 deg to the notch plane, differing from that under Mode I loading. The fracture surfaces, however, revealed intergranular facets, similar to that under Mode I loading. The addition of thiourea decreased the threshold value for SCC under Mode III and Mode I loading, which was still higher than that for dynamic charging. The threshold values of both SCC and HIC under Mode III were larger than that under Mode I,i.e., K_IIIH> K_IH, K_IIISCC > K_ISCC. Based upon the fracture mechanics analysis, this difference is attributed to the different equilibrium hydrogen concentration between Modes III and I loading. These results give strong evidence that the SCC mechanism in high strength steel under Mode III loading is also related to hydrogen induced cracking. Formerly Student at Beijing University of Iron and Steel     

Effect of Notch Root Radius on the Apparent Fracture Toughness in CNT and Carbon Fiber Reinforced, Epoxy-Matrix Hybrid Composite

Significantly improved fracture resistance (in terms of fracture toughness) can be imparted to monolithic materials by adopting composite design methodology based on fiber and nano particulate reinforcement technology. The present work describes the fracture behaviour of one such reinforced material; in this case, carbon fiber (C_f)- and carbon nanotube (CNT)- reinforced epoxy composite. The C_f and CNT reinforced, epoxy-matrix hybrid composite in longitudinal and transverse orientations with varied finite notch root radii (in the range of 120–750 μm) are subjected to mode-I (tensile) fracture. The fracture toughness/resistance (K_Q) of the material is then evaluated and analyzed by investigating the influence of varying notch root radii in longitudinal and transverse orientations. Such an analysis has revealed that the present unidirectional epoxy hybrid composite exhibits a critical notch root radius of 270 μm in longitudinal and 390 μm in transverse orientation.     

Effects of loading rate on fracture behavior of low-alloy steel with different grain sizes

Four-point bend (4PB) tests of notched specimens loaded at various loading rates, for low alloy steel with different grain sizes, were done, and the microscopic observation and finite-element method (FEM) calculations were carried out. It was found that for the coarse-grained (CG) microstructure, an appreciable drop in notch toughness with a loading rate of around 60 mm/min appeared, and further increasing the loading rate leads to a slight additional decrease in notch toughness. For the fine-grained (FG) microstructure, the effect of loading rate was not apparent. The change in toughness resulted from a change of the critical event controlling the cleavage fracture with increasing loading rate. For the CG microstructure with a lower cleavage-fracture stress (σ _f ), with an increasing loading rate, the critical event of cleavage fracture can be changed from the propagation of a pearlite colony-sized crack or a ferrite grain-sized crack, through the mixed critical events of crack propagation and crack nucleation, then to crack nucleation. This change deteriorates the toughness. For the FG microstructure with a higher cleavage-fracture stress, the critical event of cleavage fracture is the crack propagation and does not change in the loading-rate range from 120 to 500 mm/min. The measured σ _f values do not change with loading rate, as long as the critical event of cleavage fracture does not change. The higher notch toughness of the FG microstructure arises from its higher σ _f and the critical plastic strain (ε _pc ) for initiating a crack nucleus, and the fracture behavior of this FG steel is not sensitive to loading rate in the range of this work.     

The Influence of Notch Root Radius and Austenitizing Temperature on Fracture Appearance of As-Quenched Charpy-V Type AISI4340 Steel Specimens

Charpy-V type samples either step-quenched from 1200 °C or directly quenched from the usual 870 °C temperature, fractured by a slow bend test procedure, have been fractographically examined. Their notch root radius,ρ, ranged from almost zero (fatigue precrack) up to 2.0 mm. The fracture initiation process at the notch differs according to root radius and heat treatment. Conventionally austenitized samples withρ values larger than 0.07 mm approximately (ρ _eff) always display a continuous shear lip formation along the notch surface, whereas specimens with smaller notches do not exhibit a similar feature. Moreover, shear lip width in specimens withρ >ρ _eff is linearly related to the applied J-integral at fracture. In high temperature austenitized samples similar shear lips are almost nonexistent. The above findings, as well as overall fractographic features, are combined to explain why blunt notch AISI 4340 steel specimens display a better fracture resistance if they are conventionally heat treated, whereas fatigue precracked samples show a superior fracture toughness when they are step-quenched from 1200 °C. Variations of fracture morphologies with the notch root radius and heat treating procedures are associated with a shift toward higher Charpy transition temperatures under the combined influence of decreasing root radii and coarsening of the prior austenitic grain size at high austenitizing temperatures. D. FIRRAO, J. A. BEGLEY were both formerly with the Department of Metallurgical Engineering, The Ohio State University, Columbus, OH 43210.     

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.     

The effect of alloying elements and microstructure on the strength and fracture resistance of pearlitic steel

The processes of ductile and brittle fracture in fully pearlitic steel and their relation to both the scale of the microstructure and the presence of substitutional alloy elements have been investigated at room temperature using smooth tensile and over a range of temperatures using V-notched Charpy impact specimens. The results show that the early stages of cracking, revealed in both types of specimen, are largely the result of shear cracking of the pearlite lamellae. These cracks grow and can reach a size when they impinge upon the prior austenite boundary; afterward the character of fracture can be either microvoid coalescence or cleavage, depending on test conditions and metallurgical variables. Further, the carbide plates of the pearlite lamellae can act as barriers to the movement of dislocations as is the case normally with grain boundaries. For pearlite an optimum spacing of approximately 0.2 μm resulting from a balance between carbide plate thickness and interlamellar spacing was found to enhance toughness, although such changes are much smaller than corresponding changes due to varying alloy elements. Specific alloy elements used herein strengthened the lamellar ferrite in pearlite, inhibiting the movement of dislocations while also usually decreasing the lamellar cementite plate thickness for the same spacing. This dual behavior results in enhanced resistance to the initiation and propagation of microcracks leading to an improvement in strength, ductility, and toughness. The most effective alloy elements for the composition ranges studied in fully pearlitic steels are Si and Ni for strength improvement, and Ni and Mn for toughness.     

Fatigue crack growth behavior of inconel 600 at cathodic potentials

The fatigue crack growth rate of Inconel 600 is affected by applied potential, test frequency, and prior thermal treatment. At an applied cathodic potential of -700 mV (SCE), in IN H₂SO₄, a decrease in test frequency produces an increase in crack growth,da/dN, while a thermal treatment of 700 °C for 0.5 h or more produces intergranular fracture at lowA.K. Intergranular cracking of annealed samples is observed only after precharging with cathodically produced hydrogen. The enhanced FCG rate and tendency for intergranular cracking are mutually exclusive effects in that test frequencies between 1 and 10 Hz do not affect fracture mode, and thermal treatment at 700 °C for 0.5 to 100 h does not affect crack growth rate. It is suggested that since both effects are observed only at cathodic potentials, the data support a hydrogen embrittlement mechanism. G. S. WAS, formerly Research Assistant, Nuclear Engineering Dept., Massachusetts Institute of Technology formerly Postdoctoral Associate, Department of Materials Science and Engineering, Massachusetts Institute of Technology     

Evaluation of toughness in AISI 4340 alloy steel austenitized at low and high temperatures

It has been reported for as-quenched AISI 4340 steel that high temperature austenitizing treatments at 1200°C, instead of conventional heat-treatment at 870°C, result in a two-foldincrease in fracture toughness,K _Ic, but adecrease in Charpy impact energy. This paper seeks to find an explanation for this discrepancy in Charpy and fracture toughness data in terms of the difference betweenK _Ic and impact tests. It is shown that the observed behavior is independent of shear lip energy and strain rate effects, but can be rationalized in terms of the differing response of the structure produced by each austenitizing treatment to the influence of notch root radius on toughness. The microstructural factors which affect this behavior are discussed. Based on these and other observations, it is considered that the use of high temperature austenitizing be questioned as a practical heat-treatment procedure for ultrahigh strength, low alloy steels. Finally, it is suggested that evaluation of material toughness should not be based solely onK _Ic or Charpy impact energy values alone; both sharp crack fracture toughness and rounded notch impact energy tests are required. formerly with Effects Technology, Inc., Santa Barbara, CA     

Fatigue crack growth through

Fatigue cracks were grown at 25 °C and 800 °C in a titanium aluminide alloy heat-treated to give a γ+ α ₂ lamellar microstructure. These lamellae, having widths of =0.5 to 2 μm, were in colonies approximately 1.2 mm across. Crack growth was observed and photographed under high resolution conditions using a loading and heating cyclic stage for the scanning electron microscope. Stereoimaging was used to measure displacements around crack tips, from which crack opening displacements and strains were derived. Cracks were found to grow about 10 times faster at 25 °C than at 800 °C, and the threshold stress intensity for fatigue crack growth was lower at 25 °C. Strain to fracture the lamellae was determined as ≈0.08, while fatigue crack tips could sustain up to 0.3 strain at 25 °C and 0.5 strain at 800 °C. The lamellar micro- structure was found to have a strong influence on crack tip behavior.     

Research of the Continuous Casting Slab Transverse Cracking During Retarded Cooling Process

In this paper, the reason of 50Mn2V transverse cracking is studied by optical metallographic, SEM and in situ analysis system. Results show that the pearlite is surrounded by the net‐like pro‐eutectoid ferrite which locates at the former austenite grain boundary, the hydrogen content (ca.2 × 10^?4 wt%) exceeds the critical solubility in room temperature structure, divers types, and sizes inclusions (10–100 µm), plenty of fissures (2–50 µm) and classical cleavage fracture surface are obviously observed, meanwhile, a great degree of elements segregation (P, S, V, Ni, Cr correspond to 1.757, 1.674, 1.729, 1.475, 1.195, respectively) and a poorly 0.9283 integral density exist in this blank. From the above, internal gas pressure should be the dominant factor that leads the transverse embrittlement, while internal stresses, element segregation, and defects play an ancillary role.     

Cleavage fracture in pearlitic eutectoid steel

The effect of microstructure on flow and fracture properties of fully pearlitic steel has been studied by independently varying the prior austenite grain size and the pearlite interlamellar spacing through appropriate heat treatments. The yield strength is independent of the prior austenite grain size but increases as the interlamellar spacing or the temperature decreases. The microstructural dependence can be explained by using a model which assumes that yielding is controlled by dislocation motion in the ferrite lamellae. The critical tensile stress for cleavage fracture is found to be independent of prior austenite grain size, increasing as the interlamellar spacing decreases. The cleavage fracture stress is independent of temperature for fine pearlite but increases as the temperature decreases for coarse pearlite. The associated fracture in blunt notch specimens initiates at inclusions beneath notch surface near the location of maximum tensile stress. From the size of such inclusions, the effective surface energy for cleavage fracture can be directly calculated and is found to be independent of temperature and prior austenite grain size but to increase as the interlamellar spacing decreases, from about 5 to 13 J/m² for the range of microstructures and temperatures used in this study. Additional measurements of the effective surface energy and further theoretical analyses of the cleavage process are needed. D.J. ALEXANDER, formerly of Carnegie Mellon University I. M. BERNSTEIN, formerly of Carnegie Mellon University     

Hydrogen embrittlement, grain boundary segregation, and stress corrosion cracking of alloy X-750 in low-and high-temperature water

The nature of intergranular stress corrosion cracking (SCC) of alloy X-750 was characterized in low-and high-temperature water by testing as-notched and precracked fracture mechanics specimens. Materials given the AH, BH, and HTH heat treatments were studied. While all heat treatments were susceptible to rapid low-temperature crack propagation (LTCP) below 150 °C, conditions AH and BH were particularly susceptible. Low-temperature tests under various loading conditions (e.g., constant displacement, constant load, and increasing load) revealed that the maximum stress intensity factors (K _{p max}) from conventional rising load tests provide conservative estimates of the critical loading conditions in highly susceptible heats, regardless of the load path history. For resistant heats, K _{P max} provides a reasonable, but not necessarily conservative, estimate of the critical stress intensity factor for LTCP. Testing of as-notched specimens showed that LTCP will not initiate at a smooth surface or notch, but will readily occur if a cracklike defect is present. Comparison of the cracking response in water with that for hydrogen-precharged specimens tested in air demonstrated that LTCP is associated with hydrogen embrittlement of grain boundaries. Equivalent activation energies for stage II LTCP rates (11.3 kcal/mol) and hydrogen diffusion (11.5 kcal/mol) indicate that hydrogen diffusion to the peak stress region ahead of a crack is the rate-controlling process. Auger analysis showed that variability in LTCP resistance is associated with phosphorus and sulfur segregation to grain boundaries. Above 150 °C, an increase in fracture resistance and decrease in the degree of hydrogen enrichment precludes rapid intergranular cracking. The stress corrosion crack initiation and growth does occur in high-temperature water (>250 °C), but crack growth rates are orders of magnitude lower than LTCP rates. The SCC resistance of HTH heats is far superior to that of AH heats as crack initiation times are two to three orders of magnitude greater and growth rates are one to two orders of magnitude lower.