Analysis of fracture toughness in the transition-temperature region of an Mn-Mo-Ni low-alloy steel

This study is concerned with the analysis of fracture toughness in the transition region of an Mn-Mo-Ni low-alloy steel, in accordance with the ASTM E1921 standard test method. Elastic-plastic cleavage fracture toughness (K _Jc ) was determined by three-point bend tests, using precracked Charpy V-notch (PCVN) specimens, and relationships between K _Jc , the critical component of J (J _c ), critical distance (X _c ), stretch-zone width (SZW), local fracture stress, and plane-strain fracture toughness (K _Ic were discussed on the basis of the cleavage fracture behavior in the transition region. The master curve and the 95 pct confidence curves well explained the variation in the measured K _Jc , and the Weibull slope measured on the Weibull plots was consistent with the theoretical slope of 4. Fractographic observation indicated that X _c linearly increased with increasing J _c , and that the SZW had a good correlation with K _Jc , irrespective of the test temperature. In addition, the local fracture stress was independent of the test temperature, because the tempered bainitic steel used in this study showed a propagation-controlled cleavage fracture behavior.     

Tempered martensite embrittlement in SAE 4340 steel

The toughness of SAE 4340 steel with low (0.003 wt pct) and high (0.03 wt pct) phosphorus has been evaluated by Charpy V notch (CVN) impact and compact tension plane strain fracture toughness (K _1c) tests of specimens quenched and tempered up to 673 K (400°C). Both the high and low P steel showed the characteristic tempered martensite embrittlement (TME) plateau or trough in room temperature CVN impact toughness after tempering at temperatures between 473 K (200°C) and 673 K (400°C). The CVN energy absorbed by low P specimens after tempering at any temperature was always about 10 J higher than that of the high P specimens given the same heat treatment. Interlath carbide initiated cleavage across the martensite laths was identified as the mechanism of TME in the low P 4340 steel, while intergranular fracture, apparently due to a combination of P segregation and carbide formation at prior austenite grain boundaries, was associated with TME in the high P steel.K _IC values reflected TME in the high P steels but did not show TME in the low P steel, a result explained by the formation of a narrow zone of ductile fracture adjacent to the fatigue precrack during fracture toughness testing. The ductile fracture zone was attributed to the low rate of work hardening characteristic of martensitic steels tempered above 473 K (200°C).     

Effects of test temperature and grain size on the charpy impact toughness and dynamic toughness (<Emphasis Type="Italic">K</Emphasis><Subscript><Emphasis Type="Italic">ID</Emphasis></Subscript>) of polycrystalline niobium

The effects of changes in test temperature (−196 °C to 25 °C) and grain size (40 to 165 μm) on the dynamic cleavage fracture toughness (K _ID ) and Charpy impact toughness of polycrystalline niobium (Nb) have been investigated. The ductile-to-brittle transition was found to be affected by both changes in grain size and the severity of stress concentration (i.e., notch vs fatigue-precrack). In addition to conducting impact tests on notched and fatigue-precracked Charpy specimens, extensive fracture surface analyses have been performed in order to determine the location of apparent cleavage nucleation sites and to rationalize the effects of changes in microstructure and experimental variables on fracture toughness. Existing finite element analyses and the stress field distributions ahead of stress concentrators are used to compare the experimental observations with the predictions of various fracture models. The dynamic cleavage fracture toughness, K _ID , was shown to be 37±4 MPa√m and relatively independent of grain size (i.e., 40 to 105 μm) and test temperature over the range −196 °C to 25 °C.     

Effect of tempering on quasi-static and impact fracture toughness and mechanical properties for 5140 H steel

The effects of various thermal treatments,i.e., oil quench and different tempering conditions, on quasi-static and impact fracture toughness, stress-strain characteristics, hardness, and Charpy energy of 5140 H steel were examined. During quasi-static and impact loading notched round tensile specimens were used with a prefatigued crack. A specially designed device together with a pendulum hammer and electronic measuring system was used enabling testing of the opening mode fracture toughness at loading rates up to K₁ = 3 x 10⁶ MPa√m per second. It has been found that within the region of the lower tempering temperatures, 500 K≤ 650 K, the critical stress intensity factor K_Ic determined from impact testing is lower than that obtained during slow loading, whereas at the higher tempering temperatures, 650 K ≤T* ≤ 900 K, dynamic K_Iu values show a tendency to be higher than their quasi-static counterparts. This behavior was analyzed quantitatively using the Hahn-Rosenfield model which relates tensile properties to fracture toughness. A good agreement was found between quasi-static experimental results and the model. The relation between Charpy energy K_v and the critical stress intensity factor K_Ic was also evaluated. Changes of the fracture toughness are discussed within the framework of SEM fractographs taken after quasi-static and impact tests. On leave of absence from Institute of Fundamental Technological Research, Polish Academy of Sciences, Warsaw, Poland.     

Static and dynamic fracture toughness of a medium carbon microalloyed steel of three different microstructures

A multiphase ferrite-bainite-martensite (F-B-M) microstructure was developed in an automotive grade V-bearing medium carbon microalloyed steel, 38MnSiVS5. It was characterized using optical, scanning, and transmission electron microscopy. The tensile, Charpy impact, and static and dynamic fracture toughness behaviors were evaluated. The results are compared with those of ferrite-pearlite (F-P) and tempered martensite (T-M) microstructures of the same steel. Although the tensile properties of the multiphase microstructures were superior, the Charpy impact and static and dynamic fracture toughness properties were inferior compared with those of the other two microstructures. The F-P condition displayed the highest plane strain fracture toughness value (K_IC), while the T-M condition was characterized by the highest dynamic fracture toughness (conditional) value (K_IDQ). The Charpy impact energy of the T-M condition was greater than that for the other two conditions. An examination of the surfaces of fractured samples revealed predominant ductile crack growth in the F-P microstructure and a mixed mode (ductile and brittle) crack growth in the T-M and the F-B-M microstructures. Although the Charpy impact energy, plane fracture toughness (K_IC), and conditional dynamic fracture toughness (K_IDQ) of the multiphase microstructure were inferior to those of the T-M and the F-P microstructures, the toughness properties were comparable to those of medium carbon low alloy steels having bainite-martensite (AISI 4340) or tempered martensite microstructures.     

Correlations of microstructure with dynamic and quasi-static fracture in a plain carbon steel

An investigation was conducted into the effects of temperature, loading rate, and various micro-structural parameters on the initiation of plane strain fracture of a plain carbon AISI 1020 steel. Ferrite and prior austenite grain sizes were chosen as the principal microstructural features to be in-vestigated. The microstructural variations were accomplished by changing the austenitizing tempera-ture and by altering the cooling rate during normalization. Fracture toughness tests were conducted using precracked notched round bars loaded in tension to produce two stress intensity rates,viz.,K ₁ = 1 MPa √m s^-1 andK ₁ = 2 × 10⁶ MPa √m s^-1. In addition, Charpy impact tests along with quasistatic and high rate plasticity tests were conducted. The plasticity tests were done in torsion at shear strain rates of . Testing temperatures covered the range from -150 °C to 150 °C which encompassed fracture initiation modes involving transgranular cleavage to fully ductile fracture. Micromechanical processes involved in void and cleavage micro-crack formation were identified and quantified. For these purposes notched round tensile tests and subsequent metallographic observations along with TEM and SEM observations of the plane strain fracture toughness specimens were performed. The experimental results and quantitative micro-modeling using simple fracture models provide a means of correlating both quasistatic and dynamic fracture toughness with microstructures.     

Fracture behavior of C-Mn steel and weld metal in notched and precracked specimens: Part I. fracture behavior

In this investigation, fracture loads, toughness values, and the lengths of fibrous cracks on fracture surfaces were measured in Charpy V, crack tip opening displacement (COD), and precracked impact testing at various temperatures for C-Mn base steel and C-Mn and Ti-B weld metals. The uniaxial tensile properties of these metals were measured as well. By plotting the parameters related to toughness against the length of fibrous cracks measured, the energy absorbed by unit crack extension was estimated. The local cleavage fracture stresses, oy, were measured in Charpy V-notched and COD precracked specimens. The results showed σ_f about 600 MPa higher in the latter than in the former. Based on the results obtained, the factors controlling the toughness were analyzed. This was explained by the brittle transition temperature of the base metal being higher than that of the weld metal in the Charpy V test; however, it was lower in the COD test. The differences in fracture behavior between various types of toughness specimens were analyzed. The prerequisite condition for establishing the correlation between the results of Charpy V and COD tests was also discussed.     

Improved lower temperature fracture toughness of ultrahigh strength 4340 steel through modified heat treatment

A modified heat treatment has been suggested whereby lower temperature plane-strain fracture toughness (K _IC) of 4340 ultrahigh strength steel is dramatically improved in developed strength and Charpy impact energy levels. The modified heat-treated 4340 steel (MHT-4340 steel) consists of a mixed structure of martensite and about 25 vol pct lower bainite which appears in acicular form and partitions prior austenite grains. This is produced through isothermal transformation at 593 K for a short time followed by an oil quench (after austenitizing at 1133 K and subsequent interrupted quenching in a lead bath at 823 K). The mechanical properties obtained at room temperature (293 K) and 193 K have been compared with those achieved using various heat treatments. Significant conclusions are as follows: the MHT-4340 steel compared to the 1133 K directly oil-quenched 4340 steel increased theK _IC values by 15 to 20 MPa • m^1/2 at increased strength and Charpy impact energy levels regardless of the test temperature examined. At 193 K,K _IC values of the MHT-4340 steel were not less than those of the 1473 K directly oil-quenched 4340 steel, in whichK _IC values are significantly enhanced at markedly increased strength, ductility, and Charpy impact energy levels. The MHT-4340 steels compared to austempered 4340 steels at 593 K, which have excellent Charpy impact properties, showed superiorK _IC values at significant increased strength levels irrespective of test temperatures. The lower temperature improvement inK _IC can be attributed to not only the crack-arrest effect by acicular lower bainite but also to the stress-relief effect by the lower bainite just ahead of the current crack.     

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 toughness of high hardness laminar composite steel as influenced by specimen and crack orientation

The toughness behavior of high hardness laminar composite steel (high carbon, ∼Rc 60, hard layer metallurgically bonded to a medium carbon ∼Rc 50, softer layer) was investigated. The effort focused on the effect of test temperature, specimen orientation and crack location on toughness. Charpy V-notch specimens with the notch extending through both the hard and soft layers were tested over a series of temperatures to provide transition curves for both the longitudinal and transverse direction. These transition curves are compared to those obtained from specimens that were surface notched on either the hard or soft side. By precracking similarly oriented specimens, information on the fracture toughness (K _Q andW/A) was obtained over approximately the same temperature range. These data show the effect of the interface between the hard and soft layer on the various toughness parameters. Lastly, stress corrosion cracking was investigated and K_ISCC values provided.     

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     

The toughness of high hardness laminar composite steel as influenced by specimen and crack orientation

The toughness behavior of high hardness laminar composite steel (high carbon, ∼ Rc 60, hard layer metallurgically bonded to a medium carbon ∼Rc 50, softer layer) was investigated. The effort focused on the effect of test temperature, specimen orientation and crack location on toughness. Charpy V-notch specimens with the notch extending through both the hard and soft layers were tested over a series of temperatures to provide transition curves for both the longitudinal and transverse direction. These transition curves are compared to those obtained from specimens that were surface notched on either the hard or soft side. By precracking similarly oriented specimens, information on the fracture toughness (K _Q andW/A) was obtained over approximately the same temperature range. These data show the effect of the interface between the hard and soft layer on the various toughness parameters. Lastly, stress corrosion cracking was investigated andK _ISCC values provided.     

A comparison of toughness of

The low-temperature toughness of C-Mn weld steel with different grain sizes was investigated with notched and precracked specimens. The results indicated that the fine grain steel, evaluated by notched specimens (Charpy V-notch and 4 point bending specimens), is tougher than that of the coarse grain steel over a temperature range from -196 °C to -30 °C. On the other hand, the coarse grain steel, evaluated with precracked specimens, has a remarkably greater plane strain fracture toughness compared to the fine grain steel. The microstructural analysis revealed that the fracture toughness of both the fine grain and the coarse grain steel is not directly related to the distance of the fracture initiation site from the precrack tip or the size of the ferrite grain. The behavioral discrepancy can be explained in terms of the ratio of local fracture stress to yield stress,i.e., σ _f f/σ _y . The fine grain steel had a higherσ _f /σ _y in the notched specimens but a lower value in the precracked specimens compared to the coarse grain steel. The scatter of toughness data can be mainly attributed to the probabilistic distribution of the weakest particle. We suggested thatσ _f /σ _y may be a useful parameter for the engineering evaluation of toughness.     

The effect of deformation-induced transformation on the fracture toughness of commercial titanium alloys

The effect of deformation-induced transformation of metastableβ phase on the ductility and toughness of four commercial titanium alloys was investigated. Tensile tests, Charpy impact tests, and both static and dynamic fracture toughness tests were carried out at temperatures between 77 and 473 K on four titanium alloys containing metastableβ phase. Deformation-inducedα″ (orthorhombic martensite) was observed in an (α + β)-type Ti-6Al-2Sn-4Zr-6Mo alloy. The dynamic fracture toughness of this alloy increased considerably at 223 K compared to those at other temperatures. In another (α + β)-type Ti-6A1-4V alloy, the static fracture toughness at 123 K and the dynamic fracture toughness at 223 K were increased considerably by the presence of deformation-induced martensite compared to those at other temperatures. The strength increased as the temperature decreased in this alloy. An abnormal elongation of aβ-type alloy, Ti-15V-3Al-3Sn-3Cr, at 123 K was attributed to the mechanical twinning of theβ phase. However, the effect of deformation-induced transformation on the fracture toughness of Ti-3Al-8V-6Cr-4Mo-4Zr alloy was not observed. Formerly Visiting Associate Professor, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA. Formerly with the Department of Production Systems Engineering, Toyohashi University of Technology.     

Crack Arrest Toughness of Two High Strength Steels (AISI 4140 and AISI 4340)

The crack initiation toughness (K _c ) and crack arrest toughness (K _a ) of AISI 4140 and AISI 4340 steel were measured over a range of yield strengths from 965 to 1240 MPa, and a range of test temperatures from -53 to +74°C. Emphasis was placed onK _a testing since these values are thought to represent the minimum toughness of the steel as a function of loading rate. At the same yield strengths and test temperatures,K _a for the AISI 4340 was about twice as high as it was for the AISI 4140. In addition, theK _a values showed a more pronounced transition temperature than theK _c values, when the data were plotted as a function of test temperature. The transition appeared to be associated with a change in fracture mechanism from cleavage to dimpled rupture as the test temperature was increased. The occurrence of a “pop-in” behavior at supertransition temperatures has not been found in lower strength steels, and its evaluation in these high strength steels was possible only because they are not especially tough at their supertransition temperatures. There is an upper toughness limit at which pop-in will not occur, and this was found for the AISI 4340 steel when it was tempered to its lowest yield strength (965 MPa). All the crack arrest data were identified as plane strain values, while only about one-half of the initiation values could be classified this way.     

Internal hydrogen embrittlement of ultrahigh-strength AERMET 100 steel

Near-peak-aged AERMET 100 is susceptible to severe internal hydrogen embrittlement (IHE) at 23 °C, if a sufficient diffusible hydrogen content is present, compromising the high toughness of this ultrahigh-strength steel (UHSS). Evidence includes the threshold stress intensity for subcritical IHE (K _TH ) as low as 10 pct of the plane-strain fracture toughness (K _IC ) and a fracture-mode transition from microvoid coalescence to brittle transgranular (TG) cracking, apparently along martensite lath interfaces and cleavage planes. The K _TH value decreases from a K _IC value of 132 to 143 MPa√m to 12 MPa√m, and the amount of brittle TG fracture increases to nearly 100 pct as the concentration of diffusible H increases from essentially 0 to 8 wppm, with severe embrittlement in the 0 to 2 wppm H regime. The IHE is time dependent, as evidenced by increasing K _TH values with increasing dK/dt and K-independent subcritical crack growth rates, and is attributed to diffusional H repartition from reversible trap sites to the stressed crack tip. The partition distance is ∼1 μm, consistent with the fine-scale microstructure of AERMET 100. The causes of the susceptibility of AERMET 100 to TG IHE are very high crack-tip stresses and a reservoir of mobile H trapped reversibly at (Fe,Cr,Mo)₂C precipitates. These factors enable repartition of H to misoriented martensite lath interfaces and interstitial sites near cleavage planes, with each prone to decohesion along a connected path. Predissolved H also reduces the ductile fracture toughness of AERMET 100 at high loading rates, perhaps due to reduced void growth caused by H trapped strongly at undissolved metal carbides.     

The dependence of the fracture toughness of mi steel on temperature and crack velocity

The dependence of the dynamic plane-strain fracture toughness,K _Id, on temperature and crack velocity was measured for propagating cracks in 1020 steel. The dynamics of crack propagation in double-cantilevered specimens was recorded using electroresistivity techniques. The fracture surface energy was found by comparing the crack propagation to solutions of crack motion in wedged-open cantilevered specimens. The_KId behavior was investigated over a range of temperatures from —196° to —50°C and crack velocities of 3 × 10^-3 to 5 × 10^-2 of √E/p. The rate and temperature dependence ofK _Id over the range ofT and υ_c investigated is well described by:1/K _ld ²= υ₀ are experimental constants. A dynamic value ofK _Id was 70 pct ofK _Ic at the same temperature, although in the temperature and crack velocity range investigated the specific fracture surface energy varies by a factor of 6. The temperatureT _T =B/A in(υ _o/υ_c) for which1/K _Id ² = 0 is similar to Charpy impact transition temperature values whenυ _c = 3 × 10^-3√.E/p. If the plane-strain stress condition could be maintained, thenT _T would define a brittle-ductile transition temperature for dynamic plane-strain fracture toughness. The constantsA andB are interpreted by understanding the plastic energy dissipated by a moving crack. Formerly with Brown University, Providence, R. I.     

Mechanism of detrimental effects of carbon content on cleavage fracture toughness of low-alloy steel

The variation in fracture toughness of low-alloy base steels and weld steels with carbon contents of 0.08 and 0.21 wt pct was investigated using notched and precracked specimens tested at low temperatures. The attention is focused on the mechanism associated with detrimental effects on cleavage fracture toughness resulting from increasing carbon content. Analyses reveal that, in the case of constant ferrite grain sizes with increasing carbon content, the yield stress σ _y increases and the local fracture stress σ _f remains constant for notched specimens. For precracked specimens, the σ _y increases, whereas the σ _f decreases. In both cases, the ratio σ _f /σ _y decreases; this ratio is one of the principal factors inducing the deterioration in the cleavage fracture toughness of the higher carbon steels. Analyses also reveal that the critical strain for initiating a crack nucleus, which decreases with increasing carbon content and impurity elements, appears to be another principal factor that has a negative effect on the fracture toughness in both notched and precracked specimens. The results of the fracture toughness measured for weld metal with various grain sizes further support the predominant effect of grain size on the toughness of notched specimens.