The strength and fracture toughness of 18 Ni (350) maraging steel

The influence of microstructure on the strength and fracture toughness of 18 Ni (350) maraging steel was examined. Changes in microstructure were followed by X-ray and neutron diffraction and by optical and electron microscopy. These observations have been correlated with the fracture morphology established by scanning electron microscopy. Air cooling this alloy from the austenitizing temperature results in a dislocated martensite. During the initial stage of age hardening, molybdenum atoms tend to cluster (forming preprecipitates) and the cobalt assumes short range ordered positions. Subsequent aging results in Ni₃Mo and σ-FeTi with overaging being associated with the formation of equilibrium reverted austenite and Fe₂Mo. The fracture behavior is examined in terms of elementary dislocation precipitate interactions. It is suggested that the development of coplanar slip in the underaged conditions leads to its increased stress corrosion susceptibility and decreased fracture toughness. The optimum aged condition is then associated with cross-slip deformation. The fracture behavior of the overaged condition is a dynamic balance between a brittle matrix and the ductile (crack blunting) reverted austenite.     

Effect of thermal cycling on the mechanical properties of 350-grade maraging steel

The effects of retained austenite produced by thermal cycling on the mechanical properties of a precipitation-hardened 350-grade commercial maraging steel were examined. The presence of retained austenite caused decreases in the yield strength (YS) and ultimate tensile strength (UTS) and effected a significant increase in the tensile ductility. Increased impact toughness was also produced by this treatment. The mechanical stability of retained austenite was evaluated by tension and impact tests at subambient temperatures. A deformation-induced transformation of the austenite was manifested as load drops on the load-elongation plots at subzero temperatures. This transformation imparts excellent low-temperature ductility to the material. A wide range of strength, ductility, and toughness can be obtained by subjecting the steel to thermal cycling before the precipitation-hardening treatment.     

Low-temperature elastic properties of a 300-grade maraging steel

Elastic properties of an annealed 300-grade maraging steel (18 Ni, 9 Co, 5 Mo pct by weight) were studied between room temperature and liquid-helium temperature. Longi-tudinal and transverse ultrasonic velocities were determined by a pulse method. The re-ported elastic constants are: longitudinal modulus, shear modulus, Young’s modulus, bulk modulus, and Poisson’s ratio. Except for the bulk modulus, the room-temperature elas-tic constants are all lower than those of iron; and their temperature dependencies are regular in the studied temperature region.     

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.     

Fracture toughness and stress corrosion characteristics of a high strength maraging steel

The effect of heat treatment on the tensile, fracture toughness, and stress corrosion properties of a high strength maraging steel (nominal composition 16.3 Ni?12.87 Co?4.98 Mo?0.78 Ti) is described. A maximum ultimate tensile strength of 323 ksi, combined with a fracture toughnessK _Ic of 62 ksi \(\sqrt {in} \) , was achieved. This strength level appears to be the maximum which can be achieved in maraging type steels without decreasing the crack tolerance below that of currently used high strength low alloy steels. Reversion to austenite did not improve either the fracture toughness or stress corrosion resistance relative to completely martensitic microstructures with equivalent strength.     

The effects of heat treatment on fracture toughness and fatigue crack growth Rates in 440C and BG42 steels

The fatigue crack growth rates,da/dN, and the fracture toughness, K_Ic have been measured in two high-carbon martensitic stainless steels, 440C and BG42. Variations in the retained austenite contents were achieved by using combinations of austenitizing temperatures, refrigeration cycles, and tempering temperatures. In nonrefrigerated 440C tempered at 150 °C, about 10 vol pct retained austenite was transformed to martensite at the fracture surfaces duringK _Ic testing, and this strain-induced transformation contributed significantly to the fracture toughness. The strain-induced transformation was progressively less as the tempering temperature was raised to 450 °C, and at the secondary hardening peak, 500 °C, strain-induced transformation was not observed. In nonrefrigerated 440C austenitized at 1065 °C,K _Ic had a peak value of 30 MPa m^1/2 on tempering at 150 °C and a minimum of 18 MPa m^1/2 on tempering at 500 °C. Refrigerated 440C retained about 5 pct austenite, and did not exhibit strain-induced transformation at the fracture surfaces for any tempering temperature. TheK _Ic values for corresponding tempering temperatures up to the secondary peak in refrigerated steels were consistently lower than in nonrefrigerated steels. All of the BG42 specimens were refrigerated and double or quadruple tempered in the secondary hardening region; theK _Ic values were 16 to 18 MPa m^1/2 at the secondary peak. Tempered martensite embrittlement (TME) was observed in both refrigerated and nonrefrigerated 440C, and it was shown that austenite transformation does not play a role in the TME mechanism in this steel. Fatigue crack propagation rates in 440C in the power law regime were the same for refrigerated and nonrefrigerated steels and were relatively insensitive to tempering temperatures up to 500 °C. Above the secondary peak, however, the fatigue crack growth rates exhibited consistently lower values, and this was a consequence of the tempering of the martensite and the lower hardness. Nonrefrigerated steels showed slightly higher threshold values, ΔK_th, and this was ascribed to the development of compressive residual stresses and increased surface roughening in steels which exhibit a strain-induced martensitic transformation.     

The influence of grain size and stress on the morphology of a 300-grade maraging steel

The effects of austenite grain size and externally applied stress on the morphology of martensite formation for a 300-grade maraging steel were investigated. Austenite grain sizes ranging from ASTM 14 to 5 were examined. The growth pattern of the martensite was revealed by a selective aging treatment that involved heating to 815° C (austenitizing), cooling to 184° C (about 50 pct transformation to martensite), reheating to 400° C (partial aging of the martensite), and finally, cooling to room temperature (balance of austenite transforms). Blocky martensite formed in the fine-grain austenite, whereas for the coarse-grain size, a stringer-like structure developed. Electron transmission studies showed that the individual martensite units (laths or platelets) were similar in size for both types of morphologies. In both cases, the length of these units corresponded closely to the spacing between the twin boundaries or grain boundaries of the fine-grain specimens. Differences in morphology for different grain sizes are explained in terms of the relative ratios between the size of the martensite unit and the distance between boundaries intersecting the path of growing platelets. The application of an external stress to a coarse-grain specimen results in the delineation of the austenite annealing twins that normally cannot be readily detected. It is proposed that the application of a stress causes a preferential acceleration of the transformation in one of the two differently oriented, twin-related regions of an austenite grain. This argument is based on the differences in the maximum resolved shear stress for the most favorable orientation of the (112) {111} shear variants in each of the various possible twin-related austenite crystal orientations. Hardness and tensile data were also obtained. The absence of any significant variation in these properties for the different grain sizes is attributed to the similarity in size of the martensite units.     

The fracture toughness of a ferrous maraging alloy containing manganese

A Fe-20 pct Co-15 pct Mn-5 pct Mo maraging alloy has been found to exhibit low toughness and brittle fracture throughout its response to age-hardening. Intergranular embrittlement associated with segregation of manganese and with residual oxygen (15 to 300 ppm) in the early stages of aging was replaced by brittle inter-lath separation as aging progressed, the latter mode being associated with intermetallic precipitation and, possibly, the presence of finely-dispersed austenite. Formerly a Research Student in the Department of Metallurgy, University of Sheffield     

The fracture toughness of a ferrous maraging alloy containing manganese

A Fe-20 pet Co-15 pet Mn-5 pet Mo maraging alloy has been found to exhibit low toughness and brittle fracture throughout its response to age-hardening. Inter granular embrittlement associated with segregation of manganese and with residual oxygen (15 to 300 ppm) in the early stages of aging was replaced by brittle inter-lath separation as aging progressed, the latter mode being associated with intermetallic precipitation and, possibly, the presence of finely-dispersed austenite.     

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.     

Increased fracture toughness in a 300 grade maraging steel as a result of thermal cycling

Systematic changes in the fracture toughness of a 300 grade commercial maraging steel were obtained using a non-standard heat-treating process. Microstructures consisting of variable amounts of retained austenite in an aged martensitic matrix were produced. A mathematical model is presented relating these toughness values to the properties of the individual constituents. Increases in fracture toughness resulting from the non-standard heat-treatment were attributed to the nature of the distribution of the tough phase (retained austenite) in a brittle matrix of precipitation hardened martensite. In some cases, a strain-induced transformation to martensite was observed which greatly added to the toughness. Some improvements in fatigue crack propagation characteristics also resulted from this heat treatment. Formerly Postdoctoral Fellow, Department of Materials Science and Metallurgical Engineering, University of Cincinnati     

Influence of heat treatment on the fatigue crack growth rates of a secondary hardening steel

The relationships between microstructure and fatigue crack propagation behavior were studied in a 5Mo-0.3C steel. Microstructural differences were achieved by varying the tempering treatment. The amounts, distribution, and types of carbides present were influenced by the tempering temperature. Optical metallography and transmission electron microscopy were used to characterize the microstructures. Fatigue fracture surfaces were studied by scanning electron microscopy. For each heat treatment the fatigue crack growth properties were measured under plane strain conditions using a compact tension fracture toughness specimen. The properties were reported using the empirical relation of Paris [da/dN = C_oΔK^m]. It was found that secondary hardening did influence the fatigue crack growth rates. In particular, intergranular modes of fracture during fatigue led to exaggerated fatigue crack growth rates for the tempering treatment producing peak hardness. Limited testing in a dry argon atmosphere showed that the sensitivity of fatigue crack growth rates to environment changed with heat treatment.     

Influence of heat treatment on the fatigue crack growth rates of a secondary hardening steel

Metallurgical and Materials Transactions A - The relationships between microstructure and fatigue crack propagation behavior were studied in a 5Mo-0.3C steel. Microstructural differences were...     

Hydrogen-Induced subcritical crack growth of a 12 Cr-1 Mo ferritic stainless steel

Subcritical crack growth and tensile ductility measurements have been made on a 12 Cr-1 Mo ferritic stainless steel at cathodic potentials in a 1 N H₂SO₄ solution at 25 °C. The tensile ductility was found to be a minimum at −600 mV (SCE) and both the subcritical crack growth behavior and tensile ductility were similar for material in the tempered (760 °C/2.5 h) or tempered-plus-segregated (540 °C/240 h) condition. A rising-load crack growth threshold of 20 MPa √m was measured and a rising-load fracture toughness of 110 MPa √m was determined from extrapolation of the stage III crack growth curve. A K-independent stage II was observed and a stage II crack growth rate of about 1 × 10⁻⁵ mm/s was measured. The fracture mode was a mixture of intergranular and quasi-cleavage for both heat treatments and for subcritical and tensile fracture tests. Impact fracture properties were independent of heat treatment and grain boundary composition with the fracture mode predominantly transgranular. The difference in the fracture mode for hydrogen-induced crack growth and dynamic crack growth was explained by a difference in the relationship between their stress profiles and the maximum grain boundary segregation distribution.     

The effect of the austenitizing heat-treatment variables on the fracture toughness of high-speed steel

The influence of austenitizing treatment and tempering on the fracture behavior of high-speed steel (DIN 1.3333) has been investigated. The fracture behavior has been characterized by determining the K _IC and J _IC values via the performance of modified compact tension (CT) and single edge notched (SEN) tests. The micromechanisms of crack initiation and propagation have been studied by metallographic examination of the fractured specimens. The results indicate that austenitizing conditions of temperature range 1050 °C to 1190 °C and time 0.25 to 6 minutes and tempering at 550 °C to 650 °C up to 150 minutes alter the microstructure and, subsequently, the fracture toughness. It was found that cracking occurs by nucleation at the interface of matrix/vanadium-enriched large carbides, where sulfur is segregated and where linkage of the microcracks bridges ductile ligament of voids at small Mo + W enriched carbides. The improvements of the fracture toughness and hardness by short austenitizing time of 15 to 75 seconds at 1190 °C are attributed to (1) the optimum distribution of a dense network of small carbides, (2) the lack of grain growth as the boundaries are pinned down by these small carbides, and (3) the retained austenite at a level up to 16 vol pct transformed to martensite.     

Fatigue crack growth and fracture toughness behavior of an Al-Li-Cu alloy

Slip behavior, fracture toughness, and fatigue thresholds of a high purity Al-Li-Cu alloy with Zr as a dispersoid forming element have been studied as a function of aging time. The fracture toughness variation with aging time has been related to the changes in slip planarity,i.e., slip band spacing and width. Although the current alloy exhibits planar slip for all aging conditions examined, the crack initiation toughness,Klc, compares favorably with those of 2XXX and 7XXX aluminum alloys. Near threshold fatigue crack growth results in air and vacuum suggest that irregularities in the crack profile and the fracture surfaces and slip reversibility are some of the major contributing factors to the crack growth resistance of this alloy.     

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.     

The effect of step quenching on the microstructure and fracture toughness of aisi 4340 steel

The microstructure and fracture toughness of AISI 4340 steel in the direct and in the step quenched and tempered condition has been studied. Austenitizing temperatures of 1473 K followed by step quenching to either 1373 or 1143 K prior to oil quenching have been employed. A consistent drop in the fracture toughness values was observed as the intermediate holding temperature decreased or the holding time at this temperature in-creased. A concurrent increase in the amount of twinning was seen without any change in the amount and/or distribution of retained austenite. While direct evidence for segre-gation has not been found, the observed facts are consistent with segregation effects during the austenitizing treatment.