Effects of intercritical treatment and tempering on fracture behavior in a medium-carbon 2Si-3Ni steel

In the 2Si-3Ni steel intercritically treated in the range of 720 ‡C to 790 ‡C, the fracture behavior under the impact testing has been analyzed and the post-tempering effect has also been investigated. The transgranular fracture occurred in the specimens treated below and at 730 ‡C (SN73 specimen) in relatively low intercritical temperature range, but the intergranular fracture occurred in the specimens treated at 750 ‡C and 770 ‡C (SN75 and SN77 specimens) in relatively high intercritical temperature range. In the SN73 specimen, there was little coarse martensite at the prior austenite grain boundaries, whereas there was continuous, coarse martensite at those boundaries in the SN75 and SN77 specimens. The fracture behavior was mainly discussed in terms of the microstructural differences. In addition, no or a little increase in impact toughness, in spite of great decreases in hardness, in the SN75 and SN77 specimens tempered at 600 ‡C is correlated with the easy occurrence of intergranular fracture, which is caused by the carbide aggregates formed in the continuous, coarse martensite at the grain boundaries.     

Effect of hydrogen on fracture behavior of a quenched and tempered medium-carbon steel

This work examined the effects of hydrogen on fracture of quenched and tempered 1045 steel. Tests were made at room temperature on tensile, Charpy impact, and 4-point notched bend specimens. This steel exhibits tempered martensite embrittlement (TME) for tempering temperatures between 300 and 375 °C. Thus hydrogen in most cases affected fracture by increasing the amount of intergranular fracture. In bend specimens, hydrogen also induced quasicleavage (QC) fracture at points of maximum normal stress below the notch root, points which appeared to be the locations of crack initiation. Tear ridges on theseQC surfaces were at martensite lath packet boundaries. Crack orientations were largely mode I in uncharged specimens, with mode II appearing at the notch root in most hydrogen-charged specimens. These observations are in general agreement with earlier work on martensitic steel. Formerly graduate student, Carnegie-Mellon University     

On the structure of Ni9 steel and its tensile fracture behaviour

The tensile curves as well as original and selective dissoluted fracture surfaces have been studied. The function of the γ' phase on the mechanical properties of the steel has been investigated. The fracturing process consists of: formation of dimples – formation of splits in dimples – the splits in dimples join each other to form secondary cracks – formation of inner lips and tiny dimple regions. The morphology of fracture looks like ridges and peaks. It is considered that the column boundaries in ingots may split easily; this causes bedding fracture and the degradation of Z–direction properties. The formation of bedding fracture is affected markedly by heat treatment and tensile temperature. Some ways of improving mechanical properties have been discussed.     

机械搅拌对中碳钢钢液凝固组织的影响

研究了中碳钢钢液在凝固初期施加机械搅拌,改变搅拌参数对钢液凝固组织的影响。实验结果表明,随着搅拌时间的延长等轴晶比例增大,搅拌时间30 s时,等轴晶比例最大为50.784%;改变插入钢液深度,随着插入液面深度的增加,等轴晶区比例明显增大,同时晶粒得到细化,分布更均匀,插入深度为4.5 cm时,等轴晶比例和晶粒细化程度达...     

Retained carbide distribution in intercritically austenitized 52100 steel

Two 52100 steels, one containing 0.009 pct P, the other 0.023 pct P, were homogenized at 1150 °C, slowly cooled to form proeutectoid carbides and pearlite, partially spheroidized, austenitized at 850 °C for one hour, oil quenched, and tempered at 200 °C. Light microscopy and transmission electron microscopy of carbon extraction replicas showed that cementite particles were retained as three different morphologies in the fine-grained austenite formed during the 850 °C intercritical austenitizing treatment. The morphologies are characterized as follows: (1) closely spaced intragranular carbides most of which are less than 0.25 μm in diameter, (2) carbides about 1 μm in diameter, located on austenite grain boundaries, and (3) branched proeutectoid carbides arranged in networks corresponding to the coarse, 130 μm diameter austenite grains formed during homogenizing. The major effect of high phosphorus content was to retard the spheroidization of the retained carbides.     

Characterization of cryogenic Fe-6Ni steel fracture modes: A three dimensional quantitative analysis

Quantitative three dimensional analyses of fracture surfaces of Fe-6Ni cryogenic steel were used to study the effect of temperature variations on the geometry of the characteristic features in different fracture modes. Stereo SEM techniques combined with stereo photogrametry provide the tools to perform such analysis on standard Charpy specimens tested with appropriate instrumentation over a 300 deg temperature range. The characteristic features of the ductile fracture mode were found to maintain a constant aspect ratio for these temperatures, while the brittle fracture modes exhibit an aspect ratio that is temperature dependent. This geometrical factor dependence of temperature in the nonductile case resembles that of the Charpy impact energy for the same temperature range. formerly with the Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720.     

Acicular ferrite morphologies in a medium-carbon microalloyed steel

The influence of time and isothermal transformation temperature on the morphology of acicular ferrite in a medium-carbon microalloyed steel has been studied using optical and transmission electron microscopy (TEM). This study has been carried out with the analysis of the microstructures obtained with one- and two-stage isothermal treatments at 400 °C and 450 °C, following austenitization at 1250 °C. The heat treatments were interrupted at different times to observe the evolution of the microstructure at each temperature. The results show that a decrease in the isothermal transformation temperature gives rise to the development of sheaves of parallel ferrite plates, similar to bainitic sheaves, but intragranularly nucleated. These replace the face-to-edge nucleation that dominates the transformation at higher temperatures. The TEM observations reveal that the plates correspond to upper acicular ferrite and the sheaves to lower acicular ferrite. In this last case, cementite precipitates are present at the ferrite unit interiors and between the different platelets.     

Ferrite recrystallization and austenite formation in cold-rolled intercritically annealed steel

The recrystallization of ferrite and austenite formation during intercritical annealing were studied in a 0.08C-1.45Mn-0.21Si steel by light and transmission electron microscopy. Normalized specimens were cold rolled 25 and 50 pct and annealed between 650 °C and 760 °C. Recrystallization of the 50 pct deformed ferrite was complete within 30 seconds at 760 °C. Austenite formation initiated concurrently with the ferrite recrystallization and continued beyond complete recrystallization of the ferrite matrix. The recrystallization of the deformed ferrite and the spheroidization of the cementite in the deformed pearlite strongly influence the formation and distribution of austenite produced by intercritical annealing. Austenite forms first at the grain boundaries of unrecrystallized and elongated ferrite grains and the spheroidized cementite colonies associated with ferrite grain boundaries. Spheroidized cementite particles dispersed within recrystallized ferrite grains by deformation and annealing phenomena were the sites for later austenite formation.     

Fracture behavior in medium-carbon martensitic Si- and Ni-steels

A study of fracture behavior in medium-carbon martensitic Ni- and Si-steels has been made. Charpy impact testing was conducted in order to investigate fracture mode as a function of test temperature. Whereas the transgranular cleavage fracture becomes the primary brittle fracture mode with decreasing test temperature in the Si-steel, intergranular fracture is the only brittle fracture mode observed at low temperatures in the Ni-steel. The different fracture behavior between these steels appears to be due to variation in intrinsic matrix toughness. Since Si may impair the intrinsic matrix toughness, the occurrence of transgranular cleavage fracture becomes relatively easy with decreasing test temperature. On the other hand, since Ni may improve considerably the intrinsic matrix toughness, the transgranular cleavage fracture is not able to occur although the test temperature decreases. Thus, the intrinsic matrix toughness can play a significant role in controlling the fracture behavior.     

Cryogenic fracture toughness of 9Ni steel enhanced through grain refinement

A thermal cycling technique was used to refine the grain size of commercial “9Ni” cryogenic steel. The grain refined alloy was then tempered, a treatment which introduced a small admixture of retained austenite. The reprocessed alloy shows an excellent combination of strength and toughness to temperatures as low as 6 K, and shows little evidence of embrittlement in liquid helium. The microstructure and mechanical properties of the reprocessed alloy are described and compared to those obtained through conventional treatment.     

Stress-corrosion cracking behavior of an 18 Pct Ni maraging steel

Stress-corrosion cracking of an 18 pct Ni maraging steel in aqueous solutions was studied using precracked cantilever beam specimens. By appropriate heat treatments, six different structures having the same yield strength were obtained. Although significantly different plane strain fracture toughness values (K _Ic ) resulted, it was found that the threshold plane strain stress intensity (K _Iscc ) was the same for all structures.K _Iscc had the same value in 3 pct NaCl at various pH values, in 1N H₂SO₄, and in distilled water. Specimens tested in 3 pct NaCl under both anodic and cathodic applied potentials also exhibited this sameK _Iscc value. Fractographic inspection of the crack surfaces revealed no apparent differences due to changes in solution, pH, or applied potential. The crack path was intergranular in all cases. However, specimens austenitized at 1500°F exhibited crack branching, whereas in specimens austenized at much higher temperatures branching no longer occurred. Aging time and temperature seemed to change only the time to failure. The mechanism most consistent with all observations appears to be hydrogen cracking.     

Dynamic fracture initiation behavior of an HY-100 steel

An investigation was conducted into the effects of test temperature and loading rate on the initiation of plane strain fracture of an HY-100 steel. Fracture toughness tests were conducted using fatigue precracked round bars loaded in tension to produce a quasi-static stress intensity rate of ·K₁ = 1 MPa√m/s and a dynamic rate of ·K₁ = 2 × 10⁶ MPa√m/s. Testing temperatures covered the range from -150 °C to 200 °C, which encompasses fracture initiation modes involving quasi-cleavage to fully ductile fracture. The results of toughness tests show that the lower-shelf values of fracture toughness were substantially independent of loading rate, while the dynamic values exceeded the quasi-static values by about 50 pct on the upper shelf. In analyzing these results, phenomenological fracture initiation models were adopted based on the requirement that, for fracture to occur, a critical strain or stress must be achieved over a critical distance. In separate tests, the observation of microfracture processes was investigated using fractography and anin situ scanning electron microscope (SEM) fracture technique. The layered ppearance of the fracture surfaces was found to be associated with a banded structure which generally contains many MnS inclusions, probably resulting in a reduction of the fracture toughness values.     

The nature of quasicleavage fracture in tempered 5.5Ni steel after hydrogen charging

Quenched and tempered 5.5Ni steel was embrittled by hydrogen charging and broken in air at room temperature. The primary fracture mode was transgranular quasicleavage. The quasicleavage facets were studied by scanning electron fractography and by transmission electron microscopy of profile fractographic specimens. The latter were prepared by plating the fracture surface with nickel and thinning so that the fracture surface was contained within the region of the specimen that was transparent to the electron beam. The fracture surface generally followed martensite lath boundaries. In addition, interlath microcracks were frequently found in the material immediately beneath the fracture surface. These results suggest that transgranular hydrogen embrittlement in this steel is primarily an interlath cracking phenomenon. Since the lath boundary planes tend to lie in {110}, the results also explain the prevalence of {110} quasicleavage in the embrittled specimens, which contrasts with the {100} cleavage found in uncharged specimens broken below the ductile-to-brittle transition temperature.     

Ultrasonic behavior of plastically deformed and heat treated steel

Studies were made on the longitudinal ultrasonic velocity and its attenuation characteristics in medium carbon steel forging subjected to 0–60% hot upsetting followed by heat treatment. The fineness of ferrite and pearlite increased with increasing deformation and cooling rate during heat treatment. The area fraction of pearlite increased from 21% to 45% as well as the mean interlamellar spacing in pearlite decreased from 8.79 μm to 0.689 μm in the 40% deformed (upsetted) normalized sample It was observed that the ultrasonic velocity decreased with increasing degree of deformation. The velocity was found to be highest in the normalized steel acquiring a value of 5920 m/s and lowest in hardened steel acquiring a value of 4979 m/s. The attenuation decreased with increasing deformation resulting in increased back wall echo heights. The residual stresses on the surface of typical 60% deformed and annealed steel samples using x-ray diffractometry were measured to be — 39.7 MPa (compressive) and — 188.0 MPa (compressive) respectively.     

Influence of on-line tempering parameters on microstructure of medium-carbon steel

A new process involving ultra-fast cooling(UFC)and on-line tempering(OLT)was proposed to displace austempering process,which usually implements in a salt/lead bath and brings out serious pollution in the industrial application.The optimization of the new process,involving the evolution of the microstructure of medium-carbon steel during various cooling paths,was studied.The results show that the cooling path affected the final microstructure in terms of the fraction of pearlite,grain size and distribution of cementite in pearlite.Increasing the cooling rate or decreasing the OLT temperature contributes to restraining the transformation from austenite to ferrite,and simultaneously retains more austenite for the transformation of pearlite.It is also noted that bainite was observed in the microstructure at the cooling rate of 45°C/s and the OLT temperature of 500°C.Through either increasing the cooling rate or decreasing the OLT temperature,the distribution of cementite in pearlite is more dispersed and grain is refined.Taking the possibility of industrial applications into account,the optimal process of cooling at 45°C/s followed by OLT at 600°C after hot rolling was determined,which achieves a microstructure containing nearly full pearlite with an average grain size of approximately 7μm and a homogeneously dispersed distribution of cementite in pearlite.