Hydrogen assisted ductile fracture of spheroidized carbon steels

The effects of hydrogen on ductile fracture were studied in two spheroidized plain carbon steels, containing 0.16 and 0.79 pct C. A combination of fractography and quantitative metallography on sectioned, deformed specimens permitted separation of the effects of hydrogen on the initiation, growth, and link-up of voids. In both steels, hydrogen was found to have no significant effect on either the initiation of voids at carbides, or early growth of voids, prior to link-up. In the higher carbon steel the fracture surface dimple size increased after hydrogen exposure with no other evident change in the fracture surface appearance; it is therefore inferred that hydrogen primarily assists void growth during link-up in this steel. In the lower carbon steel the fracture appearance changed and a decrease in void size due to hydrogen was found fractographically; thus, both initiation and growth of voids are apparently enhanced during the link-up phase of fracture in this steel. It is hypothesized that these effects may be due largely to a void pressure mechanism if hydrogen is transported by mobile dislocations. Formaly Graduate Student, Department of Metallurgy and Materials Science, Carnegie-Mellon University     

The role of hydrogen in the ductile fracture of plain carbon steels

In this report we consider the problem of hydrogen induced ductility losses in a plain carbon spheroidized steel. Specifically, the effect of internal hydrogen on the formation of voids from second phase (cementite) particles and their subsequent growth and coalescence was studied by careful microscopic inspection of uniaxially strained bars, both initially cylindrical and circumferentially notched, with and without hydrogen. Void initiation occurred with lower strains and stresses with hydrogen, although an equally important contribution to the ductility loss was from hydrogen accelerated void growth and coalescence. This latter process takes place by the propagation of voids along the grain, and possibly subgrain, boundaries which interlink the cementite spheroids. The results indicate that hydrogen facilitates interface separation, possibly by accumulating at the boundaries during hydrogenation of the specimen and lowering the cohesive strength, thereby making void initiation and growth along them easier.     

Ferritic-bainitic structure and ductile fracture resistance of high-strength pipe steels

The formation of the products of bainitic transformation in pipe steels of strength classes K60 (X70) and K65 (X80) during thermomechanical treatment is considered. It is shown that the structural factors that affect the ductile-fracture resistance of steel are the bainite morphology and the carbon distribution over structural constituents.     

Microstructural effects on fatigue crack growth in a low carbon steel

A study of the influence of microstructure on fatigue crack growth in an AISI 1018 steel has been carried out. Two distinctly different duplex microstructures were investigated. In one microstructure ferrite encapsulated islands of martensite; in the other martensite encapsulated islands of ferrite. The latter structure resulted in a significant increase in threshold level (18 MPa√mvs 8 MPa√m) together with an increase in yield strength. Fractographic analysis was used to investigate the influence of microstructure on the mode of fatigue crack growth. Formerly at the University of Connecticut     

Fatigue crack growth in some PM low alloy steels consolidated by rotary forging and sintering

Abstract

Fatigue crack propagation rates under plane strain conditions have been investigated for three PM low alloy steels consolidated to high densities by rotary compaction followed by sintering and heat treatment. It is shown that the densities and properties are intermediate between those of pressed and sintered materials and of powder forged materials. Threshold stress intensities compare satisfactorily with those for wrought counterparts, but resistances to crack growth are inferior to those of wrought steels. Possible reasons for the properties of the rotary compacted materials are considered in the light of their microstructures and the behaviour of other PM materials.     

Bainite formation in low carbon Cr-Ni steels

A low carbon Cr-Ni steel has been used to investigate the formation of upper bainite. Experimental results indicate that the start temperatures of the three morphologies of upper bainite in this steel,i.e., carbide-free bainite, bainite with carbide between and within ferrite laths, are about 600°, 500δ, and 425 °C, respectively; the habit plane of bainitic ferrite in this steel is close to (1 7 11)_α, which is 13.3 deg away from (0 ll)_α; and the orientation relationship between cementite and ferrite is consistent with Bagaryatskii’s. By means of the superelement approach, a thermodynamic treatment which applies to Fe-C alloys is extended into that suitable for low alloy steels, and calculation shows that the driving force for bainite formation at B_S temperatures is insufficient to compensate for shear strain energy. Formerly Graduate Student, Department of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, People’s Republic of China.     

Twin roll strip casting of low carbon steels

Abstract

Strip casting of low carbon steels has been investigated using a laboratory twin roll machine with copper rolls. The following grades have been studied: a deep drawing low carbon (LC) steel, a LC steel with titanium addition, and a high strength low alloy (HSLA) grade. The casting behaviour of these steels has been examined, and the observed surface defects classified. Defects are most pronounced for the LC grade and significantly less for the HSLA steel and the LC steel with titanium addition. The as cast structure has been analysed. It can be modified by post-cast treatment, e.g. by normalisation or in line rolling. The mechanical properties of cold rolled and annealed strip materials and their textures are presented. Satisfactory sheet properties can be achieved both as hot band and as cold rolled sheet when adequate treatment steps are applied. Consequences for strip casting applications and future research are discussed.     

The tempering of low carbon steels containing tungsten

The tempering behavior of three steels each containing 0.20 pct C and having tungsten contents of 2.1, 3.9 and 5.9 pct has been followed by thermomagnetic analysis and electron microscopy. Using a Sucksmith Balance, the proportions of autotempered carbide and retained austenite in as-quenched specimens were estimated, and the amount of cementite precipitated upon subsequent tempering measured accurately. Solution of tungsten in cementite during tempering was monitored by observing changes in Curie temperature. The magnetic nature of alloy carbides precipitating at high temperatures allowed tentative identification and this directed and assisted the electron microscopy study which provided new information on the morphology of tungsten carbides. J. B. LUPTON, formerly Research Student, Metallurgy Department, University of Sheffield, England, S. MURPHY, formerly Research Fellow, Metallurgy Department, University of Sheffield,     

Solute carbon and nitrogen in the ferrite after continuous annealing of low carbon low aluminium steels

Low N_sol levels not only require reduced N contents (≤ 20 ppm) in steel but also Al/N ratios higher than 5 and C contents higher than 0.025% because of the synergic effect between N and C precipitations; scavenging of Al on nitrogen is far from complete even in steels coiled at 750°C. Low C_sol levels are achieved either at very lowC contents (C ≤ 0.003%) or at C contents higher than 0.025%. Due to high C_sol, carbon aging is expected in steels coiled at 750°C. Softer, bake hardenable steels with good aging resistance will be obtained in the range 0.025–0.030 % C independently of the coiling temperature. C supersaturation of the ferrite is reduced at higher cooling rates after continuous annealing. Changes in C and N supersaturation according to the steel composition and the continuous annealing process are explained, taking into account the carbides morphology and the annealing conditions.     

On macroscopic and microscopic analyses for crack initiation and crack growth toughness in ductile alloys

Relationships between crack initiation and crack growth toughness are reviewed by examining the crack tip fields and microscopic (local) and macroscopic (continuum) fracture criteria for the onset and continued quasi-static extension of cracks in ductile materials. By comparison of the micromechanisms of crack initiationvia transgranular cleavage and crack initiation and subsequent growthvia microvoid coalescence, expressions are shown for the fracture toughness of materials in terms of microstructural parameters, including those deduced from fractographic measurements. In particular the distinction between the deformation fields directly ahead of stationary and nonstationary cracks are explored and used to explain why microstructure may have a more significant role in influencing the toughness of slowly growing, as opposed to initiating, cracks. Utilizing the exact asymptotic crack tip deformation fields recently presented by Rice and his co-workers for the nonstationary plane strain Mode I crack and evoking various microscopic failure criteria for such stable crack growth, a relationship between the tearing modulusT _R and the nondimensionalized crack initiation fracture toughnessJ _Ic is described and shown to yield a good fit to experimental toughness data for a wide range of steels. An erratum to this article is available at .     

Hydrogen enhanced crack growth in 18 Ni maraging steels

The kinetics of sustained-load subcritical crack growth for 18 Ni maraging steels in high purity hydrogen are examined using crack-tip stress intensity,K, as a measure of crack driving force. Crack growth rate as a function of stress intensity exhibited a clearly definedK-independent stage (Stage II). Crack growth rates in an 18 Ni (250) maraging steel are examined for temperatures from -60°C to 100°C. A critical temperature was observed above which crack growth rates became diminishingly small. At lower temperatures the activation energy for Stage II crack growth was found to be 16.7 ± 3.3 kJ/mole. Temperature and hydrogen partial pressure are shown to interact in a complex manner to determine the apparentK _th and the crack growth behavior. Comparison of results on ‘250’ and ‘300’ grades of 18 Ni maraging steel indicate a significant influence of alloy composition and/or strength level on the crack growth behavior. These phenomenological observations are discussed in terms of possible underlying controlling processes. Formerly a Graduate Student and Research Assistant. Based on a thesis submitted in partial fulfillment of requirements for the M.S. degree in Metallurgy and Materials Science, Lehigh University, Bethlehem, PA.     

Internal hydrogen-induced subcritical crack growth in austenitic stainless steels

The effects of small amounts of dissolved hydrogen on crack propagation were determined for two austenitic stainless steel alloys, AISI 301 and 310S. In order to have a uniform distribution of hydrogen in the alloys, they were cathodically charged at high temperature in a molten salt electrolyte. Sustained load tests were performed on fatigue precracked specimens in air at 0 ‡C, 25 ‡C, and 50 ‡C with hydrogen contents up to 41 wt ppm. The electrical potential drop method with optical calibration was used to continuously monitor the crack position. Log crack velocityvs stress intensity curves had definite thresholds for subcritical crack growth (SCG), but stage II was not always clearly delineated. In the unstable austenitic steel, AISI 301, the threshold stress intensity decreased with increasing hydrogen content or increasing temperature, but beyond about 10 wt ppm, it became insensitive to hydrogen concentration. At higher concentrations, stage II became less distinct. In the stable stainless steel, subcritical crack growth was observed only for a specimen containing 41 wt ppm hydrogen. Fractographic features were correlated with stress intensity, hydrogen content, and temperature. The fracture mode changed with temperature and hydrogen content. For unstable austenitic steel, low temperature and high hydrogen content favored intergranular fracture while microvoid coalescence dominated at a low hydrogen content. The interpretation of these phenomena is based on the tendency for stress-induced phase transformation, the different hydrogen diffusivity and solubility in ferrite and austenite, and outgassing from the crack tip. After comparing the embrittlement due to internal hydrogen with that in external hydrogen, it is concluded that the critical hydrogen distribution for the onset of subcritical crack growth is reached at a location that is very near the crack tip. Formerly Research Assistant, Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign.     

On macroscopic and microscopic analyses for crack initiation and crack growth toughness in ductile alloys

Relationships between crack initiation and crack growth toughness are reviewed by examining the crack tip fields and microscopic (local) and macroscopic (continuum) fracture criteria for the onset and continued quasi-static extension of cracks in ductile materials. By comparison of the micromechanisms of crack initiationvia transgranular cleavage and crack initiation and subsequent growthvia microvoid coalescence, expressions are shown for the fracture toughness of materials in terms of microstructural parameters, including those deduced from fractographic measurements. In particular the distinction between the deformation fields directly ahead of stationary and nonstationary cracks are explored and used to explain why microstructure may have a more significant role in influencing the toughness of slowly growing, as opposed to initiating, cracks. Utilizing the exact asymptotic crack tip deformation fields recently presented by Rice and his co-workers for the nonstationary plane strain Mode I crack and evoking various microscopic failure criteria for such stable crack growth, a relationship between the tearing modulusT _R and the nondimensionalized crack initiation fracture toughnessJ _Ic is described and shown to yield a good fit to experimental toughness data for a wide range of steels.     

The influence of aqueous environments on low δK and high ΔK fatigue crack propagation behavior in low carbon structural steels

The influence of aqueous environments on fatigue crack propagation behavior was investigated for two types of structural steel (SB42 and HT80) in pure and 3 pct NaCl water under freely corroding conditions. In the intermediate to high ΔK region, fatigue crack propagation rates were higher in both aqueous environments and in 1 atm hydrogen than in air for both types of steel, and the acceleration effect increased power functionally with decreasing frequency from 5 to 0.0005 Hz. Such a crack growth acceleration property was explained by the mechanism of cyclically induced hydrogen embrittlement, as shown by the brittle striations formed on the fracture surfaces. On the other hand, in the lower ΔK region, both aqueous environments inversely suppressed crack growth and enhanced the threshold stress intensity factor range ΔK _th just above the ΔK _th in air, while only in aerated 3 pct NaCl water was the crack observed to grow even under the condition below the ΔK _th in air, not showing the threshold. Probable mechanisms for such fairly complex environmental effects were also suggested.     

Effects on the cementite distribution in low carbon enamelling steels

To enamel modern LC‐steels it is necessary to provide a sufficient amount of hydrogen recombination sites as well as hydrogen traps within the materials microstructure to keep the hydrogen inside the steel. Hence surface defects like fish scaling, which are related to the effusion of hydrogen, can be avoided. Therefore it is necessary to produce internal surfaces inside the steel In form of hard and brittle particles which can be fractured during cold rolling and produce voids. For LC‐steels these particles could be formed by iron carbides. Aiming at an increased amount of cementite particles inside the steel, the carbon content could be raised or the parameters of the thermomechanical treatment (TMT) could be adjusted for a given carbon content to form coarse cementite particles. In this investigation the TMT‐parameters were systematically varied in hot compression tests and the results were evaluated by quantitative metallography. The focus was laid on the variation of the final deformation temperature, the coiling temperature and the cooling rate after coiling.     

Characterizing the dynamic crack resistance of linepipe steels with different impact test configurations

Comparison of CVN-, BDWTT-and BSC (small and large specimens)-test results, depending on ligament length. Evaluation of total, initiation and propagation absorbed energies. Finding that propagation energies per unit of ligament area from different test configurations can be correlated by introduction of conversion factors, which are specific for each type of test.     

Fundamental studies on the deoxidation of low carbon steels with ferrosilicon

Laboratory size (4.6 kg) low carbon-iron melts were deoxidized using plain ferrosilicon, ferrosilicon with calcium aluminate flux, and ferrosilicon with calcium silicate flux. The dissolved oxygen and total oxygen contents in these heats were measured as a function of time and temperature using oxygen probes and quenched pin samples taken from the melts. The dissolved oxygen values, as measured by the oxygen probes, indicated that the iron-silicon deoxidation reaches equilibrium within five minutes of reaction time. However, the total oxygen (dissolved oxygen+oxygen in oxide inclusions), as measured from the quenched pin samples, took almost twenty minutes to reach a steady state. The two aforementioned features were common to all the experimental heats. Below 1978 K the ferrosilicon heats with calcium silicate or calcium aluminate flux had lower steady state total oxygen values as compared to the plain ferrosilicon heats; the difference was more significant at lower quenching temperatures. Also, below 1978 K, the dissolved oxygen-temperature relationship of flux heats was identical to their steady state total oxygen-temperature relationship, which indicated that the addition of fluxes leads to elimination of oxide inclusions. The performance of calcium aluminate and the calcium silicate fluxes in removing the oxide inclusions was found to be identical in these laboratorysize heats. U. B. PAL, formerly Senior Metallurgist, Melting and Primary Operations, Allegheny Ludlum Corporation, Technical Center, Brackenridge, PA 15014     

Effect of impurity content on creep crack growth resistance in 1Cr1Mo0.25V ferritic steels

The effect of impurity content on creep crack growth (CCG) rate and, more generally, on hot ductility of a typical lCrlMo0.25V ferritic steel was evaluated. Four heats intentionally doped with various amounts of impurities were characterized after heat treatments simulating the industrial thermal cycle taking place in a 1000-mm-diameter high-pressure rotor at two positions: near the outside surface and at the center in the cases of air cooling and oil quenching, respectively. Results indicate that the highest crack growth rates occur in the grade with a low P content (40 ppm) and Sn and Sb values (100 to 200 ppm) comparable with those characteristic of commercial steels. A marked reduction in brittleness is achieved only through a substantial reduction in the amount of Sn and Sb, even when medium-to-high P levels (100 ppm) are present. Creep resistance in terms of both time to rupture and minimum growth rate is not influenced by the impurity content, at least within the range of stresses investigated. Auger analyses on crept specimens demonstrate the presence of a selective segregation of impurity elements similar to that found in other ferritic steels: P is the only segregating element at non-cavitated grain boundaries, while cavitated areas contain Sn, Sb, and Cu in addition to P. The embrittlement at high Sn and Sb levels depends on two factors: at low P levels, cracks rapidly propagate under surface diffusivity control; at high P levels, excess P segregates at the grain boundaries, and crack propagation proceeds by an intergranular decohesion mechanism.     

Effects of hydrogen concentration on slow crack growth in stainless steels

The behavior of four austenitic stainless steel alloys containing bulk hydrogen contents up to 50 ppm by weight (0.28 at. pct) has been determined in sustained load and in slow strain rate tensile tests. Tests of these same alloys were also made in hydrogen gas. AISI 301 and two compositions of AISI 304 of different austenite stability showed typical three stage crack propagation regimes under sustained load. AISI 310S did not. The slow crack growthvs stress intensity relation was quite sensitive to hydrogen content above a minimum value and also sensitive to austenite stability. It was shown that hydrogen contents up to 40 wt ppm had no influence on the austenite stability at tensile elongations up to 60 pct. Fractographic and phase analysis results are presented, and a model for crack propagation is discussed. S. SINGH, formerly with the Department of Metallurgy, University of Illinois at Urbana-Champaign.