Effect of threshold stress intensity on fracture mode transitions for hydrogen-assisted cracking in AISI 4340 steel

Fracture mode transition in hydrogen-assisted cracking (HAC) of AISI 4340 steel has been studied from an equilibrium aspect at room temperature with 8.6-mm-thick double cantilever beam (DCB) specimens. The threshold stress intensity,K _th , necessary for the occurrence of HAC and the corresponding fracture surface morphology have been determined as a function of hydrogen pressure and yield strength. The K _th increases with decrease in hydrogen pressure at a given yield strength and also with decrease in yield strength at a given hydrogen pressure. AsK _th increases, the corresponding HAC fracture mode changes from the intergranular (IG) and quasi-cleavage (QC) modes to the microvoid coalescence (MVC) mode. The experimental results indicate that the critical hydrogen concentration for crack extension in the IG mode is higher than that for crack extension in the MVC mode. The fracture mode transition with varying hydrogen pressure and yield strength is discussed by simultaneously considering the micromechanisms for HAC and the hydrogen pressure and yield strength dependencies ofK _th .     

Fracture mechanics and surface chemistry studies of subcritical crack growth in AISI 4340 steel

Coordinated fracture mechanics and surface chemistry experiments were carried out to develop further understanding of environment enhanced subcritical crack growth in high strength steels. The kinetics of crack growth were determined for an AISI 4340 steel (tempered at 204°C) in hydrogen and in water, and the kinetics for the reactions of water with the same steel were also determined. A regime of rate limited (Stage II) crack growth was observed in each of the environments. Stage II crack growth was found to be thermally activated, with an apparent activation energy of 14.7 ±2.9 kJ/mole for crack growth in hydrogen, and 33.5 ± 5.0 kJ/mole in water. Fractographic evidence indicated that the fracture path through the microstructure was the same for these environments, and suggested hydrogen to be the embrittling species for environment enhanced crack growth in hydrogen and in water/water vapor. A slow step in the surface reaction of water vapor with steel was identified, and exhibited an activation energy of 36 ± 14 kJ/ mole. This reaction step was identified to be that for the nucleation and growth of oxide. The hydrogen responsible for embrittlement was presumed to be produced during this reaction. On the basis of a comparison of the activation energies, in conjunction with other supporting data, this slow step in the water/metal surface reaction was unambiguously identified as the rate controlling process for crack growth in water/water vapor. The inhibiting effect of oxygen and the influence of water vapor pressure on environment enhanced subcritical crack growth were considered. The influence of segregation of alloying and residual impurity elements on crack growth was also considered.     

A study of the mechanism of stress-corrosion crack propagation in AISI 4340 steel in aqueous 3.5 wt.% NaCl solution

Stress-corrosion (SC) crack propagation in AISI 4340 steel has been studied with 2 mm thick single edge-notched (SEN) specimens under constant load conditions as a function of applied potential and tempering conditions in an aqueous 3.5 wt.% NaCI solution at 30°C. The SC crack lengths were estimated by using the electrical potential method. As the amount of cathodic polarization increased, the SC crack propagation rate increased. Anodic polarization yielded opposite results. These polarization effects on the SC crack propagation are discussed in terms of absorbed hydrogen resulting from a cathodic reaction on the specimen surface. SC cracks propagated by intergranular fracture through most of the inner region, but shear lips were formed at the near subsurface, irrespective of applied potential and tempering temperature. This is explained in terms of the stress state dependency of hydrogen behaviour. Above experimental evidence well supports the theory that SC crack propagation is controlled by the hydrogen embrittlement (HE) process. The SC crack propagation rate decreased in the sequence of 300, 200, and 400°C-tempered specimens. This is discussed as being related to the microstructural and yield stress effects.     

Stress history effect on incubation time for stress corrosion crack growth in AISI 4340 steel

The effect of stress history on stress corrosion cracking of AISI 4340 steel in an aqueous environment has been studied with the use of double-cantilever beam specimens. The stress history effect was found to influence the incubation time period with changes in the stress intensity. When the stress intensity was decreased, the incubation time period was dependent on the △K and finalK _f during stress corrosion testing. When the stress intensity was increased, the incubation time period was independent of the applied stress intensity. However, the stress history effect did not influence the steady-state crack growth rates. In this report, the stress history effect is explained by using the hydrogen embrittlement mechanism.     

Effect of thickness and orientation on fatigue crack growth rate in 4340 steel

Fatigue crack growth rate in 4340 is evaluated from the viewpoint of orientation and specimen thickness. It is shown that orientation has little effect on the relation between crack growth rate and stress intensity factor. However, increasing the specimen thickness from 1/16 to 1/2 in. caused a significant increase in the value ofm in the relationda/dN α (ΔK) ^m , viz. 2.6 to 5.4. This is believed to show an effect of stress state in that, in the thicker specimens, fracture is flat and the stress state tends more toward plane strain. The data show that applying crack growth data from thin specimens to larger structural members can be misleading.     

Crack paths and hydrogen-Afinssisted crack growth response in AlSi 4340 steel

A study of the correlation between crack paths and crack growth response was undertaken to define better the elemental processes involved in gaseous hydrogen embrittlement. AISI 4340 steel fractured under sustained load in hydrogen and in hydrogen sulfide over a range of temperatures and pressures, whose crack growth kinetics have been well characterized previously, was chosen for study. Fractographic results showed that crack growth followed predominantly along prior-austenite grain boundaries, with a small amount of quasi-cleavage, at low temperatures. At high temperatures, crack growth occurred primarily by microvoid coalescence. The fracture surface morphology, which is indicative of the micromechanisms for crack growth, was essentially the same for hydrogen and hydrogen sulfide. Changes in fracture morphology,i.e., crack paths, corresponded to changes in crack growth kinetics, both of which depended on pressure and temperature. There was no evidence for crack nucleation in advance of the main crack, and this suggests that the fracture process zone is located within one prior-austenite grain diameter from the crack tip. The experimental results indicate that microstructure plays an important role in determining crack growth response. The prior-austenite grain boundaries are seen to be most susceptible to hydrogen embrittlement, followed by the (110)_α’ and (112)_α’ cleavage planes. The martensite matrix, on the other hand, is relatively immune. The observed changes in crack growth rate with temperature and pressure in the higher temperature region are explained in terms of the partitioning of hydrogen into the different microstructural elements and the consequent changes in the micromechanisms for fracture. Leave from the Department of Materials Science, Shanghai Jaio Tong University, Shanghai, People’s Republic of China. Formerly Research Associate, Department of Mechanical Engineering and Mechanics.     

Mechanical Properties and Metallurgical Characterization of Dissimilar Welded Joints between AISI 316 and AISI 4340

In the present study, the influence of six different process parameters and three interactions on joint tensile strength, toughness, fusion zone microhardness variation are studied during dissimilar tungsten inert gas welding between austenitic stainless steel AISI 316 and alloy steel AISI 4340. Detailed experimental study using fractional factorial experimental design and subsequent statistical analysis show that higher tensile strength, toughness can be achieved using ER 309 filler material and suitably selecting the other process parameters and heating conditions. Addition of small proportion of hydrogen in shielding gas increases the heat transfer efficiency, melting and subsequent penetration. Preheating of AISI 4340 material reduces the chance of solidification cracking and post-heating helps to improve the joint mechanical property. Microstructural observations show that improper selection of process parameters may lead to micro-pores and degrade the joint quality. Successful joining of the dissimilar materials greatly depends on the selection of optimum process parameters, filler material and shielding gas.     

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.     

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     

X-ray photoemission electron microscopic study of transformed shear bands in AISI 4340 steel

Abstract

Failures of structural materials under dynamic mechanical loading at high strain rates are commonly initiated by shear strain localisation along adiabatic shear bands, which act as preferential sites for crack initiation and propagation. We have used synchrotron based X-ray photoemission electron microscopy (XPEEM) to identify chemical elements and measure compositional contrast in the shear band in AISI 4340 steel. The high spatial resolution of XPEEM combined with near edge X-ray absorption fine structure (NEXAFS) spectroscopy is used to study the microstructural evolution in transformed shear bands that formed in quench hardened and tempered AISI 4340 steel under dynamic impact loading. We compared our XPEEM findings with other complementary techniques such as energy dispersive X-ray spectroscopy and scanning electron microscopy to get a complete picture. As in the case of optical and scanning electron microscopy, XPEEM images show a featureless transformed band devoid of the martensite laths found in the parent metal. Interestingly, XPEEM images and corresponding Cr 2p→3d and Ni 2p→3d NEXAFS spectra confirms a compositional contrast in the transformed shear band that resulted in more nickel inside the shear band than in the adjacent region.

Les défaillances des matériaux structuraux sous charge mécanique dynamique à des vitesses élevées de déformation sont communément initiées par la localisation de la déformation de cisaillement le long des bandes de cisaillement adiabatique, lesquelles agissent comme sites préférentiels pour l’amorçage et la propagation de fissure. Nous avons utilisé le synchrotron basé sur la spectromicroscopie d’électrons photo-excités par rayons X (XPEEM) pour identifier les éléments chimiques et pour mesurer le contraste de composition dans la bande de cisaillement de l’acier AISI 4340. La haute résolution spatiale par XPEEM, combinée avec la spectroscopie des structures fines d’absorption X proches du seuil (NEXAFS), est utilisée pour étudier l’évolution de microstructure dans les bandes de cisaillement transformées qui se forment dans l’acier AISI 4340 durci par trempe et revenu sous charge dynamique par impact. Nous avons comparé nos trouvailles par XPEEM à d’autres techniques complémentaires comme la spectroscopie aux rayons X à dispersion d’énergie et la microscopie électronique à balayage afin d’obtenir une image complète. Comme dans le cas de la microscopie optique et électronique à balayage, les images XPEEM montrent une bande transformée sans particularité et sans martensite massive trouvée dans le métal parent. Ce qui est intéressant, c’est que les images XPEEM et les spectres correspondants de NEXAFS Cr 2p →3d et Ni 2p →3d confirment un contraste de composition dans la bande de cisaillement transformée qui a pour résultat la présence de plus de nickel à l’intérieur de la bande de cisaillement que dans la région adjacente.     

Effect of postweld treatment on the fatigue crack growth rate of electron-beam-welded AISI 4130 steel

This article studies the effect ofin-chamber electron beam and ex-chamber furnace postweld treatments on the fatigue crack growth rate of electron-beam-welded AISI 4130 steel. Mechanical properties of the weldment are evaluated by tensile testing, while the fatigue properties are investigated by a fatigue crack propagation method. Microstructural examination shows that both postweld treatments temper the weldment by the appropriate control of beam pattern width, input beam energy, and furnace temperature. In addition, the ductility, strength, and microhardness of the weldment also reflect this tempering effect. The fatigue crack growth rate is decreased after both postweld treatments. This is mainly caused by the existence of a toughened microstructure and relief of the residual stress due to the fact that (1) the residual stress becomes more compressive as more beam energy is delivered into the samples and (2) postweld furnace tempering effectively releases the tensile stress into a compressive stress state. Formerly Lecturer, Department of Mechanical Engineering, Chung Cheng Institute of Technology     

On the stress corrosion crack growth rates of AISI 4340 steel near its lower temper martensite embrittlement range in 0.5 N NaCl solution

Based on the data of the literature for intercrystalline stress corrosion cracking (SCC) and hydrogen embrittlement of the High Strength AISI 4340 steel, determination of the so far unknown effects of tempering treatment around the low temper martensite embrittlement range (between 175 and 285°C) on the crack growth rates in 0.5 N NaCl solution. Effect of variation of stress intensity and applied potentials on crack growth rates. Effect of initial applied stress intensity and crack tip sharpness on crack growth characteristics. Discussion on crack growth rates for a better understanding of the SCC mechanism.     

The effect of hydrogen source on crack initiation in 4340 steel

The crack initiation site and the corresponding incubation time were determined as a function of notch radius in 4340 steel for both internally and externally supplied hydrogen. The source of hydrogen was found to affect both the crack nucleation site and the incubation time. Hydrogen cracking in cathodically charged 4340 steel initiated near the elastic-plastic boundary with incubation times which exhibited a linear dependence on notch radius. Hydrogen cracking in an aqueous solution initiated near the notch surface with incubation times which were relatively independent of notch radius. Short time diffusional flow models which include a stress dependent critical hydrogen concentration were found to predict incubation times reasonably for internally supplied hydrogen. Cherepanov's solution for the diffusion at the tip of a semi-infinite linear slit when applied in the context of a finite notch root radii problem was found to predict incubation times adequately for externally supplied hydrogen.     

Fatigue crack propagation in 4140 steel

The fatigue crack propagation rates, da/dN, of 4140 steel were measured in dry argonvs tempering temperature. In specimens 3.2 mm thick at a given ΔK between 15 and 30 MN/ m^3/2, da/dN decreases with increasing tempering temperature, reaches a shallow minimum for tempering at 400°C. The rate for as-quenched specimens increases withR ratio; this is not the case for the 400, 550 and 650°C tempers. Reducing the specimen thickness to 1.3 mm has little effect on the 650°C temper but causes a large decrease in da/dN for the asquenched condition and 200°C temper. Edge notch specimens tempered at 550 and 650°C are subject to crack arrest from cycling prior to crack initiation. The results are discussed in terms of the metallurgical structures and various fatigue crack propagation equations which have been proposed. The results cannot be explained on the basis of da/dN being determined only by Young’s modulus andK _c.     

An analysis of plastic instability in pure shear in high strength AISI 4340 steel

The phenomenon of plastic instability in pure shear was studied at room temperature in heat treated high-strength AISI 4340 steels, employing the torsion test. The instability occurs after saturation of strain hardening by the dispersed carbide particles. The strain at onset of instability is sensitively dependent on rate of straining. The effective stress (total stress minus the stress resulting from dispersion hardening) may be a result of Snoek relaxation or the Cottrell drag, depending on strain rate; the magnitude of this stress is linearly proportional to the dissolved carbon content. Experimental observations indicate that instability interfaces advance spirally in the axial direction of the specimen. It is proposed that the interface is stable in a thermodynamic sense and that the driving force for migration depends only on ratio of the speed of the front to the macroscopic strain rate. When the interface becomes stationary, fracture is nucleated at an axial distortion in the interface. Fracture by the instability occurs by a translatory motion of one now rigid body with respect to a second rigid body along the characteristic (slip) surface. Pores may be found in the fracture surface, but these are incidental to the intense flow along the characteristic, and are not the cause of the instability.     

A unified theory for some effects of hydrogen source,alloying¦elements,and potential on crack growth in martensitic AISI 4340 steel

The effects of hydrogen on crack growth in martensitic AISI 4340 steel are shown to be fundamentally the same whether the hydrogen is supplied as molecular gas, through stress corrosion, or by electrolytic charging. At a given yield strength differences observed in the values of threshold stress intensity for crack growth are proposed to be linked to the degree of dissociation of the hydrogen near the crack tip, and hence to the concentration of hydrogen developed in the critical crack-tip region. Over a range of yield strength values, an upper bound of threshold stress intensity is developed in molecular hydrogen gas and a lower bound on exposure to atomic hydrogen from cathodic charging during or prior to testing. The open circuitK _Iscc values of the steel fall always within the upper and lower bounds, but the values ofK _Iscc may be moved to the lower bound by coupling to magnesium (cathodic charging) or to the upper bound by coupling to copper (anodic polarization). Variations in the concentration of carbon or manganese in the steel at a fixed yield strength produce effects on the value ofK _Iscc similar to the effects produced by cathodic or anodic polarization. With the lower concentrations of carbon or manganese the steel acts as if it were coupled to copper and at the higher concentrations as if coupled to magnesium. Carbon and manganese are therefore proposed to shift the positions of local anodes and cathodes and so influence the proportions of molecular and atomic hydrogen which reach the critical crack-tip region. The proposal is supported by data which show that only cathodic polarization affects the threshold stress intensity of the lowest carbon and manganese steelsK _Iscc is lowered) whereas only anodic polarization affects the higher carbon or manganese steels(K _Iscc is raised).     

Further considerations on the inconsistency in toughness evaluation of AISI 4340 steel austenitized at increasing temperatures

A study has been made of the influence of austenitizing temperature on the ambient temperature toughness of commercial AISI 4340 ultrahigh strength steel in the as-quenched (untempered) and quenched and tempered at 200°C conditions. As suggested in previous work, a systematic trend ofincreasing plane strain fracture toughness(K) _Ic anddecreasing Charpy V-notch energy is observed as the austenitizing temperature is raised while the yield strength remains unaffected. This effect is seen under both static <slowbend> and dynamic (impact) loading conditions, and is rationalized in terms of a differing response of the microstructure, produced by each austenitizing treatment, to the influence of notch root radius on toughness. Since failure in all microstructures was observed to proceed primarily by a ductile rupture (microvoid coalescence) mechanism, an analysis is presented to explain these results, similar to that reported previously for stress-controlled fracture, based on the assumption that ductile rupture can be considered to be strain-controlled. Under such conditions, the decrease in V-notch Charpy energy is associated with a reduction in critical fracture strain at increasing austenitizing temperatures, consistent with an observed decrease in uniaxial and plane strain ductility. The increase in sharp-crack fracture toughness, on the other hand, is associated with an increase in “characteristic distance” for ductile fracture, resulting from dissolution of void-initiating particles at high austenitizing temperatures. The microstructural factors which affect this behavior are discussed, and in particular the specific role of retained austenite is examined. No evidence was found that the enhancement of fracture toughness at high austenitizing temperatures was due to the presence of films of retained austenite. The significance of this work on commonly-used Charpy/K_Ic empirical correlations is briefly discussed. formerly with Lawrence Berkeley Laboratory, Berkeley, CA