Factors Influencing Stress-Corrosion Cracking of High-Strength Steel Weld Metals

The stress-corrosion cracking (SCC) behavior of high-strength steel weld metals, as indexed by K_Iscc, was examined with emphasis on the relative influences of yield strength, electrochemical potential, welding process, and weld metal composition. The weld metals were from weldments fabricated by the gas metal arc (GMA) or gas tungsten arc (GTA) process. Filler metals with four different compositions—designated 120S, 140S, AX140 and HY-130—were used. The multi-pass welding procedures and their associated thermal cycles produced very complex martensitic-bainitic type microstructures. The GTA weld metals were considerably more fine-grained and more highly tempered than the GMA weld metals. This enhanced the fracture toughness of all four of the higher strength GTA weld metals but improved the SCC properties of only two GTA weld metals—HY-130 and 140S. The effectiveness of microstructural influences on SCC behavior is correlated with the sulfur content of the weld metals assuming that hydrogen is the cause of SCC in these materials. The role of sulfur is presumed to be that of catalytic poison for the hydrogen recombination reaction which increases opportunities for nascent hydrogen absorption. The results show that the weld metals with improved SCC properties contain the lower concentrations of sulfur.     

High Power Laser Powder Deposition Repair of AISI 4340 Steel in Aircraft Applications

High power laser powder deposition (LPD) has been used to investigate the potential of repairing damaged aero-grade high strength steel.Metallurgical analysis was performed to analyze the integrity of the clad layer.A 4kW fiber laser was used to deposit two separate alloys (AISI 4340 and AISI 420 stainless steel) on an AISI 4340 steel substrate and metallurgical analysis was performed to analyze the integrity of the clad layer.No microcracks was observed on the clads,but porosity and high dilution was observed on most clads.However,microstructural analysis showed a crack and porosity free clad layer with low dilution can be achieved for some laser conditions.     

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.     

Stress Corrosion Cracking of HY-180 Steel in Aqueous 3.5 Pct NaCI

Stress corrosion cracking of HY-180 steel (Fe-10 Ni-2 Cr-1 Mo-8 Co-0.12 C) was studied in aqueous 3.5 pct NaCI (pH = 6.5) at 22 °C. The alloy was austenitized, water quenched and aged at 510 °C for 5 h. Specimens were of the precracked, double cantilever beam (DCB) variety and exposure times extended up to 1000 h. The crack propagation rates (v) were studied as a function of stress intensity(K,) under both freely corroding potentials(E ≈-0.36 V_SHE) and potentials produced by coupling to Zn(E ≈ -0.82 V_SHE. Crack fractography was studied by scanning electron microscopy and corrosion products were identified by electron diffraction analysis. The stress intensity, K_ISCC, below which SCC could not be detected was ~45 MPa m^1/2 for both freely corroding and Zn-coupled conditions. Analysis of the results showed that cracking was consistent with a hydrogen embrittlement mechanism, irrespective of potential. Furthermore, comparison of the data with previous studies on a similarly heat treated and closely related alloy (HY-180 M), containing 14 Co-0.16 C, showed no significant difference in SCC behavior, provided comparison was made at similar electrochemical potentials.     

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.     

Hydrogen Uptake Enhancement and Accelerated Hydrogen Re-embrittlement of Cd-plated AISI 4340 Steel Bolts Coupled with IN718 Nuts

Hydrogen re-embrittlement on anodically coated high strength steels could be of great concern because the uptake of hydrogen from the corrosion process can cause component failure. A scratched Cd-coated AISI 4340 steel membrane has been coupled with different materials reproducing crevice conditions, and the hydrogen uptake has been measured using a modified Devanathan?CStachurski permeation apparatus. Experimental tests proved that, in presence of a crevice, metals nobler than cadmium strongly enhance local hydrogen reduction on exposed steel areas, thus possibly favoring brittle failure of high strength steel components during service. Therefore, the coupling of uncoated nuts made of noble passive alloys (like Inconel) to Cd-plated AISI 4340 steel bolts should be avoided.     

Behavior of sulfide inclusions during thermomechanical processing of AISI 4340 steel

The morphological and compositional modifications of sulfides in AISI 4340 low alloy steel, in which the sulfur level was raised to about 0.1 pct, were studied during hotrolling at 1223 K followed by homogenization at 1583 K for various lengths of time. The relative plasticity of sulfide inclusions with respect to the steel matrix increased with cooling rate during solidification, hence with iron content. The number of sulfides first increased with homogenization time, reached a maximum and subsequently decreased. Inclusion size exhibited the opposite variation. The sulfide matrix interface area per unit volume of sulfide decreased continuously with homogenization time. These variations were in agreement with observed morphological modifications of sulfides during homogenization. During early stages of homogenization the flattened and elongated sulfide plates in the as-rolled material coarsened and became cylindrical. The cylindrical sulfides, broken into segments which spheroidized and coarsened with time, assumed finally a faceted morphology. During homogenization iron was rejected from the sulfide phase into the surrounding matrix, whereas manganese was accepted, causing the formation of manganese depleted zone around the inclusions. This paper is based on a Ph.D. Thesis submitted by Y. V. Murty to the Department of Metallurgy, University of Connecticut.     

The Influence of Notch Root Radius and Austenitizing Temperature on Fracture Appearance of As-Quenched Charpy-V Type AISI4340 Steel Specimens

Charpy-V type samples either step-quenched from 1200 °C or directly quenched from the usual 870 °C temperature, fractured by a slow bend test procedure, have been fractographically examined. Their notch root radius,ρ, ranged from almost zero (fatigue precrack) up to 2.0 mm. The fracture initiation process at the notch differs according to root radius and heat treatment. Conventionally austenitized samples withρ values larger than 0.07 mm approximately (ρ _eff) always display a continuous shear lip formation along the notch surface, whereas specimens with smaller notches do not exhibit a similar feature. Moreover, shear lip width in specimens withρ >ρ _eff is linearly related to the applied J-integral at fracture. In high temperature austenitized samples similar shear lips are almost nonexistent. The above findings, as well as overall fractographic features, are combined to explain why blunt notch AISI 4340 steel specimens display a better fracture resistance if they are conventionally heat treated, whereas fatigue precracked samples show a superior fracture toughness when they are step-quenched from 1200 °C. Variations of fracture morphologies with the notch root radius and heat treating procedures are associated with a shift toward higher Charpy transition temperatures under the combined influence of decreasing root radii and coarsening of the prior austenitic grain size at high austenitizing temperatures. D. FIRRAO, J. A. BEGLEY were both formerly with the Department of Metallurgical Engineering, The Ohio State University, Columbus, OH 43210.     

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.     

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.     

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     

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.     

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.     

Observation of an adiabatic shear band in AISI 4340 steel by high-voltage transmission electron microscopy

Adiabatic shear bands, formed in a hollow AISI 4340 steel cylinder subjected to dynamic expansion by means of an explosive charge placed in its longitudinal axis, were characterized. The adiabatic shear bands formed in this quenched and tempered steel were of the classical “transformed” type. Scanning electron microscopy (SEM) of etched surfaces revealed that alignment of the lamellae along the direction of shear seems to be the event that precedes shear localization. The transmission electron microscopy of a “white”-etching shear band having undergone a shear strain of approximately 4 revealed that it containedX (Fe₅C₂) carbides in a martensitic structure. These carbides were observed to form on (112) internal microtwins. Grains could not be resolved inside of the shear band, but they could be observed in the surrounding matrix material. A traverse of the shear band was made, and there existed no definite boundary between the matrix and the shear band. No evidence of a transformation to austenite was observed. Heat transfer calculations were conducted to help explain the features observed inside of the shear band. It is concluded that the “white”-etching bands, commonly referred to in the literature as “transformed” bands, do not exhibit a transformation at values of shear strain of up to 4. The enhanced reflectivity is an etching artifact and is possibly due to microstructural changes, a very small grain size, and carbide redissolution in the bands. Formerly with the Department of Metallurgical and Materials Engineering, New Mexico Institute of Mining and Technology, Socorro, NM 87801.