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.     

A “Hydrogen partitioning” model for hydrogen assisted crack growth

A “hydrogen partitioning” model has been developed to account for the pressure and temperature dependence for hydrogen-assisted crack growth. The model gives explicit recognition to the role of hydr en-microstructure interactions in determining the distribution (or partitioning) of hydrogen among the various microstructural elements (principally between the prior-austenite grain boundaries and the matrix) and the rate of crack growth along the elements. It also takes into account the role of various rate controlling processes in determining the rate that hydrogen is being supplied to the fracture process (or embrittlement) zone. Quantitative assessment of the model indicates very good agreements between the model predictions and the observed crack growth responses for AISI 4340 and 4130 steels tested in hydrogen and for AISI 4340 steel tested in hydrogen sulfide. This model accurately characterizes the reduction in crack growth rate and the concomitant change in fracture mode at “high” temperatures. Through its integration with the earlier models, based on rate controlling processes, the model predicts the pressure and temperature dependence for K-independent crack growth over the entire range of environmental conditions.     

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.     

Effect of Hardfaced Interlayer Thickness and Low Hydrogen Ferritic Capping on Ballistic Performance of Shielded Metal Arc Welded Armour Steel Joints

Armour grade quenched and tempered steel closely confirming to AISI 4340 is well known for its superior ballistic performance and hence used in the fabrication of combat vehicles. The traditional fillers like austenitic stainless steel showed poor ballistic performance of these welded joints as compared to the base metal. Attempts have been made to deposit hardfaced interlayer between austenitic stainless steel weld metals. Though this method, marginal improvements in ballistic performance can be yielded, and cracks were observed in between base metal and hardfaced layer. Thickness of the hardfaced interlayer plays a vital role for the effective ballistic performance. Thus, an attempt has been made to investigate the effect of hardfaced interlayer thickness on ballistic performance of armour steel welds. The results of effect of buttering, low hydrogen ferritic (LHF) filler and three different hardfaced layer thicknesses (4, 5.5 and 7 mm) on ballistic performance of shielded metal arc welded armour steel joints were given.     

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.     

Effects of silicon additions and retained austenite on stress corrosion cracking in ultrahigh strength steels

A study has been made of the effects of silicon additions and of retained austenite on the stress-corrosion cracking (SCC) behavior of commercial ultrahigh strength steels (AISI 4340 and 300-M) tested in aqueous solutions. By comparing quenched and tempered structures of 4340 and 300-M i) at equivalent strength and ii) at their respective optimum and commercially-used heat-treated conditions, the beneficial role of silicon addition on SCC re-sistance is seen in decreased Region II growth rates, with no change in K’_ISCC. The beneficial role of retained austenite is demonstrated by comparing isothermally transformed 300-M, containing 12 pct austenite, with conventionally quenched and tempered structures of 300-M and 4340, containing less than 2 pct austenite, at identical yield strength levels. Here, the isothermally transformed structure shows an order of magnitude lower Region II SCC growth rates than quenched and tempered 300-M and nearly two orders of magnitude lower Region II growth rates than 4340, K ISCC values remaining largely unchanged. The results are discussed in terms of hydrogen embrittlement mechanisms for SCC in martensitic high strength steels in the light of the individual roles of hydrogen diffusivity and carbide type.     

Hydrogen trapping in precipitation-hardened alloys

The ingress of hydrogen in three precipitation-hardened alloys (Inconel 718, Incoloy 925, and 18 Ni maraging steel) exposed to an acetate electrolyte (1 mol L⁻¹ HAc/1 mol L⁻¹ NaAc where Ac = acetate) was studied using a potentiostatic pulse technique. The data were shown to fit a diffusion/trapping model under interface control, and values were determined for the irreversible trapping constants (k) and the flux of hydrogen into the alloys. The density of irreversible trap defects in Inconel 718 and Incoloy 925 was calculated from k and found to be in excellent agreement with the concentration of NbTi(CN) and TiC particles, respectively. The maraging steel was characterized by two trapping constants; one is associated with quasi-irreversible traps that saturate, leaving only irreversible traps thought to be TiC/Ti(CN) particles. The irreversible trapping constants for these alloys are consistent with their relative susceptibilities to hydrogen embrittlement. Moreover, a comparison of the trapping constants with those for AISI 4340 steel and two other nickel-base alloys (Monel K-500 and MP35N) indicates that a strong correlation exists between hydrogen embrittlement susceptibility and trapping capability over all the alloys.     

Rate controlling processes for crack growth in hydrogen sulfide for an alsl 4340 steel

Parallel fracture mechanics and surface chemistry studies were carried out to develop further understanding of environment assisted subcritical crack growth in high strength steels. The kinetics of crack growth for an AISI 4340 steel (tempered at 477 K) in high purity hydrogen sulfide have been determined as a function of pressure at room temperature and as a function of temperature at hydrogen sulfide pressures of 0.133 and 2.66 kPa. The kinetics for the reactions of hydrogen sulfide with this steel and the extent of reactions were also determined. Two rate controlling processes have been identified. At the lower pressure, the rate of crack growth varies according to T^1/2 and is controlled by the rate of transport of hydrogen sulfide to the crack tip. At the higher pressure, crack growth is controlled by the rate of diffusion of hydrogen into the steel ahead of the crack tip and exhibits an apparent activation energy of about 5 kJ/mol. Embrittlement results from hydrogen that is produced by the reactions of hydrogen sulfide with the steel. These reactions are extremely rapid and are limited in extent, leading to the formation of one to two layers of “sulfide” on the fracture surfaces. The crack growth results are discussed in terms of measured reaction kinetics and published data on diffusion, and in relation to models for transport- and diffusion-controlled crack growth. Formerly with Lehigh University, Bethlehem, PA     

Hydrogen induced interior crack-tip morphologies in high strength steel

In this study WOL specimens of high strength 4340 steel were used to obtain information on the interior, or near plane strain, crack-tip morphology resulting from hydrogen assisted crack growth, including the position of secondary cracks in relation to the region of maximum triaxial stress, crack-tip plastic zone size, and grain size. The interior, or near plane strain, regions were revealed by removing thin sections, by a grinding and polishing procedure, from the surfaces of specimens that underwent hydrogen assisted crack growth. The results appear to indicate that advance of the crack occurs by growth of protrusions, or steps, from the main crack front, followed by linkup of these protrusions by sideways growth along grain boundaries.     

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 .     

Crack growth from internal hydrogen—temperature and microstructural effects in 4340 steel

Internal hydrogen effects on stage II crack growth rates in AISI 4340 steel have been studied as a function of test temperature. A model is developed that is physically based in that classical thermodynamics relates to solubility and trapping and Fick’s second law controls hydrogen transport. Both of these are microstructurally related to how trapping affects both the crack initiation site and diffusion to it. For two tempered conditions of 4340 steel, it is shown that there is a test temperature,T ₀, for stage II crack growth, above which the crack does not grow. The fractography associated with test temperatures approachingT ₀ tends toward 100 pct intergranular for both 1340 MPa and 1620 MPa strength levels. At lower test temperatures, there is as much as 50 pct microvoid coalescence or 30 pct quasi-cleavage. In the lower strength condition, hydrogen traps at oxysulfide particles with a binding energy near 75 kJ/mol. Where these intersect the prior austenite grain boundaries, this promotes fingers of intergranular fracture which later triggers tearing of 100 μm size ligaments by microvoid coalescence. For the higher strength material, it is proposed that hydrogen traps along martensite lath intersections with prior austenite grain boundaries, the binding energy being near 27 kJ/mol. This promotes 1 μm size striations along intergranular facets. In both cases the fractography is consistent with a proposed model of stress field concentration of hydrogen, further concentration along trap sites, fracture nucleation at trap sites, and local, discontinuous fracture instabilities.     

Stress Oriented Hydrogen Induced Cracking of Linepipe Steel

The stress oriented hydrogen induced cracking (SOHIC) is a typical hydrogen embrittlement phenomenon occurring in the linepipe steels exposed to sour environment containing H 2 S gas.However,even recently,the cracking mechanism of SOHIC has not been clarified because of lacking in the empirical data on the actual failure mode of SOHIC cracking.The factors affecting SOHIC are discussed in terms of metallurgy of high strength linepipe steel and hydrogen electrochemistry.The cracking mechanisms of SOHIC are examined by comparing them with the empirical failure mode of SOHIC which is developed by observation of the actual fracture sites of the hydrogen induced blister cracking (HIBC) and secondary cracks.Finally,the correlation between SOHIC and HIC is discussed.     

Workability of commercial-purity titanium and 4340 steel during equal channel angular extrusion at cold-working temperatures

The deformation and failure of commercial-purity (CP) titanium (grade 2) and AISI 4340 steel (tempered to R _c 35) during equal channel angular extrusion were determined at temperatures between 25 °C and 325 °C and effective strain rates between 0.002 and 2.0 s⁻¹. The CP titanium alloy underwent segmented failure under all conditions except at low strain rates and high temperatures. By contrast, the 4340 steel deformed uniformly except at the highest temperature and strain rate, at which it also exhibited segmented failure. Using flow curves and fracture data from uniaxial compression and tension tests, workability analysis was conducted to establish that the failures were a result of flow localization prior to the onset of fracture. This conclusion was confirmed by metallographic examination of the failed extrusion specimens.     

Relationship between hydrogen-assisted crack propagation rate and the corresponding crack path in AISI 4340 steel

In the present investigation, relationship between hydrogen-assisted (HA) crack propagation rate and the corresponding fracture mode in AISI 4340 steel has been elucidated with critical hydrogen concentration concept. Hydrogen assisted crack-propagation rate and the corresponding fracture surface morphology were determined from double cantilever beam (DCB) specimens as a function of hydrogen pressure and temperature. As hydrogen pressure decreased, threshold stress intensity factor necessary for the onset of hydrogen-assisted crack propagation increased and the stage II (plateau) crack-propagation rate decreased. The kinetics of stage II crack propagation indicated substantial difference, i.e., positive and negative responses in the two investigated low and high temperature regions, respectively. Fractographic analysis showed that increased amounts of the microvoid coalescence mode resulted in slower crack-propagation rates. The observed changes in crack-propagation rate and the corresponding fracture mode with hydrogen pressure and temperature are discussed in terms of critical stress or strain and critical hydrogen concentration concepts.     

Effect of hydrogen on fracture of U-notched bend specimens of quenched and tempered AISI 4340 steel

U-notched bend specimens of quenched and tempered AISI 4340 steel were deformed in the uncharged condition and after either precharging or dynamically charging with hydrogen. In the uncharged condition fracture initiated at the notch surface and progressed in mode II along a characteristic slip trace. For precharged specimens, strain to fracture was markedly reduced, cracks nucleated internally in a mode I manner and the crack progressed to the surface in mode II. Dynamic charging reduced plastic strain to essentially zero at crack nucleation, which occurred when the notch root stress reached the yield stress, and the crack grew by mode I. The results are compared to those on lower strength steels and are discussed in view of theories for hydrogen degradation of mechanical properties. Formerly with the Ohio State University Formerly with the Ohio State University     

Fractographic studies of stress corrosion cracking behaviour of AISI 4340 steel in 0.5 N NaCI solution near its lower temper martensite embrittlement range

Based on the data of the literature for intercrystalline stress corrosion cracking (SCC) and hydrogen embrittlement of the high-strength steel AISI 4340, determination of the so far unknown effects of tempering treatment around the low temper-martensite embrittlement range (between 175 and 285°C) on the fracture appearance. Variation of the stress intensity and applied potentials in 0.5 N NaCI solution. Discussion on obtained fractographs for a better understanding of the SCC mechanism.     

Analytical electron microscopy of grain boundaries in high-strength steels

Phosphorus could be detected at prior austenite grain boundaries (PAGB) in high-strength alloy steels quenched and tempered at 500°C when using a VG's HB 501 dedicated field emission STEM but not with a conventional JEOL 4000FX STEM. No phosphorus was detected at PAGB's in the as-quenched materials or away from PAGB's in tempered materials of either type. The grain boundary coverage of phosphorus was, assuming a specimen thickness of 80 nm, 0.7 monolayers for the 3.5NiCrMoV rotor steel and 0.4 monolayers for the AISI 4340 steel. The grain boundary concentration of phosphorus, assuming a specimen thickness of 80 nm and a segregated layer thickness of 1 nm, for the 3.5NiCrMoV rotor steel was 6 wt% and for AISI 4340 4 wt%. Compared to the bulk concentration of about 0.01 wt% this means that the enrichment factor of P to the grain boundaries was several hundred times (610 respectively 370). Our measurements showed no correlation between the stress corrosion crack growth rate and the grain boundary phosphorus concentration. The yield strength, however, decreased after tempering while the phosphorus concentration at the grain boundaries increased.     

Effect of quench rate on microstructure and tensile properties of ALSL 4320 and 4340 steels

A study has been made of the effect of quench rate on the microstructure and tensile properties of two commercial AISI 4320 and 4340 steels having fully martensitic structures. The steels were quenched from various temperatures from 1323 to 1473 K, at two different quench rates using iced brine (fast quench treatments) and oil held at 373 K (slow quench treatments). Tensile properties of these steels, after double-tempering at 473 K with intermediate quenching and refrigeration, were determined at ambient temperature (293 K) using an Instron test machine. The microstructural changes accompanying these quench rates were examined by means of optical and thin-foil transmission electron microscopic techniques. In the 4320 steel with a relatively high Ms temperature, the slow quench treatments compared to the fast quench treatments increased both the 0.2 pct proof stress and the ultimate tensile strength at similar total elongation levels, regardless of the prior austenite grain size, while the strength data of the slowly quenched steels exhibited a large scatter as the prior austenite grain size increased. However, in the 4340 steel with a relatively low Ms temperature tensile properties were less sensitive to quench rate, while the slow quench treatments compared to the fast quench treatments increased slightly only the 0.2 pct proof stress. From microstructural results, it is suggested that the beneficial effect on the strength of the slowly-quenched steels is caused by a dispersion-hardening effect due to carbon segregation or fine carbide precipitation in the martensite during the quench(i.e., autotempering).     

Determination of Hydrogen in Steel by Thermal Desorption Mass Spectrometry

Hydrogen embrittlement has been observed since high‐strength steels have been produced in the nineteen thirties 1 . Several different analytical methods have been developed to quantify the total and diffusible hydrogen in steel, but many aspects of hydrogen determination are still to be explored. Purely quantitative determination of hydrogen is not sufficient to fully characterize the steel regarding its resistance against embrittlement. Thermal Desorption Mass Spectrometry (TDMS) allows the investigation of hydrogen absorption and desorption mechanisms to characterize hydrogen traps in different kinds of steel microstructures. This provides valuable information for the development of new materials with a higher resistance against hydrogen embrittlement. Additionally, TDMS allows the quantitative determination of very small concentrations of hydrogen (<0.1 µg/g). Such low detection limits cannot be reached with other methods. Due to time‐consuming analysis and a rather complex construction, TDMS is usually not applied for hydrogen determination in German steel mills. The present work describes the development of a thermal desorption spectrometer at ThyssenKrupp Steel Europe AG by adapting a compact quadrupole mass spectrometer to a commercially available hot solid extraction analyzer, which has proven to be a simple and efficient solution for the determination of diffusible hydrogen in steel.