Study of the effect of heat treatment on hydrogen embrittlement of AISI 4340 steel

Delayed failure tests were performed on fully-quenched AISI 4340 steel tempered at 500 and 700° C, subjected to sustained tensile loads and cathodically charged with hydrogen (current density 10 mA cm^–2) in an aqueous solution of 0.1 N sulphuric acid. The aim was to study the effect of the microstructure on the behaviour of steel towards the embrittling action of hydrogen. The tests were carried out with two different research techniques in order to highlight this behaviour better. This was done with a view to making the use of the steel safer. With the methods used the results obtained, although substantially different from each other, showed that thermodynamically more stable structures are less sensitive to the phenomenon of hydrogen embrittlement.     

Macroscopic characteristics of plasma-nitrided AISI 4340 steel

Plasma-nitrided AISI 4340 steel, in a low-pressure abnormal glow discharge was studied. The layer formation and their characteristics, were studied as a function of the main macroscopic parameters of the gas discharge: gas flow, N₂/H₂ mixture, treatment time and sample temperature. The nitrided layers were characterized using a metallographic microscope coupled to a microhardness tester and X-ray techniques. Typical results obtained for samples which were nitrided for 2 h in a gas mixture of 0.9N₂+0.1 H₂ at 793 K, showed a white layer composed of and phases, 20 m thick, and a 1100 HV20 microhardness.     

Fatigue behaviour of AISI 304 steel to AISI 4340 steel welded by friction welding

In the presented study, The weldability of AISI 304 austenitic stainless steel to AISI 4340 steel joined by friction welding in different rotational speeds and fatigue behaviour of friction-welded samples were investigated. Tension tests were applied to welded parts to obtain the strength of the joints. The welding zones were examined by scanning electron microscopy (SEM) and analyzed by energy dispersive spectroscopy (EDS). The Vıckers microhardness distributions in welding zone were determined. Fatigue tests were performed using a rotational bending fatigue test machine and the fatigue strength has been analysed drawing S-N curves and critically observing fatigue fracture surfaces of the tested samples. The experimental results indicate that mechanical properties and microstructural features are affected significantly by rotation speed and the fatigue strength of friction-welded samples decrease due to chromium carbide precipitation in welding zone with increasing rotation speed in choosen conditions.     

Hierarchical probabilistic modeling of short-crack growth behavior in AISI 4340 steel

Probabilistic analyses of non-uniform crack growth data sets require a flexible statistical framework to determine the influence of each crack on the resulting inference. Hierarchical generalized linear models provide a rigorous method to analyze such data sets properly. Bayesian techniques are well-suited to analyze these models, especially when the inference, or portions thereof, are ill-posed. A hierarchical generalized linear crack growth model is developed using a semi-conjugate formulation that enables Gibbs sampling simulation. The model is applied to create a probabilistic crack growth model from short-crack data generated from AISI 4340 steel single-edge-notch tension (SENT) specimens. Simulation of the model is performed using a Gibbs sampling procedure, and key results are discussed. Stress ratio effects on experimental scatter and crack growth rates are quantified and discussed.     

Effect of chemical banding on the local hardenability in AISI 4340 steel bar

The microstructure of a 240 mm diameter AISI 4340 mill shaft, quenched and tempered, was analyzed through its longitudinal cross section. Normally, higher cooling rates promote martensitic structure while the diffusion assisted austenite decomposition products are expected in the lower cooling rate condition. The martensitic reaction takes place inside the grains while the products diffusion starts from the prior austenite grain boundaries, nucleation and grow processes. An anomalous hardenability behavior was reported with a mixed structure containing martensite and bainite, in different proportion from the surface (higher cooling rate) to the core (lower cooling rate). This behavior was attributed to the wide banded structure. Banding due to the as cast structure segregation, is directly related to the solidification rate as well as the deformation degree due (during) to forging and rolling operations. When the deformation degree is high, the banding thickness is thin and approaches the austenite grain size. In this study the thickness of the bands embedded about four to five austenite grain sizes, in such a way that every band behaves like a particular steel composition. A semi quantitative chemical analysis was carried out through the banded structure to understand the differential hardenability behavior. The results were discussed using classical hardenability formula as well as calculated TTT diagrams for each composition.     

Microstructural evolution of AISI 4340 steel during Direct Metal Deposition process

In the current investigation AISI 4340 steel was laser deposited on a rolled mild steel substrate by Direct Metal Deposition (DMD) technology. The microstructural investigation of the clad was performed using optical and electron microscopes and X-ray diffraction techniques. The microstructure consisted of ferrite, martensite and cementite phases. Two types of martensite, lathe-type and plate-type, were observed in the microstructure. Decrease in microhardness values from the top layer to the alloy layer proves that the degree of tempering of the martensite phase increases in the same direction. The lattice parameters of the identified phases were found to be shorter than those reported in literature. The reported parameters in literature are from samples processed under equilibrium conditions.     

Improvement in the fatigue strength of chromium electroplated AISI 4340 steel by shot peening

In landing gear, an important mechanical component for high responsible applications, wear and corrosion control is currently accomplished by chrome plating or hard anodising. However, some problems are associated with these operations. Experimental results have also shown that chrome‐plated specimens have fatigue strength lower than those of uncoated parts, attributed to high residual tensile stress and microcracks density contained into the coating. Under fatigue conditions these microcracks propagate and will cross the interface coating‐substrate and penetrate base metal without impediment. Shot peening is a surface process used to improve fatigue strength of metal components due to compressive residual stresses induced in the surface layers of the material, making the nucleation and propagation of fatigue cracks difficult. This investigation is concerned with analysis of the shot peening influence on the rotating bending fatigue strength of hard chromium electroplated AISI 4340 steel. Specimens were submitted to shot peening treatment with steel and ceramic shots and, in both cases, experimental results show increase in the fatigue life of AISI 4340 steel hard chromium electroplated, up to level of base metal without chromium. Peening using ceramic shot resulted in lower scatter in rotating bending fatigue data than steel shots.     

Effect of specimen size on fatigue crack growth rate in AISI 4340 steel

An investigation has been carried out to study the influence of specimen size parameters (thickness, with and aspect ratio) on fatigue crack growth rate. Compact tension specimens with a TL orientation, prepared from aircraft quality AISI 4340 steel and heat treated to a yield strength level of 1000 MPa, were used. All testing was done at a constant δK level. The investigation demonstrates that specimen thickness and width have no significant influence on fatigue crack growth rate for AISI 4340 steel. On the other hand, fatigue crack growth rate was found to increase marginally at high aspect ratios (a/W0.55). Paris constants C and m were also evaluated.     

Corrosion fatigue of coated AISI 4340 high strength steel with dent damage

Tension‐compression fatigue behaviour of a high‐strength steel, coated with cadmium and zinc‐nickel, with dent damage was investigated under saltwater environment. Dent damage exposed coated high‐strength steel substrate to saltwater environment to study hydrogen reembrittlement. Three types of specimens were tested: cadmium and zinc‐nickel‐coated specimens with dent damage and uncoated specimens with similar dent damage that were shot‐peened in a similar way as the coated specimens. The environmentally friendly zinc‐nickel‐coated specimens displayed similar fatigue behaviour like hazardous cadmium‐coated specimens. Scanning electron microscopy with an apparatus Quanta 450 confirmed that damage mechanisms were similar in all 3 types of specimens.     

Evaluation of corrosion resistance of diamond-like carbon films deposited onto AISI 4340 steel

The corrosion resistance of amorphous diamond-like carbon (DLC) coatings deposited by radio frequency plasma enhanced chemical vapor deposition (rf-PECVD) technique on AISI 4340 steel substrates was evaluated under saline (5% NaCl) and acid (1700 ppm H₂SO₄) atmospheres. The corrosion process was investigated by surface characterization and electrochemical methods, such as potentiostatic polarization and electrochemical impedance spectroscopy (EIS). DLC coatings effectively protected the substrate after 48 h in a salt fog chamber and after the first Kesternich cycle. For comparison, under the same conditions, titanium nitride (TiN) coatings did not protect the substrate even for 2 h of saline exposure and even for the first Kesternich cycle. Although the DLC coatings resisted well to the corrosive action of the aggressive media, nucleation and growth of homogenous and micro-sized pinholes uniformly distributed on DLC coatings were observed as a result of the corrosion processes. The observed results suggest that the development of techniques which would reduce the porosity of the DLC films could promote further improvement on their corrosion protection ability.     

Residual stress influence on fatigue lifetimes of electroplated AISI 4340 high strength steel

Residual stresses play an important role in the fatigue lives of structural engineering components. In the case of near surface tensile residual stresses, the initiation and propagation phases of fatigue process are accelerated; on the other hand, compressive residual stresses close to the surface may increase fatigue life. In both decorative and functional applications, chromium electroplating results in excellent wear and corrosion resistance. However, it is well known that it reduces the fatigue strength of a component. This is due to high tensile internal stresses and microcrack density. Efforts to improve hard chromium properties have increased in recent years. In this study, the effect of a nickel layer sulphamate process, as simple layer and interlayer, on fatigue strength of hard chromium electroplated AISI 4340 steel hardness – HRc 53, was analysed. The analysis was performed by rotating bending fatigue tests on AISI 4340 steel specimens with the following experimental groups: base material, hard chromium electroplated, sulphamate nickel electroplated, sulphamate nickel interlayer on hard chromium electroplated and electroless nickel interlayer on hard chromium electroplated. Results showed a decrease in fatigue strength in coated specimens and that both nickel plating interlayers were responsible for the increase in fatigue life of AISI 4340 chromium electroplated steel. The shot peening pre-treatment was efficient in reducing fatigue loss in the alternatives studied.     

Investigation of the mechanical properties of friction-welded joints between AISI 304L and AISI 4340 steel as a function rotational speed

In this study, standard AISI 304L austenitic stainless steel and AISI 4340 steel couple were welded by friction welding process using five different rotational speeds. The joining performances of AISI 304L/AISI 4340 friction-welded joints were studied and the influences of rotational speed on the microstructure and mechanical properties of the welded joints were also estimated. The microstructural properties of heat affected zone (HAZ) were examined by scanning electron microscopy (SEM). The microhardness across the interface perpendicular to the interface was measured and the strength of the joints was determined with tensile tests. The experimental results indicate that the tensile strength of friction-welded 304L/4340 components were markedly affected by joining rotational speed selected.     

Evaluation on fatigue strength of AISI 4340 steel aluminum coated by electroplating and IVD processes

In spite of toxicity, hydrogen embrittlement susceptibility, and environmental issues, cadmium electroplating is usually applied on high strength AISI 4340 aeronautical steel due to its efficient protection against electrochemical corrosion. Ion vapor deposition (IVD) process with pure aluminum also offers good protection against corrosion with the advantages of decreasing hydrogen embrittlement susceptibility and improving the fatigue strength of metallic components. In this research, the effects of aluminum electroplating and IVD aluminum coating on the rotating bending fatigue strength of AISI 4340 steel were evaluated in comparison with cadmium electroplated specimens. Experimental fatigue results showed that both aluminum electroplating and IVD aluminum coatings are possible alternatives to cadmium electroplating.     

Effect of electroslag refining on the fracture toughness and fatigue crack propagation rates in heat treated AISI 4340 steel

The AISI 4340 steel has been electroslag refined and the improvement in mechanical properties has been assessed. Electroslag refining (ESR) has improved tensile ductility, plane strain fracture toughness, Charpy fracture energy, and has decreased fatigue crack growth rates. The K_IC values for the ESR steel are nearly twice those estimated in the unrefined steel and higher than those obtained in the vacuum arc remelted steel. Fatigue crack growth rates in region I and in region III are found to be decreased considerably in the ESR steel, while they are unaffected in region II. Measurements on heat treated samples have shown that the ESR steel has a better response to heat treatment. Both the suggested heat treatments namely austenitizing at 1140–1470 K as well as the conventional heat treatment of austenitizing at 1140 K have been followed. The improvement in the mechanical properties of ESR steel has been explained on the basis of removal of nonmetallic inclusions and reduction in sulfur content in the steel.     

Morphology of adiabatic shear bands in cylindrical specimens of AISI 4340 steel impacted by Hopkinson pressure bar

Abstract

Cylindrical specimens of AISI 4340 steel, which were heat treated by quenching in oil followed by tempering at either 315 or 425°C, were impacted in a Hopkinson pressure bar at different impacting speeds. It was found that when strain and strain rate reached certain values, adiabatic shear bands (or plastic deformation zones) were formed in the specimens. The adiabatic shear bands appeared either in a circle on the transverse section, a hyperbola on different longitudinal sections without the central axis of the cylinder, and a triangle on the longitudinal section through the central axis of the cylinder. From these observations, it can be concluded that the plastic deformation localisation zone is limited in a thin conical shell in three dimensions. It was further confirmed that the adiabatic shear bands initiated along the maximum shear stress directions. In addition, the adiabatic shear bands in the specimens tempered at 315°C appeared white, while those in specimens tempered at 425°C had deformation characteristics. This indicates that the appearance of adiabatic shear bands is related to the hardness and microstructure of the tested steel.     

Distortion and residual stress measurements of induction hardened AISI 4340 discs

10 Induction hardened discs with two initial hardness levels were used for exploring the influences of the variation of initial hardness as well as induction hardening (IH) recipes on the heat treatment distortions and hardening depth. The results show that for the same initial hardness, the larger the energy input, the higher the distortion size as well as the hardening depth. For a given induction hardening recipe, the increase in initial hardness leads to a deeper hardening depth but a smaller distortion. One disc was selected for the residual stress investigation in three orthogonal directions by neutron diffraction (ND). The corresponding stress-free lattice spacing d₀ was measured from the same material using both ND and X-ray diffraction (XRD) methods. The ND results show that the variation of d₀ in the hardened layer is significant and should be taken into account for stress calculation. However, regarding the core region, the d₀ value measured by XRD is more reliable. Accordingly, a combination of the ND-measured d₀ profiles in the hardened layer and the XRD-measured d₀ value in the core was adopted for the determination of residual stress distributions.