The effect of single and double quenching and tempering heat treatments on the microstructure and mechanical properties of AISI 4140 steel

This investigation is concerned to evaluate the effect of double quenching and tempering (DQT) with conventional quenching and tempering (CQT) heat treatment processes on microstructure and mechanical behavior of a commercially developed hot rolled AISI 4140 type steel. Comparison of microstructure and mechanical properties of DQT and CQT heat treated specimens have been established in details. Optical and scanning electron microscopies have been used to follow impurity concentration and microstructural changes, and their relation to the associated mechanical properties. The results indicate that the improvement of mechanical properties particularly impact toughness of DQT heat treated specimens is much higher than that of the CQT condition, and this observation is rationalized in terms of finer austenite grain size developed in the DQT condition providing much finer martensitic packets within the grains and a lower level of impurity concentration of sulfur (S) and phosphorus (P) near the prior austenite grain boundaries as well.     

An investigation on the microstructure and mechanical properties of direct-quenched and tempered AISI 4140 steel

Direct quenching (DQ) process is an appropriate method in steels heat treatment field. This method enhances production rate, reduces energy consumption and decreases environment contamination. In this study hot-rolled AISI 4140 steel billets with different diameters (75, 80, 85, 100, 105 and 115 mm) and 20 m length were quenched directly in a water tank. Also some samples with similar size and composition were provided by conventional reheating, quenching and tempering (RQ) heat treatment process. The quenched samples were tempered at the temperature of 630 °C for 2 h. Mechanical properties of heat treated samples including tensile strength, yield strength, elongation, hardness and impact toughness were measured. Also, the microstructure and harden-ability of this steel were investigated under various conditions and the results were compared to RQ heat treated products. The results showed that direct quenching and tempering processes (DQ–T) is due to enhance of mechanical properties such as tensile strength and harden-ability of AISI 4140 and it is affected by various parameters such as steel temperature before quenching, water temperature, quenching time and also billet size.     

Effect of magnetic treatment on fatigue life of AISI 4140 steel

The effect of magnetic field with different intensities on fatigue life of AISI 4140 steel has been investigated. For this purpose, unnotched and notched specimens were prepared from AISI 4140 steel. It was obtained that when the magnetic field especially was applied to the specimens at the stage of fatigue crack initiation, the fatigue life improved by reason of delaying formation time of slip bands due to regular direction and distribution of magnetic domains. And, it was observed that magnetic field was applied from the beginning of fatigue test up to occur the fatigue fracture, it was detrimental on the fatigue life because of over heating of the specimens.     

Influence of stabilization heat treatments on microstructure,hardness and intergranular corrosion resistance of the AISI 321 stainless steel

The AISI 321 is an austenitic stainless steel which contains Ti as stabilizing element. This material can be selected for high temperatures services due to its high creep and intergranular corrosion resistance. However, for services into the sensitization range of temperatures (450–850 °C), the steel must be previously heat treated at higher temperatures for TiC precipitation. In the present work the importance of this so-called stabilization treatment, and the best range of temperatures for its realization, were investigated by means of microscopy and electrochemical potendynamic reactivation tests. It was found that the higher temperature for stabilization must be 950 °C. Samples stabilized at 975 °C or 1000 °C and aged at 600 °C for 100 h showed the very begin of the sensitization process.     

Effect of post-weld heat treatment and electrolytic plasma processing on tungsten inert gas welded AISI 4140 alloy steel

Post-weld heat treatment (PWHT) is commonly adopted on welded joints and structures to relieve post-weld residual stresses; and restore the mechanical properties and structural integrity. An electrolytic plasma process (EPP) has been developed to improve corrosion behavior and wear resistance of structural materials; and can be employed in other applications and surface modifications aspects. In this study the effects of PWHT and EPP on the residual stresses, micro-hardness, microstructures, and uniaxial tensile properties are explored on tungsten inert gas (TIG) welded AISI-4140 alloys steel with SAE-4130 chromium–molybdenum alloy welding filler rod. For rational comparison all of the welded samples are checked with nondestructive Phased Array Ultrasonic Testing (PAUT) and to ensure defect-free samples before testing. Residual stresses are assessed with ultrasonic testing at different distances from weld center line. PWHT resulted in relief of tensile residual stress due to grain refinement. As a consequence higher ductility but lower strength existed in PWHT samples. In comparison, EPP-treated samples revealed lower residual stresses, but no significant variation on the grain refinement. Consequently, EPP-treated specimens exhibited higher tensile strength but lower ductility and toughness for the martensitic formation due to the rapid heating and quenching effects. EPP was also applied on PWHT samples, but which did not reveal any substantial effect on the tensile properties after PWHT at 650 °C. Finally the microstructures and fracture morphology are analyzed using scanning electron microscopy (SEM) and optical microscope to study the evolution of microstructures.     

On the impact of deep rolling at different temperatures on the near surface microstructure and residual stress state of steel AISI 304

The consequences of deep rolling at cryogenic temperature, room temperature and high temperature were investigated for steel X5CrNi18‐10 (AISI 304). Near surface microstructures, martensite content and residual stresses were analyzed. It is shown that, applying different deep rolling temperatures, the near surface states of the processed specimen are altered in a characteristic way. Particularly important is that different depth distributions of strain hardening, martensite volume fraction and residual stresses can be created.     

Influence of fusing temperature on microstructure, wear and corrosion resistance of induction melted bimetal of Co-based alloy and AISI 4140 steel

Two bimetals composed of a Co-based alloy and AISI 4140 steel were fabricated by induction fusing at 1200 °C and 1250 °C, respectively. Their microstructures were examined and their wear and corrosion resistances were investigated using a ball-on-disc system and immersion tests. The results show that the phases of the Co-based alloy of the bimetal, fused at 1200 °C, are inherited from its original powder and consist of a Co-rich phase, a Cr-rich phase and W₃CoB₃; however, the effect of iron dilution causes the formation of σ-CrFe and Co_0.72Fe_0.28. When fusing at 1250 °C, W₃CoB₃ decomposes to form Co₃B and increases the W content in the Co-rich phase thereby reducing the corrosion rate of the Co-rich phase and resulting in improved corrosion resistance of the bimetal. However, a higher fusing temperature causes a significant drop in hardness due to severe alloy dilution by iron diffusing from AISI 4140 steel and results in declined wear resistance. The associated wear behavior also changes from abrasion wear to oxidation wear.     

Effects of pulsed Nd:YAG laser welding parameters and subsequent post-weld heat treatment on microstructure and hardness of AISI 420 stainless steel

Martensitic stainless steels are often used in cases where high strength and medium corrosion resistance are required. In this study, pulsed Nd:YAG laser welding of AISI 420 martensitic stainless steel is considered. Welding of samples were carried out autogenously. The spacing between samples was set to almost zero. All samples were butt welded. The effect of welding parameters such as voltage, laser beam diameter, frequency, pulse duration, and welding speed on the weld dimensions were investigated and the optimum values were obtained for the 450 V voltage, 0.6 mm focal diameter, 6 Hz frequency, 5 ms pulse duration and 1.5 mm/s welding speed. Microstructure of weld pool and heat affected zone (HAZ) were investigated by optical microscopy (OM) and scanning electron microscopy (SEM). Micro-hardness studies were also carried out. The results showed the presence of some remaining delta-ferrite in the martensitic weld structure and coarsening of M₂₃C₆ carbides in HAZ. The magnitude of hardness in the HAZ was higher than that of the weld zone. To reduce the hardness of weld and HAZ and to increase the toughness in these regions, two types of post-weld heat treatments (PWHTs) were carried out. In type 1, samples tempered for 2 h. In type 2, samples austenitizied for 0.5 h at 1010 °C and then tempered for 2 h. In order to achieve high strength and toughness, optimum temper temperatures for type 1 and 2 heat treatments were obtained for 595 and 537 °C, respectively. The results showed higher toughness for type 2 than type 1.     

Effect of microstructure on KCl corrosion attack of modified AISI 310 steel

Abstract

The effect of microstructure on KCl corrosion attack was studied using a specifically chosen modified AISI 310 austenitic steel in a 15% (v/v) H₂O (g) + 5%(v/v) O₂ (g) + N₂ (g) (balance) atmosphere at 600°C for 168 h. The material was a targeted choice as it allows investigation of different microstructures i.e. as-received (without sigma phase) and heat-treated (29% σ-phase per area) microstructures. The corrosion attack was studied with light optical, scanning electron and transmission electron microscopy as well as X-ray diffraction. The heat-treated sample showed a corrosion attack that was 5 times higher than the as-received sample. In the heat-treated sample, the σ-phase was selectively attacked. At the corrosion front, chlorine (but not potassium) was detected in the selectively attacked σ- phase but not in the unattacked adjacent matrix. Therefore, the corrosion attack was propagated by preferential σ-phase attack by chlorine species.     

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.     

The influence of yield strength and negative stress ratio on fatigue crack growth delay in 4140 steel

Single tensile overloads were applied to AISI 4140 compact steel specimens with three yield strengths of 110, 165 and 205 ksi and three stress ratios R of 0, –1/2, and –1. Fatigue crack growth delay was significantly longer for the low yield strength steel with R=0. However, at R=–1 only slight differences in delay occurred for all three yield strengths. Thus delay from single tensile overloads with R0 can be quite misleading when compared with negative stress ratio delay results. This implies that small compressive cyclic stresses, which often have only a small influence on constant amplitude crack growth behavior, may have significant influence in spectrum loading due to reduction of delay from high tensile overloads. This reduction in delay may be attributed to cyclic relaxation of both crack region residual stresses and crack closure. Under constant amplitude testing, fatigue crack growth rates were more dependent upon yield strength than upon R ratio.     

Investigation on AISI 304 austenitic stainless steel to AISI 4140 low alloy steel dissimilar joints by gas tungsten arc,electron beam and friction welding

This paper presents the investigations carried out to study the microstructure and mechanical properties of AISI 304 stainless steel and AISI 4140 low alloy steel joints by Gas Tungsten Arc Welding (GTAW), Electron Beam Welding (EBW) and Friction Welding (FRW). For each of the weldments, detailed analysis was conducted on the phase composition, microstructure characteristics and mechanical properties. The results of the analysis shows that the joint made by EBW has the highest tensile strength (681 MPa) than the joint made by GTAW (635 Mpa) and FRW (494 Mpa). From the fractographs, it could be observed that the ductility of the EBW and GTA weldment were higher with an elongation of 32% and 25% respectively when compared with friction weldment (19%). Moreover, the impact strength of weldment made by GTAW is higher compared to EBW and FRW.     

Combined effect of shot peening,subcritical austenitic nitriding,and cryo-treatment on surface modification of AISI 4140 steel

Shot peening is a simple but effective severe plastic deformation process to synthesize ultrafine grains in micro- to nanometer range on metallic surfaces. In this work, shot peening on AISI 4140 steel specimens was done in a novel centrifugal air blast shot peening reactor with shot velocity of 5.8?m/s for 3?h. Characterization of the shot peened surface (XRD, micro-hardness, SEM, and TEM) showed that surface undergoes significant plastic deformation with marked increase in microstrain of lattice, dislocation density, and surface hardness. XRD profiles and TEM analysis confirmed formation of ultrafine grain structure in the nanometer range. These specimens were then subjected to austenitic nitriding at 610°C for 4?h followed by cryo-treatment at???185°C for 32?h. Characterization of pre-shot peened nitrided and cryo-treated surfaces showed that there was marked improvement in surface hardness (from 695 to 797 HV_0.05) and effective case depth (from 19 to 54?µm) in comparison with un-shot peened nitrided and cryo-treated specimens. It was demonstrated that presence of ultrafine grain structure and austenitic phase during nitriding plays synergetic role to improve content and diffusion kinetics of nitrogen in AISI 4140 steel surface.     

Effect of heat treatment on interface microstructure and bond strength in explosively welded Ti/304L stainless steel clad

Abstract

The effects of heat treatment on the microstructure and bond strength at the interface of explosively welded titanium/304L stainless steel clad have been investigated. The microstructure of the clad interface were examined using optical and scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and X-ray diffraction (XRD) techniques. At 700°C, the formation of intermetallic phases σ and Fe₂Ti besides β-Ti were confirmed, while in samples, heat treated at 800°C and 900°C, other intermetallic phases such as λ and FeTi, NiTi, NiTi₂ phases were detected in addition to σ and Fe₂Ti phases. The shear test results show that the shear strengths of heat treated samples are overall significantly lower than that of stress relieved samples. This could be due to the formation of brittle intermetallic phases at the interface. Despite of formation of the intermetallic phase at 700°C, the shear strength of the sample is still more than the minimum standard amount of 137·9 MPa. It is also observed that the higher the heat treatment temperature, the lower the interface shear strength, which can be explained by the fact that the volume fraction of intermetallics increases with increasing temperature. The shear strength values of heat treated samples are lower than those of diffusion bonded clads. This could be attributed to the fact that the total widths of intermetallic layers achieved in heat treated samples are larger than those of for diffusion bonded ones.     

Effect of heat input on the microstructure and mechanical properties of gas tungsten arc welded AISI 304 stainless steel joints

Influence of heat input on the microstructure and mechanical properties of gas tungsten arc welded 304 stainless steel (SS) joints was studied. Three heat input combinations designated as low heat (2.563 kJ/mm), medium heat (2.784 kJ/mm) and high heat (3.017 kJ/mm) were selected from the operating window of the gas tungsten arc welding process (GTAW) and weld joints made using these combinations were subjected to microstructural evaluations and tensile testing so as to analyze the effect of thermal arc energy on the microstructure and mechanical properties of these joints. The results of this investigation indicate that the joints made using low heat input exhibited higher ultimate tensile strength (UTS) than those welded with medium and high heat input. Significant grain coarsening was observed in the heat affected zone (HAZ) of all the joints and it was found that the extent of grain coarsening in the heat affected zone increased with increase in the heat input. For the joints investigated in this study it was also found that average dendrite length and inter-dendritic spacing in the weld zone increases with increase in the heat input which is the main reason for the observable changes in the tensile properties of the weld joints welded with different arc energy inputs.     

热输入对Q890高强钢焊缝组织及性能的影响

针对国内某钢厂最新研制的Q890高强钢,采用三种不同的热输入对其进行气体保护焊接,研究了不同热输入对焊缝金属组织、硬度及冲击韧性的影响.结果表明,3种热输入下,焊缝组织主要以板条贝氏体为主,并含有粒状贝氏体、少量的板条马氏体和残余奥氏体.随着热输入的增大,焊缝组织中贝氏体铁素体板条粗化,板条马氏体逐渐减少,而粒状贝氏体逐渐增多,部分残余奥氏体由薄膜状向块状转变;焊缝金属硬度随着热输入的增大而下降;焊缝金属的冲击韧性亦呈逐渐下降的趋势.     

Effect of thermochemical treatments on the strength and microstructure of SiC fibres

The room-temperature strength of commercially available polymer-derived SiC fibres degrades during the typical high-temperature thermal cycle used in ceramic matrix composite fabrication. Substantial improvements in retained room-temperature strength for two different commercially available fibres were observed after annealing in carbon powder at temperatures up to 1600 °C. Further improvements in strength were observed for both fibres when heat treated in CO atmospheres. X-ray diffraction, TEM, SEM, auger electron spectroscopy, and optical microscopy were used to characterize the microstructure and chemistry of these heattreated fibres in order to understand better the degradation mechanisms of the fibres as well as their improved strength retention.     

Evaluation of residual stress relaxation and its effect on fatigue strength of AISI 316L stainless steel ground surfaces: Experimental and numerical approaches

This paper is aimed at evaluating the residual stress relaxation and its effect on the fatigue strength of AISI 316L steel ground surfaces in comparison to electro-polished surfaces. An experimental evaluation was performed using 3-point and 4-point bending fatigue tests at R_σ = 0.1 on two sets of notched specimens finished by electro-polishing and grinding. The residual stress fields were measured at the notch root of specimens, before and after fatigue tests, by means of the X-ray diffraction technique. It was found a degradation of about −35% for the 4-point bending fatigue limit at 2 × 10⁶ cycles of the ground specimens in comparison to the electro-polished ones. This degradation is associated with a slight relaxation of the grinding residual stresses which remain significant tensile stresses at the stabilized state. While under the 3-point bending test, these residual stresses relax completely and provoke a noticeable increase of the fatigue limit estimated at about 50% in comparison to the 4-point bending fatigue test. The numerical evaluation of residual stress relaxation was carried out by FE analyses of the cyclic hardening behaviour of the ground layer. The isotropic and nonlinear kinematic model proposed by Chaboche was used and calibrated for the base material and the ground layer. The results show that residual stresses relax to a stabilized state characterized by elastic-shakedown response. This stabilization is occurred after the first cycle of the 4-point bending test corresponding to the higher stress concentration (K_t-4p = 1.66), while it requires many cycles under the 3-point bending test corresponding to the lower stress concentration (K_t-3p = 1.54). The incorporation of stabilized residual stress values into the Dang Van’s criterion has permitted to predict with an acceptable accuracy the fatigue limits under both bending modes.     

Effect of mean stress on residual stress relaxation in steel specimens

Abstract

Residual stress relaxation was investigated by subjecting specimens with various mean stress levels to strain controlled cyclic loading. The material studied was mild steel in three different conditions. The mean stress levels ranged from 100 to 200 MPa, and two strain amplitudes, 0·05 and 0·06%, were studied in detail. The residual stresses in the specimens were measured before and after mean stress relaxation experiments. It was found that experimental factors such as temperature variations and crack growth have a significant influence on the results. Based on the experimental results, it is proposed that the mean stress relaxation exponent should be divided into two parts: mean stress dependent and mean stress independent. The first includes the contribution of quasi-static relaxation, i.e. mean stress dependent plastic deformation. The second part includes the contribution of cycle dependent mean stress relaxation, which does not depend on the mean stress.     

Effect of fatigue damage on the dynamic tensile behavior of 6061-T6 aluminum alloy and AISI 4140T steel

Usually material properties are determined from damage free materials, but it is not well known how these properties vary with respect to previous fatigue damage. In the present work the dynamic response of fatigue damaged 6061-T6 aluminum alloy and AISI 4140T steel specimens subjected to impact loading was investigated. Samples subjected to previous damage under high cycle fatigue and low cycle fatigue were tested. Different fatigue damage levels were considered. In addition, the effect of previous fatigue damage on the quasi-static behavior, ductility and fracture mechanism was also evaluated for both materials. A tensile Hopkinson bar apparatus was used in this work to investigate the dynamic response of the pre-fatigued specimens. Projectile speeds ranged from 18 m/s to 30 m/s giving strains rates from 550 to 2850 s⁻¹. The quasi-static mechanical properties of aluminum are not affected by the way the fatigue damage is induced. The dynamic properties, however, are sensitive to the previous fatigue damage, but are not affected by the strain rate. In the steel case, and when damage is induced by strain control, the mechanical properties are influenced by the previous fatigue damage. The dynamic properties are sensitive to the previous fatigue damage and depend on the strain rates. The analysis results show an increase in the ductility of the aluminum alloy when increasing the fatigue damage level; the steel exhibits an opposite behavior, a decrease in the ductility when increasing the damage level. The results show how the previous fatigue damage can modify the quasi-static and dynamic mechanical properties of the tested materials.