Medium Carbon Low Alloy Steel Research Articles

Structure, properties and quality of a composite nickel-phosphorus coating deposited on steel substrates of various compositions

Possible causes of defects during deposition and subsequent heat treatment (HT) of nickelphosphorus (Ni – P) coatings containing dispersed silicon carbides SiC on steel sheets with a thickness of 65–180 mm are investigated. The phase composition, microstructure, and microhardness of 40–70 μm thick coatings chemically deposited on workpieces made of low- and medium-carbon low-alloy steels in various structural states and subjected to crystallization annealing of various durations were studied. According to X-ray diffraction studies, after deposition coatings are in an amorphous state and have low hardness and plasticity. Heat treatment in the temperature range of 400–450 ºC is accompanied by an increase in coating adhesion, microhardness to the required values (1000 HV) according to specifications, and a drop in elongation to zero. This is due to the crystallization with the formation of carbides SiC and Si5C3, nickel with a face-centered cubic (fcc) lattice, and nickel phosphide Ni3P. The conducted studies and the revealed regularities made it possible to choose 09G2S thick steel sheet for some articles of oil and gas equipment, to establish the technological parameters for deposition of composite Ni–P coatings and the final HT modes that do not lead to the appearance of defects in the form of surface cracks, micropores and other discontinuities. The more homogeneous and stable the substrate structure, the less defects will appear in the coating. Another reason for rejection may be due to the transition from a less dense amorphous initial state to a denser crystalline state upon heating, accompanied by a reduction in volume in combination with a drop in the relative elongation of the coating to zero.

Formation of nanostructured surface layer, the white layer, through solid particles impingement during slurry erosion in a martensitic medium-carbon steel

The extremely altered topmost surface layer, known as the white layer, formed in a medium-carbon low-alloy steel as result of impacts by angular 10–12 mm granite particles during the slurry erosion process is comprehensively investigated. For this purpose, the characteristics, morphology, and formation mechanism of this white layer are described based on the microstructural observations using optical, scanning and transmission electron microscopies as well as nanoindentation hardness measurements and modelling of surface deformation. The white layer exhibits a nanocrystalline structure consisting of ultrafine grains with an average size of 200 nm. It has a nanohardness level of around 10.1 GPa, considerably higher than that of untempered martensitic bulk material (5.7 GPa) achieved by an induction hardening treatment. The results showed that during the high-speed slurry erosion process, solid particle impacts brought forth conditions of high strain, high strain rate, and multi-directional strain paths. This promoted formation of a cell-type structure at first and later, after increasing the number of impacts, development of subgrains following by subgrain rotation and eventually formation of a nanocrystalline structure with ultra-high hardness. The model confirmed that high strain conditions - much higher than required for the onset of plastic deformation - can be achieved on the surface resulting in severe microstructural and property changes during the slurry erosion test.

Fatigue Analysis of a Forged Medium Carbon Low Alloy Steel Quenched in a Polymer for Vehicle Structures Applications

The fatigue properties of medium carbon low alloy forged Steels (EN 18, EN19, EN 24 and EN25) for untreated (forged) and polymer quenched samples are investigated. A dissolving temperature of 855ºC with a soaking period of 60 minutes followed by step tempering of 575ºC and 220ºC was used for the heat treatment. Thereafter quenching was performed in a polymer (polyethylene glycol 10% and 30%) separately. Fatigue tests were performed on both untreated and polymer quenched samples. The polymer quenched samples outperform the untreated samples. The fatigue analysis has been carried out using ANSYS and substantiated by the experimental results for the polymer quenched samples loaded to 30% of UTS. The heat-treated specimens had a fine-tempered martensite structure with a trace of ferrite, which enhances the toughness characteristics required for vehicles structures applications.

Friction stir processing as a strengthening technique for medium carbon low alloy steels

Friction stir processing (FSP) has shown its potential in the controlled microstructure evolution in steels. It is fact that the microstructure i.e. grain size, crystal orientation, type of phase and phase fraction, plays the most important role in the determination of mechanical properties. In this work, material gets strengthened by 1.5 times yield strength, 1.41 times ultimate strength as compared to base material after FSP. Ductility was observed to be decreased to 23% as compared to 32.5% of base material. The hardness of processed zone was observed to be 1.5 times that of the base material. The changes in the properties are studied in the context of microstructure resulted from the severe plastic deformation and thermomechanical effect of friction stir processing. The stir zone (SZ)/processed zone (PZ) is observed to have dual-phase structure, retained austenite along with nanosized carbide particles.

Effect of Machining Parameters on Tool Life and Surface Roughness of AISI 1040 Dual Phase Steel

The purpose of this research is to investigate the effect of heat treatment parameters on the tool life and surface roughness of dual phase steel. Optimization of machining parameters (cutting speed, feed and depth of cut) is carried out for the machinability tests on medium carbon low alloy steel. Taguchi’s method of design is used to carry out machinability tests. Analysis of variance (ANOVA) is carried out to determine the relative contribution of machining parameters on tool life and surface roughness. Microstructure analysis is carried out to ascertain the machining behavior of the steel. Results have shown that, depth of cut and cutting speed are the most significant factors contributing on the variation of the tool life and surface roughness. Optimized machining parameters are calculated in order to obtain higher tool life and lower surface roughness value.

Parametric Optimization under Cryogenic Machining of Medium Carbon Low Alloy Steel EN-19 Using the Integrated Taguchi-Present Worth Method

Medium carbon low alloy steel En-19 is a commercially thriving work material with 0.30 to 0.50% as carbon content and 0.60 to 1.65% as manganese content. The wide range of uses of medium carbon steel is for shafts, crankshafts, forgings, axles, couplings and gears. Given the broad industrial usage, the optimization of this work material is currently indispensable since it is a pointer to the economic management of drilling resources and a justifiable metric for performance assessment and evaluation. However, the present optimization models during machining have inaccurate predictions as they exclude the economic aspect of the material. In this article, a new combined Taguchi (T) method-present worth (PW) method is presented to account for the present worth and the inflation rate of steel material within the optimization context using literature data. The amalgamation of the two-component methods of Taguchi and present work is done at the signal-to-noise calculation. Then the response table, optimal parametric setting, the performance flow diagram and the present worth values are finally established. For the results, the optimal parametric setting was ascertained as SP3FR2DOC1, obtained at 37.48 (level 3), 35.92 (level 2) and 36.31 (level 1) for speed, feed rate and depth of cut, respectively, interpreted as 900rpm (speed), 0.1mm/min (feed rate) and 0.5mm (depth of cut). The T-PW method yields PWSP (62.85) as 1st, PWFR (-64.64) as 2nd and PWDOC (-65.13) as the 3rd position. It was found that the speed parameter is the best based on the optimization cum economic aspect assessment. The T-PW method predicts the optimization behaviour while analysing the economic aspect of the steel material and would help plan the purpose. The findings in this study would assist to clarify the approach to analyzing the economic characteristics of the machining process.

Spinning Process of D406A Ultra-High Strength Steel

D406A steel is a medium-carbon low-alloy steel, which has excellent comprehensive mechanical properties. It is widely used in the production of missiles and rocket barrels. In this paper, the spinning forming limit test and the intermediate heat treatment process of ultra-high-strength steel were used to explore the effect of spinning process and heat treatment on the properties of spinning parts. The research results showed that the reduction amount of the material made the material thinning rate approach the limit thinning rate. The final blank wall thickness was reduced from 15 mm to 3.0 mm when the cracking occurred. It was calculated that the material's power spinning limit thinning rate was 80%. The ferrite matrix after spinning showed a streamline distribution characteristic perpendicular to the thinning direction, and the precipitated carbides were uniformly distributed on the surface of the matrix, which had the characteristics of deformation and extension along the streamline. After the heat treatment, the structure of the spinning parts changed continuously. When the structure was quenched and tempered, the martensitic structure can be obtained, and the tempered martensitic structure was smaller. Furthermore a test piece for ultra-high-strength steel spinning technology has been developed, and the solutions discussed for flanging defects in the actual spinning process, and test data for the actual production of ultra-high-strength steel spinning parts accumulated.

Upsizing high-strength fail-safe steel through warm tempforming

Fail-safe (FS) steel is designed with enhanced fracture toughness through the crack deflection mechanism in an anisotropic microstructure. In this study, commercial boron (B)-bearing medium-carbon low-alloy steel bars with a cross-sectional area of 1390 mm2 were water-quenched and tempered at temperatures between 400 °C and 500 °C, followed by multi-pass caliber-rolling (i.e., warm tempforming). Tensile, Charpy V-notch (CVN) impact, and hydrogen embrittlement (HE) properties of the warm tempformed samples were investigated and compared with the rolling reductions of 45–92% to optimize the rolling conditions for upsizing the FS steel. Warm tempforming at 400 °C with a rolling reduction of 66% successfully produced an FS steel bar with a large cross-sectional area of 470 mm2. The evolution of a heterogeneous and anisotropic ultrafine-grained structure through the warm deformation of tempered martensite and pearlite generated an excellent combination of CVN toughness and HE resistance at the ultimate tensile strength of 1400 MPa. Delayed fracture resistance of the FS steels was also evaluated to be far superior to those of conventionally quenched and tempered steels, based on the balance between the HE susceptibility and hydrogen entry. Furthermore, it was observed that excessive warm tempforming to a rolling reduction of 92% tended to degrade the tensile elongation and HE resistance. The mechanism of the variations in mechanical properties during warm tempforming is discussed in association with the microstructural evolution.

Effect of Short-Term Low-Temperature Austempering on the Microstructure and Abrasive Wear of Medium-Carbon Low-Alloy Steel

Nano-bainitic steels have attracted great attention for good wear resistance. In this research, a medium-carbon low-alloyed steel was austempered at a low temperature close to its martensite-start temperature for various times to obtain mixed microstructure of nano-bainite, martensite and retained austenite. The austempered samples were characterised comprehensively by field-emission SEM and quantitative XRD. Its two-body abrasive wear property was evaluated by sliding on a SiC abrasive disc. The results revealed the formation of initial nano-width carbide-free bainitic ferrite (BF) after austempering by 10 min, whereas the BF size and amount both increased with the austempering time. The austempered samples exhibited wear coefficients lower than the quenched martensitic sample by up to 50%. SEM and TEM observations showed wear mechanisms of micro-cutting and ploughing deformation, including the formation of a nano-laminate top layer and bending deformation in the subsurface multiphase microstructure. The decreased wear loss was attributed to the role of retained austenite in the increased plasticity.Graphic abstract

Comparative Studies on Medium Carbon Low Alloy Steels and Maraging Steels

The maraging steel are considered ultra-high strength due to its yield strength greater than 1400Mpa and are part of a set of advanced materials of interest for technological development, mainly for aeronautics and aerospace industry. For this purpose they should exhibit good toughness, fatigue resistance and acceptable weldability. Since the maraging steels are of higher cost we are intended to make the Medium Carbon Low Alloy Steel (MCLA) for the purpose of aeronautical and aerospace industry instead of Maraging steel as the cost of MCLA is comparatively less. An attempt is made to study on the basic properties of MCLA steels and Maraging steels by process modifications and comparing the results with that of the standard properties of the Maraging steel.

Subsurface Crack Initiation and Propagation in an Open Dieforged High Strength Steel: A Microstructural Analysis

This paper presents an experimental investigation with the objective to determine the root causes for the cracking of a large size bar made of a medium carbon low alloy steel after open die forging and heat treatments operations. The cracks were observed below the surface during the machining step. In order to understand the mechanisms of crack initiation and propagation, micro-CT tomography and scanning electron microscope (SEM) were employed. Microstructural damage analysis revealed oxidation of different alloying elements, more specifically manganese, chromium and silicon. The presence of defects in the form of cavities and porosities were also observed at the grain boundaries. Some of the above defects were observed along the crack path, while others were on both sides of the cracks without any connection to them and finally, a third group completely isolated from any crack. The characteristics of the defects were thoroughly analyzed and it was found that the crack initiation could be attributed principally to the porosities/cavities formed during solidification. The analysis also showed that crack propagation occurred during solidification and/or forging and heat treatment steps.

Influence of the cooling rate below Ms on the martensitic transformation in a low alloy medium-carbon steel

The influence of cooling rate below the martensite start temperature, Ms, on the kinetics of martensitic transformation in a medium carbon low alloy steel was determined using high resolution dilatometry, optical microscopy and X-ray diffraction techniques. A two-stage transformation was observed for slow cooling rates while at higher cooling rates, martensitic transformation occured through a single stage process. It is shown that the Koistinen–Marburger equation cannot adequately describe the observed two-stage transformation. A new equation is proposed in order to model the evolution of martensitic transformation by considering the influence of post Ms cooling rate. The method considers contributions from both, the initial austenite and the carbon enriched austenite. The underlying mechanisms are discussed and validated with experimental findings.

Conditions of warm ausforming of low-alloy medium-carbon steel for making fully martensitic microstructure and its refining effect of martensite

This study elucidates the process conditions of ausforming low-alloy medium-carbon steel that can be used to make a fully martensitic microstructure. In the process, the austenitized steel at a high temperature is cooled to warm temperature kept in the supercooled austenite state, then deformed and quenched. The result of this study indicates that the duration time needs to be shortened sufficiently, because supercooled austenite of low-alloy steel only exists for a few seconds. The change of martensitic microstructure is also investigated. Originally, microstructure of low-alloy medium-carbon steel martensite consists of fine lath and coarse butterfly martensite. The frequency of coarse butterfly martensite is decreased by ausforming but is not zero for lower strain levels; coarse butterfly martensite does not form at higher strain levels. This change is presumed to be caused by lattice defects introduced into the austenite matrix by ausforming. In this research, changes in the martensite block width and hardness are investigated. The hardness increases as the block width decreases.

Micro-hardness variation of micro-phases during spheroidisation of AISI4340 steel

AISI4340 steel is the medium carbon low-alloy steel has got excellent prominence as a construction material. This steel is spheroidised on heating to the intercritical temperature range by different time intervals with the objective of increasing the number and fineness of the spheroids of cementite particles. A 9 hours, spheroidisation shows better spheroids of cementite particles. To study the effect of initial room temperature structure on spheroidisation primary heat treatments like normalising and hardening are carried out for as-bought steel. Tensile property (yield strength, ultimate tensile strength and percentage elongation), hardness (Rockwell C scale) and micro-structural features and micro-hardness of micro-phases (ferrite and cementite) are studied. A continuous reduction in the tensile strength, hardness and improvement in ductility, is observed with the increase in spheroidisation time is observed in all the spheroidisation paths. Also, increased spheroidisation time shows a lesser aspect ratio for spheroids. The micro-structure shows partial spheroidisation in all the conditions with more number of spheroids in normalised condition and finer size in hardened. Drastic reduction in strength and hardness values are observed in the hardened condition during spheroidisation.

Effect of cladding parameters on microstructure and defects in direct laser metal deposition of 24CrNiMo steel

Direct laser metal deposition of a medium carbon low alloy steel was performed on a round bar sample. Taguchi design was used to investigate the effect of three process parameters with three different levels on the microstructure and properties of single- and multitrack clad samples. Microstructural analysis showed that coalesced bainite and lath martensite exist in the clad area. Two different kinds of cracks occurred in multitrack clad samples. When laser power is not sufficient or scanning speed is high, cracks can occur in the laser clad area because of the accumulation of trapped defects inside the molten pool, e.g., inclusions, voids, and low melting point constituents alongside insufficient molten metal back-filling at interdendritic regions. These cracks can be considered as solidification cracks. Another type of cracks can occur in previous tracks of multitrack clads which actually are the heat affected zones (HAZ) of newly deposited clad tracks, because of high tensile stress on HAZ and the presence of oxide and sulfide constituents at interdendritic regions. These cracks can be considered as reheating cracks. According to the result of Taguchi design experiments, using optimum laser parameters like high laser power, medium scanning speed, and sufficient powder feed rate, crackfree clad layers can be obtained via the prevention of formation of unfavorable constituents at interdendritic regions.

Effect of Tempering on the Bainitic Microstructure Evolution Correlated with the Hardness in a Low-Alloy Medium-Carbon Steel

A low-alloy medium-carbon bainitic steel was isothermally tempered at 300 °C for up to 24 hours which led to a significant hardness decrease. In order to explain the decreasing hardness, extensive microstructural characterization using scanning and transmission electron microscopy, X-ray diffraction, and atom probe tomography was conducted. The experimental work was further supplemented by thermodynamic and kinetic simulations. It is found that the main underlying reason for the hardness reduction during tempering is related to dislocation annihilation, possibly also with corresponding changes in Cottrell atmospheres. On the other hand, cementite precipitate size, effective grain size of the bainite, and retained austenite fraction appear unchanged over the whole tempering cycle.

Austenite grain growth and hot deformation behavior in a medium carbon low alloy steel

To examine the austenite grain growth behavior and kinetics under isothermal austenitization process in a low alloy medium carbon forged steel, heat treatments at different temperatures (1150, 1175, 1200, and 1260 °C) and times (5, 15, and 25 min) were conducted. An Arrhenius constitutive relationship was developed to analyze and predict the austenite grain size as a function of the austenitization temperature and time during isothermal austenitization. The model predictions agreed well with the experimental austenite grain size data. Following the austenitization examinations, the hot deformation behavior of the alloy was studied by performing isothermal compression tests for different soaking times (5, 15, and 25 min) at the deformation temperatures of 1150, 1175, and 1200 °C, at a constant strain rate of 0.05 s−1, and up to a true strain of 0.6. The microstructures of the hot compressed samples were assessed to determine the dynamic softening mechanisms and potential austenite grain refinement by dynamic recrystallization (DRX). Under the investigated hot deformation conditions, the flow stress curves and microstructure observations showed DRX characteristics. The flow curves from the peak to the steady-state stress were accurately predicted using a Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation. Based on the flow curves and a mathematical equation, the DRX kinetics were also determined. Variations of the flow curves, DRX kinetics, and dynamic recrystallized (DRXed) grain size with the deformation temperature, strain, and the austenite grain size prior to deformation were analyzed.

Effect of Double Hit Hot Deformation on the Evolution of Dynamically Transformed Ferrite

Double-hit hot compression tests were carried on medium-carbon low-alloy steels using Gleeble 3800® thermomechanical simulator. The experiments were performed at strain rates of 0.25 and 0.5 s−1 and temperatures of 1150 and 1200 °C with interpass times of 5, 15, and 25 s. The onset of critical stresses for dynamic transformation (DT) for both first and second hit were detected using the double-differentiation method. It was found that the critical stress for DT increased with a decrease in temperature and an increase in strain rate. The presence of dynamically transformed ferrite was observed and quantified using electron-backscatter diffraction, kernal average misorientation, and grain boundary maps. Then, a thermodynamic analysis was carried out using JmatPro software. A method of determining the change in Gibbs energy during DT phenomenon is proposed for double hit deformation.

The Effect of Retained Work Hardening on the Driving Force for Dynamic Transformation

The driving force for dynamic transformation during double-hit hot compression of an as-cast medium-carbon low-alloy steel assessed done at 1473 K at strain rates of 0.25 and 0.50 s−1. Dynamically transformed ferrite was detected using the Kernel Average Misorientation (KAM) technique. The time interval between deformations affects the retained driving force, which decreases at a rate of 65 to 75 J/mol per second. This rate decreases with decreasing temperature due to a lower rate of recovery.

Promising bending properties of a new as-rolled medium-carbon steel achieved with furnace-cooled bainitic microstructures

The bending properties of a new thermomechanically processed medium-carbon (0.40 wt% C) low-alloy steel, intended for the slurry transportation pipeline application, have been investigated. The studied material was hot-rolled to 10 mm thick strips followed by direct quenching to two different quench-stop temperatures (QST) of 560 °C and 420 °C. The samples were subsequently cooled slowly to room temperature in a furnace, producing two different bainitic microstructures. In general, the final microstructures on the centerline consisted of different bainitic features with yield strengths of a ~700 MPa and ~1200 MPa for QST 560 °C and 420 °C, respectively. To determine the factors affecting bendability, as determined by three-point brake press bending tests, local microstructural features and texture were characterized with transmission and scanning electron-microscoy and macrohardness tests. Detailed quantitative microstructural evaluation of both subsurface and mid-thickness regions revealed that the bainitic sample produced at the higher temperature (QST 560) consisted of almost equal amounts of bainite types B1 and B2, where B1 has bainitic sheaves with a low dislocation density and intralath cementite, and B2 a very dislocation-dense morphology with mainly interlath cementite. In the QST 420 sample, the high dislocation density components B2 bainite and martensite were dominant, although martensite was only present near the strip surfaces. Different post-rolling cooling conditions did not change the general crystallographic theme but resulted in a slight increase in the texture intensity of the QST 420 sample. Neither the concave nor convex subsurface regions showed significant changes in texture after bending. The more favorable distributions of microstructural components and textural component intensities in the QST 560 sample resulted in higher elongation to fracture and work-hardening capacity, resulting a smaller minimum usable punch radius than for the stronger QST 420 sample. Fractographic examination of the cracked surfaces revealed that cracks developed by shear band formation followed by surface roughening, which promoted subsequent void and micro crack nucleation and growth.