Microstructure and mechanical properties of high-strength low alloy steel by wire and arc additive manufacturing

A high-building multi-directional pipe joint(HBMDPJ)was fabricated by wire and arc additive manufacturing using high-strength low-alloy(HSLA)steel.The microstructure characteristics and transformation were observed and analyzed.The results show that the forming part includes four regions.The solidification zone solidifies as typical columnar crystals from a molten pool.The complete austenitizing zone forms from the solidification zone heated to a temperature greater than 1100℃,and the typical columnar crystals in this zone are difficult to observe.The partial austenitizing zone forms from the completely austenite zone heated between Ac1(austenite transition temperature)and1100℃,which is mainly equiaxed grains.After several thermal cycles,the partial austenitizing zone transforms to the tempering zone,which consistes of fully equiaxed grains.From the solidification zone to the tempering zone,the average grain size decreases from 75 to20μm.The mechanical properties of HBMDPJ satisfies the requirement for the intended application.

Data-mining and atmospheric corrosion resistance evaluation of Sn-and Sb-additional low alloy steel based on big data technology

Machine-learning and big data are among the latest approaches in corrosion research.The biggest challenge in corrosion research is to accurately predict how materials will degrade in a given environment.Corrosion big data is the application of mathematical methods to huge amounts of data to find correlations and infer probabilities.It is possible to use corrosion big data method to distinguish the influence of the minimal changes of alloying elements and small differences in microstructure on corrosion resistance of low alloy steels.In this research,corrosion big data evaluation methods and machine learning were used to study the effect of Sb and Sn,as well as environmental factors on the corrosion behavior of low alloy steels.Results depict corrosion big data method can accurately identify the influence of various factors on corrosion resistance of low alloy and is an effective and promising way in corrosion research.

Recent progress in third-generation low alloy steels developed under M^3 microstructure control

During the past thirty years, two generations of low alloy steels(ferrite/pearlite followed by bainite/martensite) have been developed and widely used in structural applications. The third-generation of low alloy steels is expected to achieve high strength and improved ductility and toughness, while satisfying the new demands for weight reduction, greenness, and safety. This paper reviews recent progress in the development of third-generation low alloy steels with an M^3 microstructure, namely, microstructures with multi-phase, meta-stable austenite, and multi-scale precipitates. The review summarizes the alloy designs and processing routes of microstructure control, and the mechanical properties of the alloys.The stabilization of retained austenite in low alloy steels is especially emphasized. Multi-scale nano-precipitates, including carbides of microalloying elements and Cu-rich precipitates obtained in third-generation low alloy steels, are then introduced. The structure–property relationships of third-generation alloys are also discussed. Finally, the promises and challenges to future applications are explored.

ISOCHRONOUS STRESS-STRAIN CURVES OF LOW ALLOY STEEL CROSS-WELD-SPECIMEN AT HIGH TEMPERATURE

In this work, a parametric approach is presented and utilized to determine the creep properties of weldments; then the model of creep strain for cross weld specimen is given. On the basis of the experimental results, attempt has been made to establish equations of the isochronous stress-strain for weld joint that can predict the function of loading and service time in use of the creep data of base metal and weld metal.

Characterization of Rust Layer Formed on Low Alloy Steel Exposed in Marine Atmosphere

The iron rust phases formed on low alloy steels containing different quantities of Cr element have been characterized using EPMA, Raman spectroscopy, TEM, optical microscopy etc. The ion selective properties of synthesized rust films with the same phase constituent as the atmospheric corrosion products were investigated using self-made apparatus. The results showed that corrosion loss of steels exposed in marine atmosphere decreased rapidly as the Cr content of the steel was increased. Cr-containing steels were covered by a uniform compacted rust layer composed of fine particles with an average diameter of several nanometers. Inner rust layer of Cr-containing steel (2 mass fraction) was composed of a-CrxFe1-xOOH, with Cr content of about 5 mass fraction. Such rust layer showed cation selective property, and could depress the penetration of Cl- to contact substrate steel directly.

Numerical simulation of residual stress and deformation for submerged arc welding of Q690D high strength low alloy steel thick plate

The finite element simulation software SYSWELD is used to numerically simulate the temperature field,residual stress field,and welding deformation of Q690D thick plate multi-layer and multi-pass welding under different welding heat input and groove angles.The simulation results show that as the welding heat input increases,the peak temperature during the welding process is higher,and the residual stress increases,they are all between 330–340 MPa,and the residual stress is concentrated in the area near the weld.The hole-drilling method is used to measure the actual welding residual stress,and the measured data is in good agreement with the simulated value.The type of post-welding deformation is angular deformation,and as the welding heat input increases,the maximum deformation also increases.It shows smaller residual stress and deformation when the groove angle is 40°under the same heat input.In engineering applications,under the premise of guaranteeing welding quality,smaller heat input and 40°groove angle should be used.

Influence of chromium on the initial corrosion behavior of low alloy steels in the CO_2–O_2–H_2S–SO_2 wet–dry corrosion environment of cargo oil tankers

The influence of Cr on the initial corrosion behavior of low-alloy steels exposed to a CO2–O2–H2S–SO2 wet–dry corrosion environment was investigated using weight-loss measurements, scanning electron microscopy, N2 adsorption tests, X-ray diffraction analysis, and electrochemical impedance spectroscopy. The results show that the corrosion rate increases with increasing Cr content in samples subjected to corrosion for 21 d. However, the rust grain size decreases, its specific surface area increases, and it becomes more compact and denser with increasing Cr content, which indicates the enhanced protectivity of the rust. The results of charge transfer resistance(Rct) calculations indicate that higher Cr contents can accelerate the corrosion during the first 7 d and promote the formation of the enhanced protective inner rust after 14 d; the formed protective inner rust is responsible for the greater corrosion resistance during long-term exposure.

Abrasive Wear Characteristics of Carbon and Low Alloy Steels for Better Performance of Farm Implements

The low stress abrasion behaviours of heat treated mild, medium carbon and high C - low Cr steels, which are generally used in making farm implements, have been investigated. The simple heat treatment greatly improves the hardness, tensile strength and abrasion resistance of medium carbon and high C - low Cr steels. The results indicate that the material removal during abrasion is controlled by a number of factors, such as hardness, chemical composition, microstructure and heat treatment conditions. The conclusion is that the heat treated high C - low Cr steel and mild steel carburized by using coaltar pitch provide the best hardness and abrasion resistance and thus appear to be the most suitable materials for making agricultural tools.

Transformation Kinetics of Pearlite to Austenite in Low Alloy Steel Containing Rare Earth

The phase transformation kinetics of pearlite to austenite in low alloy steel containing RE was studied by the methods of DSC. The results show that the apparent transformation activation energy of pearlite to austenite in the low alloy steel is 1141.04 kJ·mol -1, and the transformation activation energy of pearlite to austenite decreases with increasing of the volume fraction of transformation phase. Through which, the relationship curve between the volume fraction of transformation phase and the temperature were drawn.

Effects of Mo Composites on the Corrosion Behaviors of Low Alloy Steel

By using electrochemical and weight loss methods, the effect of MoO42-on the corrosion behaviors of low alloy steel was investigated in the 55%LiBr＋0.07 mol/L LiOH solution at high temperature. The results show that MoO42 , being an anodic inhibitor, would form a passive film rapidly and impede both anodic and cathodic reactions. Moreover, Na2MoO4 effectively prevents corrosion in 55%LiBr＋0.07 mol/L LiOH solution when its concentration is higher than 200 mg/L. Some elements of alloy, such as chromium and nickel, may cause the widening of passive potential region and the decrease of passive density, which indicates that the corrosion resistance increases. AES analysis shows that molybdenum participates in forming a protection film. The synergistic effect between chromium and molybdenum induces Cr-steel to be in passive state in lower Na2MoO4 concentration.

Carbon Equivalent Fundamentals in Evaluating the Weldability of Microalloy and Low Alloy Steels

Understanding the weldability of steel in relation to the use of carbon equivalent is very necessary for the welding industry. The study was poised to unearth the fundamentals of carbon equivalent as applied in evaluating the weldability of steel. The study used a two-stage design approach to address the problem of carbon equivalence weldability of steel, thus, survey and experimental. Two different steels were tested to ascertain their chemical composition which could inform carbon equivalent calculation, and the results revealed microalloy and low alloy steels respectively. In subjecting the microalloy steel to carbon equivalent analyses of the AWS and IIW coefficients;revealed a value (CEV) = 0.11 each, suggesting that this microalloy steel has excellent weldability;no preheating is required. A successful welding operation on this steel does not depend on preheating. Also, the average results of the low alloy steel revealed a value (CEV) = 0.37 and 0.32 respectively, suggesting that this type of steel has very good weldability and may require to preheat. It is recommended that welders have a general idea about the weldability of steel with regard to carbon equivalent calculation. In addition, they should understand the chemical compositions of steels they are dealing with.

Thermal Activation Analyses of Dynamic FractureToughness of High Strength Low Alloy Steels

A formula is derived for determining the influence of temperature and loading rate on dynamic fracture toughness of a high strength low alloy steel (HQ785C) from thermal activation analysis of the experimental results of three-point bend specimens as well as introducing an Arrhenius formula. It is shown that the results obtained by the given formula are in good agreement with the experimental ones in the thermal activation region. The present method is also valuable to describe the relationship between dynamic fracture toughness and temperature and loading rate of other high strength low alloy steels.

Effects of [S] and T[O] on strength and toughness of high purification low alloy steel plates

The effect of [S] on strength and toughness of low alloy steel plates inwhich sum of [P], [N], T[O] is less than 8x10^(-5) and the effect of T[O] on strength and toughnessof the steel plates in which sum of [S], [P], [N] is less than 7x10^(-5) were investigated. It isfound that the strength of the steel plates decreases with increasing [S] content when [S] is lessthan 4x10^(-5). When [S] varies within the range of 4x10^(-5)-1.2x10^(-4), [S] has no significanteffect on strength of the steel. The strength of the steel plates increases with increasing T[O]content when T[O] is less than 30x10^(-6), but decreases with increasing T[O] when T[O] is more than3x10^(-5). The difference between the LETT in plate length direction and LETT in width directiondecreases with decreasing [S] content. However, even when [S] is decreased to 9x10^(-6), thedifference of the LETT is still 16℃. When T[O] varies between 1.8x10^(-5) and 5.2x10^(-5), the lowtemperature impact toughness of the steel plates slowly decreases with T[O] increasing. When T[O]increases to more than 5.2x10^(-5), the low temperature toughness of the steel rapidly decreaseswith increasing T[O] content.

Low temperature impact toughness of laser hybrid welded joint of high strength low alloy steel

High strength low alloy steel with 16 mm thickness was welded by using high power laser hybrid welding. Microstrueture was characterized by using optical microscopy, scanning electron microscopy （ SEM ） , transmission electron microscopy （TEM） and selected area electron diffraction （SAED）. Low temperature impact toughness was estimated by using Charpy V-notch impact samples selected from the upper part and the lower part at the same heterogeneous joint. Results show that the low temperature impact absorbed energies of weld metal are （202,180,165 J） of upper samples and （178,145,160 J） of lower samples, respectively. All of them increase compared to base metal. The embrittlement of HAZ does not occur. Weld metal primarily consists of refined carbide free bainite and a little granular bainite since laser hybrid welding owns the character of low heat input. Retained austenite constituent film ＂locates among the lath structure of bainitie ferrite. Refined bainitic ferrite lath and retained austenite constituent film provide better low temperature impact toughness compared to base metal.

The micro structure of high carbon low alloy steel for easy drawing

For better processing performance of high carbon low alloy steel wire rod,an investigation about the influence of cementite lamellar spacing on wire 'easy drawing' performance is completed.It is pointed out that too thin cementite lamellar spacing(<80 um) reduces the strain hardening level of wire drawing, and reduce the torsion performance of drawn wire at same time.For the wire or wire rod from industrial production,compared with the micro-structure with troostite,the micro-structure with sorbite or sorbite mixed with pearlite is more suitable to the drawing process with high reduction ratio.

Predicting the martensite transformation start-temperature of low alloy steel based on fuzzy identification

A method of fuzzy identification based on T-S fuzzy model was proposed for predicting temperature Ms from chemical composition, austenitizing temperature and time for low alloy steel. The degree of membership of each sample was calculated with fuzzy clustering algorithm. Kalman filtering was used to identify the consequent parameters. Compared with the results obtained by empirical models based on the same data, the results by the fuzzy method showed good precision. The accuracy of the fuzzy model is almost 6 times higher than that of the best empirical model. The influence of alloying elements, austenitizing temperature and time on Ms was analyzed quantitatively by using the fuzzy model. It is shown that there exists a nonlinear relationship between the contents of alloying elements in steels and their Ms, and the effects of austenitizing temperature and time on Ms temperature cannot be neglected.

Corrosion behavior of low alloy steels in a wet–dry acid humid environment

The corrosion behavior of corrosion resistant steel（CRS） in a simulated wet–dry acid humid environment was investigated and compared with carbon steel（CS） using corrosion loss, polarization curves, X-ray diffraction（XRD）, scanning electron microscopy（SEM）, electron probe micro-analysis（EPMA）, N2 adsorption, and X-ray photoelectron spectroscopy（XPS）. The results show that the corrosion kinetics of both steels were closely related to the composition and compactness of the rust, and the electrochemical properties of rusted steel. Small amounts of Cu, Cr, and Ni in CRS increased the amount of amorphous phases and decreased the content of γ-Fe OOH in the rust, resulting in higher compactness and electrochemical stability of the CRS rust. The elements Cu, Cr, and Ni were uniformly distributed in the CRS rust and formed CuFeO2, Cu2O, CrOOH, NiFe2O4, and Ni2O3, which enhanced the corrosion resistance of CRS in the wet–dry acid humid environment.

Comparison of corrosion mechanisms of 510L low alloy steel treated by ACS and EPS techniques under various service environments

The corrosion mechanism of 510L low alloy steel treated by acid-cleaned surface(ACS)and eco-pickled surface(EPS)techniques in three simulated solutions(S0:atmospheric environment;S1:soil environment;S2:industrial environment)and the influence of interaction between different corrosive anions on corrosion were investigated.The results show that the total corrosion rates of samples in three simulated solutions were in order of S2>S0>S1,which is simultaneously correlated with initial corrosion dissolution processes as well as after the formation of corrosion products.HCO3−will inhibit the initial corrosion owing to the formation of films,whereas HSO3−will accelerate the dissolutions of the matrix based on the synergistic action of HSO3−and Cl−.On the other hand,there is no significant difference in corrosion rates between the samples treated by ACS and EPS techniques.The EPS technique that is safe,reusable and environmentally friendly can be further widely used in future work.

Data mining to effect of key alloying elements on corrosion resistance of low alloy steels in Sanya seawater environmentAlloying Elements

In this paper,the relationship model between seawater environment,chemical composition and corrosion potential of low alloy steel is established and the distribution of corrosion potential of low alloy steel with changes in key alloying elements is excavated.The research was carried out with the following steps:Firstly,the relationship model between corrosion potential of low alloy steel and its influencing factors was established by data dimension reduction and artificial neural network(ANN).Secondly,key alloying elements of experimental steels were selected out by Pearson correlation analysis,then the corrosion resistance element model was visualized to show the effect of key alloying elements on corrosion potential of low alloy steel.Finally,corrosion potential of low alloy steel with the change of key alloying elements was classified and visualized by classification method.The mining results can reflect the validity of the proposed mining methods to a certain extent and provide an intuitive data basis for the development of high-quality and low-cost low alloy steels.

Distinct beneficial effect of Sn on the corrosion resistance of Cr-Mo low alloy steel

In this work,the beneficial effect of Sn addition on the corrosion resistance mechanism of Cr-Mo low alloy steel was studied.Results demonstrated that Sn improves the corrosion resistance of the steel matrix mainly by influencing the microstructural transformation.Sn addition and the synergistic effect of Sn,Cr,and Mo promote the formation of α-FeOOH,SnO_(2),SnO,Cr(OH)_(3),and molybdates,lead to the improved protection and stability of the rust layer.This synergistic effect also endows the inner rust layer with cation selectivity,preventing the further penetration of Cl-and inhibiting the localized corrosion of steel.