Air-cooled Steel Research Articles

Investigation on the hydrogen induced cracking behaviour of heat-treated pipeline steel

In this paper, the hydrogen induced cracking (HIC) sensitivity of the austenitized Cu-containing pipeline steels after cooling processes in air and water conditions was evaluated. Microstructural test, as well as hydrogen-charging slow strain rate tensile test and hydrogen concentration tests were conducted. Results indicated that steels after both cooling processes exhibited lower HIC sensitivity compared to the untreated steel. Elongation and percentage reduction of area after the hydrogen-charging slow strain rate tensile tests were increased. This was related to the formation of strong hydrogen trapping sites and the decrease of weak hydrogen trapping sites such as high-angle grain boundaries. Moreover, the TDS test results confirmed the presence of more stable hydrogen trapping sites in air-cooled steel than water-cooler steel. The latest phenomenon effectively hindered the diffusion behaviour of hydrogen in steel and reduced the HIC sensitivity.

The Effects of Microalloying on the Precipitation Behavior and Strength Mechanisms of X80 High-Strength Pipeline Steel under Different Processes

Pipeline steel is a special type of steel used for transporting, for example, oil and natural gas. This study focuses on X80-grade pipeline steel modified with the addition of Nb and Nb-V at different cooling rates (air cooling or quenching) after hot rolling and subjecting it to quenching and tempering heat treatment. Based on multiscale characterization techniques, the effects of microalloying and the cooling rate after hot rolling on the microstructure, precipitation behavior, and strengthening mechanisms were studied. The results showed that the strengths of quenched steels were higher than those of air-cooled steels, and the increase in strength was more pronounced with the addition of Nb-V than with the addition of Nb alone in the steels. Under the same cooling condition, the strengths of Nb-V-added steels were larger than those of Nb-added steels. Additionally, the Nb-V addition promotes the formation of lath structures. The yield stress of the steels, calculated by using measured microstructural parameters following the linear addition of strengthening, is in good agreement with the measured values.

Unraveling the significant role of retained austenite on the dry sliding wear behavior of medium manganese steel

For wear resistant steels, there is generally a linear relationship between the initial hardness and wear resistance, yet a high initial hardness limits the formability of components. The intrinsic ductility of austenite and strain hardening produced from the transformation-induced plasticity (TRIP) effect during service opens a new window for this contradiction. Herein, the wear resistance and the corresponding failure mechanisms of 5Mn steels after different heat treatment processes were evaluated using a ball-on-disk sliding test under dry conditions. The wear behaviors of the counterbodies were also studied to elucidate the wear mechanism of the tribo-system. The results show that the microstructures of the hot-rolled and air-cooled steel were mainly martensite. When the intercritical annealing temperature was relatively low (<695 °C), the microstructure primarily consisted of reversed austenite and ferrite. The samples annealed at 630 °C, 645 °C, 660 °C, and 675 °C had a similar initial hardness, while the sample annealed at 675 °C possessed a wear resistance that was 3.25 times that of the steel annealed at 630 °C. The large difference in the wear resistance was predominately due to the gradient of strain hardening provided by the varied amount and mechanical stability of the retained austenite. The transformation from austenite to martensite, which has an increased hardness, improves the work hardening. A strengthening layer formed with a certain thickness that provided stress support during the wear process and enhanced the wear resistance. When the annealing temperature reached 695 °C, a large amount of secondary martensite was generated from austenite that had a low thermal stability, which improved the initial hardness and wear resistance. The steels with a large amount of martensite had an enhanced wear resistance, but the elongation severely deteriorated, leading to unsatisfactory formability. The present work provides guidance for designing novel steels with an excellent balance of good formability and decent wear resistance by precisely adjusting the characteristics of the retained austenite.

Effect of Different Cooling Rates on the Corrosion Behavior of High-Carbon Pearlitic Steel

The present work discusses the effect of pearlitic morphology on the corrosion behavior of high-carbon fully pearlitic steel in 3.5% NaCl solution. Four different types of pearlitic steels (furnace-cooled, as-received, air-cooled and forced-air-cooled) consisting of coarse, medium, fine and very fine microstructures, respectively, were tested. Electrochemical behavior of these steels was studied with the help of dynamic and linear polarization and AC impedance spectroscopic tests. The corrosion resistance improved with fineness of the microstructure in general. However, with further reduction in interlamellar spacing beyond a limit, the corrosion resistance reduced slightly. Formation of homogeneous distribution of microgalvanic cells between cementite and ferrite lamellae of fine pearlitic steel improved the corrosion resistance. However, entanglement of the lamellae of pearlite in very fine pearlitic structure as well as breaking of cementite lamellae due to finer pearlitic colonies was attributed to the higher corrosion of the forced-air-cooled steel as compared to the air-cooled steel.

Phase transformation and mechanical behaviour of thermo-mechanically controlled processed high-strength multiphase steel

A novel low-alloy high-strength steel [Fe–0.20C–1.65Mn–1.40Si–1.50Al–1.30Cu–1.05Ni–1.07Co (wt%)] has been thermo-mechanically processed with a finish rolling temperature of 850 °C, followed by air cooling and water quenching in order to obtain a good combination of strength and ductility. Phase transformations of the above steel at different cooling rates have been studied and continuous cooling transformation (CCT) diagram has been constructed using data, obtained from dilatometric study. The phase field of CCT diagram indicates microstructure changes from a mixture of ferrite and bainite to fully martensite accompanied with the enhancement of hardness with increasing cooling rate. The microstructural investigation at lower cooling rate (≤5 °C/s) suggests the possibility of achieving pearlite-free microstructure by direct air cooling from the austenite region. Directly air-cooled steel has demonstrated primarily ferrite–bainite microstructure, which shows attractive tensile strength (>1050 MPa) and ductility (>15 %). On the other hand, directly water-quenched steels reveal predominantly lath martensitic microstructure with high dislocation density which exhibits higher tensile strength (>1600 MPa) and lower ductility (~12 %). The multiple stages of strain hardening behaviour of the investigated steel under different cooling conditions have been examined with respect to microstructural evolution.

A procedure for indirect and automatic measurement of prior austenite grain size in bainite/martensite microstructures

An alternative procedure for indirect and automatic measurement of the prior austenite grain size (PAGS) in bainite/martensite is proposed in this work. It consists in the determination of an effective grain size by means of statistical post-processing of electron backscatter diffraction (EBSD) data. The algorithm developed for that purpose, which is available on-line, has been applied to simulated EBSD maps as well as to both nanocrystalline bainitic steel and commercial hot-rolled air-cooled steel with a granular bainitic microstructure. The new proposed method has been proven to be robust, and results are in good agreement with conventional PAGS measurements. The added value of the procedure comes from its simplicity, as no parent reconstruction is involved during the process, and its suitability for low-magnification EBSD maps, thus allowing a large step size and coverage of a substantially broader area of the sample than the previous methods reported.

Effect of microstructure on the surface finish during machining of V-microalloyed steel: Comparison between ferrite–bainite–martensite and ferrite–pearlite microstructures

Multiphase ferrite–bainite–martensite microalloyed steel produced through a two-step cooling followed by annealing route and a ferrite–pearlite steel obtained through air-cooling after forging were subjected to turning operation. The influence of process parameters such as cutting speed, feed and depth of cut on surface roughness on both materials was compared. The results show that the multiphase microalloyed steel exhibited high surface finish than air-cooled steel. The analysis of variance shows that the contribution of cutting speed and depth of cut on surface roughness are insignificant for both ferrite–bainite–martensite and ferrite–pearlite microstructures.

Evolution of Microstructure and Mechanical Properties of Thermomechanically Processed Ultrahigh-Strength Steel

In the present study, low carbon microalloyed ultrahigh-strength steel was manufactured on a pilot scale. Transformation of the aforesaid steel during continuous cooling was assessed. The steel sample was thermomechanically processed followed by air cooling and water quenching. Variation in microstructure and mechanical properties at different finish rolling temperatures (FRTs) was studied. A mixture of granular bainite and bainitic ferrite along with interlath and intralath precipitation of (Ti, Nb)CN particles is the characteristic microstructural feature of air-cooled steel. On the other hand, lath martensitic structure along with a similar type of microalloying precipitates of air-cooled steels is obtained in the case of water-quenched steel also. The best combination of strength (1440 to 1538 MPa) and ductility (11 to 16 pct) was achieved for the selected range of FRTs of water-quenched steel.

Structure and Properties of a Low-Carbon, Microalloyed, Ultra-High-Strength Steel

The current study concerns the development of a low-carbon, microalloyed ultra-high-strength steel on a pilot scale. The continuous cooling transformation has been evaluated, and a flat top “C” curve with a mixed microstructure of bainite and martensite has been obtained at a lower transformation temperature. The steel has been processed thermomechanically, followed by air cooling and water quenching. In addition, a variation in microstructure and mechanical properties at different finish rolling temperatures has been studied. Although a mixture of granular bainite and bainitic ferrite with interlath and intralath precipitation of NbC/NbC(N)/TiC(N) particles are the characteristic microstructural feature of air-cooled steel, the lath martensitic structure along the fine NbC/NbC(N)/TiC(N) precipitate is obtained in case of a water-quenched steel. The high-strength value obtained in the current steel is caused by the accumulated contribution of fine-grained, pancaked austenite, highly dislocated fine lath martensitic structure along with the presence of tiny precipitates of microalloy carbide/carbonitride.

The influence of copper addition on microstructure and mechanical properties of thermomechanically processed microalloyed steels

The present study concerns the influence of Cu addition on the microstructural evolution and mechanical properties of directly air-cooled microalloyed thin-gauge steel. For this purpose, 1.5 wt% Cu was added to a Ti–B microalloyed steel. It is known that Ni addition to Cu-containing steel is beneficial to eliminate hot shortness caused by Cu. Therefore, the effect of Ni addition (half that of Cu in wt%) on the microstructure formation and mechanical properties has also been studied. Microstructures and mechanical properties of the directly air-cooled steels have demonstrated that addition of 1.5 wt% Cu along with 0.8 wt% Ni in Ti–B microalloyed steel results in a dual phase-like microstructure, which yielded attractive tensile strength (746 MPa) and ductility (31%). However, Cu addition deteriorated the impact toughness of the directly air-cooled Ti–B microalloyed steels.

Effect of tempering temperature and carbide free bainite on the mechanical characteristics of a high strength low alloy steel

Abstract The effect of tempering temperatures and the role of carbide free bainite (CFB) on the mechanical properties and susceptibility to hydrogen embrittlement of a low carbon Mn–Si–Cr steel have experimentally been investigated. The air-cooled steel consists of a mixed microstructure of carbide free bainite–martensite (CFB/M), which exhibits a combination of strength and toughness superior to a water-quenched martensite microstructure, after intermediate temperature tempering. The susceptibility to hydrogen embrittlement of the steel with the CFB/M microstructure decreases as the tempering temperature increases from 280 to 370 °C. This is attributed to the CFB/M mixed microstructure which not only possesses high temper resistance but also delays the tempered martensite embrittlement onset temperature. Films of carbon enriched retained austenite in the CFB exhibit good mechanical stability which enhances the toughness of the steel.

Influence of nickel on the fracture resistance of low-carbon steels

1. An increase in nickel content from 3.4 to 9.1% leads to an increase in the tendency of low-carbon steel toward the formation of lath martensite and in the volume share of residual (primary) austenite. After water quenching, the structure of ON3 steel is ferritic-martensitic and of ON6 and ON9 steels martensitic, while after air cooling a ferritic structure is formed in ON3 and ON6 steels and a ferritic-martensitic one in ON9 steel. 2. With an increase in nickel content in the steel from 3.4 to 9.1% the size of the pits in ductile fracture increases, and to a greater degree in the air-cooled steel. 3. After cooling from austenite-area temperatures, residual austenite is observed in the investigated steels both at the boundaries of the martensite laths and in the form of individual islands within the α-ferrite grains, and after tempering in the form of round precipitates at the boundaries and within the α-ferrite grains. 4. As the result of tempering of the nickel steels stable (secondary) austenite is formed in their structure, which leads to an increase in cold resistance and specific work for propagation of a ductile crack.

Roman arms and armour: a technical note

The equipment and arms of the Roman army have been the subjects of numerous books, most recently those of Webster (1969) and Robinson (1975). The core of the army until the 4th century AD was the “heavy”, that is fully-armoured, legionary infantry. Initially modelled on the Greek hoplite, their body armour consisted of a “muscle” cuirass made of a single piece of bronze. By the end of the Punic Wars, the Roman infantry had adopted the javelin instead of the pike, the large shield, and body armour of mail, probably of Celtic origin. At the same time both scale and lamellar were used, and all three forms of articulated defence are depicted on monuments. It has been suggested (Brewer, 1976) that the Roman swords must have been made of hardened steel. But their superiority would seem to reside as much in the shape of the swords and their use as in their greater hardness. During the 1st century AD body armour (lorica segmentata) of articulated plates supplemented the earlier forms, possible because it offered greater protection, but the weapons and tactics of the Imperial legions remained essentially the same as their Republican predecessors. Indeed, except for a greater use made of Gallic or German cavalry and field catapults (Eadie, 1967; Marsden, 1969) the techniques of the Roman army varied very little until its decline in the Late Empire. Notwithstanding the long period of its ascendancy, the Roman army seems to have left surprisingly few of its swords or iron armour in anycondition topermitscientificstudy, and to the best of my knowledge, no such study has been published. However, in the course of my research into armour, I was afforded the opportunities, through the cooperation of Drs Horn (Bonn) and Himmelein (Stuttgart), of examining a Roman gladius and a fragment of a Roman lorica segmentata. The results are described below. Both the sword and the lorica fragment are made of medium-carbon steels (0.5 0.7% C) but neither have been hardened by any attempt at heat-treatment. This is somewhat surprising in view of the known ability of the Romans to employ hardened steels on other occasions, e.g. a file from the 1st century BC was a quenched mediumcarbon steel (Schaaber, 1963) with a hardness of about 800 VPN. One must conclude that the Romans preferred to use an air-cooled steel of moderate, but predictable, hardness, instead of a quenched steel of higher, but perhaps variable, hardness with increased brittleness. The use of a steel for armour, unhardened, when it would apparently have lent itself so well to hardening, may be explained either by the inability to quench and temper a steel consistently, or perhaps by a preference (whether justified or not is immaterial) for soft armour. In the same way, some 7th century BC Greek bronze armour described by Smith (1972) was annealed rather than work-hardened.

多量のCとNを含むオーステナイト耐熱鋼における粒界反応と粒内析出の関係

The precipitation behavior of 21-4 N austenitic valve steels, in which the grain boundary reaction is apt to occur owing to the high concentrations of carbon and nitrogen, has been investigated at various aging temperatures ranging from 600 to 1050°C. At higher temperatures above about 850°C, the grain boundary reaction preceded the general precipitation occurred in the interior of grains and its rate was fast. However, it slowed down with decreasing temperature and only the general precipitation occurred below about 700°C.In order to study the driving force of the grain boundary reaction, the pre-aging was applied at 700°C to the solution treated steels prior to higher temperature aging. The grain boundary reaction took place at a much slower rate during aging at higher temperatures even in short time pre-aged steels because of the decreased supersaturation in the austenite matrix and the increased pinning force acting on the reaction front caused by finely dispersed particles due to pre-aging. It was also found that in the pre-aged steels the grain boundary nodules ceased to grow at higher supersaturation as a result of the increased pinning force for growth. For the same reason, the grain boundary reaction was considerably suppressed by cold working after the solution treatment. As regards to the effect of the cooling procedure from the solution temperature, a higher growth rate of the reaction was observed in air-cooled steels than the water-quenched ones.A small addition of phosphorus provided the sites for homogeneous precipitation and consequently very fine precipitates were uniformly formed in the matrix in phosphorus-containing steels. In phosphorus-containing steels, however, the observed growth rate of the reaction was somewhat higher than that in steels of standard composition. It was suggested that in phosphorus-containing steels an additional driving force caused by coherent precipitation strain had a strong influence on the rate of the reaction.