Acicular Ferrite Microstructure Research Articles

Effect of hot deformation and Nb precipitation on continuous cooling transformation of a high-Nb steel

In this work, the effects of hot deformation on continuous cooling transformation of a high-Nb steel were investigated on a Gleeble 3500 thermal simulator. The amounts of dissolved Nb were determined by inductively coupled plasma-atomic emission spectrometry. Furthermore, the effects of hot deformation and Nb precipitation on phase transformation were discussed. Results showed that high-Nb steel is suitable for acicular ferrite pipeline steels because the acicular ferrite microstructure can be obtained in a wide cooling rate range. Hot deformation strongly accelerates the polygonal ferrite transformation and increases the critical cooling rate to obtain a full acicular ferrite microstructure. Moreover, hot deformation markedly refines the final microstructure and improves the mechanical properties of acicular ferrite obtained at a high cooling rate. However, hot deformation can also promote Nb precipitation during holding and even cooling at low rates after hot deformation. Nb precipitation dramatically promotes the polygonal ferrite, weakens the effect of Nb in solution on phase transformation and strengthening, and decreases the microhardness.

Effects of Ti–Mg Complex Inclusions on Acicular Ferrite Nucleation

Abstract The alloys of Ti and Mg are added in molten carbon structural steel in sequence to reveal their influence on features of inclusion and microstructure. Compared with non-treated steel, it is found that they leads to the formation of Ti-Mg-O inclusion and reduction in portion of pure MnS. Experimental results indicate that average inclusion size in treated steel significantly decreases and its number per unit volume increases by 48 %. Moreover, the margin mole ratio of Mg/Ti in all Ti-Mg-O inclusions is larger than 2, which hints they are more complex than single assumption of MgTi2O4. After etching, intragranular acicular ferrite (IAF) microstructure is observed. EDS line analysis suggests that Mn-depletion zone exists in steel matrix, which is proposed to explain the IAF nucleation effect induced by Ti-Mg-O inclusions. Through comparison with only Mg-treated sample, the refinement of inclusion and increase in portion of pure MnS due to Ti adding are revealed.

Microstructure Evolution During Hot Deformation of a Micro-Alloyed Steel

In the present investigation, hot deformation by uniaxial compression of a microalloyed steel has been carried out, using a deformation dilatometer, after homogenization at 1200 °C for 20 min up to strains of 0.4, 0.8 and 1.2 at different temperatures of 900, 1000 and 1100 °C, at a constant strain rate of 2 s−1 followed by water quenching. In all the deformation conditions, initiation of dynamic recrystallization (DRX) is observed, however, stress peaks are not observed in the specimens deformed at 900 and 1000 °C. The specimens deformed at 900 °C showed a combination of acicular ferrite (AF) and bainite (B) microstructure. There is an increase in the acicular ferrite fraction with increase in strain at all these deformation temperatures. At high deformation temperature of 1100 °C, coarsening of DRXed grains is observed. This is attributed to the common limitations involved in fast quenching of the DRXed microstructure, which leads to increase in grain size by metadynamic recrystallization (MDRX). The strain free prior austenite grains promote the formation of large fraction of both bainite and martensite in the transformed microstructures during cooling. The length and width of bainitic ferrite laths also increases with increase in deformation temperature from 900 to 1100 °C and decrease in deformation strain.

Toughness improvement by Cu addition in the simulated coarse-grained heat-affected zone of high-strength low-alloy steels

The effect of microstructure and Cu addition in a simulated coarse-grained heat-affected zone (CGHAZ) of a high-strength low-alloy (HSLA) steel subjected to 100 kJ cm−1 heat input welding was studied. It has been observed that the primary microstructure in Cu-free HSLA steels is dominated by bainite, whereas, in Cu-bearing HSLA steels, the predominant microstructure is acicular ferrite. The acicular ferrite nucleated at intragranular complex inclusions consisting of Al and Ti oxides, covered with layer of MnS and CuS. The presence of high intensity of acicular ferrite and hard impingements between acicular ferrite laths or plates has contributed to the fine-grained and interlocked microstructure. The enhanced toughness in CGHAZ of Cu-bearing HSLA steel is attributed to the fine-grained interlocked microstructure of acicular ferrite.

Effect of microstructure on the crack propagation behavior of microalloyed 560 MPa (X80) strip during ultra-fast cooling

A novel thermo-mechanical controlled processing (TMCP) involving ultra-fast cooling (UFC) was used to process microalloyed 560MPa (X80) strip with critical thickness of 22mm. The microstructure and mechanical properties including tensile, Charpy v-notch impact toughness, and drop weight tear test (DWTT) properties were studied, with particular focus on the effect of microstructure on fracture and crack propagation behavior during DWTT test. The study underscores that the processed strip comprising of acicular ferrite (AF), bainitic ferrite (BF), together with finely distributed martensite/austenite (M/A) constituent provided excellent combination of strength, toughness and crack arrest property. The yield strength (618MPa), tensile strength (752MPa), and low-temperature toughness (upper shelf energy of 302J, transition temperature of −74°C) as well as DWTT shear area (100%) met the requirements of API SPEC 5L. The predominantly AF microstructure with small-size M/A constituent improved the low-temperature toughness by increasing the critical fracture stress and increased the ability to hinder crack propagation during Charpy test. The AF and finely distributed M/A constituent effectively deflected the crack propagation during DWTT indicating excellent crack arresting property of pipeline strip. It is the cooling schedule of UFC that was responsible for decreasing the effective grain size and increasing the mechanical properties.

Dissimilar welded joints operating in sub-zero temperature environment

The vast expanses of the Arctic have for some decades been the subject of considerable scientific interest as regards potential industrial exploitation. However, the cold climatic conditions of the area pose major challenges, particularly with respect to the metals used, infrastructure design, and manufacturing techniques required for cold-resistant structures. Dissimilar welding of metals can supply added value (lightness, thermal difference) in the construction of Arctic structures, but the behavior of dissimilar high-strength metal welds in very cold environments remains under-investigated and improved understanding is required before such metals can be adopted for Arctic applications. The objective of this research is to identify possible difficulties arising from the use of dissimilar metal welds in Arctic structures, in particular the causes of potential defects and failures, and to propose means to remedy or prevent such failures through the adoption of suitable procedures and the choice of suitable metals. This study covers steels used for structural design of work in cold environments such as low-carbon content steels with tensile strength from 379 to above 586 MPa along with alloyed steels containing nickel (Ni) up to 4.5 % or chrome (Cr) and molybdenum (Mo) and stainless steels like austenitic stainless steels which keep their ductile properties even in sub-zero temperature. These steels are known for their better weldability and toughness. The methodology consists of a review of scientific articles and research experiments on the microstructures and physical and mechanical properties of dissimilar welded structures for low-temperature environments. In-service behavior is analyzed on the basis of literature data and the relationship between the types of structure failures and weld defects investigated. Finally, appropriate joints and applicable methods are proposed. The analysis shows that a microstructure with very large grains and a large HAZ are features that increase the size and prevalence of cracks. An acicular ferrite microstructure is required in the weld metal. Migration of carbon is a further aggravating factor for cracking. With nonferrous metals, the risk of undesired intermetallic compound hardness and brittleness increases, and processes that minimize the temperature of the HAZ are recommended. Heat treatment is recommended to refine the grains. The major implication of the work is that significant care is required in the selection of the base metal, electrodes and welding parameters, and appropriate heat treatments. Effective control of such factors can lead to improvement in the quality, reliability, and service life of Arctic welded structures.

A study on the relationship between microstructure and mechanical properties of acicular ferrite and upper bainite

Acicular ferrite and bainite are microstructural constituents commonly found in ferritic weld metal and many other ferrous alloys. These often highly localised microstructural volumes are known to have a significant impact on the mechanical properties of the bulk material they constitute. It is well established that acicular ferrite and bainite have substantially different morphologies and features that can be directly linked to their intrinsic mechanical properties. However, in the bulk material they usually co-exist and interact with other microstructural constituents and features. Therefore, the individual contributions of acicular ferrite and bainite still remain unclear. This work utilises micro-testing techniques to evaluate and compare the mechanical properties of these two micro-constituents with an ultimate objective to understand their impact on bulk properties of ferrous alloys. Microscopic regions consisting of either acicular ferrite or upper bainite were first selected and then characterised using a high resolution Scanning Electron Microscope (SEM) and Electron Backscattered Diffraction (EBSD). Conventional nano-indentation and an advanced characterisation procedure were implemented to evaluate elastic modulus, yield strength, hardness and strain hardening exponent of both micro-constituents. The fracture resistance was estimated from micro-fracture tests that were conducted within the selected regions. The experimental studies have indicated that, at the micro-scale, despite their different microstructures, acicular ferrite and upper bainite have very similar mechanical properties. Nevertheless, the fracture resistance of upper bainite was more dominated by the contribution of plastic deformations.

The influence law of cooling rate on microstructure and properties of low-carbon high-grade AF pipeline steel

To explore influence laws of cooling rate on microstructure and properties of low-carbon high-grade acicular ferrite (AF) pipeline steel. The microstructure and properties influence of alloy composition and super-cooled austenite continuous cooling transformation (CCT) has been investigated by thermodynamic analysis and calculation. Studies have shown that the optimised composition was C0.06, Si0.2, Mn1.85, Nb0.04, Ni0.33, Cr0.4, Mo0.15, P ≤ 0.01 and S ≤ 0.002, when the cooling rate was less than 1°C/s and pearlite (P) transition has been suppressed. When the cooling rate exceeds 20°C/s, structure mainly consists of AF and small amount of retained austenite (RA), and the mechanical properties partially met and exceed X90 performance levels of API SPEC 5L-45th. This paper offers a novel research method of low-carbon AF high-grade steel. The conclusion provides theoretical and practical guidance for production and study of such types of steel.

The influence law of cooling rate on microstructure and properties of low-carbon high-grade AF pipeline steel

To explore influence laws of cooling rate on microstructure and properties of low-carbon high-grade acicular ferrite (AF) pipeline steel. The microstructure and properties influence of alloy composition and super-cooled austenite continuous cooling transformation (CCT) has been investigated by thermodynamic analysis and calculation. Studies have shown that the optimised composition was C0.06, Si0.2, Mn1.85, Nb0.04, Ni0.33, Cr0.4, Mo0.15, P ≤ 0.01 and S ≤ 0.002, when the cooling rate was less than 1°C/s and pearlite (P) transition has been suppressed. When the cooling rate exceeds 20°C/s, structure mainly consists of AF and small amount of retained austenite (RA), and the mechanical properties partially met and exceed X90 performance levels of API SPEC 5L-45th. This paper offers a novel research method of low-carbon AF high-grade steel. The conclusion provides theoretical and practical guidance for production and study of such types of steel.

Effect of Ultra Fast Cooling on the Alloy Cost Reduction of the X80 Pipeline Steel

The microstructures and mechanical properties of X80 pipeline steels produced by both novel ultra fast cooling and conventional‐accelerated continuous cooling modes are investigated. Results showed that different levels of Mo addition had a remarkable effect on the microstructures and mechanical properties of the investigated pipeline steels. The proeutectoid ferrite and pearlite formation is inhibited in the high‐Mo steel and acicular ferrite is obtained over a wide range of cooling rates, whereas the dominant acicular ferrite microstructure can only be obtained when the cooling rates reach up to 5 C s−1. Very similar microstructures and mechanical properties are obtained in the low‐Mo steel produced with ultra fast cooling and in the high‐Mo steel produced by the conventional‐accelerated continuous cooling. It was proved by simulation and industrial trials that high‐strength low‐alloy steels such as pipeline steels, can be produced using the novel ultra fast cooling which also reduce alloy cost.

Novel Cu-bearing high-strength pipeline steels with excellent resistance to hydrogen-induced cracking

There is a great challenge to simultaneously improve strength and resistance to hydrogen-induced cracking (HIC) for pipeline steels. In this study, through copper (Cu) alloying, a novel type of pipeline steel was developed to meet the requirement for both high strength and excellent resistance to HIC. The HIC tests of the tentative pipeline steels and the comparison steel were conducted in a hydrogen sulfide (H2S) saturated solution according to NACE standard. The results showed that an acicular ferrite microstructure with uniformly distributed fine Cu-rich precipitates was obtained by aging at 500°C for 1h. Nano-scale Cu-rich precipitates in the new steels were speculated to play a significant role in making precipitation strengthening and providing beneficial hydrogen traps. The Cu-bearing pipeline steel has achieved a high strength grade of API X120 with excellent resistance to HIC.

Effect of microstructure variation on the corrosion behavior of high-strength low-alloy steel in 3.5wt% NaCl solution

The effect of microstructure variation on the corrosion behavior of high-strength low-alloy (HSLA) steel was investigated. The protective property of the corrosion product layer was also explored. Experimental results reveal that the type of microstructure has significant effect on the corrosion resistance of HSLA steel. The measurement results of weight loss, potentiodynamic polarization curves, and electrochemical impedance spectroscopy indicate that the steel with acicular ferrite microstructure exhibits the lowest corrosion rate. Martensite exhibits a reduced corrosion resistance compared with polygonal ferrite. It is found that the surface of the acicular ferrite specimen uniformly covered by corrosion products is seemingly denser and more compact than those of the other two microstructures, and can provide some amount of protection to the steel; thus, the charge transfer resistance and modulus values of the acicular ferrite specimen are the largest. However, corrosion products on martensite and polygonal ferrite are generally loose, porous, and defective, and can provide minor protectiveness; thus, the charge transfer resistance values for polygonal ferrite and martensite are lower.

HIC and SSC Behavior of High-Strength Pipeline Steels

In this study, hydrogen-induced cracking (HIC) and sulfide stress corrosion cracking (SSC) behaviors of high-strength pipeline steels in four different strength grades (X70, X80, X90 and X100) with the microstructure of acicular ferrite were estimated. The results showed that both of X70 and X80 steels exhibited better HIC resistance, and their susceptibility to HIC increased with the strength grade. HIC parameters, including cracking length ratio, cracking thickness ratio (CTR) and cracking sensitivity ratio, were all increased, and among these, the CTR increased most, with the increase in the strength grade. HIC was found to initiate and grow along the hard boundaries such as large size martensite/austenite (M/A) islands and bainitic ferrite. In addition, the density of hydrogen-induced blister on the steel surface was increased with the decrease in pH value for the same-grade pipeline steels. SSC susceptibilities of X80, X90 and X90-C were revealed to subsequently decrease, which was related to the large size M/A islands.

Fine-Grained X80 Pipeline Steels Produced by a Novel Ultra Fast Cooling Process

ABSTRACT Zhou, F.; Wu, K.; Zhou, W., and Matrosov, M., 2015. Fine-grained X80 pipeline steels produced by a novel ultra fast cooling process. The relationship between microstructure and the mechanical properties of the X80 pipeline steels produced by an ultra fast cooling process was investigated using scanning and transmission electron microscopy and an energy dispersive spectrometer. Predominantly fine-grained acicular ferrite microstructure was obtained from the ultra fast cooling process. The developed X80 pipeline steel had low yield ratio and low temperature impact toughness. The ultra-fast cooling resulted in work hardened austenite before the phase transformation and also pushed the transformation to a lower temperature. Also, the Nb-Ti carbonitride precipitation was inhibited during ultra fast cooling. Therefore, the fine-grained microstructure and superior mechanical properties were attributed to grain refinement and the inhibiting of Nb-Ti carbontride precipitation, which was caused by ultra fa...

Study of Pipeline Steels with Acicular Ferrite Microstructure and Ferrite-bainite Dual-phase Microstructure

Three kinds of X70 steels with the same chemical composition and different microstructures fabricated by varying processes were compared in aspect of the microstructures and mechanical properties using SEM, TEM and tensile testing machine. The experimental results showed that the steel 1 with acicular ferritic microstructure fabricated by thermal-mechanical processing with online accelerated cooling (TMCP) exhibited an excellent combination of strength and toughness, but provided high yield ratio of 0.85, low uniform elongation of 8.3% as well as low strain hardening exponent of 0.09, indicating poor deformability. In contrast to above steel 1, the steel 2 and steel 3 having ferrite-bainite dual-phase microstructure respectively fabricated by TMCP and intercritical annealing exhibited the improved deformability in terms of the low yield ratio of 0.69 and 0.68, high uniform elongation of 12.8% and 11.8%, and strain hardening exponent of 0.157 and 0.155. It is argued that the optimum properties combination of strength, toughness and deformability can best be achieved by obtaining a ferrite-bainite dual-phase microstructure. This kind of ferrite-bainite dual-phase pipeline steel is appropriate to transmitting oil and natural gas in seismic zone and permafrost.

Research of Strain Aging in Pipeline Steel with a Ferrite/Martensite Dual‐Phase Microstructure

By applying ferrite–martensite microstructural control, high‐deformability linepipes of grade X70 have been developed. The steel with an acicular ferrite microstructure was intercritically heat‐treated at 820°C for 10 min, followed by rapidly water cooling, in order to get a ferrite/martensite dual‐phase microstructure. After aging at 200–250°C for 5–15 min, the microstructure and mechanical properties of the ferrite–martensite microstructural steel were studied. The pipe with a ferrite/martensite dual‐phase microstructure exhibited a superior strain aging resistance aging at 200–250°C for 5–15 min. The yield ratios were below 0.77 and the pipe still exhibited continuous yielding behavior, meanwhile tensile strength, uniform elongation and impact toughness did not experience significant change. By intercritical annealing, carbides and carbonitride containing Nb and Ti precipitated out of the ferrite phase further, at the same time, solute carbon atoms could adequately diffused from the ferrite to the uniformly distributed austenite derived from the acicular ferrite initial microstructures, leading to the reduction of the amount of free carbon and nitrogen to pin the mobile dislocations in the ferrite phase, therefore, strain aging effects were not pronounced in the temperature and time range investigated.

Effects of Boron on Toughness in High Heat Input Welds of High Strength Low Alloy Steel

The microstructure and toughness are investigated to elucidate the influence of boron on toughness in the high heat input welds of high strength low alloy(HSLA) steel. The result indicates that appropriate addition of boron effectively promotes the formation of acicular ferrite, and as a result microstructure is refined. Whereas the percentage of acicular ferrite decreases and microstructure is coarsen as the boron content further increases. Meanwhile, moderate amount of boron obviously suppresses proeutectoid ferrite, and bainite is favored. The fraction and size of M-A constituent accordingly increases from 2.4% and 2.14 μm with no addition of boron to 5.7% and 2.83 μm with 0.008 8%B. As result, the weld toughness is improred at first and then detoriorat as the boron overdoses. When the boron content is 0.005 2% with the titanium content of 0.03%, the highest toughness can be attained at low temperature for the high heat input welds of HSLA.

Transformation behavior of a Ti–Zr deoxidized steel: Microstructure and toughness of simulated coarse grain heat affected zone

Continuous cooling and isothermal transformation behavior of a Ti–Zr deoxidized microalloyed steel was studied and compared with that of C–Mn steel, in conjunction with the evolution of microstructure and measurement of toughness of simulated high heat input welding coarse grain heat-affected zone (CGHAZ). The study indicates that the inclusions in Ti–Zr steel were primarily composite inclusions (ZrO2·MnS) with high density of inclusions of size less than 1μm. Polygonal ferrite, acicular ferrite, multi-directional bainite, and segmented martensite were the typical microstructures obtained with increase in cooling rate. Each of these phases was analyzed with decreasing isothermal temperature. Intragranular ferrite nucleation was significantly promoted by ZrO2·MnS inclusions. Inclusions of smaller size were more effective with decreasing temperature such that greater fraction of ferrite plates nucleated on one inclusion. Mn-depletion is the operating mechanism and precipitation of TiN was complementary. The simulated CGHAZ was characterized by interlock acicular ferrite microstructure and high impact toughness such that the dimple fracture and fine cleavage facets contributed to high impact energy.

Development of Economic-Type X70 Linepipe Steel Hot Strip

In order to reduce the production cost, economic-type X70 pipeline steels with the thickness of 14.6 and 15.9mm were redesigned . The latest alloy system of pipeline steel designed by non-molybdenum C-Mn-Cr-Nb alloy system, which replaces the high-molybdenum C-Mn-Mo-Nb alloy system, was adopted along with acicular ferrite microstructure. The microstructure of X70 strip is homogeneous and ferrite grains are fine, resulting in high strength, excellent low-temperature toughness and weldability.

In Situ Observations of the Formation of Fine‐Grained Mixed Microstructures of Acicular Ferrite and Bainite in the Simulated Coarse‐Grained Heated‐Affected Zone

The formation of fine‐grained mixed microstructure of acicular ferrite and bainite in the simulated coarse‐grained heated‐affected zone was investigated by means of in situ observation and EBSD analysis. Single, multiple, and sympathetic nucleation of acicular ferrite were observed. Acicular ferrite laths or plates were formed prior to bainite and effectively partitioned austenite grains into many smaller and separated regions. The later formed bainite at lower temperature was confined in those smaller regions, thus resulting in the fine‐grained mixed microstructures of acicular ferrite and fine bainite plate.