Massive Ferrite Research Articles

Strengthening, strain hardening and fracture of heat-treated interstitial-free steel with massive ferrite conceptualizing the interface controlled reaction phenomena

Interstitial-free steel is extremely soft and ductile with allotriomorphic ferrite in the microstructure as a diffusional transformation product of austenite. In order to alter the mechanical property, the present work performed austenitization of the steel at 925 °C for 5 min and thereafter a rapid water quenching, resulting in massive ferrite. The yield strength (86 MPa) and ultimate tensile strength (181 MPa) increased to 362 MPa and 531 MPa, respectively, along with 18 % ductility. The statistically stored dislocation density and substructures (sub-boundaries, cells) at the grain body contributed 68 % of the yield strength. The strain hardening was observed to be lower than that with allotriomorphic ferrite, due to heterogeneous deformation of the substructures, causing an inhomogeneous distribution of geometrically necessary dislocation density at the bulk. The preferential localization of strain at cell walls initiates cracks by the de-cohesion of massive grains. The cell-substructure conceived by massive ferrite therefore has a strong role both in strain hardening and fracture. The larger the grain size is, the more the cell-substructure is, dictated by an interface-controlled growth of austenite to massive ferrite. The atomic jump at the transformation interface is usually considered to be an equilibrium process; meaning slow, not standing with the so-called faster growth kinetics of an interface-controlled reaction. In order to bridge the gap, the present work demonstrates a slip deformation at the transformation interfaces for fast-tracking the lattice reconstruction of massive ferrite from austenite, a novel.

Study on metal corrosion behavior of heat-affected zone of oil pipeline

Due to the influence of welding technology, a series of non-uniform and continuous heat-affected zones formed by welding seams is the weakest link of pipeline steel. Compared with the X70 steel, the heat-affected area is in the form of coarse acicular and massive ferrite, granular bainite, and a small amount of pearlite, with weak strength and corrosion resistance. Therefore, this paper simulated the heat-affected zone of the high-strength steel welding seam by the Gleeble 3500 thermal/force simulation testing machine for two heat cycles. Taking Shengli crude oil as the experimental medium, the metal corrosion morphology of heat-affected zones at different temperatures and transmission rates during hot oil transportation was analyzed through immersion experiment and corrosion morphology characterization. The results show that the increase in temperature and flow rate will increase the overall corrosion rate of the metal in the heat-affected area. In the range of 30∼50°C, pitting corrosion was the main corrosion feature of the metal, and in the range of 50∼70°C, the corrosion morphology of the metal changed from pitting corrosion characteristics to comprehensive corrosion characteristics. In the range of the flow rate studied (1∼4 m/s), the metal in the heat-affected area was mainly the overall corrosion rate, and the metal surface showed the erosion-corrosion characteristics along the flow rate direction.

Formation, microstructure and mechanical properties of double-sided fiber laser welded ultra-high strength steel

The ultra-high strength steel with a thickness of 20 mm was welded by double-sided high-power fiber laser welding. The welding parameters were optimized according to the behavior of molten metal and formation of the weld. Necking, spatters and uneven formation occurred when the welding parameters were improper. Microstructure of the weld at most region was made up of martensite, while austenite could be found at the final solidified position where the solidification rate was relatively slow. The thermal effects of the second welding process resulted in the austenization of original martensite and then transformed to ferrite and carbide within the interlaminar weld. Hardness of the weld was much higher than that of base metal owing to the formation of martensite. Hardness decreased at the austenite zone and the interlaminar zone made up of massive ferrite. Solid solution of C element within the martensite improved the tensile strength but weakened the toughness of the weld, leading to the formation of cleavage planes at fracture surfaces of the top and the mid welds. Large angle between the grain growth direction and force loading direction was benefit to improve the tensile properties.

A novel observation of Widmanstätten ferrite and pearlite by thermo-mechanical processing of interstitial-free steel

ABSTRACT In the present work, interstitial-free steel is rapidly cooled from the austenitic state, followed by isothermal holding at 630°C for 5 minutes in a thermo-mechanical simulator (Gleeble 3800®). The heat-treatment provides massive ferrite, Widmanstätten ferrite and pearlite, as the triplex-phase mixture. Massive ferrite, as the early decomposed product of austenite, arises by rapid cooling. Subsequent formation of Widmanstätten ferrite and pearlite during the isothermal holding is elusive in steel. The phase transformations account for a dramatic improvement in tensile strength by two-fold with a negligible reduction in elongation. Morphologically, massive ferrite is granular and contains recovered sub-grain boundaries inside the phase. The pearlite nucleates within a lath-shaped grain, unconventionally. Due to the low-temperature phase transformation (630°C) and a ppm level carbon in the alloy, the observed pearlitic structure is either underdeveloped or fragmented, which promotes cracks against the stronger Widmanstätten ferrite at an advancing front of the crack tip.

Recrystallization in commercial grade interstitial-free steel, discussing criticality of martensite and massive ferrite nucleation along with mechanical property

Interstitial-free steel is very soft and ductile, with a fully ferritic microstructure at room temperature. In order to address this issue in the present work, the cold-rolling increases tensile strength vividly by strain hardening. The heat-treatment thereafter optimizes strength-ductility, with an excellent combination, through fine recrystallized grains. The warm-rolled sample provides relatively an inferior property by partial recrystallization of the sample. Irrespective of unannihilated dislocations, both the samples after the recrystallization treatment in the austenitic domain at 925 °C, were water quenched in a laboratory scale to explore phase transformations additionally in the system. The microstructural characterization reveals that the nucleation of massive ferrite under rapid cooling requires austenite conditioning, similar to martensite in the alloy, constrained by strained lattice. The outcome fundamentally argues against the notion that the accumulation of crystal defects/dislocations promotes an extra driving force for a reconstructive phase transformation leading to massive ferrite.

Effect of Tempering Time on Microstructure and Properties of 65Si2CrV Spring Steel

The microstructure evolution and mechanical properties of 65Si2CrVspring steel at 420 °C with different tempering time were studied in this paper. The results show that the extension of tempering holding time will make the martensite recover more fully, so as to transform into massive ferrite, and the precipitated carbide increases at the same time. With the extension of tempering holding time, the strength of the sample decreases gradually. When the tempering time exceeds 300s, the ratio of length and width of carbide decreases gradually, spheroidization occurs and plasticity decreases sharply. Excellent comprehensive properties were obtained when holding for 300s. The tensile strength was 2083MPa, the elongation was 8.3%, and the product of strength and plasticity was 17.28 GPa %.

Path planning and forming of wire multi-arc additive collaborative manufacture for marine propeller bracket

The propeller bracket is an essential structural component of the large ship, with large structural size, high precision and performance. Multi-arc collaborative additive manufacturing is an effective method for forming high precision and performance large-scale propeller bracket. This paper studies the high-precision and performance multi-arc collaborative additive manufacturing propeller bracket forming strategy and path planning method. In the multi-arc cooperative additive manufacturing of the bracket, the two arcs in the front are used to form the contour of the depositing layer, while the rear three arcs are used to fill the contour. Finally, the five arcs collaborated additive manufacturing bracket is formed. Based on the structural characteristics of the bracket, it is divided into four regions: the hub, support arm, cross arm and intersecting area. The hub is a rotating body of equal thickness and cross-section. The relative positions of the five arcs are fixed, and the path is calculated by adjusting the attitude angle of the device in real-time during the printing process. The intersecting area, support arm and cross arm are all curved bodies with variable thickness and cross-section. The contour torches are used to form the two sides of the contour separately, and then the filling torch is used to variable amplitude swing filling the contour. The five arcs collaborative additive manufacture propeller bracket with a length of 3.5 m is formed by the above forming strategy and path planning method. The forming accuracy of the bracket is ±0.6 mm. The microstructure is composed of pearlite and massive ferrite. The tensile strength is 774 MPa, and the yield strength is 642 MPa. The mechanical properties are higher than the castings one with the same composition, and the forming efficiency reaches 1800 cm3/h.

Establish time-temperature-transformation diagram based on dilatometry results and microstructural evolutions in an AISI 1010 steel

A time-temperature-transformation (TTT) diagram has been constructed using dilation results. The phase transformations have been confirmed through microstructural changes of AISI 1010 steel. The strength of this steel has been improved for various applications using heat treatments. Experiments for a series of steel samples have been conducted between 310-730°C isothermal temperature range using a thermo-mechanical simulator Gleeble®3800. The austenitised microstructure of the steel has been transformed into a combination of ferrite, pearlite, widmanstätten ferrite, massive ferrite, upper and lower bainite, and martensite, etc., when held at different isothermal temperatures. Ferrite along with pearlite have been observed at higher isothermal temperatures while bainite with martensite have been observed at lower isothermal temperatures. Two C-curves have been observed in pearlite and bainite transformation regions.

Effect of cooling rate on microstructure and mechanical properties of a low-carbon low-alloy steel

The advanced electron backscatter diffraction (EBSD) technique was used to examine the microstructure of a widely used A517GrQ low-carbon low-alloy steel after different heat treatments. Three distinguishable microstructures were studied. Slow cooling in the furnace after austenitization led to the formation of a granular structure that consisted of massive ferrite and randomly distributed M–A constituents. Medium rate cooling in air produced granular bainite that was composed of lath ferrite, and M–A constituents were distributed between the laths. Lath martensite was formed by fast cooling into ice brine. EBSD analysis revealed that, in one austenite grain, the massive ferrite in the granular structure and the lath ferrite in the granular bainite were predominately separated by high-angle boundaries, whilst the ferrite laths in the martensite were separated by low-angle boundaries. The specimens with granular bainite formed by medium rate cooling had higher strength (both yield strength and tensile strength), and also almost 5 times higher Charpy impact energy than that of the specimens containing granular structure obtained at the slow cooling. The strength of the specimens with lath martensite after quenching into ice brine was slightly higher than the granular bainite but were associated with much lower Charpy impact energy. The present work indicates that it is critical to control the cooling rate after austenitization in order to simultaneously achieve high strength and high toughness of low-carbon low-alloy steels.

Microstructure Characterization of Massive Ferrite in Laser-Weldments of Interstitial-Free Steels

Laser welding, which is known for its precision and high welding rate, offers a high-cooling-rate processing environment. In this study, laser welding was applied to interstitial-free steel plates with phosphorous additions of 0.002 and 0.009 wt%. After laser welding was performed and the weldment was cooled by protective gas, massive ferrite was produced. The base metal, heat affected zone, and weld region were observed and compared by optical microscopy (OM). It was found that increasing the phosphorous content led to refinement of the grain size of the massive ferrite. In addition, the allotriomorphic ferrite in the base metal and the massive ferrite in the weld were characterized and analyzed under scanning electron microscopy–electron backscatter diffraction (SEM-EBSD). The substructures of massive ferrite in OM can be resolved to be low-angle sub-boundaries in kernel average misorientation (KAM) analysis conducted on SEM. Furthermore, TEM analysis revealed that the substructures of massive ferrite were associated with the dislocation cell structures; it is presumed that during the growth of massive ferrite, the rapid migration of incoherent boundaries generated a high dislocation density, and subsequent cooling led to auto-tempering.

Effect of Tempering Temperature after Thermo-Mechanical Control Process on Microstructure Characteristics and Hydrogen-Induced Ductility Loss in High-Vanadium X80 Pipeline Steel.

In this study, an optimum tempering temperature after a thermo-mechanical control process (TMCP) was proposed to improve the hydrogen-induced ductility loss of high-vanadium X80 pipeline steel. The results showed that with increasing tempering temperature from 450 to 650 °C, the size and quantity of granular bainite decreased but the spacing of deformed lath ferrite and the fraction of massive ferrite increased. The number of fine vanadium carbides increased as well. However, as the tempering temperature increased to 700 °C, the microstructure of T700 steel completely converted to massive ferrite and the grain size became larger. Additionally, the amount of nanoscale precipitates decreased again, and the mean size of precipitates evidently increased in T700 steel. The steel tempering at 650 °C, containing the most vanadium precipitates with a size less than 20 nm, had the lowest hydrogen diffusion coefficient and the best resistance to hydrogen-induced ductility loss.

Effect of Austenite Conditioning on Martensitic Transformation in Commercial Grade Interstitial-Free Steel

Martensite is an important phase for the strengthening of interstitial-free (IF) steel. The as-received sample is soaked in the austenitic temperature domain at 930 °C for 5 minutes, followed by water quenching in a Gleeble 3800® thermo-mechanical simulator. The outcome shows a dramatic increase in strength by lath martensite and massive ferrite. Hot deformation is also a part of the comparative study. The as-received sample is processed in 0.5 compressive strain at the rate 0.01/s in the same austenitic temperature domain, followed by water quenching. Dynamic recrystallization has not been observed. Instead, plastic deformation in austenite leads to grain boundary nucleation of diffusional ferrite, inhibiting nucleation and growth of massive ferrite in a triplex phase mixture of lath martensite. The austenite to martensite transformation follows Kurdjumov–Sachs (K–S) orientation relationships. The calculated variants based on the K–S orientation relationships between parent and product phase designate that the transformation of differently oriented martensite can originate under constraints from deformed austenite. Thus, the strength-ductility combination is verified with martensite obtained under two different austenite conditions.

Massive Ferrite Causes Toughness Decrease in Low Cr-Mo Steel Weld

Massive ferrite belts were observed in a low Cr-Mo steel multilayer weldment, which consisted of numerous massive ferrite lath blocks (200 μm × 200 μm × 800 μm). The massive ferrite lath blocks were brittle, which initiated and provided a path to propagate cleavage cracking and reduced the steel weldment toughness significantly. In the low Cr-Mo steel multilayer weldment, local low toughness regions were caused by the massive ferrite belts.

High-strength wire + arc additive manufactured steel

Abstract High-strength 690-MPa steel was prepared using a wire + arc additive manufacturing (WAAM) technology. The phase composition, microstructure, and crystal structure of highstrength 690-MPa steel samples were analysed, and the results show that a sample prepared using WAAM technology achieves a good formation quality. The metallographic structure was mainly acicular ferrite, massive ferrite, and granular bainite. The microhardness distribution of the vertical and horizontal sections of the samples is uniform. Excellent mechanical properties of the specimen were shown, including a horizontal yield strength of 536 MPa, a tensile strength of 760 MPa, an elongation of 23.5%, a Charpy impact value of 70 J at -508C, a vertical yield strength of 486 MPa, a tensile strength of 758 MPa, an elongation of 21.5%, and a Charpy impact value of 51 J at -508C.

Numerical and Experimental Study of Plastic Deformation During Flash Butt Welding of X65 Pipeline Steel

AbstractThe plastic deformation during flash-butt welding (FBW) and its effects on weld quality are investigated by using numerical and experimental methods. The electro-thermo-mechanical coupling model of FBW is validated by comparing the calculated temperature and plastic deformation to measured one, obtaining reasonable agreement. The calculation results reveal that a thin liquid metal film forms at the contact interface during accelerating flash stage to provide temperature conditions for upsetting. The length of liquid metal (including burning and expelled losses) is 29.7 mm for one piece pipeline tube under the given condition. The stress and strain at contact surface are both almost zero at the initial stage of upsetting due to the thin liquid metal film existing at the contact interface, and they rapidly increase to 58.0 MPa and 17.7, respectively, while the liquid metal are excluded from the contact interface between two tubes to be welded. The maximum plastic deformation is 18.1 mm at the given condition under the action of upsetting force. The experimental results illustrate that the microstructure of X65 FBW joints consists of massive ferrite, grain boundary pre-eutectic ferrite, pearlite, and widmannstatten, while the microstructure in heat-affected zone is fine ferrite and pearlite. The coarse grain size and gray spots in the butt joint severely decrease the tension strength and impact toughness.

High-strength wire + arc additive manufactured steel

Abstract High-strength 690-MPa steel was prepared using a wire + arc additive manufacturing (WAAM) technology. The phase composition, microstructure, and crystal structure of highstrength 690-MPa steel samples were analysed, and the results show that a sample prepared using WAAM technology achieves a good formation quality. The metallographic structure was mainly acicular ferrite, massive ferrite, and granular bainite. The microhardness distribution of the vertical and horizontal sections of the samples is uniform. Excellent mechanical properties of the specimen were shown, including a horizontal yield strength of 536 MPa, a tensile strength of 760 MPa, an elongation of 23.5%, a Charpy impact value of 70 J at -508C, a vertical yield strength of 486 MPa, a tensile strength of 758 MPa, an elongation of 21.5%, and a Charpy impact value of 51 J at -508C.

Investigation of massive ferrite in an interstitial-free steel

The formation of massive ferrite in an interstitial-free (IF) steel with three different austenitization conditions (at 1200, 1100 and 1000 °C respectively for 3 min) has been investigated. To conduct the rapid continuous-cooling transformations at a cooling rate of 500 °C/s, hollow rod specimens were employed in a dilatometer. Thermal arrests, which were associated with the phase transition, were detected in the cooling curves, and thereby the massive ferrite transformation temperatures were determined. The corresponding treated samples were studied by optical metallography and transmission electron microscopy. The features of massive ferrite and its transformation characterization are discussed.

Hybrid Laser-Arc Welding of X90 Pipeline Steel: Effect of Laser Power on Microstructure and Mechanical Properties

In this work, hybrid laser-arc welding process was applied to X90 pipeline steel which has wide potential applications in the future pipeline project. The effect of different laser power (1.0, 1.5 and 2.5 kW) on microstructure and mechanical properties of weld joints was investigated. It has been found that a macroscopic morphology of “wine cup like” is observed in the weld joint with increasing laser power, where fusion zone (FZ) and heat-affected zone (HAZ) can be clearly identified. The FZ microstructure mainly includes massive ferrite, acicular ferrite (AF), and increased laser power resulting in a decrease in AF content. The HAZ consists of coarse-grained HAZ (CGHAZ), fine-grained HAZ (FGHAZ) and mixed-grained HAZ (MGHAZ). The hardness ranging from the weld center to base metal decreases and then increases, and the effect of laser power on hardness is not significant. The increased laser power leads to an evident decrease in the ultimate tensile strength and impact toughness of weld joint. The highest ultimate tensile strength and impact energy are 815 MPa, 239.1 J respectively at a laser power of 1.0 kW. A number of inclusions are observed at the bottom of dimples, which may be the (Ti,Mn)2O3 particles.

Effect of microstructure inhomogeneity on hydrogen embrittlement susceptibility of X80 welding HAZ under pressurized gaseous hydrogen

Hydrogen embrittlement (HE) of high-grade pipeline welded joint is a threat to hydrogen gas transport. In this research, slow strain rate tension (SSRT) tests in high-pressure hydrogen gas, combined with hydrogen permeation tests and microstructure analysis were conducted on X80 steel, intercritical heated-affected zone (ICHAZ), fine-grained heat-affected zone (FGHAZ) and coarse-grained heat-affected zone (CGHAZ). The change of HE susceptibility from high to low was CGHAZ, FGHAZ, ICHAZ, and base metal. Microstructure was the important factor influencing hydrogen permeation and susceptibility to HE. Susceptibility to HE was increased in the order of “fine-grained massive ferrite (MF) and acicular ferrite (AF)”, “fine-grained granular bainite (GB) and MF”, “coarse-grained GB and bainite ferrite (BF) embedded with martensite-austenite (M-A) constitute”. The fine-grained MF and AF in base metal with lower hydrogen diffusivity can impede the embrittlement behaviour, while the coarse-grained GB and BF with higher hydrogen diffusivity in CGHAZ increased its susceptibility to HE.

The effect of chemical composition on grain structure and texture evolution of hot rough rolled carbon steels

Combination of thin slab casting and direct hot rolling to obtain thin steel plate products is an energy efficient process in commercial steel plants. Generally, cracking near the surface is the most serious problem especially during rough rolling of as cast thin slabs. Therefore, it is highly recommended that the microstructure near the surface should be controlled to have enough ductility to withstand the strain. In this study the effect of chemical composition on microstructure evolution and texture has been investigated for the steel plates rough rolled from 70 to 25mm at a commercial plant. In low carbon content of 0.05wt%, the micrograph was seen to comprise mainly ferrites with minor pearlite islands which changed to ferrite–pearlite structure with increasing carbon to 0.17wt%. A detailed investigation was carried out to examine the behavior of the grain structure formed in rough rolled steels, using high resolution SEM microscope fitted with EBSD camera. In all investigated steel plates, a fine grain structure was observed in the plate top surface due to development of rapid static recrystallization after rough rolling which gradually coarsened in mid-section region. Investigation was carried out to understand the effect of alloying elements such as Mn on the texture. Mainly fiber textures of both ND and RD directions were observed after transformation showing inhomogeneity with increasing distance from the plate surface towards the depth. Massive ferrite (bcc) grains transforming from austenite (fcc) has been detected during the rough rolling process. Therefore, tensile test was carried out at 600°C in order to investigate the failure mechanism of ferrite as the work roll chilling has a significant effect on temperature reduction in the plate surface which may lead to lower ductility of massive ferrites and thus formation of crack at the surface during rough rolling process.