Granular Bainite Research Articles

Effect of Post-Weld Heat Treatment on Microstructure and Fracture Toughness of X80 Pipeline Steel Welded Joint.

In the current study, post-weld heat treatment (PWHT 580 °C) was used for an X80 pipeline steel-welded joint, and the fracture toughness of the welded joint was investigated using a crack tip opening displacement (CTOD) test. The relationship between microstructure evolution and fracture toughness is also discussed in this study. The results showed that the weld center mainly consisted of acicular ferrite (AF). The subcritical heat-affected zone (SCHAZ) consisted of a large amount of fine polygonal ferrite and some AF, and it maintained the rolling state of the base metal. The microstructure of the coarse-grained heat-affected zone (CGHAZ) was composed of granular bainite (GB) and M/A constituents, the latter of which decreased after the PWHT. The CTOD values of the weld center were in the range of 0.18–0.27 mm, while those of the CGHAZ were in the range of 0.02–0.65 mm. A brittle fracture occurred in the CGHAZ for both the as-welded and PWHT samples; the CTOD values were 0.042 mm and 0.026 mm, respectively. The CTOD values of the SCHAZ’s location were in the range of 0.8–0.9 mm. The PWHT did not deteriorate the microstructure of the CGHAZ and had little influence on the fracture toughness of the X80 pipeline steel-welded joint; it ensured the fracture toughness of the welded joints and reduced the welding residual stress.

Effect of deformation amount distribution in the austenite recrystallization temperature region on the microstructure and mechanical properties of the X70 pipeline steel

ABSTRACT In this study, the effect of deformation amount distribution of X70 pipeline steel on the microstructure and mechanical properties in the austenite recrystallization temperature region are investigated. The results reveal that the austenite grain size is smaller and more uniform by increasing the last-pass rough rolling deformation amount in the austenite recrystallization region. An increase in the last-pass rough-rolling deformation amount can also improve the strength and low-temperature toughness. The deformation mechanism and microstructural evolution based on the last-pass in the austenite recrystallization temperature region are clarified. The final microstructure refinement effect of granular bainite and acicular ferrite attribute to the finer and more uniform original austenite grain size. The main strengthening mechanisms are grain boundary strengthening and dislocation strengthening. The high proportion of the high-angle grain boundary is significantly greater for improving the strength and toughness, which impedes dislocations to slip across grain boundaries.

Microstructure evolution and strength-toughness matching mechanism of Fe-Cr-Ni gradient alloy steel prepared by direct laser deposition

A novel 70Cr8Ni2Y alloy steel powder was designed and prepared to meet the requirement for composition design and preparation of gradient alloys with strength-toughness matching the preparation of nuclear power camshafts by direct laser deposition (DLD). A three-layer gradient structure alloy sample with strength-toughness matching was successfully prepared by DLD using 70Cr8Ni2Y, 50Cr6Ni2Y and 12CrNi2Y alloy steel powder. The microstructure evolution and strength-toughness matching mechanism of the gradient sample were studied. The interfaces between the gradient layers were metallurgical bonding and had no defects such as cracks and pores. With the change of the alloy composition of the three gradient layers, the main microstructure of the sample changed from granular bainite to lath bainite, and to martensite in turn. The mechanism of strength-toughness matching was that the changes of the three alloy components made the strength of microstructure increase, and a metallurgical bond formed at the interfaces of the gradient layers. The gradient sample demonstrated strength-toughness matching of high-strength surface and good toughness core through adjusting of the gradient layers and change of microstructure. The high-strength 70Cr8Ni2Y surface layer ensured the high hardness and wear resistance of the gradient sample, and the 50Cr6Ni2Y gradient layer acted as an intermediate layer to support the surface and protect the bottom layer. The 12CrNi2Y gradient layer with good toughness played a role in delaying fracture and hindering crack propagation. The microhardness of the gradient sample showed a gradient distribution of 739-534-321 HV. The average tensile strength was 1076 MPa, the elongation was 1.6%, and the specific wear rate (SWR) of the sample surface was 9.5 × 10−5 mm3/(N·m). This paper can provide a useful reference for the preparation of nuclear power camshaft parts with gradient properties of good toughness of the core and high hardness and wear resistance of the surface by DLD.

Investigation of the hydrogen-induced cracking of E690 steel welded joint in simulated seawater

Hydrogen-induced cracking (HIC) behavior and mechanism of E690 welded joint in simulated seawater were studied via microstructure observations, electrochemical hydrogen charging, slow strain rate tension (SSRT) tests and hydrogen content measurement. The results show that the welded joint of E690 steel has a higher HIC susceptibility than that of base metal, in which the inter-critical heat affected zone (ICHAZ) is composed of polygonal ferrite and granular bainite is the final fracture location in the joint. ICHAZ with the lowest yield strength deformed preferentially under the tensile stress, and subsequently, the proliferating dislocations with the deformation strengthening and the continuously charged hydrogen led to the HIC and brittle fracture at the ICHAZ of the welded joint. Moreover, the HIC susceptibility was linearly dependent on the logarithm of total hydrogen concentration in the steel, and the hydrogen concentration had a more obvious effect on the welded joint than the base metal. The higher density of piled dislocations adjoining the M/A islands in the welded joint exhibited a significant effect on the hydrogen agglomeration and diffusion, which enhanced the HIC susceptibility.

Generation of Acicular Ferrite in the Laser‐Welded Weld of X100 Pipeline Steel Induced by CO2 Shielding Gas

Laser welding has the potential to be used for pipeline laying. Acicular ferrite (AF) is one of the ideal microstructures to improve the strength and toughness of welds. Herein, the effect of CO2 shielding gas on the microstructure evolution in the laser‐welded weld of X100 pipeline steel is studied. The results show that CO2 shielding gas can induce a large number of Ti2O3‐Al2O3‐MnO‐SiO2 composite inclusions with a size of about 1 μm in the weld. The Ti2O3 in the inclusions promotes the formation of the Mn‐depletion zone (MDZ), resulting in the generation of AF with a volume fraction of 90.6% in the weld. The sizes of AF grains surrounded by high‐angle grain boundaries range from 1 to 3 μm, while the microstructure of the Ar shielded weld is mainly granular bainite. Inclusion mainly contains Al2O3, which cannot promote the formation of MDZ in the Ar shielded weld. The study provides a new method to induce AF in the weld of X100 pipeline steel during laser welding, and it is also useful for steels containing Ti and Mn elements.

Effect of post-heat treatment on microstructure and properties of EH47 steel CGHAZ

To clarify the effect of post-heat treatment on the microstructure and mechanical properties of the coarse-grained heat-affected zone (CGHAZ) for EH47 steel, simulated CGHAZ specimens were subjected to post-heat treatment at 250°C for 1.0 h. The results showed that the microstructure of CGHAZ was composed of granular bainite (GB) and lath bainite (LB), and a large number of iron-carbon particles (∼81 nm) were precipitated after post-heat treatment. The relationship between dislocations, low-angle grain boundaries and iron-carbon particles was revealed by KAM values. The iron-carbon particles hindered the movement of sub-grain boundaries and dislocations, thereby increased the yield strength (∼120 MPa). The precipitation of iron-carbon particles caused the cracking of bainite ferrite and reducing the fracture toughness of CGHAZ.

Genetic relationship of microstructure and mechanical properties between ferrite/pearlite steel plate and billet

This paper proposed the double-pass compression thermal/force simulation experiment to in situ analyze the phase transformation of continuous casting billet during the controlled rolling and controlled cooling process. The genetic relationship between the central segregation of the billet and solute element distribution of the steel plate, and also the genetic relationship of microstructure and mechanical properties between them were experimentally studied. The results show that the microstructure and mechanical properties between the billet and corresponding batch of steel plate have the genetic relationship, and the link of the relationship is central segregation. The phase transformation of supercooling austenite will be affected by the central segregation of Mn and C elements, and the granular bainite abnormal segregation band is the transformation product of Mn and C element segregation region.

Effect of Ni addition upon microstructure and mechanical properties of hot isostatic pressed 30CrMnSiNi2A ultrahigh strength steel

Hot isostatic pressing is an advanced powder metallurgical technology for the production of complex and special-shaped components made of ultrahigh strength steel. However, due to the low hardenability of 30CrMnSiNi2A steel, it is difficult to achieve the expected martensitic structure in the sintering process. In this study, the hardenability of the alloy is improved by adding Ni into 30CrMnSiNi2A steel powder. The microstructure, tensile properties and fracture behavior of the sintered 30CrMnSiNi2A-xNi steels (x = 0, 1.5 wt%, and 3.0 wt%) are systematically investigated. Experimental results show that with the increase in Ni addition, the matrix structure changes from granular bainite (0 wt% Ni) to lath bainite (1.5 wt% Ni) and lath martensite (3.0 wt% Ni); and the ultimate tensile strength increases from 1312 MPa to 1466 MPa and 1747 MPa. The improvement in strength is mainly related to the solid solution strengthening of C and Ni atoms in the matrix structure. In addition, the increase in dislocation density and grain boundary density in the matrix structure also promotes the improvement in strength, according to the kernel average misorientation and grain boundary analysis of EBSD. After the addition of Ni powder, the fracture mode changes from the transgranular quasi-cleavage to the interparticle debonding. This is mainly due to the increase in the oxide density at the particle boundaries and the matrix strength.

Impact of welding simulated heat treatment on hydrogen embrittlement behavior of high-strength fine-grained steels

Hydrogen embrittlement (HE) sensitivity of S690QL structural steel and X80 pipeline steel in both as delivered and welding simulated heat treatment conditions have been investigated by slow strain rate test (SSRT) with in-situ hydrogen charging. Both investigated steel grades show low strength losses, however, high ductility losses, represented by losses in total elongation and area reduction after in-situ hydrogen charging at a constant current density. The resulting diffusive hydrogen contents after charging are associated with the material microstructure, which are 2–3 ppm for S690QL and 1–2 ppm for X80. The lath martensite dominated microstructure in heat-treated S690QL specimens revealed extremely high HE sensitivity as demonstrated by fracture occurring within the elastic deformation regime. Hydrogen induced damage was found associated with the brittle nature of the quenched lath martensite, the severe grain coarsening, and the large fraction of high angle grain boundaries. The X80 pipeline steel reveals a transition from granular bainite dominated microstructure to lath bainite/martensite dominated microstructure after welding simulated heat treatment. The uptake of diffusive hydrogen during the in-situ charging process of X80 specimens was less compared to S690QL specimens, which was also desorbed at higher temperatures as characterized by hydrogen thermal desorption analysis. The results suggest a controlled bainitic/martensitic microstructure transformation with designed precipitation hardening would contribute to both strength enhancement and the suppression of hydrogen mobility.

Corrosion behavior and mechanism of the high‐strength low‐alloy steel joined by multilayer and multipass welding method

AbstractThe corrosion behavior and mechanism of the high‐strength low‐alloy steel‐welded joint fabricated by the multilayer and multipass welding method were investigated using a scanning Kelvin probe, electrochemical measurements, and so forth. The results revealed that the microstructure of the first layer welding zone was dominated by granular bainite and acicular ferrite and was fine and uniform, which exhibited the best corrosion resistance. Whereas, since the average cooling rate decreased with the increase of welding pass, the grain size of the second and third layer weldings gradually grew, and the voltaic potential gradually decreased. In addition, the microstructure of the heat‐affected zone (HAZ) changed from the tempered sorbite structure of the equilibrium phase to the granular bainite or bainite structure of the nonequilibrium phase under the action of heat transfer. The HAZ became the weakest link for corrosion of welded joint, on account of the nonequilibrium organization and galvanic coupling among base metal, weld metal and HAZ.

Effect of different strains on acicular ferrite and martensite-austenite islands in oxide metallurgy steel

The microstructure transformation of oxide metallurgy steel was studied by deforming to strains of 0.1–1 at strain rates of 1 s−1 at 900 °C. The variation trends of AF laths and M-A islands size with increasing strain were investigated. The kernel average misorientation(KAM) data obtained by EBSD are used to calculate the geometrically necessary dislocation(GND) density of AF laths (ρAFGND) induced by inclusions and matrix (ρAveGND). The results showed that the large number of dispersed spherical inclusions in experimental steel can effectively induce the nucleation of AF under different strains, which is composed of Al–Ti–Mg–Zr–O and MnS. With the increase of strain, the deformation-induced ferrite transformation(DIFT) behavior dominates the microstructure evolution, the nucleation and growth of more ferrite led to the gradual increase of HAGBs and decrease of ρAveGND, and the microstructure transformed from granular bainite(GB) to granular structure(GS). The size and aspect ratio of AF laths and M-A islands gradually decreased, and the M-A islands gradually replaced by more carbides distributed inside the grains and on the grain boundaries. The growth of AF laths induced by inclusions is hindered by the influence of pre-nucleated proeutectoid ferrite, resulting the increasing of ρAFGND. Meanwhile, the AF laths nucleated on inclusions satisfies the K–S orientation relationship with parent austenite.

Effect of Heat-Input on Microstructure and Toughness of CGHAZ in a High-Nb-Content Microalloyed HSLA Steel.

The effect of various heat inputs on the microstructure and impact toughness of the simulated coarse-grained heat-affected zone (CGHAS) of a niobium microalloyed (0.14 wt.%) low-carbon steel was studied. The results showed that higher impact toughness was achieved at a low heat input of 20 kJ/cm, which resulted from the formation of acicular ferrite laths/plates. They sectioned large prior austenite grains into many smaller regions, resulting in smaller crystallographic grains and high-angle grain boundaries. Conversely, when specimens were simulated with larger heat-inputs (100, 200 kJ/cm), the microstructure of the CGHAZ was predominantly composed of granular bainite plus massive MA constituents, thus impairing the impact toughness.

The Influence Mechanism of HSLA Weld Metal with Different Composition on High‐Temperature Strength

In order to study the influence mechanism of HSLA weld metal with different composition on high‐temperature strength, four kinds of HSLA weld metal are designed by using low‐carbon Si–Mn and Si–Mn–Ni alloy system. Different samples are prepared by simple process of normalizing and high‐temperature tempering. The experimental results show that the weld metal is mainly composed of polygonal ferrite, quasipolygonal ferrite, pearlite, and granular bainite. Ni can change the microstructure shape of reheating zone and inhibit the formation of granular bainite. When the tempering temperature is lower than 580 °C, the strength increases. After tempering at 580 °C, the tensile strength and yield strength reach the peak value. When the tempering temperature exceeds 580 °C, the strength begins to decrease. The microhardness of the two alloy systems decreases gradually from columnar crystal zone to recrystallization zone. The impact absorption energy of the four samples is greater than 200 J at 600 °C. Under different temperature conditions, the content of Ni in all second‐phase particles is less than 10%, the content of Mn is lower, and the content of Si is higher.

Corrosion Behavior of Multiphase Bainitic Rail Steels

Pearlitic steel experiences excessive corrosion in a hot and humid atmosphere. The multiphase bainitic/martensitic structure was developed for a better combination of strength and ductility, especially rolling contact fatigue, but little attention to corrosion has been investigated. Corrosion behaviors of multiphase steels obtained from bainitic-austempering (BAT) and bainitic-quenching and -partitioning (BQ&P) processes were investigated via immersion and electrochemical tests in 3.5 wt.% NaCl solution. The corroded surface and rust after immersion and electrochemical tests were analyzed via electron microscopy, Fourier transform infrared spectra, and x-ray diffraction. The multiphase bainite + martensite/retained austenite island showed higher corrosion resistance than that of the pearlitic one. The acicular bainite obtained from the BQ&P process showed slightly higher corrosion resistance than the granular bainite + martensite structure obtained from the BAT process.

Effects of granular bainite and polygonal ferrite on yield strength anisotropy in API X65 linepipe steel

Linepipe steels fabricated using thermo-mechanically controlled processes exhibit microstructural inhomogeneity and a characteristic texture; therefore, they often lead to anisotropic yield strength properties. Although yield strength anisotropy is considered as a major guaranteed property, particularly in pipe forming processes, systematic verification of the dominant microstructural component affecting the yield strength is challenging owing to complicated microstructures and inhomogeneous distributions. In this study, the microstructures were controlled by varying the rolling reduction ratios and start rolling temperatures, specifically for polygonal ferrite (PF) and granular bainite (GB); the Taylor factor (TF) and orientation distribution function (ODF) of each microstructure were quantitatively analyzed by electron backscatter diffraction to study the effects of microstructural characteristics on the yield strength. The deviation of TF with tensile direction in GB was larger than that in PF; the intensity of {113}<110> components in ODF maps was greater in GB than in PF. The results indicated that GB induced more yield strength anisotropy than PF; thus, steels with a greater GB fraction exhibited greater yield strength anisotropy. This study can be used for lowering the yield strength anisotropy in linepipe steel plate design, with promising prospects for wider industrialization of high-strength linepipe steels.

Effect of Microstructural Evolution on the Mechanical Properties of Intercritical Heat‐Affected Zone of Quenched‐and‐Tempered Ultrahigh‐Strength Steel

In order to investigate the effect of microstructural evolution on the mechanical properties in the intercritical heat‐affected zone (ICHAZ) of the quenched‐and‐tempered ultrahigh‐strength steel, the simulated ICHAZ samples with different austenite transformation degrees (0%, 25%, 50%, 75%, and 100%) are prepared. These samples are named IC‐0%, IC‐25%, IC‐50%, IC‐75%, and IC‐100%, respectively. The fine‐grained fresh martensite (FM) is produced in the partial austenite transformation during cooling. The austenite in IC‐100% with the lowest C and Mn content is transformed into a mixture of FM and granular bainite (GB). The IC‐0% and the IC‐25% are softened by about 20 HV10 compared with that of the base metal. When the matrix is transformed into quasi‐polygonal ferrite (QPF), the large difference in microhardness between FM and QPF causes the instability of their interfaces, resulting in the deterioration of the impact toughness in IC‐50% and IC‐75%. However, from IC‐75% to IC‐100%, the impact toughness has been improved from 18.0 to 28.9 J. In addition to the less embrittlement of microstructure in IC‐100%, the lower microhardness difference between FM and GB is the main reason.

Effect of Surface Hardness and Hydrogen Sulfide Partial Pressure on Sulfide Stress Cracking Behavior in Low Alloy Linepipe Steel

TMCP (thermo-mechanical controlled process) linepipes have long been used in severe sour environments, but recently sulfide stress cracking (SSC) caused by local hard zones has become a concern. In order to clarify the hardness threshold that leads to SSC, four-point bend (4PB) SSC tests as specified in NACE TM0316 were conducted under several H2S partial pressure conditions. At H2S partial pressures of 1 bar and higher, the surface hardness threshold (at 0.25 mm from the surface) observed in 4PB SSC specimens without SSC cracking was approximately correlated to a maximum acceptable hardness level of 250 HV0.1. A stable low surface hardness of 250 or less HV0.1 was achieved by suppressing hard lath bainite (LB) and obtaining a soft granular bainite (GB) microstructure, resulting in a superior SSC-resistant property. It was found that a SSC crack propagated when the surface hardness increased due to an increasing volume fraction of the LB microstructure. Under a 16 bar H2S partial pressure condition, the crack growth rate increased in the sour environment, and hydrogen embrittlement by H2S was promoted. However, in the 4PB SSC test at 16 bar, the stress concentration and the transition to a crack were suppressed because the shape of localized corrosion was semicircular due to low localized corrosivity. This can be the reason why the SSC susceptibility was similar to 1 bar condition, especially in the 4PB SSC test using the samples with lower surface hardness level of 250 or less HV0.1.

Effects of cooling media on the formation of Martensite-Austenite microconstituent in a HSLA steel

The effect of different cooling conditions on the formation of Martensite-Austenite (MA) in a High - Strength Low - Alloy (HSLA) steel was assessed. The MA constituent is detrimental to impact toughness in pipeline applications, so the purpose of this research was to minimize its presence through the choice of effective cooling media and optimal parameters such as the cooling rate and final cooling temperature. The volume fraction, size and morphology of MA was evaluated by selective etching and corroborated trough SEM and EBSD. Vickers hardness testing agreed with the measured MA volume fractions. The sample cooled with helium gas and salt bath with the lowest final cooling temperature of 460 °C, exhibited a fine mixture of ferritic bainite, granular bainite and the lowest volume fraction of MA, along with MA smaller particle average size. A high cooling rate and a decrease in the final cooling temperature resulted in a decrease in the volume fraction and average particle size of MA.

Effect of Austenite Grain Size on the Bainitic Transformation in a 690 MPa Grade High-Strength Multi-Functional Construction Steel

A high-strength low-carbon construction structural steel was investigated in the laboratory. The various austenite grain sizes were obtained by austenitizing the steel at different temperatures. The effect of austenite grain size on bainite transformation was studied by the dilatometer. The results show that the microstructure of high-strength low-carbon structural steels mainly includes granular bainite, lath-like bainite and martensite-austenite (M-A). The microstructure changes from granular bainite to lath-like bainite with the increase in austenitizing temperature or austenite grain size. When the samples were heated at the lower temperature of 860 °C, the bainite starting temperature was relatively high, which was mainly attributed to the promotion of the granular bainitic nucleation and the formation of the solute-depleted regions in the austenite. Compared to 860 and 1260 °C, the bainite transformation rate in the specimen austenitized at 1000 °C is the highest because of the small prior austenite grain size and larger transformation driving force.

Effects of granular bainite and polygonal ferrite on yield point phenomenon in API X65 line-pipe steels

Yield strength and yield-point (YP) phenomenon are regarded as crucial properties to be assured in practical applications such as line-pipe forming. In this work, the impacts of microstructural constituents on YP and yield ratio were explored in four API X65 line-pipe steel plates fabricated by altering the start or finish rolling temperature and plate thickness. The four steel plates were predominantly made up of granular bainite (GB) and polygonal ferrite (PF), along with the greater volume proportion of PF in the case of the lower start and finish rolling temperatures and the thicker plate thickness. According to the skin-pass rolling to reduce the YP elongation, the increased amount of mobile dislocations was greater in the PF than in the GB, indicating that the PF was more responsible for the YP alleviation than the GB because the more mobile dislocations were formed in the PF than in the GB. Thus, the decreasing rate of YP elongation was faster for the thicker plate and the low start or finish rolling temperature. On the other hand, the lower yield ratio was preferred for the thicker plates as they exhibited the shorter YP elongation and the higher strain-hardening rate, thereby leading to the rapid enhancement of yield strength in the YP range and consequently to the continuous increase in yield ratio. Thus, this work would be favorably utilized for designing optimal microstructural and TMCP parameters for securing the yield-strength guarantee in line-pipe steels.