Effect of simulated thermal cycles on the microstructure of the heat-affected zone in HSLA-80 and HSLA-100 steel plates

The influence of weld thermal simulation on the transformation kinetics and heat-affected zone (HAZ) microstructure of two high-strength low-alloy (HSLA) steels, HSLA-80 and HSLA-100, has been investigated. Heat inputs of 10 kJ/cm (fast cooling) and 40 kJ/cm (slow cooling) were used to generate single-pass thermal cycles with peak temperatures in the range of 750 °C to 1400 °C. The prior-austenite grain size is found to grow rapidly beyond 1100 °C in both the steels, primarily with the dissolution of niobium carbonitride (Nb(CN)) precipitates. Dilatation studies on HSLA-80 steel indicate transformation start temperatures (T _s ) of 550 °C to 560 °C while cooling from a peak temperature (T _p ) of 1000 °C. Transmission electron microscopy studies show here the presence of accicular ferrite in the HAZ. The T _s value is lowered to 470 °C and below when cooled from a peak temperature of 1200 °C and beyond, with almost complete transformation to lath martensite. In HSLA-100 steel, the T _s value for accicular ferrite is found to be 470 °C to 490 °C when cooled from a peak temperature of 1000 °C, but is lowered below 450 °C when cooled from 1200 °C and beyond, with correspondingly higher austenite grain sizes. The transformation kinetics appears to be relatively faster in the fine-grained austenite than in the coarse-grained austenite, where the niobium is in complete solid solution. A mixed microstructure consisting of accicular ferrite and lath martensite is observed for practically all HAZ treatments. The coarse-grained HAZ (CGHAZ) of HSLA-80 steel shows a higher volume fraction of lath martensite in the final microstructure and is harder than the CGHAZ of HSLA-100 steel.     

Effect of simulated thermal cycles on the microstructure of the heat-affected zone in HSLA-80 and HSLA-100 steel plates

The influence of weld thermal simulation on the transformation kinetics and heat-affected zone (HAZ) microstructure of two high-strength low-alloy (HSLA) steels, HSLA-80 and HSLA-100, has been investigated. Heat inputs of 10 kJ/cm (fast cooling) and 40 kJ/cm (slow cooling) were used to generate single-pass thermal cycles with peak temperatures in the range of 750 °C to 1400 °C. The prior-austenite grain size is found to grow rapidly beyond 1100 °C in both the steels, primarily with the dissolution of niobium carbonitride (Nb(CN)) precipitates. Dilatation studies on HSLA-80 steel indicate transformation start temperatures (T _s ) of 550 °C to 560 °C while cooling from a peak temperature (T _p ) of 1000 °C. Transmission electron microscopy studies show here the presence of accicular ferrite in the HAZ. The T _s value is lowered to 470 °C and below when cooled from a peak temperature of 1200 °C and beyond, with almost complete transformation to lath martensite. In HSLA-100 steel, the T _s value for accicular ferrite is found to be 470 °C to 490 °C when cooled from a peak temperature of 1000 °C, but is lowered below 450 °C when cooled from 1200 °C and beyond, with correspondingly higher austenite grain sizes. The transformation kinetics appears to be relatively faster in the fine-grained austenite than in the coarse-grained austenite, where the niobium is in complete solid solution. A mixed microstructure consisting of accicular ferrite and lath martensite is observed for practically all HAZ treatments. The coarse-grained HAZ (CGHAZ) of HSLA-80 steel shows a higher volume fraction of lath martensite in the final microstructure and is harder than the CGHAZ of HSLA-100 steel.     

Effect of microstructure on hydrogen embrittlement of weld-simulated HSLA-80 and HSLA-100 steels

HSLA-80 and HSLA-100 steels have been subjected to weld-simulated grain-coarsened heat-affected zone (GCHAZ) and grain-refined heat-affected zone (GRHAZ) treatments at peak temperatures of 1350 °C and 950 °C, respectively, followed by varying cooling rates to approximate the weld heat inputs of 10 to 50 kJ/cm. Subsequent slow strain rate testing in synthetic seawater has been employed to assess the hydrogen embrittlement (HE) propensity of the materials. It is indicated that in spite of an increase in strength after weld simulation, further ductility deterioration, compared to the base material under similar testing conditions, did not occur in GCHAZ HSLA-100 steel and for low heat input condition of GRHAZ HSLA-80. This has been attributed to their HE resistant microstructures. Predominant acicular ferrite or lath martensite or a combination of both imparts resistance to HE, as observed in the case of grain-coarsened HSLA-100 and for the low heat input grain-refined HSLA-80 steels. The deleterious effect of bainitic-martensitic microstructure has been reflected in the ductility values of grain-coarsened HSLA-80, which is in agreement with the observation of higher susceptibility of the as-received HSLA-100 steel having a similar structure. However, contrary to its beneficial effect in the as-received HSLA-80, an acicular ferrite structure has shown vulnerability toward HE for high heat input grain-refined HSLA-80. This has been attributed to the presence of polygonal ferrite and to the development of an HE susceptible substructure on GRHAZ weld simulation.     

Austenite decomposition during continuous cooling of an HSLA-80 plate steel

Decomposition of fine-grained austenite (10-μm grain size) during continuous cooling of an HSLA-80 plate steel (containing 0.05C, 0.50Mn, 1.12Cu, 0.88Ni, 0.71Cr, and 0.20Mo) was evaluated by dilatometric measurements, light microscopy, scanning electron microscopy, transmission electron microscopy, and microhardness testing. Between 750 °C and 600 °C, austenite transforms primarily to polygonal ferrite over a wide range of cooling rates, and Widmanst?tten ferrite sideplates frequently evolve from these crystals. Carbon-enriched islands of austenite transform to a complex mixture of granular ferrite, acicular ferrite, and martensite (all with some degree of retained austenite) at cooling rates greater than approximately 5 °C/s. Granular and acicular ferrite form at temperatures slightly below those at which polygonal and Widmanst?tten ferrite form. At cooling rates less than approximately 5 °C/s, regions of carbon-enriched austenite transform to a complex mixture of upper bainite, lower bainite, and martensite (plus retained austenite) at temperatures which are over 100 °C lower than those at which polygonal and Widmanst?tten ferrite form. Interphase precipitates of copper form only in association with polygonal and Widmanst?tten ferrite. Kinetic and microstruc-tural differences between Widmanst?tten ferrite, acicular ferrite, and bainite (both upper and lower) suggest different origins and/or mechanisms of formation for these morphologically similar austenite transformation products. Formerly Graduate Student, Department of Metallurgical and Materials Engineering, Colorado School of Mines. This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.     

Microstructural changes in HSLA-100 steel thermally cycled to simulate the heat-affected zone during welding

The microstructural changes that occur in a commercial HSLA-100 steel thermally cycled to simulate weld heat affected zone (HAZ) behavior were systematically investigated primarily by transmission electron microscopy (TEM). Eight different weld thermal cycles, with peak temperatures representative of four HAZ regions (the tempered region, the intercritical region, the fine-grained austenitized region, and the coarse-grained austenitized region) and cooling rates characteristic of high heat input (cooling rate (CR) = 5 °C/s) and low heat input (CR = 60 °C/s) welding were simulated in a heating/quenching dilatometer. The as-received base plate consisted of heavily tempered lath martensite, acicular ferrite, and retained austenite matrix phases with precipitates of copper, niobiumcarbonitride, and cementite. The microstructural changes in both the matrix and precipitate phases due to thermal cycling were examined by TEM and correlated with the results of (1) conventional optical microscopy, (2) prior austenite grain size measurements, (3) microhardness testing, and (4) dilatometric analysis. Many of the thermal cycles resulted in dramatic changes in both the microstructures and the properties due to the synergistic interaction between the simulated position in the HAZ and the heat input. Some of these microstructures deviate substantially from those predicted from published continuous cooling transformation (CCT) curves. The final microstructure was predominantly dependent upon peak temperature(i.e., position within the HAZ), although the cooling rate(i.e., heat input) strongly affected the microstructures of the simulated intercritical and finegrained austenitized regions. A. MATUSZESKI, formerly Summer Student, Physical Metallurgy Branch, Naval Research Laboratory.     

Modeling macro-and microstructures of Gas-Metal-Arc Welded HSLA-100 steel

Fluid flow and heat transfer during gas-metal-arc welding (GMAW) of HSLA-100 steel were studied using a transient, three-dimensional, turbulent heat transfer and fluid flow model. The temperature and velocity fields, cooling rates, and shape and size of the fusion and heat-affected zones (HAZs) were calculated. A continuous-cooling-transformation (CCT) diagram was computed to aid in the understanding of the observed weld metal microstructure. The computed results demonstrate that the dissipation of heat and momentum in the weld pool is significantly aided by turbulence, thus suggesting that previous modeling results based on laminar flow need to be re-examined. A comparison of the calculated fusion and HAZ geometries with their corresponding measured values showed good agreement. Furthermore, “finger” penetration, a unique geometric characteristic of gas-metal-arc weld pools, could be satisfactorily predicted from the model. The ability to predict these geometric variables and the agreement between the calculated and the measured cooling rates indicate the appropriateness of using a turbulence model for accurate calculations. The microstructure of the weld metal consisted mainly of acicular ferrite with small amounts of bainite. At high heat inputs, small amounts of allotriomorphic and Widmanstätten ferrite were also observed. The observed microstructures are consistent with those expected from the computed CCT diagram and the cooling rates. The results presented here demonstrate significant promise for understanding both macro-and microstructures of steel welds from the combination of the fundamental principles from both transport phenomena and phase transformation theory.     

Effect of Heat Input and Niobium on the Microstructure and Toughness of Coarse Grain Heat-Affected Zone in HSLA Steels

 The influence of Nb on the coarse grain heat-affected zone (CGHAZ) microstructure, mechanical property and the transformation kinetics of HSLA steels under different heat inputs, has been investigated. When welded at higher heat inputs (100~60kJ cm-1), impact toughness values of the steel without Nb are much higher than the steel with Nb, and the lowest span is 153J at 60kJ cm-1. But only a little higher values are observed at lower heat inputs (40~30kJ cm-1), and the highest span is 68J at 30kJ cm-1. Dilatation studies indicate continuous cooling transformation start temperatures (Ts) of CGHAZ for the steel with Nb are approximately 15~30°C lower than the steel without Nb at all heat inputs. Under higher heat inputs Nb in solid solution suppress ferrite transformation and promote the formation of granular bainite which has detrimental effect on impact toughness. Under lower heat inputs higher Charpy impact energy values in the steel with Nb are associated with the formation of low carbon self-tempered martensite.     

不同冷却制度下X100管线钢组织特征及力学性能

采用热模拟试验机和试验轧机研究了X100管线钢连续冷却相变规律及不同冷却制度下显微组织特征及力学性能变化规律.研究结果表明:随冷却速度升高及终冷温度降低,试验钢显微组织由针状铁素体过渡至板条贝氏体及马氏体,非淬火条件试验钢中马氏体岛或M-A岛为微孪晶马氏体;轧制后直接以30℃/s冷却至450℃左右时,试验钢具有良好强韧...     

Transformation Characteristics of Ferrite/Carbide Aggregate in Continuously Cooled, Low Carbon-Manganese Steels

Transformation characteristics and morphological features of ferrite/carbide aggregate (FCA) in low carbon-manganese steels have been investigated. Work shows that FCA has neither the lamellae structure of pearlite nor the lath structure of bainite and martensite. It consists of a fine dispersion of cementite particles in a smooth ferrite matrix. Carbide morphologies range from arrays of globular particles or short fibers to extended, branched, and densely interconnected fibers. Work demonstrates that FCA forms over similar cooling rate ranges to Widmanstätten ferrite. Rapid transformation of both phases occurs at temperatures between 798 K and 973 K (525 °C and 700 °C). FCA reaction is not simultaneous with Widmanstätten ferrite but occurs at temperatures intermediate between Widmanstätten ferrite and bainite. Austenite carbon content calculations verify that cementite precipitation is thermodynamically possible at FCA reaction temperatures without bainite formation. The pattern of precipitation is confirmed to be discontinuous. CCT diagrams have been constructed that incorporate FCA. At low steel manganese content, Widmanstätten ferrite and bainite bay sizes are significantly reduced so that large amounts of FCA are formed over a wide range of cooling rates.     

Continuous cooling transformations and microstructures in a low-carbon,high-strength low-alloy plate steel

A continuous-cooling-transformation (CCT) diagram was determined for a high-strength low-alloy plate steel containing (in weight percent) 0.06 C, 1.45 Mn, 1.25 Cu, 0.97 Ni, 0.72 Cr, and 0.42 Mo. Dilatometric measurements were supplemented by microhardness testing, light microscopy, and transmission electron microscopy. The CCT diagram showed significant suppression of polygonal ferrite formation and a prominent transformation region, normally attributed to bainite formation, at temperatures intermediate to those of polygonal ferrite and martensite formation. In the intermediate region, ferrite formation in groups of similarly oriented crystals about 1 μm in size and containing a high density of dislocations dominated the transformation of austenite during continuous cooling. The ferrite grains assumed two morphologies, elongated or acicular and equiaxed or granular, leading to the terms “acicular ferrite” and “granular ferrite,” respectively, to describe these structures. Austenite regions, some transformed to martensite, were enriched in carbon and retained at interfaces between ferrite grains. Coarse interfacial ledges and the nonacicular morphology of the granular ferrite grains provided evidence for a phase transformation mechanism involving reconstructive diffusion of substitutional atoms. At slow cooling rates, polygonal ferrite and Widmanstätten ferrite formed. These latter structures contained low dislocation densities and e-copper precipitates formed by an interphase transformation mechanism.     

Intragranular Ferrite Formation Mechanism and Mechanical Properties of Non‐quenched‐and‐tempered Medium Carbon Steels

The type and size distribution of inclusions in non quenched‐and‐tempered medium carbon steels were investigated quantitatively. The effect of Ti containing complex inclusions on the formation of intragranular ferrite was studied. The continuous cooling transformation (CCT) diagrams of the tested steels were obtained for determining the cooling rate range of the formation of intragranular ferrite. The mechanical properties of the tested steels were determined at room temperature. The results show that with increasing Ti content, the fraction of Ti containing complex inclusions, which could act as nuclei for intragranular ferrite, increased and the inclusion size became smaller. In the cooling rate range of 0.5~2.5°C/s, plenty of intragranular ferrite formed, while at the rate of 2°C/s, the microstructure was mainly acicular ferrite. With the formation of intragranular ferrite, the toughness was enhanced by about 50% keeping the same strength level for the studied steels.     

Welding-Induced Microstructure Evolution of a Cu-Bearing High-Strength Blast-Resistant Steel

A new high strength, high toughness steel containing Cu for precipitation strengthening was recently developed for naval, blast-resistant structural applications. This steel, known as BlastAlloy160 (BA-160), is of nominal composition Fe-0.05C-3.65Cu-6.5Ni-1.84Cr-0.6Mo-0.1V (wt pct). The evident solidification substructure of an autogenous gas tungsten arc (GTA) weld suggested fcc austenite as the primary solidification phase. The heat-affected zone (HAZ) hardness ranged from a minimum of 353 HV in the coarse-grained HAZ (CGHAZ) to a maximum of 448 HV in the intercritical HAZ (ICHAZ). After postweld heat treatment (PWHT) of the spot weld, hardness increases were observed in the fusion zone (FZ), CGHAZ, and fine-grained HAZ (FGHAZ) regions. Phase transformation and metallographic analyses of simulated single-pass HAZ regions revealed lath martensite to be the only austenitic transformation product in the HAZ. Single-pass HAZ simulations revealed a similar hardness profile for low heat-input (LHI) and high heat-input (HHI) conditions, with higher hardness values being measured for the LHI samples. The measured hardness values were in good agreement with those from the GTA weld. Single-pass HAZ regions exhibited higher Charpy V-notch impact toughness than the BM at both test temperatures of 293 K and 223 K (20 °C and –50 °C). Hardness increases were observed for multipass HAZ simulations employing an initial CGHAZ simulation.     

奥氏体状态对 Mn-Cr 齿轮钢连续冷却相变组织的影响

使用Cleeblel500热模拟试验机研究了成分(%)为：0.23C,0.74Mn,0.90Cr 齿轮钢奥氏体晶粒尺 寸和变形(真应变量0.4)对连续冷却相变组织的影响和连续转变冷却(CCT)曲线。实验结果表明,当齿轮钢 未变形时,获得完全多边形铁素体+珠光体混合组织的临界冷速为0.5~1℃/s,冷速较快时,中温相变产物 由贝氏体及针状铁素体组成;奥氏体变形时,多边形铁素体相变开始温度升高,获得完全多边形铁素体+珠光 体混合组织冷速增大,为1~2℃/s,中温相变产物没有出现贝氏体,只有针状铁素体。     

Weldability of 610MPa Grade High Strength Plate Steel 12MnNiVR for Oil Storage Tanks

The weldability of 12MnNiVR was examined in terms of the simulated HAZ continuous cooling transformation (SH-CCT) diagram,microstructure and mechanical properties of the simulated coarse grain heat-affected zone (CGHAZ).When t 8/5 is shorter,the microstructure mainly consists of lath bainite.When t 8/5 is 60 s,the microstructure becomes coarser bainite.Some acicular ferrite appears beside lath bainite when t 8/5 =100s.Finally,a microstructure composed of polygonal ferrite,acicular ferrite,and small amount pearlite is obtained with a small amount of bainite at t 8/5 >100s.With the increase of t 8/5,the hardness of CGHAZ decreases considerably.The minimum impact toughness of CGHAZ appears at t 8/5 =100s.The hardness and the toughness of CGHAZ remain above the specified values for steel 12MnNiVR.     

Microstructural analysis of fracture in heat-affected zone of two ×70 pipeline steel weldments

In the previous study, different crack propagation behaviours (ductile fracture and brittle cleavage fracture) were observed in two ×70 pipeline steel weldments (13.4 and 17.8-mm-thick) during single-edge notched bend testing. To further understand these two fracture behaviours, detailed microstructures of the base metal (BM), fine-grained heat-affected zone (FGHAZ), and coarse-grained heat-affected zone (CGHAZ) of these two ×70 pipeline steel weldments have been analysed. The results show that the initial structure of the two pipe BMs and different welding cooling rates owing to different thicknesses contributed to structural variations of the correlated sub-regions of the HAZ. For both weldments, the FGHAZ close to the BM has the highest fraction of the high-angle grain boundaries, the finest grain size, the lowest local strain levels, and the highest fraction of recrystallised ferrite grains. The CGHAZ of the 17.8-mm-thick pipe welds exhibits the lowest toughness with the highest hardness, a high frequency of deformed grains, the highest local strain level, and the highest density of preferred {100} cleavage planes than the other sub-regions in the HAZ. The high density of the {100}<011> texture components in the HAZ may cause the cleavage micro-cracks to propagate toward the BM at an approximate 45° angle to the original crack plane during bending tests.     

Effects of Mo and Controlled Cooling on Microstructure and Properties of Thick-Wall High-Strength Pipeline Steel

Herein, the effect of molybdenum (Mo) and postrolling cooling processes on the mechanical properties and microstructure transformation characteristics of an X80 thick-wall high-strength pipeline steel are deeply investigated. The results reveal that the yield strength and tensile strength of the steel are enhanced with Mo addition at the equalization temperature of 480 °C at a cooling rate of 25 °C s⁻¹, accompanied by the improvement in the yield ratio. However, when the postrolling cooling temperature is reduced to 380 °C, the mechanical properties of steels with Mo-free and 0.29 wt% Mo are comparable. The mechanistic study indicates that the Mo addition would inhibit the transformation from deformed austenite to polygonal ferrite, and promote the transformation of acicular ferrite/granular bainite at medium–low temperatures, as well as significant differences in the volume fraction, size, and morphology of martensite/austenite (M/A) in the matrix. Notably, the volume fraction of M/A decreases from 7.2% to less than 1.0% with Mo content increasing, while its average size also reduces from 1.5 to less than 1.0 μm. And the fine, spheroidal, and dispersed M/A is found to play a vital role in the high-strength and excellent low-temperature toughness of the steel.     

Effect of Boron on CGHAZ Microstructure and Toughness of High Strength Low Alloy Steels

Effect of boron on the microstructure and impact toughness in the coarse-grained heat-affected zone(CGHAZ)of two high strength low alloy steels,boron-free and boron-containing,was investigated by means of weld thermal simulation test.The result shows that,for the boron-free steel,a microstructure consisting of grain boundary ferrite degenerates pearlite and granular bainite for longer t8/5(the cooling time from 800 to 500 ℃),while lath bainite for shorter t8/5.For the boron-containing steel,granular bainite is dominant for a wide range of t8/5.Continuous cooling transformation(CCT)study on the CGHAZ indicates that the transformation start temperature decreases by about 50-100℃under different t8/5,for the boron-containing steel compared with the boron-free steel.The presence of boron suppresses the nucleation of ferrite at prior austenite grain boundaries and hence enlarges the range of t8/5for granular bainite transformation.However,the addition of boron deteriorates the impact toughness of CGHAZ,which may be due to a markedly increased fraction of martensite-austenite(M-A)constituents and decreased fraction of high angle grain boundaries.     

Influence of V(C,N) Precipitates on Microstructure and Mechanical Properties of Continuous Cooled C-Mn-V

The effect of chemical composition and processing parameters on the formation of acicular ferrite and/or bainite has been investigated.In particular,this paper deals with the influence that N through its combination with V,as V(C,N) precipitates,has on the decomposition of austenite.Likewise,the intragranular nucleation potency of V(C,N) precipitates is analyzed through the continuous cooling transformation diagrams (CCT) of two C-Mn-V steels with different contents of N.Results reported in this work allow us to conclude that acicular ferrite can only be achieved alloying with vanadium and nitrogen,meanwhile bainite is promoted in steels with a low level of nitrogen.It is concluded that higher strength values are obtained in acicular ferrite than in bainitic steel but a similar brittle-ductile transition temperature (BDT),and lower values of impact absorbed energy (KV) has been recorded in nitrogen-rich steel.     

Failure behavior of heat-affected zones within HSLA-100 and HY-100 steel weldments

The deformation and fracture behavior of simulated heat-affected zones (HAZ) within HSLA-100 and HY-100 steel weldments has been studied as a function of stress state using notched and unnotched axisymmetric tensile specimens. For the case of the HSLA-100 steel, the results for fine-grained, as well as coarse-grain HAZ (CGHAZ) material, show that, despite large differences in the deformation behavior when compared to base plate or weld metal, the failure strains are only weakly dependent on the thermal history or microstructure. Ductile microvoid fracture dominates the failure of the HSLA-100 steel with small losses of ductility occurring in the HAZ conditions only at high stress triaxialities. In contrast, the HY-100 steel is susceptible to a large loss of ductility over all of the stress states when subjected to a severe, single-pass simulation of a CGHAZ. The ductility loss is greatest at the high stress triaxiality ratio in which case failure initiation occurs by a combination of localized cleavage and ductile microvoid fracture.