Simulated Coarse Grain Research Articles

Simulated Coarse Grain Heat‐Affected Zone Transformation Behavior of Newly Developed Zr–Ti–RE Deoxidized High‐Strength Low‐Alloy Steels with Extremely High Impact Toughness

The continuous cooling transformation behavior of the Zr–Ti–RE deoxidized high‐strength low‐alloy steel in the simulated coarse grain heat‐affected zone at different cooling rates (corresponding to different t8/5) is investigated by a Gleeble 3800 thermomechanical simulator in combination with microstructure and mechanical properties analysis. The continuous cooling transformation curve in the simulated heat‐affected zone is plotted, and the transformation law of M/A constituents is systematically investigated. It is found that with the increase of the cooling rate, the dominated microstructure is transformed from pearlite + ferrite into granular bainite and, finally, into lath bainite with the further refinement of M/A constituents. The steel exhibits excellent V‐notch Charpy impact properties at −20 °C under typical heat inputs (20, 100, 500 kJ cm−1). Moreover, acicular ferrite formed on spherical Zr–Ti–RE composite inclusions are observed at the heat input of 100 kJ cm−1. The low‐temperature impact absorbed energy gradually decreases with the increasing heat input due to the coarsening of the microstructure, larger effective grain size, and the reduction in the number of high‐angle variant pairs.

Creep-fatigue interaction behavior of simulated microstructures and the actual weldment of P91 steel

The paper presents first of its kind study on the creep-fatigue interaction (CFI) behavior of simulated coarse grain (CG), fine grain (FG) and inter-critical (IC) heat-affected zones (HAZs) of P91 steel weldment. Total axial strain-controlled fatigue tests were conducted at 550 °C using trapezoidal waveform and introducing 30-min hold at peak tensile strain of ±0.6%. Irrespective of the initial microstructure, the alloy exhibited continuous cyclic softening throughout the deformation under CFI condition. Strain localization together with interlinkage of subsurface creep cavities caused enhanced crack propagation that resulted in the lowest fatigue life in the case of ICHAZ. The actual weldment failed in the interface between FGHAZ and ICHAZ. The mode of failure was found to be largely governed by Type IV cracking.

Direct Observation of Austenite and Pearlite Formation in Thermally Simulated Coarse Grain Heat-Affected Zone of Pearlite Railway Steel

In the present study, coarse grain heat affected zone (CGHAZ) of the pearlite railway steel was thermally simulated at different cooling rates ranging from 1 to 4 °C/s to in–situ observe the austenite and pearlite formation by high temperature laser scanning confocal microscopy (HT–LSCM). During heating, austenite nucleation was started at 811 °C at fast heating rate of 9.1 °C/s. Pearlite was completely decomposed into austenite in 21 s at 1005 °C. At peak temperature of 1300 °C, austenite was composed of two microstructural compositions, i.e., high-carbon austenite and low-carbon austenite. Homogenization of austenite and formation of grain boundaries were continued even during cooling when the temperature was above the pearlite transformation. During cooling, Low-carbon austenite has increased the pearlite transformation by providing additional nucleation sites. By increasing the cooling rate from 1 to 4 °C/s, the pearlite transformation temperature was reducing, pearlite growth rate was increasing, and pearlite interlamellar spacing was becoming narrow, respectively. Mathematical interpretation of pearlite growth rate was developed that gives true changing behavior of pearlite growth rate with the varying cooling rate. Present study provides direct observation and unique quantitative information of austenite and pearlite formation in CGHAZ in pearlite railway steel.

Effect of boron on intragranular ferrite nucleation mechanism in coarse grain heat-affected zone of high-nitrogen steel

Abstract The effect of boron on intragranular ferrite (IGF) nucleation mechanism in the simulated coarse grain heated-affected zone (CGHAZ) of a high-nitrogen steel was investigated. When boron (B) was added to the steel, the ferrite transformation temperature increased from 775 °C to 800 °C and the amount of polygonal ferrite increased from 63.2% to 78.6%. Consequently, the ductile-to-brittle transition temperature decreased from −30 °C to −50 °C. (Ti,V)(C,N)-MnS-BN, MnS-BN, Al2O3-MnS-BN, and BN served as IGF nucleation sites. The core of the complex precipitates was formed by undissolved (Ti,V)(C,N)-MnS, Al2O3-MnS, and MnS, with a BN cap formed on the undissolved precipitates, which increased the IGF nucleation potency.

Effective microstructure unit in control of impact toughness in CGHAZ for high strength bridge steel

The high strength bridge steel was processed with the simulated coarse grain heat affected zone (CGHAZ) thermal cycle with heat input varying from 30 to 60 kJ/cm, the microstructures were investigated by means of optical microscope (OM), scanning electron microscope (SEM), electron backscattering diffraction (EBSD) and transmission electron microscope (TEM), and the impact properties were evaluated from the welding thermal cycle treated samples. The results indicate that the microstructure is primarily composed of lath bainite. With decreasing heat input, both bainite packet and block are significantly refined, and the toughness has an increasing tendency due to the grain refinement. The fracture surfaces all present cleavage or fracture for the samples with different heat inputs. Moreover, the average cleavage facet size for the CGHAZ is nearly equal to the average bainite packet size and the bainitic packet boundary can strongly impede the crack propagation, indicating that the bainitic packet is the most effective unit in control of impact toughness in the simulated CGHAZ of high strength bridge steel.

The Role of the Bainitic Packet in Control of Impact Toughness in a Simulated CGHAZ of X90 Pipeline Steel

X90 pipeline steel was processed with the simulated coarse grain heat affect zone (CGHAZ) thermal cycle with heat input varying from 30 kJ/cm to 60 kJ/cm, the microstructures were investigated by means of optical microscope (OM), scanning electron microscope (SEM), electron backscattering diffraction (EBSD), and transmission electron microscope (TEM), and the impact properties were evaluated from the welding thermal cycle treated samples. The results indicate that the microstructure is primarily composed of lath bainite. When decreasing the heat input, both bainite packet and block are significantly refined, and the toughness has an increased tendency due to the grain refinement. The fracture surfaces all present cleavage fracture for the samples with different heat inputs. Moreover, the average cleavage facet size for the CGHAZ is nearly equal to the average bainite packet size, and the bainitic packet boundary can strongly impede the crack propagation, indicating that the bainitic packet is the most effective unit in control of impact toughness in the simulated CGHAZ of X90 pipeline steel.

Effect of Heat Input on Cleavage Crack Initiation of Simulated Coarse Grain Heat-affected Zone in Microalloyed Offshore Platform Steel

The combined effects of martensite-austenite (MA) constituent and pearlite colony on cleavage crack initiation in the simulated coarse-grained heat-affected zone (CGHAZ) of V-N-Ti microalloyed offshore platform steel under different heat inputs were investigated. The results of welding simulation, instrumented impact test, and quantitative analysis indicated that the size of the MA constituent decreased with the increase in cooling time, and by contrast, the size of the pearlite colony increased. According to Griffith theory, the critical sizes of cleavage microcracks were calculated. With the increase of cooling time, the calculated microcrack size could be characterized by the size of the MA constituent first, and then fitted with the size of the pearlite colony. Moreover, the calculated microcrack size variation was opposite to the microcrack initiation energy. This phenomenon is probably due to the combined effects of the MA constituent and pearlite colony with increasing the cooling time of the specimen's temperature from 800 to 500 °C.

Effect of heat input on impact toughness in transition temperature region of weld CGHAZ of a HY 85 steel

Physical weld simulated coarse grain heat affected zone (CGHAZ) specimens of HY 85 steel with heat inputs 15, 22, 50kJ/cm were prepared by thermo mechanical simulator. Impact toughness was determined at test temperatures from room temperature to −196°C. Ductile to brittle transition temperature (DBTT) were determined by two methods (i) average impact toughness value of upper and lower shelf energy and (ii) fracture appearance transition temperature (FATT) from the SEM fractographs. Impact toughness and hardness values have been found to decrease with increase in heat input. This decrease is attributed to slower cooling rate, increase in prior austenite grain size and increase in width of bainitic ferrite lath. DBTT was determined for base metal and weld CGHAZ specimens based on upper and lower shelf energy criterion. FATT method based on SEM fractographs is applicable only for base metal but not for weld CGHAZ specimens because they were fully brittle in character. Best impact toughness and hardness for heat input 22kJ/cm observed are 66J and 322 VHN respectively.

Effect of nitrogen content on the second phase particles in V–Ti microalloyed shipbuilding steel during weld thermal cycling

The microstructures, mechanical properties, and second-phase particles in the base plate and simulated coarse grain heat affected zone (CGHAZ) of V–Ti microalloyed steels with differing amounts of nitrogen were investigated. A Gleeble-3800, submerged arc welding, and transmission electron microscope were used during this investigation. The results showed that austenite grain size was refined and microstructure and toughness of CGHAZ improved with increasing nitrogen. Furthermore, in the case of low- nitrogen steel, base plate consisted mainly of Ti-rich (Ti, V) (C, N), which were coarsened after welding thermal cycling, thereby leading to coarsening of the austenite grains. In the case of high-nitrogen steels, base plate consisted mainly of Ti-rich (Ti, V) (C, N), V-rich (V, Ti) (C, N), and V (C, N) particles. During welding heating, Ti-rich (Ti, V) (C, N) dissolved only partially, whereas V-rich (V, Ti) (C, N) and V (C, N) particles dissolved completely. Fine Ti-rich (Ti, V) (C, N) and V (C, N) re-precipitated during cooling stage of welding. The sub-50-nm Ti-rich (Ti, V) (C, N) particles present at peak temperature and during cooling, were effective in refining the austenite grain size, by preventing migration of austenite grain boundaries. The nitrogen-enhanced V–Ti steels exhibit excellent toughness for high heat inputs.

Effect of thermo-mechanical cycling on the microstructure and toughness in the weld CGHAZ of a novel high strength low carbon steel

We have studied the microstructural evolution in the simulated coarse grain heat affected zone (CGHAZ) of novel low carbon microalloyed steel with yield strength of 1000MPa using electron microscopy, while the crystallographic characteristics were studied by electron backscatter diffraction (EBSD). The deterioration in low temperature toughness after the simulated welding cycle was attributed to the formation of coarse blocky M–A constituent. However, the lower bainite microstructure of the matrix was beneficial for low temperature impact toughness because of similarity in the crystal structure of variants of lath martensite, which were present in high proportion at the high angle grain boundaries. A high fraction of small M–A constituent also influenced impact toughness.

Some Studies on Heat-Affected Zone (HAZ) Toughness Behavior of API 5L X52 Steel

Heat Affected Zone (HAZ) simulation by Gleeble is a useful method for evaluating welding process parameters to improve the toughness of weld HAZ by optimizing process parameters. The HAZ Toughness in low alloy steels is strongly influenced by the welding parameters (as HAZ microstructure is influenced by the heat input and cooling rate). In the present work different regions of weld HAZ of API 5L X52 steel have been simulated by using Gleeble @ 3800 Thermal- Mechanical Simulator. Two cooling rates (t 8/5 ) 10 s to simulate low heat input (LHI) welding conditions without preheating, (t 8/5 ) 40s to simulate high heat input (HHI) welding were used. Charpy V-notch impact tests were conducted on the simulated as well as on the base material at room temperature (25° C) and at -40° C. Results showed that HHI simulated coarse grain HAZ (CGHAZ) region attributed highest Impact energy of 300 J in comparison to 199 J of base material at 25° C. In HAZ regions, lowest room temperature toughness was observed in sub critical HAZ (SCHAZ) for both HHI and LHI conditions. Lowest impact energy at -40° C was observed in fine grained HAZ (FGHAZ) for both HHI and LHI conditions. Hardness test showed that LHI samples exhibited slightly higher hardness than HHI condition and the hardness profile for HAZ regions was similar for both HHI and LHI conditions.

Some Studies on Heat-Affected Zone (HAZ) Toughness Behavior of API 5L X52 Steel

Heat Affected Zone (HAZ) simulation by Gleeble is a useful method for evaluating welding process parameters to improve the toughness of weld HAZ by optimizing process parameters. The HAZ Toughness in low alloy steels is strongly influenced by the welding parameters (as HAZ microstructure is influenced by the heat input and cooling rate). In the present work different regions of weld HAZ of API 5L X52 steel have been simulated by using Gleeble @ 3800 Thermal- Mechanical Simulator. Two cooling rates (t 8/5 ) 10 s to simulate low heat input (LHI) welding conditions without preheating, (t 8/5 ) 40s to simulate high heat input (HHI) welding were used. Charpy V-notch impact tests were conducted on the simulated as well as on the base material at room temperature (25° C) and at -40° C. Results showed that HHI simulated coarse grain HAZ (CGHAZ) region attributed highest Impact energy of 300 J in comparison to 199 J of base material at 25° C. In HAZ regions, lowest room temperature toughness was observed in sub critical HAZ (SCHAZ) for both HHI and LHI conditions. Lowest impact energy at -40° C was observed in fine grained HAZ (FGHAZ) for both HHI and LHI conditions. Hardness test showed that LHI samples exhibited slightly higher hardness than HHI condition and the hardness profile for HAZ regions was similar for both HHI and LHI conditions.

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.

Effect of Cooling Rate on Microstructure of Simulated CGHAZ for Modified High-Cr Ferritic Heat-Resistant Steel

The microstructure and carbide precipitate of simulated coarse grain heat affected zone(CGHAZ) in modified high Cr ferritic heat-resistant steel at different cooling rates have been investigated by means of thermal simulator, optical microscope, SEM and TEM . It was found that the microstructure of CGHAZ of testing steel was mainly lath martensite and δ-ferrite under the different welding thermal cycles. However, the prior austenite grain size reduced with increasing the cooling rate. Furthermore, with increasing the cooling rate, the amount of carbide precipitate inside laths of martensite increased, and the size and morphology of precipitates have changed from elongated and coarse to needlelike and fine.

Study in Simulated Heat-Affected Zone of Ship Steel

The experiments were carried out upon the determination of simulated heat-affected zone continuous cooling transformation (SH-CCT) diagrams, the characteristics of microstructure and Vickers hardness of SH-CCT specimens, and impact toughness in simulated coarse grain heat-affected zone (CGHAZ) of ship steels under different heat input based on physical simulation. The SH-CCT diagram reveals that bainite is always obtained in a wide range of cooling rates. When the maximum cooling rate reaches 100 °C/s (t8/5=3 seconds), the maximum fraction of martensite (8%) is obtained and the microstructures mainly consist of lath bainite and the hardness is only 255 HV. This demonstrates that the steel has a low quench-hardening tendency and excellent resistance to cold cracking. There are no obvious hardening and softening phenomena in simulated CGHAZ. Test results of impact toughness under different heat input in simulated CGHAZ show that the impact energies reach over 30 J at -40 °C when t8/5 is less than 20 s, meeting the stipulated requirements of ship steel (≥22 J at -40 °C) but no great allowance. Thus, to meet the requirement of properties during welding, it is proposed to choose t8/5 ranging from 5 to 20 s, correspondently the line energies ranging from 14 to 37 KJ/cm for 30 mm thick plate.

Microstructures and Toughness in Simulated Coarse Grain Heat-Affected Zones of Copper-Bearing Steel

Microstructures and toughness of simulated coarse grain heat-affected zone of hot continuously rolled copper-bearing steel were investigated using physical simulation. The results showed that brittlement is easy to happen in the region of CGHAZ with slower thermal cycles (t8/5≥45s). Granular bainite transformed from austenite led to brittlement. The dimensions of granular martensite and austenite (M-A) constituents are main factors influencing the impact toughness. There is no visible effect on the toughness when the dimensions of M-A constituents are less than 1μ m. However, the toughness decreases greatly once the dimensions exceed 1μ m. Therefore, decreasing the dimensions of M-A constituents by controlling weld heat input will do good to improve the impact toughness of copper-bearing steel.

Microstructures and Properties in Simulated Heat-Affected Zones of 685 MPa Grade Copper-Bearing Steel

Abstract The experiments were carried out upon the determination of SH-CCT diagrams and the characteristic of microstructures and properties in simulated coarse grain heat-affected zone of hot continuously rolled copper-bearing steel under single and double welding thermal cycle using physical simulation. The results show that with the help of SH-CCT diagram of copper-bearing steel, it is proposed to choose t8/5 ranging from 7s to 35s during welding. The copper-bearing steel has a narrow range of heat-input and brittlement is easy to happen in the region of CGHAZ with higher heat-input. Granular bainite transformed from austenite leads to brittlement. Softening starts when t8/5 is more than 7s. The dissolution of ɛ-Cu and coarse lath bainite and more ferrite cause softening. Moreover, the impact toughness decreases dramatically and obvious brittlement happens in the intercritical region of CGHAZ. The reason is pearlite formed on the interface of original austenite and coarse granular bainite, which can reduce the impact toughness.

Effect of Welding Heat Input on Simulated HAZ Microstructure and Toughness of a V-N Microalloyed Steel

Abstract This study investigates the correlation among the welding heat input, microstructure and impact toughness of the simulated coarse grain heat-affected zone (CGHAZ) of a V-N microalloyed steel which contained 140 ppm of nitrogen using Gleeble simulation technique, light microscope, electron microscope and Charpy-V-Notch (CVN) testing. CVN toughness estimated at −20°C supported that a relatively low energy input is recommended for welding of low carbon V-N microalloyed steel, according to a fact that an impact energy as high as of more than 180j attained in the CGHAZ with heat input less than 25Kj-cm−1, while a CGHAZ with impact energy less than 80j was obtained at a heat input more than 40 Kj-cm−1. Quantitative metallography of samples showed that CVN toughness decreased with the increase of the amount of grain boundary ferrite (GBF). Quantitative metallography of two-stage electrolytically etched samples showed that the averaged size and ratio of lath martensite-austenite (M-A) constituent increased with increasing heat input, while the linear density and amount of M-A decreased. It is indicated that the toughness of CGHAZ was severely dependent on the characteristic parameters of M-A and the amount of GBF. Microhardness of M-A constituents estimated by nanoindentation technique were much higher than that of the neighboring matrix. Severer brittle cracking susceptibility can occur in the CGHAZ with higher heat input based on Chen et al's suggestion that the stress concentration and triaxiality of the neighboring matrix are increased by the hard phase particles such as M-A constituents.