Simulated Welding Thermal Cycles Research Articles

Morphology and crystallography of microstructures in Mg-deoxidized offshore engineering steels after simulated welding thermal cycles

ABSTRACT The morphology and crystallography of microstructures in heat-affected zone of offshore engineering steels with 0.0002 and 0.0042 wt% Mg were studied. With increasing Mg content from 0.0002 to 0.0042 wt%, the prior austenite grain (PAG) size increases from 92 to 163 μm, which decreases the PAG boundary density and increases the bainite transformation temperature. So the bainite nucleation sites and driving force reduce, affecting bainites with morphology and crystallography. For morphology, the sympathetic nucleation reduces and widening growth rate of bainitic laths increases, coarsening microstructures. For crystallography, the variant selection is intensified, coarsening the unevenly distributed Bain zones with specific variants and reducing the high-angle grain boundary (HAGB) density from 1.4 to 0.6 μm–1. The coarsened microstructure with lower crack resistance increases the ductile-brittle transition temperature, contributing to the ductile fracture transforming to cleavage fracture and the HAZ toughness at –40 °C decreasing from 201 to 58 J.

Relationship of the Microstructure and Toughness of the Coarse Grain Heat-Affected Zone of TiNbV Microalloyed Steels Based on Electron Backscatter Diffraction Analysis

The microstructure evolution and impact toughness of the coarse grain heat-affected zone (CGHAZ) of TiNbV microalloyed steels were investigated by using a thermal simulation test. The samples were treated with various simulated welding thermal cycles. The phase constituents and grain sizes were analyzed by using electron backscatter diffraction analysis. The microstructure of the CGHAZ of the treated samples consisted of ferrite, acicular ferrite, pearlite, and bainite. The samples have a higher impact toughness under a lower welding heat input. This is because the microstructure of the CGHAZ is dominated by the higher volume fraction of the high-angle grain boundaries of acicular ferrites. The presence of bainite and coarsening grains are two key factors deteriorating the toughness of the CGHAZ of TiNbV microalloyed steels. The volume fraction of bainite sharply increased as the welding heat input increased, leading to a decrease in the impact toughness of the CGHAZ. For a higher welding heat input, both the severe coarsening of the grain size and a higher bainite content would result in poor impact toughness.

In Situ Observation of Phase Transformations in the Coarse-Grained Heat-Affected Zone of P91 Heat-Resistant Steel During Simulated Welding Process

The solid-state phase transformation in the coarse-grained heat-affected zone of P91 steel under simulated welding thermal cycle has been investigated in situ by confocal scanning laser microscope. It is found that γ-austenite initiates from the triple point of δ-ferrite grain boundaries (GBs), then at ordinary δ-GBs, and eventually from grain interiors. Upon further cooling, martensite laths are observed to nucleate and growth instantaneously, with the growth rate being estimated ranging from 175 to 454 μm/s.

Morphological evolution of HAZ microstructures in low carbon steel during simulated welding thermal cycle

It is necessary to reveal the derivation of bainite microstructures with different appearance because these complex appearances are closely related to the transformation nature and their final mechanical properties. In this work, the morphological evolution of HAZ microstructures during simulated welding thermal cycle was investigated in a low carbon steel that was intentionally treated with combined addition of Ti and Mg elements. The inclusions present in the matrix have a complex core/shell composite structure composed of Al-Ca-Mg-Ti-O and MnS. However, only very few inclusions with large size (about 2∼6μm) can stimulate the intragranular nucleation of ferritic laths. Thus, this behavior did not show an obvious refinement effect on HAZ microstructure. Partial bainite microstructure revealed the ferritic lath structures always first form at the beginning of the transformation irrespective of the final morphologies of microstructure (granular bainite or bainitic ferrite), implying that the granular bainite is actually derived from the ferritic lath structures although these lath structures will become gradually obscure at the later stages of the transformation.

Influence of arc interactions on heat and mass transfer during a two-arc hybrid welding

The quantitative analysis of the heat and mass transfer in weld pool is of great significance for the optimization of welding parameters in manufacturing process with a high-quality weld bead. In this study, the arc deflections caused by the electromagnetic force of two-arc hybrid welding was calculated. The heat source and arc force models of the hybrid welding were proposed considering the influence of the arc deflection and surface deformation of weld pool. Compared with single heat welding, the distributions of the arc heat energy, arc pressure, and arc shear stress in two-arc hybrid welding were studied based on three-dimensional transient computational models. The temperature field and the fluid flow on the upper surface and the longitudinal and cross -sections of the weld pool were analyzed to reveal the heat and mass transfer in workpiece. Experiments were carried out and the results showed that the simulated cross-section weld bead, thermal cycle, temperature field, and flow field agreed well with the experimental values.

Study of Nb non-equilibrium segregation at prior austenite grain boundary in welding using atom probe tomography and modeling

The non-equilibrium segregation of Nb at prior austenite grain boundary (PAGB) induced by welding is experimentally investigated by means of atom probe tomography (APT). The Nb concentration at PAGB appeared to be 0.115 at.% for experimental steel containing 0.06 at.% Nb at the simulated welding thermal cycle for 10 s from 1320 to 950 °C. The critical time tc for achieving maximum segregation is calculated to be 565 s for the experimental steel. For the first time, the diffusion coefficient of Nb in desegregation process is calculated for the temperature of Ti = 950 °C using non-equilibrium segregation model and APT experimental result and is confirmed to be 9.23 × 10−11 m2/s. Based on it, the kinetics of Nb non-equilibrium segregation at the PAGBs in welding process is outlined, and the segregation process and desegregation process are discussed. The Nb segregation at PAGB is also predicted over the t8/5 range of 3–600 s. The critical t8/5, at which the highest Nb concentration of 0.144 at.% at PAGB is predicted to be achieved, is ~ 29 s.

Effect of simulated welding thermal cycles on microstructure and mechanical properties of coarse-grain heat-affected zone of high nitrogen austenitic stainless steel

The microstructural evolution, strength, ductility change, and precipitates in the base metal and coarse-grain heat-affected zone (CGHAZ) of high nitrogen austenitic stainless steel with different thermal cycles were investigated. The research results indicate that the grain size of austenite and the precipitates reduced while the strength improved with increasing cooling rate. Furthermore, the shape and formation location of the precipitates that were proved to be M23C6 and Cr2N by transmission electron microscopy (TEM) varied with the cooling rate. M23C6 precipitated from the catenary to the round type and from the grain boundary to the grain interior with increasing cooling rate, causing the transformation of the fracture mode from an intergranular fracture to a transcrystalline fracture. Cr2N precipitated cellularly and was detrimental to the mechanical properties. The low cooling rate (<5 °C/s) deteriorated the strength and ductility owing to the mass of intermetallic compounds, while the properties of the simulated specimens above 5 °C/s were better than those of the base metal. However, the ductility was the best at 5 °C/s and the fracture surface consisted of many deep dimples. Meanwhile, the grains deformed and increased as the cooling rate changed from 1 °C/s to 30 °C/s according to electron backscatter diffraction (EBSD). Further, the results of Taylor factor are consistent with the variation in ductility.

The Microstructure and Pitting Resistance of 2002 Lean Duplex Stainless Steel after the Simulated Welding Thermal Cycle Process.

In this paper, thermal cycles with different heat inputs and cooling rates were investigated for a novel lean duplex stainless steel 2002 using a welding simulation. The microstructure and pitting resistance of the simulated heat-affected zones were studied. With the increasing heat input, the amount and size of the austenite phase both increased, along with a transformation from rods to dendritic structures. The critical pitting temperature (CPT) and the pitting potential (Epit) both increased first and then declined as the heat input increased, indicating a strong dependence of pitting resistance on the heat input. For the different cooling rates, the amount of ferrite increased as the cooling rate increased from 0.25 °C/s to 20 °C/s. The CPT and Epit both increased with the increasing cooling rates, indicating an improved pitting resistance. The pits initiated preferentially at the boundaries of ferrite and austenite due to the precipitation of M23C6 in the specimens with different cooling rates.

Variation in Morphology and Kinetics of Granular Bainite with Welding Thermal Cycles in High-Nb Fire-Resistant Steel: Experiments and Theoretical Calculations

The microstructure and kinetics of granular bainite (GB) were investigated in novel fire-resistant steel containing high niobium and subjected to simulated welding thermal cycles. The dilatometric data were analyzed to determine the activation energy and Avrami exponents. It was observed that austenite transformed to GB over a wide range cooling rates of 0.5-100 K/s. Theoretical calculations indicated that Avrami exponent (n) decreased with increased transformed volume fraction ( $$f_{b}$$ ) and cooling rate. At a moderate cooling rate of 20-6 K/s, bainitic ferrite as sheaves formed from austenite and the corresponding Avrami exponent was 2 3. The local activation energy for austenite transformation to bainitic ferrite decreased from ~ 198 to ~ 182 kJ/mol with increasing $$f_{b}$$ from 0.05 to 0.95, while the local activation energy for austenite transformation to martensite decreased from ~ 80 to ~ 71 kJ/mol with increasing $$f_{b}$$ from 0.05 to 0.95.

Effect of heat input on austenite microstructural evolution of simulated heat affected zone in 2205 duplex stainless steel

The effect of simulated welding thermal cycle on the microstructure and impact toughness of heat affected zone (HAZ) in 2205 duplex stainless steel was investigated by optical microscopy, scanning electron microscopy, transmission electron microscopy and room temperature impact test. The results show that the morphology and volume fraction of austenite change greatly with heat input. The amount of residual austenite and grain boundary austenite (GBA) decreases while Widmanstatten austenite (WA) laths and intergranular austenite increase with the increase in heat input. Only the fine equiaxed austenite exists in the HAZ when the heat input is increased up to 61.8 kJ/cm. WA laths nucleate initially either at the ferrite and GBA phase boundaries or directly in ferrite grains and begin to decompose into diamond-shaped austenite with the heat input larger than 25.2 kJ/cm. The impact toughness shows a non-monotonic variation, which is related to the increase in austenite fraction and the formation and the decomposition of WA laths.

Influence of Multiple Gleeble®-Simulated Weld Thermal Cycles on Maraging 17-4 and 13-8+Mo

Influence of Multiple Gleeble®-Simulated Weld Thermal Cycles on Maraging 17-4 and 13-8+Mo

Effect of Martensite–Austenite Constituent on Low-Temperature Toughness in YS 500 MPa Grade Steel Welds

The effect of martensite–austenite (M–A) constituents and simulated microstructure on low-temperature toughness was investigated in YS 500 MPa grade structural steel welds. The specimens were fabricated using a direct quenching and tempering process. After simulated weld thermal cycles, the coarse-grained heat-affected zone (CGHAZ) and intercritically reheated coarse-grained heat-affected zone (IRCGHAZ) were produced using a Gleeble tester and real welded joint to support the simulation results. The largest low-temperature toughness was observed in the fine-grained heat-affected zone (FGHAZ) owing to the fine-ferrite microstructure. However, the toughness decreased in the IRCGHAZ because of the slender morphology of the M–A constituents that formed primarily along the prior austenite grain boundaries in the IRCGHAZ.

Inclusion Evolution Behavior of Ti-Mg Oxide Metallurgy Steel and Its Effect on a High Heat Input Welding HAZ

We have studied here the evolution of inclusions in ladle furnace (LF), Ruhrstahl &amp; Heraeus furnace (RH), and simulated welded samples during Ti-Mg oxide metallurgy treatment and the mechanical properties of the heat-affected zone (HAZ) after high heat input welding. The study indicated that inclusions in an LF furnace station are silicomanganate and MnS of size range ~0.8–1.0 μm. After Mg addition, fine Ti-Ca-Mg-O-MnS complex oxides were obtained, which were conducive to the nucleation of acicular ferrite (AF). The corresponding microstructure changed from ferrite side plate (FSP) and polygonal ferrite (PF) to AF, PF, and grain boundary ferrite (GBF). After a simulated welding thermal cycle of 200 kJ/cm, disordered arrangements of acicular ferrite plates, fine size cleavage facets, small inclusions, and dimples all promoted high impact toughness.

In-situ observation of grain refinement in the simulated heat-affected zone of high-strength low-alloy steel by Zr-Ti combined deoxidation

The effect of Zr-Ti combined deoxidation on the grain refinement in the simulated coarse-grained heat-affected zone of a high-strength low-alloy steel was investigated by means of analytical characterization techniques such as in-situ microscopy, transmission electron microscopy, and electron backscattered diffraction analysis. Owing to the Zr-Ti combined deoxidation, a large amount of fine Zr-Ti oxide particles were formed in the steel and retarded the austenite grain growth during simulated welding thermal cycle. The austenite grains were small and uniform. The Mn can diffuse spontaneously from austenite to Zr-Ti oxide inclusion and MnS precipitated on ZrO2, which can form Mn depleted zone in the vicinity of inclusion. The acicular ferrite grains nucleated on intragranular Zr-Ti oxide inclusions in austenite grains grew in different directions and effectively divided the austenite grain into several finer and separate regions at intermediate temperature. The crystallographic grain size became small in the simulated coarse-grained heat-affected zone of Zr-Ti-killed steel due to the effective pinning effect by Zr-Ti oxide particles and acicular ferrite formation.

Characterization of coarse bainite transformation in low carbon steel during simulated welding thermal cycles

Coarse austenite to bainite transformation in low carbon steel under simulated welding thermal cycles was morphologically and crystallographically characterized by means of optical microscope, transmission electron microscope and electron backscattered diffraction technology. The results showed that the main microstructure changes from a mixture of lath martensite and bainitic ferrite to granular bainite with the increase in cooling time. The width of bainitic laths also increases gradually with the cooling time. For a welding thermal cycle with relatively short cooling time (e.g. t8/5 is 30s), the main mode of variant grouping at the scale of individual prior austenite grains changes from Bain grouping to close-packed plane grouping with the progress of phase transformation, which results in inhomogeneous distribution of high angle boundaries. As the cooling time is increased, the Bain grouping of variants becomes predominant mode, which enlarges the effective grain size of product phase.

Effect of welding thermal cycles on the structure and properties of simulated heat-affected zone areas in X10CrMoVNb9-1 (T91) steel at a state after 100,000 h of operation

The article presents results of structural tests (light, scanning electron and scanning transmission electron microscopy) of X10CrMoVNb9-1 (T91) creep-resisting steel after approximately 100,000h of operation. It was ascertained that the parent metal of T91 steel is characterized by the microstructure of tempered martensite with M23C6 carbide precipitates and few dispersive precipitates of MX-type niobium and vanadium carbonitrides. The most inconvenient change in T91 steel precipitate morphology due to long-term operation is the appearance of the Laves Fe2Mo phase which along with M23C6 carbide particles forms elongated blocks and conglomerates on grain boundaries. The article also presents results of tests related to the effect of simulated welding thermal cycles on selected properties of X10CrMoVNb9-1 (T91) grade steel at a state after approximately 100,000h of operation. The tests involved the determination of the chemical composition of the steel tested as well as impact tests, hardness measurements and microscopic metallographic examination (based on light microscopy) of simulated heat-affected zone (HAZ) areas for a cooling time (t8/5) restricted within a range between 3s and 120s, with and without heat treatment. The tests revealed that, among other results, hardness values of simulated HAZ areas in X10CrMoVNb9-1 (T91) steel do not guarantee cold crack safety of the steel at the state without additional heat treatment. It was also observed that simulated welding thermal cycles of cooling times t8/5=3, 12, 60 and 120s do not significantly affect the toughness and hardness of simulated HAZ areas of the steel tested.

An Assessment of the Mechanisms of Transformation Plasticity in SA508 Grade 3 Steel during Simulated Welding Thermal Cycles

Residual stresses in welded joints must be quantified in order to carry out structural integrity assessments on critical nuclear components. This usually requires the application of finite element models for components with wall thicknesses exceeding 50 mm. In ferritic steels, the development of residual stresses is made more complex by the strains associated with the solid-state phase transformations that occur during heating and cooling. Finite element models often do not account for factors that contribute to anisotropy in the transformation strains, such as Greenwood-Johnson plasticity and variant selection. In this work, we search for evidence that might reveal which mechanism (s) contributes to this anisotropy. Coupons of SA508 steel were subjected to simulated welding thermal cycles, with and without external loading, and in-situ X-ray diffraction was used to track changes in crystal structure. The results were checked for evidence of plastic deformation in austenite and variant selection in its daughter phases.

Analysis of Simulated Welding Thermal Cycles S700MC Using Thermal Imaging Camera

In this paper an influence of simulated thermal cycle on properties and HAZ structure of 10 mm thick S700MC steel plates. The introduction of thermomechanically processed steels with high yield and relatively low carbon equivalent, will significantly reduce the time of welding works by reducing the preheating temperature, or even complete removal of this processing step, furthermore reduction of cross-sectional areas of structural elements, making welded structures with the same capacity will be more slender and lighter. The simulation of thermal cycles was carried out on a specially built test stand equipped with resistive heating source infrared camera VarioCam Head HR with 50 mm lens and a computer with software IRBIS 3 plus. Simulation was prepared for simple and complex thermal cycle. Simulation studies of thermal cycles consisted of resistive heating of samples prepared for the impact test. Single thermal cycles were simulated at temperatures ranging from 400 to 1300 oC, 100 oC and the cycle complex. For each temperature three repeats were carried out. Specimens were tested on impact, strength test and also hardness and metallographical tests. The study showed that there is a possibility to use a purpose built system to simulate simple and complex thermal cycles of S700 MC steel in specific ranges of the cooling time t8/5. Analysis of the results of the study showed that the welding thermal cycle strongly influences the structural changes and phase in the HAZ zone of S700 MC steel. Areas of HAZ heated to high temperatures above 1000 [°C], show a sudden drop of toughness to unacceptable levels of impact strength (27 [J/cm2]). This sharp decrease in toughness is associated with uncontrolled separation processes of MX phases and dissolution of carbides, niobium and vanadium carbonitrides in austenite during heating. The study also showed that the chemical composition of steel and especially titanium and aluminium content is sufficient to bind in the HAZ free nitrogen and reduce the aging process. The control of the amount of heat introduced into the joint area during welding will reduce the adverse separation processes in the weld and HAZ which will ensure adequate toughness of the connection. Precise knowledge of the phenomena occurring in the HAZ during the thermal cycle can impact the ability to control properties and structure of the welded joint.

Investigation of heat affected zone softening in armour steels Part 1 – Phase transformation kinetics

Heat affected zone (HAZ) softening in two armour steels (high hard and flash processed) was investigated by monitoring phase transformations during simulated welding thermal cycles. The high hard steel was produced by conventional thermomechanical processing followed by quench and tempering. Flash processed steel was produced by rapid heating and cooling of a spheroidised steel to produce a mixed microstructure. Heat affected zone softening in high hard steel was observed on heating to a peak temperature below Ac1 due to overtempering of the original microstructure. In flash processed steel, the softening was associated with allotriomorphic ferrite formation when heated to a peak temperature between Ac1 and Ac3. The above results demonstrate the importance of initial microstructure on HAZ on phase transformation transients and associated softening in armour steels.

Continuous Cooling Bainite Transformation Characteristics of a Low Carbon Microalloyed Steel under the Simulated Welding Thermal Cycle Process

Continuous cooling transformation of a low carbon microalloyed steel was investigated after it was subjected to the simulation welding thermal cycle process and the interrupted cooling test. Microstructure observation was performed by optical microscopy and transmission electron microscopy. On the basis of the dilatometric data and microstructure observation, the continuous cooling transformation (CCT) diagram was determined, which showed that the main microstructure changes from a mixture of lath martensite and bainitic ferrite to full granular bainite with the increase in the cooling time t8/5 from 10 to 600 s, accompanied with a decrease in the microhardness. The interrupted cooling test confirmed that the bainitic ferrite can form attached to grain boundaries at the beginning of transformation even if the final microstructure contains a mixture of granular bainite and bainitic ferrite.