Cleavage initiation in the intercritically reheated coarse-grained heat affected zone: Part II. Failure criteria and statistical effects

In part I of this article, cleavage initiation in the intercritically reheated coarse-grained heat affected zone (IC CG HAZ) of high-strength low-alloy (HSLA) steels was determined to occur between two closely spaced blocky MA particles. Blunt notch, crack tip opening displacement (CTOD), and precracked Charpy testing were used in this investigation to determine the failure criteria required for cleavage initiation to occur by this mechanism in the IC CG HAZ. It was found that the attainment of a critical level of strain was required in addition to a critical level of stress. This does not occur in the case of high strain rate testing, for example, during precracked Charpy testing. A different cleavage initiation mechanism is then found to operate. The precise fracture criteria and microstructural requirements (described in part I of this article) result in competition between potential cleavage initiation mechanisms in the IC CG HAZ. formerly Graduate student with the Department of Materials Science and Metallurgy, University of Cambridge     

Evolution of microstructure in reheated coarse-grained zone of G115 novel martensitic heat-resistant steel

Based on the thermal simulation method,a systematical analysis was conducted on the effect of welding peak temperature and the cooling time that takes place fro...     

A process model for the heat-affected zone microstructure evolution in duplex stainless steel weldments: Part I. the model

The present investigation is concerned with modeling of the microstructure evolution in duplex stainless steels under thermal conditions applicable to welding. The important reactions that have been modeled are the dissolution of austenite during heating, subsequent grain growth in the delta ferrite regime, and finally, the decomposition of the delta ferrite to austenite during cooling. As a starting point, a differential formulation of the underlying diffusion problem is presented, based on the internal-state variable approach. These solutions are later manipulated and expressed in terms of the Scheil integral in the cases where the evolution equation is separable or can be made separable by a simple change of variables. The models have then been applied to describe the heat-affected zone microstructure evolution during both thick-plate and thin-plate welding of three commercial duplex stainless steel grades: 2205, 2304, and 2507. The results may conveniently be presented in the form of novel process diagrams, which display contours of constant delta ferrite grain size along with information about dissolution and reprecipitation of austenite for different combinations of weld input energy and peak temperature. These diagrams are well suited for quantitative readings and illustrate, in a condensed manner, the competition between the different variables that lead to structural changes during welding of duplex stainless steels.     

Toughness improvement by Zr addition in the simulated coarse-grained heat-affected zone of high-strength low-alloy steels

The effect of Zr addition on the microstructure and impact toughness in the coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steels subjected to 100?kJ?cm^-1 heat input was investigated. The second- phase particles were mainly Al–Ti complex oxides and (Ti,Nb)N precipitates in Zr-free steel, whereas lots of finer Zr–Al–Ti complex oxides and (Al,Ti,Nb)N precipitates were formed in Zr-bearing steel because of Zr addition. These finer oxides and precipitates effectively restricted the austenite grain growth by pinning effect during welding thermal cycle, and smaller and more uniform prior austenite grains were obtained in CGHAZ of Zr-bearing steel. Furthermore, more acicular ferrite grains nucleated on Zr–Al–Ti complex oxides, inducing formation of fine-grained microstructure in CGHAZ of Zr-bearing steel. The toughness improvement in CGHAZ of Zr-bearing steel with dimple fracture surface was attributed to the grain refinement by pinning effect and acicular ferrite formation.     

Microstructure and toughness of intercritically reheated coarse- grained heat affected zone in a 1300MPa grade ultra- high strength steel

It was generally recognized that intercritically reheated coarse- grained heat affected zone (ICCGHAZ) was the poorest toughness zone in the welded joint. Welding thermal cycles of ICCGHAZ with different intercritical peak temperatures for 1300MPa grade ultra- high strength steel were carried out on the thermomechanical simulator. The influence of peak temperature on microstructure and impact toughness of ICCGHAZ was discussed on the basis of impact test results and microstructural observation. The results show that the impact absorbed energy tested at -20?? increases from 21. 47J to 76. 05J when the peak temperature improves from 720?? to 800?棬and the crack initiation energy and crack propagation energy have both been greatly increased. When the peak temperature is below 760??, the near- connected coarse M- A constituents form along the prior austenite grain boundary not only weaken the grain boundary but also serve as crack initiation position, which is the main reason for the decrease of toughness. The best toughness will be obtained in this study when the peak temperature reaches up to 800??. That??s because of the disappearance of near- connected coarse M- A constituents, the decrease of dislocation density and the reduction of carbide size in the microstructure.     

Effect of magnesium addition in low carbon steel part 2: toughness and microstructure of the simulated coarse-grained heat-affected zone

A critical investigation into the role of Mg on the toughness and microstructure of coarse grain heat-affected zone (CGHAZ) in low carbon steel has been investigated. In this research, the specimens (Mg-free and Mg-added) underwent weld thermal cycle with heat input of 54, 80, and 100?kJ?cm^?1 at 1350°C peak temperature using a thermal simulator. The typical inclusions characteristics were characterised by means of scanning electron microscopy and equilibrium calculations. The precipitates were characterised by transmission electron microscopy and energy-dispersive spectroscopy. It is revealed that the occurrence of Mg in steel mostly exists in the form of Mg-Al-O oxide inclusions, but a few in the form of solid solution state and (Nb,Ti)(C,N)+MgO precipitates when the concentration of Mg is 0.0026%. The improvement of CGHAZ toughness is obtained when the heat input is 80 and 100?kJ?cm^?1. The possible reasons about the effects of Mg on the toughness of CGHAZ, including Mg-Al-O inclusions, precipitates, and soluble Mg, are discussed in detail.     

原位观察TiN粒子对低合金高强度钢模拟焊接热影响区粗晶区晶粒细化作用

采用高温激光共聚焦显微镜原位观察和电子背散射衍射技术研究TiN粒子在低合金高强度钢模拟大线能量焊接热循环过程中晶粒细化效果.研究发现合理的Ti和N含量能形成大量细小弥散分布的纳米级TiN粒子,在焊接热循环过程中有效钉扎热影响区粗晶区奥氏体晶界,抑制晶粒粗化.同时,TiN附着在Al₂O₃表面析出,在冷却过程中有效促进针状铁素体形核,得到有效晶粒尺寸非常细小的由少量针状铁素体和大量贝氏体构成的复合组织.     

Brittle fracture initiation associated with the strain localization in a heat-affected zone of a low carbon steel

Brittle fracture initiation in the ductile-brittle fracture transition region in the heat-affected zone (HAZ) of weldments of a low carbon steel has been investigated. Consistent with the previous results from blunt notch Charpy tests, brittle fracture initiation was observed in the case of J-integral tests to take place at the intersection of small bainitic ferrite grains of different orientations within a mixed area of bainitic ferrite and quasipolygonal ferrite in proximity to the boundary between a coarse bainitic ferrite. Partial load drop during loading, pop-in phenomena, in fracture mechanics tests in the low-temperature region is caused by essentially the same mechanism as for unstable brittle fracture initiation. Inhomogeneous microstructure in the HAZ gives rise to intense strain localizations in the mixed area of bainitic ferrite and quasipolygonal ferrite due to the constraint of plastic deformation therein and may produce accumulated defects that form an incipient crack for the brittle fracture. Partial load drop proceeds in association with repetitive initiations of brittle facets and their ductile linking. The strong temperature dependence of the magnitude of partial load drop is likely to show that the temperature dependence of the brittle fracture initiation is controlled by the first initiation of a brittle facet and the ductile linking with the following induced facets. Existence of coarse bainitic ferrite grains is a prerequisite for the extension of an incipient crack.     

Microstructures relevant to brittle fracture initiation at the heat-affected zone of weldment of a low carbon steel

Charpy toughness of the heat-affected zone (HAZ) of weldment of a low carbon steel has been investigated by means of an instrumented Charpy test and fractographic analysis. Microstructures were varied with thermal cycles simulating double-pass welding. The ductile-brittle transition temperature is the most deteriorated at an intermediate second-cycle heating temperature. The origin of the difference in the transition temperatures has been analyzed to exist in the brittle fracture initiation stage. Fractographic examination correlating with microstructural features has revealed that the brittle fracture initiation site is associated with the intersection of bainitic ferrite areas with different orientations rather than the martensite-austenite constituents. The role of the constraint of plastic deformation on the brittle fracture initiation is discussed.     

A process model for the heat-affected zone microstructure evolution in duplex stainless steel weldments: Part II. Application to electron beam welding

In the present investigation, a process model for electron beam (EB) welding of different grades of duplex stainless steels (i.e. SAF 2205 and 2507) has been developed. A number of attractive features are built into the original finite element code, including (1) a separate module for prediction of the penetration depth and distribution of the heat source into the plate, (2) adaptive refinement of the three-dimensional (3-D) element mesh for quick and reliable solution of the differential heat flow equation, and (3) special subroutines for calculation of the heat-affected zone (HAZ) microstructure evolution. The process model has been validated by comparison with experimental data obtained from in situ thermocouple measurements and optical microscope examinations. Subsequently, its aptness to alloy design and optimization of welding conditions for duplex stainless steels is illustrated in different numerical examples and case studies pertaining to EB welding of tubular joints.     

A process model for the heat-affected zone microstructure evolution in duplex stainless steel weldments: Part II. Application to electron beam welding

In the present investigation, a process model for electron beam (EB) welding of different grades of duplex stainless steels (i.e. SAF 2205 and 2507) has been developed. A number of attractive features are built into the original finite element code, including (1) a separate module for prediction of the penetration depth and distribution of the heat source into the plate, (2) adaptive refinement of the three-dimensional (3-D) element mesh for quick and reliable solution of the differential heat flow equation, and (3) special subroutines for calculation of the heat-affected zone (HAZ) microstructure evolution. The process model has been validated by comparison with experimental data obtained from in situ thermocouple measurements and optical microscope examinations. Subsequently, its aptness to alloy design and optimization of welding conditions for duplex stainless steels is illustrated in different numerical examples and case studies pertaining to EB welding of tubular joints.     

Nonuniform distribution of carbonitride particles and its effect on prior austenite grain size in the simulated coarse-grained heat-affected zone of thermomechanical control-processed steels

The spatial distribution of carbonitride particles in the simulated coarse-grained heat-affected zone (HAZ) of Nb-Ti microalloyed thermomechanical control-processed (TMCP) steels was investigated using a scanning transmission electron microscope (STEM). It was found that the particles in quenched coarse-grained HAZ were frequently distributed in a nonuniform way, forming clusters and arrays of particles. This nonhomogeneity is defined by the grouping tendency of particles and described by the closeness of the average number density (the mean particle number per unit area) to the average local number density (the mean particle number per unit area, excluding the examined areas without particles). A high concentration of Nb (0.04 mass pct in this article) promoted the formation of carbonitride particle arrays and clusters because of its high segregation tendency at grain and subgrain boundaries during the cooling of a slab. Some of these particles remain undissolved at the peak temperature of a welding thermocycle and may result in sympathetic nucleation of new particles on them. The effectiveness of the particle groups to restrict grain growth is discussed.     

Dispersoid-free zones in the heat-affected zone of aluminum alloy welds

The local microstructure in the heat-affected zone 1 (HAZ1) of a laser beam-welded Al-Mg-Si-Cu aluminum alloy is investigated closely. Dispersoid-free zones (DFZs), where the dispersoids of the base material (BM) are dissolved, are found in the vicinity of the fusion line (FL). They are not uniformly surrounding a grain, but oriented toward the FL. Their width can be as large as 10 μm. Detailed analysis has revealed a decreased silicon concentration as well as a decreased hardness of the oriented dispersoid-free zones (ODFZs). Two mechanisms for the formation of this welding metallurgical feature are discussed.     

Microstructural evolution in the heat-affected zone of a friction stir weld

The microstructure and crystallographic texture spanning the soft region at the thermomechanically affected zone/heat-affected zone (TMAZ/HAZ) boundary of a friction stir weld in 2519 Al were systematically investigated to determine their contributions to the properties of that region. The microstructure was shown to be the primary cause of softening at the TMAZ/HAZ boundary. During welding, fine ϑ′ precipitates responsible for much of the strength in this alloy coarsen and transform to the equilibrium ϑ phase in the HAZ and into the TMAZ, accounting for the observed softening through the HAZ region. The higher temperatures achieved in the TMAZ partially resolutionize the precipitates and allow the subsequent formation of Guinier-Preston (GP) zones during cooling. These two processes are responsible for the variation in microhardness observed in the TMAZ/HAZ region. Texture analyses revealed significant differences in the crystallographic texture across this region that were primarily due to macroscopic rigid-body rotations of the grains, but do not account for the observed softening. The effect of the observed microstructural evolutions on the friction stir welding (FSW) deformation field and on the fracture behavior of the weld are also discussed.     

Monte Carlo simulation of grain boundary pinning in the weld heat-affected zone

A methodology for obtaining a one-to-one correlation between Monte Carlo (MC) and real parameters of grain size and time is described. Using the methodology, and the MC grain growth algorithm, the grain structure in the weld heat-affected zone (HAZ) of a 0.5 Mo-Cr-V steel has been simulated. The simulations clearly show that the kinetics of grain growth can be retarded by the presence of steep temperature gradients in the weld HAZ. Additional pinning due to the formation of grain boundary liquid near the solidus temperature has also been simulated. It is shown that in order to accurately predict the observed grain size in the weld HAZ of the 0.5Cr-Mo-V steel, the retardation in growth kinetics due to temperature gradients as well as liquid pinning should be considered.     

A process model for the heat-affected zone microstructure evolution in Al-Zn-Mg weldments

In the present investigation, process modeling techniques have been applied to unravel the sequence of reactions occurring during welding and subsequent natural aging of Al-Zn-Mg extrusions. The model uses a combination of chemical thermodynamics and diffusion theory to capture the heat-affected zone (HAZ) dissolution and aging kinetics, with the particular feature of writing the constitutive evolution equation in a differential form. Separate response equations are then developed to convert the calculated values for the particle volume fraction and the matrix solute content into engineering quantities such as hardness or strength, for a direct comparison to experiments. The model indicates that particle dissolution is the main factor contributing to strength loss during welding. At the same time, growth of the remaining particles occurs during cooling, leading to solute depletion within the aluminium matrix. This, in turn, reduces the precipitation potential and contributes to the development of a permanent soft zone within the partly reverted region after prolonged room-temperature aging. It is concluded that the combination of a microstructure model with an appropriate heat-flow model creates a powerful tool for alloy design and optimization of welding conditions for Al-Zn-Mg extrusions, and an illustration of this is given toward the end of the article.     

Kinetics of grain growth in the weld heat-affected zone of alloy 718

Grain-boundary liquation occurs in the weld heat-affected zone (HAZ) of the Ni-base superalloy 718 at locations where the peak temperatures are greater than about 1200 ‡C. The evolution of the grain structure at these HAZ locations depends upon the interaction between the grains and the grain-boundary liquid. The evolution of grain structure in the presence of grain-boundary liquid was simulated by subjecting samples to controlled thermal cycles using resistance heating. A measurement of grain size as a function of isothermal hold at two peak temperatures of 1200 ‡C and 1227 ‡C indicated that in alloy 718, the kinetics of grain growth depended upon the prior thermal history of the alloy. In the solution-treated alloy, the presence of grain-boundary liquid did not arrest grain growth at either peak temperature. In the homogenized and aged alloy, a grain refinement was observed at the peak temperature of 1227 ‡C, while an arrest of grain growth was observed at a peak temperature of 1200‡C. Liquid film migration (LFM) and subgrain coalescence, either acting alone or simultaneously, are shown to explain most of the observed microstructural phenomena and the kinetics of grain growth in the alloy. Formerly Research Associate, with the Department of Materials Science and Engineering, University of Alabama at Birmingham, Birmingham, AL     

Influence of different microstructural features on impact toughness and crack initiation behavior of coarse grain heat-affected zone in X80 pipeline steel

Coarse grain heat-affected zone samples of X80 pipeline steel under different heat inputs were obtained through thermal welding simulation experiments with Gleeble 3500.Charpy impact tests and a combination of multiscale characterizations were conducted to investigate the influence of various microstructural features on impact toughness and crack initiation behavior.The results prove that,as the heat input increases,the number of M/A components increases,thereby degrading toughness and increasing hardness.Meanwhile,more M/A constituents tend to aggregate on prior austenite grain boundaries(PAGBs),and the overall dimensions of M/A and the width and volume fraction of the lath martensite substructure inside M/A islands would increase as well.These changes make intersections between boundary M/As and PAGBs become one of the preferred sites for crack initiation.In addition,only large-sized grotesque inclusions can act as a direct inducement of crack initiation.