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.     

Mechanisms and modeling of cleavage fracture in simulated heat-affected zone microstructures of a high-strength low alloy steel

The effect of the welding cycle on the fracture toughness properties of high-strength low alloy (HSLA) steels is examined by means of thermal simulation of heat-affected zone (HAZ) microstructures. Tensile tests on notched bars and fracture toughness tests at various temperatures are performed together with fracture surface observations and cross-sectional analyses. The influence of martensite-austenite (M-A) constituents and of “crystallographic” bainite packets on cleavage fracture micromechanisms is, thus, evidenced as a function of temperature. Three weakest-link probabilistic models (the “Master-curve” (MC) approach, the Beremin model, and a “double-barrier” (DB) model) are applied to account for the ductile-to-brittle transition (DBT) fracture toughness curve. Some analogy, but also differences, are found between the MC approach and the Beremin model. The DB model, having nonfitted, physically based scatter parameters, is applied to the martensite-containing HAZ microstructures and gives promising results.     

Mechanisms and modeling of cleavage fracture in simulated heat-affected zone microstructures of a high-strength low alloy steel

The effect of the welding cycle on the fracture toughness properties of high-strength low alloy (HSLA) steels is examined by means of thermal simulation of heat-affected zone (HAZ) microstructures. Tensile tests on notched bars and fracture toughness tests at various temperatures are performed together with fracture surface observations and cross-sectional analyses. The influence of martensite-austenite (M-A) constituents and of “crystallographic” bainite packets on cleavage fracture micromechanisms is, thus, evidenced as a function of temperature. Three weakest-link probabilistic models (the “Master-curve” (MC) approach, the Beremin model, and a “double-barrier” (DB) model) are applied to account for the ductile-to-brittle transition (DBT) fracture toughness curve. Some analogy, but also differences, are found between the MC approach and the Beremin model. The DB model, having nonfitted, physically based scatter parameters, is applied to the martensite-containing HAZ microstructures and gives promising results.     

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.     

Brittle fracture in austenitic steel

The fracture mode of austenitic steel may feature a ductile to brittle transition (DBT), depending on alloy composition and temperature. The DBT variation can be explained in terms of the actual deformation structure evolving during cold work and the correlated internal stresses. The crucial features of microstructure causing brittle fracture are found to be the intersections of deformation twins and the total density of free dislocations. To avoid brittle fracture, the stresses of intersecting twins must be screened by dislocations. Manganese and nitrogen promote brittle fracture since they lower the stacking fault energy and thus shift the onset of twinning to low strain levels where the total dislocation density is low. Nickel additions oppose this trend. The results of the microstructural fracture model agree well with experimental results and the model is confirmed by continuum-mechanical considerations.     

Simulation study on heat-affected zone of high-strain X80 pipeline steel

The microstructure evolution and impact-toughness variation of heat-affected zone(HAZ)in X80 highstrain pipeline steel were investigated via a welding thermal-simulation technique,Charpy impact tests,and scanning electron microscopy observations under different welding heat inputs and peak temperatures.The results indicate that when heat input was between 17 and 25kJ·cm~(-1),the coarse-grained heat-affected zone showed improved impact toughness.When the heat input was increased further,the martensite-austenite(M-A)islands transformed from fine lath into a massive block.Therefore,impact toughness was substantially reduced.When the heat input was 20kJ·cm~(-1) and the peak temperature of the first thermal cycle was between 900 and 1300°C,a higher impact toughness was obtained.When heat input was 20kJ·cm~(-1) and the peak temperature of the first thermal cycle was 1300°C,the impact toughness value at the second peak temperature of 900°C was higher than that at the second peak temperature of 800°C because of grain refining and uniformly dispersed M-A constituents in the matrix of bainite.     

Microstructure of the heat-affected zone in 17–4 PH stainless steel

The microstructure of the heat-affected zone (HAZ) in bead-on-plate welded 17–4 PH stainless steel was studied with special reference to the roles of prior heat treatment and heat input during welding. The HAZ in solution-annealed condition consists of three different microstructural zones containing: (i) retransformed martensite and reformed austenite; (ii) overaged martensite; and (iii) under-aged martensite. In aged condition the HAZ consists of zones (i) and (ii), while in overaged condition it consists almost entirely of zone (ii). The HAZ in solution-annealed and aged conditions is characterised by steep gradients in hardness, while in overaged condition it has uniform hardness throughout. A good correlation was obtained between the calculated temperature distribution in the HAZ and the observed microstructural features.     

Cleavage initiation in the intercritically reheated coarse-grained heat-affected zone: Part I. Fractographic evidence

Tensile, crack opening displacement (COD), blunt notch, and Charpy impact tests were used to investigate cleavage initiation in the intercritically reheated coarse-grained heat-affected zone (IC CG HAZ) of three steels. The steels were chosen to provide different distributions and morphologies of MA (high-carbon martensite with some retained austenite) particles within the IC CG HAZ structure. Observation of minimum impact toughness values for the IC CG HAZ was found to be associated with a particular microstructure containing a near-connected grain boundary network of blocky MA particles, the MA particles being significantly harder than the internal grain microstructure. The initiation mechanism for this structure was determined to be from a combination of an overlap of residual transformational induced stress fields, due to the formation of the MA particles, between two closely spaced particles and stress concentration effects resulting from debonding of the particles.     

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.     

低合金钢焊接热影响区的微观组织和韧性研究进展

对钢结构而言,诸如海洋平台、船舶、桥梁、建筑和油气管线等,焊接后的性能直接决定了其服役寿命和安全性,重要性不言而喻.在针对焊接相关问题的研究中,焊接热影响区的韧性提升一直是重点和难点.焊接热影响区会经历高达1400℃的高温,从而形成粗大的奥氏体晶粒,如果焊接参数控制不当,不能通过后续冷却过程中的相变细化组织,就会造成韧性的降低.而多道次焊接的情况更为复杂,前一道次形成的粗晶区还会在后续焊接过程中经历二次热循环,从而形成链状M-A,造成韧性的急剧下降.本文旨在对一些现有焊接热影响区的相关研究结果进行总结,探讨母材的成分、第二相及焊接工艺等因素对热影响区微观组织和性能的影响,为低温环境服役的大型钢结构的焊接性能改善提供一些设计思路.     

Metallographic examination of the heat-affected zone of a test weld in a type X 20 CrMoV 12 1 steel

Metallographic examination of a similar test weld in a type X 20 CrMoV 12 1 steel revealed a martensitic phase within the heat-affected zone of the base metal adjacent to the fusion boundary. As a consequence, the heat-affected zone displayed an excess microhardness compared with the unaffected base metal. It is shown that the occurrence of this martensitic phase can be correlated with segregations in the base metal. Due to its martensitic nature, this phase can be transformed to α-ferrite and carbide by an additional tempering treatment subsequent to the standard post-weld heat treatment. By this additional tempering treatment the uniformity of the microstructure of the heat-affected zone can be considerably improved.     

Dissolution kinetics of NbC particles in the heat-affected zone of type 347 austenitic stainless steel

Dissolution kinetics of NbC particles in the heat-affected zone (HAZ) of Type 347 austenitic stainless steel were experimentally studied by a rapid cooling method. Coupons with 1.5-mm thickness were water quenched on a GLEEBLE thermomechanical simulator at various instances during the heating portion of a welding thermal cycle. Particle dissolution kinetics data were obtained by statistical analysis of digital images of resulting microstructure. For most of the test alloys, a good correlation exists between the dissolution kinetics and the susceptibility to cracking in the HAZ. The faster the Nb-carbide particles dissolve, the more resistant the alloy is to constitutional liquation cracking. The rate at which particles dissolve seems to be affected by their stoichiometry. If Nb:C ratios in bulk alloys are closer to the ideal stoichiometry of 7.7, NbC particles formed in the alloy are more likely to be stable and, hence, slower to dissolve.     

Martensite morphology and strain aging behaviours in intercritically treated low carbon steel

Abstract

The strain aging behaviour of intercritically annealed low carbon steel with different martensite morphologies has been investigated. Aging experiments after 4% prestrain were carried out at 180°C for different times ranging from 10 to 160 min. It was found that the variation in bake hardening response ΔY, lower yield stress and ultimate tensile stress with aging time describes a similar trend for all three microstructural variants, but the absolute values of bake hardening response, lower yield stress and ultimate tensile stress are higher for the microstructure containing fibrous, more uniformly distributed martensite. The aging response of fibrous martensite was also found to be slower, and the fibrous martensite morphology provided the best combination of strength and ductility as desired for dual phase steels.     

Texture evolution induced by strain path changes in low carbon steel sheets

Two-stage sequences of simple shear performed on low carbon steel sheets were supplemented by texture measurements. A detailed analysis of the texture evolution during the simple shear sequences was realized by studying the orientation distribution functions. The calculation of the orientation stability map (using the classical viscoplastic Taylor model) allows explanation of the observed texture development: the initial texture is shown to be partly stable and hence the measured textures differ from the usually reported ones. The influence of the texture evolution on the shape of the stress-strain curves, as well as on the remaining symmetries of the material, is also discussed. In particular, it is demonstrated that both intragranular and geometrical contributions have to be considered to explain the shape of the stress-strain curves after a strain path change.     

Dynamic fracture behaviour of simulated weld heat affected zone of a high strength low alloy steel

Fracture toughness properties of a simulated weld coarse grain heat affected zone (CGHAZ) in a newly developed 785 MPa grade high strength low alloy (HSLA) steel were investigated at different loading rates and a series of temperatures. It is indicated that the fracture toughness K_Ic is a single-value function of the parameter ΔH=kT In (A/ε?), implying that just like the plastic deformation, the event of fracture initiation is also controlled by dislocation thermal activation movement.     

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.     

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.     

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.