Study on local embrittlement of welding heat-affected zone in XSO pipeline steels

The relationship between the microstructure and toughness of welding heat-affected zone in XSO grade pipeline steels is studied. It is found that the intercritical reheated coarse-grained heat-affected zone （ICCGHAZ） of experimental steels has the lowest toughness values when the secondary peak temperature is at intercritical （ α ＋ γ ） region during multi-pass welding. The local embrittlement is mainly attributed to the morphology, amount and size of M-A constituent. It is also found that the microstructural inhabitanee at ICCGHAZ has a deleterious effect on the toughness. On the basis of above experimental results, it is suggested that the local embrittlement should be prevented by using pre-heating thermal cycle which could eliminate the microstructural inhabitance and using post-heating thermal cycle which could improve the morphology, amount and size of MA constituent.

Improving creep strength of the fine-grained heat-affected zone of novel 9Cr martensitic heat-resistant steel via modified thermo-mechanical treatment

The infamous type Ⅳ failure within the fine-grained heat-affected zone (FGHAZ) in G115 steel weldments seriously threatens the safe operation of ultra-supercritical (USC) power plants.In this work,the traditional thermo-mechanical treatment was modified via the replacement of hot-rolling with cold rolling,i.e.,normalizing,cold rolling,and tempering (NCT),which was developed to improve the creep strength of the FGHAZ in G115 steel weldments.The NCT treatment effectively promoted the dissolution of preformed M_(23)C_(6)particles and relieved the boundary segregation of C and Cr during welding thermal cycling,which accelerated the dispersed reprecipitation of M_(23)C_(6) particles within the fresh reaustenitized grains during post-weld heat treatment.In addition,the precipitation of Cu-rich phases and MX particles was promoted evidently due to the deformation-induced dislocations.As a result,the interacting actions between precipitates,dislocations,and boundaries during creep were reinforced considerably.Following this strategy,the creep rupture life of the FGHAZ in G115 steel weldments can be prolonged by 18.6%,which can further push the application of G115 steel in USC power plants.

High-Efficiency Dynamic Scanning Strategy for Powder Bed Fusion by Controlling Temperature Field of the Heat-Affected Zone

Improvement of fabrication efficiency and part performance was the main challenge for the large-scale powder bed fusion(PBF)process.In this study,a dynamic monitoring and feedback system of powder bed temperature field using an infrared thermal imager has been established and integrated into a four-laser PBF equipment with a working area of 2000 mm×2000 mm.The heat-affected zone(HAZ)temperature field has been controlled by adjusting the scanning speed dynamically.Simultaneously,the relationship among spot size,HAZ temperature,and part performance has been established.The fluctuation of the HAZ temperature in four-laser scanning areas was decreased from 30.85℃to 17.41℃.Thus,the consistency of the sintering performance of the produced large component has been improved.Based on the controllable temperature field,a dynamically adjusting strategy for laser spot size was proposed,by which the fabrication efficiency was improved up to 65.38%.The current research results were of great significance to the further industrial applications of large-scale PBF equipment.

Effect of Dynamic Strain Aging on Hardness in the Heat-Affected Zone of SUS316 Steel Welds

DSA(dynamic strain aging)phenomenon in SUS316 steel was investigated using isothermal and non-isothermal tensile tests of simulated HAZ(heat-affected zone)thermal cycles.Isothermal tensile tests were performed on SUS316 in the peak temperature range of 20-700°C,with strain rates varying from 4.2×10^(-3)to 4.2×10^(-5)s^(-1).Based on the appearance of discontinuous plastic flows,expressed as serrations,and the hardening phenomenon of the tensile samples,the conditions for the occurrence of DSA in the SUS316 steel were investigated.Furthermore,the extent of hardening due to DSA was evaluated by comparing the hardness values of the SUS316 and SUS316EHP steels after the tensile tests.To confirm the effect of DSA on hardness in the HAZ of the welded SUS316 steel,non-isothermal tensile tests of the simulated HAZ thermal cycles were performed using a Thermec Master.The relationship between the increase in Vickers hardness due to DSA and the strain in the HAZ was determined;the effect of DSA on hardness in the HAZ could be predicted.The DSA in SUS316 steel was found to be mainly attributed to the dynamic interaction of dislocations with C and N interstitial atoms during high-temperature deformation.

Effect of welding parameters on the heat-affected zone of AISI409 ferritic stainless steel

One of the main problems during the welding of ferritic stainless steels is severe grain growth within the heat-affected zone （HAZ） In the present study, the microstmctural characteristics of tungsten inert gas （TIG） welded AISI409 ferritic stainless steel were investigated by electron backscattered diffraction （EBSD）, and the effects of welding parameters on the grain size, local misorientation, and low-angle grain boundaries were studied. A 3-D finite element model （FEM） was developed to predict the effects of welding parameters on the holding time of the HAZ above the critical temperature of grain growth. It is found that the base metal is not fully recrystallized. During the welding, complete recrystallization is followed by severe grain growth. A decrease in the number of low-angle grain boundaries is observed within the HAZ. FEM results show that the final state of residual sWains is caused by competition between welding plastic strains and their release by recrystallization. Still, the decisive factor for grain growth is heat input.

Microstructure transformation of X70 pipeline steel welding heat-affected zone

The continuous cooling transformation curve of heat-affected zone （HAZ） of X70 pipeline steel was mea- sured by Gleeble-3500 thermal mechanical simulator, optical microscope （OM） and hardness analysis. The microstructure transformation rule at different cooling rates and solution behaviors of microalloy carbonitride during heating process of simulated specimens were investigated. When the cooling rate changes from 10 to 20 ℃.s-l, microstructures at HAZ are identified as granular bainite, lathy bainite, and quasi-polygonal ferrite. This micro- structure is featured with fine ferrite grains, martensite/ austenite islands dispersed, high-density dislocations, and fine carbonitride particles, resulting in improving the strength and toughness of HAZ. With the cooling rate increasing to above 40 ℃.s-1, the microstructure is pre- dominantly coarse lathy bainite with clear primary aus- tenite grain boundary. While the cooling rate decreases to below 1 ℃.s-1, a fairly small amount of pearlite can be observed at the boundaries. The strength and toughness of HAZ are deteriorated because of coarse grains among these microstructures. Most of microalloy carbonitrides in HAZ could be dissolved in the matrix during heating process. A few of TiN particles existing as residues in the matrix can prevent austenite grain from growing, and then improve the strength and toughness of HAZ.

A hybrid machine learning model for predicting continuous cooling transformation diagrams in welding heat-affected zone of low alloy steels

Continuous cooling transformation diagrams in synthetic weld heat-affected zone(SH-CCT diagrams)show the phase transition temperature and hardness at different cooling rates,which is an important basis for formulating the welding process or predicting the performance of welding heat-affected zone.However,the experimental determination of SH-CCT diagrams is a time-consuming and costly process,which does not conform to the development trend of new materials.In addition,the prediction of SHCCT diagrams using metallurgical models remains a challenge due to the complexity of alloying elements and welding processes.So,in this study,a hybrid machine learning model consisting of multilayer perceptron classifier,k-Nearest Neighbors and random forest is established to predict the phase transformation temperature and hardness of low alloy steel using chemical composition and cooling rate.Then the SH-CCT diagrams of 6 kinds of steels are calculated by the hybrid machine learning model.The results show that the accuracy of the classification model is up to 100%,the predicted values of the regression models are in good agreement with the experimental results,with high correlation coefficient and low error value.Moreover,the mathematical expressions of hardness in welding heat-affected zone of low alloy steel are calculated by symbolic regression,which can quantitatively express the relationship between alloy composition,cooling time and hardness.This study demonstrates the great potential of the material informatics in the field of welding technology.

Effect of Zr-Ti combined deoxidation on impact toughness of coarse-grained heat-affected zone with high heat input welding

In this study, the effects of Zr-Ti combined deoxidation and AI deoxidation on the impact toughness of coarse- grained heat-affected zone in high-strength low-alloy steels were investigated. More fine oxides were formed in the Zr-Ti-killed steel than in Al-killed steel. It was also found that more acicular ferrite grains were formed in the coarse-grained heat-affected zone in the Zr-Ti-killed steel than in Al-killed steel. The impact toughness of coarse-grained heat-affected zone of Zr-Ti-kiUed steel was higher than that of Al-killed steel. The good impact toughness was attributable to the pinning effect of fine oxides and the formation of acicular ferrite grains on fine oxides.

Influence of the secondary welding thermal cycle on the microstructure and property of coarse grain heat-affected zone in an X100 pipeline steel

The influence of the secondary thermal cycle on the microstructure of coarse grain heat-affected zone in an XIO0 pipeline steel was investigated by means of a thermal simulation technique and microscopic analysis method. The property of coarse grain heat-affected zone was characterized by Charpy V-Notch impact properties testing. The results indicated that the experimental steel exhibited local brittleness of intercritically reheated coarse-grained heat-affected zone when the peak tempera- ture of secondary thermal cycle was in the range of two phases region （ ~ and 3/）. There were two main reasons for the local brittleness. The first was that the microstructures of intercritically reheated coarse-grained heat-affected zone were not fined although partial grain recrystallization occurred. The second was that M-A islands, which had the higher content, larger size and higher hardness, existed in intercritically reheated coarse-grained heat-affected zone.

Welding Effect at the Heat Affected Zone by Joining a Gray Cast Iron with Stainless Steel E308-16

Different investigations of the union of dissimilar materials such as stainless steel and different castings have been carried out, but rapid cooling immediately after welding has not been considered, in this work it was investigated how rapid cooling affects the metallurgical microstructure and consequently the mechanical properties. The effect of welding parameters on the microstructure and mechanical properties of the joint between dissimilar metals, an E-308-16 austenitic stainless steel and Gray Cast Iron was also analyzed. Gray cast iron samples (GCI) were fabricated, welded and cooled. The main welding parameters studied in this work are the welding technique and the type of filler electrodes. Flux-coated electrode E-308-16 was applied for this different joint. An experimental study was carried out for the analysis of welded joints of similar and dissimilar steels. The microstructure of the welded joints was analyzed using an optical microscope, in the base metals, heat affected zone (HAZ) and filler metal. The mechanical properties of the welded joints were evaluated by Vickers microhardness and tensile strength tests. The hardness profile showed differences in hardness between the base metals, the heat affected zone and the filler metal. The metallurgical microstructures observed along the welded areas corresponded to the profile. The hardness differences determined the effect on the mechanical and metallurgical characteristics of the welded samples as a result of the cooling rate differences. This research work is important because it allows us to analyze the possibility of reworking pieces of dissimilar materials by welding or, failing that, to determine if this may or may not be possible.

Liquation cracking in the heat-affected zone of IN939 superalloy tungsten inert gas weldments

The main aim of this study was to investigate liquation cracking in the heat-affected zone(HAZ)of the IN939 superalloy upon tungsten inert gas welding.A solid solution and age-hardenable filler metals were further studied.On the pre-weld heat-treated samples,upon solving the secondaryγ′particles in the matrix,primaryγ′particles in the base metal grew to"ogdoadically diced cubes"of about 2μm in side lengths.The pre-weld heat treatment reduced the hardness of the base metal to about HV 310.Microstructural studies using optical and fieldemission scanning electron microscopy revealed that the IN939 alloy was susceptible to liquation cracking in the HAZ.The constitutional melting of the secondary,eutectic,and Zr-rich phases promoted the liquation cracking in the HAZ.The microstructure of the weld fusion zones showed the presence of fine spheroidalγ′particles with sizes of about 0.2μm after the post-weld heat treatment,which increased the hardness of the weld pools to about HV 350 and 380 for the Hastelloy X and IN718 filler metals,respectively.Application of a suitable solid solution filler metal could partially reduce the liquation cracking in the HAZ of IN939 alloy.

High-temperature creep properties of fine grained heat-affected zone in P92 weldment

The simulated fine grained heat-affected zone (FGHAZ) specimens for P92 welded joints were prepared by heat treatment, then the creep tests were carried out at 650 ℃ under the applied stress of 90-120 MPa to investigate high-temperature creep behavior of FGHAZ. The results show that the creep property of FGHAZ is much inferior to that of the base metal, which exhibits the much higher steady creep rate and shorter time to creep fracture. The power law equation can describe the steady creep rate dependence on applied stress, indicating that the stress exponent n of FGHAZ is distinguished between two regions with n=15.1 at high stresses (more than 100 MPa) and n=8.64 at lower stresses. Based on Monkman-Grant equation, the relationship between the secondary creep rate and time to rupture is obtained to evaluate the creep life of FGHAZ with the applied stress above 100 MPa.

Study on laser welded heat-affected zone in new ultralow carbon bainitic steel

800 MPa grade ultralow carbon bainitic （NULCB） steel is the recently developed new generation steel, which was produced by thermo mechanical controlled processing ＆ relaxation-precipitation controlling transformation （TMCP＆RPC） tech- nique. The microstructure and the mechanical properties of the heat-affected zone （HAZ） in NULCB steel under laser welding conditions were investigated by using a Gleeble-1500 thermal simulator. The experimental results indicate that the simplex microstructure in the HAZ is granular bainite that consists of bainite-ferrite （BF） lath and M-A constituent when the cooling time from 800 to 500℃ （t8/5） is 0.3-30 s, and the M-A constituent consists of twinned martensite and residual austenite. As t8/5 increases, the hardness and tensile strength of HAZ decreases, but they are higher than that of the base metal, indicating the absence of softened zone after laser welding. The impact toughness of HAZ increases at first and then decreases when t8/5 increases. The impact energy of HAZ is much higher than that of the base metal when t8/5 is between 3 and 15 s. It indicates that excellent low temperature toughness can be obtained under appropriate laser welding conditions.

Simulation research on heat-affected zone of the NiTi alloy plasma arc welded joints

In order to study the compositions and properties of NiTi alloy plasma arc welded joints, different temperatures of the heat-affected zone（HAZ） are simulated. Temperatures are set at 20 ℃, 400 ℃, 500 ℃, 600 ℃, 700 ℃, 800 ℃, 900 ℃ and 1 000 ℃. X-ray diffraction results of the thermal simulated specimens show that NiTi2 phase emerges in the alloy when the temperature is between 500 ℃ and 800 ℃, while NiTi2 and Ni3 Ti phases emerge when the temperature is between 800 ℃ and 1 000 ℃. Tensile strengths of specimens at 600 ℃ and 900 ℃ are only 68. 0% and 61.3% of the strength at the room temperature respectively due to the emergence of NiTi2 and Ni3 Ti phases.

In situ observation of austenite grain growth behavior in the simulated coarse-grained heat-affected zone of Ti-microalloyed steels

The austenite grain growth behavior in a simulated coarse-grained heat-affected zone during thermal cycling was investigated via in situ observation. Austenite grains nucleated at ferrite grain boundaries and then grew in different directions through movement of grain boundaries into the ferrite phase. Subsequently, the adjacent austenite grains impinged against each other during the α→γtransformation. After the α→γ transformation, austenite grains coarsened via the coalescence of small grains and via boundary migration between grains. The growth process of austenite grains was a continuous process during heating, isothermal holding, and cooling in simulated thermal cycling. Abundant finely dispersed nanoscale TiN particles in a steel specimen containing 0.012wt% Ti effectively retarded the grain boundary migration, which resulted in refined austenite grains. When the Ti concentration in the steel was increased, the number of TiN particles de- creased and their size coarsened. The big particles were not effective in pinning the austenite grain boundary movement and resulted in coarse austenite grains.

Microstructure and embrittlement of the fine-grained heat-affected zone of ASTM4130 steel

The mechanical properties and microstructure features of the fine-grained heat-affected zone（FGHAZ） of ASTM4130 steel was investigated by optical microscope（OM）,scanning electron microscope（SEM）,transmission electron microscope（TEM）,and welding thermal simulation test.It is found that serious embrittlement occurs in the FGHAZ with an 81.37% decrease of toughness,compared with that of the base metal.Microstructure analysis reveals that the FGHAZ is mainly composed of acicular,equiaxed ferrite,granular ferrite,martensite,and martensite-austenite（M-A） constituent.The FGHAZ embrittlement is mainly induced by granular ferrite because of carbides located at its boundaries and sub-boundaries.Meanwhile,the existence of martensite and M-A constituent,which distribute in a discontinuous network,is also detrimental to the mechanical properties.

Study on the microstructure of 12%chromium low carbon stainless steel in a high temperature heat-affected zone

Coarsening, embrittlement and corrosion sensitization in a high temperature heat-affected zone （HTHAZ） are the major problems when 12% chromium low carbon stainless steel is being welded, which induce the deterioration of the impact toughness at a low temperature and intergranular corrosion resistance property. This study investigates the corresponding microstructures in HTHAZ with different chemical compositions and heat inputs through thermal simulation tests. The results show that the martensite content increases with the descending of ferrite factor （FF） when FF is below 9.0 and heat input influences the microstructure of high FF steel in HTHAZ. Martensite of 12% Cr stainless steel in HTHAZ with only Nb stabilization reticularly distributes at ferrite grain boundaries.

Quantitative research on the heat affected zone of weave bead welding for Invar alloy

Quantitative research on the heat affected zone （ HAZ） o f weave bead welding （ WBW） joint fo r Invar alloy is carried out in this paper. Based on the morphology and related data analysis of the weld seam, the width difference o f each layer and the forming mechanism are analyzed. Results show that the bottom layer （ Layer 1 ） has the widest HAZ and the smallest fluctuation, which reaches 1 200 ｜jLm. HAZ width o f layer 2 to 5 is relatively narrower which is basically below 600 jjim, while the amplitude fluctuation is greater. The main reason lies in the welding path. The long straight welding without weave causes the base metal near the groove fully melts which causes by the long straight welding without weave, while welding with weave leads to the uneven and inadequate melting of metal near groove.

MICROSTRUCTURE TRANSFORMATION IN THE WELDING HEAT AFFECTED ZONE OF 800MPa GRADE ULTRA FINE STRUCTURED STEEL

The transformation behavior and microstructure development in the heat affected zone(HAZ)of 800MPa grade ultra fine structured steel was investigated.It was found that the HAZ has intermediate temperature transformation characteristics in a wide range of cooling rates,with the bainite sheaves consisting of bainite ferrite plates without carbide precipitation and retained austenite in the fast cooling regime.At relatively high cooling rates,which corresponded to low heat inputs,the hardness of the simulated HAZ was above that of the base metal.When the cooling rate was below 9C/s,the welding HAZ would have an obvious softening.The analysis of transformation rates in continuous cooling processes was completed by numerical differential method.The result indicated that the microstructure transformation rate of the HAZ in 800MPa grade ultra fine structured steel changed sharply to slow speeds when the cooling time t8/5 is longer than 7s.

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.