Characterization and prediction of the heat-affected zone in a laser-assisted mechanical micromachining process

Laser-assisted mechanical micromachining (LAMM) is a micro-cutting method that employs highly localized thermal softening of the material by continuous wave laser irradiation focused in front of a miniature cutting tool. However, since it is a heat-assisted process, it can induce a detrimental heat-affected zone (HAZ) in the part. This paper focuses on characterization and prediction of the HAZ produced in a LAMM-based micro-grooving process. The heat-affected zone generated by laser heating of H-13 mold steel (42 HRC) at different laser scanning speeds is analyzed for changes in microstructure and microhardness. A 3-D transient finite element model for a moving Gaussian laser heat source is developed to predict the temperature distribution in the workpiece material. The model prediction error is found to be in the 5–15% range with most values falling within 10% of the measured temperatures. The predicted temperature distribution is correlated with the HAZ and a critical temperature range (840–890 °C) corresponding to the maximum depth of the HAZ is identified using a combination of metallography, hardness testing, and thermal modeling.     

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 measured 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 °Cás-1,microstructures at HAZ are identified as granular bainite,lathy bainite, and quasi-polygonal ferrite. This microstructure 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 °Cás-1, the microstructure is predominantly coarse lathy bainite with clear primary austenite grain boundary. While the cooling rate decreases to below 1 °Cás-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.     

The softening prediction of HSLA steel heat-affected zone based on the grey system

The high-strength low-alloy( HSLA ) steel heat-affected zone (HAZ)softening was predicted using a grey model.HSLA steel DILLIMAX690E,NK-HITEN610U2 and BHW35 were taken as examples in the research on ul...     

Precipitation of carbonitrides and high-temperature strength in heat-affected zone of high-Nb containing fire-resistant steel

The morphology and structure of precipitates in the heat-affected zone (HAZ) of fire-resistant steel were studied by transmission electron microscopy, while the simulated elevated temperature strength studies were carried out to elucidate the effect of precipitation state on strength at high temperature. The precipitates were identified as (Nb,Ti)(C,N) with FCC structure. A high density of fine (Nb,Ti)(C,N) precipitated in subcritical HAZ. As the peak temperature of thermal cycle was increased, the density and size of particles were observed to decrease. During austenisation, precipitates were dissolved mostly in the coarse-grained heat-affected zone and dissolved totally in the fusion line. The higher strength at 600?°C was present in coarse-grained heat-affected zone and lower in intercritical heat-affected zone. The different precipitate condition in sub-HAZs led to the different evolution behaviour of carbonitrides on heating and thus high-temperature strength.     

Continuous cooling transformation behavior and impact toughness in heat-affected zone of Nb-containing fire-resistant steel

Simulated heat-affected zone continuous cooling transformation diagram was developed for advanced fireresistant steel. Over a wide range of cooling rates, corresponding to t_8/5 from 6 s to 150 s, granular bainite was the dominant transformation constituent, while the morphology of less dominant martensite-austenite (M-A) constituent changed from film-like to block-type constituent; but the hardness remained similar to the average value of 190-205 HV (0.2). The start and finish transformation temperature was high at 700 °C and 500 °C, and is different from the conventional high strength low alloy steels. It is believed that the high-content (0.09 wt%) of Nb may promote bainite transformation at relatively high temperatures. Martenistic matrix was not observed at high cooling rate and the film-like M-A constituent and blocky M-A constituent with thin film of retained austenite and lath martensite were observed on slow cooling. Excellent impact toughness was obtained in the heat-affected zone with 15-75 kJ/cm welding heat input.     

智能型耐火钢焊接热影响区高温拉伸性能研究

采用热模拟的方法模拟智能型抗震耐火钢Q420FRE的焊接热影响区,并进行600℃的高温拉伸实验。在经受高温拉伸的过程中,单道次的高温拉伸强度在粗晶区最高,逐渐向细晶区和不完全重结晶区降低,到未相变区又升高。双道次焊接热影响区也呈现相同的规律。随着焊接热输入的提高,焊接热影响区粗晶区的高温拉伸强度降低。在焊接热循环作用下,碳氮化物(Nb,Ti)(C,N)的溶解、长大和粗化引起了焊接热影响区不同区域高温拉伸强度的差异。     

Q460q钢模拟焊接热影响区组织及韧性研究

采用热模拟试验机对Q460q高强度桥梁钢热影响区的组织及韧性进行了分析、研究。结果表明,峰值温度Tp为900℃时,模拟焊接热影响区组织为细小的铁素体+少量细小的珠光体,韧性最好。Tp为1100～1350℃时,组织为粗大的粒状及板条状贝氏体,韧性最差。Tp为1350℃时的粗晶区,随着冷却时间t8/5的延长,组织变得粗大,先共析铁素体数量增加,M-A组元尺寸变大,且数量先增加后减少,韧性降低。     

Microstructure and toughness of coarse grain heat-affected zone for Nb-microalloyed X80 pipeline steel

Based on welding thermal simulation on Nb-microalloyed X80 pipeline steel using Gleeble-3500 thermal simulation equipment,microstructure and impact toughness in coarse grain heat-affected zone (CGHAZ) ...     

Numerical simulation of heat-affected zone microstructure in GMAW

The grain size in the heat-affected zone (HAZ) of workpieces is an important factor for evaluating the welding quality. The Monte Carlo technique of grain growth in the heat-affected zone of alloy is widely used. A finite element model (FEM) was employed to simulate the temperature field of gas metal arc welding (GMAW) ; the data of thermal cycles were used in the Monte Carlo model to calculate the grain sizes in different welding heat inputs. The equation for the relationship between Monte Carlo Step and real time has been estimated using regression analysis. Then mathematics model of the grain growth could be worked out by both the kinetic model and the Monte Carlo model. The dynamic process of grain growth was simulated by the result of the Monte Carlo model. The experimental result was used to prove the validity of this method in simulating of microstructure.     

In-situ microscopy study of grain refinement in the simulated heat-affected zone of high-strength low-alloy steel by TiN particle

High-strength low-alloy steels subjected to high heat input welding are susceptible to failure due to low toughness caused by grain coarsening. The effect of TiN on grain refinement in the simulated heat-affected zone (HAZ) was investigated. Because of small amount of Ti addition, abundant dispersed nanoscale TiN precipitates were formed. The TiN precipitates tended to be stable at high temperature and effectively retarded the austenite grain growth by refining the grain size during thermal cycle. Furthermore, the TiN also covered on the surface of Al–Ti complex oxide with MnS and caused low interface energy with ferrite. The acicular ferrite grains nucleated on complex inclusion in austenite grains at intermediate temperature and induced the austenite grain transform to the fine-grained mixed microstructure of acicular ferrite and bainite. The crystallographic grain size became small in the simulated HAZ due to the effective pinning effect and acicular ferrite formation.     

Grain growth simulation with the second phase particle pinning for heat-affected zone of microalloyed steel welds

The second phase particle dispersed in microaUoyed steel has different effects on grain growth depending on their size and volume fraction of the second phase particles which will change during welding thermal cycles.The particle coarsening and dissolution kinetics model was analyzed for continuous heating and cooling.In addition,based on experimental data,the coupled equation of grain growth was established by introdacing limited size of grain growth with the consideration of the second phase particles pinning effects.Using Monte Carlo method based on experimental data model,the grain growth simulation for heat-affected zone of microalloyed steel welds was achieved.The calculating results were well in agreement with that of experiments.     

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.     

Prediction of heat-affected zone using Grey theory

The amount of deformation and the residual stress, which influence welded plates, are related directly to three aspects of the welding process: the method of heat input; the welding speed and the geometrical conditions; limitations of the weldment. The temperature behavior in the heat-affected zone usually determines the strength of the welded plate as a whole. It is important to carry out research into the temperature behavior in this zone, not only because the temperature in this area influences the material composition (the continuous cooling transformation (CCT) diagram) but also because it can be used to supply a feedback signal to the welding system for monitoring and control of the welding process. This study uses Grey theory to predict the temperature message of the heat-affected zone. The first stage of the project is to use differential equations to construct a two-dimensional temperature model of the dynamic conditions during the welding process, and then to model the temperature distribution of the dynamic welding temperature field in order to obtain the range of the heat-affected zone. In addition, the modeled message of the heat-affected zone around the welded bead, and temperature data collected at a dynamic state measurement point, which moves at the same uniform speed as the heat source, are used as inputs to a Grey theory predictor. Using a G(1, 1) modeling method and a G(1, 1) unified-dimensional new message method the temperature behavior in the heat-affected zone is then predicted. The number of temperature data points collected and input to the Grey predictor models varies between 5 and 15. The weld heat input source is rated at 2800–5400 W. The welding speed is 0.6–1.2 cm/s. From the results, it is learnt that the higher the number of temperature data points input to the predictor models, the more precise is the prediction obtained. Additionally, it is found that the G(1, 1) unified-dimensional new message method is more accurate than the G(1, 1) method of prediction, and that the average error is approximately 0.2–0.3%.     

焊接热循环对ASTM4130钢热影响区组织及韧性影响

采用金相、扫描电镜(SEM)和焊接热模拟方法,研究了不同峰值温度和焊接线能量对ASTM4130钢焊接热影响区(HAZ)显微组织、冲击韧性和断口形貌的影响.结果表明,ASTM4130钢热影响区除回火软化区外均发生脆化现象.当峰值温度为1200 ℃和1350℃时,由于晶粒粗大,且产生了贝氏体、未回火马氏体和M-A组元等非平衡组织,其冲击韧性损失达母材的94.5%,脆化现象最严重.当峰值温度为950℃,冲击韧性较低的原因是该区产生了未回火马氏体和块状铁素体.当峰值温度为800℃时,晶界附近碳化物聚集和不均匀分布,以及块状铁素体的存在,造成该区发生脆化.焊态下焊接线能量对ASTM4130钢粗晶区的冲击韧性影响较小.     

Impurity-induced embrittlement of heat-affected zone in welded Mo-based alloys

Characteristics of strength and plasticity, fracture mode and grain boundary segregation for two Mo-based alloys with different bulk compositions, recrystallized by either furnace annealing or rapid heating followed by quenching, are studied as a function of heating temperature by mechanical test, scanning electron microscopy, Auger electron spectroscopy and computer simulation. There exists an essential difference in both segregation behaviour and mechanical properties between as-annealed and as-quenched structural states. The rapid quenching causes strong oversaturation of the grain boundaries. In this case, intergranular enrichment is approximately twice as high as that in as-annealed alloys, and spontaneous nucleation of brittle microcracks is observed at certain embrittled boundaries. The proposed high-speed heat treatments are considered as a promising method for modelling of the structural states of the heat-affected zone of weldments. The results obtained are discussed from the viewpoint of possible reasons of impurity-induced embrittlement of Mo-based alloys.     

Carbide and hardness development in the heat-affected zone of tempered and postweld heat-treated 2.25Cr-1Mo steel weldments

The temper-bead and the conventional weaving, multipass welding procedures have been developed to eliminate postweld heat treatment (PWHT) of creep resistant 2.25Cr-1Mo steel. Both procedures aim to refine and temper the heat-affected zone (HAZ). The temper-bead procedure resulted in a martensitic HAZ and a homogeneous fine-grain size. Hardness was not decreased by high heat input weaving fillout passes. Upper bainite developed in the conventional weaving HAZ, although grain refinement was inhomogeneous and some martensite was present. However, the conventional weaving procedure appears to produce a lower as-welded hardness than the temper-bead procedure. The carbides within the temper-bead HAZ aged more rapidly than those in the conventional weaving HAZ as a result of the initial martensitic temper-bead HAZ microstructure. Previous work indicated that the temper-bead HAZ toughness decreased after 1000 h of tempering at 538°C. This correlated with the coarsening and the agglomeration of the carbides. The maximum hardness occurred within the as-welded coarse-grained HAZ. The PWHT resulted in the greatest decrease in hardness and also reduced hardness variability throughout the HAZ. The hardness decreased to postweld heat-treated values after approximately 1000 h at 538°C and softening was associated with the precipitation and coarsening of acicular carbides and the development of coarse grain-boundary carbides.     

In situ observation and electron backscattered diffraction analysis of granular bainite in simulated heat-affected zone of high-strength low-alloy steel

In this study, the formation of granular bainite (GB) which may form in the heat-affected zone of double-sided double arc welding joint was in situ observed. The crystallographic characteristics of GB were also compared with those of microstructure mainly comprising lath bainite (LB). The results show that bainite packets exist at the initial stage of GB transformation and can be distinguished by the distribution of retained austenite and martensite–austenite constituents. The bainite blocks of GB are larger than those of LB which makes both the length and fraction of high-angle grain boundaries in GB less than those of LB which partly lead to the brittleness of granular bainite.