EFFECT OF Zr ADDITION TO Ti-KILLED STEEL ON INCLUSION FORMATION AND MICROSTRUCTURAL EVOLUTION IN WELDING INDUCED COARSE-GRAINED HEAT AFFECTED ZONE

Effects of Zirconium on the chemical component and size distribution of Ti-bearing inclusions, favored the grain refinement of the welding reduced, coarse-grained heat affected zone （CGHAZ） with enhanced impact toughness in Ti-killed steels, which were examined based on experimental observations and thermodynamic calculations. It indicated that the chemical constituents of inclusions gradually varied from the TiO oxide to the Ti-O＋Zr-O compound oxide and a single phase of the ZrO2 oxide, as the Zr content increased from zero to 0.0100%. A trace of Zr （0.0030%-0.0080%, depending on the oxygen content in liquid steel） provided a large amount of nucleating core for Ti oxide because of the larger specific density of ZrO2 oxide, and produced a small size distribution of the inclusions favorable for acicular ferrite transformation with a high nucleation rate in the CGHAZ, and a high volume fraction of acicular ferrite was obtained in the CGHAZ, with enhanced impact toughness. Otherwise, a high content of Zr （-0.0100%） produced a single phase Zr02, which was impotent to nucleate acicular ferrite, and a microstructure composed of ferrite side plate and grain boundary ferrite developed in the CGHAZ. The experimental results were confirmed by thermodynamic calculations.

HEAT-AFFECTED ZONE FRACTURE TOUGHNESS OF HIGH STRENGTH STEELS EVALUATED USING THE WEAKEST LINK MODEL

A probabilistic model is derived based on the weakest link theory to evaluate the specimen thickness effect and the size effect of coarse-grained heat-affected zone (CGHAZ) on CTOD toughness values in heat-affected zones (HAZ) of high strength steels. The scatter of CTOD fracture toughness values in heterogeneous weldments is predicted using this model. The specimen thickness has influence on the CTOD toughness values of HAZ. With increase in the specimen thickness the estimated and experimental mean CTOD values decrease while the lower boundary CTOD values remain almost unchanged. The size of CGHAZ has strong negative effect on the mean CTOD values of HAZ. The estimated mean CTOD values and lower boundary CTOD values agree with experimental CTOD testing results of HAZ notched specimens. The facture toughness scatter is dependent on the Weibull shape parameter a and specimen thickness.

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.

Effect of Magnesium on Inclusion Formation in Ti-Killed Steels and Microstructural Evolution in Welding Induced Coarse-Grained Heat Affected Zone

Effects of Mg on the chemical component and size distribution of Ti-bearing inclusions favored grain refinement of the welding induced coarse-grained heat affected zone （CGHAZ）, with enhanced impact toughness in Ti-killed steels, which were examined based on experimental observations and thermodynamic calculations. The results indicated that the chemical constituents of the inclusions gradually varied from the Ti-O＋Ti-Mg-O compound oxide to the Ti-Mg-O＋MgO compound oxide and the single-phase MgO, as the Mg content increased from 0.002 3M to 0.006%. A trace addition of Mg （approximately 0. 002%） led to the refinement of Ti-bearing inclusions by creating the Ti-Mg-O compound oxide and provided favorable size distribution of the inclusions for acicular ferrite transformation with a high nucleation rate in the CGHAZ, and a high volume fraction of acicular ferrite was obtained in the CGHAZ with enhanced impact toughness. Otherwise, a high content of Mg （approximately 0. 006%） produced a single-phase MgO, which was impotent to nucleate an acicular ferrite, and a microstructure comprised of a ferrite side plate and a grain boundary ferrite developed in the CGHAZ. The experimental results were confirmed by thermodynamic calculations.

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 from 800 to 500℃ on microstructure,precipitates,substructure and microhardness of the reheated coarse-grained heat-affected zone(CGHAZ)of G115 novel martensitic heat-resistant steel.As revealed from the results,the microstructure of un-altered CGHAZ(UACGHAZ)and supercritically CGHAZ(SCCGHAZ)was lath martensite,and structural heredity occurred.Intercritically reheated CGHAZ(IRCGHAZ)exhibited martensite and over-tempered martensite,and subcritical CGHAZ(SCGHAZ)displayed martensite and under-tempered martensite.The austenite in UACGHAZ and SCCGHAZ was transformed with the diffusion mechanism during the first thermal cycle,but with the non-diffusion mechanism during the second thermal cycle.For this reason,A_(c1) and A_(c3) during the second thermal cycle were significantly lower than those during the first thermal cycle,and A_(c1) and A_(c3) were reduced by nearly 14 and 44℃,respectively.Since the content and stability of the austenite alloy during the second thermal cycle of IRCGHAZ were lower than those during the first thermal cycle,M_(s) increased by nearly 30℃.There were considerable precipitates in the over-tempered region of IRCGHAZ,and the Laves phase was contained,which was not conducive to high-temperature creep property.Moreover,the dislocation density and the number of sub-grains in the region were lower,resulting in a sharp decrease in the microhardness,and it was the weak area in the reheated CGHAZ.

Microstructure and defect of titanium alloy electron beam deep penetration welded joint

The microstructure, phase composition and cold shut defect of thick titanium alloy electron beam welded joint were studied. The results showed that the microstructure of weld zone was composed of α′ phase; the heat affected zone was divided into fine-grained zone and coarse-grained zone, the microstructure of fine-grained zone was primary α phase ＋ β phase ＋ equiaxed α phase, and the microstructure of coarse-grained zone was primary α phase ＋ acicular α′ phase; the microstructure of base metal zone basically consisted of primary α phase, and a small amount of residual β phase sprinkled. The forming. reason of cold shut was analyzed, and the precaution of cold shut was proposed.

国产06Ni9DR钢焊接热影响区组织和低温韧性

采用扫描电镜(SEM)、背散射电子衍射(EBSD)和焊接热模拟技术,研究了单次热循环不同峰值温度对国产06Ni9DR钢焊接热影响区(HAZ)显微组织和低温冲击韧性的影响.结果表明,06Ni9DR钢HAZ的-196℃冲击吸收能量均低于母材,HAZ整体发生了脆化.粗晶区脆化最为严重,原因是原始奥氏体晶粒粗大及其导致的有效大角度晶界较少,残余奥氏体量少且不稳定,以及较大的位错密度和粗大马氏体的存在.晶界呈链状分布的大块逆转奥氏体和M-A组元的存在导致回火区脆化程度仅次于粗晶区.细晶区和不完全脆化区的韧性低于母材,主要是因为淬火马氏体的存在和残余奥氏体的低温稳定性差.

交流干扰下X100管线钢及其热影响区在库尔勒土壤模拟液中的腐蚀行为

通过Gleeble热模拟实验机模拟了X100管线钢的粗晶热影响区(CGHAZ)及再热临界粗晶热影响区(ICCGHAZ)微观组织.采用电化学测试、浸泡实验及表面分析技术研究了交流干扰下X100管线钢母材、CGHAZ及ICCGHAZ在库尔勒土壤溶液中的腐蚀行为.结果表明:交流干扰下X100管线钢母材、CGHAZ及ICCGHAZ都表现为活性溶解,平均腐蚀速率随交流电流密度的增大而增加.交流干扰造成的极化电位振荡幅值及微观组织对X100管线钢母材、CGHAZ及ICCGHAZ的平均腐蚀速率和腐蚀形貌有着重要影响.在5 mA·cm^(-2)交流电流密度干扰下,母材的腐蚀电位最负、平均腐蚀速率最大,ICCGHAZ的腐蚀电位最正、平均腐蚀速率最小,CGHAZ的腐蚀电位及平均腐蚀速率都居中;在20 mA·cm^(-2)及50 mA·cm^(-2)交流电流密度干扰下,ICCGHAZ腐蚀电位最负、平均腐蚀速率最大,母材的腐蚀电位最正、平均腐蚀速率最小,CGHAZ的腐蚀电位及平均腐蚀速率都仍居中.在20 mA·cm^(-2)交流电流密度交流干扰下,X100管线钢发生局部腐蚀,CGHAZ、ICCGHAZ发生明显的晶界腐蚀,GCHAZ晶界腐蚀形貌呈缝隙状、ICCGHAZ晶界腐蚀形貌为连续孔洞.

Structure Character of M-A Constituent in CGHAZ of New Ultra-Low Carbon Bainitic Steel under Laser Welding Conditions

800 MPa grade new ultra-low carbon bainitic （NULCB） steel is the recently developed new generation steel. The microstructure in the coarse-grained heat affected zone （CGHAZ） of NULCB steel under laser welding conditions was investigated by thermal simulation. The influence of the cooling time from 800℃ to 500℃.t8/5 （0.3-30 s）, on the microstructure of the CGHAZ was discussed. The experimental results indicate that the microstructnre of the CGHAZ is only the granular bainite which consists of bainitic ferrite （BF） lath and M-A constituent while t8/5 is 0.3-30 s. The M-A constituent consists of twinned martensite and residual austenite, and the change of the volume fraction of the residual austenite in the M-A constituent is very small when t8/5 is between 0.3 and 30 s. The morphology of the M-A constituent obviously changes with the variation of t8/5.As t8/5 increases, tile average width, gross and shape parameter of the M-A constituent increase, while the line density of the M-A constituent decreases.

复杂回转体的电火花线切割加工及表面特性分析

论述了回转体电火花线切割的原理及应用 ,实验并分析了电火花脉冲宽度、线电极进给速度和工件旋转速度等加工参数对加工表面特性的影响 ,借助于扫描电子显微镜识别和量化了加工表面的重凝固层和热影响层 .

Microstructure evolution and corrosion resistance in simulated CGHAZ of X80 high-deformability pipeline steel

In this study, the microstructure evolution and corrosion resistance in O. 5 M Na2CO3 - 1 M NaHCO3 solution of X80 high-deformability （ X8OHD ） pipeline steel coarse-grained heat-affected zone （CGHAZ） with several heat input levels were investigated. It is shown that the microstructure of CGHAZ changes from bainite ferrite to granular bainite as the heat input increasing. In addition, the corrosion resistance and the stability of passive film of base material are better than those of CGHAZ with several heat input levels. Too small or too big heat input is inadvisable and better corrosion resistance of CGHAZ is attained when heat input is 30 kJ/cm.

Analysis on SH-CCT curves of HD15Ni1MnMoNbCu steel for nuclear power station

The microstructural evolution and Vickers hardness measurement in the welding heat-affected zone （HAZ） of HD15 Nil MnMoNbCu steel for nuclear power station were investigated by Gleeble-3180 thermal mechanical simulator, and the simulated HAZ continuous cooling transformation curves （SH-CCT） were measured simultaneously. With ts/5 inereasing from 3.75 s to 15 000 s, the product was obtained martensite, bainite, ferrite and pearlite, successively. The result of microstructure and Vickers hardness in the heat-affected zone was in good agreement with those measured by SH-CCT diagram with the heat input 16. 2 kJ/cm as an example to weld the HD15Ni1MnMoNbCu steel pipe using TIG/SMAW/SAW welding methods.

Effect of Heat Input on Cleavage Crack Initiation of Simulated Coarse Grain Heat-affected Zone in Microalloyed Offshore Platform Steel

The combined effects of martensite-austenite（MA）constituent and pearlite colony on cleavage crack initiation in the simulated coarse-grained heat-affected zone（CGHAZ）of V-N-Ti microalloyed offshore platform steel under different heat inputs were investigated.The results of welding simulation,instrumented impact test,and quantitative analysis indicated that the size of the MA constituent decreased with the increase in cooling time,and by contrast,the size of the pearlite colony increased.According to Griffith theory,the critical sizes of cleavage microcracks were calculated.With the increase of cooling time,the calculated microcrack size could be characterized by the size of the MA constituent first,and then fitted with the size of the pearlite colony.Moreover,the calculated microcrack size variation was opposite to the microcrack initiation energy.This phenomenon is probably due to the combined effects of the MA constituent and pearlite colony with increasing the cooling time of the specimen′s temperature from800 to 500 ℃.

Effect of cooling time t8/5 on microstructure and toughness of Nb-Ti- Mo microalloyed C-Mn steel

In order to further optimize welding process of Nb-Ti-Mo microalloyed steel, welding thermal cycles on coarse-grained heat-affected zone （CGHAZ） of welded joints were simulated using Gleeble 1500. The microstructure and low-temperature impact fracture were investigated using a scanning electron microscope and a pendulum impact machine, respectively. Moreover, the relationship between cooling time ts/5 and the microstructure of CGHAZ was discussed, and the effect of microstructure on impact toughness was also studied. As cooling time increased, martensite fraction decreased from 97.8% （3 s） to 3.0% （60 s）. The fraction of martensite/austenite （M/A） constituent increased from 2.2% （3 s） to 39.0% （60 s）, its shape changed from granular to strip, and the maximum length increased from 2.4 μm （3 s） to 7.0 μm （60 s）. As cooling time increased, the prior austenite grain size increased from 34.0 μm （3 s） to 49.0 gm （60 s）, the impact absorption energy reduced from 101.8 J （5 s） to 7.2 J （60 s）, and the fracture mechanism changed from quasi-cleavage fracture to cleavage fracture. The decreased toughness of CGHAZ was due to the reduction of lath martensite-content, coarsening of original austenite grain, and increase and coarsening of M/A constituent. The heat input was controlled under 7 kJ cm-1 during actual welding for these steels.

Effect of welding heat input on microstructure and impact toughness in CGHAZ of X100Q steel

The coarse-grained heat-affected zones (CGHAZs) of X100Q steel were reproduced via simulating their welding thermal cycles with the varying heat input (Ef) from 10 to 55 kJ/cm in Gleeble3500 system. The microstructures were characterized, and the impact toughness was estimated from each simulated sample. The results indicate that the microstructure in each simulated CGHAZ was primarily constituted of lath-like bainite. With the decreased heat input and accordingly the lowered Ar3 (the onset temperature for this transition), the prior austenite grain and the bainitic packet/block/lath substructure were refined remarkably, and the impact toughness was enhanced due to the microstructure refinement. The bainitic packet was the microstructural unit most effectively controlling the impact properties in CGHAZ of X100Q steel, due to their close correlation with the 50% fracture appearance transition temperatures, their size equivalent to the cleavage facet and their boundaries impeding the crack propagation.

Embrittlement and toughening in CGHAZ of ASTM4130 steel

In the present investigation, a thermal welding simulation technique was used to investigate the mechanical properties and microstructure features of the coarse-grained heat-affected zone (CGHAZ) of ASTM4130 steel. The effect of post welding heat treatment (PWHT) and welding heat inputs on the toughness of CGHAZ was also analyzed. The results show that CGHAZ has the lowest toughness, which is only 5.5%-7.1% of the base metal. CGHAZ is mainly composed of dislocation martensite, up-per and lower bainite, and M-A constituents. But after PWHT, carbides precipitate from non-equilibrium microstructures of CGHAZ accompanying some retained austenite which transforms into low bainite, thereby enhancing toughness over the base metal. Therefore, the key microstructure factors affecting fracture toughness are lathlike non-equilibrium microstructure and lowered supersaturation before and after PWHT respectively. When welding heat input is between 12 kJ/cm and 28 kJ/cm, the mechanical properties in CGHAZ of ASTM4130 with single-pass welding can satisfy the requirements when PWHT is applied.

Effect of Boron on CGHAZ Microstructure and Toughness of High Strength Low Alloy Steels

Effect of boron on the microstructure and impact toughness in the coarse-grained heat-affected zone（CGHAZ）of two high strength low alloy steels,boron-free and boron-containing,was investigated by means of weld thermal simulation test.The result shows that,for the boron-free steel,a microstructure consisting of grain boundary ferrite degenerates pearlite and granular bainite for longer t8/5（the cooling time from 800 to 500 ℃）,while lath bainite for shorter t8/5.For the boron-containing steel,granular bainite is dominant for a wide range of t8/5.Continuous cooling transformation（CCT）study on the CGHAZ indicates that the transformation start temperature decreases by about 50-100℃under different t8/5,for the boron-containing steel compared with the boron-free steel.The presence of boron suppresses the nucleation of ferrite at prior austenite grain boundaries and hence enlarges the range of t8/5for granular bainite transformation.However,the addition of boron deteriorates the impact toughness of CGHAZ,which may be due to a markedly increased fraction of martensite-austenite（M-A）constituents and decreased fraction of high angle grain boundaries.

Experimental investigation and design of aluminum columns with longitudinal welds

This paper presents an experimental investigation of longitudinally welded aluminum alloy I-section columns subjected to pure axial compression.The specimens were fabricated using 6061-T6 heat-treated aluminum alloy.The test program included 20 column tests which were separated into 2 test series of different types of welding sections.Each test series contained 10 columns.All the specimens were welded using the Tungsten Inert Gas welding method.The length of the specimens ranged from 442 to 2433 mm in order to obtain a column curve for each test series.The observed failure mode for the column tests includes mainly flexural buckling around the minor axis and the major axis by applying support except for one column(ZP 1217-1)which buckled in the local zone and some columns which failed in the weld.The test strengths were compared with the design strengths predicted by the European Code and China Code for aluminum structures.The purpose of this paper is to present the tests results of two typically longitudinally welded I-section columns,and to check the accuracy of the design rules in the current specifications.