Effects of Niobium on the Mechanical Properties and Corrosion Behavior of Simulated Weld HAZ of HSLA Steel

Simulated microstructures of the TZ, ICHAZ, FGHAZ, and CGHAZ of weld joints made from two kinds of HSLA steels with 0 or 0.079 wt pct Nb were prepared by means of heat treatment. Optical microscopy and transmission electron microscopy were used to observe microstructures and the distribution of nanosized precipitates in the simulated weld heat-affected zone (HAZ). Mechanical properties of the simulated HAZ were measured by tensile tests, and the corrosion behavior in simulated seawater was studied using electrochemical and immersion tests. It was shown that the ICHAZ and CGHAZ possess the worst overall mechanical properties in both kinds of HSLA steels, and the corrosion resistance in the descending order was the BM, TZ, FGHAZ, ICHAZ, and CGHAZ. Contrasting Nb-bearing and Nb-free steel demonstrated that the strength and corrosion resistance of the simulated HAZ were enhanced by Nb microalloying, which resulted in precipitation, homogeneous microstructures, and other relative sequences. Moreover, the surface of the Nb-bearing steel formed compact corrosion product films with higher resistance to ion migration; thus, the initiation and propagation of pitting holes were effectively inhibited.     

High Strength Steel Plates for Large Container Ships

Heavy thickness steel plates with a good combination of strength,toughness and weldability have been demanded for building of large container ships.High strength steel plates with heavy gauge of EH36,EH40 and EH47 grades were developed by optimizing chemical compositions and TMCP process parameters.Micro alloying elements of Ti and Nb were added to the three steel grades for enhancing the strength and toughness of base plates.The strength of base plates of the EH47 grade was further enhanced with the help of the increased amount of substitutional solid solutes,such as manganese,copper,nickel,or chromium.EH36 steel plates for high heat input over 550 kJ/cm were manufactured by improving thermal stability of TiN particles for better weld heat-affected zone toughness.Thermally stabilized TiN particles effectively suppress grain growth at weld heat-affected zone,leading to better toughness.These steel plates showed excellent mechanical properties of base plates and welded joints.     

Low Cost High Performance Steel Plate With Superior HAZ Toughness for Large Heat Input Welding

In this paper, a laboratory study has been made to develop low cost high performance steel plates with superior HAZ toughness for large heat input welding. Simulated results show that the absorbed impact energy of heat-affected zone (HAZ) at -20℃reaches above 200J when large heat inputs of 100 to 400kJ/cm were applied, suggestive of superior HAZ toughness for large heat input welding of developed steel plate. The microstructures in HAZ are transformed from mainly fine ferrite and bainite at 100kJ/cm, through an intermediate stage of ferrite, bainite and pearlite at 200 and 300kJ/cm, to nearly fine ferrite and pearlite at 400kJ/cm. The prior austenite grain size and ferrite grain size in HAZ are controlled to ～50 and ～20μm, respectively. The high HAZ toughness is due to the inhibition of prior austenite grain size at high temperatures and the formation of beneficial microstructures to HAZ toughness during continuous cooling.     

780MPa级重载汽车用大梁钢的工业试制

以低碳复合添加微合金元素铌和钛为成分设计思路,综合运用细晶强化、相变强化和析出强化三种强化机制,在国内某厂1750mm半连续热连轧机组进行了780MPa级大梁钢的工业试制.结果表明,终轧温度需控制在780～860℃,卷取温度需控制在450～550℃.大梁钢的显微组织为贝氏体和少量的细晶铁素体,并获得了大量弥散的尺度为1...     

Microstructure development in high-strength low-alloy steel welds

The reliability of steel welds becomes more critical issue with increasing steel strength,because brittle phases are more likely to form in the weld metals and heat-affected zone(HAZ) and thereby the toughness and ductility of the welds are degraded.Therefore,refinement of microstructure and minimization of the brittle phases are necessary to improve the reliability of the high-strength steel welds.In this presentation,microstructure formation that controls the toughness of weld metals and HAZ in high-strength low-alloy(HSLA) steel welds is reviewed and possible routes to the improvement of the weld microstructure and weld toughness are discussed.     

大线能量焊接用海工钢的夹杂物与组织

 海洋工程领域对可大线能量焊接且低温韧性优良的厚钢板需求迫切。宝钢通过微合金化技术及调质工艺，开发出了满足大线能量焊接的E550级海洋工程用钢。通过冶炼、轧制、调质试验、焊接热模拟试验、电镜观察统计及金相观察，研究了大线能量焊接用海工钢H1夹杂物、显微组织及力学性能，并与宝钢现有海工钢E550进行了对比。研究结果表明，H1中夹杂物为Al2O3、MnS、Al2O3 Ti3O5、Al2O3 MnS、TiN MnS、Al2O3 Ti3O5 MnS；E550中夹杂物为Al2O3、CaO·xAl2O3(CAx)、CaO CaS、CAx CaS、CAx CaS TiN。H1力学性能满足E550级海工钢要求，且满足50和100 kJ/cm线能量焊接要求。其HAZ韧性改善的机理为，低硅低铝质量分数有利于减少局部脆性区；钛质量分数的降低，有利于抑制TiC脆化，提高HAZ韧性。     

Laser Welding of Ultra-Fine Grained Steel SS400

Steeliswidelyusedbecauseofitsgoodcompre hensive properties ,plentyofresourceandlowerprice .Thestrengthandtoughnessaretwoimpor tantpropertiesofsteels ,andpeoplemakeeffortstoincreasetheirvalues .Addingalloyingelementandcontrollingmicrostructurearetwobasicwaystoac complishtheaim .Therefinedmicrostructureob tainedbyprocessingtechniqueenablesthestrengthandtoughnessofsteeltobeincreasedwithoutaddingalloyingelementandtheratioofperformance costtobeincreased .Theultra finegrainedsteelshavefer ritegrains…     

Welding-Induced Microstructure Evolution of a Cu-Bearing High-Strength Blast-Resistant Steel

A new high strength, high toughness steel containing Cu for precipitation strengthening was recently developed for naval, blast-resistant structural applications. This steel, known as BlastAlloy160 (BA-160), is of nominal composition Fe-0.05C-3.65Cu-6.5Ni-1.84Cr-0.6Mo-0.1V (wt pct). The evident solidification substructure of an autogenous gas tungsten arc (GTA) weld suggested fcc austenite as the primary solidification phase. The heat-affected zone (HAZ) hardness ranged from a minimum of 353 HV in the coarse-grained HAZ (CGHAZ) to a maximum of 448 HV in the intercritical HAZ (ICHAZ). After postweld heat treatment (PWHT) of the spot weld, hardness increases were observed in the fusion zone (FZ), CGHAZ, and fine-grained HAZ (FGHAZ) regions. Phase transformation and metallographic analyses of simulated single-pass HAZ regions revealed lath martensite to be the only austenitic transformation product in the HAZ. Single-pass HAZ simulations revealed a similar hardness profile for low heat-input (LHI) and high heat-input (HHI) conditions, with higher hardness values being measured for the LHI samples. The measured hardness values were in good agreement with those from the GTA weld. Single-pass HAZ regions exhibited higher Charpy V-notch impact toughness than the BM at both test temperatures of 293 K and 223 K (20 °C and –50 °C). Hardness increases were observed for multipass HAZ simulations employing an initial CGHAZ simulation.     

EAF—CSP流程Nb微合金管线钢的开发研究

基于对Nb微合金化技术的系统研究，珠钢利用Nb—Ti复合微合金化技术，运用60mm厚度铸坯在EAF—CSP流程成功地开发了9．50mm厚度X60管线钢。结果表明，通过合理的冶金成分设计和控制连铸工艺、均热及除鳞工艺和控轧控冷工艺，解决了EAF—CSP流程Nb微合金化X60管线钢的组织混晶问题。开发的含NbX60管线钢组织均匀细小，晶粒度11．5～12．0级，具有优异的强韧性能和良好的焊接性能。     

Study of welding ultra‐fine grained ferrite steel by CO2 laser

Ultra‐fine grained ferrite steels have higher strength and better toughness than the normal ferrite steels because of their micrometer or sub‐micrometer sized grains. In this paper the ultra‐fine grained steel SS400 is welded by CO₂ laser. The shape of weld, cooling rate of HAZ, width of HAZ, microstructures and mechanical properties of the joint are discussed. Experimental results indicate that laser beam welding can produce weld with a large ratio of depth to width. The cooling rate of HAZ of laser beam welding is fast, the growth of prior austenite grains of HAZ is limited, and the width of weld and HAZ is narrow. The microstructures of weld metal and coarse‐grained HAZ of laser beam welding mainly consist of B_L + M (small amount). With proper laser power and welding speed, good comprehensive mechanical properties can be acquired. The toughness of weld metal and coarse‐grained HAZ are higher than that of base metal. There is no softened zone after laser beam welding. The tensile strength of a welded joint is higher than that of base metal. The welded joint has good bending ductility.     

Effect of Nb on Mechanical Properties of HAZ for High-Nb X80 Pipeline Steels

The mechanical properties of heat affected zone (HAZ) of two commercial high-Nb X80 grade pipeline steels with different alloy elements were investigated using thermal simulation performed on a Gleeble-3500 thermal simulator. The results showed that the high-Nb steels have excellent weldability. Embrittlement regions appear in coarse grain heat affected zone (CGHAZ) and intercritically heat affected zone (ICHAZ); Softening region appears in fine-grain heat affected zone (FGHAZ), and the strength here was even lower than 555 MPa as required in the standard. Meanwhile, with the increase of heat input, the strength and the toughness of HAZ of steel with high Nb, C and lower alloy decrease notably. Therefore, take into account the welding procedure during manufacture of weld pipe, suitable amount of alloy elements, such as Cr, Ni, Cu, Mo and so on, is necessary for high Nb X80 heavy-thick steel plate.     

Microstructural Characterization of the Heat-Affected Zones in Grade 92 Steel Welds: Double-Pass and Multipass Welds

The microstructure in the heat-affected zone (HAZ) of multipass welds typical of those used in power plants and made from 9 wt pct chromium martensitic Grade 92 steel is complex. Therefore, there is a need for systematic microstructural investigations to define the different regions of the microstructure across the HAZ of Grade 92 steel welds manufactured using the traditional arc welding processes in order to understand possible failure mechanisms after long-term service. In this study, the microstructure in the HAZ of an as-fabricated two-pass bead-on-plate weld on a parent metal of Grade 92 steel has been systematically investigated and compared to a complex, multipass thick section weldment using an extensive range of electron and ion-microscopy-based techniques. A dilatometer has been used to apply controlled thermal cycles to simulate the microstructures in distinctly different regions in a multipass HAZ using sequential thermal cycles. A wide range of microstructural properties in the simulated materials were characterized and compared with the experimental observations from the weld HAZ. It has been found that the microstructure in the HAZ can be categorized by a combination of sequential thermal cycles experienced by the different zones within the complex weld metal, using the terminology developed for these regions based on a simpler, single-pass bead-on-plate weld, categorized as complete transformation, partial transformation, and overtempered.     

Weldability of Low Carbon Transformation Induced Plasticity Steel

Transformation induced plasticity （TRIP） steel exhibited high or rather high carbon equivalent （CE） because of its chemical composition, which was a particularly detrimental factor affecting weldability of steels. Thus the weldability of a TRIP steel （grade 600） containing （in mass percent, %） 0.11C-1. 19Si-1.67Mn was extensively studied. The mechanical properties and impact toughness of butt joint, the welding crack susceptibility of weld and heat affected zone （HAZ） for tee joint, control thermal severity （CTS） of the welded joint, and Y shape 60° butt joint were measured after the gas metal arc welding （GMAW） test. The tensile strength of the weld was higher than 700 MPa. Both in the fusion zone （FZ） and HAZ for butt joint, the impact toughness was much higher than 27 J, either at room temperature or at -20 ℃, indicating good low temperature impact ductility of the weld of TRIP 600 steel. In addition, welding crack susceptibility tests revealed that weldments were free of surface crack and other imperfection. All experimental results of this steel showed fairly good weldability. For application, the crossmember in automobile made of this steel exhibited excellent weldability, and fatigue and durability tests were also accomplished for crossmember assembly.     

Crack-initiation toughness and crack-arrest toughness in advanced 9 pct Ni steel welds containing local brittle zones

The present study investigates the influence of local brittle zones (LBZs) on the fracture resistance of the heat-affected zones (HAZs) in quenched, lamellarized, and tempered (QLT) 9 pct Ni steel weld joints. The results of Charpy impact tests using simulated coarse-grained, heat-affected zone (CGHAZ) specimens show that the intercritically reheated (IC) CGHAZ and unaltered (UA) CGHAZ are the primary and secondary LBZs, respectively, of the steel at cryogenic temperature. Compact crack arrest (CCA) tests and crack-tip opening displacement (CTOD) tests were conducted at a liquefied natural gas (LNG) temperature to measure the variations in crack-arrest toughness and crack-initiation toughness along the distance from the fusion line (FL) within the actual HAZ. While CTOD tests show a decrease in toughness when approaching the FL, i.e., the regions containing LBZs, the crack-arrest-toughness values are found to be higher than those in the regions near the base materials. This is due to the fact that the crack-arrest toughness is governed by the fraction of microstructures surrounding LBZs instead of the LBZs themselves. By direct comparison of the brittle-crack-arrest toughness (K _a ) with the brittle-crack-initiation toughness (K _c ), this investigation has determined that, with regard to crack-arrest behavior, the LBZs of QLT-9 pct Ni steel do not limit the practical safety performance of the weld joints in LNG storage tanks.     

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.     

Fatigue crack propagation in austenitic stainless steel weldments

The fatigue crack propagation rate (FCPR) in 316L austenitic stainless steel (ASS) and its weldments was investigated, at two loading amplitudes, 7 and 8.5 kN, under tension-tension mode. Two welding techniques, submerged arc welding (SAW) and manual arc welding (MAW), have been used. Magnetic δ-ferrite, depending upon Ni and Cr content in the metal, in the weld zone upon solidification was considered. The ferrite number (FN) of δ-ferrite formed in the SAW zone was much higher (maximum 9.6) compared to the corresponding value (maximum 0.75) in the MAW zone. A fatigue starter notch was positioned at different positions and directions with respect to the weld zone, in addition to the heat-affected zone (HAZ). Regions of high and low FCPRs as the fatigue crack propagated through and across the weld zone have been noticed. This is related to the direction of the tensile residual stresses present in weld zone, resulting from solidification of the weld metal. The FCPR was higher along through the HAZ and weld zone because of the microstructural change and direction and distribution of tensile residual stresses. The FCPR was much lower when crack propagated perpendicular to the weld zone, particularly in the case of SAW in which higher δ-ferrite volume fraction was noticed. A lower FCPR found across the weld zone, in both SAW and MAW, was accompanied by rubbed areas in their fractures.     

Characterization of HAZ of API X70 Microalloyed Steel Welded by Cold-Wire Tandem Submerged Arc Welding

High-strength low-carbon microalloyed steels may be adversely affected by the high-heat input and thermal cycle that they experience during tandem submerged arc welding. The heat-affected zone (HAZ), particularly the coarse-grained heat-affected zone (CGHAZ), i.e., the region adjacent to the fusion line, has been known to show lower fracture toughness compared with the rest of the steel. The deterioration in toughness of the CGHAZ is attributed to the formation of martensite-austenite (M-A) constituents, local brittle zones, and large prior austenite grains (PAG). In the present work, the influence of the addition of a cold wire at various wire feed rates in cold-wire tandem submerged arc welding, a recently developed welding process for pipeline manufacturing, on the microstructure and mechanical properties of the HAZ of a microalloyed steel has been studied. The cold wire moderates the heat input of welding by consuming the heat of the trail electrode. Macrostructural analysis showed a decrease in the CGHAZ size by addition of a cold wire. Microstructural evaluation, using both tint etching optical microscopy and scanning electron microscopy, indicated the formation of finer PAGs and less fraction of M-A constituents with refined morphology within the CGHAZ when the cold wire was fed at 25.4 cm/min. This resulted in an improvement in the HAZ impact fracture toughness. These improvements are attributed to lower actual heat introduced to the weldment and lower peak temperature in the CGHAZ by cold-wire addition. However, a faster feed rate of the cold wire at 76.2 cm/min adversely affected the toughness due to the formation of slender M-A constituents caused by the relatively faster cooling rate in the CGHAZ.     

大线能量焊接用EH420海工钢生产工艺及焊接性能

河钢集团有限公司开发了利用钢液中形成TiO_x?MgO?CaO细小粒子改善焊接粗晶热影响区韧性的ITFFP技术（Improve the toughness of HAZ by forming TiO_x?MgO?CaO fine particles in steel）,成功试制生产出大线能量焊接用30 mm厚度规格（H30）和60 mm厚度规格（H60）EH420海洋工程用钢。母材力学性能试验结果表明,H30和H60试制钢屈服强度分别达到461 MPa和534 MPa,抗拉强度分别达到570 MPa和628 MPa,延伸率分别为26%和24.5%,满足EH420海洋工程用钢国家标准要求。采用Gleeble-3800型热模拟试验机对试制钢进行了200 kJ·cm?1条件下热模拟试验,并对焊接热影响区中的显微组织和?40 ℃冲击韧性进行了分析和测试。结果表明,试制钢中形成的CaO(?MgO)?Al₂O₃?TiO_x?MnS夹杂物可以有效地诱导针状铁素体析出,显著提高钢材的冲击韧性。另外,利用气电立焊设备对H30和H60试制钢分别进行了焊接线能量为247 kJ·cm?1和224 kJ·cm?1的实焊试验,结果显示,H30试制钢焊接接头表面和根部焊缝处?40 ℃冲击吸收功值≥74 J,焊接热影响区≥115 J,H60试制钢焊接接头表面和根部焊缝处?40 ℃冲击吸收功值≥91 J,焊接热影响区≥75 J,焊接接头的冲击性能远高于国家标准值42 J。      

Microstructural characterization of INCOLOY 903 weldments

The fusion zone and the heat-affected zone (HAZ) microstructures obtained during electron beam welding of thermomechanically processed INCOLOY 903 have been characterized by analytical electron microscopy. The microsegregation observed during solidification in the fusion zone indicates that while Fe and Co segregate in the gamma dendrites, Nb, Ti, and C are extensively rejected into the interdendritic liquid. Electron diffraction and energy dispersive X-ray microanalyses on secondary phases extracted from the fusion zone, from the HAZ microfissures, and from the HAZ grain boundaries on carbon replicas established the major secondary solidification constituent formed from the interdendritic liquid to be cubic niobium-rich MC carbides. Laves phase was observed only in trace amounts. Continuous sheets of the MC carbides were observed in the HAZ grain boundary microfissures, while the HAZ grain boundaries that resisted microfissuring were devoid of the continuous sheets of carbides and/or extensive fine carbide precipitation. The microstructure observed in the HAZ microfissures suggests grain boundary liquation and formation of low melting liquid films enriched in niobium and carbon. The liquid originates primarily from the constitutional liquation of primary MC carbides, MNP phosphides, and fine MC carbides already present on the grain boundaries of the base metal. The solidification pattern of the liquid films on the microfissures is observed to follow a similar pattern as that of the fusion zone. The observation of microfissuring on boundaries which show continuous and/or semicontinuous sheets of resolidified structure suggests that HAZ microfissuring occurs due to the presence of low melting liquid films. The existence of continuous and/or semicontinuous liquid films and the added presence of phosphorus in them appear to be one of the major causes for extensive HAZ microfissuring in thermomechanically processed INCOLOY 903.     

Microstructural Evolution and Mechanical Properties of CSEF/M P92 Steel Weldments Welded Using Different Filler Compositions

In the present investigation, P92 steel weld joints were prepared using a shielded metal arc welding (SMAW) process for two different fillers, E911 and P92. A comparative study was performed on the microstructural evolution, tensile strength, microhardness, and Charpy toughness across the P92 steel weldments in the as-welded and post-weld heat-treated (PWHT) conditions. The PWHT was performed at 760 °C for 2 hours. To study the effect of the different filler metals and PWHT on the mechanical properties, longitudinal and transverse tensile tests were carried out at room temperature for a constant cross-head speed of 1 mm/min. In the longitudinal direction, the tensile strength of the P92 steel welds was measured as 958 ± 35 and 1359 ± 38 MPa for the E911 and P92 filler, respectively. In the as-welded condition, the transverse tensile specimens were fractured from the fine-grained heat-affected zone or inter-critical heat-affected zone (FGHAZ/ICHAZ) and, after PWHT, the fracture location was shifted to over-tempered base metal from the FGHAZ/ICHAZ. After the PWHT, the tempering reaction resulted in lowering of the hardness throughout the weldment. After PWHT, the Charpy toughness of the weld fusion zone and heat-affected zone (HAZ) of the E911 filler weldments was measured as 66 ± 5 and 142 ± 8 J, respectively. The minimum required Charpy toughness of 47 J (EN1557: 1997) was achieved after the PWHT for both E911 and P92 filler.