Microstructure features of 12MnNiVR pressure vessel steel welded joint deposited by the high heat input electrogas welding have been systematically investigated. It is revealed that the welded joint is featured by a heterogeneous juxtaposition. The coarse grained heat-affected zone (CGHAZ) primarily consists of lath bainites and minor granular bainites. The fine grained heat-affected zone (FGHAZ) is dominated by polygonal ferrites, pearlites, and fine cementite particles. Moreover, electron backscatter diffraction results further demonstrate that the CGHAZ is populated by coarse prior austenite grains (PAGs) with high frequency (61.3%) of low angle grain boundaries (LAGBs). On the other hand, the FGHAZ is filled with fine PAGs with a lower frequency (19.6%) of LAGBs. Such microstructural differences may likely contribute to differed mechanical properties for samples tested at designed positions.

设计了一种新型1500 MPa级Si-Mn-Cr-Ni-Mo多组元系低合金超高强度结构钢, 对比研究了控轧+控冷(TMCP)、控轧+空冷、控轧+直接淬火、控轧+直接淬火+250 ℃回火4种不同工艺对其微观组织和力学性能的影响. 结果表明: 直接淬火态钢板抗拉强度最高, 可达1890 MPa, 屈服强度为1280 MPa,  延伸率为13\%; 250 ℃回火30 min后抗拉强度降低为1820 MPa, 而屈服强度升高为1350 MPa, 分析认为这归因于位错亚结构的回复软化过程与残余奥氏体分解为马氏体、析出ε-碳化物强化机制的综合作用; 空冷与TMCP工艺获得板条贝氏体+马氏体+少量残余奥氏体的复相组织, 贝氏体分割马氏体板条束, 使实验钢具有良好的强塑性. 低碳马氏体相变过程存在C扩散现象. 研究发现, 回火过程不仅包含残余奥氏体分解, 也包含C从马氏体或贝氏体向奥氏体的分配过程. 证明了立方结构的析出粒子在奥氏体中形核, 在整个冷却过程长大、粗化, 而相变后的马氏体或贝氏体未出现大量第二相析出核心.

In recent years, ultra-high strength steel (UHSS) has been widely utilized in engineering structures, mining machinery, and military equipment. However, UHSS is prone to brittle fracture and fatigue failure due to high strength and relatively low plasticity. Moreover, residual stress induced by welding process affects both brittle fracture and fatigue failure. In this work, a single-pass butt-welded joint was fabricated by metal inert-gas welding. The base metal was 1600 MPa grade UHSS with a 5 mm thickness, and the filler metal was ER307Si. The distributions of welding residual stress and hardness of the butt-welded joint were measured using the hole drilling method and a microhardness tester, respectively. Based on measured values of hardness in the heat-affected zone (HAZ) and softening zone (SZ), SYSWELD software was used to develop an advanced computational approach with consideration of “thermal-metallurgical-mechanical” coupling behaviors. In addition to the strain hardening and annealing effects of weld metal, the established computational model accounted for both the solid-state phase transformation (SSPT) of HAZ and softening effect of SZ. The temperature field and residual stress distribution of the UHSS single-pass butt-welded joint were simulated. Furthermore, the simulated results were compared with the corresponding measured data. The simulation results revealed the effect of SSPT and softening on welding residual stress. The numerical results indicated that SSPT has a strong influence on both the magnitude and distribution of the longitudinal residual stress; however, it has a limited effect on transverse residual stress. Meanwhile, the softening effect drastically affects the peak values of the longitudinal residual stress, while it hardly influences transverse residual stress. When both SSPT and softening effects are simultaneously considered in the numerical model, the computed results of welding residual stress are in good agreement with the experimental measurements.

以板厚为5 mm的1600 MPa级超高强钢为研究对象，采用熔化极惰性气体保护焊(MIG)焊接方法和ER307Si焊丝制备了单道对接接头，分别利用盲孔法和显微硬度仪测量残余应力和接头硬度分布。基于接头热影响区和软化区的硬度测量结果，以SYSWELD软件为平台，开发了考虑超高强钢固态相变和软化效应以及焊缝金属加工硬化和退火软化的“热-冶金-力学”多场耦合的有限元计算方法。采用该方法模拟了超高强钢单道对接接头的温度场及残余应力，并与实验结果进行了比较。基于数值模拟结果探讨了固态相变和软化效应对焊接残余应力的影响机制。数值模拟结果表明，固态相变对纵向残余应力的大小和分布有显著影响；对横向残余应力的大小和分布有一定程度的影响。软化效应对纵向残余应力的峰值有较显著的影响，对横向残余应力几乎没有影响。将数值模拟结果与实验结果比较可知，在同时考虑固态相变和软化效应的情况下，平板对接接头的焊接残余应力计算结果与实验测量结果最为吻合。

采用自行研究开发的一种新型药芯焊丝, 通过实验室大热输入焊接实验, 研究分析了热输入量对焊缝熔敷金属组织与冲击韧性的影响规律. 结果表明: 在大热输入焊接条件下, 熔敷金属中形成了大量细小、弥散分布的夹杂物, 夹杂物周围诱导生成的大量相互交叉互锁的针状铁素体晶粒, 这是焊缝熔敷金属具有较高低温冲击韧性的主要原因; 随着焊接热输入量的增加, 尺寸为1 μm以下的夹杂物数量减少, 1 μm以上夹杂物分布无显著变化, 针状铁素体形核点减少, 熔敷金属中针状铁素体晶粒尺寸略有增加、低温冲击韧性略有降低.

In recent years, high and ultra-high strength steels have been developed and used in light-weight constructions such as the structural members of mobile equipment in order to reduce weight and fabrication costs and to enhance the performance. Welding of steels with yield strength of more than 900 MPa is particularly challenging because of the toughness requirements for the weld metal, which calls for welding consumables of high strength and good toughness. Weld metals have been produced for a variety of welding methods with yield strength up to or above 1000 MPa, but their impact toughness remained only at medium yield strengths. Proper microstructure is the key to meeting this requirement, and its final microstructure depends on the chemical composition and cooling rate. For the deposited metal produced by gas metal arc welding (GMAW), the composition is dependent on the welding wire and shielding gas. The cooling rate of the weld metal is controlled by a combination of heat input and heat extraction. It is known that the addition of CO2 to argon based shielding gas is effective for improvement of productivity in GMAW welding of steel. Through the chemical reaction in the welding arc, CO2 in the shielding gas can affect the chemical composition of the weld metal, and its microstructure. The 1000 MPa grade deposited metals was welded with GMAW, and the effects of shielding gas composition (Ar+(5%~30%)CO2, volume fraction) on the general compositional and microstructural characteristics of deposited metals, including nonmetallic inclusions, were experimentally characterized with SEM, EBSD and TEM. The microstructure of the deposited metals is mainly composed of martensite and bainite. With the increase of CO2 content (5%~30%), the strength of the deposited metals decrease slightly and the impact toughness increases first and then decreases. Meanwhile, the transformation range (B50-Ms) of the deposited metal increases, the bainite content increases (8%~29.6%) with the quantity of inclusions that are suitable for bainite nucleation increases, and the nucleation position changes from the original austenite grain boundary to the common nucleation on the original austenite grain boundary and inclusions within the grain. At the same time, the microstructure morphology of the deposited metal changes from parallel to intertexture, which presented an intersected configuration and microstructure refinement.

通过附带EDS的FEGSEM、EBSD、TEM等实验方法，研究了保护气成分(Ar+5%CO₂、Ar+10%CO₂、Ar+20%CO₂、Ar+30%CO₂，体积分数)对1000 MPa级高强熔敷金属组织特征的影响，阐明了保护气成分对组织转变的影响机制。结果表明，随着保护气中CO₂含量增加，1000 MPa级熔敷金属强度略有下降，而冲击韧性先升高后降低。不同保护气熔敷金属均由马氏体/贝氏体混合组织及板条间残余奥氏体组成。随着保护气中CO₂含量增加，熔敷金属中贝氏体相变体积分数为50%时的温度(B₅₀)与马氏体相变开始温度(M_s)相变温度区间增大，适宜贝氏体形核的夹杂物数量增多，随贝氏体含量(体积分数)由8%增加到29.6%，其形核位置从原始奥氏体晶界向原始奥氏体晶界及晶内夹杂物处共同形核转变，熔敷金属组织形貌由“平行状”向“交织状”转变，分割细化组织，有利于高强熔敷金属强韧性的改善。

The C content in high strength steel must be controlled at a lower level for the good weldability. However, the lower level of C content will reduce the C partitioning efficiency and influence the stability of retained austenite, which leads to the decrease of the product of tensile strength and elongation of high strength steel. A novel preparation mechanism of high strength steel is to employ some kind of substitutional alloying elements, for example Mn, instead of C to partitioning to enhance the austenitic stability, which would not remarkably reduce the weldability of the steel. One low alloy C-Si-Mn steel was used in present work. The Mn partitioning behavior and its effect on the stability of the retained austenite and the mechanical property were studied by means of intercritical annealing, subsequent austenitizing, then quenching and partitioning process (I&Q&P). The results show that in the process of intercritical annealing at 760 ℃, by extending the annealing time, austenite volume fraction increases gradually until it reaches the saturation, meanwhile the Mn partitioning behavior occurs and Mn content increases gradually from ferrite to austenite until it reaches the chemical potential balance in two phases. The sample is heated to 930 ℃ for 120 s, then rapidly quenching to 220 ℃, the carbon diffuses from martensite to austenite phase in the process of partitioning. After I&Q&P process, the tensile strength of experimental steel is 1310 MPa, elongation up to 12%, the product of strength and elongation up to more than 15000 MPa.%. The steel only contains a small amount of retained austenite by only C partitioning after traditional Q&P process, while the steel contains more Mn-rich retained austenite after I&Q&P process. Hence, the content and stability of retained austenite of steel can be improved significantly, which enhance the formability at room temperature.

采用双相区保温+奥氏体化淬火+低温退火的热处理工艺, 研究了合金元素配分行为对C-Si-Mn系高强钢微观组织和力学性能的影响. 结果表明, 在760 ℃随着保温时间的延长, 双相区中奥氏体相的体积分数逐渐增多直至达到饱和, 而铁素体向奥氏体扩散的Mn元素含量也逐渐增多直至在两相间达到化学势平衡, 后加热至930 ℃保温120 s, 再淬火至220 ℃, 配分过程中发生了C从马氏体向奥氏体中的扩散偏聚. 经该工艺处理后实验用钢的抗拉强度为1310 MPa, 延伸率可达12%, 强塑积达到15720 MPa.%, 相比传统淬火+碳配分工艺, 双相区保温+奥氏体化淬火+低温退火的热处理工艺过程中Mn配分和C配分共同作用能够显著提高钢中残余奥氏体的含量和稳定性, 从而提高高强钢的室温成形能力.

以Mn-Ni-Mo-Ti-B为主要合金系, 研制出适用于低温服役环境下的高强高韧管线钢埋弧焊丝, 并应用于30.8 mm厚K65管线钢现场焊接实验. 结果表明, 焊缝金属屈服强度达到583~689 MPa, 抗拉强度达到714~768 MPa, -40 ℃冲击功均在90 J以上, 焊缝具有优异的强韧性匹配. 焊丝直径为4.0 mm, 适用于四丝双面埋弧焊, 效率高, 且热影响区(HAZ) 低温韧性优异(-40 ℃冲击功>100 J). 采用OM, TEM和LePera方法对焊缝金属组织的观察表明, 焊缝组织主要为精细的针状铁素体、少量的先共析晶界铁素体、侧板条铁素体和弥散分布的细小马氏体/奥氏体(M/A) 岛状颗粒. 焊缝金属中0.2%Mo可以有效抑制先共析晶界铁素体及侧板条铁素体的生成, 晶粒细化作用显著. Mn和Ni的适量增加会促进针状铁素体的形成, 显著提高焊缝金属低温韧性. 但Mn, Ni配比不当而超过某个范围时将会导致马氏体或其它低温相变产物形成, 削弱低温韧性. 当K65焊缝金属中含(1.5%~2.0%)Mn, (0.9%~1.2%)Ni, (0.2%~0.25%)Mo时, 可以使其具有高强度的同时低温冲击韧性优异, 且在Mn与Ni配比含量不越过马氏体形成线(M_s线)的前提下, 可以采用加Mn减Ni的方法配比其合金含量.

Marine engineering steel is the key material for the construction of major marine infrastructure projects. Due to the harsh environment in the deep sea, the mechanical properties such as strength, low temperature toughness and so on of the marine steel are required to be higher. In this work, the weldability of a Fe-Cr-Ni-Mo high-strength steel was studied, and the microstructure and impact toughness of the steel after welding thermal cycling at different peak temperatures were analyzed. The results show that the average impact toughness of characteristic heat affected zone under different temperatures increases first and then decreases with the increase of peak temperature (Tp). The microstructures of coarse grain heat-affected zone (CGHAZ, Tp=1320 ℃) and fine grain heat-affected zone (FGHAZ, Tp=1020 ℃) are quenched martensite. Because of the coarse grain size, the impact toughness of CGHAZ is poor, which is lower than that of FGHAZ. The microstructure of inter-critical heat-affected zone (ICHAZ, Tp=830 ℃ and Tp=760 ℃) is composed of quenched martensite and tempered martensite. Due to the randomness of the proportion of the interfaces between the mixed microstructures near the V-notch, the impact energy values of ICHAZ fluctuates greatly. The homogeneous fine grain structure in ICHAZ (Tp=830 ℃) has a crack arrest effect during the impact deformation, which makes the characteristic zone have the best impact toughness. Although the grain size in ICHAZ (Tp=760 ℃) is also fine, the existence of the ultra-fine grain zones (the grain size in which is only 1~2 μm) benefits the formation of secondary voids under the impact load. The undissolved M2C and MC precipitations in matrix promote the connecting of secondary voids and then form the secondary cracks. As a result, the impact toughness of the characteristic zone is poor, and becomes the weak region of HAZ.

研究了一种Fe-Cr-Ni-Mo高强钢经不同峰值温度(T_p) (760、830、1020和1320 ℃)焊接热循环后的显微组织与冲击韧性。结果表明,随着T_p的升高,特征热影响区的平均冲击功先增大后减小。粗晶区(CGHAZ,T_p=1320 ℃)和细晶区(FGHAZ,T_p=1020 ℃)的显微组织为淬火马氏体。由于晶粒粗大,造成CGHAZ冲击韧性较差,低于晶粒细小的FGHAZ冲击韧性。部分相变区(ICHAZ,T_p=760 ℃和T_p=830 ℃)的显微组织为淬火马氏体和回火马氏体组成的混合组织,由于冲击试样V型缺口处的混合组织界面所占比例具有随机性,造成ICHAZ冲击功波动较大。ICHAZ (T_p=830 ℃)中均匀的细晶组织对冲击裂纹具有止裂作用,使得该微区具有最佳的冲击性能。尽管ICHAZ (T_p=760 ℃)晶粒细小,但存在的极细晶组织(尺寸为1~2 μm)在遭受冲击载荷时易形成密集分布的次生微孔,基体中未溶的M₂C及MC析出相促使微孔连接形成裂纹,导致该微区冲击韧性最差,成为热影响区的薄弱区域。

As China's economy enters a stage of high-quality development, it is important to study high-performance bridge steels with high strength, high toughness, high efficiency, and easy welding. Presently, coarse-grained heat-affected zones (CGHAZs) of high-performance bridge steels are prone to coarse-grain embrittlement, which reduces its impact toughness. To improve their low-temperature toughness, the relationship between their microstructure and impact toughness has been extensively researched and discussed. However, the control connection and internal mechanism between the bainite microstructure and impact toughness have not been clarified. In this study, the simulated samples of CGHAZs in Q500qE steel with varying heat inputs, from 15 to 30 kJ/cm, were reproduced in Gleeble 3500 thermal simulation-testing machine. The effect of different heat inputs on the microstructure, impact toughness of CGHAZs, and their inherent mechanism was discussed and analyzed in-depth using OM, SEM, and EBSD. The results indicate that the microstructure of each simulated sample of CGHAZs in Q500qE steel was composed of lath-like bainite (LB) and granular-like bainite (GB). The LB increased, the GB reduced, phase-transition temperature (Ar3) declined, and the bainitic packet/block substructure refined as the welding heat input decreased. In addition, with the decreased heat inputs, the Charpy impact energy (KV2) at -40oC of CGHAZs is enhanced because of the refined microstructure. The impact fracture of all samples showed cleavage fracture characteristics, and the cleavage face size of the simulated samples of CGHAZs decreased due to the reduced heat inputs. Compared with prior austenite grain boundary and bainite block, the bainite packet is the most effective microstructural unit for controlling the impact toughness of CGHAZs in Q500qE steel, and its boundary effectively facilitates the prevention of further propagation of a secondary crack.

采用Gleeble-3500热模拟试验机模拟了Q500qE钢在不同热输入(E_j = 15~30 kJ/cm)下的焊接热循环过程，结合OM、SEM以及EBSD等手段探讨了热输入对粗晶热影响区(CGHAZ)微观组织和冲击韧性的影响规律及其作用机理。结果表明，随着焊接热输入的降低，板条贝氏体(LB)增多，粒状贝氏体(GB)减少，相变温度A_r3降低，贝氏体块(packet)和贝氏体束(block)明显细化；CGHAZ的冲击功KV₂ (-40℃)随着热输入的降低而提高，冲击断口均表现出明显的解理断裂特征，其解理面尺寸D_f随着热输入的降低而减小；贝氏体packet是Q500qE钢中控制CGHAZ冲击韧性最有效的结构单元。

By virtue of their high thermal conductivity, low thermal expansion coefficient, and excellent high-temperature creep strength, high-Cr (mass fraction: 9%-12%) martensitic heat-resistant steels are the putative main constituents of the key equipment in ultra-supercritical (USC) power plants. However, the harsh environment caused by enhancing the steam parameters has recently challenged the high-temperature properties and the continually deteriorating creep strength during service has seriously threatened the safety and reliability of these steels. Previously, the creep strength of high-Cr martensitic heat-resistant steels was enhanced by optimizing the alloying compositions to promote the dispersed precipitation of strengthening phases, but the enhancement effect of reinforced single-precipitate strengthening is limited. In recent years, synergistic strengthening reinforcement of dislocation-precipitate-interface has emerged as a promising solution because the introduced dislocations promote various precipitations and the phase transformation can be controlled to tailor the lath structure, thus reinforcing the dislocation-precipitate-interface interactions and synergistically enhancing various strengthening effects. This paper overviews the synergistic strengthening of dislocation-precipitate-interface and microstructure control in high-Cr martensitic heat-resistant steels subjected to thermo-mechanical treatments. The review covers alloying optimization to improve the creep strength, the phase transformations during heating treatments, and the mechanism of microstructural degradation at high temperatures. It also compares the effects of single-precipitate and synergistic strengthening processes on creep strength and introduces microstructure control in welded joints by thermo-mechanical treatments in terms of creep failure behaviors. This research aims to guide the design and engineering applications of high-Cr martensitic heat-resistant steels and other precipitate-strengthening heat-resistant steels for USC power plants.

高Cr (9%~12%，质量分数)马氏体耐热钢因其较高的热导率、较低的热膨胀系数以及优异的高温蠕变强度等优点而被认为是超超临界火电机组关键设备升级改造的主选材料。然而，服役过程中高Cr马氏体耐热钢高温蠕变强度的不断弱化严重影响了其安全可靠性。以往提升高Cr马氏体耐热钢高温蠕变强度的主要手段是通过合金成分优化设计来促进沉淀相弥散析出，但单一析出强化效应对蠕变强度的提升效果非常有限。近年来，位错-沉淀相-界面协同强化效应在提升高Cr马氏体耐热钢高温蠕变性能方面表现出显著效果。其原理是通过形变热处理引入位错来促进多种沉淀相弥散析出，同时通过控制相变来细化板条组织，增强位错、沉淀相及界面3者之间的交互作用，从而实现多类蠕变强化效应的协同提升。本文总结了高Cr马氏体耐热钢的协同强化机制及形变热处理组织调控，从高温蠕变强度提升角度回顾了合金成分的优化历程，阐述了热处理过程中的相变行为及高温组织退化机理，对比分析了单一析出强化效应及形变热处理后位错-沉淀相-界面协同强化效应对其高温蠕变强度的影响规律，并基于焊接接头蠕变失效行为探索了形变热处理对焊接热影响区的组织调控机制，以期为高Cr马氏体耐热钢及其他火电机组用沉淀型强化耐热钢的材料设计及工程应用提供指导。

建立了Fe-Co-Ni超高强度钢板电子束焊热源模型和“热-冶金-力学”耦合有限元模型，通过焊缝截面形貌和残余应力的模拟结果和实测结果对比，验证了耦合有限元模型的可靠性。利用耦合有限元模型模拟了Fe-Co-Ni超高强度钢长臂梁构件的“电子束焊-真空气淬”过程，预测了在焊接-淬火过程中组织转变规律和应力与变形。研究发现，真空气淬是导致超高强度钢构件产生明显变形的主要原因，考虑固态相变的真空气淬过程模拟结果可以获得准确的变形方向和大小。

Fossil-fired thermal power generation has dominated China's electricity production for a long time, contributing to around 70% of the total capacity. Developing long-life ultra-supercritical thermal power units is essential for improving coal-fired power generation efficiency, reducing harmful gas emissions, and achieving national energy conservation and emission reduction targets. The assembly and manufacture of advanced heat-resistant steel grades are required to address the above demands, serving as crucial components driving the technological advancement of thermal power units. Heat-resistant steel grades P11, P22, and P91, which are Cr-Mo based ferritic, possess a range of highly attractive properties, such as excellent mechanical properties, excellent corrosion resistance, and relatively low construction costs. These steel grades are widely used in pressure vessels and pipelines focused on high-temperature applications. Fusion welding techniques are invariably necessary to weld such heat-resistant-grade steels before they are positioned in high-temperature service. However, it is worth noting that drastic solid-state phase transformations in the heat-affected zones (HAZs) during thermal welding cycles can profoundly influence the heterogeneous microstructures of welded joints, determining their final mechanical properties to a large extent. Furthermore, it seriously threatens the safe and stable operation of thermal power plants. High-temperature confocal scanning laser microscopy (CSLM) revolutionized traditional metallographic experiments, enabling real-time morphology and quantitative analysis tracking. This innovation has facilitated investigations into the kinetic phase transformation process and microstructure evolution in steels at high temperatures. In this work, the kinetics of phase transformation and microstructural evolution in the HAZs of P11, P22, and P91 ferritic heat-resistant steels during continuous cooling processes were systematically investigated using CSLM. The results revealed that bainite laths preferentially nucleate in the order of increasing difficulty in the energy barrier on austenite grain boundaries, inclusions, internal grain distortion areas, previous bainite laths, and grain interiors. Meanwhile, the growth characteristics of bainite/martensite laths were documented as the phase transformation progressed. It is revealed that bainite laths attach to prior austenite grain boundaries and the previous bainite, while martensite laths grow radially inside the prior austenite grains. Both bainite and martensite laths cease growing when they encounter grain boundaries or other laths, eventually forming an interlocking microstructure. Additionally, the growth rates of bainite/martensite laths in the HAZs of P11, P22, and P91 ferritic heat-resistant steels exhibited considerable variations as the temperature decreased. The analysis revealed that as the temperature decreased, the growth rate of laths in the coarse-grained heat-affected zone was considerably higher than that in the fine-grained heat-affected zone, which can be attributed to the increase in the degree of supercooling and prior austenite grain size.