Maraging stainless steel (MSS) is widely employed in aerospace and pressure vessels due to its high strength, high toughness and good corrosion resistance. Microstructure design and combined deoxidation are important strategies for developing high-performance alloy steel. However, the evolution behavior and characteristics of non-metallic inclusions and their effects on the mechanical properties of additively manufactured MSS have not been conclusive for a long time. This work investigated the microstructure, inclusion characteristics, and mechanical properties of laser-directed energy deposited (LDED-ed) MSS from three kinds of raw materials with different deoxidizer contents. The results indicated that the microstructure of As-deposited (AD) MSS specimens primarily consisted of martensite with a minor fraction of retained austenite. After heat treatment (HT), the MSS specimens became fine and equiaxed grains with fine martensite packets/blocks, some reverted austenite, and dispersive Fe2Mo nano-size particles. The HT MSS specimens exhibited an increased strength to 1200 MPa, primarily due to the synergistic effects of Hall–Petch strengthening and precipitation strengthening. The inclusions in the AD MSS specimen were oxides, while those in the HT MSS specimen were composite inclusions, which consisted of oxide, TiN, and MnS. After heat treatment, a slight coarsening of the inclusions was observed; however, their number density remained constant. Compared to the HT counterpart specimen (MSS-I), both the HT MSS-LZr specimen with Zr content of 0.003 wt pct and the HT MSS-HZr specimen with Zr content of 0.026 wt pct by Ti–Al–Zr combined deoxidation had a higher impact toughness, due to the reduced amount of inclusions. The findings elucidate the critical role of combined deoxidation and heat treatment in tailoring microstructure and enhancing the mechanical properties of LDED-ed MSS.

Maraging steels are ultrahigh-strength, low-carbon steels requiring strict control of impurity elements to ensure optimal strength and toughness. This study aims to elucidate the role of oxygen content in controlling oxide inclusions and cryogenic toughness in laser powder bed fusion (L-PBF) fabricated maraging steels. Two types of powders were used: vacuum induction gas atomization (VIGA) powder with 0.034wt

In this study, AF9628 high-strength low-alloy steel was fabricated using laser melting deposition. The initial non-equilibrium microstructure exhibited cellular segregation of elements such as Cr and Mo, with cellular structure dimensions of 20-30 mu m. Samples were heated to 950 degrees C and held there for 10 min before being cooled at four different cooling rates-that is, 0.5, 1, 10, and 50 degrees C/s-to research the impact of cooling conditions on the microstructure. The results demonstrated that under specific cooling conditions (1 degrees C/s), cellular segregation could effectively limit the growth of bainitic/martensitic substructures. This becomes an effective component interface that can hinder the phase transformation, in addition to the grain boundaries. When the cooling rate was too fast or too slow, martensite and bainite grew throughout the cellular structure, the phase-transformation restriction by cellular segregation was limited. At a cooling rate of 1 degrees C/s, its mechanical properties (tensile strength of 1865 MPa and elongation of 10.3 %), exceeding those of samples prepared by other heat treatment routes. The results of grain reconstruction confirmed the restrictive influence of cellular segregation on bainitic/ martensitic transformation. This approach to microstructure control opens up new routes for the optimization of the microstructure and properties in additive manufacturing of low alloy steel.

The cellular structure, a distinctive feature of steels fabricated by laser powder bed fusion (L-PBF), can enhance strength and ductility in austenitic steel. Its effect in maraging stainless steel undergoing austenite-martensite transformation is more complex. This study compares the microstructure and mechanical properties of L-PBFed Fe-Cr-Ni-Co-Mo maraging steel with and without the cellular structure. The as-built steel shows a similar to 0.52 mu m cellular structure with Cr and Mo segregation at cellular walls, which disappears after homogenization. Direct solution-aging of the as-built (775) specimen yields 22.6 % retained austenite with an island-like morphology, whereas the homogenized (1775) specimen has 18.6 % with a lath-like morphology. This difference results from elemental segregation, which stabilizes austenite at cellular walls and disrupts lath-like austenite continuity. At 23 degrees C, the 1775 specimen shows higher strength and elongation than the 775 specimen. At -196 degrees C, this advantage increases, with yield strength of 1697 vs. 1606 MPa, ultimate strength of 1737 vs. 1658 MPa, elongation of 26 % vs. 16 %, and impact energy of 44 vs. 7 J. The island-like austenite in the 775 specimen provides limited crack resistance, causing more secondary cracks. In contrast, the lath-like austenite with heterogeneous shells in the 1775 specimen undergoes greater strain-induced martensitic transformation, producing a stronger transformation-induced plasticity (TRIP) effect. Eliminating the cellular structure via homogenization treatment is essential for optimizing L-PBF maraging steels.

CF170 is an ultralow-carbon, cobalt-free maraging stainless steel with a tensile strength of 1700 MPa, making it an ideal material for high-load gears in aerospace robotic arms. However, these gears are subjected to a long ageing process during ion nitriding, resulting in remarkable variations in the austenite volume fraction (Va), which strongly affects the dimensional accuracy of the product. Therefore, accurately predicting Va is crucial for optimizing the chemical heat treatment process of CF170 steel. In this work, the nonisothermal and isothermal ageing process, combined with Kissinger analysis and the Johnson‒Mehl‒Avrami (JMA) model, proved that the nonisothermal kinetic model was applicable for the prediction of Va. Moreover, models of the preexponential factor K0 and maximum austenite volume fraction (Vmax) were proposed, and a kinetic model of austenite phase transformation after ageing at 482–593 °C for 0–6000 min was established, which predicted and controlled the microstructure within a large temperature and time range. The nucleation mechanism of austenite in CF170 steel was investigated by scanning electron microscopy (SEM), electron backscattering diffraction (EBSD), and transmission electron microscopy (TEM). Nucleation was regulated by a shear mechanism, with nucleation occurring primarily at the grain boundary, packet, block, and subblock interfaces. The nucleation was the reversion of γ → α → γ, and the austenite inherited the crystal orientation features of the prior austenite, demonstrating the “austenite memory” phenomenon.

采用激光熔化沉积技术制备JBK-75合金,选取750℃直接时效和1180℃高温固溶+750℃时效两种不同热处理工艺路线,分析了其沉积态的显微结构,对比两种不同热处理态的组织和力学性能.结果表明,沉积态JBK-75合金组织表现为各向异性,存在柱状晶、胞状组织和Ti元素偏聚.直接时效处理JBK-75合金打印组织未发生溶解,打印组织对强化相(γ'相)的析出行为几乎没有影响,但是Ti元素的偏聚促进了晶界有害相(η相)的生成.高温固溶+时效处理JBK-75合金打印组织消失,获得细小均匀的γ晶粒且在晶界未发现η相.高温固溶+时效处理激光熔化沉积JBK-75合金表现出最佳的强塑性配比,抗拉强度为1055 MPa、屈服强度为679 MPa、断后伸长率为29％,达到锻件JBK-75合金热处理态力学性能水平.

超高强度钢大型深盲孔锻件在芯轴拔长过程中采用单一压下率和连续 45°翻转工艺,锻件头部的实心锻透性及整体均匀性较差.通过DEFORM-3D有限元模拟和分析,确定了头、尾部变形角与压下率的关系,制定了压下率为 8%、10%和 45°、60°新翻转工艺的不同组合的对比实验.结果表明:深盲孔锻件头部采用 10%压下率与 60°翻转的组合工艺,可改善锻件头部的锻透性及变形均匀性;尾部采用 8%压下率与 45°翻转的组合工艺,能够有效地控制尾部内孔畸变率并保证变形均匀性.在此基础上,合理设计头部锥度,可使头部变形量达到与尾部相当的水平,进一步提升头部的锻透性,完成近终成形.研究结果在工业化试制中得到了应用和验证,采用新工艺成形的大型深盲孔锻件的头尾性能均匀性明显改善.

通过光学显微镜(OM)、扫描电镜(SEM)、透射电镜(TEM)和X射线衍射等研究了不同温度淬火对AF9628超高强度钢力学性能和微观组织的影响.结果表明:随着淬火温度的升高,AF9628钢的晶粒逐渐长大,未溶相逐渐溶解,抗拉强度和屈服强度呈现逐渐下降趋势,冲击吸收能量、伸长率和断面收缩率先升高后降低.当淬火温度为970℃时,试验钢的晶粒较为均匀,未溶相大部分溶解,仅剩余微量的碳化物,此时钢的塑性较好,同时有足够的强度,其伸长率、断面收缩率、冲击吸收能量、抗拉强度和屈服强度分别为 15％、59％、100 J、1746 MPa 和 1357 MPa.

采用扫描电镜、能谱分析和力学试验等研究了回火温度对30Cr3Si2NiMoWNb超高强度钢组织和性能的影响.结果表明,回火温度变化可实现对力学性能的大幅度调控.200～350℃回火,微观组织为回火马氏体与细小弥散的ε-碳化物,此阶段强韧性变化幅度较小,抗拉强度等级1700 MPa、屈服强度等级1300 MPa;350～500℃回火由于渗碳体的不均匀析出,强度和韧性同时下降,其中500℃左右回火脆性最为严重,冲击吸收能量下降至最低点;500～700℃回火生成较稳定的球状渗碳体,强度大幅下降,韧性大幅上升.回火温度对强韧性的影响机理为ε-碳化物、渗碳体等析出相演变过程的影响;一定含量的Si元素可以提高渗碳体形成温度和回火脆性温度.

采用Thermo-Calc热力学软件计算了一种新型低钴二次硬化钢高温区间的析出相种类和含量,结合光学显微镜、扫描电镜和力学检测等试验方法,研究了淬火温度对其组织和性能的影响.结果表明,在较低温度淬火时,板条马氏体基体上存在大量富W、Mo的球状M6 C析出相.升高淬火温度,M6 C相迅速回溶,并在1060℃时完全溶解.M6 C相的溶解使得二次硬化效果增强、冲击性能提升,同时导致原奥氏体晶粒明显粗化,进而对强韧性产生不利影响,最终试验钢经1060℃淬火后获得最佳力学性能配合.

采用激光增材制造技术制备了Ferrium M54钢,研究了传统热处理对其组织和力学性能的影响.利用光学显微镜(OM)、扫描电镜(SEM)、X射线衍射仪(XRD)、拉伸试验机及维氏硬度计分析了沉积态和热处理后试验钢的微观组织和力学性能.结果表明,激光增材制造M54二次硬化钢是由沿沉积方向生长的柱状晶构成,沉积态试样纵向的抗拉强度和屈服强度分别为1832 MPa和997 MPa,断后伸长率和断面收缩率分别为9.5％和28％;经过传统热处理后,定向凝固形成的胞状结构消失,得到马氏体组织.经1075℃固溶+1060℃油淬+-73℃深冷+510℃时效处理后激光增材制造Ferrium M54钢的性能最好,抗拉强度为1863 MPa,屈服强度为1594 MPa,断后伸长率为15％,断面收缩率为59％,硬度为603 HV.

采用Gleeble-3800热模拟试验机研究了含有W、Mo等多种碳化物形成元素的新型中合金超高强度钢的热变形行为,变形温度为800～1200℃,应变速率为0.01～10 s-1,最大应变量为0.7.热模拟试验得到了试验钢的高温流变应力曲线,其变形抗力随变形温度的降低和应变速率的提高而增加.在变形温度1000℃以上进行热压缩时,试验钢可发生动态再结晶;变形温度的升高会促进晶粒粗化及二次再结晶的发生,而应变速率的提升有利于促进再结晶晶粒的细化和均匀化.根据试验钢的高温流变应力曲线,计算出试验钢的热加工本构方程,并建立了真应变为0.4的热加工图.结合微观组织演变的分析结果,得出试验钢的最佳热加工区域应为:变形温度为1000～1100℃、 应变速率为1～10 s-1.

The effect of aging on transformation behavior of reverted austenite and impact toughness in Co-free maraging stainless steel were investigated via thermodynamic calculation, transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). Excluding the film-shaped austenite growing along the phase interface, other shapes of reverted austenite are evolved from the growth and aggregation of acicular austenite in the range of 300-600 °C. Under N-W orientation relationship, {111}γ grows inside the martensite lath along <100>α, and austenite merges in <100>α and <110>α simultaneously. Under K-S orientation relationship, the growth direction of {111}γ is 60° or parallel to <112>α. From 300 to 500 °C, Ni prefers to diffuse into η-Ni3Ti and matrix. The precipitation of Ni3Ti hinders the formation of reverted austenite and significantly deteriorates the toughness. Above 500 °C, due to the coarsening of Ni3Ti and the recovery of matrix, the resistance to the formation of austenite is obviously weakened, and then austenite plays a leading role in the improvement of toughness. When the aging temperature reaches 600 °C, the dissolution of Ni3Ti promotes the formation of austenite and η-Ni3Ti changes to γ'-Ni3Ti. The interaction between Ni3Ti and reverted austenite essentially depends on the diffusion behavior of Ni.

Aging is an important heat treatment process for maraging stainless steel. During aging, the interaction between intermetallic compounds and dislocations has a significant impact on the mechanical properties of the material. In this article, the effect of aging temperature on the precipitation behavior and mechanical properties of Fe–Cr–Ni maraging stainless steel was studied by means of a three-dimensional atom probe (3DAP) and high-resolution transmission electron microscopy (HRTEM). The results show that coherent Ni3(Ti, Al) precipitates by heterogeneous nucleation from Ni/Ti/Al coclusters formed at defects (mainly dislocations) in the range of 350–450 °C, which leads to a rapid increase in strength and a sharp decrease in toughness. At 450–500 °C, the interface between Ni3(Ti, Al) and the matrix gradually changes from coherent to semicoherent, and an increase in the equivalent precipitate radius is accompanied by a decrease in their density. The tensile strength of the material reaches its peak value, and the toughness is improved. Within the range of 500–600 °C, Ni3(Ti, Al) continues to grow, aggregate and coarsen, and the amount of reverted austenite increases significantly. Both effects lead to a prominent decrease in the tensile strength and a substantial increase in toughness. The Ni3(Ti, Al) grows axially along the <111> dislocations through tube diffusion. The radial growth is parallel to [110]α and [11‾2]α, and the process is affected by the climbing and slipping of misfit edge dislocations. The strength increment of precipitates with different sizes formed at the peak aging temperature is calculated according to the critical transition radius, and the superposed results of the cutting and bypassing mechanisms during the yielding stage are found to be consistent with the experimental data, indicating that the two mechanisms work simultaneously. The findings provide a more accurate method for predicting the yield strength of materials.

Hydrogen embrittlement of steels is directly linked to hydrogen diffusion and trapping in the microstructure, which can hardly be precisely measured by modern experimental techniques. A phase-field model, in which a chemical potential well of hydrogen in the grain boundaries is introduced, is proposed to simulate hydrogen diffusion and trapping in the polycrystalline iron. It was interestingly found that grain boundaries, as connected trap sites, have a complex influence on the effective diffusivity of hydrogen, which are strongly linked to grain boundary diffusivity and binding energy.

采用光学显微镜、扫描电镜和物理化学相分析技术等方法研究了固溶温度对G33新型超高强度钢组织和性能的影响.结果表明:G33钢在860℃固溶时板条马氏体基体上存在M6 C、VC和NbN未溶相并且以M6 C碳化物为主;随着固溶温度升高,未溶相快速溶解,VC和M6 C相分别在940℃和980℃完全溶解;M6 C、VC未溶相的溶解使微裂纹缺乏形核点而不易萌生,同时提升了基体中合金元素固溶量,增强了固溶强化效果,让G33钢在保持高强度(2000 MPa级)的同时提高了冲击性能.

利用Formast-FⅡ膨胀仪测定了G31L钢的热膨胀曲线,研究其在不同冷速下的组织演变及硬度变化规律,绘制出G31L钢过冷奥氏体连续冷却转变(CCT)曲线,并分析了实际大型深盲孔锻件的锻后热处理工艺可靠性.结果表明:G31L钢的Ac1=740℃、Ac3=816℃,Ms=315℃、Mf=138℃.当冷速≤0.028℃/s时,获得珠光体、铁素体、贝氏体混合组织;当冷速在0.028～0.84℃/s之间,随冷速增大,珠光体-铁素体消失,贝氏体量逐渐降低,直至转变为全马氏体组织;当冷速大于0.84℃/s时,获得全马氏体组织.实际生产的大型深盲孔锻件经915℃保温6 h后油淬至室温并进行回火处理,锻件实心头部的心部为马氏体和少量贝氏体的混合组织,头部R/2处、头部边部及尾部为全马氏体组织,且锻件的强度和塑性均满足产品质量要求.

采用Gleeble-3800型热模拟试验机对新型低成本超高强度G31L钢的热变形行为进行了系统研究,变形温度为750～1250℃、 应变速率为0.01～10 s-1,最大应变量为0.9.取不同变形条件下的峰值应力,计算G31L钢热变形激活能Q及其本构方程;并基于动态材料模型绘制应变量为0.2、0.4、0.6和0.8的功率耗散图和流变失稳图,建立该材料热加工图.分析发现合理加工区主要分布在1050～1250℃,0.01～0.32 s-1范围内,且功率耗散因子 η大于0.3,在此区域内合理选择变形温度和应变速率,使基体发生动态再结晶并避免晶粒过分长大,可获得性能优良的锻件.

针对1 700 MPa级新型超高强度钢大规格盲孔锻件局部出现的探伤缺陷问题,采用横截面酸浸试验、热处理后力学性能试验、断口及裂纹面观察等手段进行综合分析,鉴定缺陷为氢致裂纹(白点),并详细分析了其可能产生的工艺环节,对此提出了提高渣料烘烤温度和时间、延长扩氢退火时间、加强心部锻透性等改进措施.

研究了两种新型超高强度钢30Cr3SiNiMoWNb和30CrNi5Si2MoNb奥氏体化后以30～3.5℃/min速度冷却的相变产物,及其对随后回火材料强韧性的影响.结果表明,30Cr3SiNiMoWNb钢奥氏体化后以30和15℃/min冷却得到马氏体组织;以7℃/min冷却,过冷奥氏体的相变产物为马氏体和25％～30％的下贝氏体;以3.5℃/min冷却,过冷奥氏体的相变产物为珠光体、贝氏体和马氏体.30CrNi5Si2MoNb钢降低冷却速度后回火强度上升,韧性下降不大,在3.5℃/min冷速时强度达到最高值.与30Cr3 SiNiMoWNb钢相比,30CrNi5Si2MoNb钢因其合金元素含量高,马氏体形成能力强,更难形成贝氏体和珠光体组织.