Enhancement of Strength–Ductility Balance of the Laser Melting Deposited 12CrNi2 Alloy Steel Via Multi-step Quenching Treatment

A ferrite–austenite 12CrNi2 alloy steel additively manufactured by laser melting deposition(LMD) was heat treated by direct quenching(DQ) and tempering inter-critical quenching(TIQ) at 800 ℃ for enhancing its strength–ductility balance. Both heat-treated alloy steels have the martensite–ferrite dual-phase(DP) microstructures. The volume fractions of martensite in the two treated alloy steels are nearly similar(～ 85 vol%), while the sizes of the prior austenitic grain for martensite are different. The martensite-dominated DP microstructure resulted in an obvious improvement in strength–ductility balance of the alloy steel. Compared with the DQ treatment, the multi-step TIQ treatment caused the strength–ductility balance of the alloy steel to be enhanced due to its higher total elongation. The better ductility of the TIQ-treated alloy steel can be attributed to the optimization of the microstructure. The preferred orientation of ferritic grain in the as-deposited alloy steel which was adverse to plastic deformation through dislocation slip was eliminated via the multi-step TIQ treatment. Moreover, the TIQ treatment promoted the formation of finer-grained martensite with larger areas of grain boundaries and twinning boundaries which resulted in the enhancement of the coordinated deformability of the martensite with the ferrite.     

Simulation of the Dendrite Morphology and Microsegregation in Solidification of Al–Cu–Mg Aluminum Alloys

Since most typical alloys in industrial applications are multicomponent with three or more components, and various CA models proposed in the past mainly focus on the binary alloys, a two-dimensional modified cellular automaton model allowing for the quantitatively predicting dendrite growth of multicomponent alloys in the low Pe′clet number regime is presented. The elimination of the mesh-induced anisotropy is achieved by adopting a modified virtual front tracking method. A new efficient method based on the lever rule is applied to calculate the solid fraction increment of the interfacial cells. The thermodynamic data such as liquidus temperature, the partition coefficients, and the slope of liquidus surface, needed for determining the dynamics of dendrite growth, are obtained by coupling with Pan Engine. This model is applied to simulate the dendrite morphology and microsegregation of Al–Cu–Mg ternary alloy both for single and multidendrites growth. The simulated results demonstrate that the difference of the concentration distribution profiles ahead of the dendrite tip for each alloying element mainly results from the different partition coefficients and solute diffusion coefficients. Comparison with the prediction of analytical model is carried out and it reveals the correctness of the model.Consequently, the difference in interdendritic microsegregation behavior of different components is analyzed.     

Wear Resistance of Austenitic Steel Fe–17Mn–6Si–0.3C with High Silicon and High Manganese

To solve the problem of poor wear resistance in conventional Hadfield steels under medium and low stress,a new kind of steel with high silicon and high manganese Fe–17Mn–6Si–0.3C was designed and its wear resistance was studied.The results showed that it exhibited better wear resistance than conventional Hadfield steel in both dry friction and abrasive friction.The better wear resistance of the new steel with high silicon and high manganese resulted from the stressinduced γ→ε martensitic transformation.     

Effects of Titanium and Silicon on the Swelling Behavior of 15–15Ti Steels by Heavy-Ion Beam Irradiation

The swelling behavior of 20% cold-worked 15–15 Ti steels with(0.23–0.40) wt% titanium and(0.42–0.81) wt%silicon content after heavy-ion beam irradiation has been investigated by transmission electron microscopy.The results show that 15–15 Ti steel with higher titanium content displays lower swelling.As silicon content increases,the cavity size and the swelling of the 15–15 Ti steels decrease obviously.Titanium and silicon may play important roles in suppressing the cavity nucleation and growth in 15–15 Ti steel.     

Development of Ti–V–Mo Complex Microalloyed Hot-Rolled 900-MPa-Grade High-Strength Steel

A new Ti–V–Mo complex microalloyed hot-rolled high-strength steel sheet was developed by controlling a thermo-mechanical controlled processing(TMCP) schedule, in particular with variants in coiling temperature. The effects of coiling temperature(CT) on various hardening mechanisms and mechanical properties of Ti–V–Mo complex microalloyed high-strength low-alloy steels were investigated. The results revealed that the steels are mainly strengthened by a combined effect of ferrite grain refinement hardening and precipitation hardening. The variation in simulated coiling temperature causes a significant difference in strength, which is mainly attributed to different precipitation hardening increment contributions. When the CT is 600 C, the experimental steel has the best mechanical properties: ultimate tensile strength(UTS) 1000 MPa, yield strength(YS) 955 MPa and elongation(EL) 17%. Moreover, about 82 wt% of the total precipitates are nano-sized carbide particles with diameter of 1–10 nm, which is randomly dispersed in the ferrite matrix.The nano-sized carbide particles led to a strong precipitation hardening increment up to 310 MPa.     

FEM Simulation of the Hydrogen Diffusion in X80 Pipeline Steel During Stacking for Slow Cooling

The influence of temperature on the hydrogen diffusion behavior in X80 pipeline steel during stacking for slow cooling was studied using electrochemical penetration method, the temperature field and the hydrogen diffusion in this pipeline steel during stacking for slow cooling were simulated by ABAQUS finite element method(FEM) software. The results show that in this process there is a reciprocal relationship between the natural logarithm of hydrogen diffusion coefficient and temperature. The cooling rate decreases gradually with the increase of steel plate thickness. The hydrogen content is higher at high temperature(500–400 °C) than that in low temperature region(300–100 °C). The FEM simulation results are consistent with the experimental ones, and the model can be used to predict the hydrogen diffusion behavior in industrial production of X80 pipeline steel.     

Preparation and properties of W–Cu–Zn alloy with low W–W contiguity

The W–Cu–Zn alloy with a-brass matrix and low W–W contiguity was prepared by method of electroless copper plating combined with spark plasma sintering(SPS) method.The effects of process and parameters on the microstructure and mechanical properties of the alloy were investigated.The W–Cu–Zn alloy with a relative density of 96 % and a W–W contiguity of about 10 % was prepared by original fine tungsten particles combined with wet mixing method and SPS solid-state sintering method at 800 °C for 10 min.The microstructure analysis shows that Cu–Zn matrix consists of nano-sized a-brass grains,and the main composition is Cu_3Zn electride.The nano-sized Cu was coated on the surface of tungsten particles by electroless copper plating method,and the fairly low consolidation temperature and short solid-state sintering time result in the nano-sized matrix phase.The dynamic compressive strength of the W–Cu–Zn alloy achieves to1000 MPa,but the alloy shows poor ductility due to the formation of the hard and brittle Cu_3Zn electrides.The fine-grain strengthening and the solution strengthening of the Cu–Zn matrix phase are responsible for the high Vickers microhardness of about 300 MPa for W–Cu–Zn alloy.     

Application of nanoparticles in cast steel:An overview

The innovative and environmentally friendly methodologies for comprehensively enhancing the performances of high-strength steels without damage to plasticity,toughness and heat/corrosion/fatigue resistance are being developed.In recent years,nanoparticles elevate the field of high-strength steel.It is proposed that nanoparticles have the potential to replace conventional semi-coherent intermetallic compounds,carbides and alloying to optimize the steel.The fabrication process is simplified and the cost is lower compared with the traditional methods.Considerable research effort has been directed towards high-performance cast steels reinforced with nanoparticles due to potential application in major engineering.Nanoparticles are found to be capable of notably optimizing the nucleation behavior and precipitate process.The prominently optimized microstructure configuration and performances of cast steel can be acquired synchronously.In this review,the lattice matching and valence electron criterion between diverse nanoparticles and steel are summarized,and the existing various preparation methods are compared and analyzed.At present,there are four main methods to introduce nanoparticles into steel:external nanoparticle method,internal nanoparticle method,in-situ reaction method,and additive manufacturing method.These four methods have their own advantages and limitations,respectively.In this review,the synthesis,selection principle and strengthening mechanism of nanoparticles in cast steels for the above four methods are discussed in detail.Moreover,the main preparation methods and microstructure manipulation mechanism of the steel reinforced with different nanoparticles have been systematically expatiated.Finally,the development and future potential research directions of the application of nanoparticles in cast steel are prospected.     

Microstructure and properties of aging Cu–Cr–Zr alloy

The crystallography and morphology of precipitate particles in a Cu matrix were studied using an aged Cu–Cr–Zr alloy by transmission electron microscopy(TEM) and high-resolution transmission electron microscopy(HRTEM). The tensile strength and electrical conductivity of this alloy after various aging processes were tested. The results show that two kinds of crystallographic structure associated with chromium-rich phases, fcc and bcc structure, exist in the peak-aging of the alloy. The orientation relationship between bcc Cr precipitate and the matrix exhibits Nishiyama–Wasserman orientation relationship. Two kinds of Zr-rich phases(Cu4Zr and Cu5Zr)can be identified and the habit plane is parallel to {111}Cu plane during the aging. The increase in strength is ascribed to the precipitation of Cr- and Zr-rich phase.     

Strain Hardening Associated with Dislocation,Deformation Twinning,and Dynamic Strain Aging in Fe–20Mn–1.3C–(3Cu) TWIP Steels

The effects of Cu on stacking fault energy,dislocation slip,mechanical twinning,and strain hardening in Fe–20Mn–1.3C twinning-induced plasticity(TWIP) steels were systematically investigated.The stacking fault energy was raised with an average slope of 2 mJ/m2 per 1 wt% Cu.The Fe–20Mn–1.3C–3Cu steel exhibited superior tensile properties,with the ultimate tensile strength reached at 2.27 GPa and elongation up to 96.9% owing to the high strain hardening that occurred.To examine the mechanism of this high strain hardening,dislocation density determination by XRD was calculated.The dislocation density increased with the increasing strain,and the addition of Cu resulted in a decrease in the dislocation density.A comparison of the strain-hardening behavior of Fe–20Mn–1.3C and Fe–20Mn–1.3C–3Cu TWIP steels was made in terms of modified Crussard–Jaoul(C–J) analysis and microstructural observations.Especially at low strains,the contributions of all the relevant deformation mechanisms—slip,twinning,and dynamic strain aging—were quantitatively evaluated.The analysis revealed that the dislocation storage was the leading factor to the increase of the strain hardening,while dynamic strain aging was a minor contributor to strain hardening.Twinning,which interacted with the matrix,acted as an effective barrier to dislocation motion.     

Mapping the parent austenite orientation reconstructed from the orientation of martensite by EBSD and its application to ausformed martensite

A new method is developed for reconstruction of the local orientation of the parent austenite based on the orientation of lath martensite measured by electron backscattered diffraction. The local orientation of austenite was obtained by least squares fitting as the difference between the experimental data and the predicted martensite orientation was minimal, assuming the specific orientation relationship (OR) between martensite and the parent austenite. First, the average OR between austenite and lath martensite was precisely determined and it was shown that both close packed planes and directions between martensite and the parent austenite deviated by more than 1° in low carbon martensite. The quality of the reconstructed austenite orientation map depended strongly on the OR used for the calculation. When Kurdjumov–Sachs (K–S) or Nishiyama–Wasserman (N–W) ORs were used the austenite orientation was frequently mis-indexed as a twin orientation with respect to the true orientation because of the mirror symmetry of (0 1 1)_α stacking in the K–S or N–W ORs. In contrast, the frequency of mis-indexing was significantly reduced by using the measured OR, where the close packed planes and directions were not parallel. The deformation structure in austenite was successfully reconstructed by applying the proposed method to ausformed martensite in low carbon steel.     

17CrNiMo6钢中板条马氏体的形态与晶体学分析

利用电子背散射衍射(EBSD)技术和TEM分析方法研究了17CrNiMo6钢中一个形态上的packet内的板条马氏体的形态与晶体学特征.叙述了马氏体K-S关系的晶体学分析方法.结果表明:虽然真实马氏体取向与理想K-S关系有一定的分散,但是packet内6种变量符合K-S关系,6种K-S关系的变量之间有特定的结合.     

焊缝金属中针状铁素体晶粒长大行为

以针状铁素体(AF)组织为基体组织的大热输入焊缝金属作为研究对象,采用金相显微镜(OM)、扫描电子显微镜电子背散射衍射装置(EBSD)、全自动静态相变仪等手段表征了焊缝金属内细长状的针状铁素体(AF)组织晶粒的取向特征,分析不同焊接热输入对焊缝金属内AF晶粒形核以及长大行为的影响规律. 结果表明,在大热输入焊接条件下,当焊缝金属的冷却速度低于临界冷却速度时,AF晶粒以多边形铁素体形态从夹杂物边缘开始形核时,此刻与母相奥氏体(γ)间偏离K-S关系,当焊缝金属温度降低至相变开始温度后,AF晶粒以细长的针状开始长大,并且与母相奥氏体间满足K-S关系. 但是随着焊接热输入降低,冷却速度逐步的加快,AF晶粒形核尺寸将越来越小,并且向着与母相奥氏体满足K-S关系的取向偏转速度加快,当焊缝金属的冷却速度超过临界冷却速度时,AF晶粒不用形核就可以迅速长大.     

On the crystallographic characteristics of ion beam synthesised β–FeSi2

Nanometre-scale β–FeSi₂ precipitates were introduced in a single crystal silicon substrate by low-dose ion-beam synthesis (IBS). The crystallographic relationship between these nanometre β precipitates and the silicon substrate has been studied by high resolution electron microscopy (HREM). The results show that the orientation relationship (OR) between the nanometre β precipitates and the silicon substrate is [100]_β//[110]_Si and (001)_β//(1 1)_Si, with abnormally large strain between the precipitates and the substrate. This OR is important for the formation of 90°-OD boundaries within β–FeSi₂ grains. Also, the relationship between various reported low-index ORs has been analysed and a new low-index OR is predicted.     

中碳合金结构钢端淬曲线的非线性方程法模拟

利用余柏海非线性模型模拟计算了中碳合金结构钢的端淬曲线，结果表明，因为该模型未考虑合金元素对钢淬透性影响的交互作用，计算所得复合合金化的中碳钢端淬曲线与实测结果偏差较大。给出了中碳合金结构钢端淬曲线模拟过程中合金元素交互作用的数学模型，引入合金化交互当量参数(L0)，并利用该改进模型对多元合金结构钢的端淬曲线进行模拟计算，结果与文献给出的曲线吻合得较好。     

Microstructural Characteristics and Mechanical Properties of Low-Alloy,Medium-Carbon Steels After Multiple Tempering

The microstructure and mechanical properties of NiCrMoV-and NiCrSi-alloyed medium-carbon steels were investigated after multiple tempering. After austenitising, the steels were hardened by oil quenching and subsequently double or triple tempered at temperatures from 250 to 500 °C. The samples were characterised using scanning electron microscopy and X-ray diffraction, while the mechanical properties were evaluated by Vickers hardness testing, V-notched Charpy impact testing and tensile testing. The results showed that the retained austenite was stable up to 400 °C and the applied multiple tempering below this temperature did not lead to a complete decomposition of retained austenite in both steels. It was also found that the microstructure, hardness and impact toughness varied mainly as a function of tempering temperature,regardless of the number of tempering stages. Moreover, the impact toughness of NiCrMoV steel was rather similar after single/triple tempering at different temperatures, while NiCrSi steel exhibited tempered martensite embrittlement after single/double tempering at 400 °C. The observed difference was mainly attributed to the effect of precipitation behaviour due to the effect of alloying additions in the studied steels.     

Analysis of the orientation relationships between austenite, Widmanstätten carbides, and martensite in the 150G4 high-carbon steel after isothermal γ → α transformation

The orientation relationships (ORs) between the initial austenite, Widmanstätten carbide (WC), and martensite have been analyzed by the electron-microscopic method. An OR between WC and martensite has been experimentally determined in addition to the well-known OR between WC and austenite and the Kurdjumov-Sachs OR between austenite and martensite. The realization of a concrete variant of the Bagaryatskii ORs between martensite and WC is determined by specific features of the kinetics of austenite transformation in hypereutectoid steels.     

Dynamic Compression Behavior and Microstructure of a Novel Low-Carbon Quenching-Partitioning-Tempering Steel

A 0.2C-1.5Mn-1.5Si-0.6Cr-0.05Nb （wt%） steel is treated respectively by novel quenching-partitioning-tempering （Q-P-T） process and traditional quenching and tempering （Q＆T） process for comparison. X-ray diffraction analysis indicates that Q-P-T steel has about 10% retained austenite, but Q＆T steel hardly has one. With the increase of com- pression strain rate from 7 × 10^2 to 5 × 10^3 s^-1, the flow stress of Q-P-T steel increases, which demonstrates the positive strain rate effect, but does not exist in Q＆T steel. The characterization of scanning electron microscopy indicates that a large number of long, straight martensite laths in Q-P-T steel will bend or be destroyed by large compressive strain of 35% at 5 × 10^3 s^-1. However, relative small compressive s~xain of about 5% at 7× 10^2 s^-1 almost does not have any effect on the original lath morphology. The characterization of transmission electron microscopy further reveals the origin of the positive strain rate effect and the microstructural evolution during dynamic compressive deformation.     

The morphology and crystallography of lath martensite in Fe-C alloys

The morphology and crystallography of lath martensite in Fe-C alloys containing various carbon contents from 0.0026 to 0.61% were studied by analyzing electron back scattered diffraction patterns in scanning electron microscopy and Kikuchi diffraction patterns in transmission electron microscopy. As carbon content increases, the sizes of both packet and block decrease. In low carbon steels (0.0026–0.38%C), a block which is observed as having different contrasts under optical microscopy contains two groups (sub-blocks) of laths which are of two K-S variants with a misorientation of about 10 degrees. On the other hand, in the high carbon alloy (0.61%C), a block consists of laths of a single K-S variant.     

超高强度马氏体时效不锈钢中逆转变奥氏体的析出与长大行为

研究了一种Cr-Ni-Co-Mo系超高强度马氏体时效不锈钢中逆转变奥氏体的析出与长大行为。结果表明,经540℃×4 h时效后钢的强度达到峰值1940 MPa,并有薄膜状逆转变奥氏体沿马氏体板条界上非连续析出,使钢具有良好的综合力学性能。时效初期逆转变奥氏体在马氏体板条、板条界或原奥氏体晶界处形核,随着保温时间的延长,薄膜状的逆转变奥氏体通过与扩散有关的形核长大方式生长,最终形成块状,并与板条马氏体间保持着K-S或N-W关系。