Improvement of Toughness of Ultrahigh Strength Steel Aermet 100

The influence of double aging on the microstructure and mechanical properties of ultrahigh strength steel Aermet 100 was analyzed. Under the double aging, there is no apparent decrease in the strength of steel. However, the impact fatigue life can be prolonged by 35.5% and dynamic fracture toughness be raised by 22.6% respectively, as compared with the normal aging. Based on the observation of microscopic structure, the physical mechanism of the prolongation of impact fatigue life and the enhancement of stability of the reverted austenite, AR, is analyzed further. The results show that this new technique is a breakthrough of combination optimization between strength and toughness for Aermet 100 steel. In the light of the current understanding on this subject, the volume fracture of soften and tough AR formed in process of heat preservation at higher temperature of double aging increases drastically. Moreover, during the treatment of lower temperature of double aging, the carbon separating from the martensitic ferrite will diffuse into AR, resulting that the martensitic brittleness decreases and the stability of AR increases.

Effects of Si on the stability of retained austenite and temper embrittlement of ultrahigh strength steels

Effects of silicon （Si） content on the stability of retained austenite and temper embrittlement of ultrahigh strength steels were investigated using X-ray diffraction （XRD）,transmission electron microscopy （TEM）,and other experimental methods.The results show that Si can suppress temper embrittlement,improve temper resistance,and hinder the decomposition of retained austenite.Reversed austenite appears gradually with the increase of Si content during tempering.Si has a significant effect on enhancing carbon （C） partitioning and improving the stability of retained austenite.Si and C atoms are mutually exclusive in lath bainite,while they attract each other in austenite.ε-carbides are found in 1.8wt% Si steel tempered at 250℃,and they get coarsened obviously when tempered at 400℃,leading to temper embrittlement.Not ε-carbides but acicular or lath carbides lead to temper embrittlement in 0.4wt% Si steel,which can be inferred as cementites and composite compounds.Temper embrittlement is closely related to the decomposition of retained austenite and the formation of reversed austenite.

High-cycle Fatigue Fracture Behavior of Ultrahigh Strength Steels

The fatigue fracture behavior of four ultrahigh strength steels with different melting processes and therefore different inclusion sizes were studied by using a rotating bar two-point bending fatigue machine in the high-cycle regime up to 107 cycles of loading. The fracture surfaces were observed by field emission scanning electron microscopy （FESEM）. It was found that the size of inclusion has significant effect on the fatigue behavior. For AtSI 4340 steel in which the inclusion size is smaller than 5.5 μm, all the fatigue cracks except one did not initiated from inclusion but from specimen surface and conventional S-N curve exists. For 65Si2MnWE and Aermet 100 steels in which the average inclusion sizes are 12.2 and 14.9 μm, respectively, fatigue cracks initiated from inclusions at lower stress amplitudes and stepwise S-N curves were observed. The S-N curve displays a continuous decline and fatigue failures originated from large oxide inclusion for 60Si2CrVA steel in which the average inclusion size is 44.4 pro. In the case of internal inclusion-induced fractures at cycles beyond about 1×10^6 for 65Si2MnWE and 60Si2CrVA steels, inclusion was always found inside the fish-eye and a granular bright facet （GBF） was observed in the vicinity around the inclusion. The GBF sizes increase with increasing the number of cycles to failure Nf in the long-life regime. The values of stress intensity factor range at crack initiation site for the GBF are almost constant with Nf, and are almost equal to that for the surface inclusion and the internal inclusion at cycles lower than about 1×10^6. Neither fish-eye nor GBF was observed for Aermet 100 steel in the present study.

ON THE STRENGTHENING MECHANISM OF HIGH Co-Ni ULTRAHIGH STRENGTH STEEL

In this paper, the microstructure and strengthening mechanism of 23NiCo steel were studied by transmission electron microscopy and atom-probe field-ion microscopy. The results show that the peak strength obtained when the steel is tempered at 440455℃is due to the precipitation of coherent zones of fine carbides.The peak in toughness attained at the absence of cementite and the formation of reverted and stable austenite.Tempering at higher temperatures resulted in loss of both strength and toughness. The drop of the strength of the steel tempered at high temperature is due to precipitation coarsening and loss of coherence with the matrix.

Phase Transformation and Mechanical Properties of an Ultrahigh Strength Steel

The present study deals with the development of a low carbon high strength steel by thermomechanical controlled processing on a pilot scale.The continuous cooling transformation has yielded a flat top "C" curve with the phase field occupied by a mixture of bainite and martensite.The microstructure of water quenched steel essentially consists of highly dislocated lath martensite along with fine (Ti,Nb)CN precipitates and twins.High strength steel with 1217-1298 MPa yield strength and 1372-1513 MPa ultimate tensile strength along with 16-12% total elongation has been obtained in the range of 850 to 750℃ finish rolling temperature.The impact toughness value in the range of 45-72J was also achieved in the present steel.

Microstructural Evolution and Mechanical Behavior of Friction-Stir-Welded DP1180 Advanced Ultrahigh Strength Steel

Friction stir lap welding of a DP1180 advanced ultrahigh strength steel was successfully carried out by using three welding tools with different pin lengths. The effects of the welding heat input and material flow on the microstructure evolution of the joints were analyzed in detail. The relationship between pin length and mechanical properties of lap joints was studied. The results showed that the peak temperatures of all joints exceeded A c3, and martensite phases with similar morphologies were formed in the stir zones. These martensite retained good toughness due to the self-tempering effect. The formation of ferrite and tempered martensite was the main reason for the hardness reduction in heat-affected zone. The mechanical properties of the lap joints were determined by loading mode, features of lap interface and the joint defects. When the stir pin was inserted into the lower sheet with a depth of 0.4 mm, the lap joint exhibited the maximum tensile strength of 12.4 kN.

Electroslag Refining of CrNiMoWMnV Ultrahigh-Strength Steel

Increasing demands for ultrahigh-strength steels in commercial as well as military applications have raised interest in finding alternatives to the high-cost high-alloyed steel and super-alloys currently used, e.g. the use of economic low-alloy compositions processed via low-cost air induction melting and electroslag refining (ESR). In this work the yield of alloying elements and the removal of the impurities nitrogen, sulphur and phosphorus as a result of electroslag refining (ESR) in a newly developed CrNiMoWMnV ultrahigh-strength steel (UHSS) have been studied in relation to their activities in the molten metal pool. Six experimental heats of CrNiMoWMnV UHSS with different chemical compositions were designed, melted in an induction furnace (IF) and refined using ESR. This was followed by hot forging of the ingots at 1100°C to 950°C. ESR using a CaF2-CaO-Al2O3 slag system led to a high yield in Cr, Ni, Mo, W, Mn and V, while the yield of Si is low. The desulphurization of all six UHSS grades was pronounced with most of the sulphur removed either to the slag or by gas reactions. The degree of dephosphorization was only 5% irrespective of the steel composition. On the other hand, denitrification (removal of nitrogen) was achieved. It ranged from 8% to 63% depending on the steel composition. The yield of the alloying elements and removal of impurities from the steel during ESR depends on the chemical and physical properties of the ESR slag and the activity of the elements in the molten state, taking into account elemental interactions.

钢管超高强混凝土受弯力学性能试验研究

以官盛渠江大桥为依托,通过3组共6个受弯试件的模型试验,探讨钢管超高强混凝土(抗压强度f_(cu)=80.3~115.2MPa)的受弯力学性能,研究混凝土强度对钢管超高强混凝土的抗弯承载力、变形特征与失效模式的影响。试验结果表明:钢管超高强混凝土的受弯破坏模式与普通钢管混凝土相同,主要为挠度过大而失效,呈整体弯曲破坏,受压区有局部鼓屈;试件进入屈服阶段后,承载力降低较少,具有很好的弯曲延性性能;管内混凝土主要对钢管提供横向约束,避免钢管过早受弯压陷屈曲;在截面含钢率一定时,管内混凝土强度增长对钢管超高强混凝土受弯构件的受弯破坏模式、承载力与延性性能的影响较小。

Simulation of dynamic recrystallization in 23Co13Ni11Cr3Mo steel using a modified cellular automaton

A modified cellular automaton(CA) program was developed to simulate the process of dynamic recrystallization(DRX) for 23Co13Ni11Cr3Mo ultrahigh strength steel.In this model,influences of deformation parameters on hardening rate and solute drag effect were considered.Moreover,an inverse analysis method was proposed for parameters identification of dislocation model and solute drag effect based on the results of isothermal compression tests on Gleeble-1500.Then,simulated microstructures under different deformation conditions were compared with those of experiments.A good agreement is achieved.Furthermore,influences of deformation parameters on microstructure evolution for 23Co13Ni11Cr3Mo steel were investigated in details.High strain is an effective measure to refine grain and improve homogeneity.Meanwhile,the desired deformation parameters are temperature of 1000-1050 °C and strain rate of 0.008-0.01 s-1 for obtaining grains smaller than 22.5 μm.

HRTEM Study on Precipitates in High Co-Ni Steel

The secondary hardening reaction is accompanied with precipitation of fine carbides in high CoNi ultrahigh strength steel. The crystal structure of the precipitating carbides is unambiguously determined by microbeam diffraction in transmission electron microscopy. It is identified that the needle-shaped carbides are M2C with a hexagonal structure. The concentration of substitutional alloying elements in the carbides quantified by energy dispersive X-ray spectroscopy (EDS) also supports the result above. The spatial structure of M2C is identical with L'3 type. Metal atoms are in a close packed hexagonal structure, the carbon atoms partly distribute with random in the octahedral interstices and the filling probability is less than 1/2. Particular attention was paid to the relationship of needle-shaped carbides/ferrite matrix at secondary hardening peak tempered at 482癈 for 5 h. Observation by high resolution transmission electron microscopy (HRTEM) confirms that carbides with black-white contrast are fully coherent with ferrite and have individual crystal structure, and the coarsened carbides with moire' fringe are partially coherent with matrix. The orientation relationship between M2C and ferrite matrix is directly observed and identified with the well established P-S relationship [001]a //[111]c.

SOLUTION TREATMENT EFFECTS IN COBALT-NICKEL SECONDARY HARDENING STEEL

Effect of the austenitizing temperature on the microstructure,strength, and toughness of 16NiCo and 23NiCo alloys was studied. With increasing austenitizing temperature, the reductions in strength and hardness are probably caused by the coarsening of M_2C carbides. The use of higher austenitizing temperature resulted in improved impact toughness due to the dissolution of undissolved (Cr, Fe, Mo)_(23)C_6 carbides.As the austenitizing temperature is increased to 1050℃ for 16NiCo and 23NiCo alloys impact toughness levels are reduced on average by 31 and 53 J/cm￣2 respectively.The use of 1050℃ austenitizing temperatures resulted in intergranular fracture for 23NiCo alloy in 482℃ aged condition. It was suggested that this grain boundary embrittlement was the result of the formation of cementite particles during ageing.

挤压成形和回火温度对DT300钢力学性能的影响分析

本文研究了挤压成形和不同回火温度对DT300钢力学性能的影响,并分析了各工艺条件下的影响规律,以获得最佳材料力学性能,为指导企业生产、促进DT300钢的推广应用提供了有效的试验数据。

Austenite/martensite structure and corresponding ultrahigh strength and high ductility of steels processed by Q&P techniques

The microstructure of steels treated by Q&P(quenching and partitioning) process was characterized,a method of controlling retained austenite fraction based on inhomogeneous martensitic transformation was proposed,and the mechanical properties of steels treated by Q&P process were measured.The results show that the microstructure of the studied steels is mainly composed of initial martensite,fresh martensite and retained austenite.The initial marteniste formed at the first quenching step is easily etched;the fresh martensite formed at the final quenching step looks like 'blocky' type phase,and the retained austenite is mainly located on the packet boundary and initial austenite grain boundary.The inhomogeneous microstructure causes the experimental optimum quenching temperature corresponding to maximum retained austenite fraction to be higher than the calculation based on CPE(constrained paraequilibrium) model.The product of tensile strength and total tensile elongation is 47.5 GPa%,and tensile strength of 1760 MPa was obtained for the steel with carbon content of 0.51 wt%.The TRIP(transformation induced plasticity) effects of the large fractioned metastable austenite make a main contribution to the high ductility improvement,and the martensitic matrix provides high strength.

Ultrastrong steel strengthened by multiple shearable nanostructures

Precipitation of multiple strong nanoprecipitates is crucial for the development of ultrahigh-strength structural materials with a strength of 2.5 GPa or above.Nevertheless,the ductility usually loses rapidly with strength due to limited dislocation mobility and high cracking tendency if coarse non-deformable precipitates are employed.Herein,we report a 2.5 GPa maraging steel strengthened by an ultrahigh den-sity of intermeshed shearable nanostructures consisting of Ni(Al,Fe)nanoprecipitates and Mo-rich(∼30 at.%)disordered clusters,both of which assume coherent interfaces.The fully coherent B2-Ni(Al,Fe)par-ticles precipitate in an extremely fast fashion,effectively accelerating local aggregation of low-diffusivity Mo atoms and promoting the formation of Mo-rich clusters surrounding them.This elemental partition was found to be further enhanced by Co addition via depleting both residual Al and Mo within the ma-trix,leading to the formation of copious yet fine intermeshed nanostructures.During plastic deformation,the interlocked nanostructures not only enhance local cutting stress by combining long-range elastic and short-range chemically ordering effects but also improve dislocation activity and resist shear-induced plastic instability.The multiple shearable nanostructures endow decent ductility(>6%)of the 2.5 GPa steel,suggesting a new paradigm for designing ultrastrong steels.

Characterization of( Nb,Ti,Mo) C Precipitates in an Ultrahigh Strength Martensitic Steel

A study on ultrahigh strength steel plate subjected to novel thermo-mechanical control process was presented. The mechanical properties examination showed that the investigated steel exhibited excellent combination of ultra-high strength（ 2 200 MPa） and toughness（ 26 J）. The microstructure of the experimental steel was observed by scanning electron microscope and transmission electron microscope. Desired martensitic lath with width of about 180- 250 nm was obtained. Nanostructured carbide precipitates with sizes of 20-50 nm,which contained Nb,Ti and Mo,were observed in the lath martensitic microstructure,and confirmed to be MC-type carbides with B1 structure by means of selected area electron diffraction.The compositional characteristics revealed by energy dispersive X-ray spectrometer mapping implied that the carbide forming elements Nb,Ti and Mo distributed in the precipitates evenly. Three-dimensional atom probe tomography reconstruction further indicated that Mo incorporated into the precipitates without enrichment in the carbide-matrix interface and probably substituted for Nb and Ti to form the（ Nb,Ti,Mo） C carbides.

奥氏体化温度对超强耐热齿轮轴承钢显微组织和强韧性的影响

新型超强耐热齿轮轴承钢具有优越的强韧性。通过改变钢的淬火加热温度,结合拉伸、冲击、断裂韧度等力学性能测试以及TEM,SEM,EDS等微观分析技术,研究不同奥氏体化温度下钢的显微组织与力学性能。结果表明:1060℃奥氏体化后,钢中存在未溶碳化物M_(6)C,冲击功和断裂韧度较低;1080~1100℃奥氏体化后,M_(6)C碳化物固溶,冲击功和断裂韧度显著增加。在1060~1100℃奥氏体化后,抗拉强度和塑性变化不大,规定塑性延伸强度随奥氏体化温度的增加略有降低。M_(6)C碳化物加速裂纹的萌生与扩展,导致韧性下降。在1080~1100℃奥氏体化后,超强耐热齿轮轴承钢可获得超高强度和高韧性,抗拉强度不小于2000 MPa,规定塑性延伸强度不小于1800 MPa,断裂韧度不小于100 MPa·m^(1/2)。

THE VALENCE ELECTRON STRUCTURE OF AUSTENITE IN LOW ALLOY ULTRAHIGH-STRENGTH STEELS AND ITS INFLUENCES ON KINETICS OF PHASE TRANSFORMATION

Based on S. H. Yu's empirical electron theory of solids and molecules, the valence electron structure of austenite in low alloy ultrahigh-strength steels, 30CrMnSiNi_2A and 40 CrMnSiMoVA is established. The behavior of various main-added elements in the kinetics of phase transformation is discussed on the basis of C-Me segregation caused by the valence electron structure, The influence of alloying elements on the structure and morphology of transformed products is discussed from the viewpoint of interaction between the driving force of phase transformation and segregating force. Then on the basis of the valence electron structure of the alloy, the composition design of ultrahlgh-strength steels is discussed.

Strain-induced formation of a gradient nanostructured surface layer on an ultrahigh strength bearing steel

In the present work, an ultrahigh strength bearing steel（AISI 52100） was subjected to surface mechanical rolling treatment（SMRT） at room temperature. Microstructural observations showed that martensitic laths, twins and cementite particles in the initial microstructure underwent distinct plastic strains and were gradually refined into nanostructures. Consequently, a gradient nanostructured（GNS） surface layer with a mean grain size of -24 nm at the top surface was obtained on the bearing steel, resulting in an increment of -20% in the surface hardness. Analyses based on microstructural evolution, phase constitution and in-depth hardness distribution revealed a mechanically induced formation mechanism of the GNS surface layer. The multiple surface severe plastic deformation under fine lubrication and cooling during SMRT contributed to the formation of a thick hardened surface layer on the bearing steel.

阴极电流密度对超高强钢锌镍镀层腐蚀及氢脆性能的影响

碱性电镀锌镍因生产过程更环保、镀层耐蚀性好等优点,已成为代镉技术的首选。但是,由于氢脆等原因,暂未应用于超高强钢A100的表面防护。采用一种碱性镀液配方,在不同阴极电流密度下对超高强钢A100表面进行电镀及钝化,同时对钝化后的镀层进行了物相、微观形貌、电化学性能、盐雾、氢脆等组织及性能进行表征。结果表明:在阴极电流密度为2—4 A∙dm^(−2)范围内,可得到单一γ相镀层,并且镍含量在12%—13%左右;当阴极电流密度为2 A∙dm^(−2)时,析氢反应进行得比高电流密度时剧烈,电镀效率较低,需要更长的电镀时间才能得到与阴极电流密度为3—4 A∙dm^(−2)时耐蚀性相近的镀层。本镀液配方制备的镀层氢脆性能好,带有镀层的试棒均通过了200 h缺口拉伸测试,且在3—4 A∙dm^(−2)阴极电流密度电镀时渗氢电流也保持在了一个较低水平。

超高强度钢小螺纹孔加工工艺研究

目前,超高强度钢在各领域的应用越来越广泛,而在这类材料上加工小螺纹孔往往会产生断刀和乱牙等缺陷。为提高生产率,降低生产成本,急需研究出一种能够显著提高小螺纹孔质量的加工工艺。对超高强度钢上的小螺纹孔加工进行工艺试验,首先在同一件试验件上采用4种不同的加工工艺分别加工4组M2螺纹孔,然后使用螺纹塞规对这4组螺纹孔进行检测,可知热处理过程会对螺纹产生烧蚀,并且丝锥无法在热处理后的螺纹底孔上加工出有效螺纹。对工艺试验所得到的工艺路线进行优化,在热处理前使用手动成组丝锥中的Ⅰ锥加工螺纹,热处理时对螺纹孔施加保护措施,热处理后使用手动成组丝锥中的Ⅱ锥加工螺纹,最终所得到的小螺纹孔质量和加工效率都有了显著提升。