A new step-cooling process for strength and toughness matching control of vanadium-containing railway wheels:effect of intragranular ferrite

To improve the competitive relationship between strength and toughness,the effect of low undercooling in austenite(γ)on the microstructure and mechanical properties of commercial vanadium-containing wheel steels was studied using an optical microscope(OM),a scanning electron microscope(SEM),a transmission electron microscope(TEM),and mechanical property tests.The results show that when the wheel steel is slightly cooled to an appropriate temperature above A c3 point for a short time after it has been austenitized at an elevated temperature,the solid-solved vanadium is pre-precipitated in the form of V(C,N)second phase semicoherent with the matrix in the originalγgrain.This phase hardly participates in matrix strengthening.Due to the small mismatch between V(C,N)and ferrite(α),during the subsequent-cooling phase transformation stage,the pre-precipitated second phase becomes theαnucleation point,causing granular and ellipsoidal intragranular ferrite(IGF,with an average size of 4-6μm)to nucleate in the originalγ.The IGF production and strength loss increases with the increasing undercooling degree.Based on this,Masteel Co.,Ltd.has developed a new heat-treatment step-cooling process that can promote the formation of IGF,considerably improving the level and uniformity of fracture toughness on the premise that the strength and hardness of the wheel are almost unchanged.

Dynamic fracture toughness of high strength metals under impact loading:increase or decrease

An elusive phenomenon is observed in previous investigations on dynamic fracture that the dynamic fracture toughness （DFT） of high strength metals always increases with the loading rate on the order of TPa.m1/2.s-1. For the purpose of verification, variation of DFT with the loading rate for two high strength steels commonly used in the aviation industry, 30CrMnSiA and 40Cr, is studied in this work. Results of the experiments are compared, which were conducted on the modified split Hopkinson pressure bar （SHPB） apparatus, with striker velocities ranging from 9.2 to 24.1 m/s and a constant value of 16.3 m/s for 30CrMnSiA and 40Cr, respectively. It is observed that for 30CrMnSiA, the crack tip loading rate increases with the increase of the striker velocity, while the fracture initiation time and the DFT simultaneously decrease. However, in the tests of 40Cr, there is also an increasing tendency of DFT, similar to other reports. Through an in-depth investigation on the relationship between the dynamic stress intensity factor （DSIF） and the loading rate, it is concluded that the generally increasing tendency in previous studies could be false, which is induced from a limited striker velocity domain and the errors existing in the experimental and numerical processes. To disclose the real dependency of DFT on the loading rate, experimentsneed to be performed in a comparatively large striker velocity range.

Effect of Plastic Deformation and H_2S on Dynamic Fracture Toughness of High Strength Casing Steel

The effects of plastic deformation and H2 S on fracture toughness of high strength casing steel（C110 steel） were investigated. The studied casing specimens are as follows： original casing, plastic deformation（PD） casing and PD casing after being immersed in NACE A solution saturated with H2S（PD＋H2S）. Instrumented impact method was employed to evaluate the impact behaviors of the specimens, meanwhile, dynamic fracture toughness（JId） was calculated by using Rice model and Schindler model. The experimental results show that dynamic fracture toughness of the casing decreases after plastic deformation. Compared with that of the original casing and PD casing, the dynamic fracture toughness decreases further when the PD casing immersed in H2 S, moreover, there are ridge-shaped feature and many secondary cracks present on the fracture surface of the specimens. Impact fracture mechanism of the casing is proposed as follows： the plastic deformation results in the increase of defect density of materials where the atomic hydrogen can accumulate in reversible or irreversible traps and even recombine to form molecular hydrogen, subsequently, the casing material toughness decreases greatly.

Enhanced strength and toughness of high nitrogen stainless bearing steel by controlling interstitial partitioning via V-microalloying

High-nitrogen stainless bearing steel(HNSBS)with ultra-high tensile strength(∼2403 MPa)and good toughness(∼80.0 J)was obtained by V-microalloying,overcoming the strength-toughness trade-off of conventional V-free HNSBS.In this work,since V-microalloying facilitated the enrichment of interstitial atoms(C and N)in precipitates,the content of interstitial atoms in the matrix was reduced accordingly(i.e.,interstitial partitioning).On the one hand,V-microalloying reduced the substantial intergranular precipitates and transformed the precipitates from M_(23)C_(6)+M_(2)N into V-containing M_(23)C_(6)+M_(2)N+MN with multi-scale particle sizes,causing a coupling strengthening effect,which contributed to the toughness and additional strength increase.On the other hand,V-microalloying controlled interstitial partitioning,effectively refined coarse retained austenite(RA),increased the fraction of dislocation martensite,and reduced the fraction of twin martensite.The more film-like RA and dislocation martensite with high dislocation density coordinated plastic deformation and prevented crack propagation,thus obviously enhancing the strength and toughness of 0.2 V steel.This study provides a new route to develop high-performance HNSBS for aerospace applications.

Optimization Evaluation Test of Strength and Toughness Parameters for Hot-Stamped High Strength Steels

Use of hot-stamped high strength steels （HSHSS） not only reduces the vehicle weight, but also improves the crash safety, therefore more and more mentioned steels are used to produce automobile parts. However, there are several problems especially the low ductility and toughness, which have restricted the application of HSHSS in automobile body. Suitable process parameters are very crucial to improve strength and toughness. In order to study the effect of austenization temperature, soaking time and start deformation temperature on strength and toughness of boron steel 22MnB5, an L9 （34） orthogonal experiment which was analyzed by means of comprehensive evaluation was carried out based on Kahn tear method to obtain the value of fracture toughness. The results indicate that the ex- cellent formability, high strength and toughness of boron steel 22MnB5 with 1.6 mm in thickness are obtained when the austenization temperature is in the range of 920- 950 ℃, the soaking time is 1 min and the start deformation temperature is in the range of 650- 700 ℃. The optimal parameters were used for typical hot stamping structural parts tests. Properties of samples such as tear strength, unit initiation energy and ratio of strength to toughness （RST） were improved by 10.91%, 20.32% and 22.17%, respectively. Toughness was increased substantially on the basis of a small decrease of strength.

Effect of W on microstructure of high strength and toughness steels

The effect of W on the microstructure and the mechanical properties of ultrahigh strength low alloy steels was carried out. The microstructure of 30Cr3Si2Mn2NiMoNb and 30Cr3Si2Mn2NiMoNbW steels under quenched conditions were investigated by metallographic microscope, scanning electron microscope （SEM）, X-ray diffrac- tion （XRD）, and transmission electron microscope （TEM）. Thermodynamic cal- culation was also conducted. The results showed that the addition of W made undissolved carbides more and finer, which exerted strong pinning force on migrat- ing packet boundary and improved tensile strength significantly. M6C particles in 30Cr3Si2Mn2NiMoNb steel were disappeared above 1193 K, while the M6C particles in 30Cr3Si2Mn2NiMoNbW steel were disappeared above 1253 K, the calculation results were in agreement with the experimental.

Development of High Strength and Toughness Non-Heated Al–Mg–Si Alloys for High-Pressure Die-Casting

Based on the 3 factors and 3 levels orthogonal experiment method,compositional effects of Mg,Si,and Ti addition on the microstructures,tensile properties,and fracture behaviors of the high-pressure die-casting Al-x Mg-y Si-z Ti alloys have been investigated.The analysis of variance shows that both Mg and Si apparently infl uence the tensile properties of the alloys,while Ti does not.The tensile mechanical properties are comprehensively infl uenced by the amount of eutectic phase(α-Al+Mg2Si),the average grain size,and the content of Mg dissolved intoα-Al matrix.The optimized alloy is Al-7.49 Mg-3.08 Si-0.01 Ti(wt%),which exhibits tensile yield strength of 219 MPa,ultimate tensile strength of 401 MPa,and elongation of 10.5%.Furthermore,contour maps,showing the relationship among compositions,microstructure characteristics,and the tensile properties are constructed,which provide guidelines for developing high strength and toughness Al–Mg–Si–Ti alloys for high-pressure die-casting.

In situ synthesis and phase analysis of low density O'-sialon-based multiphase ceramics

In situ formed low density O＇-sialon-based multiphase ceramics were prepared by liquid-phase sintering method at 1400°C with Si3N4, SiO2 and Al2O3 as raw materials.Crystalline phases were identified by X-ray diffraction（XRD）.The quantitative phase analysis was finished by matrix-flushing method and the substitution parameter x value of O＇-sialon was estimated.The effects of sintering additives on the phase composition of the material were studied.The results show that, when using Y2O3 alone, Al6Si2O13 phase can be formed in the material, but when using Y2O3 and MgO, MgAl2O4 phase can be preferentially formed and the Al6Si2O13 is not observed.The mechanical properties of the material were measured and the relationships between microstructure and mechanical properties were discussed.The sample with Y2O3 and MgO sintering additives, using fused quartz alone as SiO2 source, displays a combination of high bending strength（163 MPa） and good fracture toughness（3.11 MPa·m1/2）.Bending strength and fracture toughness of the samples increase with the increase of the content and aspect ratio of elongated grains and decrease with the increase of the porosity.

Extraordinary mechanical performance in charged carbyne

Carbyne,the linear chain of carbon,promises the strongest and toughest material but possesses a Peierls instability(alternating single-bonds and triple-bonds)that reduces its strength and toughness.Herein,we computationally found that the gravimetric strength,strain-to-failure,and gravimetric toughness can be improved from 74 GPa·g^(-1)·cm^(3),18%,and 9.4 k J·g^(-1)for pristine carbyne to the highest values of 106 GPa·g^(-1)·cm^(3),26%,and 19.0 k J·g^(-1)for carbyne upon hole injection of+0.07 e/atom,indicating the charged carbyne with record-breaking mechanical performance.Based on the analyses of the atomic and electronic structures,the underlying mechanism behind the record-breaking mechanical performance was revealed as the suppressed and even eliminated bond alternation of carbyne upon charge injection.

Predictive fatigue crack growth law of high-strength steels

The fatigue resistance of metallic materials is generally attributed to both strength and toughness.Unfortunately,these properties are mutually exclusive in most materials.Classical theories like Paris’law only provide some data correlation schemes rather than a predictive capability,which cannot satisfactorily guide the anti-fatigue design.In this study,for the first time,the predictive fatigue crack growth rate law is proposed by considering the effects of both strength and toughness.Accordingly,a quantitative criterion is established for judging the fatigue crack resistance of high-strength steels.The predictive law would provide a unique view to the quantitative anti-fatigue design of metallic materials.

High-performance flexible nanocomposites with superior fire safety and ultra-efficient electromagnetic interference shielding

High-performance multifunctional polymeric materials integrated with high fire safety,excel-lent mechanical performances and electromagnetic interference(EMI)shielding properties have great prospects in practical applications.However,designing highly fire-safe and mechanically ro-bust EMI shielding nanocomposites remains a great challenge.Herein,hierarchical thermoplastic polyurethane/cyclophosphazene functionalized titanium carbide/carbon fiber fabric(TPU/CP-Ti_(3)C_(2)T_(x)/CF)nanocomposites with high fire safety and mechanical strength and toughness were prepared through the methods of melt blending,layer-by-layer stacking and thermocompression.The TPU/CP-Ti_(3)C_(2)T_(x)showed improved thermal stability.Moreover,the peak of heat release rate and total heat release of the hi-erarchical TPU sample containing 4.0 wt.%CP-Ti_(3)C_(2)T_(x)were respectively reduced by 64.4%and 31.8%relative to those of pure TPU,which were far higher than those of other TPU-based nanocomposites.The averaged EMI shielding effectiveness value of the hierarchical TPU/CP-Ti_(3)C_(2)T_(x)-2.0/CF nanocomposite reached 30.0 dB,which could satisfy the requirement for commercial applications.Furthermore,the ten-sile strength of TPU/CP-Ti_(3)C_(2)T_(x)-2.0/CF achieved 43.2 MPa,and the ductility and toughness increased by 28.4%and 84.3%respectively compared to those of TPU/CF.Interfacial hydrogen bonding in combination with catalytic carbonization of CP-Ti_(3)C_(2)T_(x)nanosheets and continuous conductive network of CF were re-sponsible for the superior fire safety,excellent EMI shielding and outstanding mechanical performances.This work offers a promising strategy to prepare multifunctional TPU-based nanocomposites,which have the potential for large-scale application in the fields of electronics,electrical equipment and 5 G facilities.

Experimental Investigation on Friction Welding of UNS S32205 Duplex Stainless Steel

UNS S 32205 duplex stainless steel specimens were joined by continuous drive friction welding process. The experiments were conducted as per the Taguchi（L16 orthogonal array） method. The friction welding process parameters such as heating pressure, heating time, upsetting pressure, upsetting time, and speed of rotation were fixed with low,medium, and high levels of range based on the machine capacity, and the required knowledge was acquired from the preliminary experiments. The joint characterization studies included micro structural examination and evaluation of mechanical properties of the joints. Microhardness variation, impact toughness, and tensile strength of the joints were evaluated. Neither a crack nor an incomplete bonding zone was observed. The tensile strength of the joints was higher than the strength of the base material, and the friction and upsetting pressures were found to influence the joint strength. The tensile strength of all the welds was observed to be increasing with an increase in the rotational speed. The toughness of the friction welds was evaluated at room temperature and also at subzero（cryo） temperature conditions. The toughness for friction welds was found to be superior to the fusion welds of duplex stainless steel at room temperature and cryo conditions. Weldments exhibited better corrosion resistance than the parent material.

Water molecule-induced hydrogen bonding between cellulose nanofibers toward highly strong and tough materials from wood aerogel

Lightweight,highly strong and bio-based structural materials remain a long-lasting challenge.Here,inspired by nacre,a lightweight and high mechanical performance cellulosic material was fabricated via a facile and effective top-down approach and the resulting material has a high tensile strength of149.21 MPa and toughness of 1.91 MJ/m^(3).More specifically,the natural balsawood(NW) was subjected to a simple chemical treatment,removing most lignin and partial hemicellulose,follow by freeze-drying,forming wood aerogel(WA).The delignification process produced many pores and exposed numerous aligned cellulose nanofibers.Afterwards,the WA absorbed a quantity of moisture and was directly densified to form above high-performance cellulosic material.Such treatment imitates highly ordered"brick-and-mortar" arrangement of nacre,in which water molecules plays the role of mortar and cellulose nanofibrils make the brick part.The lightweight and good mechanical properties make this material promising for new energy car,aerospace,etc.This paper also explains the strengthening mechanism for making biomimetic materials by water molecules-induced hydrogen bonding and will open a new path for designing high-performance bio-based structural materials.

Ausforming effects on anisotropy of mechanical properties in HSLA martensitic steel

In order to clarify effects of prior pancaked austenitic structure on microstructure and mechanical properties of transformed martensite in ausformed steel,a super-thin pancaked austenite was processed by multi-pass rolling in a 0.03-2.6Mn0.06Nb-0.01Ti(wt%) low alloy steel.The evolution of prior pancaked austenite grain during multi-pass rolling was studied using Ni-30Fe model alloy.Related with the structure and texture in the prior super-thin pancaked austenite in Ni-30Fe alloy,the texture and anisotropy of mechanical properties of transformed martensite in the studied ausformed steel were focused on.There were mainly three kinds of rolling texture components in the super-thin pancaked austenite:Goss {110} 001,copper {112} 111 and brass {110} 112.They were further transformed into the weak {001} 110 and strong {112} 110,{111} 112 texture components in the martensitic structure.The orientation relationship(OR) of lath martensite transformation from pancaked austenite in the ausformed steel deviated larger from the exact Kurdjumov-Sachs(K-S) OR than in the case of equiaxed austenite without deformation.The tensile and yield strengths of the ausformed martensitic steel first decreased and then increased as the angle between tension direction and rolling direction increased.The main reason for the anisotropy of strength was considered as the texture component {112} 110 in martensite.However,the anisotropy of impact toughness was more complex and the main reasons for it are unknown.

Porosity Effects on Mechanical Properties of 3D Random Fibrous Materials at Elevated Temperatures

In this study,we prepare the specimens of three-dimensional random fibrous(3D RF)material along its through-the-thickness(TTT)and in-plane(IP)directions.The experimental tests of tensile and compressive properties as well as fracture toughness of 3D RF material are performed at elevated temperatures.Then,the porosity(83%,87%and 89%)and temperature dependence of the tensile and compressive strength,elastic modulus,fracture toughness and fracture surface energy of the 3D RF materials for both the TTT and IP directions are analyzed.From the results of the tensile strength and elastic modulus versus material porosities at various temperatures,we find that tensile strength and elastic modulus for the TTT direction are more sensitive to the porosity,but not for the IP direction.Fracture toughness increases firstly and then decreases at a certain critical temperature.Such critical temperature is found to be the lowest for the porosity of 83%.On the other hand,at below 1073 K,the temperature-dependent fracture surface energies with three porosities for the TTT direction show similar variation trends.