Effect of heat treatment on the structural transformations and properties of high-nitrogen chromium steels

Conclusions  

Super-resolution spectral analysis and signal reconstruction of magnetic Barkhausen noise

An investigation was performed based on frequency domain analysis and reconstruction for the magnetic Barkhausen noise (MBN). The power spectrum density (PSD) of MBN was analyzed based on autoregressive (AR) model. The relations between PSD peaks and mechanical properties of high strength steel sheet were studied. The energy of low frequency signal components decreases as hardness increases, and increases as tensile stress increases. The energy of high frequency signal components increases as hardness increases, and decreases as tensile stress increases. After identification, the MBN signal was reconstructed by discrete Gabor expansion which remains desired signal components and removes undesired ones.     

Effect of the magnetic properties of cementite on the coercive force of high-carbon steels after quenching and tempering

From an analysis of temperature dependences of the coercive force, contributions of different mechanisms to the formation of the coercive force of steel U9A samples after quenching and tempering have been determined. It is shown that the main contribution to the formation of the coercive force upon tempering at intermediate and high temperatures T _temp comes from the magnetic hardness of cementite. The coercive force of cementite is determined by its structural state and in the range of tempering temperatures equal to 300–700°C changes along a curve with a maximum, thereby forming a maximum in the H _c(T _temp) dependence of this steel as well.     

Tempering kinetics and corrosion resistance of quenched and tempered AISI 4130 medium carbon steel

The kinetics and the effects of tempering on the corrosion resistance of AISI 4130 steel were studied. The tempering activation energy was close to that for the diffusion of C in α-iron. This implies that the tempering kinetics in this steel is controlled by carbon diffusion. By increasing the time and temperature of tempering, the corrosion current density (i_corr) in the polarization curves decreased whereas the diameter of the semicircular arc in the Nyquist plots increased, which implies higher corrosion resistance. The i_corr was successfully related to hardness, which actually indicates the effect of microstructure. Based on these findings, the quench and tempering treatment was used to adjust both the hardness and corrosion resistance of the AISI 4130 steel.     

Effects of ageing on bainitic and thermally induced martensitic transformations in ductile Cu–Al–Mn-based shape memory alloys

The effects of ageing at 473–573 K on the hardness, microstructure and thermoelastic martensitic transformation in Cu–Al–Mn-based shape memory alloys were investigated. It was found that hardness was dramatically increased by ageing due to the formation of fine bainitic plates and that the volume fraction of the bainite phase with ageing time can be described by the Austin–Rickett equation. The martensitic transformation temperatures decreased with the formation of bainite plates, mainly due to the composition change of the β-matrix. Moreover, the growth of thermally induced martensite plates was disturbed by the existence of bainite plates. Consequently, the transformation intervals (M_s–M_f and A_f–A_s) and transformation hystereses (A_f–M_s and A_s–M_f) increased with the progress of bainitic transformation.     

Kinetics of phase separation in iron-based ternary alloys. I. Theoretical analysis of phase separation behavior in iron-based ternary alloys

Phase sepation behavior in an Fe-based ternary system was investigated by using a model based on the Cahn–Hilliard equation. The asymptotic behavior of the minor element, Y, in an Fe–X–Y ternary alloy along a trajectory of a peak top of the major element, X, is classified into three groups according to the sign of the second derivative of the chemical free energy f₁ with respect to the concentration of X, c_X and the concentration of Y, c_Y, f′_XY≡∂²f₁/∂_CX∂_CY. If f′_XY>0,, the value of c_Y along the trajectory of the peak top of Y decreases with time. When f′_XY<0 at 0t<∞, peaks of c_Y formed at the peak tops of Y. The amplitute of the peak increases with time. In the case that f′_XY<0 at 0t<t₀and f′_XY>0 at t₀<t<∞, peaks of c_Y formed at the peak tops of Y at the initial stage. Bifurcation of peaks occurs. The concentration of the major element, c_X, along the trajectory of its peak increases with time regardless of the sign of f′_XY.     

The effect of nickel hardness and grain size on acoustic and electromagnetic Barkhausen emission

Acoustic Barkhausen emission (ABE) and electromagnetic Barkhausen emission (EBE) have been measured during the magnetization of nickel. Of particular interest is the effect on the Barkhausen emission envelope as the microstructure of the material is varied. Samples of nickel were heat treated at temperatures up to 1100°C and the effect on ABE and EBE measured. Measurements on ‘as machined’, unannealed material indicate that EBE is unrelated to ABE since EBE evaluates surface condition, while ABE is generated within the bulk of material, and the machined surface and bulk conditions are different for an unannealed sample. On heat treatment the correlation between surface condition and bulk condition returns (as indicated by Barkhausen emission envelopes) and the correlation is considered to be due to the relative energies of break away of 71°, 180° and 109° domain walls. Measurements were made of hardness and grain size and the characteristics of the Barkhausen emission signals compared to hardness and grain size. Parameters used to characterize the Barkhausen emission signals include the time corresponding to the peak in the EBE signal (t_p) and the rms level of the ABE signal (V_rms). These parameters correlate well with material hardness and grain size and offer some potential for measuring these quantities.     

Tb doped CoNbZr soft magnetic films with high anisotropy field for applications in GHz frequency region

CoNbZr films for an adjustable magnetic anisotropy field H_k by doping with rare-earth of different atomic ratio Tb element were obtained in this work. The effect of Tb addition ranged from 0 to 4 at.% on the magnetic properties of the amorphous CoNbZr films was further investigated. The results show that the magnetic anisotropy field H_k increases sharply with the addition of Tb while the coercivity H_ch along the hard axis and H_ce along the easy axis change slightly with increasing Tb content in the films. As a consequence, CoNbZr film doped with 2 at.% of Tb exhibits excellent soft magnetic properties with a saturation magnetization 4πM_s of 8.9 kG, a hard axis coercivity H_ch of 1.79 Oe, a easy axis coercivity H_ce of 1.4 Oe and a magnetic anisotropy field H_k of 87 Oe. The measured ferromagnetic resonance frequency f_FMR of this film is 2.30 GHz. The real permeability μ′ is about 100, which is maintained up to 2 GHz. In addition, there is a broad band of the imaginary permeability μ″ over a large frequency band, indicating high losses. Therefore, Tb doped CoNbZr film is an excellent candidate for high frequency applications such as electromagnetic interference suppressors.     

Shot noise and statistical parameters for the estimation of corrosion mechanisms

In this paper, electrochemical noise measurements on aluminium AA2014 are reported. Samples were exposed to a range of solutions to promote different corrosion rates and mechanisms. Subsequently, statistical parameters (electrochemical noise resistance, coefficient of variation of current and localization index) and parameters derived from shot noise theory (the low frequency limit of the amplitude of the noise impedance, Z_n,f→0, the average charge in each event, q, and the frequency of events, f_n) were obtained and related with the type and rate of corrosion. Finally, pairs of shot noise parameters were represented in maps to relate their values with the corresponding corrosion rates and mechanisms.     

The effect of Si addition on crystallization behavior of amorphous Al-Y-Ni alloy

This article reports the effect of silicon (Si) addition upon the crystallization behavior and mechanical properties of an amorphous AlYNi alloy. An amount of 1 at.% Si was added to a base alloy of Al₈₅Y₅Ni₁₀ either by substitution for yttrium (Y) to form Al₈₅Y₄Ni₁₀Si₁, or by substitution for nickel (Ni) to form Al₈₅Y₅Ni₉Si₁. Differential scanning calorimetry (DSC) of all three alloys showed three exothermic peaks. Comparing the peak temperature for the first exothermic peak, a significant shift occurs toward the lower temperature. This indicates that 1 at.% substitutions of Y or Ni by Si decreases the stability of the amorphous phase. DSC study of these amorphous alloys during isothermal annealing at temperatures about 5–15 K lower than their first crystallization peaks showed that the formation of α-Al nanocrystals via primary crystallization occurred without an incubation period. The Avrami time exponent (n) of the primary crystallization from the amorphous structure was determined to be 1.00–1.16 using the Johnson-Mehl-Avrami (JMA) analysis. This suggested a diffusion-controlled growth without nucleation. However, a DSC study of these amorphous alloys during isothermal annealing at higher temperatures between 585 and 605 K showed a clear incubation period during the formation of the Al₃Ni and Al₃Y intermetallic phases. An n value of 3.00–3.45 was determined using JMA analysis. This suggested that the transformation reaction involved a decreasing nucleation rate and interface-controlled growth behavior. The tensile strength σ_f and Vickers hardness for these amorphous alloys are in the range 1050–1250 MPa and 380–398 diamond pyramid hardness number (1 diamond pyramid hardness number=1 kg/mm²=9.8 MPa), respectively.     

Investigation of the microwave dielectric properties of Ca1−xMgxLa4Ti5O17 ceramics for application in coplanar patch antenna

In this paper, the dielectric properties of Ca_1−xMg_xLa₄Ti₅O₁₇ ceramics at microwave frequency have been studied. The diffraction peaks of Ca_(1−x)Mg_xLa₄Ti₅O₁₇ ceramics nearly unchanged with x increasing from 0 to 0.03. Similar X-ray diffraction peaks of Ca_0.99Mg_0.01La₄Ti₅O₁₇ ceramic were observed at different sintering temperatures. A maximum density of 5.3 g/cm³ can be obtained for Ca_0.99Mg_0.01La₄Ti₅O₁₇ ceramic sintered at 1500 °C for 4 h. A maximum dielectric constant (_r) and quality factor (Q × f) of Ca_0.99Mg_0.01La₄Ti₅O₁₇ ceramic sintered at 1500 °C for 4 h are 56.3 and 12,300 GHz (at 6.4 GHz), respectively. A near-zero temperature coefficient of resonant frequency (τ_f) of −9.6 ppm/°C can be obtained for Ca_0.99Mg_0.01La₄Ti₅O₁₇ ceramic sintered at 1500 °C for 4 h. The measurement results for the aperture-coupled coplanar patch antenna at 2.5 GHz are presented. With this technique, a 3.33% bandwidth (return loss <−10 dB) with a center frequency at approximately 2.5 GHz has been successfully achieved.     

Two-stage quenching of steel Kh12M

Conclusions  

Fracture toughness properties of high-strength martensitic steel within a wide hardness range

Fracture toughness tests were carried out on six grades of high-strength martensitic steel within the hardness range from 270 to 475 HB. Four types of tests were performed: (a) Charpy V-notch (CVN) impact over the temperature range −120 to 60 °C, (b) plane strain fracture toughness, K _IC , near the onset of crack growth, (c) fracture toughness, J _IC , near the initiation of slow crack growth, and (d) fracture toughness, J _iC , and crack tip opening displacement (CTOD _iC ) at the onset of slow crack growth using direct current potential drop (DCPD) technique. Further, true plane strain fracture toughness, K _o , at the onset of crack initiation was determined. Fracture toughness behavior including the measured and determined values of CVN, K _IC , K _o , J _IC , K _iC , and CTOD _iC have been interrelated over the entire hardness range using the various analytical and empirical correlations reported in the literature. The results indicate that the steel acquires the optimum fracture toughness properties at a hardness of 305 HB, corresponding to a tempering temperature of 630 °C. Further, the steel exhibits a slight 300 °C temper embrittlement phenomenon.     

On the wide range of mechanical properties of ZTA and ATZ based dental ceramic composites by varying the Al2O3 and ZrO2 content

In this work the influence of pressureless sintering on the Vickers hardness and fracture toughness of ZrO₂ reinforced with Al₂O₃ particles (ATZ) and Al₂O₃ reinforced with ZrO₂ particles (ZTA) has been investigated. The ceramic composites were produced by means of uniaxial compacting at 50 MPa and the green compacts were heated to 1250 °C using a heating rate of 10 °C min⁻¹, then to 1500 °C at 6 °C min⁻¹ and maintained at this temperature during 2 h. After sintering, relative density over 94%, hardness values between 9.5 and 21.9 GPa, and fracture toughness as high as 3.6 MPa m^1/2 were obtained. The presence of TZ-3Y particles on the grain boundaries suggests that they inhibit notably the alumina grain growth. The grain sizes of pure Al₂O₃ and TZ-3Y as well as Al₂O₃ and TZ-3Y in the 20 wt% Al₂O₃+80 wt% TZ-3Y composite were 1.27 ± 0.51 μm, 0.57 ± 0.12 μm, 0.65 ± 0.19 μm and 0.41 ± 0.14 μm, respectively. The 20 wt% Al₂O₃ + 80 wt% ZrO₂ + 3 mol% Y₂O₃ (TZ-3Y) composite showed a hardness of 16.05 GPa and the maximum fracture toughness (7.44 MPa m^1/2) with an average grain size of 0.53 ± 0.17 μm. On the other side, the submicron grain size and residual porosity seem to be responsible for the high hardness and fracture toughness obtained. The reported values were higher than those obtained by other authors and are in concordance with international standards that could be suitable for dental applications.     

Ultra-precision surface finish of the hardened stainless mold steel using vibration-assisted ball polishing process

A vibration-assisted spherical polishing system driven by a piezoelectric actuator has been newly developed on a machining center to improve the burnished surface roughness of hardened STAVAX plastic mold stainless steel and to reduce the volumetric wear of the polishing ball. The optimal plane surface ball burnishing and vibration-assisted spherical polishing parameters of the specimens have been determined after conducting the Taguchi's L₉ and L₁₈ matrix experiments, respectively. The surface roughness R_a=0.10 μm, on average, of the burnished specimens can be improved to R_a=0.036 μm (R_max=0.380 μm) using the optimal plane surface vibration-assisted spherical polishing process. The improvement of volumetric wear of the polishing ball was about 72% using the vibration-assisted polishing process compared with the non-vibrated polishing process. A simplified kinetic model of the vibration-assisted spherical polishing system for the burnished surface profile was also derived in this study. Applying the optimal plane surface ball burnishing and vibrated spherical polishing parameters sequentially to a fine-milled freeform surface carrier of an F-theta scan lens, the surface roughness of R_a=0.045 μm (R_y=0.65 μm), on average, within the measuring range of 149 μm×112 μm on the freeform surface, was obtainable.     

Through-thickness cleavage fracture stress of a Ti-V-N plate steel

Through-thickness cleavage fracture stresses, σ* _f , have been determined for six microstructures of a Ti-V-N plate steel directly by through-thickness four-point bending (4PB) notched specimens. Results showed that σ* _f is higher for ferrite-bainite microstructures than for ferrite-banded pearlite. For an identical finish-rolling temperature (FRT), the plates with a high cooling rate have a higher value of σ* _f than their counterparts with a low cooling rate. Following the same cooling rate, the highest values of σ* _f are obtained for steels finish rolled above A _r3 but below T _NR (nonrecrystallization) and the lowest for steels finish rolled below A _r3, which contain deformed ferrite (DF) with texture components. Cleavage microcracks are observed to initiate at second-phase particle or pearlite-ferrite interface and then to propagate into ferrite matrix. Growing microcracks could be arrested by bainite phase distributed uniformly in ferrite matrix, which contributes to a high value of σ* _f . The low value of σ* _f was attributed to elongated ferrite-deformed ferrite and martensite/austenite (MA) microstructures.     

The gallium-iron system: Enthalpy of formation in liquid state

Through the use of a very high-temperature, automated calorimeter, the enthalpy of formation, Δ_f H=f(X _Fe), of the Fe-Ga liquid system was measured in the temperature and molar fraction ranges 0<X _Fe< 0.591 and 1373 <T< 1573 whereT is inK. The molar enthalpies of formation of these liquid alloys are negative with an extremum point atX _Fe = 0.55 and Δ_f H = -5.9 ± 0.6 kj/mol (all results are referred to the liquid state). From measurements performed on the Ga-rich mole fraction side, the limiting partial molar enthalpy was deduced Δh _m ⁰ (Fe_L in Ga_L) = -2 ± 0.2 kJ/mol. These results were compared with those obtained previously for the two similar systems, Ni-Ga and Co-Ga. Moreover a point of the liquidus line was obtained (X _Fe = 0.48 atT = 1466 K).     

不同脉冲频率条件下2219-T87高强铝合金焊缝成形行为

采用复合超高频脉冲方波变极性钨极氩弧焊接技术完成了2219-T87高强铝合金平板堆焊试验,分析了脉冲电流频率对焊缝成形特征的影响及其规律.结果表明,脉冲电流频率对电弧特性及熔池流动行为有较大影响,造成焊缝熔宽、熔深及熔透率出现显著变化.脉冲频率f_H<60 kHz时,焊缝熔宽、熔深随脉冲频率的增加而增大,熔透率在f_H<35 kHz时基本保持不变,在f_H>35 kHz时出现显著提升,较常规变极性氩弧焊(VP-GTAW)至少提高了34%;当脉冲频率达到60 kHz时,焊缝熔透率达到最大,较VP-GTAW提高了约60%;脉冲频率f_H>65 kHz时,熔宽、熔深及熔透率呈现回落趋势.     

Complete model for effects of silicon in 5%Cr hot work tool steels

Abstract

Recent modifications in chemical composition have been applied commercially to high alloy tool steels to improve toughness and tempering resistance. A common point in all compositions is the reduction of silicon content from the 1·0% used in AISI H11 and H13 down to 0·3% or lower levels. The present work investigates in detail the effect of silicon on tempering sequence and alloy carbide formation, proposing an explanation for the mechanical properties. Laboratory heats with silicon contents between 0·05 and 2·0% were cast and forged under industrial conditions. Mechanical tests were based on impact toughness and hardness measurements, after hardening from 1020°C and tempering at temperatures between 400 and 650°C. Secondary carbides were evaluated through transmission electron microscopy, mainly on extraction replicas, and matrix features were observed in thin foils. High resolution scanning electron microscopy was also applied, especially on fracture surface samples, to correlate toughness results with secondary carbide distributions. The effect of Si on cementite formation was found to be the major factor for the differences observed for the mechanical properties. During the initial tempering stages, cementite formation is delayed or inhibited in high Si steels, anticipating alloy carbide formation with preferred M₇C₃ precipitation on high energy interfaces. After longer tempering, M₇C₃ particles coarsen and may act as preferential cracking routes, explaining the lower toughness of high Si steels. In low Si steels, cementite is stabilised by Cr, Mo and V in solid solution, delaying alloy carbide precipitation and thus increasing tempering resistance.     

基于磁噪声和增量磁导率的塑性变形定量无损评价

针对机械结构在加工、制造和使用过程中常出现的塑性变形损伤,研究了碳素钢材料塑性变形与巴克豪森噪声及增量磁导率之间的关联性。试验结果表明,在小变形时,巴克豪森噪声信号随塑性变形变化明显;增量磁导率值随塑性变形的增大而规律性变化。不同碳素钢材料的试验结果表明,巴克豪森磁噪声和增量磁导率可用于碳素钢材料中塑性变形的定量无损评价。