Tooth Root Bending Fatigue Strength of High-Density Sintered Small-Module Spur Gears: The Effect of Porosity and Microstructure

The present paper is aimed at investigating the effect of porosity and microstructure on tooth root bending fatigue of small-module spur gears produced by powder metallurgy (P/M). Specifically, three steel variants differing in powder composition and alloying route were subjected either to case-hardening or sinter-hardening. The obtained results were interpreted in light of microstructural and fractographic inspections. On the basis of the Murakami a r e a method, it was found that fatigue strength is mainly dictated by the largest near-surface defect and by the hardness of the softest microstructural constituent. Owing to the very complicated shape of the critical pore, it was found that its maximum Feret diameter is the geometrical parameter that best captures the detrimental effect on fatigue.

Link between Microstructure and Mechanical Behavior of Double Annealed Medium Mn Steel

Double annealing of low carbon medium Mn steel was studied. The second intercritical annealing was done at 650°C within a range of holding time: 3min to 30h. Tensile properties of the steel were measured as a function of holding time and the relation between microstructure and mechanical behavior was analyzed. Furthermore, a model, based on the mixture law combined with the considerations of equivalent increment of work in each microstructural constituent during mechanical loading, was proposed. The individual mechanical behavior of each considered microstructural constituent was described with the approaches existing in the literature. The complete model shows a very good agreement with the experimental stress-strain curves and predicts well the optimum strength-ductility balance after 2h holding.

Characterisation of Fracture and HAC Resistance of an Individual Microstructural Constituent with Micro-Cantilever Testing

This paper focuses on the application of miniaturized fracture tests to evaluate the fracture and hydrogen assisted cracking (HAC) resistance of a selected microstructural constituent (acicular ferrite, AF) which only occurs in microscopic material volumes. Site-specific Focused Ion Beam (FIB) micro-machining was used to fabricate sharply notched micro-cantilevers into a region fully constituting of AF. The micro-cantilevers were subsequently tested under uncharged and hydrogen charged conditions with a nanoindenter. The load displacement curves were recorded and analysed with a simplified plastic hinge model for the uncharged specimen, as AF demonstrated an essentially ductile behaviour. The simplified model assisted with FE simulations provided values of the critical plastic crack tip opening displacement (CTOD). A value of the conditional fracture toughness was thereby determined as 12.1 MPa m1/2. With LEFM, a threshold stress intensity factor, Kth, to initiate hydrogen crack propagation in AF was found to range between 1.56 MPa m1/2 and 4.36 MPa m1/2. All these values were significantly below the corresponding values reported for various ferrous alloys in standard macro-tests. This finding indicates that the fracture and HAC resistance at the micro-scale could be very different than at the macro-scale as not all fracture toughening mechanisms may be activated at this scale level.

Multiscale Simulation of Cold Axisymmetric Deformation Processes

This paper proposes a method for simulation of axisymmetric cold plastic deformation processes with allowance for the microstructure of steel. The results of the method approbation relating to rod drawing process for low carbon non alloy steel (0.2% carbon) are described. A comparative analysis of the stress and strain state parameters and their distribution at macro and micro levels of a metal in the deformation zone was carried out. The micro model, in contrast to the macro model, has allowed more accurate values of stress and strain state to be obtained and multiple stress and strain localizations to be revealed due to the microstructural constituent interaction.

Development of Hot Rolled Ship Plate with High Strength and High Toughness

A new hot-rolled ship plate with high strength and high toughness is successfully developed through chemical composition design and TMCP process. Experimental methods, such as OM, TEM and X-EDS, were used to study the microstructure and precipitates of steel. The primary microstructural constituent is acicular ferrite, quasi-polygonal ferrite with second constituents along grain boundaries. Lath width of acicular ferrite is about 1μm. Cubic particles about several hundreds nanometers and nanometer particles exist in experimental steel. It can be concluded that acicular ferrite is the main reason for high strength and super toughness. precipitation hardening due to dispersed precipitations of carbonitrides can not be overlooked.

Effect of Spheroidizing Annealing Time on Microstructure and Hardness of GCr15 Bearing Steel

In order to shorten the spheroidizing annealing time, the effects of annealing time on microstructure and hardness of GCr15 have been researched by using OM, SEM and Vickers hardness tester. The original microstructural constituent of bearing steel is pearlite and cementite. Prolonged time at 805°Cwill decrease the number and increase size of cementite particles. After incomplete austenization at 805°C, prolonged time at 720°C induces increase of particle size, and uniform distribution of divorced pearlite. The hardness of specimens treated with different spheroidization process fluctuates around 200HV, and is equivalent to that with conventional spheroidization process. It is realizable to shorten the annealing time on the premise of good quality of bearing steel.

Effect of Vanadium on Development of Acicular Ferrite Microstructure in Low Carbon Steel

The influence of vanadium on the development of an acicular ferrite microstructure has been investigated in a low carbon steel. Optical and electron microscopy were carried out to identify the precipitates, inclusions and constituents of the acicular ferrite microstructures. By the addition of vanadium, the main microstructural constituent was changed from a side plate ferrite to an acicular ferrite. VCN precipitates, which were known to favor the nucleation of acicular ferrite, were formed on the (Mn,Si) oxide and MnS particles. The presence of vanadium in alloys suppressed the formation of a side plate ferrite and reduced the transformation of ferrite during an isothermal transformation. Nucleation of intragranular ferrite and a subsequent sympathetic nucleation of ferrite within austenite grains were favored in the vanadium containing steel and an acicular ferrite microstructure was developed.

Development of a τ-Type Mg-Zn-Al Alloy: An Investigation of the Microstructure and Solidification Characteristics

The practical phase constituent diagram has been used to determine the composition of a low alloying τ-type Mg-Zn-Al alloy, which has a nominal element content of 7wt%Zn and 3wt%Al (ZA73), as a basis for further improvement of τ-type alloy through appropriate thermal processing and micro-alloying. The microstructure and solidification characteristics of the alloy have been experimentally examined using optical microscopy, scanning electron microscopy, X-ray diffraction, and differential scanning calorimetry analysis. The results confirm that the as-microstructure of ZA73 alloy consists of globular equiaxed dendrite with τ phase as the only secondary phase evenly distributed in interdendritic spacing. Solidification sequence has been proposed with the help of DSC analysis and Mg-Zn-Al ternary liquidus projection phase diagram. Consistent with previous observed results for τ-type Mg-Zn-Al alloy, due to the decrease of element contents, ZA73 alloy has higher liquidus temperature (~627°C) and wider solidification range (~283°C), while at the mean time lower starting temperature of the second phase transformation (~354°C), compared to higher element containing ZA104 alloy. The phase constituent diagram has been shown to be a practical and effective tool for predicting the as-cast microstructural constituent of high zinc Mg-Zn-Al alloys under normal permanent mould solidification condition.

Effect of Ag and Cu Concentrations on Creep of Sn-Based Solders

The creep behavior of Sn1Ag0.5Cu, Sn2.5Ag1Cu and Sn4Ag0.5Cu ball grid array (BGA) solder balls and 99.99% pure polycrystalline Sn bulk was studied using impression creep. The microstructures of the as-reflowed solders was characterized. It was found that SnAgCu solders consist of primary dendrites/grains of β-Sn, and a eutectic microconstituent comprising fine Ag3Sn and Cu6Sn5 particles in β. With increasing concentrations of Ag and Cu in the alloy, the proportion of the eutectic microconstituent in relation to the primary β phase increases. In pure Sn and Sn-1Ag-0.5Cu, the β grains form the continuous matrix, whereas in Sn2.5Ag1Cu and Sn4Ag0.5Cu, the eutectic microconstituent forms a continuous network around the β grains, which form isolated islands within the eutectic. The steady state creep behavior of the alloys was dominated by the response of the continuous microstructural constituent (β-Sn or solid solution β for pure Sn and Sn1Ag0.5Cu, and the eutectic microconstituent for Sn2.5Ag0.5Cu and Sn4Ag0.5Cu). In general, the steady-state creep rate decreased with increased alloy content, and in particular, the volume fraction of Ag3Sn and Cu6Sn5 precipitates. The rate-limiting creep mechanism in all the materials investigated here was core diffusion controlled dislocation climb. However, subtle changes in the stress exponent n and activation energy Q were observed. Pure Sn shows n = 5, Q = 42kJ/mole, Sn1Ag0.5Cu shows n = 5, Q = 61kJ/mole, whereas both Sn2.5Ag1Cu and Sn4Ag0.5Cu show n = 6 and Q = 61kJ/mole. Rationalizations for the observed changes of n and Q are provided, based on the influence of the microstructure and the solute concentrations.