Debdulal Das

Effect of Deep Cryogenic Treatment on the Carbide Precipitation and Tribological Behavior of D2 Steel

The influence of deep cryogenic processing in between quenching and tempering (QCT) on the carbide precipitation and the tribological behavior of a commercial AISI D2 steel has been examined. The developed microstructures have been characterized with an emphasis to understand the influence of QCT vis-à-vis conventional quenching and tempering (QT) on the nature, size, morphology, and distribution of carbide particles. The mechanical properties such as hardness and wear resistance of the samples treated by QT and QCT have been evaluated employing Vickers indentation and sliding wear techniques, respectively. It has been demonstrated that deep cryogenic treatment leads to considerable microstructural changes which result in enhanced tribological properties.

On the enhancement of wear resistance of tool steels by cryogenic treatment

The present article aims to resolve the debate on the degree of benefit of processing tool or die steels by cryogenic treatment. This has been done by measuring transition loads for quenched and tempered die steel with and without cryogenic treatment. The wide range of reported degrees of improvement of wear resistance by cryotreatment is explained by the operative modes and mechanisms of sliding wear considering the wear rate ratio of conventionally treated to cryotreated steels on a two-dimensional map.

On the refinement of carbide precipitates by cryotreatment in AISI D2 steel

Refinement of carbide particles by cryotreatment is often proposed as a major factor for the improvement of wear resistance in tool steels. However, this proposition is not substantiated by experimental evidence. This has been examined in this report by (i) detailed micro-structural analyses of the nature, volume fraction, size, population density and distribution of carbide particles, (ii) XRD and EDX micro-analysis on the bulk samples and electrochemically extracted carbides, and (iii) measurement of hardness and wear rate of a series of differently cryotreated AISI D2 steel. The results conclusively establish that (i) cryotreatment, in comparison to conventional treatment, induces precipitation of finer carbides with higher volume fraction and more uniform distribution, and (ii) population density and the size of secondary carbide particles significantly increases with holding time up to a critical duration at 77 K in cryotreatment. The latter observation indicates the pioneering direction towards optimization of cryotreatment design for techno-economic benefit.

Inconsistent wear behaviour of cryotreated tool steels: role of mode and mechanism

Abstract This report aims to reveal the cause of wide variation in the reported degree of improvement in wear resistance of cryotreated tool steels. Sliding wear tests at different normal loads have been carried out on conventional and cryotreated AISI D2 steel specimens together with SEM examinations and EDX microanalyses of the surfaces and subsurfaces of the worn specimens and that of the generated debris. The obtained results reveal that when the modes and mechanisms of wear are similar for both types of specimens, mild oxidative at lower load or severe delaminative at higher load, the improvement in wear resistance is 1·6–2·2 times. At the intermediate load, the modes and mechanisms are dissimilar, and the observed improvement is as high as 53·2 times. The reported varied degree of improvement in wear resistance by cryotreatment has been attributed to the operating test conditions that govern the modes and mechanisms of wear.

A MEASURE OF ENHANCED DIFFUSION KINETICS IN MECHANICAL ALLOYING OF Cu-18 at.% Al BY PLANETARY BALL MILLING

Metallurgical and Materials Engineering Department, Indian Institute of Technology,Kharagpur 721 302 (W.B.), India(Received January 7, 1999)(Accepted in revised form May 21, 1999)Keywords: Mechanical alloying; Diffusion; Kinetics; X-ray diffraction; Mathematical modelingIntroductionMechanical alloying (MA) by high energy ball milling has established itself as a versatile solid stateprocessing route for producing metals, alloys and intermetallics in the nanocrystalline state [1]. In MAof ductile metals, powder particles trapped between the colliding balls and vial undergo repeateddeformation, fragmentation and cold welding resulting into agglomerates of multilayered structureswith clean interfaces [2,3]. Intermixing in such an aggregate during MA is actuated by the deformationprocess [1]. Indeed, it is well known over three decades that mechanical deformation enhances thediffusion rate and the process has been termed as mechanical interdiffusion by Balluffi and Rouff [4].Gleiter [5] has shown that a large potential gradient leads to high rate of diffusion in the vicinity of adislocation even at a temperature where self diffusion is practically negligible. In analogy to the masstransport in ion irradiation, Martin and Gaffet [6] have suggested that ballistic diffusion, which isindependent of temperature, may account for the enhanced rate of intermixing during MA. Similarly,Bellon and Averback [7] have considered shear along randomly selected glide planes as one of thepossible mechanisms of intermixing that accompanies MA during ball milling. In a recent work, Pabiet al. [8] have developed a rigorous mathematical model of MA by means of the modified iso-concentration contour migration (MICCM) method to estimate the effective intermixing rate duringMA, which is equivalent to volume diffusion at an elevated temperature (T

Mathematical modelling of the mechanical alloying kinetics

Abstract A rigorous mathematical model, based on the modified iso-concentration contour migration method, has been developed to predict the kinetics of diffusive intermixing in a binary miscible system in the course of mechanical alloying (MA). The present model considers the variation of diffusion coefficient with composition, and interface shift due to both interdiffusion and mechanical deformation. Comparison of the kinetics predicted by the present model with the relevant experimental data from Cu–Ni and Cu–Zn shows that the effective mass transport operative in MA attains a rate intermediate between that for volume or grain boundary diffusion. An effective temperature for diffusion (Teff) has been proposed to simulate the observed alloying kinetics. Teff is a function of composition but is not related to the adiabatic temperature rise at the point of ball-powder collision. Finally, the ratio of Teff to the corresponding liquidus temperature for all the composition studied lies between 0.4 and 0.5.

PVC-Cu composites with chemically deposited ultrafine copper particles

PVC-Cu composites with chemically deposited ultrafine (0.1 to 0.3 μm diameter) copper particles were prepared by hot-pressing copper-coated PVC powder (−106, +150 μm) at 120° C. Metallic copper in fine-particle form was deposited on the PVC particles by reducing an ammoniacal cupric acetate solution with hydrazine at 85° C. The electrical resistivity (d.c.) and transverse rupture strength of these composites were measured. Measurement of electrical resistivity indicated that in these composites copper particle network formation was initiated at a copper content of about 0.2 vol%; with further increase of copper content the resistivity dropped sharply from about 1014 (for pure PVC) to about 105 MN m−2 Ωcm at a copper content of about 0.5 vol%. Increase of copper loading beyond 0.5 vol% did not decrease resistivity significantly whereas the rupture strength increased continuously from 120 MN m−2 (for pure PVC) to a value of about 300 MN m−2 with 4 vol% copper loading. This high value of resistivity even after copper particle chain formation and the continuous increase of rupture strength, is thought to be due to formation of a thin layer of polymer film between the copper particles introducing a “quasi-random” character to the otherwise segregated network of copper particles.

Synthesis of Bulk Nano-Al2O3 Dispersed Cu-Matrix Composite Using Ball Milled Precursor

Kinetics of solid-state reduction reaction during ball milling of CuO-Al and CuO-prealloyed Cu(Al) powder blends in dry and wet condition has been investigated by using X-ray diffraction (XRD), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM) techniques. Direct reduction of CuO by Al has resulted into Al2O3 dispersed Cu-matrix composite through a self-propagating reaction only during milling in dry condition. However, indirect reduction of CuO by prealloyed Cu(Al) resulted into formation of nano-Al2O3 dispersed Cu-matrix composite either by continued ball milling in dry condition or by subsequent thermal treatment of wet milled powder precursor. The influence of milling conditions, that is, milling speed, and milling media, on the occurrence of reduction of CuO by elemental Al or Al in prealloyed Cu(Al) during ball milling have been explained by considering their effects on the rise of powder temperature due to collisions between balls and powder particles, and the rate of reduction of ignition temperature of the reaction due to microstructural refinement. TEM investigation has revealed that the size of Al2O3 particles in the composite power blend formed by the indirect reaction route (CuO-prealloyed Cu(Al)) is much finer than the same in case of direct reaction route (CuO-Al). It is suggested that the kinetics of the reduction reaction in the indirect reaction route is relatively sluggish in nature and amenable to processing of large amount of nano-Al2O3 dispersed Cu-matrix composite powder for industrial purpose.

On the mechanism of wear resistance enhancement of tool steels by deep cryogenic treatment

This study elucidates the underlying mechanism associated with microstructural modifications responsible for significant enhancement of wear resistance of tool steels by deep cryogenic treatment (DCT). It is demonstrated that DCT not only reduces the amount of retained austenite, but also conditions the martensite that, in turn, leads to the precipitation of higher amount of ultrafine secondary carbides. These favorable microstructural alterations by DCT enhance the resistance of tool steels against dynamic changes during wear resulting in improved wear resistance.

X-ray diffraction and Mossbauer spectroscopy studies of cementite dissolution in cold-drawn pearlitic steel

Cementite dissolution in cold-drawn pearlitic steel (0.8 wt.% carbon) wires has been studied by quantitative X-ray diffraction (XRD) and Mössbauer spectroscopy up to drawing strain 1.4. Quantification of cementite-phase fraction by Rietveld analysis has confirmed more than 50% dissolution of cementite phase at drawing strain 1.4. It is found that the lattice parameter of the ferrite phase determined by Rietveld refinement procedure remains nearly unchanged even after cementite dissolution. This confirms that the carbon atoms released after cementite dissolution do not dissolve in the ferrite lattice as Fe-C interstitial solid solution. Detailed analysis of broadening of XRD line profiles for the ferrite phase shows high density of dislocations (∼1015/m2) in the ferrite matrix at drawing strain 1.4. The results suggest a dominant role of ⟨1 1 1⟩ screw dislocations in the cementite dissolution process. Post-deformation heat treatment leads to partial annihilation of dislocations and restoration of cementite phase. Based on these experimental observations, further supplemented by TEM studies, we have suggested an alternative thermodynamic mechanism of the dissolution process.