Haroldo Cavalcanti Pinto

Nanosized precipitates in H13 tool steel low temperature plasma nitriding

Abstract A comprehensive study of pulsed nitriding in AISI H13 tool steel at low temperature (400xa0°C) is reported for several durations. X-ray diffraction results reveal that a nitrogen enriched compound (e-Fe 2-3 N, iron nitride) builds up on the surface within the first process hour despite the low process temperature. Beneath the surface, X-ray Wavelength Dispersive Spectroscopy (WDS) in a Scanning Electron Microscope (SEM) indicates relatively higher nitrogen concentrations (up to 12xa0at.%) within the diffusion layer while microscopic nitrides are not formed and existing carbides are not dissolved. Moreover, in the diffusion layer, nitrogen is found to be dispersed in the matrix and forming nanosized precipitates. The small coherent precipitates are observed by High-Resolution Transmission Electron Microscopy (HR-TEM) while the presence of nitrogen is confirmed by electron energy loss spectroscopy (EELS). Hardness tests show that the material hardness increases linearly with the nitrogen concentration, reaching up to 14.5xa0GPa in the surface while the Young Modulus remains essentially unaffected. Indeed, the original steel microstructure is well preserved even in the nitrogen diffusion layer. Nitrogen profiles show a case depth of about ~xa043xa0μm after nine hours of nitriding process. These results indicate that pulsed plasma nitriding is highly efficient even at such low temperatures and that at this process temperature it is possible to form thick and hard nitrided layers with satisfactory mechanical properties. This process can be particularly interesting to enhance the surface hardness of tool steels without exposing the workpiece to high temperatures and altering its bulk microstructure.

Casting in the Semi-Solid State of ZK60 Magnesium Alloy Modified with Rare Earth Addition

In this work, the casting process under mechanical agitation in the semi-solid state was investigated for the production of ZK60 magnesium alloy modified with the addition of 2.5% wt of mischmetal. The results show that this process enables the production of ingots with homogeneous chemical composition and free of shrinkage, inner defects and internal oxidation. The as-cast microstructure consists of an α-Mg matrix with globular grains reinforced by a grid of distinct intermetallics of Mg-Zn, Mg-Zn-RE and Mg-RE type along the grain boundaries. The yield strength at room temperature undergoes more than 50% increase during direct T5 aging, thus reaching 170 MPa. At 300°C, however, the dispersion of nanometric precipitates does not modify the hot deformation behavior of the aged alloy, which undergoes dynamic recrystallization in a similar manner to the as-cast alloy. DRX at 300°C is fastest for the alloy solution-treated at 500°C.

Solution and Ageing Heat Treatments of ZK60 Magnesium Alloys with Rare Earth Additions Produced by Semi-Solid Casting

Rare earth elements reportedly improve high temperature strength and creep resistance of Mg alloys. In the present work, ZK60 Mg alloys containing different amounts (0.5, 1.5 and 2.5 wt.%) of rare earth additions were prepared by thixocasting and submitted to solution and ageing heat treatments (T4, T5 and T6). The as-cast and heat treated microstructures were investigated by scanning electron microscopy and hardness was evaluated as a function of heat treatment parameters. In the as-cast state, the alloys are formed by globular α-Mg grains reinforced by a network of composite Mg-Zn-RE precipitates with either smooth or lamellar/acicular morphologies. Solution of the smooth precipitates took place in alloys containing 0.5 and 1.5wt.%RE for T4-500 oC but no precipitates were dissolved with T4-380 oC. The optimum temperature for T5 and T6 was identified as 175 oC, while T6-500 oC led to the highest hardness, followed by T5 and T6-380 oC, respectively.

Effect of Mischmetal Additions and Solution Heat Treatments (T4) on the Microstructure and Mechanical Properties of Thixocast ZK60-RE Magnesium Alloys

Solution treatments (T4) at 380 °C for 16 h and 500 °C for 8 h were performed for ZK60 magnesium alloys modified with addition of 0.5, 1.5 and 2.5 wt% of mischmetal (combination of rare-earth (RE) elements). The compression behaviour was investigated at room temperature and at 300 °C correlated with the microstructure and differential scanning calorimetry (DSC) data. The as-cast microstructure is formed by a-Mg matrix with globular grains reinforced by a semi continuous network of Mg-Zn, Mg-Zn-RE and Mg-RE intermetallic particles. Solution-treated alloys show lower yield strengths due to partial dissolution of precipitates. Work hardening was not observed for the alloys compressed at 300°C with the compression speed of 10-3 s-1, whereas it was observed for the compression speed of 10-2 s-1 for the all as-cast, ZK60-1.5RE-T4 at 380 °C and ZK60-1.5RE-T4 at 380 °C.

3D-FIB Characterization of Wear in WC-Co Coated Composites

In this work 3D visualization of wear in milling inserts has been investigated by Focused Ion Beam tomography. It has been observed that the morphology of the cracks differs in the z-axis direction, allowing particular characteristics of the microstructure and wear evolution to be visible. Two types of cracks develop: principal and lateral cracks. The formation of lateral cracks is strongly influenced by the degradation of the binder phase in the regions surrounding the principal crack. The lateral cracks and the deflection of the main cracks present a particular semi-elliptical geometry, which correlates with the stress field originated during the input of a cycling load in a fatigue condition.

Dissimilar Friction Stir Welding of HSLA Steel to Austenitic High-Mn TRIP Steel

The microstructure and mechanical properties of dissimilar butt-joints between a high-strength low alloyed (HSLA) grade and an austenitic high Mn TRIP steel were investigated. The tool rotation and the tool offset toward the TRIP steel were varied between 300–500 rpm and 1–2 mm, respectively. Tool advancing speed amounted to 100 mm/min. Maximum tension stress was observed for the butt-joint welded with 300 rpm and 2 mm offset. The lowest increase in hardness within the stirred zone also occurred for this FSW condition, indicating that this tool rotation is more promising for welding dissimilar joints of commercial HSLA and high Mn TRIP steels. The weld microstructure consisted mainly of a stirred zone, and neither significant HAZ nor TMAZ are observed. However, two main lobular regions are observed, one at the bottom and another one at the top side of the welds. Besides, the HSLA develops a multiphase microstructure consisting of bainite, martensite and retained austenite phases, whereas no e/a martensite is found in the stirred zone of the austenitic high-Mn TRIP steel.

Effect of Low Temperature Nitriding of 100Cr6 Substrates on TiN Coatings Deposited by IBAD

In this paper we studied the influence of pre-treating the surface of 100Cr6 steel by ion beam nitriding at low temperature (380°C) on the surface topography and wear resistance of thin TiN coatings deposited by reactive ion beam assisted deposition. The specimens were characterized by grazing incidence X-ray diffraction, scanning electron microscopy and atomic force microscopy. The wear resistance of the TiN-coated specimens was evaluated by means of ball on disc tests. The results showed that application of a preliminary ion beam nitriding treatment slightly increased the surface roughness but improved the wear resistance of the 100Cr6 steel due to the formation of a diffusion zone containing the γ-Fe4N nitride combined with the TiN coating.

Microstructure and Residual Stress Formation during Friction-Stir Welding of Semi-Solid Cast ZK60-RE Magnesium Alloy

In this work, we report on the friction stir weldability of a semi-solid cast ZK60 alloy modified with 1.5 wt% mischmetall in the lap-joint configuration using a 120WV4 steel tool with concave shoulder and conical pin. The coarser solidification microstructure in the semi-solid cast ZK60-1.5%RE alloy requires low strain rates and increased heat input to produce lap-joints without inner defects. This was achieved with 250 rpm tool rotation and 50 mm/min welding speed. Friction stir welding results in a very fine grained microstructure in the stir zone probably due to dynamic recrystallization. In the thermomechanically affected zones dynamic recrystallization seems to occur within the solute enriched intergranular zones. The distribution of longitudinal residual stresses exhibit stress maxima at both thermomechanically affected zones. A compression peak is observed at the retreating side, whereas a tensile stress maximum occurs at the advancing side.

Microstructure and Residual Stress Analysis of Dissimilar Metal Composite Plates Produced by Explosion Welding

In the present work, different corrosion resistant materials (AL-6XN superaustenitic stainless steel, ZERON 100 superduplex stainless steel and Inconel 625 Ni alloy) were joined with ASME SA516-70 carbon steel by explosion welding to form bimetal composite plates. The microstructure of cladded plates was characterized by optical and scanning electron microscopy and the hardness variation across the interface was determined by applying Vickers microhardness. The residual stresses generated by the cladding process were determined by X-ray diffraction. All materials adhered well to the ASME SA516-70 base plate and the cladded interface exhibited the wavy morphology usually associated with strong bond strengths. Hardness increased near the interface due to strain hardening caused by plastic deformation and tensile residual stresses were found to develop on all clad metals.© 2014 ASME

Investigation of Compression Behavior of Mg-4Zn-2(Nd,Gd)-0.5Zr at 350°C by In Situ Synchrotron Radiation Diffraction

As-cast Mg-4Zn-0.5Zr, Mg-4Zn-2Gd-0.5Zr and Mg-4Zn-2Nd-0.5Zr alloys were investigated by in situ synchrotron radiation diffraction during hot compression at 350 °C using the facilities of P07 beamline of Petra III at Deutsches Elektronen Synchrotron (DESY), Hamburg, Germany. The specimens were heated at a rate of 100 K/min and compressed with an initial strain rate of 1.1 x 10–3 s-1 up to 30 % strain. The addition of rare earth elements improved the yield strength from 23 MPa in the Mg-4Zn-0.5Zr alloy up to 40 MPa in the alloy with Nd and Gd. Continuous dynamic recrystallization played an important role in the Mg-4Zn-0.5Zr alloy during deformation and twinning was not dominant. Discontinuous dynamic recrystallization was observed in the in the Mg-4Zn-2Gd-0.5Zr along the grain boundary regions while the grains remained largely without any recrystallization. In the Mg-4Zn-2Gd-0.5Zr and Mg-4Zn-2Nd-0.5Zr alloys the contribution of twinning to deformation was observed at 350 °C. Reasons for these differences will be discussed with respect to microstructures of the alloys.