Microhardness Survey Research Articles

Si dilution mechanisms in BNi 5/metal joints

The ability of Ni brazes to join Si3N4 is degraded by dissociation of the ceramic at the bonding temperatures and ingress of Si to form brittle silicides. Attempts to mitigate this problem [1-3] have focused on brazes containing Si, which may not be as susceptible to ingress. Thus, use of the BNi 5 braze, an Ni-19Si-19Cr (at%) alloy with solidus and liquidus temperatures of 1080 and 1135 °C has been suggested, but has also been found to suffer from Si ingress [1]. Therefore, in a recent work the use of BNi 5 has been supplemented by two other approaches: sealing of the ceramic surface with a diffusion barrier such as TiN, and using interactions with the metal workpieces to dilute the Si contents of the joints to below the 10% needed to stabilize silicides [4]. This letter describes studies of two dilution processes: dissolution of the workpiece material in the molten braze to increase the solvent volume, and diffusion of Si out of the braze into the workpieces. The first of these options is difficult to model for a braze joint geometry, but the egress of Si can be analysed using standard diffusion equations. Plates of 2 mm thick Fe-17Cr-12Ni-2Mo AISI 316 steel and of 99.9% pure W were chosen for the study. AISI 316 contains the main constituents of many alloys used in demanding applications where performance could be enhanced by ceramic inserts, and the relatively good match with the thermal expansion coefficient of Si3N 4 has led to the selection of W as a buffer insert material in ceramicmetal components. BNi 5 obtained from GTE Wesgo as 0.080 mm foil was used to braze sandwiches of finely abraded and degreased workpieces by heating in a vacuum of 0.01 Pa at 1150 °C for 15 rain and then holding at 1050 °C for 1-15 h. The condition of the joints was assessed by metallographic examination supplemented by Vickers microhardness (VHN) surveys, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). These observations provided direct evidence as to whether the time spent at 1150 or at 1050 °C was the more important and hence whether workpiece dissolution or Si egress was the dominant process. The joints between AISI 316 workpieces held at 1050 °C for only 1 h had continuous seams of two hard and frequently cracked phases embedded in a

Effect of Process Variables on Bead Geometry of the Submerged-Arc Strip-Overlay Welds

A study has been made of the effect of process variables on the bead geometry and dilution in a submerged-arc stripoverlay process. Weld deposits were made on a 2.25 Cr-1 Mo steel plate using a 60 X.5 mm 309 L stainless steel strip. The effect of current, voltage, welding speed, electrode stickout, and preheat temperature was assessed on the bead width, bead height, penetration, and dilution. It was found that with the exception ofpreheat temperature all other variables had an influence on one or more features of the weld deposits. Welding parameters can be specified from these results so that a clad of any required shape and dilution can be deposited. Metallographic examination revealed the microstructures in and adjacent to the fusion line and a microhardness survey was carried out to determine the hardness distribution in these areas. A few deposits were analyzed by microprobe techniques and it was found that the welding conditions had a significant effect on the Ni and Cr content of the clad.

Fatigue of stainless steel in hydrogen

The fatigue crack growth rates of two austenitic stainless steel alloys, AISI 301 and 302, were compared in air, argon, and hydrogen environments at atmospheric pressure and room temperature. Under the stresses at the crack tip the austenite in type 301 steel transformed martensitically to a’ to a greater extent than in type 302 steel. The steels were also tested in the cold worked condition under hydrogen or argon. Hydrogen was found to have a deleterious effect on both steels, but the effect was stronger in the unstable than in the stable alloy. Cold work decreased fatigue crack growth rates in argon and hydrogen, but the decrease was less marked in hydrogen than in argon. Metallographic, fractographic, and microhardness surveys in the vicinity of the fatigue crack were used to try to understand the reasons for the observed fatigue behavior.

Microstructural aspects of aluminized coatings on nickel-base alloys

AbstractThe influence of base-alloy composition on the microstructure of aluminide coatings has been studied for a series of nickel-base alloys aluminized by the pack-cementation process. The microstructural features of the coatings formed on Nickel 200. Brightray Alloy C, Nimonic 80A, and Inconel 718 were evaluated by optical metallographic examination, together with microhardness surveys and scanning electron microscopy. The coating composition was determined from electron microprobe traverses of the coating layer and the aluminide phases were identified by X-ray diffraction examination of the bulk sample. The coatings were found to consist primarily of the nickel aluminide NiAI. although some Ni2Al3 was found to a lesser extent as a Widmanstatten structure in the coating on the Brightray alloy. A discontinuous, lamellar-type layer, enriched in chromium and other base-alloying elements with low solubility in the NiAI, was also observed at the coating/substrate interface. The coating could be precipitate...

On the mechanism behind the punch-blank surface conformation in stretch-forming and deep-drawing

In industrial sheet-metal forming operations such as deep-drawing and stretching, friction and evolution of surface texture are very important factors, especially to secure optimum forming depth and acceptable surface finish, and to prevent defects in the final products. The punch profile region experiences a severe tribological condition as the blank elements are subjected to sub-surface plastic flow and moderate-to-high tool-contact-pressure at extremely low relative sliding speeds. In the present paper a brief literature survey is presented concerning the surface interaction and friction between a workpiece experiencing bulk plastic flow and a polished, rigid tool. A friction measuring apparatus is described which could be used to investigate various tribological relationships pertaining to the sliding friction characteristics of the tool—workpiece interface. Results of a series of sliding friction tests, surface profile traces and their digital analyses and micro-hardness surveys are discussed and, based on these, an attempt has been made to understand the mechanism leading to the high tool—blank conformation at the punch profile in stretch-forming and deep-drawing operations.

On the explosive hardening of one end of a metallic block

Aluminium and mild steel plates were subjected to shock loading on one end by detonating Metabel explosive sheet. The residual stresses in the principal directions were measured using the X-ray, back reflection method, through the specimen thickness. Macro- and micro-structural analyses and micro-hardness surveys of the unloaded specimens were made to examine how the influence of explosive thickness decays with distance from the loaded surface. A central fracture under explosive loading is explained by reference to unloading waves.

Fatigue stability of residual stress in shot peened alloys

Fatigue specimens of Ni-20 pct Co and Ni-60 pct Co alloys were shot peened to an Almen intensity of 0.008N2 and cycled at two levels of alternating stress. The surface residual stress was monitored during cycling by the two exposure X-ray method. Microhardness surveys were made across cross sections of specimens representing each test condition. It was found that the surface residual stress decayed partially during the plastic cycling of the lower strength-higher stacking fault energy (SFE) 20 pct Co alloy and that little such decay occurred in either the plastic or elastic cycling of the higher strength-lower SFE 60 pct Co alloy. There was virtually no change of microhardness during cycling in either alloy, hence the residual stress relaxation in the 20 pct Co alloy is interpreted as a dynamic recovery process.

The dry sliding wear characteristics of copper, with and without 0.08% silver

Microhardness surveys and metallographic examination shows that recrystallisation occurs in tough-pitch copper but not silver-bearing copper during dry sliding wear experiments. Silver reduces the mild-severe transition load and increases the wear rate in the severe region. Severe wear is by a prow-wedge mechanism involving adhesion, work hardening and fracture. Prow formation is inhibited by in situ recrystallisation.

Direct methods of making molar bands

Metallurgical joining of high thermal conductivity materials like aluminum has been technically challenging. This paper illustrates a novel method for joining of bulk metallic materials through microwave heating. Joining of aluminum alloy in bulk form has been carried out using microwave energy in a multimode applicator at 2.45 GHz and 900 W. SiC was used at both top and bottom of the workpiece as susceptor material to facilitate microwave hybrid heating (MHH). Aluminum in plate forms have been successfully joined through microwave heating within 600 s of exposure time. A sandwich layer of aluminum powder with approximately 0.5 mm thickness was introduced between the two candidate surfaces. Near complete melting of the powder particles in the sandwich layer does take place during the microwave exposure leading to metallurgical bonding of the bulk surfaces. Characterisation of the joints has been carried out through microstructure study, elemental analysis, phase analysis, microhardness survey, X-ray diffraction (XRD) pattern indicates that some aluminum powder particles got transformed into aluminum oxides. A dense uniform microstructure with good metallurgical bonds between the sandwich layer and the interface was obtained. The hardness of the joint area was observed to be 72.4 ± 10 Hv.