N.F. Fiore

Electrochemical studies of hydrogen diffusion and permeability in Ni

Values of permeability and diffusivity of H in annealed Ni, measured from 25 through 90°C in an electrolytic permeability cell, compared well with existing gas-phase data. The diffusivity was independent of H concentration in solid solution, but evidence of trapping of H by impurity atoms was found. There were no indications that planar or line defects contributed to trapping. Subsidiary electrochemical studies in the cell indicated that H fugacity is governed by overpotential η, temperature T, and an electrochemical parameter Z, whose value was 3, in agreement with the literature. The necessity of controlling H fugacity in permeability and embrittlement studies was underscored by the experimental data.

The effect of carbide volume fraction on the low stress abrasion resistance of high Cr-Mo white cast irons☆

Abstract High chromium white cast irons display a number of advantages in abrasive wear applications. Their composition may be adjusted so that they contain moderate amounts of chromium yet solidify to produce massive chromium-rich carbides in an austenitic matrix of sufficient hardenability to be transformed to martensite with relatively simple heat treatment. In this study the role of carbide volume fraction (CVF) in developing abrasion resistance was investigated using a series of alloys with varying CVF but with constant matrix and carbide compositions. The low stress abrasion resistance of the alloys against quartz and Al 2 O 3 was measured with a dry abrasive rubber wheel abrasion test. The abrasion resistance passed through a maximum at an intermediate CVF, near the eutectic composition, for the softer quartz, while the abrasion resistance increased monotonically with the CVF for tests against the harder Al 2 O 3 . Scanning electron microscopy showed that for quartz abrasion the reversal in the trend of wear versus CVF which occurred at the eutectic composition was caused by spalling and pitting of the massive primary M 7 C 3 carbides in the hypereutectic alloys. This phenomenon did not occur in the Al 2 O 3 abrasive tests, in which a monotonic increase in wear resistance with CVF was observed.

Effect of carbide size on the abrasion of cobalt-base powder metallurgy alloys

Abstract A study of the effect of carbide size on the abrasion resistance of two cobalt-base powder metallurgy alloys, alloys 6 and 19, was conducted using low stress abrasion with a relatively hard abrasive, A1 2 O 3 . Specimens of each alloy were produced with different carbide sizes but with a constant carbide volume fraction. The wear test results show a monotonie decrease in wear rate with increasing carbide size. Scanning electron microscopy of the worn surfaces and of wear debris particles shows that the primary material removal mechanism is micromachining. Small carbides provide little resistance to micromachining because of the fact that many of them are contained entirely in the volume of micromachining chips. The large carbides must be directly cut by the abrasive particles. Other less frequently observed material removal mechanisms included direct carbide pull-out and the formation of large pits in fine carbide specimens. These processes are considered secondary in the present work, but they may have greater importance in wear by relatively soft abrasives which do not cut chips from the carbide phase of these alloys. Some indication of this is provided by limited studies using a relatively soft abrasive, rounded quartz.

A study of abrasive wear mechanisms using diamond and alumina scratch tests

Abstract Scratch tests using alumina (Al 2 O 3 ) abrasive particles and Vickers diamond pyramids were employed to study material removal mechanisms in the abrasion of cobalt-base powder metallurgy alloys 6 and 19. The alloys were specially prepared to produce either fine or coarse carbides in order to study the effects of carbide size. Scanning electron microscopy was used to analyze the scratch grooves, the scratch tools and the wear debris particles. Comparison of scratch tests with Al 2 O 3 and diamond pyramids shows that many features produced by the extremely hard regularly shaped diamond tools are different from those produced by irregular Al 2 O 3 particles. Except for differences produced by tool wear, multiple-pass Al 2 O 3 scratch tests provide excellent reproduction of the material removal processes which occur in low stress Al 2 O 3 abrasion. Al 2 O 3 scratches produced both chip-like and fine irregular debris particles similar to those extracted from spent abrasive used in wear testing. Material removal in the fine carbide alloys is facilitated by the direct removal of entire carbides within the volume of micromachining chips removed from the scratch groove. In coarse carbide alloys, machining chips from large carbides are observed, but the depth of cut in the carbide phase is less than that in the f.c.c. matrix and this leads to a decrease in the volume of material removed. Direct comparison of chips removed from fine and coarse carbide alloys by the same Al 2 O 3 particle shows larger chips from the fine carbide material. The effects of subsurface deformation and surface irregularities on material removal were studied by carrying out scratch tests on specimens subjected to prior abrasion and by investigating multiple-pass scratches in the same scratch groove.

Dislocation damping and hydrogen pinning in austenitic stainless steels

The room‐temperature damping of three Fe‐18Cr‐Ni alloys was determined as a function of vibrational strain amplitude at 80 kHz. The data are consistent with the Granato‐Lucke theory of dislocation damping. Values of Lc (distance between minor pins) measured are consistent with those obtained from a thermodynamic analysis based on Suzuki locking. Hydrogen charging decreases both the damping and modulus defect, indicating that hydrogen pins dislocations in these alloys at room temperature. The pinning corresponds to four hydrogen atoms per 100 lattice atoms along the dislocation, and a hydrogen‐dislocation (Hd) binding energy of 0.14 eV. The origin of the Hd interaction lies in the elastic misfit interaction, Suzuki interaction, or combination of both. These estimates of H concentration at dislocations and Hd interaction energy are consistent with the observed release rate of hydrogen from H‐charged specimens undergoing plastic deformation.

Hydride formation in a Ni-base superalloy

X-ray diffraction, optical- and scanning-electron metallographic and microhardness studies have been conducted in pure Ni and a Ni-base alloy (HASTELLOY Alloy C-276) in an effort to determine if hydrides form in the alloy during cathodic charging in the presence of a strong H-recombination poison. The studies indicate that hydrides do form and that the extent of surface damage caused by them is a function of the alloy thermomechanical state. In the annealed alloy, hydrides produce a network of crystallographic cracks. In the 50 pct cold-worked alloy, extensive intergranular blistering and lamellar peeling are observed in addition to the crack network. In the cold-worked and aged alloy, damage similar to, but less extensive than, that of the cold-worked alloy is found. The average depth of damage penetration may be accounted for by the bulk diffusion of H in the alloy. It is not clear whether the observed susceptibility of the cold-worked and aged alloy to H embrittlement can be attributed to hydride formation.

Hydrogen-assisted ductile fracture in a Ni-base superalloy

Hydrogen-assisted ductile fracture was evaluated for conditions of electrochemical cathodic charging. Hastelloy C was selected as the material. Room temperature cathodic charging produces a steep hydrogen concentration gradient in the material as opposed to a uniform distribution of hydrogen. (FS)

A study of abrasive wear mechanisms in cobalt-base alloys

Abstract Single-point scratch tests were used to investigate material removal mechanisms in two cobalt-base powder metallurgy alloys 6 and 19. Each alloy was produced with fine, medium and coarse chromium-rich M 7 C 3 carbides in a cobalt-rich f.c.c. matrix phase. In a separate study, the low stress abrasion resistance was found to increase with carbide size. The present scratch test study was designed to simulate low stress abrasion conditions by using single quartz abrasive particles as scratch tools, and the results are compared with those from scratches made using regularly shaped diamond tools. Single- and multiple-pass scratches were made with several different loads using Vickers diamond pyramids and single quartz abrasive particles on metallographically prepared surfaces. Single-pass scratches were also made on preworn surfaces by using a Vickers diamond. Single-pass diamond scratches exhibited ploughing and extensive coarse slip bands in the matrix phase of metallographic specimens. Evidence of carbide deformation and of slip band cracking of the matrix material was observed also. Coarse slip bands were not observed on preworn surfaces. On both polished and preworn surfaces, thin layers of the matrix phase were often smeared over the carbides, and coarse carbides were extensively cracked. Multiple-pass diamond scratches on metallographic specimens exhibited thin detached layers of matrix material in the scratch groove and tongue-like extruded lips of material stretching away from the ridge. Particles of similar size and shape were found on the diamond pyramid. Scratch tests with quartz particles exhibited slip band cracking, smeared matrix material over the carbides and cracking and pulling out of carbides similar to observations with diamond tools. However, extensive ploughing and shear lip formation were not observed in quartz scratches, and wear debris particles in the scratch and on the tool were rounded and very much smaller than those produced by multiple-pass diamond scratches. Wear debris in multiple-pass quartz scratches were observed to pile up at the leading edges of the carbides, which protruded from the surface because of preferential wear of the matrix phase, as observed in low stress quartz abrasion. With both quartz and diamond tools, the material with the finest carbides (alloy 19A) exhibited large pits where cracks had traveled along the carbide-matrix interface and between carbides. Very little evidence of pulling out of carbides in the fine carbide materials was found.

The effect of aging on hydrogen embrittlement of a nickel alloy

The effect of aging at 500° C on the hydrogen embrittlement tendency of a cold worked Ni-base superalloy was investigated in a series of experiments which included hydrogen charging studies, mechanical tests in hydrogen and in air, and fractographic and slip line investigations. Embrittlement tendency increased (time-to-failure decreased) markedly during the first hour of aging and then remained constant until about 1000 h aging time, whereupon it increased rapidly again. The short-time embrittlement could be accounted for either by a mechanism involving segregation of P to grain boundaries or by one involving planar slip induced by short-range order. The hydrogen charging studies indicated that hydrogen uptake decreases during aging, a result which is not consistent with the P segregation hypothesis. The increase in embrittlement at long aging times is most readily explained in terms of planar slip induced by long range order. Tensile tests over a range of strain rates suggested that accelerated transport of hydrogen by dislocation dragging of hydrogen atmospheres is involved in embrittlement.

Abrasive wear-microstructure interactions in a Ni-Cr white iron

Abstract The low-stress and gouging wear behavior of a series of Ni-Cr white iron (Ni-Hard 4) samples has been characterized. The samples were processed to contain 5–85% retained austenite in their microstructures so that their Rockwell C hardness ranged from 63 to 47. The low-stress abrasion behavior was measured with loose SiO 2 and Al 2 O 3 abrasives in a rubber wheel test system. The gouging abrasion behavior was determined in a bonded Al 2 O 3 wheel test system. Wear scars were characterized by scanning electron microscopy used in conjunction with energy dispersive X-ray spectroscopy. Both low-stress and gouging wear behavior were strong functions of test condition and microstructure, and weight losses passed through maxima or minima as the volume fraction retained austenite or abrasion condition varied. Carbides controlled wear behavior in the low-stress test against SiO 2 , and their attrition occurred by uniform scratching, preferential chipping at leading edges and cracking-spalling. In low-stress and gouging tests against Al 2 O 3 carbides and matrix underwent attrition by uniform micromachining. The test results indicated that retained austenite content could be used to optimize wear resistance in a variety of abrasion situations.