The chemistry and structure of wear-resistant, iron-base hardfacing alloys
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I.
INTRODUCTION
C O B A L T is a key constituent of many wear-resistant hardfacing alloys. Anthony tlj attributes the good galling wear resistance and cavitation erosion resistance of these cobalt-base alloys to the low stacking fault energy of cobalt and to the deformation-induced martensitic transformation of the cobalt alloy matrix from a metastable face-centered cubic (fcc) structure to a hexagonal closepacked (hcp) structure, which leads to an increased rate of work hardening. Also, hcp structures have fewer operative slip systems than fcc structures, rendering them more susceptible to fracture than to deformation. Two motives exist to reduce the cobalt content of hardfacing alloys: (1) primary cobalt is a high-priced commodity, and because its supply is dominated by a few countries, its price fluctuates significantly, and (2) cobalt released by the wear and corrosion of nuclear plant components is largely responsible for the occupational radiation exposure of plant maintenance personnel. Cobalt-base alloys are very resistant to galling wear; which is an extreme form of adhesive wear. High resistance to galling wear refers to the ability of contacting surfaces to resist material deformation and transfer across the interface at high contact stresses. Ohriner et al. t2] have reported galling wear test results obtained on iron-base alloys. These tests used a pin-onplate test geometry and were performed both in air and deionized water at ambient temperature. Applied stresses were 20, 40, and 60 ksi (140, 275, and 415 MPa). A surface damage value was calculated by comparing preand post-test wear scars, which were traced with a profdometer. Lower surface damage values indicate more galling-resistant alloys. Wear data were obtained on castings and on gas tungsten arc welding (GTAW) and plasma arc welding (PAW) overlays of the iron-base al-
loys. A number of heats of the cast iron-base alloys showed resistance to galling wear as high as that of a cobalt-based standard. Surface damage values measured in air at 60 ksi were - 5 /xm, and over 90 pct of the compositions studied showed surface damage values < 5 0 / z m under these test conditions. Tests in air resulted in more surface damage than those in deionized water, which acted as a lubricant. Gas tungsten arc welding and plasma arc welding deposits of the iron-base alloys and the cobalt-base standard showed higher resistance to galling wear than did castings, with surface damage values dropping to - 1 /~m for the best-performing heats. The wear resistance of these iron- and cobalt-base alloys is suggested by a comparison of these damage values with those obtained on a number of commercial wearresistant, nickel-base alloys, which ranged from - 5 0 to 75/zm. Recently, Johnson et al.[31 have reported that the crosion rates of these iron-base alloys are very low in experiments using the cavitating jet erosion technique. This article discusses the chemistry and structure of these new wear-resistant alloys. Because alloy structure is affected by processing, the techniqu
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