Lesson 8 - Hardsurfacing Electrodes

good heat and corrosion resistance. They retain their hardness and temperatures up to 1200°F. The nickel base alloys lend themselves to flame spray and plasma arc applica- tions, and are available largely in powder form. The cost of nickel base alloys is approxi- mately five to six times that of the iron base alloys. 8.2.3 Cobalt Base Alloys - The cobalt base alloys consist of 45-63% cobalt, 24-29% chromium, 5.50-13.5% tungsten and 1.10-3.20% carbon. They are probably the most versatile of the hardfacing alloys because they resist heat, corrosion, abrasion, moderate impacts, galling, and metal-to-metal wear. Some alloys in this group remain substantially hard at temperatures up to 1500°F. Applications would include hot work equipment such as hot punches, valve parts, shear blades, etc. 8.2.3.1 In recent years, the price of cobalt has risen sharply since there are few sources in the world. The price of cobalt alloys per pound exceed that of the iron base alloys by approximately eighteen times. 8.2.4 Tungsten Base Alloys - The tungsten base alloys produce the most wear resis- tant deposits of the hard surfacing materials. They consist of hard granules of tungsten carbide distributed in a matrix of iron, carbon steel, cobalt alloy, or nickel alloy. The matrix, being somewhat softer than the carbides, wears away to a degree, leaving the hard car- bides protruding. This roughness of the deposit renders these alloys useless for metal-to-metal applications, but ideal for applications such as rock drill bits and other min- ing, quarrying and digging applications. 8.2.4.1 These rods or electrodes are usually supplied as carbon steel tubes filled with tungsten carbide granules by weight. The steel matrix produced is not soft by any means, because when the tube melts, it dissolves enough of the tungsten and carbon to form a hard matrix and is capable of supporting the carbide granules. 8.2.4.2 Despite their excellent abrasion resistance, tungsten carbide alloys can only withstand impacts that do not produce compressive stress above their yield strength. Tungsten carbide alloys have low resistance to oxidation and low resistance to corrosion, unless deposited in a nickel or cobalt matrix. Hardness at high temperatures is approxi- mately equal to the higher alloy iron base alloys if the tungsten carbide granules are in an iron or steel matrix. If in a nickel or cobalt matrix, better hot hardness can be achieved. 8.2.4.3 The cost of rods or electrodes consisting of tungsten carbide granules in a car- bon steel matrix is approximately nine times that of the iron base alloys. If the matrix is a nickel or cobalt base alloy, costs will be higher.

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