Lesson 8 - Hardsurfacing Electrodes

8.1.5.6 The chart in Figure 7 lists the recommended preheat temperatures for welding the various grades of steel with the proper welding electrode. In hardfacing, the deposit is quite different from the base metal and variations dictated by experience may be neces- sary. 8.2 CLASSIFICATION OF HARDFACING ALLOYS Unlike the various electrodes, wires and filler metals in the previous lessons, hardfacing electrodes and filler metals are frequently proprietary alloys made to each manufacturer’s specifications from formulas proven over the years. Very few of them are classified accord- ing to an AWS specification. Hardfacing systems may be divided into four basic catego- ries: iron base, nickel base, cobalt base, and tungsten. 8.2.1 Iron Base Alloys - The iron base alloys as a group are the most widely used of all the hardfacing systems, and include a wide range of alloy types. These range from low alloy steels containing 2-12% alloying elements to high alloys containing 12-50% of these elements. This group includes a number of buildup alloys, as well as excellent hardfacing alloys. The iron based alloys are characterized by excellent resistance to abrasion in varying degrees or excellent resistance to impact, depending on alloy content. The higher alloy versions afford good wear resistant properties up to 1,000°F. Filler metal is available as coated electrodes, bare electrodes for oxyacetylene welding or gas tungsten-arc weld- ing, solid or cored wires for submerged arc welding, and cored wires for open arc welding. 8.2.1.1 When surfacing with the high chromium-iron base alloys or other brittle alloys, a number of small cracks across the weld will appear. These cracks (known as checking or check cracks) are not detrimental because they do not penetrate into the tougher base metal or buildup alloy. They are, in fact, helpful in relieving stress buildup which would cause eventual longitudinal cracking in the fusion zone, leading to spalling of the hardfacing material. On heavy weldments where heat buildup is great, check cracks may not appear. They should be induced by a light water spray or by an occasional hammer blow on the weld surface. 8.2.1.2 The iron base alloys are the lowest in cost of the various hardfacing systems. 8.2.2 Nickel Base Alloys - The nickel base alloys contain 70-80% nickel, 11-17% chromium, 2.50-3.70% boron, and 0.30-4.50% silicon. The forming of various carbides and borides in the nickel matrix results in a deposit with excellent resistance to low temperature abrasion, and makes these the best alloys for metal-to-metal wear. These alloys also have

Current
Chapter
Table of Contents