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HARDFACING
 
Hardfacing involves protecting parts exposed to different types of wear in order to obtain a certain specifi c wear resistance.
 
Hardfacing is primarily used to restore worn out parts back to usable condition in order to extend their service life but it can be successfully employed in new component manufacture as well. This technique will help in manufacturing a component from cheaper material with a wear resistant overlay providing the surface properties.
 
A higher hardness level does not necessarily imply improved wear resistance or longer life. The effectiveness of any hardfacing alloy depends of its suitability in the operating conditions.
 
So the appropriate hardfacing alloy has to be selected considering the following aspects
  • The wear factors
  • The base material of the component
  • The surface fi nish required
  • The process to be used
WEAR FACTORS
Wear factors are the action of different agents on the metallic surface leading to degeneration and disintegration of the metal. A number of wear factors exist, which act individually or in combination. The weld metals have to be selected properly to counter these factors effectively.
 
The major types of wear factors are as follows
  • Friction
    Wear from metal parts that roll or slide against each other. It accounts for 15% of the total spectrum of industrial wear. This type of wear is likely to be most severe when parts rub together under load with little or no lubrication. Generally, contact between surface materials of the same hardness will result in excessive wear. E.g. shafts against bearing surfaces, chain links against a roll, sprockets, steel mill rolls.
  • Impact
    The sudden action of a very large force for a very short time period gives rise to wear after some time. The mechanism is attributed to fatigue failure that has an incubation period prior to the appearance of surface damage. E.g. crusher rolls, impact hammers, railway points and crossings.
  • Abrasion
    About 50% of all industrial wear is abrasion in different forms. The wear takes place when metal is removed from a surface by the cutting or gouging action of hard non-metallic particles. Abrasion may be classifi ed into three types
    • Gouging abrasion : This class of abrasion involves the removal of sizable particles from a metallic surface by the action of a coarse material. The high pressure and impact cause the particles to cut into the surface and produce large gouge marks and scratches. Eg. shovel diggers, chute impact areas, pulveriser mills.
    • Low stress abrasion : This form of wear results from the sliding action of free moving hard particles along a surface. Material is removed by scratching or micro-machining process. Eg. chutes, mineral conveyors.
    • High stress abrasion : It occurs where abrasive particles are forced between two metal parts and crushed under heavy loads. The wear involves surface damage accompanied by plastic deformation. Eg. rock drills, scraper blades, ball mills.
  • Erosion
    This wear is similar to abrasion but the particles are carried by a fl uid stream like water, steam, etc., generally at a higher velocity compared to low-stress abrasion. Eg. slurry transport systems, shot blasting equipment.
  • Cavitation
    When a liquid is subjected to rapid changes of pressure, vapour or gas bubbles form in the lower pressure regions of the liquid. Entering high pressure areas at any metal/ liquid interface, these bubbles collapse and the immense force causes cyclic stress and fatigue on the metal surface. E.g. ship propellers, pump impellers.
  • Heat
    When metals are exposed to high temperature for long periods, they lose their durability mainly due to thermal fatigue cracking. Moreover, the metals tend to lose their strength and hardness at higher temperatures. These factors add up to wear. Eg. hot forging dies, extrusion dies, stamping dies, sinter crushing equipment.
  • Corrosion
    It involves reaction between a metallic surface and a corrosive environment whence the former is dissolved away leading to wear. In the presence of any mechanical force, the corrosion products may be removed leading to virgin surface coming in contact with thecorrosive environment again causing continued wear. E.g. valves, seating rings, screw conveyors.
  • Oxidation
    In an oxidizing atmosphere, possibly aided by high temperatures, the metal surface builds .uP an oxide layer, which is mostly brittle in nature. This may break due to expansion and the entire oxidation operation is repeated. E.g. blast furnace parts, exhaust valves of internal combustion engines, hot working shears.
BASE MATERIAL
The two main groups of base materials for hardfacing are
  • Carbon or low-alloy steels : These steels require preheating, post-weld heat treatment, slow cooling for hardfacing according to the chemical composition and the section thickness. The general guidelines for preheating are given at the end of this section.
  • Austenitic manganese steels : These steels should be welded without any preheating or post-weld heat treatment at all. The interpass temperature should be kept as low as possible.
SURFACE FINISH
The surface fi nish requirements must be kept in mind while selecting the hardfacing alloy as they cover the entire range from easily machinable to non-machinable. Additionally, many of these deposits will contain "relief checks", which are formed across the bead as the bead releases the stress generated while the hard weld metal cools. These relief checks are not harmful to the deposit but may propagate into the base metal if the component is subjected to heavy impact or flexing.
 
So, the issues of acceptability of relief checks and finishing requirements should be addressed to when the hardfacing deposit is decided.
 
WELDING PROCESSES
Shielded Metal Arc Welding
  • Covers the widest range of weld metals
  • Is a versatile process for on - site repairs and outof- position work
  • Is inexpensive
Flux - Cored Arc Welding
  • The same range of alloys available as in case of coated electrodes
  • On-site use is possible
  • Self-shielded wires do not require any additional shielding gas
  • High deposition rate
Submerged Arc Welding
  • High deposition rate no spatter loss
  • Product range limited
  • Positional welding not possible
 
 
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