Introduction to Hardfacing

WHAT IS HARDFACING?

• Hard facing is generally used for surfacing components exposed to mechanical wear, abrasion , impact galling , erosion and cavitation .  Components in the field will be exposed to corrosion, high temperature scaling, impact, heat, metal to metal wear, abrasion or a combination of these factors.
• The metals used for applications must have properties like wear resistance, corrosion resistance or temperature resistance. But metal with all those properties are highly expensive. Instead of making the entire part from a highly expensive metal which has all the resistance properties it is economical to manufacture the part from metal of low cost and then cover the critical surfaces with a metal which has all the properties such as wear, corrosion and temperature resistance.  The application of a coating of an alloy to a metallic surface of a component by the means of welding or spraying with the intention of extending the working life of the component is called as Hardfacing.
• In reclamation or repair identifying the reason for the wear incurred by the component will be the deciding factor to establish the process and alloys to be used to complete the Hard – facing procedure.
• Most applications fall into three classes
        1.       Protection of the cutting edge
        2.       Protection  of  a single surface
  3.     Wear from Steel parts rolling or sliding against each other with or without lubrication.
• Wear is defined as “the damage to a solid surface caused by the removal or displacement of material by mechanical action of a contacting solid, liquid or gas“.   
• Hard – facing is used to prevent or control various forms of surface degradation such as metal to metal wear and or abrasive wear. Here we will discuss   metal to metal and abrasive wear. However it should be noted that Hardfacing is also effective in countering other forms of wear such as impact loading , erosion and cavitation or thr combination of these factors . One noted such combination is wear and corrosion for example slurry pumps , knives in food processing industry , valves etc .

• TYPES OF WEAR

• Adhesive wear
• Abrasive wear
• Surface fatigue
• Fretting wear
• Erosive wear
• Corrosion and oxidation wear

A number of different wear phenomena are impact-, cavitation-, diffusive- and corrosive- wear are all such examples.

These wear mechanisms, however, do not necessarily act independently and multiple wear mechanisms occurring in unison is common. Wear mechanisms and / or sub-mechanisms frequently overlap and occur in a synergistic manner, producing a greater rate of wear than the sum of the individual wear mechanisms.

Adhesive wear:

Adhesive wear can be found between two poorly lubricated surfaces during frictional contact and generally refers to unwanted displacement and attachment of wear debris and material compounds from one surface to another.  This process is also referred to as seizing, galling or scuffing.

Abrasive wear:

Abrasive wear occurs when a hard rough surface slides across a softer surface under pressure. ASTM International (formerly American Society for Testing and Materials) defines it as the loss of material due to hard particles or hard protuberances that are forced against and move along a solid surface.

The two modes of abrasive wear are known as two-body and three-body abrasive wear. Two-body wear occurs when the grits or hard particles remove material from the opposite surface. The common analogy is that of material being removed or displaced by a cutting or plowing operation. Three-body wear occurs when the particles are not constrained, and are free to roll and slide down a surface.

Deep 'groove' like surface indicates abrasive wear over cast iron.

There are a number of factors which influence abrasive wear and hence the manner of material removal.  Three commonly identified mechanisms of abrasive wear are,

Plowing:

It occurs when material is displaced to the side, away from the wear particles, resulting in the formation of grooves that do not involve direct material removal. The displaced material forms ridges adjacent to grooves, which may be removed by subsequent passage of abrasive particles.

Cutting:

It occurs when material is separated from the surface in the form of primary debris, or microchips, with little or no material displaced to the sides of the grooves. This mechanism closely resembles conventional machining.

Fragmentation:

It occurs when material is separated from a surface by a cutting process and the indenting abrasive causes localized fracture of the wear material. These cracks then freely propagate locally around the wear groove, resulting in additional material removal by spalling.

Fretting wear:

Fretting wear is the repeated cyclical rubbing between two surfaces. Over a period of time fretting which will remove material from one or both surfaces in contact. It occurs typically in bearings, although most bearings have their surfaces hardened to resist the problem.

Erosive wear:

Erosive wear can be defined as an extremely short sliding motion and is executed within a short time interval. Erosive wear is caused by the impact of particles of solid or liquid against the surface of an object.  The impacting particles gradually remove material from the surface through repeated deformations and cutting actions. It is a widely encountered mechanism in industry. Due to the nature of the conveying process, piping systems are prone to wear when abrasive particles have to be transported.

Any metal capable of being fusion welded can be Hard-faced. The base metals suitable for Hard - facing are stainless steels, manganese steels, cast iron, low and high alloy steels, nickel alloys and many more.

PROCESSES FOR HARDFACING

The processes used for the purpose of Hardfacing are,

• Oxyacetylene process (OAW) with flame spray or manual torch
• TIG or GTAW 
• SMAW
• SAW
• Open Arc Welding  
• PTAW (Plasma transferred arc welding)
• Plasma Spray
• HVOF 
• Detonation Flame Spraying
• Laser Cladding 
• ESW ( Electro slag welding )

                                  Selection of consumables for surfacing

Condition required	Type of deposit	Approx. hardness
High impact resistance with medium abrasion resistance	Low alloy	350 HV
Medium impact and high abrasion resistance	Medium alloy	650 HV
Excellent impact resistance and work-hardening under impact to resist abrasion	13% Manganese steel or Austenitic steel	250 HV (500 HV when work hardened)
Maximum resistance to impact and abrasion	Chromium carbine	700 HV
Maximum abrasion resistance with moderate impact resistance	Tungsten carbide	1800 HV

APPLICATIONS

Hardfacing technology has wide spread use in the following Industries 

• Aerospace
• Oil & Gas
• Automotive
• Rubber
• Power Generation
• Steel
• Timber
• Plastics
• Glass
• Forging
• Dental
• Earthmoving
• Mining
• Food Processing
• Valves & Fittings

REFERENCE: 

https://en.wikipedia.org/wiki/Wear

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