CLASSIFICATION OF HARDFACING ALLOYS

These are based on four systems: Tungsten, Cobalt, Nickel and Iron based.
Relative cost ratio of the Tungsten, Cobalt, Nickel and Iron based systems is roughly 30:15:5:1.
To meet the wide requirements of wear applications in various industries a large number of proprietary alloys have been developed made to each manufacturer’ specifications from formulas proven over the years. A wide variety of Hardfacing electrodes, powders and filler metals are supplied for a huge number of applications in terms of both properties and economics. Some of these alloys are covered by AWS Spec A5.13 and also ASM specifications. The proprietary alloys are however not covered by any specifications.
AWS Specification A5.13 Surfacing welding rods and electrodes include:

Table 1. Surfacing Alloys Covered by AWS Specification: A5.13

AWS Classification		Nominal Chemical Composition, %
Welding Rods	Electrodes	C	Mn	Si	Cr	Ni	Other
Iron-Base Alloys
1. RF35-A	(1) EFe5-A	0.85	0.50	0.50	4.0	-	5.0 Mo, 2.0V
2. RFe5-B	(1) EFe5-B	0.70	0.50	0.50	4.0	-	7.0 Mo, 1.0V
3.	(1) EFe5-C	0.40	0.50	0.70	4.0	-	7.0 Mo, 1.0V
4.	(2) EFeMn-A	0.70	14.0	0.8	0.50	3.5
5.	(2) EFeMn-B	0.70	14.0	0.8	0.50	-	1.0 Mo
6. RFeCr – A1	(3) EFeCr-A1	4.0	6.0	2.0	28.0	-
	(3) EFeCr-A2	4.0	<1.0	1.5	28.0	3.5
Cobalt-Base Alloys
7. RCoCr-A	(4) ECoCr-A	1.0	1.0	2.0	29.0	3.0	4.5W, 3.0Fe
8. RCoCr-B	(4) ECoCr-B	1.5	1.0	2.0	29.0	3.0	8.0W, 3.0Fe
9. RCoCr-C	(4) ECoCr-C	2.5	1.0	2.0	29.0	3.0	12.0W,3.0Fe
Nickel-Base Alloys
10. RNiCr-A	(4) ENiCr-A	0.45	-	2.3	11.0	80	2.25 Fe, 2.5B
11. RNiCr-B	(4) ENiCr-B	0.60	-	4.0	13.0	76	4.0 Fe, 3.0 B
12. RNiCr-C	(4) ENiCr-C	0.75	-	4.5	15.0	70	4.5 Fe, 3.5 B

Tungsten Base Alloys:

The tungsten base alloys produce the most wear resistant deposits of the hard surfacing materials having excellent abrasive wear resistance and with proper selection of matrix alloy excellent corrosion resistance.
They consist of hard wear resistant granules of tungsten carbide distributed as discrete particles of predetermined composition in a matrix of iron, carbon steel, cobalt alloy or nickel alloy. Tungsten based consumables are available in the form of tube rod in which tungsten carbide granules are contained in a steel tube. Tungsten based hard facing alloys are also produced as filler powders derived from a casting process. Sintered powder metallurgy rods are available in which tungsten carbide is mixed with high performance nickel or cobalt based alloy.
Tungsten carbide melts at a much higher temperature than iron, cobalt or nickel alloys to form the matrix. The steel matrix produced is not soft by any means because when the tube melts, it dissolves a portion of the tungsten carbide to form a hard matrix which is capable of supporting the carbide granules. The matrix being somewhat softer than the carbides wears away to a degree leaving the hard carbides protruding. It is ideal for applications such as rock drill bits and other mining, quarrying and digging applications.

Iron Base Alloys:

The iron base alloys as a group are one of the most widely used of all the hardfacing systems. The hardening mechanism is carbide formation, work hardening and martensitic transformation. Hardness range is from HRB80 to HRC55 or 60.
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 welding, solid or cored wires for submerged arc welding and cored wires for open arc welding. The iron base alloys are the lowest in cost of the various hardfacing systems.

Nickel Base Alloys:

Nickel based alloys containing chromium and boron is very commonly used for hardfacing applications. The nickel base alloys contain 17% chromium, 2.50-3.75% boron, and 0.30-4.50% silicon balance nickel.
The forming of various carbides and borides in the nickel matrix results in a hard deposit (HRC 67) with excellent resistance to low stress abrasion and makes these the best alloys for metal-to-metal wear. These nickel base and relatively high chromium content alloys result in good heat and corrosion resistance. They retain their hardness and temperatures up to1200°F. The nickel base alloys lend themselves to flame spray and plasma arc applications, and are available largely in powder form. They are normally used for the applications combining low stress abrasion with corrosion or high temperature .Typical applications for nickel alloys are centrifuge screws and pump plunger suckerods.

Cobalt Base Alloys:

These are Cobalt and chromium alloys with varying amounts of tungsten and carbon. With the best “all round' properties’ they combine excellent mechanical wear resistance especially at high temperatures with very good corrosion resistance.
They are typically used in the temperature range 315-600˚C (600-1112˚ F.) They can be finished to exceptional levels of surface finish with a low coefficient of friction to give good sliding wear.
They are probably the most versatile of the hardfacing alloys because they resist heat, corrosion, cavitation, erosion, abrasion, moderate impacts, galling, and metal-to-metal wear. Some alloys in this group remain substantially hard at temperatures up to 1500°F.
Cobalt base alloys with high carbon content are so brittle they may develop cross check cracks upon deposition. Applications would include hot work equipment such as hot punches, valve parts, shear blades etc.

Table 2. ASM Classification of Hard Facing Alloy Groups

Groups	Total alloy content, %	Principal alloying elements
Low Alloy Ferrous Materials
1A	2 to 6	Cr, Mo, Mn
1B	6to12	Cr, Mo, Mn
High-Alloy Ferrous Materials
2A	12to25	Cr,Mo
2B	12to25	Mo,Cr
2C	12t025	Mn,Ni
2D	30 to 37	Mn, Cr, Ni
3A	25 to 50	Cr, Ni, Mo
3B	25 to 50	Cr,Mo
3C	25to 50	Co, Cr
Nickel-Base and Cobalt-Base Alloys
4A	50 to 100	Co, Cr, W
4B	50 to 100	Ni, Cr, B
4C	50 to 100	Cr, Ni, Co
Tungsten Carbide
5	75 to 96	WC in metal Matrix

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