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WD-Co21

PRODUCT

Introduction

WD-Co21 (previously known as WD-Co8) was developed in the mid 1930s as a corrosion resistant CoCr alloy, and rapidly found application as a biocompatible hip implant and denture alloy. Many of the alloys currently used in medical applications are variants of the original WD-Co21 composition. It was also one of the first heat-resistant alloys trialled for use in jet engines.

WD-Co21 consists of a CoCrMo alloy matrix containing dispersed hard carbides which strengthen the alloy and increase its hardness, but also decrease the ductility. The type, shape, size, and distribution of the carbides is strongly influenced by the processing history of the alloy, and for this reason the mechanical properties of WD-Co21 are very dependent upon the manufacturing route and any subsequent heat treatments.

Due to the low volume fraction of carbides, the Co-based alloy matrix dominates the wear and corrosion properties. WD-Co21 has excellent cavitation, galling and metal-to-metal sliding wear resistance, but is not recommended for severe hard particle abrasion. The surface can work harden considerably during wear or even during machining, and the use of correct machining tools and techniques is important to achieve optimal results.

WD-Co21 has excellent resistance to thermal and mechanical shock. Optimum high temperature strength is obtained by solution heat treatments at 1200-1240ºC (2190 - 2265ºF) followed by quenching, and aging in the temperature range 700-1150°C (1290-2100°F).

WD-Co21 can be cast, powder metallurgically processed, or applied as a weld hardfacing. It is recommended for applications involving cavitation, erosion, corrosion and high temperature, such as valve trim for petrochemical and power generation. Due to its good impact resistance, it has been widely used in the building up of forging or hot stamping dies. The oxyacetylene weld deposition method is not recommended for this alloy.

Common Name: Stellite® 21, Polystel® 21

Chemical Composition(%)

C	Si	Mn	Mo	Cr	W	Ni	Fe	Co	Others
0.2	2.0	≤1.0	5.5	28.0	0.2	≤3.0	2.0	Bal	<1.0

Deposit Characteristic

Abrasion Resistance	Moderate
Impact Resistance	Excellent
Corrosion Resistance	Good
Hardness	HRC 20-40
Deposit Layers	Multiple
Deposit Cross Checks	No
Magnetic	No
Hot Wear Hardness	Good
Machinability	Carbide Tools