Stellite 21

Description

STELLITE ® 21 ALLOY

 

Co	Cr	Mo	C	Ni	Others	Hardness**	Density	Melting Range
Base	26-29	4.5-6.0	0.20-0.35	2.0-3.0	Fe, Si, Mn	27-40 HRC** 290-430 HV**	8.33 g/cm3 0.301 lb/ni 3	2360-2615 °F 1295-1435 °C

 

Stellite® 21 (previously known as Stellite® 8) was devel- oped 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 Stellite® 21 composition. It was also one of the first heat- resistant alloys trialled for use in jet engines.

Stellite® 21 consists of a CoCrMo alloy matrix contain- ing 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 Stellite® 21 are very dependent upon the manufacturing route and any subsequent heat treatments.

Due to the low volume fraction of carbides, the Co-based

Optical Micrograph of a Stellite® 21 investment casting (as-cast, etched, 200X). The carbides in Stellite® 21 are usually of the type

 

alloy matrix dominates the wear and corrosion properties. Stellite® 21 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.

Stellite® 21 has excellent resistance to thermal and mechanical shock. Optimum high temperature strength is ob- tained by solution heat treatments at 1200-1240ºC (2190 – 2265ºF) followed by quenching, and aging in the tempera- ture range 700-1150°C (1290-2100°F).

Stellite® 21 can be cast, powder metallurgically processed, or applied as a weld hardfacing. It is recommended for ap- plications involving cavitation, erosion, corrosion and/or high temperatures, 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 stamp- ing dies. The oxyacetylene weld deposition method is not recommended for this alloy.

Corrosion Resistance

Stellite® 21 is resistant to oxidizing and reducing gaseous atmospheres up to 1150°C (2100°F). Because its ternary alloy- ing element is Mo and not W, it has higher resistance to reducing or complex environments (e.g. sulphuric acid, hydro- chloric acid, and sour gas) than CoCrW alloys such as Stellite® 6. The typical electrode potential in sea water at room temperature is approx. -0.3 V (SCE). Like stainless steels, Stellite® 21 corrodes primarily by a pitting mechanism and not by general mass loss in seawater and chloride solutions. More information regarding corrosion resistance can be pro- vided on request.

 

100°C (212°F) 200°C (392°F) 300°C (572°F) 400°C (752°F) 500°C (932°F) 600°C (1112°F) 700°C (1292°F) 800°C (1472°F) 900°C (1652°F) µm/m.K 11.0 11.2 12.0 12.65 13.1 13.6 14.3 14.7 15.21 µ-inch/inch-°F 6.1 6.2 6.7 7.0 7.3 7.6 7.9 8.2     2 8.45

Nominal Thermal Expansion Coefficient (from 20°C to stated temperature)

 

Nominal Tensile Properties at Room Temperature

Note: The Mechanical properties of Stellite® 21 are very dependent upon the manufacturing route and heat treatment.
	Ultimate Tensile Strength Rm		Yield Stress Rp(0.2%)		Elongation	Elastic Modulus
	ksi	MPa	ksi	MPa	A(%)	psi	GPa
Investment Castings or undiluted weld metal	103	710	82	565	9	36.2×10 6	250
Stellite® HS-21 (*)	145	1000	87	650	20	35.5×106	245

(*) “HS” = HIP -consolidated from thpeowder form.

Nominal Hot Hardness (DHP) of undiluted weld deposit

 

20°C (68°F)	100°C (212°F)	200°C (392°F)	300°C (572°F)	400°C (752°F)	500°C (932°F)	600°C (1112°F)	700°C (1292°F)	800°C (1472°F)	900°C (1652°F)
347	279	248	228	208	197	181	153	123	92

 

Product Forms and Cross Reference Specifications

SPECIFICATION	PRODUCT FORM		SPECIFICATION	PRODUCT FORM
UNS	Rod, Castings		AWS A5.21 / ASME BPVC IIC ASF5.21 ERCorC-	Rod
UNS W73041	Wire		AWS A5.21 / ASME BPVC IIC ASF5.21 ERCCoCr-	ire
UNS W73021	Electrode		AWS A5.13 / ASME BPVC IIC ASF5.13 ECoCr-	lectrode
AMS 5385	Castings		SAE	HIP – Consolidated
AMS 5819	Rod, Wire