Aug 15, 20 the carbides present in the cocrwbased stellite alloys are of chromiumrich m 7 c 3 type. The distinctive difference between cobalt 6k proprietary name stellite 6k and cobalt 6b the alloy used in talonite is that 6k contains about 50% more carbon than 6b, making. These hardfacing layers are applied as an overlay onto a steel. All indents were visually verified in the software for accuracy and the. Most often these cracks are transverse cross cracks, which are very beneficial to relieve the residual stress in the weld bead. Gandy2 in recent years, several incidents of cracking and failures have been observed in stellite. The term cladding includes not only weld overlays but also solidstate processes such as explosive cladding, mechanical cladding or rollbonding. Cracking has been observed in valve components at the stellitesteel. Most of the time, this material is stellite, a cobaltbase. In stellite alloys this generally has the following effects. The material itself has been around since the 1900s and was originally seen as a substitute for traditional metal cutlery which suffered from general wear and tear and was also prone to staining. The dilution region is the region where melted substrate forms a mixture with coating material and possesses microstructure similar to the coating.
Since investigators did not know whether deloro cobaltbased hardfacing alloys were used in the fabrication of these valves, the generic names alloy 6 and alloy 21 are used here. The cocrw system alloy retains its hardness at elevated temperatures and is especially effective for wear applications at high temperatures or in a corrosive environment, and are widely used as a wear, corrosion, and heatresistant material 43. Stellite alloys chemical composition, mechanical properties. During cladding, a corrosion or oxidation resistant alloy is applied to a less corrosion resistant base material. Laser cladding, hvof, plasma spraying, stellite 6 coating, friction and wear, nickel alloy.
Important to note is that the two stellite alloys typically used for hardfacing of valve seats and discs are stellite 6 and stellite 21, trademarked products of deloro stellite group. Stellite failure on a p91 hp valve failure investigation. Because of the presence of the carbide network, the overlay is somewhat prone to cracking, although resistance to cracking is very good for an alloy of this hardness level. Stellite 6 welds pretty clean unless it gets oxidized and then the puddle gets sluggish. Cocrw meets or exceeds ams 5894, pwa 1196, mcs 1012 and uns r30016. Delamination failures of stellite hardfacing in power plants. Stellite is a nonmagnetic alloy which contains a variety of different metal components with the main elements being cobalt and chromium. The trademark owners of stellite claim that stellite 6 is the most widely used of their range of stellite alloys, offering a proven industry standard for generalpurpose wear resistance, and high mechanical and chemical perfromance in hostile environments. Influence of cutting parameters and tool wear on the surface. The different process steps are coupled in order to transfer the residual conditions from preceding processes to the ones following. Since the thickness is 5mm, we have to do 2 layers. A strategy is given to improve the ductility and thermal fatigue cracking feature of stellite 6 coating by adding ti and ni. The following 19 answers to frequently asked questions may help you select.
The microstructure of stellite 6 alloy coating mainly consists of. The alloy currently most suited for cutting tools, for example, is stellite 100, because this alloy is quite hard, maintains a good cutting edge at high temperature, and resists hardening and annealing. This paper deals with the investigation of the microstructure and wear behavior of the stellite 6 cladding on precipitation hardening martensitic stainless steel 174ph using gas tungsten arc welding gtaw method. Status of stellite 6 friction testing unt digital library. Delamination failures of stellite hardfacing in power. This alloy also has higher hot hardness and resists galling like all stellite alloys when mated with another stellite alloy. Microstructure evolution and failure behavior of stellite 6. The pta process is typically used to apply relatively thick i. Hightemperature hardness and wear resistance of stellite alloys. Hard facing hf and corrosionresistancealloy cra with hot wire, materials. The microstructure of stellite 6 contains hard m 7 c. With ultimet alloy, cracking of the overlay is not a consideration, and multiple layers may be applied, with little or no preheat.
Stellite 6b hpa cobalt 6b uns r30016 high performance. The torch manipulation is generally automated and hence can achieve uniform overlays on complex parts. Compositions of grade 91 substrate, stellite 21 butter layer and stellite 6 hardfacing alloys. Stellite 6b is supplied in the solution treated condition to ams 5894 and its wear resistant. How to avoid stellite 6 crackings american welding society. Evaluation of microstructure and mechanical properties at the. When they are applied to steels by hardfacing, there is intermixing of the stellite alloy and the steel, resulting in a higher iron content in the overlay, especially in the first layer. Cobalt alloy 6b stellite 6b is a cobaltbased, chromiumtungsten alloy used extensively in wear environments where its low friction coefficient minimises the risk of seizing and galling, even in applications where lubrication is not practical. This alloy is very popular since it can retain hardness even at elevated temperature. Stellite 6 is a cobase wear resistant alloy containing 1% c, 27% cr, 4. The material is also resistant to erosion and most. When we finished the first stellite 6 layer, there is no cracking found.
Introduction 1 stellite 6 is a very versatile material that is used for hardfacing of various component parts for applications requiring wear resistance cobaltbase rods 1982. Stellite 21 or stellite 6 or ultimet uns r31233 weld overlay where a combination of corrosion and wear resistance is required weld cladding is typically between 2 and about 20 mm thick. Wear characteristic of stellite 6 alloy hardfacing layer by plasma. It is easily deposited by a number of methods but you do have to be careful about contamination from sulphur or phosphorus. Bonding stellite 6 to alloy steel asme mechanical code. Stellite is a family of completely nonmagnetic and corrosionresistant cobalt alloys of various compositions that have been optimised for different uses.
Ti addition made the m 23 c 6 eutectic carbide change to the tic carbide. Stellite 6 is a well know overlay in the pump and valve industry. For the past several years, researchers at the idaho national engineering and environmental laboratory, under the sponsorship of the us nuclear regulatory commission, office of nuclear regulatory research, have been investigating the performance of motoroperated valves subjected to design basis flow and pressure loads. Stellite 21 is much less prone to cracking in our experience when we can talk the customer into it over 6 we do it every time. Most crackings are perpendicular to the welding seam. Dudzinski 2 1 mechanical laboratory of materials and plant maintenance lr3mi. Since most hardfacing deposits have nearly no ductility, the welds will crack. However, limited reports can be found to study the thermal fatigue cracking property of a stellite 6 coating modified with both titanium and nickel. Dry sliding wear map for stellite 6 against a wcco 9010 disc 12 49 figure 214.
Two layers of stellite 6 were coated for a maximum thickness of 6 mm or higher. Stellite definition of stellite by the free dictionary. Pdf stellite alloy mixture hardfacing via laser cladding for control. Aug 12, 2011 the alloy stellite 6 has less tendency to crack than for example stellite 12, but only when multilayer technique is used. It is very easy to apply as an overlay, compared with the traditional cobaltbase wear alloys. Developing the ductility and thermal fatigue cracking property of. Mar 08, 2005 this robot is hardfacing an agricultural tool. The present program was initiated to examine the effects of. Hpa cobalt 6b is a wear resistant alloy used for stems, balls, rails and bushings. Stellite 720 ultraflex technical data stellite 720 ultraflex provides a metallurgicallybonded cladding with minimal dilution to a metal substrate, providing superior wear resistance and exceptionally high corrosion resistance in reducing atmospheres. Oct 10, 20 the interface area in laser cladding, microstructures and mechanical properties of which determine the structural integrity of the laserdeposited coating, consists of two distinctive zones. Understanding some of the basics about hardfacing can go a long way toward instilling confidence in your hardfacing product selection.
The sequence of the primary welding process and the following secondary processes of machining, and heat treatment has been modelled to predict the residual conditions in a stellite 6 overlay weld on a p91 steam turbine valve. However, when we finished the second layer and let it cool gradually wrapping the cone with insulate material, we could see some crackings on the second. Higher carbide level is associated with lower corrosion resistance, since, in. A few years ago, i had to develop a procedure for multi layer crack free stellite 6, applied to a multi faceted c. Influence of yttrium additive on wear behavior of stellite 6 at elevated temperatures 25 54. Stellite 6 alloys coated on stainless steel are used for valve. The phase diagrams were constructed by using the pandat software with.
Microstructure and wear behavior of stellite 6 cladding on 17. View detailed import data, price, monthly trends, major importing countries, major ports of stellite 6 wire. Inspect steam valves for stellite delamination welcome to. Besides, stellite 6 exhibits favorable thermal fatigue and creep.
We built an insulated box with a piece of grating to place the parts on for welding. Also we can apply other materials such as stellite 6 or 12 or nickel ni 100%. So it is possible to apply the material without cracks but in cases where the hard facing also is used for corrosion resistance or for a perfect. The modified stellite 6 coating with improved performance may play an important role in prolonging the service life of brake discs. This cladding may be applied to internal diameters. Stellite 6k is one of a series of cobaltchromium alloys, which are basically a combination of cobalt 50%plus, chromium between 2530%, and several other elements. In single layer applications you probably will have small cracks. For small, simple parts, cracking is not a usually a problem. Erosion test curves of stellite alloys 7 51 figure 215.
Pdf influence of cutting parameters and tool wear on the surface. Recharge by welding a carbon steel valve with inconel625 825 overlay, cladding. Ref 8 examined the dry milling machinability of stellite 6 alloy layers. Argon gas was used as the shielding gas as well as the medium for generation of plasma in pta process. Stellite 3 is superior and this improved resistance increases as loads are increased or speeds are increased to 10 times to 25 times that of stellite alloy 6 or 12. At first glance, hardfacing can be confusing and troublesome. Developing the ductility and thermal fatigue cracking. Epri has established a committee on cracking and disbonding of hardfacing.
Pta hardfacing of stellite 6 on stainless steel has been carried out in plasma transferred arc pta welding machine since it is a semiautomated process. Today, the hardening of sealing surfaces is performed by welding a material harder than the valve body material. These carbides provide high hardness level of the alloy. In other words, the overlaid surfaces met at 90 degree angles, inside and outside corners. Cobalt alloy 6b stellite 6b ams 5894 aircraft materials. Oct 24, 2002 stellite 6 is a cobalt based alloy but is very tolerant of dilution from other materials and is often used on stainless to give a wear resistant area, e. This type of cracking will not affect the wear resistance. Alternate stellite process material selection and properties. The chemical composition of stellite 6 is given in table 3. Plasma transferred arc welding of stellite 6 alloy on. Effect of welding variables on cracking in cobaltbased sma.
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