4 Cobalt in Metallurgical Uses

Transcription

1 4 Cobalt in The use of cobalt as a constituent of metal alloys systems (not including magnets) is based on its ability to impart high temperature strength, though this is a simple phrase which describes what can be a complex effect and in the Co/Fe/V systems to provide controlled expansion. This section will cover the main uses of cobalt in a simplified manner. 4.1 Superalloys Superalloys are simply defined as alloys developed for elevated temperature service, usually based on group VIIA elements, where relatively severe mechanical stressing is encountered and high surface stability is frequently required (Sims The Superalloys). Three classes of alloys have appeared - cobalt-base, nickel-base, iron-base to meet this definition. The driving force behind their development has been the jet engine which has required ever higher operating temperatures. The use of the alloys has, however, extended into many other fields all types of turbines, space vehicles, rocket motors, nuclear reactors, power plants, chemical equipment and possibly 20% of alloys have arisen for corrosion resistant applications. Leaving aside the cobalt-based alloys, the greater majority of alloys are what might loosely be termed super stainless steels. The strength in these alloys arises from strengthening the close packed face centred cubic (fcc) austenitic lattice which is at the heart of the alloy. Until the 1930 s, this matrix, either in iron- or nickel-base, was strengthened by carbide precipitation. However, cubic gamma prime γ -Ni 3 Al was created in the matrix at this time and the Nimonic 80 of the period (Ni76/Cr19/Al 1-4%/Ti 2-4%) contained this phase. This phase itself is similar to the matrix and can also be hardened. It has a yield strength which perversely increases with temperature. So, nickel-based alloys which form the bulk of alloys produced, are basically nickel-chrome alloys with a fcc solid solution matrix containing carbides and the coherent intermetallic precipitate γ(ni 3 (Al 1 Ti). This latter precipitate provides most of the alloy strengthening and results in useful operating temperatures up to 90% of the start of melting. Further additions of aluminium, titanium, niobium and tantalum are made to combine with nickel in the γ phase and of molybdenum, tungsten and chromium which strengthen the solid solution matrix. At last cobalt the role of cobalt is not completely understood but it certainly increases the useful temperature range of nickel-based alloys. γ also occurs as γ which has a body centred tetragonal structure (i.e. two cubes stacked). Cobalt is thought to raise the melting point of this phase thus enhancing high temperature strength. Figure 1 shows the plethora of elements which have become part of the superalloys stew, some to provide hardening, some to prevent other unwanted phases from forming and some to stop required phases disappearing. As small additions can make large differences, it is not surprising that impurities are of equal importance, hence the superalloy user s insistence on quality. As well as structure, processing has been responsible for enhancement of these alloys. From the 1930 s to the 50 s, these alloys were packed with increasing structure to strengthen them. However, in the 1950 s, problems occurred with embrittling phases such as σ and Laves. The 1950 s saw the generation of complex grain boundaries with carbides engloved in γ, creating a dispersion strengthened layer bonding the grains together. By 1970, hafnium was being added and the engloved structure was less essential as Hf contorts the grain boundary to create strength and ductility in a more mechanical fashion as well as generating additional γ. Cobalt Facts, 2006 CDI 8

2 Figure 1 A qualitatively comparative view of trends in superalloy chemical compostion Figure 2 depicts (at 10,000x) the 50-year development of nickel superalloy appearance. The upper 2/3 of the picture show structures that have enhanced performance and the bottom part those which cause brittleness, lower strength or have other problems. Figure 2 The microstructure. Panorama of development of nickel superalloy microstructure showing both useful and deleterious phases Cobalt-Based Alloys The cobalt-base superalloys have their origins in the Stellite alloys patented in the early 1990 s by Elwood Haynes. These names live on with the Stellite name now belonging to the Deloro Stellite company and the later (Haynes-Stellite alloys) Hastelloys being a registered name for the large range of corrosion resistant superalloys made by the Haynes International company. Cobalt Facts, 2006 CDI 9

3 The Co-Cr-Mo alloys started with the 1930 s Vitallium which was castable and used for dental prosthetics. It was followed by HS21 which became the lead material for gas turbine applications in the 1940 s. Lastly, X- 40 which was synonymous with HS-31 and is still used today was invented in 1943 and this alloy has served as a based for new generations of cobalt-based superalloys (X-40 Co-based 25% Cr, 10%Ni, 8%W). Although in terms of properties the (γ ) hardened nickel-based alloys have taken the lion s share of the superalloy market, cast and wrought cobalt alloys continue to be used. WHY? Because: 1. Cobalt alloys have higher melting points than nickel (or iron) alloys. This gives them the ability to absorb stress to a higher absolute temperature. 2. Cobalt alloys give superior hot corrosion resistance to gas turbine atmospheres, this is due to their high chromium content. 3. Cobalt alloys show superior thermal fatigue resistance and weldability over nickel alloys. Composition and Structure Cobalt alloys are termed austenitic in that the high temperature Face Centred Cubic phase is stabilised at room temperature. As such, they are comparable to stainless steels. They are hardened by carbide precipitation, thus carbon content is critical. Chromium provides hot corrosion resistance and other refractory metals are added to give solid solution strengthening tungsten and molybdenum and carbide formation tantalum, niobium, zirconium, hafnium. Processing is of course vital and whilst the above metals are helpful, others such as dissolved oxygen are not. Vacuum melting is therefore becoming the norm to give close alloy control. It is also critical that the specified compositions are adhered to, as excess of the soluble metals, W, Mo, Cr, will tend to form unwanted and deleterious phases similar to the nickel alloys σ and Laves (Co 3 Ti tetragonal close packed TCP phases). Table 1 shows the effect of some additions and typical alloy compositions. Table 1 Function of Alloying Element Groups in Cobalt Superalloys Nickel Chromium Tungsten Ti, Zr, Cb, Ta C Principal Austenite Surface Stability Solid-Solution MC Formers Carbide Function Stabiliser + Carbide Former Strength Formation Problems a Lowers Forms TCP Forms TCP Harms Surface Decreases Corrosion Phases Phases Stability Ductility Resistance Examples X MM Ta, 0.5Zr, Ti L HS a when added in excess Casting is important for cobalt-based alloys and directionally solidified alloys (DS) have led to increased rupture strength and thermal fatigue resistance. Cobalt Facts, 2006 CDI 10

4 The newest class of alloys as the (pm) materials and the finer dispersion of carbides and the small grain size have increased properties above that of cast alloys (pm = powder metallurgical). Further process development by hot isostatic pressing (HIP) has even further improved the properties by removal of possible failure sites. Figure 3 finally illustrates why cobaltbased alloys continue to be used in gas turbines. Figure 3 Stress Rupture Properties Compared to the nickel alloys, the curve for the cobalt alloy (TD-CoCr) is flatter and shows much lower strength up to around 1800 F (980 C). The greater stability of the carbides, which provide the cobalt alloy strengthening compared to γ which strengthens the nickel alloys then asserts itself. This factor is the primary reason cobalt alloys are used in the lower stress, higher temperature stationary vanes of gas turbines. Discussion Superalloys in general are a complex subject and there is probably no other group of alloys in history into which more effort had been channelled. The jet turbine is a hostile environment and superalloy metallurgists have developed alloys more structured and used at higher fractions of their melting point than any other alloy grouping. It is difficult to give a list of all superalloys available, both cast and wrought, corrosion resistant, nickel-, ironand cobalt-based. The tables in Annex 1 give a summary of typical alloys. Over the last 30 years, the use of directly solidified (DS) alloys has become well established. The purpose of directional solidification is to produce alloys with a columnar-grained structure, so eliminating the transverse grain boundaries in alloys cast by conventional means. DS superalloys have vastly superior fatigue life, rupture life and rupture ductility than conventional alloys. Even further improvements in strength and temperature resistance have been achieved by the development of single crystal alloys. By definition, these alloys do not contain grain boundaries, so do not need elements which are traditionally added to strengthen the boundaries, but which at the same time depress the alloys melting point. Both these trends have allowed the development of higher thrust jet engines which operate at even higher temperatures. Cobalt Facts, 2006 CDI 11

5 4.2 Wear Resistant Alloys & Coatings Cobalt is used in two ways to give hard, corrosion/erosion resistant, high temperature coatings. Firstly, carbide coatings containing up to 17% cobalt (see section 7) can be deposited by flame and plasma guns on to softer substrates to give the finish and hardness of carbide work rolls, mixers, grinders, etc. The main interest in this section is the metallurgical alloys based on cobalt, whose primary aim is wear resistance and which may be applied by surface coating or used as castings and forgings. Alloys Once again in the history of cobalt, we have to go back to Elwood Haynes. The alloys used in this field are again based on the Stellite alloys developed in the early 1900 s, although coatings have moved on into cobalt-containing nickel based alloys as well, strictly for corrosion resistance. The Stellites were originally used as cutting tools and whilst this use has mainly been replaced by carbide, it does remain. More often now however, the CoCrW alloys are used to coat other metals or are used as castings wherever their unique erosion resistance and high temperature properties are needed. They also form the basis of the prosthetic alloys used to produce hip and knee replacement joints. Figure 1 shows a few typical castings and Table 1 compositions of the alloys used. Figure 1 Cobalt-based alloy castings The spray alloys used for plasma or flame spray are in powder form and contain silicon and boron to form a low melting point eutectic which allows fusion with the substrate with minimum distortion. In general, the cobalt-based alloys can be deposited by: a) Welding both rods and strip are available MIG, TIG, submerged arc, oxy-acetylene, etc. b) Plasma/flame spray powders are available for both these processes or rod feed can be used c) They can be cast and used as complete parts or as inserts i.e. titanium hip joint with Co/Cr ball Cobalt Facts, 2006 CDI 12

6 Table 1 Standard Cobalt-Base Alloys Nominal Compositions Co Cr C W Mo Ni Si B Fe Mn Others STELLITE 1 BASE STELLITE 2 BASE STELLITE 4 BASE STELLITE 6 BASE STELLITE 12 BASE STELLITE 20 BASE STELLITE 21 BASE STELLITE 31 BASE STELLITE 190 BASE STELLITE 238 BASE STELLITE 306 BASE Nb = 5 STELLITE 694 BASE V = 1 SF1 BASE Cu = 5 SF6 BASE Cu = 5 SF12 BASE Cu = 5 SF20 BASE Cu = 5 STELLITE 250 BASE STELLITE 251 BASE Nb = 2 STELLITE 100 BASE STELLITE 314 BASE Nb = 6 Cu = 2 STELLITE 703 BASE Surgical (ASTM BASE F75/82BSS:3351) TRIBALOY T400 BASE TRIBALOY T800 BASE TRIBALOY T900 BASE 18 < DENTAL A range of alloys to meet national/international compositions Cobalt Facts, 2006 CDI 13

7 4.3 High Speed Steel What is a high-speed steel? One should say before we get carried away, that unfortunately not all high-speed steels contain cobalt, but as we shall see, possibly the newest and the best ones do. The evolution of high-speed cutting tools commenced with Musket s self-hardening tungsten-manganese steel in The usefulness of these steels for cutting was only appreciated in 1900 when Taylor and White developed the forerunner to modern high speed steels. Table 1 typical High-Speed Steel Compositions C Cr Mo W Co V M K M M ASP ASP ASP M = Molybdenum type ASP = Asea Stora Process Steel is of course an iron-carbon alloy. Highspeed steels are also steel but with large additions of refractory metals tungsten, chromium, molybdenum, vanadium and, of course, in specialised cases, cobalt. The other element in steel, namely carbon, forms carbides in carbon steels with just iron and in high-speed steels, with all the alloying additions except cobalt which has other functions. So, in essence, a high speed steels is a steel containing large amounts of refractory carbides which proved hardness, high temperature strength, wear resistance to tempering, with cobalt enhancing high temperature strength (Table 1). How are modern high-speed steels made? As with many other modern alloys, high-speed steels have not changed greatly in basic composition since their conception. What changed are the manufacturing techniques. Structure is of paramount importance in tools steels and the aim is to get a very fine distribution of carbides. To this end, complex heat treatment schedules have been devised, often with two or even three tempering stages. Three current methods of manufacture seem to have evolved: I) air melt cast and work, ii) vacuum melt cast and work, iii) atomise cold isostatic press sinter hot isostatic press (see Cobalt News July 1991) and work. As far as cobalt is concerned, one would suppose that the route did not matter this, to some extent, is true. However, the newer ASP alloys made by method (iii) are superior to other grades and the best of these contain high levels of cobalt (8-10%). Figure 1 Structures at x100 magnification The benefit of the powder route is in the structure. Casting produces segregation by its very nature and further work and heat do little to change it. Atomising a homogeneous molten metal gives such rapid cooling that each miniingot (powder particle) is homogeneous unlike its large cast Cobalt Facts, 2006 CDI 14

8 brethren. The rest of the process is to stick these little ingots back together into a pore-free, homogeneous form. The final structures shown in Figure 1, illustrate the difference and in practice this can mean: a) better stability in heat treatment, b) easier to grind, and c) FASTER CUTTING with higher cutting edge strength. Why is cobalt in high-speed steels? A good question as it doesn t form carbides. The reasons that have brought cobalt to prominence in these latest alloys are the same as they always were. Cobalt dissolved in iron (ferrite and austenite) and strengthens it at the same time imparting high temperature strength (temperature on cutting surfaces can be 850 C. During solution heat treatment (to dissolve the carbides), cobalt helps to resist grain growth so that higher solution temperatures can be used which ensures a higher percentage of carbides being dissolved. Steels are quenched after solution annealing and the structure is then very hard martensite, plus the retained high temperature phase austenite plus carbides peppered throughout the structure. Tempering will precipitate the ultrafine carbides still in solution and maximum hardness will be attained. Here, cobalt plays another important role, in that it delays their coalescence. This is important as it means that during cutting, the structure is stable up to higher temperatures. Thus, cobalt-containing tool steels are capable of retaining strength to higher temperatures They cut faster for longer. Figure 2 Effect of temperature hardness of various types of tool steel Figure 2 is the classic diagram showing the effectiveness of cobalt in retaining strength/hardness at high temperature by resisting tempering and strengthening the matrix. Use, New and Future The types of steels we are talking about are aimed at the tope end of the cutting tool market. The workhorse cutting alloys is M2 and this may well continue. The newer alloys are undoubtedly more expensive initially (NB: ASP30 is four times the cost of M20. It is, however, easy to demonstrate that in many operations, the initial tool cost is of little significance and the benefits of less tool wear, more holes/cuts, etc., soon far outweigh it, so things may chance. Figure 3 shows typical tools gear hobs, mills, taps and drills reamers, broaches, single point tools for parting and final finishing could also be included. Tools, however, are no longer as simple as they were. The surface can be modified by coating with TiN or TiC for example, put on by plasma or vapour deposition. These coatings increase cutting life by large factors (4 and 5 times) and do so even after regrinding. This latter phenomenon looks inexplicable but consider for example a gear hob. One would normally cut its way down a tapering gear tooth and failure (much later on) is by crater wear. Cobalt reduces this wear by increasing hot strength. Even after regrinding, the TiN coating remains on the flanks and life is still increased. Cobalt Facts, 2006 CDI 15

9 The Market In spite of these developments, the trend to achieve high cutting speeds for improved productivity has meant there has been a steady move from high-speed steels to carbide tipped tools in recent years. Figure 3 Typical tools In 2001, the CDI estimated that refined cobalt demand in hardfacing and high speed steels totalled about 2,800 tonnes or 7% of the world market. However, it is unlikely that much refined cobalt finds its way into high speed steels as most is expected to be in the form of cobalt-containing scrap. 4.4 Cobalt Prosthetic Alloys The use of metals to repair and replace parts in the human body, may go back a long way. In 1775, iron wire is reported as having been used to fix a fractured bone. It became known however that not all materials were what we would now term biocompatible. Several metals were used platinum, gold, etc. but in 1924, it was concluded by Zierold that a cobaltchrome alloy had the best combination of properties. In 1937, Vitallium arrived on the scene, this being a CoCrMo alloy which had good strength, corrosion resistance and above all, was tolerated by the body. Cobalt-chrome alloys of the Vitallium type are still used in the production of knees and hips, though they are only used when wear resistance is paramount as they are relatively heavy. Figures 1a & 1b Typical knee and hip components Typical alloys are 62%Co, 30%Cr, 5%Mo and 52%Co, 26%Cr, 14%Ni, 4%Mo. They are therefore basically Stellites being close to HS21 and 31. Cobalt Facts, 2006 CDI 16

10 In recent years, Ti.318 (6Al4V) has moved into the prosthetic field but the Co/Cr alloys have maintained their place and one now sees composite hip joints with CoCr balls and titanium stems. This is also an expanding market with replacement joints being fitted more and more routinely to younger and younger people. Figures 1 and 1b show typical knee and hip components made by casting and powder metallurgy respectively. Figure 2 shows an X-ray of a typical hip insert. Figure 2 X-ray showing cast hip replacement in position 4.5 Controlled Expansion Alloys These alloys are based on nickel/iron alloys with the 36% nickel version demonstrating the lowest expansion coefficient 1 x 10-6 per C. The coefficient is however very temperature dependent (see Figure 1) and even small changes affect it. Other binary alloys such as 48% nickel are more stable but one sacrifices the very low values that can be obtained. Figure 2 shows a general curve, relating L/L the expansion coefficient to temperature and this type of curve applies to all the low expansion materials. Figure 1 Expansion coefficients of nickel-iron-cobalt alloys (0-1 C) Figure 2 Typical expansivity curve of lowexpansion alloys Cobalt Facts, 2006 CDI 17

11 Naturally, we would like a material where the flat part of the curve C-D is at a low level and T 2 the temperature, where the curve inflexes and climbs again is at high level. In practice, substituting cobalt for nickel goes some way towards this. The lowest expansion occurs with 6% Co substitution into the original 36% Ni alloy Invar and we have Super-Invar. In fact, various levels of cobalt can be used to vary L/L and create a series of alloys with specific expansion coefficients (Table 1). Table 1 Low-Expansion Cobalt Alloys Trade or Composition % Coefficient Common of expansion Name Co Fe Ni Cr Mn C 10-6 per C Super-Invar Ditto Super-Nilvar 4-6 Bal 31 Stainless Invar Japanese Invar Fernico Fernichrome Kovar min. min. 4.0 The original alloys Invar, etc. may have had their applications linked to mechanical devices where expansion was a problem like clocks, watches, measuring devices, etc. but the Super-Invar and Kovar now find their uses in the electronic age. They use their controlled expansion to match that of glass for glass to metal seals, and also in the electronic packaging industry where they can match various substrates to provide hermetic seals which can stand the rigours of the +55 C 55 C expansion test and also provide corrosion resistance. 4.6 Cobalt in Steels Cobalt is not an element commonly added to alloy steels. It does have some effects but these are also obtainable by other additives at lower cost and mostly with better results Molybdenum, nickel, etc. We have seen in other areas that cobalt does not form carbides and that in fact, it decreases hardenability (a measure of the depth to which a steel hardens on quenching). It hardens ferrite but only marginally and has only a small influence on the transformation temperature of iron. Figure 1 Effect of cobalt on tempering resistance of martensitic stainless steel (0.1C, 12Cr, 4Mo) The factors above ensure that cobalt is unlikely to ever find a use in high tonnage low alloy steel production. It does however have some niche markets in steel. In martensitic steels, cobalt has the effect of delaying tempering and this can be shown by plotting hardness against a Cobalt Facts, 2006 CDI 18

12 time/temperature parameter as in Figure 1 (T = temperature, t = hours). Increasing cobalt levels produces increased hardness and steels of this type 1%C, 12%Cr, 4%Mo and 7%Co have attractive properties. Some steels such as Jethete M120 have been developed for use at high temperature using the effect of cobalt to give high temperature strength in the range below which superalloys are more usual. The steels where cobalt has found its home (apart from the high speed variety) are termed maraging. This name is derived from the fact that they are aged in the martensitic form. The original steels used were 20/25% nickel steels with small additions of Al, Ti and Nb. The secret of these steels was and is, that simple air-cooling is sufficient to transform the Austenite high temperature phase to Martensite, the hard unstable lower temperature form. On reheating, the temperature to return to Austenite is found to be much higher than the Martensite forming temperature of 250 C and is in fact over 500 C. Reheating (aging) at an intermediate temperature retains the Martensite but allows precipitation of various hardening phases such as Ni 3 Mo, Ni 3 Ti, FeMo and these raise the hardness to up to 900 Vickers. The early steels were found to be brittle and cobalt additions solved this problem. As usual, the role of cobalt is obscure but it enhances the properties and accelerates the process. Table 1 shows a range of typical steels. Table 1 Composition and Properties of Maraging Steels Composition, wt.% UTS Elong. Hardness Grade Ni Co Mo Ti Al 10 3 psi MN/m2 % Rc 18Ni(200) Ni(250) Ni( Ni(350) Ni(400) Their properties are not the highest possible but they score in that they can be air-cooled without distortion, machined without difficulty and finally, develop their properties with a relatively simple low temperature aging process. Maraging steels have found many uses in the aerospace and military industries where their strength coupled with workability has got them the job over possibly stronger materials. Typical applications are landing gears, arrestor hooks, torque shafts, rocket motor casings, gun barrels, bolts, fasteners, extrusion arms, etc., etc. There are the areas where cobalt steels are best. 4.7 Other Alloys Cobalt is used in other alloys, Co/Pt magnets, 36%Ni12%Cr spring alloys which can be varied to provide given temperature expansion coefficients against elastic moduli. The main uses however are covered in the preceding sections. One use left until last is the role of cobalt in cancer treatment and flaw detection with the 60 Co isotope. Cobalt-59 is irradiated in a reactor for a long period and some of the metal is converted to 60 Co. This isotope has a half-life of 5.3 years and emits γ-rays as it decays. These rays can be used in portable machines in lieu of X-rays for flaw detection. They have the advantage of portability and greater penetration over X-rays tough they do not provide quite the same definition in photographic terms. The rays can also be targeted at cancer cells and used to destroy them. This is the basis of radiation therapy in cancer treatment. Food also has its life prolonged by irradiation after packing. Cobalt Facts, 2006 CDI 19

13 4.8 Further Reading 1. Cobalt in Superalloys CDI Cobalt and its Alloys w. Betteridge (Horwood ltd) Superalloys II Sims & Hagel (Wiley) 4. The role of cobalt in wear/corrosion resistant materials THE Cobalt Conference 1998, CDI 5. Powder metallurgical processing improves high carbon Co-Cr-Mo alloy orthopaedic implants THE Cobalt Conference 2001, CDI 6. Cobalt in superalloys THE Cobalt Conference 2004, CDI Note: Stellite is a registered trade name of the Deloro Stellite company, Hastelloy is registered by Haynes International and Nimonic by Inco Alloys. Cobalt Facts, 2006 CDI 20

14 Summary Tables Nominal Chemistry and Density of Cast Nickel- and Cobalt-Base Alloys Density Alloy Ni Cr Co Mo W Ta Cb Al Ti Fe Mn Si C B Zr Others lb/ in. 3 g/ cm 3 Nickel-Base Alloys Alloy 713C IN V IN V MM Hf MM Hf MM Hf MM Hf MM Hf René René Udimet Cobalt-Base Alloys FSX MAR-M Mar-M X-40/X Cobalt Facts, 2006 CDI 21

15 Nominal Chemistry and Density of Wrought Nickel-, Cobalt- and Iron-Base Alloys Alloy Ni Cr Co Mo W Ta Cb Al Ti Fe Mn Si C B Zr Others lb/ in. 3 g/ cm 3 Cobalt-Base Alloys Haynes MAR-M MP35N Iron-Base Alloys Haynes N 0.02La Incoloy N N Nominal Chemistry and Density of Wrought Nickel-, Cobalt- and Iron-Base Alloys Alloy Ni Cr Co Mo W Ta Cb Al Ti Fe Mn Si C B Zr Others lb/ in. 3 g/ cm 3 Nickel-Base Alloys Hastelloy V C-22 Hastelloy V C-276 Hastelloy Cu G-30 Hastelloy S La Hastelloy X IN V Inconel Inconel Nimonic Nimonic Nimonic Nimonic René Udimet Udimet Udimet Waspaloy Cobalt Facts, 2006 CDI 22