Wednesday, July 17, 2013
Nickel Chrome Alloys
The nickel-chromium system shows that chromium is quite soluble in nickel. This is a maximum at 47% at the eutectic temperature and drops off to about 30% at room temperature. A range of commercial nickel chromium alloys is based on this solid solution. Such nickel chromium alloys have excellent resistance to high temperature oxidation and corrosion and good wear resistance.
The introduction of small amounts (<7%) of chromium to nickel increase the sensitivity of the nickel chromium alloy to oxidation. This is because the diffusion rate of oxygen in the scale is increased. This trend reverses after addition levels increase above 7% chromium and increases up to an addition level of approximately 30%. Above this level, there is little change.
Oxidation resistance can be attributed to the formation of a highly adherent protective scale. The adherence and coherence of the scale can be improved by the addition of small amounts of other reactive elements such as zirconium, silicon, cerium, calcium or similar. The scale thus formed is a mixture of nickel and chrome oxides (NiO and Cr2O3). These combine to form nickel chromite (NiCr2O4), which has a spinel-type structure.
A marked increase in electrical resistivity is observed with increasing chromium additions. An addition level of 20% chromium is considered the optimum for electrical resistance wires suitable for heating elements. This composition combines good electrical properties with good strength and ductility, making it suitable for wire drawing. Commercial grades include Nichrome and Brightray. Small modifications of to this composition may be made to optimise it for particular applications.
The addition of the appropriate reactive alloying elements will affect the properties of the scale. The operating conditions of the alloy will largely influence the composition that should be used. Table 1 outlines the compositional differences between alloys used for intermittent and continuous usages.
While the compositional changes have a negligible effect on mechanical properties, higher additions of reactive elements tend to prevent flaking of the scale during cyclic heating and cooling. This effect is less of an issue with continuously operating heating elements, so addition levels do not need to be as high.
The binary 90/10 Ni/Cr alloy is also used for heating elements, and has a maximum operating temperature of 1100°C. Other uses for this alloy are thermocouples.
The 90/10 nickel chromium alloy is commonly used in thermocouples, in conjunction with a 95/5 Ni/Al alloy. This combination is called chromel-alumel, and similar to heating elements has a maximum operating temperature of 1100°C. This couple becomes susceptible to drift in the region of 1000°C due to preferential oxidation after prolonged usage. The addition of silicon has been found to overcome this effect. Commercial grades include Nicrosil (containing 14% Cr and 1.5% Si) and Nisil (containing 4.5% Si and 0.1% Mg).
High Temperature Corrosion Resistant Alloys
The 80/20 nickel chromium alloy is often used for wrought and cast parts for high temperature applications, as it has better oxidation and hot corrosion resistance compared to cheaper iron-nickel-chromium alloys. This nickel chromium alloy is highly suited to applications that are subject to oxidation.
In applications where fuel ashes, and/or deposits such as alkali metal salts such as sulphates are encountered, higher chromium content alloys are more suitable. This is because fuel ashes react with the oxide scale. Ashes containing vanadium are particularly aggressive in the respect and have a fluxing effect on the scale, increasing the susceptibility of the alloy to attack by oxidation.
In sulphur containing environments, chromium sulphide (Cr2S3, melting point 1550°C) is formed preferentially to nickel sulphide. Formation of nickel sulphide is preferred as this hinders the formation of the nickel/nickel-sulphide eutectic which has a low melting point. Eventually, local chromium supplies can be exhausted, leaving sulphur to react with nickel to form the low melting point eutectic compound, leading to liquid phase attack. Alloys that have suffered this form of attack have wart-like growths on their surface. Due to the preferential formation of chromium sulphides, it follows that higher chromium containing nickel chromium alloys are more resistant to this type of attack.
Nickel/chromium alloys containing more than 30% chromium have a two phase structure which consists of a-chromium and γ-nickel. The a-chromium phase brittle and hence the alloy decreases in ductility with increasing chromium content. Properties for some binary alloys are given in table 2. The addition of about 1.5% niobium induces improved strength and ductility, while at the same time reducing embrittlement after high temperature exposure provided impurities such as carbon, nitrogen and silicon are controlled.
Alloys with chromium contents up to approximately 35% are suitable for hot working. Above this level, they are generally only suited to casting. Some ductility gains can be achieved by the addition of zirconium or titanium. Inconel 671, (containing 48% Cr and 0.35% Ti is such an alloy and is used in applications including duplex tubing for coal-fired superheating tubing.
Wear Resistant Alloys
Wear mechanisms are complex, but high hardness and good corrosion resistance contribute to good wear resistance. nickel chromium alloys provide an economical alternative to materials such as weld deposited cobalt-chrome alloys with additions of carbon and tungsten which are commonly used wear resistant applications. An example of a nickel chromium alloy for this type of application is 8-12% Cr, 0.3-1.0% C, 3-4% Si, 1.5-2.5% B, 1-4% Fe and the balance Ni. A coating of this material deposited by inert gas shielded arc techniques would be in the range 40-50 Rockwell C.