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The nickel-chromium system reveals that chromium is fairly soluble in nickel. It has a maximum soluble rate of 47% at eutectic temperature, which then decreases to approximately 30% at room temperature. Several commercial alloys are based on this solid solution. These alloys have superior resistance to high-temperature corrosion and oxidation, and optimal wear resistance.
Adding small quantities (less than 7%) of chromium to nickel can increase the sensitivity of the alloy to oxidation. The reason behind this is the increase in the diffusion rate of oxygen in the scale. This trend reverses once addition levels go beyond 7% chromium, and increases up to an addition level of about 30%. There is minimal change above this level.
Oxidation resistance can arise due to the formation of an extremely adherent protective scale. The adherence and coherence of the scale can be enhanced by adding small quantities of other reactive elements like cerium, silicon, zirconium, calcium, etc. The scale thus formed is a blend of nickel and chrome oxides (NiO and Cr2O3). These join together and form nickel chromite (NiCr2O4), with a spinel-type structure.
Increased addition of chromium leads to a noticeable increase in electrical resistivity. An addition level of 20% chromium is said to be ideal for electrical resistance wires, which can be used in heating elements.
This composition integrates optimal electrical properties with optimal ductility and strength, making it ideal for wire drawing. Commercial grades include Brightray and Nichrome. Small alterations to this composition may be done to enhance it for specific applications.
The incorporation of suitable reactive alloying elements will influence the properties of the scale. The alloy’s operating conditions largely govern the composition that should be used. Table 1 shows the differences in composition between alloys used for intermittent and continuous applications.
Table 1. Suitable compositions for heating elements used intermittently and continuously.
While the effect of the compositional alterations on the mechanical properties is negligible, increased addition of reactive elements helps to inhibit flaking of the scale during cyclic heating and cooling. This effect does not pose much problem to heating elements that operate nonstop; therefore, it is not necessary for the addition levels to be very high.
The binary 90/10 Ni/Cr alloy is also used for heating elements. This alloy has the highest operating temperature of 1100 °C. Thermocouples are the other applications of this alloy.
The 90/10 Ni/Cr alloy is usually used in thermocouples, together with a 95/5 Ni/Al alloy. This combination, which is referred to as chromel-alumel, has a maximum operating temperature of 1100 °C, similar to heating elements. This thermocouple becomes vulnerable to drift in the region of 1000 °C because of preferential oxidation, following continuous usage for a prolonged period. It has been discovered that the addition of silicon overcomes this effect. Commercial grades include Nisil (which has 4.5% Si and 0.1% Mg) and Nicrosil (which has 14% Cr and 1.5% Si).
High-Temperature Corrosion Resistant Alloys
The 80/20 Ni/Cr alloy has better resistance to hot corrosion and oxidation than inexpensive iron-nickel-chromium alloys. Hence, it is mostly used for cast and wrought parts for high-temperature applications. This alloy is ideal for applications that are prone to oxidation.
Alloys with higher chromium content are more ideal for applications that are prone to fuel ashes and/or deposits such as, alkali metal salts like sulfates. This is because, fuel ashes tend to react with the oxide scale. Ashes that contain vanadium are quite aggressive and have a fluxing effect on the scale, thus increasing the vulnerability of the alloy to degradation caused by oxidation.
In sulfur-containing environments, chromium sulfide (Cr2S3, with a melting point of 1550 °C) is formed preferentially to nickel sulfide. But, formation of nickel sulfide is favored, as this deters the formation of the nickel/nickel-sulfide eutectic that has a low melting point. Ultimately, local chromium supplies can be depleted, leaving sulfur to react with nickel and form the eutectic compound with a low melting point. This results in a liquid phase attack.
Alloys that have undergone this type of attack have wart-like formations on their surface. The preferential formation of chromium sulfides shows that alloys with higher chromium levels are more resilient to this form of attack.
Nickel/chromium alloys with over 30% chromium have a two-phase structure including γ-nickel and α-chromium. Since the α-chromium phase is brittle, the ductility of the alloy decreases with an increase in chromium content.
Table 2 illustrates the properties of certain binary alloys. The addition of approximately 1.5% niobium boosts ductility and strength, while simultaneously minimizing embrittlement after high-temperature exposure, given that impurities such as nitrogen, carbon, and silicon are minimized.
Table 2. Tensile and ductility properties for some Ni/Cr alloys at room temperature
|Cr Content (%)
||Tensile Str (MPa)
Alloys with up to about 35% chromium content can be hot worked. Beyond this level, they are mostly suited only for casting. A specific level of ductility gain can be realized by adding titanium or zirconium. One such example is Inconel 671 (with 48% Cr and 0.35% Ti) that is used in applications like duplex tubing for coal-fired superheating tubing.
Wear mechanisms are complicated; however, good corrosion resistance and high hardness add to good wear resistance. Ni/Cr alloys offer a cheaper alternative to materials like weld-deposited cobalt-chrome alloys with added tungsten and carbon, that are usually used in wear-resistant applications.
An example of a Ni/Cr alloy for this type of application is an alloy containing 8%–12% Cr, 1%–4% Fe, 3%–4% Si, 0.3%–1.0% C, 1.5%–2.5% B, and the remaining portion Ni. The hardness of a coating of this material deposited by inert gas shielded arc methods would fall in the range of 40 to 50 Rockwell C.