Oct. 28, 2024
Overview
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Whenever corrosion-resistant metal materials are mentioned, stainless steel and nickel alloys (Monel, Inconel, Incoloy and Hastelloy) are the two most common materials that people tend to think of.
However, the same corrosion-resistant materials, can you tell the difference between them? Why do their prices vary so much? Which material should you use?
In this article, we will introduce the differences between nickel alloys and stainless steel in detail from six aspects: operating temperature, chemical composition, mechanical properties, corrosion resistance, application and price.
Chemical Composition
First of all, we need to understand the chemical composition of the two metals.
From the point of view of chemical composition, stainless steel mainly belongs to iron alloy. Alloys such as Monel, Inconel, Incoloy and Hastelloy are mainly nickel alloys.
Therefore, the main component of stainless steel is iron. Its iron content can reach more than 65%. The iron content of some ferritic and martensitic stainless steels even reaches more than 80%. In contrast, nickel alloys tend to contain less than 50% iron. Certain nickel alloys such as Hastelloy C- have even less than 3% iron.
The nickel content of nickel alloys such as Monel, Inconel, Incoloy and Hastelloy is often greater than 30%, and can reach more than 70%. The nickel content of stainless steel is less than 30%. Partially ferritic and martensitic stainless steels are even completely nickel-free.
Both stainless steel and nickel alloys contain around 20% chromium. Chromium is a very effective anti-corrosion element. It is also a major source of corrosion resistance for stainless steel and nickel alloys.
Molybdenum may also be present in some stainless steels and nickel alloys. Nickel alloys tend to contain more molybdenum than stainless steels.
Ni: 30% ~ 75%
Cr: 0% ~ 35%
Mo: 0% ~ 32%
Fe: 1% ~ 39%
Ni: 0% ~ 30%
Cr: 10% ~ 29%
Mo: 0% ~ 8%
Fe: 45% ~ 86%
In addition to the above differences, the composition of nickel alloys is often more complex than that of stainless steel. The metal element content of the former can reach more than 20 kinds at most, while the element types of stainless steel are often below 10. In addition, the control of trace elements in nickel alloys is often stricter than that of stainless steel.
C
Si
Mn
P
S
Ni
Cr
Mo
Al
Ti
Nb
Ta
Cu
W
N
B
Co
V
Zr
Y
C
Si
Mn
P
S
Ni
Cr
Mo
Al
Ti
Nb
Ta
Cu
W
N
Operating Temperature
High
Temperature
Room
Temperature
The service temperature is the biggest difference between stainless steel and nickel alloy.
In general, stainless steel tends to be used for room temperature applications. Nickel alloys such as Inconel, Incoloy and Hastelloy are basically used in high temperature environments.
One of the most important factors determining this characteristic is the nickel content. Since the iron element cannot maintain a stable structure at high temperature, it is difficult to guarantee the performance of stainless steel with iron as the main element at high temperature. Nickel can perfectly maintain the austenitic structure of the alloy at high temperature. Therefore, only nickel-based alloys can perform well at high temperatures. Although nickel is also present in some austenitic stainless steels, their nickel content is still low.
In addition, Monel alloys are often used at room temperature because the corrosion resistance of copper elements decreases at high temperatures.
Corrosion Resistance
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The main elements that determine the corrosion resistance of stainless steel and nickel alloys are nickel, chromium and molybdenum.
Chromium forms an oxide film on the metal surface when it comes into contact with oxygen. This layer of oxide film can effectively prevent the alloy from being further oxidized. The difference between the chromium content of nickel alloy and stainless steel is not very large. Therefore, their antioxidant properties are both excellent.
It is nickel and molybdenum that really make the difference between nickel alloys and stainless steels.
Nickel itself is an excellent corrosion-resistant element, unlike iron, which is easily corroded in the air. Therefore, nickel alloys naturally have a better corrosion resistance foundation than stainless steel.
In addition, molybdenum is a metal element with good resistance to reduction corrosion. As mentioned above, nickel alloys tend to have higher molybdenum content. A typical alloy is Hastelloy B-3. It has a molybdenum content of up to 30%.
In summary, nickel alloys are superior to stainless steel in comprehensive corrosion resistance.
Mechanical Properties
When it comes to the mechanical properties of two materials, it is far more complicated than we think. In stainless steel and nickel alloys, there are both higher strength materials and lower strength materials. Therefore, we cannot make a comparison as a whole.
First of all, in stainless steel and nickel alloys, there are some materials that can greatly increase the strength of the material through precipitation strengthening.
In stainless steels, they are called precipitation hardening stainless steel, such as 17-4PH.
In nickel alloys, they are called precipitation strengthening alloy, such as Inconel 718.
The strength of these alloys far exceeds that of ordinary materials. Among them, precipitation strengthened alloys can maintain high strength at high temperatures. The high strength of precipitation hardening stainless steels will fail at high temperatures. In practical applications, precipitation strengthened alloys are used more frequently than precipitation hardening stainless steels.
Monel K-500 (UNS N)
Inconel 718 (UNS N)
Inconel X-750 (UNS N)
Incoloy A-286 (UNS S)
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15-5PH (UNS S)
PH15-7Mo (UNS S)
17-4PH (UNS S)
17-7PH (UNS S)
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For ordinary stainless steels and nickel alloys, we also need to discuss the situation.
First, we tend to compare austenitic stainless steels with general nickel alloys. Because their structure is very similar. Of the two materials, nickel alloys can be strengthened by adding more solid solution elements. Therefore its strength tends to be higher than that of austenitic stainless steel. Likewise, it maintains mechanical properties better than austenitic stainless steels at elevated temperatures.
Monel 400 (UNS N)
Inconel 600 (UNS N)
Incoloy 800 (UNS N)
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Hastelloy C-276 (UNS N)
...
201 (UNS S)
304 (UNS S)
316 (UNS S)
310S (UNS S)
...
In addition, there is another type of stainless steel called martensitic stainless steel. This stainless steel can be strengthened by quenching. After quenching, martensitic stainless steels will be stronger than common nickel alloys. However, the high strength of this type of stainless steel is also only maintained at room temperature. Martensitic stainless steels also tend to sacrifice some corrosion resistance.
Applications
Nickel alloys have a lot of overlap with stainless steel in terms of applications. Because they are all corrosion-resistant alloys. There are three main differences between them in application.
First, as mentioned above, stainless steel is not suitable for high temperature applications. Therefore, nickel alloys are required for most high temperature applications. Such as: engines, reactors of nuclear power plants, deep oil wells, etc.
Secondly, stainless steel has a wide range of applications in the civilian field. Such as: tableware, medical treatment, construction, etc. Nickel alloys such as Monel, Inconel, Incoloy and Hastelloy are basically only used in industrial and military fields.
Finally, the corrosion resistance of stainless steel is limited to oxidizing environments. If the corrosive environment becomes special and complicated, most stainless steels cannot resist this kind of corrosion. Nickel alloys have developed various targeted grades according to different corrosive environments.
Price
high
LOW
Needless to say, the price of nickel alloys is much higher than that of stainless steel. This is determined by the huge difference between nickel and iron prices. Secondly, the price of molybdenum is even several times higher than that of nickel. Nickel alloys, which are generally higher in molybdenum, are therefore more expensive.
In addition, in the normal state, the strength of nickel alloy is higher than that of stainless steel. This also caused an increase in the difficulty of processing nickel alloys. This high processing difficulty is also an important factor in making nickel alloys more expensive.
Finally, due to the more complex composition of nickel alloy and stricter control of trace elements, it also has higher requirements for raw materials. This also increases the price of the alloy.
FAQ
How should I decide which material to use?
We suggest to use materials with lower price if the performance is sufficient. For example, when you need building materials, we recommend the less expensive stainless steel, although nickel alloys are also perfectly fine. But if you need to use materials on aeroengine blades, we recommend nickel alloys more because stainless steel cannot withstand such a large pressure at such a high temperature.
What is the difference between Monel, Inconel, Incoloy and Hastelloy?
In general, they are all nickel alloys. Monel has more copper elements and is suitable for normal temperature marine fields. Inconel and Incoloy have more chromium, and they are suitable for corrosion-resistant applications at high temperatures. Incoloy has more iron content than Inconel, so it costs less. On the basis of Inconel, Hastelloy adds more molybdenum, which improves its overall corrosion resistance.
What are the most commonly used stainless steels and nickel alloys?
The most commonly used stainless steels are: 304(L), 316(L, Ti), 321, 310S, & 904L. The most commonly used nickel alloys are: Monel 400, Inconel 600, Inconel 625, Inconel 718, Incoloy 800, Incoloy 825 & Hastelloy C-276.
Conclusion
Both stainless steel and nickel alloys are corrosion resistant materials. Nickel alloys perform better than stainless steel in most cases. An important difference between them is that stainless steel is often used at room temperature, and nickel alloy is often used at high temperature. Of course, the better performance of nickel alloys will inevitably bring higher prices.
We offer a variety of nickel alloy materials for you to choose from. If you have any inquiries, you can contact us by .
Inconel is a nickel-chromium-based superalloy often utilized in extreme environments where components are subjected to high temperature, pressure or mechanical loads. Inconel alloys are oxidation- and corrosion-resistant. When heated, Inconel forms a thick, stable, passivating oxide layer protecting the surface from further attack. Inconel retains strength over a wide temperature range, attractive for high-temperature applications where aluminum and steel would succumb to creep as a result of thermally-induced crystal vacancies. Inconel's high-temperature strength is developed by solid solution strengthening or precipitation hardening, depending on the alloy.[1][2]
Inconel alloys are typically used in high temperature applications. Common trade names for various Inconel alloys include:
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The Inconel family of alloys was first developed before December , when its trademark was registered by the US company International Nickel Company of Delaware and New York.[5][6] A significant early use was found in support of the development of the Whittle jet engine,[7] during the s by research teams at Henry Wiggin & Co of Hereford, England a subsidiary of the Mond Nickel Company,[8] which merged with Inco in . The Hereford Works and its properties including the Inconel trademark were acquired in by Special Metals Corporation.[9]
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Inconel alloys vary widely in their compositions, but all are predominantly nickel, with chromium as the second element.
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When heated, Inconel forms a thick and stable passivating oxide layer protecting the surface from further attack. Inconel retains strength over a wide temperature range, attractive for high-temperature applications where aluminium and steel would succumb to creep as a result of thermally induced crystal vacancies (see Arrhenius equation). Inconel's high temperature strength is developed by solid solution strengthening or precipitation strengthening, depending on the alloy. In age-hardening or precipitation-strengthening varieties, small amounts of niobium combine with nickel to form the intermetallic compound Ni3Nb or gamma double prime (γ&#;). Gamma prime forms small cubic crystals that inhibit slip and creep effectively at elevated temperatures. The formation of gamma-prime crystals increases over time, especially after three hours of a heat exposure of 850 °C (1,560 °F), and continues to grow after 72 hours of exposure.[22]
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The most prevalent hardening mechanisms for Inconel alloys are precipitate strengthening and solid solution strengthening. In Inconel alloys, one of the two often dominates. For alloys like Inconel 718, precipitate strengthening is the main strengthening mechanism. The majority of strengthening comes from the presence of gamma double prime (γ&#;) precipitates.[23][24][25][26] Inconel alloys have a γ matrix phase with an FCC structure.[25][27][28][29] γ&#; precipitates are made of Ni and Nb, specifically with a Ni3Nb composition. These precipitates are fine, coherent, disk-shaped, intermetallic particles with a tetragonal structure.[24][25][26][27][30][31][32][33]
Secondary precipitate strengthening comes from gamma prime (γ') precipitates. The γ' phase can appear in multiple compositions such as Ni3(Al, Ti).[24][25][26] The precipitate phase is coherent and has an FCC structure, like the γ matrix;[33][27][30][31][32] The γ' phase is much less prevalent than γ&#;. The volume fraction of the γ&#; and γ' phases are approximately 15% and 4% after precipitation, respectively.[24][25] Because of the coherency between the γ matrix and the γ' and γ&#; precipitates, strain fields exist that obstruct the motion of dislocations. The prevalence of carbides with MX(Nb, Ti)(C, N) compositions also helps to strengthen the material.[25] For precipitate strengthening, elements like niobium, titanium, and tantalum play a crucial role.[34]
Because the γ&#; phase is metastable, over-aging can result in the transformation of γ&#; phase precipitates to delta (δ) phase precipitates, their stable counterparts.[25][27] The δ phase has an orthorhombic structure, a Ni3(Nb, Mo, Ti) composition, and is incoherent.[35][29] As a result, the transformation of γ&#; to δ in Inconel alloys leads to the loss of coherency strengthening, making for a weaker material. That being said, in appropriate quantities, the δ phase is responsible for grain boundary pinning and strengthening.[33][32][29]
Another common phase in Inconel alloys is the Laves intermetallic phase. Its compositions are (Ni, Cr, Fe)x(Nb, Mo, Ti)y and NiyNb, it is brittle, and its presence can be detrimental to the mechanical behavior of Inconel alloys.[27][33][36] Sites with large amounts of Laves phase are prone to crack propagation because of their higher potential for stress concentration.[31] Additionally, due to its high Nb, Mo, and Ti content, the Laves phase can exhaust the matrix of these elements, ultimately making precipitate and solid-solution strengthening more difficult.[32][36][28]
For alloys like Inconel 625, solid-solution hardening is the main strengthening mechanism. Elements like Mo [clarification needed] are important in this process. Nb and Ta can also contribute to solid solution strengthening to a lesser extent.[34] In solid solution strengthening, Mo atoms are substituted into the γ matrix of Inconel alloys. Because Mo atoms have a significantly larger radius than those of Ni (209 pm and 163 pm, respectively), the substitution creates strain fields in the crystal lattice, which hinder the motion of dislocations, ultimately strengthening the material.
The combination of elemental composition and strengthening mechanisms is why Inconel alloys can maintain their favorable mechanical and physical properties, such as high strength and fatigue resistance, at elevated temperatures, specifically those up to 650°C.[23]
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Inconel is a difficult metal to shape and to machine using traditional cold forming techniques due to rapid work hardening. After the first machining pass, work hardening tends to plastically deform either the workpiece or the tool on subsequent passes. For this reason, age-hardened Inconels such as 718 are typically machined using an aggressive but slow cut with a hard tool, minimizing the number of passes required. Alternatively, the majority of the machining can be performed with the workpiece in a "solutionized" form,[clarification needed] with only the final steps being performed after age hardening. However some claim[who?] that Inconel can be machined extremely quickly with very fast spindle speeds using a multifluted ceramic tool with small width of cut at high feed rates as this causes localized heating and softening in front of the flute.
External threads are machined using a lathe to "single-point" the threads or by rolling the threads in the solution treated condition (for hardenable alloys) using a screw machine. Inconel 718 can also be roll-threaded after full aging by using induction heat to 700 °C (1,290 °F) without increasing the grain size.[citation needed] Holes with internal threads are made by threadmilling. Internal threads can also be formed using a sinker electrical discharge machining (EDM).[citation needed]
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Welding of some Inconel alloys (especially the gamma prime precipitation hardened family; e.g., Waspaloy and X-750) can be difficult due to cracking and microstructural segregation of alloying elements in the heat-affected zone. However, several alloys such as 625 and 718 have been designed to overcome these problems. The most common welding methods are gas tungsten arc welding and electron-beam welding.[37]
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Delphin 3.0 rocket engine, used on Astra Rocket. 3D-printed in InconelInconel is often encountered in extreme environments. It is common in gas turbine blades, seals, and combustors, as well as turbocharger rotors and seals, electric submersible well pump motor shafts, high temperature fasteners, chemical processing and pressure vessels, heat exchanger tubing, steam generators and core components in nuclear pressurized water reactors,[38] natural gas processing with contaminants such as H2S and CO2, firearm sound suppressor blast baffles, and Formula One, NASCAR, NHRA, and APR, LLC exhaust systems.[39][40] It is also used in the turbo system of the 3rd generation Mazda RX7, and the exhaust systems of high powered Wankel engine and Norton motorcycles where exhaust temperatures reach more than 1,000 °C (1,830 °F).[41] Inconel is increasingly used in the boilers of waste incinerators.[42] The Joint European Torus and DIII-D tokamaks' vacuum vessels are made of Inconel.[43] Inconel 718 is commonly used for cryogenic storage tanks, downhole shafts, wellhead parts,[44] and in the aerospace industry -- where it has become a prime candidate material for constructing heat resistant turbines.[45]
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Rolled Inconel was frequently used as the recording medium by engraving in black box recorders on aircraft.[64]
Alternatives to the use of Inconel in chemical applications such as scrubbers, columns, reactors, and pipes are Hastelloy, perfluoroalkoxy (PFA) lined carbon steel or fiber reinforced plastic.
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Alloys of Inconel include:
In age hardening or precipitation strengthening varieties, alloying additions of aluminum and titanium combine with nickel to form the intermetallic compound Ni3(Ti,Al) or gamma prime (γ&#;). Gamma prime forms small cubic crystals that inhibit slip and creep effectively at elevated temperatures.
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