In this article, I will introduce what Hastelloy, Monel and Inconel are, how the three families differ, where each one is specified, and the questions a buyer should answer before the alloy is ordered.
What are Hastelloy, Monel and Inconel?
Hastelloy, Monel and Inconel are trade names for families of nickel alloys. They are not stainless grades with a new label. Hastelloy is a Haynes family, generally nickel with molybdenum and chromium, used where the chemical is more aggressive than a stainless grade was selected for. A widely specified grade in that family is C-276, a nickel-molybdenum-chromium alloy known for reducing acids. The drawing may name a different Hastelloy grade. Only that grade is the order.
Monel is a nickel-copper family. Monel 400 is the common grade, used in seawater, some hydrofluoric and alkaline services, and other duties where a nickel-copper alloy is already on the drawing. Inconel is a nickel-chromium family used where corrosion resistance and high temperature sit together. Alloy 625 is a common corrosion and temperature grade. Alloy 718 is a high-strength grade used where the drawing also asks for strength at temperature. A note that says only nickel alloy does not tell the mill which family, or which grade inside that family, to prepare.
How these alloys are supplied
Each family is supplied in the same product forms as stainless: sheet, plate, coil or strip, pipe, and bar. The form changes the mill route. Flat product is rolled. Pipe is welded from strip or made seamless. Bar is a long product for machining. The alloy is melted and processed to the chemistry and the mechanical properties of the grade standard named on the drawing, such as an ASTM B or ASME SB specification. The trade name does not replace that standard.
These alloys cost more than 304 or 316 because of the nickel, and because the grades are made in smaller lots. That is a reason to copy the grade exactly, not a reason to substitute a stainless grade that looks similar on a thickness list. Substitution is a design change. It belongs with the end user, not in the enquiry.
What a correct alloy order protects
The point of naming the family and the grade is to keep the corrosion and temperature behavior the designer already chose.
The chemistry the media was chosen for
C-276, Monel 400 and Inconel 625 do not resist the same chemicals. A strong reducing acid, seawater and a high-temperature oxidizing gas are three different problems. The grade on the drawing is the record of which problem was designed for.
The form the shop can process
A plate quote does not fill a pipe order. Sheet, coil, pipe and bar have different sizes, tests and lead times. Naming the form keeps the offer on the product the shop will cut, weld or machine.
A certificate that can be read
Nickel-alloy lots are accepted against a mill test certificate: chemistry, and the mechanical results the standard requires. If the enquiry names the grade and the standard, the certificate has a line to match. If it says only nickel alloy, the certificate cannot close the order.
When these alloys are specified
Specify a Hastelloy grade when the process fluid is outside the range of the stainless grade already considered, especially strong acids and other severe corrosion, and the drawing names that Hastelloy grade. Specify Monel when the duty is a nickel-copper service such as seawater or a chemical the drawing has already assigned to Monel. Specify Inconel when the part sees high temperature as well as corrosion, or when the drawing asks for a high-strength nickel-chromium grade such as 718.
Do not use these families as a general upgrade from 316. If 316 meets the drawing, the extra nickel is not a benefit the service asked for. Do not use them where a listed stainless grade, such as a duplex or 904L, is the grade the designer already wrote down.
Hastelloy versus Monel versus Inconel
Hastelloy grades are the usual conversation for severe wet corrosion that stainless was not specified for. Monel is the nickel-copper conversation, often seawater and selected chemicals, not a high-temperature alloy. Inconel is the nickel-chromium conversation, where heat and corrosion are both on the drawing. The three names are not a ladder of quality. A higher price does not mean the alloy survives a medium it was not designed for.
Inside one family the grade still matters. C-276 is not every Hastelloy grade. 625 is not 718. The grade comparison on the alloy series is there for buyers who are still matching a designation. Once the drawing prints a UNS number or a trade grade, that is the line to copy.
FAQ
These points stop a nickel-alloy enquiry from being quoted as a generic special metal.
Are these names stainless grades?
No. They are nickel-alloy families. Stainless grades such as 316 remain the right order when the drawing says 316. Moving to a nickel alloy is a different specification.
Is C-276 the same as Hastelloy?
C-276 is one grade in the Hastelloy family. Other grades in that family have different chemistry. Order the grade the drawing names, not the family name alone.
Can Monel replace Inconel?
Not as a shortcut. Monel is nickel-copper. Inconel is nickel-chromium and is the family used when temperature is part of the duty. They are specified for different services.
Which product form should be written?
Sheet, plate, coil, pipe or bar, matching the drawing. Pipe also needs welded or seamless if the drawing says so, plus outside diameter and wall. Bar needs the section and the length.
What does the certificate have to show?
The grade, the standard, the heat chemistry, and the mechanical results that standard requires. Ask for the certificate on the enquiry if the project requires it. Do not assume every lot is supplied with the same pack of reports.
What to send with the enquiry
Name the alloy grade as printed on the drawing, the form, and the standard. Add size, quantity and whether a mill test certificate is required. The alloy series keeps Hastelloy, Monel and Inconel as separate lines. The grade, the size and the certificate are confirmed on the enquiry before the lot is made.