Alloy 601, designated UNS N06601, is a nickel-chromium-iron heat-resisting alloy whose thermal expansion behaviour is one of the first properties a design engineer looks up. Expansion data for this grade is frequently quoted as a single number, which is misleading: the coefficient of thermal expansion is only meaningful when the temperature interval it refers to is stated at the same time. This article sets out how the coefficient is defined, how the values for Alloy 601 vary with temperature, and what that means for furnace, burner and process equipment design.
Alloy 601 (UNS N06601) at a Glance
The alloy combines a nickel-rich austenitic matrix with a relatively high chromium content and a controlled aluminium addition. The aluminium forms a tightly adherent oxide layer at high temperature, and that layer is the reason the grade is chosen for furnace internals, radiant tubes, retorts and combustion components. It is a solid-solution alloy: it cannot be strengthened by aging, and it is supplied in the annealed condition. The European designation is NiCr23Fe with Werkstoff number 2.4851, and the relevant specifications are ASTM B166 and ASTM B167 with their ASME equivalents SB-166 and SB-167.
| Property | Reference information |
|---|---|
| UNS number | N06601 |
| EN / DIN designation | NiCr23Fe, Werkstoff 2.4851 |
| Product standards | ASTM B166 / ASME SB-166 for rod and bar; ASTM B167 / ASME SB-167 for seamless tube and pipe |
| Delivery condition | Annealed; the grade is not age hardenable |
| Typical product forms | Tube, pipe, sheet, plate, rod, bar, wire and forgings |
What the Thermal Expansion Coefficient Actually Means
The linear coefficient of thermal expansion expresses how much a material lengthens per unit length for each degree of temperature rise. For a nickel alloy it is normally reported as the mean coefficient over a stated interval, for example from 20 degrees Celsius to 100 degrees Celsius, and it is written in micrometres per metre per degree Celsius. Because the mean coefficient is an average over the interval, it rises as the upper temperature of the interval rises: the same alloy will show a low value for a narrow room-temperature window and a noticeably higher value when the interval is extended to 1000 degrees Celsius.
That is the single most common error in published expansion tables for heat-resisting alloys. A figure such as 13 micrometres per metre per degree Celsius belongs to the room-temperature to about 100 degrees Celsius interval; it is not the mean coefficient over a span reaching 1000 degrees Celsius. Quoting one number for the whole range understates the expansion at working temperature and can lead to insufficient clearance in supports, wrong cold-set on spring hangers and excessive thermal stress in restrained pipework.
Thermal Expansion of Alloy 601 Across Temperature
Published datasheet values for Alloy 601 follow a consistent pattern: the mean coefficient increases steadily as the upper limit of the interval increases, and the total expansion over a wide range is best calculated by integration rather than by multiplying one coefficient by the whole temperature difference.
| Interval | Commonly published mean coefficient (micrometre per metre per degree Celsius) |
|---|---|
| 20 - 100 degrees Celsius | approximately 13.1 - 13.8 |
| 20 - 200 degrees Celsius | approximately 13.9 - 14.2 |
| 20 - 400 degrees Celsius | approximately 14.4 - 14.8 |
| 20 - 600 degrees Celsius | approximately 15.0 - 15.4 |
| 20 - 800 degrees Celsius | approximately 15.7 - 16.0 |
| 20 - 1000 degrees Celsius | approximately 16.1 - 16.4 |
Ranges are given rather than a single figure because published tables differ slightly according to the product form, the test method and the edition of the source standard. For design work the expansion value should always be taken from the interval-matched table of the standard edition that governs the purchase, and the same source should be used consistently throughout a calculation.
The other physical properties of the grade are equally temperature dependent. Room-temperature density is approximately 8.1 grams per cubic centimetre, thermal conductivity is around 11 watts per metre kelvin at room temperature and rises with temperature, and elastic modulus falls as temperature increases, which together with the expansion data defines the thermal stress behaviour of the component.
Chemical Composition of Alloy 601 and a Note on Grade Data
The composition ranges below are those specified for UNS N06601 in the applicable nickel alloy product standards.
| Element | Specified range |
|---|---|
| Ni | 58.0 - 63.0 |
| Cr | 21.0 - 25.0 |
| Fe | Balance |
| Al | 1.0 - 1.7 |
| C | 0.10 max |
| Mn | 1.0 max |
| Si | 0.5 max |
| Cu | 1.0 max |
| S | 0.015 max |
Two common transcription errors appear in secondary sources for this grade and should be avoided when specifications are copied. The first is a copper figure written as a nominal one percent: copper is a controlled residual with a maximum of one percent, not an intentional addition at that level. The second is the appearance of titanium as a specified element; titanium is not part of the specified composition range for UNS N06601, and its presence in a table usually indicates that data for another nickel alloy has been mixed in. Aluminium, not titanium, is the element that produces the protective oxide scale in this grade.
Heat Treatment and High-Temperature Service
Alloy 601 is a solid-solution alloy and is not strengthened by aging. Published recommendations for the grade therefore centre on annealing or solution annealing in the region of 1100 to 1150 degrees Celsius followed by rapid cooling, which softens the material after cold working and restores a uniform structure. Statements that describe an aging treatment for this grade are incorrect and should be disregarded; there is no precipitation-hardening response to exploit.
In service the alloy forms a thin, adherent, self-healing oxide layer that protects it in oxidising atmospheres. Published datasheet guidance places continuous service capability in oxidising conditions at up to roughly 1150 degrees Celsius, with lower limits of about 1000 degrees Celsius recommended for cyclic conditions where the oxide must survive repeated thermal shock. Resistance falls in strongly reducing or sulphidising atmospheres and in environments where molten salts or heavy carbon deposition are present, so the actual furnace atmosphere must always be checked against the material guidance.
Design and Fabrication Consequences of Expansion
Support and guide clearances must be calculated with the interval-matched expansion data for the actual metal temperature.
Restrained pipework and tube assemblies need a thermal stress check, because the load generated is proportional to expansion and to elastic modulus, which changes with temperature.
Dissimilar joints between Alloy 601 and austenitic stainless steel or carbon steel should be located where thermal gradients are moderate.
Long components such as radiant tubes and muffle linings should be designed with allowance for growth rather than with tight fits.
Hot forming and welding are followed by a full anneal where the specification requires the original creep and oxidation performance to be restored.
Typical Applications
The grade is found in furnace muffles, radiant tubes, retorts, annealing baskets, thermocouple protection tubes, burner nozzles, heat exchanger components in high-temperature processes, and combustion chamber and afterburner parts. It is also used in the chemical and petrochemical industries for equipment handling high-temperature oxidising streams, and in heat treatment fixtures where resistance to scaling and to repeated thermal cycling determines service life.
Frequently Asked Questions
Q: What is the thermal expansion coefficient of Alloy 601?
The mean coefficient depends on the temperature interval. Commonly published values are about 13.1 to 13.8 micrometres per metre per degree Celsius for 20 to 100 degrees Celsius, rising to about 16.1 to 16.4 when the interval is extended to 1000 degrees Celsius.
Q: Why do different sources give different expansion figures for the same grade?
Because a single number is only valid for the interval it was measured over. Product form, test method and the edition of the source standard also introduce small differences, so interval-matched data from one consistent source should be used for design.
Q: Can Alloy 601 be strengthened by aging?
No. It is a solid-solution strengthened austenitic alloy with no precipitation-hardening response. Recommended heat treatment is annealing or solution annealing at about 1100 to 1150 degrees Celsius followed by rapid cooling.
Q: Which standards apply to the grade?
UNS N06601 is covered by ASTM B166 and ASME SB-166 for rod and bar and by ASTM B167 and ASME SB-167 for seamless tube and pipe, with the European designation NiCr23Fe and Werkstoff number 2.4851.
Q: What temperature can the alloy withstand in service?
Published datasheet guidance indicates continuous service in oxidising atmospheres up to roughly 1150 degrees Celsius, with about 1000 degrees Celsius recommended under cyclic conditions. Reducing, sulphidising and molten salt environments reduce these limits.
Q: How should expansion be handled in design calculations?
Use the mean coefficient for the actual metal temperature interval, integrate expansion over the temperature profile where the gradient is steep, and combine the result with the temperature-dependent elastic modulus to obtain the thermal stress.
