0Cr13Al6Mo2 high resistance electric heating alloy is a high-quality and efficient electric heating element material

The name of this alloy is its recipe, and reading it is the fastest way to understand what it can and cannot do. The zero in front of the chromium means a very low carbon content, which is a statement about weldability and about ductility after service. The 13 is the chromium level, which is low for a heating alloy. The 6 is aluminium, which is high, and the 2 is molybdenum.

Each of those three numbers buys something and costs something. Chromium forms the oxide that protects the wire, and it is the expensive element in the recipe, so lowering it lowers the metal cost. Aluminium forms an oxide that protects at least as well and costs far less, so raising it raises the service temperature without raising the alloy cost. Molybdenum raises the resistivity and helps the strength at temperature. The cost of the aluminium is paid in ductility, and that is the property a buyer notices first.

This page places the grade against the published family it belongs to, shows what the family data says about the aluminium and chromium trade, and sets out the two things a specification has to say about a high aluminium alloy that it does not have to say about a nickel-chromium grade.

The trade between chromium and aluminium is the central fact of the iron-chromium-aluminium family, and it is visible in the published data for the grades around this one. The table below reproduces the composition and property values for six grades from the national standard for high resistance heating alloys.

Three regularities can be read straight off the table, and they explain the design of the grade this page is about.

The first is that the maximum element temperature rises with aluminium. The grade at 3.0 to 4.2 % aluminium is rated to 1,100 °C, and the grades at 5.0 to 7.0 % are rated from 1,250 to 1,400 °C depending on chromium and on the niobium or molybdenum addition. Aluminium is the element that buys temperature in this family, and it buys it more cheaply than chromium does.

The second is that resistivity rises with aluminium and with molybdenum. The low aluminium grade sits at 1.23 μΩ·m, the 5 to 7 % grades sit at 1.42 to 1.43, and the grade with 2 % molybdenum sits at 1.53. A higher resistivity means a shorter wire for the same power at the same voltage, which is a real saving in a heating element.

The third is that density falls as aluminium rises, from 7.40 g/cm3 in the low aluminium grade to 7.10 in the high aluminium grades. Aluminium is the light element here, so a wire of the same diameter weighs less, and the element designer gets slightly less mass for the same length.

Those three regularities also show why a grade with only 13 % chromium can be interesting. Chromium is the expensive part of the recipe, and if aluminium can carry most of the oxidation resistance, then a lower chromium level reduces the metal cost while keeping a useful temperature capability. That is the design logic of 0Cr13Al6Mo2, and it is the reason the grade exists in a family where most members sit above 20 % chromium.

An alloy with 6 % aluminium is a ferritic iron alloy with a large volume of aluminium in solution, and that structure is not ductile at room temperature. The published family data shows the trend before the aluminium reaches 6 %: the elongation at rupture falls from above 16 % in the 4 % aluminium grade to above 10 % in the 7 % aluminium grade with molybdenum, and the reduction of area falls from the 65 to 75 % band toward the lower end of the same band.

The practical consequence is a fabrication rule that a nickel-chromium alloy does not need. A high aluminium iron-chromium wire is normally coiled at temperature or with a minimal bend radius, and a cold bend that a 20 % chromium grade would accept can crack a 6 % aluminium grade. A drawing that specifies a tight coil and a cold forming operation has specified a manufacturing route that the alloy may not survive, and the failure appears as a crack at the inside of the coil and not a short life in service.

There is a second brittleness that belongs in the same paragraph. Iron-chromium-aluminium alloys pass through a brittle range around 475 °C, where a ferritic structure separates into two phases with different chromium contents and the alloy loses toughness. A heating element that cools slowly through that range, or that is held in it, can become brittle without any change in its composition. The published description of the higher temperature grade in the same family names 475 °C as the brittle temperature range, and the same caution applies to any ferritic grade in the family.

The distinction between element temperature and furnace temperature in the third row is the one that causes most arguments about a failed heater. The rated temperature in the standard is the temperature of the element, and the element runs hotter than the furnace because it has to transfer its heat somewhere. A furnace at 1,100 °C can contain an element that is well above its rating if the element is poorly supported or if the load has been lost.

Both films protect, and they behave differently at temperature. A chromium oxide film forms quickly and is thin, and it protects the alloy below the temperature at which chromium oxide begins to evaporate. An aluminium oxide film is more stable at higher temperature, which is why the high aluminium grades in the family reach 1,250 to 1,400 °C while the 13 % chromium grade with 4 % aluminium is rated at 950 °C.

The aluminium oxide film also grows more slowly once it is established, which is the reason for the higher temperature rating. It is less forgiving during the first hours of service, because the film has to form before it protects, and a heating element is usually given a controlled pre oxidation before it is put into service. A supplier that ships a wire without a stated surface condition has left that step to the customer.

That is the second thing a specification should say about a high aluminium alloy and a nickel-chromium grade does not need. The surface condition at delivery decides how the film forms on the first heat, and a wire with drawing marks or residual lubricant forms it unevenly. A bright annealed surface and a pickled surface are not interchangeable for this purpose, and the drawing should name one.

The comparison explains why both families exist. Where the atmosphere is oxidising, the temperature is high, and the element can be supported, the iron-chromium-aluminium grade is cheaper and runs hotter. Where the element is unsupported, where the atmosphere cycles between oxidising and reducing, or where the wire has to be coiled cold, the nickel-chromium grade is the one that survives the fabrication and the service.

The national standard for high resistance heating alloys covers drawn wire, rolled strip, bar, and coiled rod, and the grade is ordered by form as well as by composition. The forms that matter for a heating element are wire for a coiled element, strip for an etched foil or a flat element, and rod for a heavy section.

DLX supplies iron-chromium-aluminium heating alloys in Changzhou, Jiangsu, in wire from 0.025 mm, round bar to 50 mm, and strip from 0.50 mm to 2.5 mm in thickness, together with the nickel-chromium grades used where a high aluminium alloy is the wrong choice. Material is supplied against the standard named on the order, with an EN 10204 3.1 certificate on request and the heat number traceable to the test results.

The two questions that decide a quotation are the element temperature and the fabrication route. A grade that is correct for the temperature can still fail at the coiling operation, and a supplier who knows the bend radius and the coil diameter can say so before the wire is cut.

The zero is a very low carbon content. The 13 is about 13 % chromium, the 6 is about 6 % aluminium, and the 2 is about 2 % molybdenum. The carbon level is kept low so that less chromium is consumed as carbide, the chromium forms the base oxide, the aluminium raises the temperature the oxide can take, and the molybdenum raises the resistivity and the strength at temperature.

Because chromium is the expensive element and aluminium can carry much of the oxidation resistance. The published family data shows grades at 21 to 27 % chromium reaching 1,250 to 1,400 °C, and a grade at 12 to 15 % chromium with 4 to 6 % aluminium reaching 950 °C. A grade with 13 % chromium and 6 % aluminium is an attempt to buy more temperature with aluminium while keeping the chromium cost low.

A high aluminium ferritic alloy has limited room temperature ductility, so a tight cold coil is a risk. The published data for the family shows elongation at rupture falling from above 16 % in the 4 % aluminium grade to above 10 % in the 7 % aluminium grade with molybdenum. Coiling at temperature, a minimum bend radius, or an anneal before forming are the usual answers, and the drawing should name one.

It is a loss of toughness in ferritic iron-chromium-aluminium alloys around 475 °C, caused by the ferrite separating into two phases with different chromium contents. A heating element that cools slowly through that range, or that is held in it, can become brittle without any change in composition. The published description of the higher temperature grade in the same family names 475 °C as the brittle temperature range.

Because the element has to run hotter than the furnace to transfer heat into it. The maximum element temperature in the standard is the temperature of the wire, and a furnace at a given temperature can contain an element that is above its rating if the element is poorly supported or if the load has been lost. A specification that quotes the furnace temperature has quoted the wrong number.

The iron-chromium-aluminium grade has a higher resistivity, a higher element temperature, a lower alloy cost, and much lower room temperature ductility. The nickel-chromium grade has a lower resistivity, a lower element temperature, a higher alloy cost, and enough ductility to be coiled cold. Where the element is supported and the atmosphere is oxidising, the iron-chromium-aluminium grade is usually the cheaper answer.

The name 0Cr13Al6Mo2 is a design statement: low carbon for weldability, low chromium for cost, high aluminium for temperature, and molybdenum for resistivity and strength. The family data confirms the logic, with maximum element temperature rising from 950 °C at 4 % aluminium to 1,400 °C at 7 % aluminium with molybdenum, and resistivity rising from 1.23 to 1.53 μΩ·m across the same set of grades.

The cost of the design is ductility, and it appears twice. A high aluminium wire resists cold coiling, and a ferritic alloy becomes brittle around 475 °C if it cools slowly through that range. A specification that names the element temperature, the coiling route, and the surface condition has covered the three items that decide whether the wire survives both the fabrication and the service.

GB/T 1234, high resistance alloys for electrical heating, as the national standard for the grades, forms, and property values quoted for the iron-chromium-aluminium family.

Published property table for iron-chromium-aluminium high resistance heating alloys to GB/T 1234, giving composition ranges, maximum element temperature, resistivity at 20 °C, density, thermal conductivity, expansion coefficient, melting point, tensile strength, elongation, reduction of area, and hardness for 1Cr13Al4, 0Cr21Al4, 0Cr21Al5, 0Cr21Al6, 0Cr21Al6Nb, and 0Cr27Al7Mo2.

Published description of 0Cr27Al7Mo2 to GB/T 1234-2012, noting that cold formability and weldability are poor in the as supplied condition and that 475 °C is the brittle temperature range for the ferritic structure.

Published technical data for iron-chromium-aluminium heating alloys, describing the aluminium oxide scale, its stability at high temperature, and the pre oxidation treatment used before service.

Published comparison of iron-chromium-aluminium and nickel-chromium resistance alloys, covering resistivity, maximum temperature, ductility, and behaviour in reducing atmospheres.

GB/T 1234 product forms, covering drawn wire, rolled strip, bar, and coiled rod as the delivery forms for the grades named.

ASTM B603, standard specification for iron-chromium-aluminium alloy wire for electrical resistance and heating, as the comparable document outside the Chinese standard system.

EN 10204, inspection documents, for the definition of the 3.1 certificate as a document carrying actual test results and endorsed by the manufacturer independent inspection representative.

The composition ranges and property values quoted here are the published values for the family members named, and they are not a certificate for a delivered lot. The properties of 0Cr13Al6Mo2 itself should be taken from the current edition of the applicable standard and from the supplier data sheet for the specific form and size, because the values for a grade are affected by the drawing and annealing route. Maximum element temperature is a property of the wire in a stated atmosphere and not a property of the furnace, and it assumes a supported element with a defined heat transfer. The 475 °C brittleness and the cold coiling limitation apply to the ferritic family generally and their severity depends on the aluminium content and on the thermal history. Product specifications are revised, and the edition named on the purchase order governs. Nothing in this page is a substitute for an engineering release or for the qualification testing agreed between buyer and supplier.