Melting Point of Steel and Stainless Steel: Values by Grade, and Why the Service Limit Is Far Lower

The melting point of steel is not a single temperature but a range, because steel is an alloy and its constituents solidify at different temperatures. Plain carbon steel melts between about 1,425 and 1,540 degrees Celsius, the exact range falling as the carbon content rises. Austenitic stainless steel melts lower: AISI 304 between about 1,400 and 1,450 degrees Celsius, and AISI 316, with its molybdenum, between about 1,375 and 1,400 degrees Celsius. Martensitic and ferritic stainless steels, which carry less nickel, melt higher, at about 1,480 to 1,530 degrees Celsius for AISI 410. Pure iron, the reference for all of them, melts at 1,538 degrees Celsius. The temperature at which a steel can be used in service is a different and much lower figure, set by the loss of strength and the growth of oxide scale rather than by melting.

Why steel melts over a range

A pure metal melts at one temperature. An alloy begins to melt at its solidus, the temperature at which the first liquid forms, and is fully liquid at its liquidus. Between the two the metal is a mixture of solid and liquid. The width of the range depends on how far the alloying elements depress the melting point of iron and how unevenly they are distributed as the metal freezes. Carbon is the strongest depressant that steel routinely contains: the liquidus falls by roughly 90 degrees Celsius for every percentage point of carbon, from 1,538 degrees Celsius for pure iron to 1,147 degrees Celsius at the eutectic composition of 4.3 percent, which is cast iron territory rather than steel. Nickel, chromium, molybdenum, manganese and silicon each lower the range by smaller amounts, and it is their combined effect that puts an 18 percent chromium, 8 percent nickel austenitic steel about 100 degrees Celsius below a low carbon steel.

Stainless steel grades therefore melt in an order that follows their composition. The ferritic and martensitic grades, which are chromium steels with little or no nickel, sit highest. The austenitic grades, with 8 to 14 percent nickel and in the case of 316 an addition of molybdenum, sit lowest. The duplex grades, which balance the two structures, fall in between.

Melting point by grade

The values below are the melting ranges published for the annealed composition of each grade in the ASM Specialty Handbook on stainless steels, the Atlas Steels technical handbook and the Outokumpu handbook. Published ranges for one grade differ by 10 to 20 degrees Celsius between those sources, because the composition window of a grade is wide and each source rounds differently, and a given heat melts inside the range at a point set by its own analysis.

AISI 310S has the same melting range as 304 but a far higher service temperature, because scaling resistance is set by the chromium and nickel content of the grade and not by how close the metal is to melting.

Melting point is not the service limit

No steel is used anywhere near its melting range. Long before it melts, a steel loses the strength that a design relies on, and its surface begins to oxidise faster than the protective scale can keep up. The usable limit is therefore set by two properties: strength at temperature, which for sustained loads means creep, and scaling resistance, which depends on the chromium content and on whether the temperature cycles.

For AISI 304 the Atlas Steels and Outokumpu handbooks give a maximum service temperature in air of about 870 degrees Celsius for intermittent exposure and 925 degrees Celsius for continuous exposure, where "intermittent" means the part is repeatedly heated and cooled and the scale cracks off. The ASME Boiler and Pressure Vessel Code, Section II Part D, stops listing allowable stresses for 304 above 816 degrees Celsius, and a pressure part is designed against those allowables, not against the scaling limit. AISI 316 carries similar limits. The stabilised grade AISI 321 is chosen for the 425 to 870 degrees Celsius band because titanium keeps its chromium out of carbides during long exposure. AISI 310S, with 25 percent chromium and 20 percent nickel, resists scaling to about 1,035 degrees Celsius intermittent and 1,150 degrees Celsius continuous, and it is the grade specified for furnace parts, radiant tubes and heat treatment fixtures. The martensitic AISI 410 is limited to about 705 degrees Celsius continuous and the ferritic AISI 430 to about 815 degrees Celsius, both well short of their high melting ranges.

Creep sets in for the austenitic grades from roughly 550 degrees Celsius. Above that temperature a bar under a steady load stretches slowly and continuously, and the design stress is set by the creep rupture data for the grade rather than by its room temperature yield strength. A building fire reaches 1,000 degrees Celsius or more, and structural steel loses most of its strength in it without melting: at 600 degrees Celsius carbon steel retains about half of its yield strength, at 800 degrees Celsius about a tenth.

The intermittent figure is lower than the continuous one for the austenitic grades and higher for the chromium steels. On an austenitic steel the oxide has a different thermal expansion from the metal and spalls off with every cooling cycle, exposing fresh metal; on a chromium steel the scale adheres under cycling but the metal loses strength under long continuous heating.

How a melt shop reaches the temperature

Stainless steel is melted in an electric arc furnace, in which three graphite electrodes strike an arc onto a charge of scrap and ferroalloys. The arc temperature exceeds 3,000 degrees Celsius at the tip, and the bath is brought to a tapping temperature of about 1,600 to 1,650 degrees Celsius, which is 150 to 250 degrees Celsius above the liquidus of the grade. This margin, the superheat, is what keeps the metal fluid through tapping, refining and casting. The liquid steel is then transferred to an argon oxygen decarburisation converter, where a mixture of oxygen and argon blown through the bath burns the carbon down to the grade's limit without burning out the chromium, and the bath temperature is held by the heat of that reaction. In continuous casting the metal enters the mould at a superheat of roughly 20 to 40 degrees Celsius above its liquidus, low enough for the shell to form quickly against the water cooled copper and high enough not to freeze in the tundish nozzle. The technology page describes the route from furnace to bar.

Hot working takes place far below the solidus. A stainless billet is reheated to about 1,100 to 1,250 degrees Celsius before rolling, where the austenite is soft enough to deform and recrystallise but the metal is entirely solid. Forging of the martensitic and precipitation hardening grades uses a similar band, and the heat treatment that follows, described in the article on heat treatment of steel, runs between about 480 and 1,120 degrees Celsius depending on the grade and the condition ordered.

Other metals for comparison

Steel sits in the middle of the range of engineering metals. Aluminium melts at 660 degrees Celsius, which is why aluminium castings are made in furnaces that could not begin to melt steel. Copper melts at 1,085 degrees Celsius and its brasses and bronzes lower still. Nickel melts at 1,455 degrees Celsius, chromium at 1,907, titanium at 1,668, and the refractory metals far above: molybdenum at 2,623 degrees Celsius and tungsten at 3,422, the highest of any metal. The strongest metals article ranks these metals by a different property, and the two rankings do not coincide: tungsten is both the highest melting and one of the strongest, but chromium, which melts higher than iron, is brittle at room temperature, and titanium is used for its strength to weight ratio rather than its melting point.

Semi-finished steel from the melt

Laxcon Steels melts stainless, alloy and carbon grades in its own electric arc furnace and refines them in the argon oxygen decarburisation converter before casting. The liquid steel is cast as continuous cast billets and blooms for the rolling mills and as forging quality ingots for forge shops, and every heat is analysed by optical emission spectrometry before it is tapped, so the grade on the certificate is the grade in the ladle. The heat resisting grades in the table above, 310S, 321 and 304H, are listed in the grade reference with their composition windows.

Frequently asked questions

What is the melting point of stainless steel?

Between about 1,375 and 1,530 degrees Celsius depending on the grade. AISI 304 melts at 1,400 to 1,450 degrees Celsius, AISI 316 at 1,375 to 1,400, and the martensitic grade AISI 410 at 1,480 to 1,530. The figure of about 1,400 degrees Celsius is the usual single answer for the common austenitic grades.

Does stainless steel have a higher melting point than carbon steel?

No. The austenitic stainless grades melt about 100 degrees Celsius lower than low carbon steel, because nickel, chromium and molybdenum all depress the melting range of iron. The ferritic and martensitic stainless grades melt in about the same range as carbon steel.

At what temperature does steel lose its strength?

Carbon steel keeps most of its yield strength to about 400 degrees Celsius, retains about half at 600 degrees Celsius and about a tenth at 800 degrees Celsius. Austenitic stainless steel begins to creep under sustained load from roughly 550 degrees Celsius, and its scaling limit in air is 870 to 925 degrees Celsius for 304 and 316, and up to 1,150 degrees Celsius for 310S.

Why is molten steel tapped at 1,600 degrees Celsius if it melts at 1,450?

The margin above the liquidus, called superheat, keeps the steel fluid through tapping, refining in the converter, ladle treatment and casting. Without it the metal would begin to freeze in the ladle and the nozzle before it reached the mould.

Sources

• ASM Handbook, Volume 1, Properties and Selection: Irons, Steels, and High-Performance Alloys: melting ranges and elevated temperature properties of carbon and stainless steels.
• ASM Specialty Handbook: Stainless Steels: physical properties of the wrought stainless grades, including melting ranges.
• Atlas Steels, Technical Handbook of Stainless Steels: melting ranges and maximum service temperatures in air, grade by grade.
• Outokumpu, Handbook of Stainless Steel: physical properties and temperature resistance of the austenitic, ferritic, martensitic and duplex grades.
• ASME Boiler and Pressure Vessel Code, Section II Part D: allowable stress tables for austenitic stainless steels.
• Eurocode 3, EN 1993-1-2: strength reduction factors for carbon steel at elevated temperature.
• ASM Handbook, Volume 3, Alloy Phase Diagrams: the iron-carbon diagram, for the liquidus and the eutectic temperature.