Carbon Steel vs Stainless Steel vs Alloy Steel: The Differences

Carbon steel, alloy steel and stainless steel are the three families into which nearly all steel is classified, and the classification is by composition. Carbon steel is iron with carbon and no other element added on purpose beyond manganese and silicon; it is graded by its carbon content, low (up to 0.30 percent), medium (0.30 to 0.60) and high (above 0.60). Alloy steel is carbon steel with deliberate additions of chromium, nickel, molybdenum, vanadium or other elements, normally under about 5 percent in total, to raise hardenability and strength. Stainless steel is iron with at least 10.5 percent chromium, which forms a passive oxide film that gives it corrosion resistance. The practical differences follow from those definitions: carbon steel is the least expensive of the three and rusts, alloy steel reaches the highest strength after heat treatment and also rusts, and stainless steel resists corrosion at a price per tonne that is a multiple of carbon steel and that moves with the chromium and nickel markets. The choice between them turns on the environment the part will see, the strength it must have through its section, and the price.

Carbon steel

Carbon steel accounts for about 90 percent of the steel made in the world. Its properties are set almost entirely by the carbon content and by the condition. The carbon content sets three bands, and the band decides what the steel can be asked to do. Low carbon steel, also called mild steel, has up to about 0.30 percent carbon: IS 2062 E250, EN 10025 S275, ASTM A36 and EN3 are low carbon structural and general engineering grades with a yield strength of about 250 MPa. There is too little carbon to form enough martensite on quenching for a useful hardness, so the band is used as rolled or normalised, and it welds and forms readily for the same reason. Medium carbon steel, 0.30 to 0.60 percent, holds enough carbon to harden and temper to a chosen strength: EN8 (080M40, 0.40 percent carbon) and SAE 1045 are the common shaft and axle grades, and preheat becomes necessary in welding as the carbon rises. High carbon steel, above 0.60 percent, reaches the hardness wanted for springs, blades, dies and wire rope, at the cost of ductility and of weldability. All carbon steels are magnetic, all have a density close to 7,850 kilograms per cubic metre, and all rust in damp air unless painted, galvanised or otherwise protected. The carbon steel family page lists the grades.

Alloy steel

Alloy steel is carbon steel with additions made to change the response to heat treatment. Chromium, molybdenum, nickel, manganese and vanadium slow the transformation of austenite on cooling, so a bar hardens through its full section rather than at the surface only, and the same additions raise the strength after tempering and the toughness. The EN grades of the British Standard 970 series and the SAE four-digit grades are the common designations: EN19 (709M40, a 1 percent chromium molybdenum steel, equivalent to SAE 4140), EN24 (817M40, nickel chromium molybdenum, close to SAE 4340), EN36C and SAE 8620 for case hardening, and EN31 for bearings. Hardened and tempered EN19 reaches a tensile strength of 850 to 1,000 MPa and EN24 of 850 to 1,550 MPa, above the range a plain carbon steel of the same section holds after the same treatment, because the alloying lets the section harden through rather than at the surface. Alloy steel is magnetic, has the density of carbon steel, and rusts as carbon steel does: the alloying elements are there for strength, not for corrosion resistance. Grades with more than about 5 percent of alloying element in total (tool steels, high speed steels) are usually classed separately. The alloy steel family page lists the grades.

Stainless steel

Stainless steel is defined by its chromium. At 10.5 percent and above, chromium forms a chromium oxide film on the surface that stops the iron beneath from oxidising and re-forms whenever the surface is cut or scratched. The other elements set the structure and the properties within the family: nickel gives the austenitic 300 series (304, 316), which is non-magnetic, very ductile and the most used; chromium alone gives the ferritic grades (430), which are magnetic and cheaper; chromium with more carbon gives the martensitic grades (410, 420), which harden by quenching like alloy steel; chromium, nickel, molybdenum and nitrogen in balance give the duplex grades (2205), with twice the yield strength of 304. Density is 7,700 to 8,000 kilograms per cubic metre according to grade. The manufacturing article describes the melt route and the composition article explains each element.

Comparison table

Corrosion: the deciding difference

Carbon and alloy steels rust because iron oxide is porous and does not protect the metal beneath it; the rust grows until the section is gone unless a coating keeps water and oxygen away. Every coating has a life, and where recoating is difficult or the downtime is expensive the uncoated stainless part can cost less over the service life than the coated carbon steel one, which is the usual economic case for stainless steel in water, food, chemical and outdoor service. Stainless steel is not immune: chlorides pit 304 in seawater and in swimming pools, and the film needs oxygen to re-form, so the grade is chosen for the environment, 316 or duplex for chlorides and 304 for fresh water and atmosphere. The 304 and 316 comparison covers that choice, and the mild steel and stainless steel comparison works through the case a buyer meets most often. Where the part stays dry, oiled, painted or indoors, carbon steel corrodes slowly and the corrosion argument for stainless steel does not apply.

Strength and hardness

In the annealed condition the austenitic stainless steels are no stronger than mild steel, and 304 at 205 MPa yield is below E250. Their strength in service comes from cold work (a cold drawn 304 bright bar is 400 MPa yield or more) and from the duplex and precipitation hardening grades. Alloy steel is the family chosen when a part must be strong and tough through a large section: a hardened and tempered EN24 shaft at 1,000 MPa yield has no austenitic equivalent, and the martensitic stainless grades that reach that strength (431, 17-4 PH) cost more and are harder to machine. Carbon steel sits at the bottom of the range and is chosen when strength beyond 250 to 400 MPa is not needed. The strength article defines the terms and gives the certificate values for each grade.

Fabrication

Low carbon steel is the easiest of the three to weld, form and machine. Alloy steel needs preheating and tempering after welding, because the hardenability that makes it strong also makes the weld zone hard and brittle on cooling. Austenitic stainless steel welds well and needs no preheat, but it distorts more because it conducts heat at a third of the rate of carbon steel and expands half as much again; it also work hardens under the tool, so machining needs rigid setups, sharp tools and steady feed. Ferritic and martensitic stainless steels behave more like carbon and alloy steels in the shop, and the martensitic grades need the same care in welding as alloy steel.

Where each is specified

Carbon steel is specified for structures, buildings, pipelines, vehicles, general machinery, fasteners and every application where the part is painted, dry or expendable. Alloy steel is specified for shafts, gears, axles, crankshafts, high-tensile bolts, pressure parts and tooling: parts that carry load through a section and are protected from the weather by their own housing or by oil. Stainless steel is specified where the part meets water, food, chemicals, steam, the outdoors or the body, and where a clean surface must stay clean: process plant, food and dairy equipment, marine fittings, architecture, medical devices, fasteners in exposed positions and shafting in pumps and valves.

When stainless steel is the wrong choice

Several conditions count against stainless steel: an environment that does not corrode carbon steel, a strength requirement above what the affordable stainless grades reach, volume machining of an austenitic grade, and a part that has to conduct heat. A gear that runs in oil inside a sealed housing meets no corrodent, so the chromium it would carry is not working. A drive shaft to 1,000 MPa in EN24 costs a fraction of the same shaft in a precipitation hardening stainless grade. Stainless steel also brings galling between austenitic parts, higher distortion in welding, and a thermal conductivity around a third of carbon steel, which matters in heat exchangers and moulds. Against those sit the costs that fall on the other side of the comparison and are easy to leave out of it: coating and recoating, the downtime to do it, and replacement of a section lost to rust.

The three families as bar

Laxcon Steels melts and rolls stainless, alloy and carbon grades as long products. Hot rolled round bar is supplied from 16 to 125 mm and bright bar from 5 to 115 mm, in the stainless grades of the grade reference and in the EN and SAE alloy grades listed on the family pages, each with the heat analysis on the certificate.

Frequently asked questions

Is stainless steel stronger than carbon steel?

Not necessarily. Annealed 304 has a yield strength of 205 MPa, below mild steel at 250 MPa. Duplex, martensitic and precipitation hardening stainless grades are stronger than most carbon steels, and hardened alloy steel is stronger than most stainless steels. Strength depends on the grade and its condition, not on the family.

What is the main difference between carbon steel and stainless steel?

Chromium. Stainless steel contains at least 10.5 percent chromium, which forms a self-healing oxide film that stops rust. Carbon steel has none and rusts in damp air unless it is coated. Everything else, including the higher cost, the lower thermal conductivity and the non-magnetic condition of the austenitic grades, follows from the alloying.

Is alloy steel the same as stainless steel?

No. Alloy steel is carbon steel with additions of chromium, molybdenum, nickel or vanadium, under about 5 percent in total, made to improve hardenability and strength; it still rusts. Stainless steel is defined by chromium at 10.5 percent or more and resists corrosion. Technically stainless steel is a high-alloy steel, but the two names are used for different families.

Which is cheaper, carbon steel or stainless steel?

Carbon steel, by a wide margin per tonne, because stainless steel carries chromium and nickel and carbon steel carries neither. The multiple is not fixed: it moves with the chromium and nickel markets and is quoted at the time of the enquiry. Over the life of a part exposed to water or weather the stainless part can cost less, because it needs no coating, no recoating and no replacement of a corroded section.

Sources

• ASTM A276/A276M, Standard Specification for Stainless Steel Bars and Shapes.
• EN 10088-1, Stainless steels, list of stainless steels.
• BS 970-1, Wrought steels for mechanical and allied engineering purposes (the EN grade series).
• SAE J404, Chemical Compositions of SAE Alloy Steels; SAE J403, Chemical Compositions of SAE Carbon Steels.
• IS 2062, Hot rolled medium and high tensile structural steel.
• ASM Handbook, Volume 1, Properties and Selection: Irons, Steels, and High-Performance Alloys.