Chromium, nickel, molybdenum: how the elements define a grade
12Х18Н10Т is a composition right in the name: 0.12% carbon, 18% chromium, 10% nickel, titanium. It is chromium that makes steel stainless: it creates a passive film of oxides on the surface.
Nickel maintains the austenitic structure and is responsible for toughness, molybdenum adds resistance to chlorides, titanium binds carbon and protects the weld seam. Understanding these roles explains why grades behave differently although they look the same.
What stainless steel consists of: the composition of 12Х18Н10Т
The base of any stainless steel is iron. Chromium makes it stainless, and there must be enough of it for a stable passive film; the specific threshold is set by the standard for the grade. Beyond that the composition is assembled for the task: some elements increase resistance, others are responsible for the structure, yet others for manufacturability.
| Grade | ГОСТ designation | What the code means |
|---|---|---|
| AISI 304 | 08Х18Н10 | 0.08% C, 18% Cr, 10% Ni |
| AISI 304L | 03Х18Н11 | 0.03% C, 18% Cr, 11% Ni |
| AISI 316 | 10Х17Н13М2 | 0.10% C, 17% Cr, 13% Ni, 2% Mo |
| AISI 316L | 03Х17Н14М2 | 0.03% C, 17% Cr, 14% Ni, 2% Mo |
| AISI 321 | 12Х18Н10Т | 0.12% C, 18% Cr, 10% Ni, Ti |
| AISI 430 | 12Х17 | 0.12% C, 17% Cr, no Ni |
| AISI 201 | 12Х15Г9НД | 0.12% C, 15% Cr, 9% Mn, Ni, Cu |
| AISI 310S | 20Х23Н18 | 0.20% C, 23% Cr, 18% Ni |
The weight formula is the same for all grades: the volume is multiplied by the density of stainless steel — about 7.9 g/cm³. The difference in composition affects the mass only slightly, so the weight of a sheet or a pipe is calculated without specifying the grade.
The class of steel is set not by a single element but by their balance. Austenitic grades hold nickel in a sufficient proportion, ferritic ones manage without it, martensitic ones gain strength by quenching. The same chromium with a different nickel content gives different materials with different magnetic properties, ductility and weldability.
| Element | What it is responsible for | Where it shows |
|---|---|---|
| Chromium | passive film, heat resistance | basic corrosion resistance |
| Nickel | austenitic structure, toughness | ductility, non-magnetic behaviour after annealing |
| Molybdenum | resistance to chlorides | pitting, sea water, chemicals |
| Titanium | binds carbon | weld seam, intergranular corrosion |
| Manganese | partly replaces nickel | economy grades, strength |
| Copper | additional alloying | specific corrosive environments |
| Carbon | strength, but also a risk for the seam | the lower, the better for welding |
Chromium
Chromium is the main element of stainless steel. In air it forms a thin layer of oxides that shields the metal from the environment. The film is dense, strongly bonded to the base and restores itself if damaged in air.
Two practical consequences follow. First: as long as the film is intact, the metal does not rust. Second: as soon as it is destroyed mechanically or chemically and there are no conditions for restoration, corrosion begins. That is exactly why stainless steel rusts under deposits, in gaps and in stagnant water where no oxygen arrives.
Chromium is responsible not only for corrosion but also for heat resistance. The more chromium, the higher the temperature at which the steel holds its scale. Hence high-chromium grades for furnaces and hot components.
An important caveat: the amount of chromium in the grade is not the only factor. The condition of the surface, the presence of weld seams and contamination, the temperature and the composition of the environment all matter.
The passive film can be strengthened. After welding, machining or contamination the metal is passivated: the surface is cleaned of iron and scale and the film is allowed to restore itself in air. This is not a coating but a return of the surface to its normal state. If this is not done, areas with impaired protection will remain on the seam.
The environment changes the requirements for chromium. In fresh water and an ordinary atmosphere basic grades are enough, in a chloride environment a margin is needed, and in hot gases scale resistance is more important. The same composition behaves like different materials under different conditions, so a grade is selected for a specific duty, not "in general".
There is another side to it. High chromium increases heat resistance, but in combination with a large amount of carbon it makes the steel prone to embrittlement and complicates welding. That is why a grade is assessed as a whole, not by a single figure in the name.
Nickel
Nickel stabilises the austenitic structure. It is what makes 304 and 316 ductile, tough and weakly magnetic after annealing. Austenite bends, stretches and welds well, so the bulk of stainless rolled steel is austenitic grades.
The second consequence is behaviour at low temperatures. Austenitic grades retain toughness on cooling, whereas ferritic ones become more brittle. For cryogenic components this is fundamental.
The third is economics. Nickel is expensive, and its share determines the price of the grade. Hence economy solutions appeared: in 201 part of the nickel was replaced with manganese, and in 430 there is no nickel at all. More on this in the breakdowns of AISI 201 and AISI 430.
Nickel does not work against chlorides. Molybdenum is responsible for that part, so the difference between 304 and 316 is not in the nickel but in the molybdenum.
If there is little nickel, the structure shifts to ferrite or martensite. Ferrite is always magnetic, martensite too, and austenite only after deformation. That is why magnetism is linked to composition but does not identify a grade unambiguously.
Austenite is not stable under all conditions. After severe deformation part of the structure turns into martensite, and the metal begins to attract a magnet. Nickel and manganese hold back this transition, so 304 and 316 have a higher margin than the economy grades.
Molybdenum and titanium
Molybdenum increases the resistance of the passive film in a chloride environment. It raises the threshold at which the chloride ion breaks through the film and starts pitting corrosion. That is why 316 goes into sea water, brines, pools and chlorine-containing chemicals, where 304 does not hold up. A comparison of the grades is set out in the article AISI 304 or 316.
Titanium solves a different problem. On heating, the carbon in the steel combines with chromium and precipitates along the grain boundaries as carbides. Around the boundaries the chromium is depleted, and the seam begins to corrode. Titanium binds the carbon earlier than chromium would, and intergranular corrosion does not develop. That is why 321 and 12Х18Н10Т are taken for welded structures that cannot be annealed after welding.
Low carbon has the same goal as titanium: in 304L and 316L there is little carbon, and the carbides simply have nothing to form from. This is the second way to protect the seam.
Carbon by itself does not make steel stainless, but it often determines how a weld seam will behave. The lower the carbon, the lower the risk of intergranular corrosion, so for welding grades with an L index or with a stabiliser in the composition are taken.
Manganese and copper are auxiliary elements. Manganese partly replaces nickel and adds strength, but does not give the same resistance. Copper is introduced into specific grades for specific environments.
Nitrogen in austenitic grades works together with molybdenum: it too increases resistance to pitting corrosion and strengthens the metal. Niobium in some grades plays the same role as titanium — it binds carbon and protects the seam from intergranular corrosion.
How to read a designation
A ГОСТ grade is read from left to right. The figure at the beginning is the carbon content in hundredths of a percent. Then come the letters of the elements, and the figures after a letter are their proportion in percent. The order of the letters does not matter, their combination does.
| Designation | Element | What it gives |
|---|---|---|
| Х | chromium | passive film, heat resistance |
| Н | nickel | austenitic structure, toughness |
| М | molybdenum | resistance to chlorides |
| Т | titanium | protection of the seam from intergranular corrosion |
| Г | manganese | nickel substitute, strength |
| Д | copper | alloying for the environment |
| Б | niobium | stabilisation, analogous to the role of titanium |
| С | silicon | special properties |
| Ф | vanadium | special properties |
Examples of reading:
- 12Х18Н10Т — 0.12% carbon, 18% chromium, 10% nickel, titanium. An analogue of AISI 321.
- 08Х18Н10 — 0.08% carbon, 18% chromium, 10% nickel. An analogue of AISI 304.
- 03Х17Н14М2 — 0.03% carbon, 17% chromium, 14% nickel, 2% molybdenum. An analogue of AISI 316L.
- 12Х17 — 0.12% carbon, 17% chromium, no nickel. An analogue of AISI 430.
Separately, on the confusion with chrome plating. Chrome-plated steel is ordinary steel with a thin electroplated coating. A scratch opens the coating, and underneath it the base begins to rust. Stainless steel is alloyed with chromium throughout its volume, so a scratch does not expose unprotected metal. These are different materials, and they behave differently.
It is impossible to determine the composition without a laboratory. A magnet does not distinguish grades, the appearance of 304, 201 and 430 is identical, and a spark and colour do not give a reliable answer. The working source is the certificate for the batch, and when in doubt a spectral analysis.
The American and European systems have a different logic. AISI and EN do not encode the composition in the name: AISI 304 or 1.4301 are numbers behind which a certain set of requirements stands. To understand what exactly is hidden behind a number, a correspondence table or a certificate is needed. That is why when purchasing, not only the name of the grade matters but also confirmation of the composition in the documents for the batch.
Other letters also occur in ГОСТ grades: А — nitrogen, Е — selenium, Ю — aluminium, Ц — zirconium. In stainless grades they are rare, but when reading a certificate it is useful to understand that the code is not limited to the basic set.
In practice the choice is fixed in the documents. The specification states the grade, the certificate states the composition. If an order simply says "stainless steel", the supplier is entitled to bring whichever grade is in stock.
A practical example. If a component is welded and works in a damp environment, they look not only at chromium but also at carbon: 08Х18Н10 behaves worse in a weld seam than 03Х18Н11 or 12Х18Н10Т. The same base composition with different carbon and stabiliser gives a different service life.
Common mistakes when working with composition
- Judging a grade by a magnet. Austenite becomes magnetic after deformation, and ferrite always is. The test does not answer the question of composition.
- Confusing chrome-plated steel with stainless. The coating protects only until the first scratch, alloying works through the whole thickness.
- Assuming that more nickel is always better. A grade is selected for the environment. In a dry room extra nickel gives nothing but price.
- Ignoring carbon for welding. 304 in a welded structure without heat treatment loses to 304L, because carbon precipitates along the grain boundaries.
- Believing that polishing changes the composition. The surface affects the service life but does not turn one grade into another.
- Storing different grades in one place without marking. Mixed-up batches lead to the wrong metal ending up in a critical component.
- Ordering "stainless steel" without stating the grade. The specification must carry a specific designation, otherwise the delivery will be random.
Frequently asked questions
What is the chemical composition of 12Х18Н10Т
0.12% carbon, 18% chromium, 10% nickel and titanium. This is the Russian analogue of AISI 321. The exact composition of a batch is stated in the certificate, because the tolerances are set by the standard for the grade.
What is the composition of AISI 304
About 18% chromium and 8–10% nickel, the rest iron and small additions. The Russian analogue is 08Х18Н10. In everyday use this grade is called 18/10 after the proportions of chromium and nickel.
How does the composition of AISI 316 differ from 304
By molybdenum. The composition of 316 has 2–3% molybdenum, while 304 has none at all. There is slightly more nickel in 316. It is molybdenum that gives resistance to chlorides, which is why the grades are changed in the sea, in brines and in chemicals.
How does chrome-plated steel differ from stainless
Chrome-plated is steel with a thin coating: a scratch opens the base and it rusts. Stainless is alloyed with chromium through the whole thickness, so damage to the surface does not expose unprotected metal.
What is nickel responsible for in stainless steel
For the austenitic structure. Thanks to nickel the grade becomes ductile, tough and weakly magnetic after annealing. Nickel is expensive, so its share determines the price and the appearance of economy grades.
What does molybdenum do in the composition
It increases the resistance of the passive film in a chloride environment. Molybdenum raises the pitting corrosion threshold, so 316 works where 304 quickly goes into pitting.
Can the composition of steel be determined without a laboratory
Reliably — no. A magnet, a spark and colour give no answer, and the appearance of the grades is identical. Look at the certificate for the batch, and when in doubt order a spectral analysis.
What do the letters in the grade 12Х18Н10Т mean
Х — chromium, Н — nickel, Т — titanium. The figures show the proportions of the elements in percent, and the first figure is the carbon content in hundredths of a percent. A detailed table of analogues is in the breakdown of stainless steel grades.
Place an order with the required composition
The composition of a grade is not discussed after matching, so state it in advance. Choose a position in the catalogue and record the grade, size and surface in the order — exactly that metal will pass into the trade.
- Catalogue of stainless steel products
- AISI 304 or 316 — where molybdenum decides
- AISI 430 — a ferritic grade without nickel