Definition:
Stainless steel is a Carbon Steel that contains between 3% and 30% chromium.
Stainless iron is a stainless steel with less than 0.05% carbon.
As a rule of thumb;
If %Cr:%C ≤ 90 the stainless steel should be Martensitic
If %Cr:%C > 90 and contains nickel the stainless steel should be Austenitic.
If %Cr:%C > 90 and contains no nickel the stainless steel should be Ferritic.
There are exceptions to the above rule e.g. AISI 502, which is ferritic but should be martensitic in that its %Cr:%C = 50, however, it has insufficient chromium (≈5%) to allow martensite to generate naturally.
As the level of chromium is increased it is advisable to also increase the carbon content in order to minimise the quantity of free chromium. This is because chromium and carbon together generate very hard and stable chromium carbides but too much free chromium would cause grain growth and thus embrittlement.
Austenitic stainless steels contain high levels of nickel (3.5% to 22%) and exist as a face centre cubic lattice structure in their primary phase when cooled to room temperature. The basis for austenitic stainless steel is the AISI 302 (18-8) grade steel that contains 18% chromium and 8% nickel. The amounts of chromium and nickel can be varied but the face centre cubic lattice structure must remain in place at room temperature for these steels to be austenitic. Austenitic stainless steels cannot be hardened with heat treatment; they need to be cold worked to increase hardness.
| AISI | SMYS (MPa) |
UTS (MPa) |
e (%) |
BHN | Chemistry (%) |
Cr:C |
|---|---|---|---|---|---|---|
| 201 | 379 | 793 | 55 | 185 | C ≤0.15; Cr 16≤18; Ni 3.5≤5.5; Mn ≤7.5; Si ≤1 |
113 |
| 202 | 379 | 724 | 55 | 185 | C ≤0.15; Cr 17≤19; Ni 4≤6; Mn ≤10; Si ≤1 |
120 |
| 301 | 276 | 758 | 60 | 165 | C ≤0.15; Cr 16≤18; Ni 6≤8; Mn ≤2; Si ≤1 |
113 |
| 302 | 276 | 621 | 50 | 165 | C ≤0.15; Cr 17≤19; Ni 8≤10; Mn ≤2; Si ≤1 |
120 |
| 303 | 241 | 621 | 50 | 145 | C ≤0.15; Cr 17≤19; Ni 8≤10; Mn ≤2; Si ≤1; S ≤0.15 |
120 |
| 304 | 241 | 586 | 60 | 150 | C ≤0.08; Cr 18≤20; Ni 8≤10.5; Mn ≤2; Si ≤1 |
238 |
| 305 | 262 | 586 | 50 | 156 | C ≤0.12; Cr 17≤19; Ni 10.5≤13; Mn ≤2; Si ≤1 |
150 |
| 308 | 241 | 586 | 50 | 150 | C ≤0.08; Cr 19≤21; Ni 10≤12; Mn ≤2; Si ≤1 |
250 |
| 309 | 310 | 621 | 45 | 165 | C ≤0.2; Cr 22≤24; Ni 12≤15; Mn ≤2; Si ≤1 |
115 |
| 310 | 310 | 655 | 45 | 165 | C ≤0.25; Cr 24≤26; Ni 19≤22; Mn ≤2; Si ≤1.5 |
100 |
| 314 | 345 | 689 | 45 | 170 | C ≤0.25; Cr 23≤26; Ni 19≤22; Mn ≤2; Si ≤3 |
98 |
| 316 | 207 | 552 | 60 | 142 | C ≤0.08; Cr 16≤18; Ni 10≤14; Mn ≤2; Si ≤1; Mo 2≤3 |
213 |
| 316L | 290 | 558 | 50 | 147 | C ≤0.03; Cr 16≤18; Ni 10≤14; Mn ≤2; Si ≤1; Mo 2≤3 |
567 |
| 317 | 276 | 586 | 50 | 160 | C ≤0.08; Cr 18≤20; Ni 11≤15; Mn ≤2; Si ≤1; Mo 3≤4 |
238 |
| 321 | 207 | 586 | 55 | 165 | C ≤0.08; Cr 17≤19; Ni 9≤12; Mn ≤2; Si ≤1; Ti ≤5xC |
225 |
| 347 | 241 | 621 | 50 | 160 | C ≤0.08; Cr 17≤19; Ni 9≤12; Mn ≤2; Si ≤1; Cb-Ta ≤10xC |
225 |
Ferritic stainless steels contain no nickel and exist as a body centre cubic lattice structure in their primary phase when cooled to room temperature. The basis for ferritic stainless steel is the AISI 430 grade steel that contains 18% chromium. The amount of chromium can be varied but the body centre cubic lattice structure must remain in place at room temperature for these steels to be ferritic. Ferritic stainless steels cannot be hardened with heat treatment; they need to be cold worked to increase hardness.
| AISI | SMYS (MPa) |
UTS (MPa) |
e (%) |
BHN | Chemistry (%) |
Cr:C |
|---|---|---|---|---|---|---|
| 405 | 276 | 483 | 30 | 150 | C ≤0.08; Cr 11.5≤14.5; Mn ≤1; Si ≤1; Al 0.1≤0.3 |
163 |
| 430 | 276 | 517 | 30 | 160 | C ≤0.12; Cr 16≤18; Mn ≤1; Si ≤1 |
133 |
| 434 | 365 | 531 | 23 | 159 | C ≤0.12; Cr 16≤18; Mn ≤1; Si ≤1; Mo 0.75≤1.25 |
142 |
| 436 | 365 | 531 | 23 | 159 | C ≤0.12; Cr 16≤18; Mn ≤1; Si ≤1; Mo 0.75≤1.25 |
142 |
| 442 | 310 | 552 | 20 | 185 | C ≤0.2; Cr 18≤23; Mn ≤1; Si ≤1 |
103 |
| 446 | 345 | 552 | 25 | 170 | C ≤0.2; Cr 23≤27; Mn ≤1.5; Si ≤1 |
125 |
| 502 | 207 | 483 | 30 | 150 | C ≤0.1; Cr 4≤6; Mn ≤1; Si ≤1; Mo 0.4≤0.65 |
50 |
Martensitic stainless steels are carbon steels with relatively high levels of carbon when compared to chromium. These stainless steels are excellent for producing clean, hard, sharp edges as they can be hardened through heat treatment without generating corrosion. They are therefore ideal for knives, surgical equipment, valve seats, and high-strength springs.
| AISI | SMYS (MPa) |
UTS (MPa) |
e (%) |
BHN | Chemistry (%) |
Cr:C |
|---|---|---|---|---|---|---|
| 403 | 276 | 517 | 30 | 155 | C ≤0.15; Cr 11.5≤13; Mn ≤1; Si ≤0.5 |
82 |
| 410 | 276 | 517 | 30 | 155 | C ≤0.15; Cr 11.5≤13.5; Mn ≤1; Si ≤1 |
40 |
| 414 | 655 | 827 | 17 | 235 | C ≤0.15; Cr 11.5≤13.5; Ni 1.25≤2.5; Mn ≤1; Si ≤1 |
40 |
| 416 | 276 | 517 | 30 | 155 | C ≤0.15; Cr 12≤14; Mn ≤1.25; Si ≤1; S ≤0.15 |
87 |
| 420 | 345 | 655 | 25 | 195 | C ≥0.15; Cr 12≤14; Mn ≤1; Si ≤1 |
65 |
| 431 | 655 | 862 | 20 | 260 | C ≤0.15; Cr 15≤17; Ni 1.25≤2.5; Mn ≤1; Si ≤1 |
80 |
| 440A | 414 | 724 | 20 | 215 | C 0.6≤0.75; Cr 16≤18; Mn ≤1; Si ≤1; Mo ≤0.75 |
25 |
| 440B | 427 | 738 | 18 | 220 | C 0.75≤0.95; Cr 16≤18; Mn ≤1; Si ≤1; Mo ≤0.75 |
20 |
| 440C | 448 | 758 | 13 | 230 | C 0.95≤1.2; Cr 16≤18; Mn ≤1; Si ≤1; Mo ≤0.75 |
30 |
| 501 | 207 | 483 | 28 | 160 | C ≥0.1; Cr 4≤6; Mn ≤1; Si ≤1; Mo 0.4≤0.65 |
33 |
Duplex stainless steels contain roughly equal amounts of ferritic and austenitic stainless steels in their structure. They are very difficult to create as both materials must remain in their solid state during manufacture
They exhibit higher strengths than their austenitic counterparts, while retaining their corrosion resistance. They also offer good resistance to chloride ion stress corrosion cracking.
Additional alloying elements are included in the manufacture of stainless steels, which include not only manganese, phosphorus, sulphur, molybdenum, chromium, nickel, tungsten, vanadium and silicon, all of which are discussed elsewhere on this website, but also: selenium, titanium, tantalum, cobalt, aluminium and copper, which are discussed below:
Selenium is similar to sulphur and sometimes used in its stead. It forms selenide globules that act as chip breakers when machining. It is typically (but not exclusively) used in austenitic stainless steels.
Austenitic Steels: Titanium is added to steels with high carbon content to increase resistance to intergranular corrosion. It also improves mechanical properties at elevated temperatures.
Ferritic Steels: Titanium improves toughness, and corrosion resistance.
Martensitic Steels: Titanium reduces hardness by attaching to free carbon thereby reducing the effects of tempering
Tantalum is extremely corrosion resistant and when alloyed to stainless steel significantly increases its corrosion resistance without adversely affecting its strength. It is, however, very expensive and therefore used only for very special applications such as parts that come into contact with strong acids.
Cobalt (RAM = 58.9332) is chemically similar to nickel (RAM = 58.6934). Small amounts of cobalt (≤0.2%) significantly increase the red-heat strength and hardness of metals making them suitable for extreme applications in the aerospace and machine-tool industries. It is also used in non-ferrous magnets.
Conversely, it is also used in large quantities (e.g. 8%) along with nickel in marageing steels (see Marageing Steels below), which contain less than 0.03% carbon but retain the strength of higher carbon steels with much improved toughness and ductility.
Aluminium is used to minimise corrosion in heat resistant steels at elevated temperatures. They can also improve the strength of aged steels (see Marageing Steels below).
Copper improves resistance to certain acids and promotes an austenitic microstructure. It also reduces work hardening for improved machinability and assists with forming and drawing.
A marageing steel is an age hardenable martensitic stainless steel that contains between 12% and 18% nickel and so little carbon that they are referred to as low carbon martensites.
The tempering temperature for marageing steels is about 870°C at which they will become austenitic. They revert to a martensitic structure over about a week or so of cooling in air at ambient temperature.
Iron-carbon martensite is hard and brittle in the quenched condition and more ductile when tempered. On the other hand, carbon-free iron-nickle martensite is relatively soft but toughens, hardens and strengthens with ageing.
Therefore they can be formed or machined when soft and left to harden naturally resulting in minimal post-machining distorsion.
These steels are not included in the AISI grade range.
You will find further reading on this subject in reference publications(2, 3 & 44)