Enter one JIS (or similar) material designation to see the meaning of each part in a table. For example, SCM440 reads as S (steel) + CM (chromium and molybdenum) + 4 (class of the amount of main alloying elements) + 40 (representative carbon content, about 0.40 %). Differences in letter case, full-width characters, spaces and hyphens are normalized automatically.
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Compositions are guides based on specified ranges or typical values checked against published sources, not the full text of any standard. JIS is revised, so names, grades and ranges can change between editions. For design and purchasing, check the latest standard and the manufacturer's data.
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Designations run “material + use or form + grade”
JIS designations for iron and steel are in principle built from three parts. The first is a letter for the material: steels begin with S (Steel), and cast irons and similar materials with F (Ferrum). Next come letters for the product form or use, such as plate (P), tube (T), wire (W), tool (K) or casting (C), and the last part is a number or mark for the grade. SS400 is built as S + S + 400, and SUJ2 as S + UJ + 2. Steels for machine structural use are the exception: their second part lists the symbols of alloying elements, so the grade is expressed by its composition. Aluminum alloys begin with A and copper and copper alloys with C, and a product form mark and a temper (the processing condition) follow the four-digit alloy number.
The same digits mean different things in different steels
What the digits in a designation stand for differs from one kind of steel to another. In SS400, SM490A and FC250 the number is the guaranteed minimum tensile strength (N/mm²), so SS400 has 400 N/mm² or more. In S45C, on the other hand, 45 is the representative carbon content (about 0.45 %) multiplied by 100, and the final 40 in SCM440 is also the carbon content. The number in SK105 is likewise a representative carbon content (about 1.05 %), but the numbers in SUJ2, SUP9, SKD11 and SUS304 are grade numbers and do not express amounts. Many SUS numbers are the same as AISI numbers. The trick is not to assume from the digits alone that they mean strength or carbon content, but to check first which standard the designation belongs to.
How alloy steels for machine structural use are numbered
Designations of low-alloy steels for machine structural use (JIS G 4053) such as SCM440 put the symbols of the main alloying elements after S, followed by a one-digit main alloying element content code and a two-digit representative carbon content. Alloying elements are written Mn or Cr when alone, and in combinations chromium is written C, molybdenum M, nickel N and aluminum A, so SCM is chromium-molybdenum steel and SNCM nickel-chromium-molybdenum steel. The code is a class of the amount of alloying elements: in the JIS G 4053 table, Ni is 0.40–0.70 % in SNCM2xx, 1.60–2.00 % in SNCM4xx and 4.00–4.50 % in SNCM8xx. The representative carbon content is roughly the middle of the specified range; SCM440 contains 0.38–0.43 % C. A final H denotes an H steel with guaranteed hardenability.
Designations for stainless steels, aluminum alloys and copper alloys
Stainless steels have a three-digit number after SUS, followed by marks such as L (low carbon, C 0.030 % or less), N (nitrogen added) and J1 or J2 (a grade differing in part of its composition from the grade with the same number). They fall into five types: austenitic, ferritic, martensitic, austenitic-ferritic (duplex) and precipitation-hardening. In the four-digit number of an aluminum alloy, the first digit gives the series (1000 series pure aluminum, 5000 series Al-Mg, 6000 series Al-Mg-Si and so on), and the 2000, 6000 and 7000 series are heat-treatable, gaining strength by heat treatment. In the temper after the hyphen, O means annealed, H work-hardened and T heat-treated; A6061-T6 has been solution heat-treated and then artificially aged. In the four digits after C in a copper alloy, the first digit gives the alloy family (2 for Cu-Zn brass, 3 for leaded free-cutting brass, 5 for phosphor bronze and so on).
AA rolled steel for general structure with a tensile strength of at least 450 MPa
BA carbon steel for machine structural use with about 0.45% carbon
CA cast iron with about 4.5% carbon
Show answer and explanation
Answer: B. A carbon steel for machine structural use with about 0.45% carbon
In S45C, S stands for steel and C for carbon, and the number in between gives the nominal carbon content in units of 0.01%, so it is a carbon steel for machine structural use containing about 0.45% carbon (roughly 0.42–0.48%), close to AISI 1045. Its strength can be adjusted by quenching and tempering, and it is widely used for shafts and gears. In SS400 (JIS), the rolled steel for general structure, the number gives not the carbon content but the guaranteed tensile strength (at least 400 MPa), so the choice reading S45C as rolled steel for general structure with a tensile strength of at least 450 MPa confuses how the two designations are read. Above 2.14% carbon the material is no longer steel but cast iron, and 4.5% is the mistake of reading the number in units of 0.1%. In the hypoeutectoid range, more carbon means higher hardness and strength but lower ductility and weldability.
AIts high Cr content makes it a martensitic stainless steel, so it hardens greatly on quenching in oil or water
BDespite its Ni content it is a BCC ferritic steel, strongly attracted to a magnet even in the annealed condition
CIt is FCC austenite, does not harden on quenching, and is nearly nonmagnetic in the annealed condition
Show answer and explanation
Answer: C. It is FCC austenite, does not harden on quenching, and is nearly nonmagnetic in the annealed condition
SUS304 contains about 8% Ni, which stabilizes austenite, so it keeps an FCC austenitic structure even at room temperature. Because it does not transform on cooling, it does not harden by quenching; it is strengthened by cold working instead. In the annealed condition it is nearly nonmagnetic, but heavy cold working converts part of it to strain-induced martensite, and it becomes attracted to a magnet. Being FCC, it shows no ductile-to-brittle transition at low temperatures and can be used for cryogenic vessels. Steels that harden on quenching are martensitic grades such as SUS420J2 (similar to AISI 420), with about 13% Cr and higher carbon. Ferritic grades such as SUS430 (AISI 430) contain almost no Ni and are BCC and strongly magnetic.
ASolution heat-treated and then naturally aged at room temperature
BSolution heat-treated and then artificially aged
CWork-hardened by cold working
Show answer and explanation
Answer: B. Solution heat-treated and then artificially aged
In aluminum alloy designations, the four-digit number gives the alloy series (the 6000 series is Al-Mg-Si) and the temper designation after the hyphen gives the processing condition. T6 means solution heat-treated and quenched, then artificially aged by heating to about 150–200 °C, strengthened by fine Mg₂Si-type precipitates. Solution heat-treated and naturally aged at room temperature is T4, somewhat weaker than T6. Work hardening by cold working is designated H and is used for non-heat-treatable alloys such as the 5000 series. In addition, O means annealed (the softest condition) and F means as-fabricated. Because strength varies widely with temper even for the same alloy, drawings specify both.
FAQ
What is the difference between SS400 and S45C?
SS400 is a rolled steel for general structure; its number is the minimum tensile strength (400 N/mm²), and only upper limits for P and S are specified in its composition. S45C is a carbon steel for machine structural use whose composition is set to contain about 0.42–0.48 % C, and it is used after quenching and tempering to adjust its strength. Because the digits mean different things, these are materials chosen on different grounds even though the names look alike.
Can a “similar” foreign grade be used as a direct substitute?
Not necessarily. The foreign grades shown by this tool are regarded as similar in composition, but composition ranges, impurity limits, guaranteed mechanical properties and heat treatment conditions differ from standard to standard. Before substituting, compare the values in both standards.
Why can some designations not be read?
This tool reads, by rule, the main designations of machine structural steels, structural steels, steel sheet, tool steels, bearing steel, spring steel, stainless steels, aluminum alloys, copper alloys and cast irons. It cannot yet read cast steels (SC), forged steels (SF), heat-resisting steels (SUH), steel tubes, aluminum alloy castings (AC, ADC) and others. Even when a designation can be read by rule, that grade may not actually exist in the standard.