Stainless Steel Knowledge and FAQ不鏽鋼技術知識與常見問答

Stainless steel is an iron-based alloy that contains chromium (usually at least 10.5%). Chromium reacts with oxygen and forms a very thin passive oxide layer on the surface. This layer can repair itself and blocks corrosion. It is the main reason stainless steel resists corrosion.

Carbon steel does not have enough chromium, so its surface rusts easily. It usually needs painting or surface treatment. Stainless steel contains chromium and resists corrosion better, but it is not completely rust-proof. The two also differ in strength, workability, and cost. Please choose based on your use.

They are grouped by the metal's internal structure. Austenitic grades (such as 301, 304, and 316) contain nickel and have good ductility and corrosion resistance. In the annealed soft condition they are usually not magnetic, but they may become slightly magnetic after cold working. Ferritic grades (such as 430) have low nickel and are magnetic. Martensitic grades (such as 420J2) have more carbon and can be hardened by quenching and tempering. They are magnetic.

The 400 series (SUS400 series) is magnetic. The 300 series (SUS300 series) in soft temper is usually not magnetic. After cold working, it may become slightly magnetic. 301 hard temper is also magnetic because it is cold worked.

Stainless steel resists corrosion better than ordinary carbon steel, but it is not completely rust-proof. How fast it rusts depends on the type of stainless steel and the environment. The surface may also change color, and how fast depends on the environment. Common causes are: leftover pickling solution or cleaner on the surface, marks left after water dries, oxidation colors caused by heat, environments or cleaners that contain chlorides, and contamination from iron chips or carbon steel dust.

When stainless steel touches other metals such as carbon steel in a wet environment, galvanic corrosion (also called dissimilar metal corrosion) may happen. It makes the more active metal, such as carbon steel, corrode faster. It may also stain the stainless steel surface. Iron chips that stick to stainless steel can also leave rust spots on its surface.

Compared with carbon steel, stainless steel has lower thermal conductivity and lower electrical conductivity. Austenitic grades (such as 304) have a larger coefficient of thermal expansion. Ferritic grades (such as 430) have a smaller one. For exact values, please contact us or check the material data.

Both are austenitic stainless steels. (The names austenitic, ferritic, and martensitic describe the crystal structure of the steel.) 316 contains molybdenum (Mo 2.00–3.00%) and more nickel. 304 has no molybdenum. Under CNS 8499 (the Taiwan national standard), the specified yield strength, tensile strength, and elongation are the same for both. The main differences are chemical composition and corrosion resistance.

Not always. It depends on what 'better' means to you. Under CNS 8499, the specified mechanical properties are the same for both. The advantage of 316 is better corrosion resistance, which comes from its molybdenum. It suits harsher settings, such as places with chlorides. In normal settings, 304 is usually enough. 316 costs more than 304.

430 is a ferritic stainless steel. It contains 16.00–18.00% chromium. Its nickel content must not exceed 0.60%. In other words, nickel is not added on purpose.

Both are martensitic stainless steels. 420J2 has more carbon (0.26–0.40%, while 410 has 0.15% or less).

Both are martensitic stainless steels, and both can be hardened by heat treatment. The main difference is carbon content. 420J2 has more carbon (0.26–0.40%). After quenching, it can reach a higher hardness and wears less. 410 has less carbon (0.15% or less), so its maximum hardness is lower. It suits knives or industrial parts that do not need high hardness or long edge life. Please choose the knife material based on how it will be used.

Yes. 420J2 can be hardened by heat treatment.

L stands for low carbon. N stands for added nitrogen. Under CNS 8499, the carbon content of 316 is 0.08% or less, and 316L1 and 316L2 are 0.030% or less. The nitrogen content of 316N is 0.10–0.22%. 316L belongs to the 316 family, but it is a different grade from 316 because its carbon limit is different. In ASTM, 316L1 and 316L2 are usually written as 316L.

304, 316, and 430 are commonly used. Please check your use environment and local regulations before you choose.

Here are the common cross references for the stainless steel grades we supply. CNS 8499 and ASTM A240 use the same grade numbers. JIS G4305 adds SUS in front of the grade number. UNS and EN numbers are for reference only. If your order follows a specific standard, use the grade named in that standard.

Not exactly. They are close matches, but they are not identical. 420J2 is a JIS grade (JIS is Japanese Industrial Standards). AISI 420 (UNS S42000) and EN 1.4028 (X30Cr13) have similar carbon content and other composition ranges, but they are not exactly the same. AISI is the American Iron and Steel Institute. UNS is the Unified Numbering System. EN is the European standard. If you must meet a specific standard, use the grade named in that standard.

301 has no single matching grade in EN 10088. Two close grades are common: 1.4310 (X10CrNi18-8) and 1.4319 (X5CrNi17-7). Their maximum carbon contents are different. If you must meet a specific standard, please use the grade that standard names.

SUS304 is the grade name used in Japan's JIS standard (Japanese Industrial Standards). CNS 8499 uses the symbol '304' and lists JIS G4305 in its reference section. UNS S30400 (US) and EN 1.4301 (Europe) closely match it. Other grades follow the same pattern.

The main differences between 301 and 304 stainless steel are chemical composition and mechanical strength. 301 has more carbon and less nickel and chromium. This gives it higher strength and more springiness. 304 has better corrosion resistance and ductility (it stretches and bends more easily without breaking).

Under CNS 8499, in the solution annealed condition (heated to a high temperature, then cooled quickly, which leaves the metal soft): yield strength is 205 N/mm² or more, tensile strength is 520 N/mm² or more, elongation is 40% or more, and hardness is HBW 187 or less.

Under Table 9 of CNS 8499: the tensile strength of 1/2H (half hard) is 1,030 N/mm² or more, 3/4H (three-quarter hard) is 1,210 or more, and H (full hard) is 1,270 or more. These values apply to thickness of 0.30 mm and above.

They are all names of standards. CNS is from Taiwan, JIS is from Japan, and ASTM is from the US. The same material may have different names under different standards. Before you look for a material, check which names are common in each region.

2B is cold-rolled, then heat-treated and pickled (cleaned in acid), then given a light cold roll. This gives a moderately bright surface. BA is cold-rolled, then bright annealed. The surface is brighter. No.4 is polished with P150 to P180 abrasives. HL has continuous polishing lines (a hairline finish).

Material with a 2B or BA surface is usually supplied in soft temper. Surface finish and temper are separate things. Soft temper has lower hardness. Hard temper comes in these levels, from softer to harder: quarter hard (1/4H), half hard (1/2H), three-quarter hard (3/4H), full hard (H), extra hard (EH), and super extra hard (SEH). Sursun Metals supplies both soft and hard temper stainless steel. Please contact us. Hardness ranges for reference (HV) are as follows. For 304: 1/2H is 250–310, 3/4H is 310–370, and H is 370 and above. For 301: 1/2H is 310–370, 3/4H is 370–430, H is 430–490, EH is 490–530, and SEH is 530 and above.

Sursun Metals supplies stainless steel grades 304, 316, 410, 420J2, and 430 in soft (annealed) temper. We also supply 301 and 304 in hard temper. If you have other special needs, please call or email us.

Please give us the grade (such as 304), the temper and surface (for example soft temper 2B, soft temper BA, or a hard temper level), the size, and the quantity.