Understanding Materials

If the material is wrong, everything else is useless

Even if you perfectly match the dimensions according to the drawing, if the material is different, that part is defective. These are the kinds of accidents that actually happen frequently on site.

• Using SS400 in a welded structure causes cracking in the weld area
• Steel plates without a mill stamp are mixed into shipbuilding blocks and are all rejected during inspection
• Using SUS304 instead of SUS316 in parts exposed to seawater causes holes to form within a few months (official)
• Quenching S45C on a thick shaft causes the core not to harden, and it breaks during use

You don't need to memorize 30 material symbols — you need to know how to read them. If you know the rules, you can understand even symbols you've never seen before.

Decoding Steel Symbols

KS symbols are generally structured as [Material] + [Purpose·Composition] + [Number].

S S 4 0 0 │ │ └─┴─┴─ Minimum tensile strength 400 N/mm² │ └─────── Structure (general structural use) └───────── Steel (steel)

S 4 5 C │ └─┴─ Carbon content 0.45% (specification range 0.42~0.48%) │ C = Carbon └───── Steel

S C M 4 3 5 │ │ │ │ └─┴ Carbon 0.35% (specification 0.33~0.38%) │ │ │ └──── Series number (SCM4xx group) │ │ └────── Molybdenum (molybdenum) │ └──────── Chromium (chromium) └────────── Steel

S U S 3 0 4 └─┴─┴────── Steel Use Stainless └─┴─┴ 300 series = Austenitic (non-magnetic)

Here, the meaning of the numbers varies by series.

> SS400 is currently being changed to yield strength-based notation (SS275) due to recent KS revisions. SM490 used for welded structures is also being changed to SM355. Since old and new symbols may appear together on drawings, check the title block and material test certificate together.

Four Representative Steels

Why SS400 should not be used in welded structures

SS400 has only upper limits for P and S specified, with no carbon content specification. Even though it's the same SS400, the carbon content can vary by batch, making it impossible to guarantee the quality of the weld. Therefore, for structures subjected to welding loads, welding structural rolled steel (SM series) with specified composition and carbon equivalent is used.

Carbon determines the properties

Carbon is the biggest variable that changes the properties of steel.

Carbon ↑ → Strength ↑ Hardness ↑ Quenching effect ↑ Elongation ↓ Toughness ↓ Weldability ↓ Machinability ↓

> If carbon is less than 0.3%, it won't harden even after quenching. If a request comes in saying "SS400 part, but make it a bit harder," you should suggest material change or carburizing, not quenching.

Carbon Equivalent (Ceq) — the number that determines whether welding is possible

Looking at carbon alone is not enough; alloying elements like Mn, Cr, Mo also negatively affect weldability. This value, calculated as if it were just carbon, is called the carbon equivalent. The IIW (International Institute of Welding) formula is the most widely used.

Ceq = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15

Let's try calculating it. (Based on representative composition; actual calculation should be done with measured values from the test certificate)

① Welding structural steel SM355 (C 0.18, Mn 1.40) Ceq = 0.18 + 1.40/6 = 0.18 + 0.233 = 0.41

② Structural steel S45C (C 0.45, Mn 0.75, Cr 0.20, Ni 0.20, Cu 0.30) Ceq = 0.45 + 0.75/6 + 0.20/5 + (0.20+0.30)/15 = 0.45 + 0.125 + 0.040 + 0.033 = 0.65

③ Chromium molybdenum steel SCM435 (C 0.35, Mn 0.75, Cr 1.05, Mo 0.22) Ceq = 0.35 + 0.75/6 + (1.05+0.22)/5 = 0.35 + 0.125 + 0.254 = 0.73

> If you weld S45C (0.65) and SCM435 (0.73) without preheating, cracking is almost certain. Conditions vary depending on plate thickness, restraint, and hydrogen content of the welding rod, so actual preheating temperature should follow the WPS (Welding Procedure Specification). You must not set it arbitrarily.

SUS304 and SUS316

Mo prevents pitting caused by chloride ions. In environments exposed to sea breeze, such as Busan, Geoje, and Ulsan, 304 stainless steel often develops rust spots within a few months. This is why 316 stainless steel is used for ship and marine plant parts.

> SUS must be strictly separated from general steel tools and workbenches. If iron particles are transferred to the surface, rust will start from that spot (contamination). This can become a claim reason in shipbuilding and plant deliveries. Grinders, wire brushes, clamps are also kept separately as SUS专用.

When receiving materials, check the following

• Engraving/marking on the actual material matches the specified material on the drawing
• The Heat No. on the test certificate (Mill Sheet) matches the engraving on the actual material
• If it's a welded structure, carbon equivalent is present on the test certificate
• If it's certified material from a mill or prime contractor, certification stamp is applied
• If the material is to be cut and used in pieces, marking has been transferred before cutting

> Cutting removes the engraving from the original plate. If you don't transfer the grade and Heat No. to the remaining pieces, those pieces become unidentifiable materials and cannot be used. In block factories with many foreign workers, combining color tape identification and multilingual labeling is highly effective.