SS400 and SM400 both deliver 400–510 MPa tensile strength, yet specifying the wrong one for a welded structure can invalidate your welding procedure, fail a government inspection, or cause a hydrogen-induced crack weeks after fabrication. This guide explains the difference — and what to order when sourcing outside Japan.

Note for International Readers: JIS vs. Global Standards

Both SS400 and SM400 are JIS designations. They do not appear by these names in ASTM, EN, or DIN catalogs. The equivalence is more nuanced than a simple 1-to-1 mapping, because the distinction between the two grades — guaranteed weldability — is handled differently in Western standards.

How to Read These JIS Codes

Grade Equivalents Overview

The Carbon Equivalent (Ceq) — The Core Difference

The key metric that separates SM400 from SS400 is the Carbon Equivalent (Ceq), calculated as:

How Ceq Compares Across Standards

1. Grade Codes and Standard Positioning

2. Composition and Mechanical Properties — Same Numbers, Different Guarantees

Sources: JIS G3101:2015, JIS G3106:2017

3. Three Situations Where SS400 Causes Real Problems

Situation 1: Thick-plate welded structures (t > 16 mm)

When welding plates thicker than 16 mm (0.63 in), the carbon content and heat input must be controlled to prevent HAZ cold cracking. Because SS400 has no carbon upper limit, the Welding Procedure Specification (WPS) cannot be written with a clear basis for eliminating preheat. A mill certificate saying “SS400” tells you nothing about whether Ceq is 0.30% or 0.48% — and the difference determines whether you need to preheat to 50°C or 150°C (122°F or 302°F).

Situation 2: Government-specified and structural-permit work

In Japan, bridges, buildings requiring confirmation (建築確認), and public infrastructure typically mandate SM-series or SN-series steel by specification. Submitting SS400 mill sheets for an SM400B requirement will fail the acceptance inspection — regardless of the actual tensile properties.

Situation 3: Overseas procurement and third-party inspection

When sourcing internationally, SS400’s undefined carbon spec allows carbon contents above 0.28% from some producers. If a weld cracking incident occurs, the absence of Ceq documentation creates a grey area for liability. SM400B’s Ceq ≤ 0.36% is verifiable from the mill certificate and provides a clear chain of quality evidence.

4. SM400B and the Ceq Guarantee — The Basis for Preheat-Free Welding

SM400A vs SM400B vs SM400C — Which to Specify?

5. Substitution Possibility Matrix

6. Procurement and Design Checklist

For designers:

• Does the part have any welded joints? → If yes, specify SM400B as a minimum
• Is plate thickness > 16 mm (0.63 in)? → Verify Ceq on mill certificate
• Is the part used in temperatures below 0°C (32°F)? → Specify SM400B or SM400C
• Is this a public works or building-permit project? → Check whether SM or SN material is mandated
• Is there a possibility of overseas procurement? → Make SM400B (or equivalent with Ceq ≤ 0.36%) a contractual requirement

For procurement / purchasing:

• When SS400 is specified: confirm whether welding is involved before placing the order
• For SM400B/C: verify the Ceq value on the mill certificate before accepting delivery
• When substituting from stock: obtain written designer approval before using SS400 in place of SM400B
• For overseas-sourced material: require individual heat analysis and Ceq documentation, not just tensile certification
• Provide separate line items in quotations when spec differences affect price

7. JIS and International Standard Cross-Reference

Sources: JIS G3101:2015, JIS G3106:2017, JIS G3136:2012, ASTM A36/A36M-19, EN 10025-2:2019

8. Application Guide: Which Grade to Specify

Summary

Original article (Japanese): SS400とSM400について調べてみた — tasuichi.jp