“Can we just use SS400 for the shaft?” — a common question on the mechanical design floor. The answer depends on load conditions, heat treatment requirements, and whether welding is involved. This guide walks you through a decision framework for Japan’s three most common engineering steels, with ASTM/EN equivalents, chemical composition data, and practical sourcing advice for engineers outside Japan.

Note for International Readers: JIS vs. Global Standards

This article is based on Japanese Industrial Standards (JIS). SS400, S45C, and SCM435 will not appear by these names in ASTM, EN, or DIN catalogs. The sections below explain how to read JIS codes, where the compositions differ from Western equivalents, and what to actually order if you are sourcing outside Japan.

How to Read JIS Material Codes

Once you know this system, other JIS grades become self-explanatory — S35C, S55C, SCM440, and so on.

Grade Equivalents Overview

Chemical Composition Comparison

SS400 vs. ASTM A36

Key difference: SS400 does not specify a carbon maximum; A36 caps it at 0.25%. In practice, both are functionally interchangeable for structural and welded applications.
Sources: JIS G3101:2015, ASTM A36/A36M-19

S45C vs. ASTM 1045 / EN C45

Key difference: S45C has tighter P and S limits, reflecting slightly stricter cleanliness requirements. For most engineering applications the difference is negligible; for precision fatigue-critical parts, the JIS spec provides a small quality margin.
Sources: JIS G4051:2016, ASTM A29/A29M, EN 10083-2

SCM435 vs. ASTM 4135 / 4140

Key differences: SCM435 allows slightly higher Cr (up to 1.20%) and Mo (up to 0.30%) than 4135, giving marginally better hardenability. ASTM 4140 has higher carbon (0.38–0.43%) — it achieves higher hardness but slightly lower toughness at the same tempering temperature. Note: 4140 corresponds more closely to JIS SCM440, not SCM435.
Sources: JIS G4105:2015, ASTM A29/A29M

Practical Sourcing Guide for Engineers Outside Japan

SS400, S45C, and SCM435 will not appear in Western steel distributor catalogs. Here is what to order instead:

1. Basic Comparison of the Three Materials

2. Material Selection Decision Flow

Step 1: Is welding required?

If the part has welded joints, SS400 (≈ A36) is the most practical choice. S45C (≈ 1045) requires preheat to 75–150°C (167–302°F) before welding and carries a risk of cold cracking. SCM435 (≈ 4135) is generally considered unweldable in structural applications and should not be used in welded assemblies.

Step 2: Is hardening or surface hardening required?

If high surface hardness (40 HRC or above) is needed, choose S45C or higher. SS400 has low carbon content and shows almost no response to hardening. For large cross-sections (diameter 50 mm / 2 in or more) where uniform hardness through the full section is required, SCM435 (Cr-Mo steel) is necessary.

Step 3: Is high tensile strength required?

If the design calls for tensile strength ≥800 MPa (116 ksi) or yield strength ≥600 MPa (87 ksi), select SCM435 after quench and temper (Q&T). After Q&T, S45C reaches approximately 700–800 MPa (100–116 ksi) tensile strength. SS400 cannot be strengthened through heat treatment.

Step 4: Is fatigue strength or long-term durability required?

For shafts, gears, and bolts subjected to cyclic loading, SCM435 is the best choice. The Cr and Mo content suppresses temper embrittlement, and SCM435 offers superior toughness and fatigue strength compared to S45C.

3. Application Guidelines by Material

When SS400 (≈ A36) is the right choice

Frames, bases, brackets, and plate structures. Welded assemblies where high hardness or high strength is not required. When cost is the top priority and tensile strength ≥400 MPa (58 ksi) is sufficient. For non-heat-treated applications, SS400 is often entirely adequate.

When S45C (≈ 1045) is the right choice

Small to medium shafts, gears, pins, and jigs/fixtures. When induction (surface) hardening to 55–60 HRC is needed. S45C offers the best balance of cost and strength and is widely used as the “standard material” for machine components.

When SCM435 (≈ 4135/4137) is the right choice

Large-diameter shafts, high-strength bolts (Grade 10.9 / 12.9 equivalent), crankshafts, and connecting rods. When the cross-section is large and uniform through-hardening is required, or when high fatigue strength and toughness are critical for power-transmission components.

4. Hardness and Effective Case Depth: How They Differ

5. Case Studies: Real-World Failures

6. Substitution Possibility Matrix

7. Material Selection Checklist

• Have you confirmed whether any weld joints are present? (If yes, SS400 or SM-series structural steel is the baseline choice.)
• Have you confirmed whether heat treatment (quench and temper) is required?
• If the cross-section exceeds 30 mm (1.2 in) diameter, have you verified that S45C hardenability is adequate?
• If tensile strength ≥800 MPa (116 ksi) is required, have you specified SCM435?
• Are you using SS400 in power-transmission components subject to cyclic loading?
• Are you using S45C for high-strength bolts (Grade 10.9 or higher)?

Summary

Original article (Japanese): SS400・S45C・SCM435の選び方をやさしく解説 — tasuichi.jp