JIS G4804 Free-Cutting Steel Guide: All SUM Grades, Machinability, and Selection

JIS G4804 defines 15 free-cutting steel grades under the SUM designation, organized by the type and quantity of machinability additions: sulfur (S), phosphorus (P), and lead (Pb) — each with a distinct mechanism and performance profile. Understanding which addition is present in each grade removes the mystery from the numbering: SUM11/12 are sulfur-only grades (resulfurized); SUM21–25 add phosphorus (resulfurized and rephosphorized); SUM22L, SUM23L, SUM24L, SUM31L, and SUM43 add lead (leaded grades); the SUM3x and SUM4x series have higher carbon for applications requiring moderate strength alongside machinability. This guide maps the full series, explains why each grade exists, and provides the selection criteria for the decision points most engineers encounter: SUM22 vs SUM24L, the Pb-restriction question, and when to step up to a higher-carbon SUM grade.

1. Full Grade Composition Table

Source: JIS G4804:2008. “L” suffix grades add Pb (0.10–0.35%). P values above 0.04% indicate rephosphorized grades. S values above 0.10% indicate resulfurized grades.

2. Three Machinability Mechanisms

The three mechanisms are additive: S+P (SUM22) provides better chip breaking than S alone (SUM12); S+P+Pb (SUM24L) adds the liquid lubricant effect for the highest surface finish and lowest tool wear. Each addition also has a corresponding penalty: S reduces corrosion resistance; P reduces weldability and ductility slightly; Pb has regulatory implications.

3. Grade Family Structure

The 15 JIS G4804 grades fall into four families based on carbon content and machinability addition type:

4. Machinability by Grade

5. Higher-Carbon SUM Grades

The SUM3x and SUM4x grades serve applications where a free-cutting steel must also have moderate mechanical strength — not achievable with the near-zero-carbon SUM22 base:

SUM41–43 cannot be welded reliably (medium carbon + sulfur causes hot cracking in HAZ) and cannot be case-carburized (too high base carbon). They are used in the as-drawn or stress-relieved condition. When both machined finish and induction hardening is needed, SUM42/43 provides surface hardening to ~55–60HRC via induction on the machined surface, with the free-cutting core enabling high turning productivity.

6. Regulatory Note: Lead-Containing Grades

Grades with “L” suffix (SUM22L, SUM23L, SUM24L, SUM31L) contain 0.10–0.35% Pb. Current regulatory status:

Always check customer material specifications and IMDS requirements before specifying L grades for automotive or electronics supply chains. For non-automotive industrial applications, L grades remain a valid and common specification.

7. Selection Guide

8. FAQ

Q: Why are there so many free-cutting grades in JIS G4804?

Each grade represents a specific combination of machinability additions and carbon content that serves a distinct application niche: some applications need maximum machinability with minimum strength (SUM22), others need moderate machinability with moderate strength (SUM32, SUM41), and others need maximum machinability in a Pb-free formulation (SUM21, SUM23). The 15 grades cover this range more precisely than a smaller grade set would, allowing users to select the minimum necessary addition for their specific machinability and strength combination.

Q: Can SUM grades be cold-drawn to improve strength?

Yes — SUM bar stock is commonly supplied in the cold-drawn condition, which raises tensile strength by 10–20% over hot-rolled material through work hardening. Cold drawing also improves dimensional tolerance and surface finish of the bar. Cold-drawn SUM22 at 450–550 MPa tensile is the standard supply condition for CNC turning bar feeders. Further cold working (swaging, drawing to smaller diameter) increases strength further but reduces ductility — check elongation remains adequate for the application.