1. What JIS G4051 Covers
JIS G4051 is the Japanese standard for carbon steels for machine structural use, covering round bars, flat bars and other sections used in shafts, gears, bolts and machine parts. The standard defines the chemical composition and the mechanical properties of grades from S10C to S58C, where the letter S stands for steel, the number is the nominal carbon content in hundredths of a percent, and C indicates carbon steel.
2. The Grade Designation System
The grade name directly tells the carbon content: S45C has a nominal carbon content of 0.45 percent, S20C has 0.20 percent, and S58C has 0.58 percent. The standard also distinguishes the steelmaking and delivery condition by suffixes, so S45C can be supplied normalized, annealed or as rolled, and the mechanical properties are guaranteed according to the delivery condition and the size range.
3. Mechanical Properties by Carbon Content
As the carbon content rises, the strength increases and the ductility and weldability decrease. The low-carbon grades S10C to S25C are soft, formable and weldable, and are used for lightly stressed parts and for carburizing. The medium-carbon grades S35C to S55C are hardened and tempered for shafts, gears and pins, while the high-carbon S58C is used for springs and wear parts after suitable heat treatment.
4. Related Standards and Alloy Grades
JIS G4051 covers plain carbon grades only. The alloy steel grades for machine structural use are covered by JIS G4105 for chromium steels such as SCM440, and JIS G4106 for nickel-chromium and nickel-chromium-molybdenum steels such as SNCM439. The naming follows the same logic, with SCM440 meaning a chromium-molybdenum steel with 0.40 percent carbon, and the mechanical properties are defined for the hardened and tempered condition.
5. Ordering and Testing Practice
When ordering round bar to JIS G4051, the specification states the grade, the diameter, the length, the delivery condition and the heat treatment. Mill certificates document the heat number, the chemical composition and the mechanical properties tested in the delivery condition. For critical parts, the buyer may specify additional testing such as ultrasonic examination or hardenability testing to confirm the bar is suitable for the intended heat treatment.
Frequently Asked Questions
Q: What does S45C mean?
S means steel, 45 is the nominal carbon content in hundredths of a percent, and C indicates a carbon steel, so S45C contains about 0.45 percent carbon.
Q: Which standard covers alloy round bars in Japan?
Alloy steels for machine structural use are covered by JIS G4105 for chromium steels and JIS G4106 for nickel-chromium steels, while JIS G4051 covers plain carbon grades.
Q: Is S45C weldable?
S45C can be welded with care, using preheat and low-hydrogen consumables, because the medium carbon content increases the hardening risk in the heat-affected zone.
Q: What are the low-carbon JIS G4051 grades used for?
Grades S10C to S25C are used for lightly stressed parts, forming work and carburized components where a soft, tough core is needed.
Q: How is the delivery condition specified?
The order states whether the bar is delivered as rolled, normalized or annealed, and the mechanical properties are guaranteed for that condition.
Q: What is SCM440?
SCM440 is a chromium-molybdenum steel of JIS G4105 with about 0.40 percent carbon, used hardened and tempered for shafts, gears and heavy-duty parts.