One Steel, Three Designations
SCM440, 42CrMo4 and AISI 4140 are different names for the same chromium-molybdenum steel grade: SCM440 per JIS G4105, 42CrMo4 per EN 10083-3 with material number 1.7225, and 4140 per ASTM A29. The grade contains about 0.38 to 0.45 percent carbon with chromium and molybdenum, which give deep hardenability and high strength after quenching and tempering.
Because the designations are equivalent, the grade can be ordered to the standard required by the end-user's country, with the same general properties.
Chemical Composition
The composition across the three designations falls in the following typical ranges:
Carbon: 0.38 to 0.45 percent.
Silicon: 0.10 to 0.40 percent.
Manganese: 0.60 to 1.00 percent.
Chromium: 0.80 to 1.20 percent.
Molybdenum: 0.15 to 0.30 percent.
Mechanical Properties After Heat Treatment
The grade is normally supplied soft-annealed or in the quenched and tempered condition. For quenched and tempered bars in the common 16 to 40 mm range, typical properties are:
Tensile strength: 900 to 1100 MPa.
Yield strength: 650 MPa minimum.
Hardness: 270 to 330 HB depending on tempering.
Elongation: minimum 12 percent.
In the soft-annealed condition the bar is supplied at about 240 HB maximum for good machinability, and larger diameters show slightly lower guaranteed values after hardening.
Forged and Rolled Supply
Round bars are produced either by hot rolling for diameters up to about 250 mm, or by open-die forging for larger diameters up to 600 mm and above. Forged bars have a more uniform center quality and finer grain, which is important for highly stressed parts and for further forging. Both forms are delivered black, turned or ground, in 3 to 6 m lengths.
Applications
The grade is used for shafts, spindles, gears, connecting rods, crankshafts, bolts, axles, tool holders, mould parts and hydraulic components. Its deep hardenability allows induction hardening and nitriding for wear resistance, and its fatigue strength suits rotating and reciprocating machinery.
Frequently Asked Questions
Q: Are SCM440, 42CrMo4 and 4140 the same steel?
They are equivalent chromium-molybdenum grades under JIS, EN and ASTM, with the same general composition and property range.
Q: What does 1.7225 mean?
It is the EN material number for 42CrMo4, used in European specifications and certificates.
Q: What is the difference between forged and rolled round bar?
Forging compresses the center of the bar, giving denser structure and better mechanical properties for large diameters; rolling is more economical for standard sizes.
Q: Can the steel be induction hardened?
Yes, the chromium and molybdenum content gives excellent response to induction hardening for gear teeth and shaft journals.
Q: What condition is best for machining?
The soft-annealed condition at about 240 HB maximum gives the best machinability before final hardening.