4121

Alloy Steel

4121

Cr-Mo carburizing steel. (AISI; equiv GB 20CrMo / DIN 20CrMo4 / JIS SCM420). Nominal values—confirm mill certificate.

Chemical Composition (%)

C0.18
MN0.7
CR0.4
MO0.08

Mechanical Properties

Tensile:≥885 MPa;Yield:≥685 MPa

Equivalent Grades

AISI 4121 / GB 20CrMo / DIN 20CrMo4 / JIS SCM420

Applications

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Available Forms

Forging"

Uses

Carburizing (case-hardening) steel with low carbon core for toughness and a hard, wear-resistant surface after carburizing. Used for gears, camshafts, pinions, spline shafts, bushings and transmission parts subjected to fatigue and surface wear.

Heat Treatment

Typically normalized (880–920 °C, air cool) or carburized (900–930 °C) for case-hardening grades, then quenched and tempered at 150–200 °C. Carburize 900–930 °C, direct/intake quench, temper 150–200 °C; useful case depth 0.5–1.5 mm.

Precautions

Weldable with standard procedures; preheat may be needed for thick sections or higher-carbon/ alloy grades. Protect from decarburization during heating (controlled atmosphere or protective coating). Machine the core stock before surface hardening; leave grinding allowance for the hardened case.

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