AISI / Alloy Steel 4125

Chemical Analysis
C% Carbon
0.23 – 0.28
Mn% Manganese
0.70 – 0.90
P% Phosphorus
0.040 max
S% Sulfur
0.040 max
Si% Silicon
0.20 – 0.35
Cr% Chromium
0.40 – 0.60
Mo% Molybdenum
0.20 – 0.30

General characteristics of Alloy Steel 4125

This is a chromium-molybdenum alloy steel used in the case-hardened condition and displaying good strength and toughness properties.


The alloy is used for components of medium and large cross section, requiring high strength and toughness properties for automotive engineering and gear and engine construction, such as forged crankshafts, steering knuckles, connecting rods, spindles, intermediate gears, pump and gear shafts.


The alloy may be forged at a temperature in the region of 2200ºF (1205ºC) down to a temperature of around 1700ºF (925ºC) and subsequently air cooled.

Heat treatment

Annealing: The alloy may not normally be annealed since optimum machining performance at this carbon level may be found in either an as-forged or normalized structure, the normalized structure being best suited to smaller sections.

Normalizing: At around 1650ºF (900ºC) and air cooling. Normalizing is advantageous prior to carburizing.

Hardening:Austenitize at 1545 – 1615ºF (840 – 880ºC) and water or oil quench depending on size and complexity of part.

Tempering: Temper at a temperature that will be based on past experience and will give the required mechanical properties.


This alloy is readily machinable providing the appropriate heat treatments are employed.


This alloy is readily weldable by normal fusion methods, but should always be welded prior to carburizing and other heat treatments.

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