AISI / SAE 4150 Alloy Steel

Chemical Analysis
C% Carbon
0.48 – 0.53
Mn% Manganese
0.75 – 1.00
P% Phosphorus
0.035 max
S% Sulfur
0.040 max
Si% Silicon
0.20 – 0.35
Cr% Chromium
0.80 – 1.10
Mo% Molybdenum
0.15 – 0.25

General characteristics of 4150 Alloy Steel

AISI or SAE 4150 grade is a low-alloy steel containing chromium and molybdenum as strengthening agents. Its chemical composition is as follows:


This alloy is used as forgings in the aerospace and oil and gas industries, and has myriad uses in the automotive, agricultural and defense industries, Typical uses are forged gears, shafts, and spindles,


This steel should be forged between 2200 and 1600 º F (1200 and 870 º C.) The lower the finishing temperature from forging, the finer will be the grain size. This alloy should ideally not be forged below 1600 º F (870 º C) and should be slow cooled after forging.

Heat treatment

The alloy is heat treated to render it suitable for machining, and to meet the mechanical property ranges specified for its particular applications.


Annealing of 4150 forgings may be carried out by transferring the part straight from the forging operation to a furnace held at a suitable temperature, around 1500 º F (815 º C), for annealing, holding for a suitable time then furnace cooling, forming a structure suitable for machining. This kind of treatment is best used for parts with simple shapes. If some areas of a forging will finish much colder than others then a uniform structure will not be obtained, in which case a spheroidizing anneal at around 1380 º F (750 º C) might be used. It is safe to say that experience alone will decide the best type of annealing treatment to be used prior to machining.


Normalizing may be defined as heating a steel to a temperature above the ferrite to austenite transformation range and then cooling in air to a temperature well below this transformation range. This treatment may be carried out on forged or rolled products as a conditioning treatment prior to final heat treatment. Normalizing also serves to refine the structure of forgings that might have cooled non-uniformly from their forging operation. The nominal normalizing temperature range for 4150 grade is 1600 to 1700 º F (870 to 925 º C). followed by air cooling. In fact when forgings are normalized before, say, carburizing or hardening and tempering, the upper range of normalizing temperatures is used. When normalizing is the final heat treatment, the lower temperature range is used.


This heat treatment results in the formation of martensite after quenching and hence an increase in strength and hardness together with a significant loss of ductility. The steel should be austenitized at 1500 to 1550 º F (815 to 845 º C), the actual temperature being a function of chemical composition within the allowed range, section size and cooling method. Smaller sections of 4150 might be quenched in oil, heavier sections in water.

Tempering is carried out to relieve stresses from the hardening process, but primarily to obtain the required mechanical properties. The actual tempering temperature will be chosen to meet the required properties, and in many cases will be a matter of trial and error.


The alloy is readily machinable after annealing. The optimum structure for machining of this alloy is normally one of coarse lamellar pearlite to coarse spheroidite.


Weldability of 4150 is good, and the alloy may be welded using any commercial method. Parts should be preheated before welding at around 1100 º F (590 º C) and stress relieved after. Parts in the hardened and tempered condition should not be welded since mechanical properties will be adversely affected : parts should only be welded in the annealed condition.

Cold formability

This alloy may be cold formed on material that has a spheroidized structure. Further heat treatment, such as hardening and tempering, is carried out, as required on the finally – formed part.

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