AISI / SAE 4140 Alloy Steel: Forging Facts

Chemical Analysis
C Carbon
0.38 – 0.43
Mn Manganese
0.75 – 1.00 max
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 AISI / SAE 4140

AISI or SAE 4140 grade is a low-alloy steel containing chromium and molybdenum as strengthening agents. Its chemical composition is seen to the right:

AISI/SAE 4140 grade is a versatile alloy with good atmospheric corrosion resistance and reasonable strength up to around 600º F (315º C.) It shows good overall combinations of strength, toughness, wear resistance and fatigue strength.

Forging 4140 Alloy Steel

Forging of this steel should be carried out between 2200 and 1650 º F (1200 and 900 º C). The lower the finishing temperature from forging, the finer will be the grain size. This alloy should ideally not be forged below 1650º F (900º C) and should be slow cooled after forging.

Applications of AISI / SAE 4140

This alloy finds many applications as forgings for the aerospace and oil and gas industries, along with myriad uses in the automotive, agricultural and defense industries, Typical uses are forged gears and shafts, spindles, fixtures, jigs and collars.

Heat Treatment

Heat treatment is carried out after hot working to render the steel suitable for machining, and to meet the mechanical property ranges specified for the steel’s particular applications. There are several ways to heat treat a given steel part and it is only through practice that the optimum treatments, temperatures and conditions may be defined. The following information should be taken as a guide only.


Forgings of 4140 grade may be annealed by transferring the parts straight from the forging operation to a furnace held at a suitable temperature, between 1450 and 1550 º F (790 and 840 º C), 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) may 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 4140 Alloy Steel

This process is 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 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 for 4140 grade is 1600 º F (870 º C), but production experience may necessitate a temperature either 50 º F (10 º C) above or below this figure. 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, hence a great increase in hardness and tensile strength together with some 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. Austenitizing should ensure all micro-constituents in the steel are transformed to austenite. Smaller sections of 4140 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. Tempering should not be carried out between 400 – 790 º F (200 – 420 ºC) to avoid the danger of embrittlement.


The alloy is readily machinable. Simple shapes might be machined following a normalizing treatment, whereas more complex shapes will require annealing. At the carbon level of this grade, a structure of coarse lamellar pearlite to coarse spheroidite is normally optimum for machinability.


This grade has good weldability and may be welded using any commercial method. Parts should be preheated before welding at 400-600 º F (200-300 º C), slow cooled from welding and stress relieved after. Parts in the hardened and tempered or surface-hardened conditions should not be welded since mechanical properties will be adversely affected. Parts should be welded in the annealed condition only.

Low-hydrogen electrodes are recommended together with preheat at 300 – 500 º F (150 – 260 º C.) to be maintained during welding, Cool slowly and stress relieve where possible.

 Cold formability
Any cold forming on this alloy should be carried out on material with a spheroidized structure. Further heat treatment, hardening and tempering, is carried out where applicable on the finally formed part.

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