AISI/SAE 4130:: Forging Facts & Characteristics

Chemical Analysis
C% Carbon
0.28 – 0.33
Mn% Manganese
0.40 – 0.60
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 or SAE 4130 grade is a low-alloy steel containing chromium and molybdenum as strengthening agents. The steel has good strength and toughness, weldability and machinability.


This low-alloy steel finds many applications as forgings in the aerospace and oil and gas industries – as forged valve bodies and pumps –, as well as in the automotive, agricultural and defense industries.


Type 4130 steel should be forged between 2250 and 1750 º F (1230 and 950 º C.) The lower the finishing temperature from forging, the finer will be the grain size. If the alloy is forged at too low a temperature, there is a risk of the formation of a non-uniform structure in certain areas of the forged part, necessitating a normalizing treatment – prior to further heat treatment – after forging.

Heat treatment

Heat treatment after forging is carried out to render the steel suitable for machining, and to meet the mechanical property limits 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 temperature and conditions may be defined. The following information should be taken as a guide only.


Annealing of 4130 forgings may be carried out by transferring the part straight from the forging operation to a furnace held at a suitable temperature, around 1575 º F (860º C) for annealing, holding for a suitable time then furnace cooling. In this way a structure suitable for machining may be obtained. This treatment is best used for parts with simple shapes. If the forging is such that some sections (of the forging) will finish much colder than others then a uniform structure will not be obtained and for best results 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.


This process is defined as heating a steel to a temperature above the ferrite to austenite transformation temperature, then cooling in air to a temperature well below this transformation temperature. The 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 for 4130 grade is 1650 º F (900 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, thus a great increase in hardness and tensile strength, together with some brittleness. The steel should be austenitized – i.e. all micro constituents transformed to austenite – at 1500 to 1600 º F (815 to 870 º C). The actual austenitizing temperature is a function of chemical composition within the analysis range, section size and cooling method. Smaller sections of 4130 might be quenched in oil, heavier sections in water.


Tempering is carried out to relieve stresses from the hardening process, but primarily to obtain mechanical properties required for the final application. 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. Simple shapes might be machined following a normalizing treatment, whereas more complex shapes will require annealing. For optimum machinability a coarse pearlitic structure with minimum ferrite is normally recommended.

Weldability of 4130 is good, and the alloy may be welded using any commercial method.

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