General Characteristics

Alloy Steel 4145 AISI

This is a direct hardening chromium-molybdenum low alloy steel that may be readily forged, machined and welded. It may be heat treated to a wide range of strength and ductility levels.

Chemical Analysis

C% Mn%  Si%  P% S%    Cr Mo%
0.43/0.49 0.85/1.10 0.10/0.35 0.035 max 0.040 max 0.80/1.10 0.15/0.25


This alloy is used in the oil and gas industry as forged down-hole drilling tools such as drill collars. Other applications are forged gears, shafts for hydraulic presses, rolls for paper mills, pump shafts and tool holders.


Stock should be carefully preheated then the temperature raised to 2100-2200ºF (1150-1200ºC) for forging. Forging should not be carried out below 1600ºF (850ºC) and parts should be slow cooled after forging to prevent possible cracking.

Heat Treatment

Parts should be heated slowly to 1470-1560ºF ( 800 – 850ºC), soaked and furnace cooled to 900ºF (480ºC) then air cooled.


A typical normalizing temperature for 4145 forgings is 1600ºF (870ºC), but based on production experience this temperature may vary from 100ºF (38ºC) above to 50ºF (10ºC) below this figure. If normalizing is carried out before a hardening and tempering treatment, then the upper temperature range is used; if normalizing is the final treatment then the lower range is used.


The austenitizing temperature for this grade is in the range 1500-1550ºF (815-845ºC). Parts should be slowly heated to this temperature, soaked and oil quenched. Parts should be tempered as soon as they reach room temperature.


Parts will be tempered between 1020-1300ºF (550-700ºC) for 2 hours per inch (25 mm), then air cooled. Reduced impact values will be obtained on tempering at 500ºF (260ºC) or at 750 to 1025ºF (340 to 550ºC).
The optimum combination of strength and toughness is obtained from a microstructure that is tempered martensite. If very high strength is required then tempering is carried out at a low temperature – in fact this is a stress relief – when lightly-tempered martensite will result.


Best machinability in this alloy is obtained from a structure that runs from coarse lamellar pearlite to coarse spheroidite. Heat treatment providers should be consulted for the best treatment required to obtain these structures, always bearing in mind the complexity and section size of the parts to be machined.


The grade may be welded by fusion or resistance methods, but preheating and post-weld stress relief are recommended. The grade should not be welded in the hardened and tempered condition.