General Characteristics AISI Alloy Steel 4337
Alloy steel 4337 is a nickel-chromium-molybdenum engineering alloy steel that may be heat treated to a full range of strength, ductility and toughness levels.
Commercial and military aircraft, automotive systems, forged hydraulic and other machine tool applications, forged crankshafts.
This alloy steel should be forged between 2250ºF (1230 ºC) and 1700ºF (980ºC) and slow cooled after forging.
Heat treatment after forging is carried out to render the part suitable for machining and to meet certain mechanical property requirements. Treatments, temperatures and conditions will be learned through practice, and any parameters given here should be taken as a guide only.
To obtain a predominantly pearlitic structure, the parts should be slowly heated to around 1525ºF (830ºC), cooled to 1350ºF (730ºC) and furnace cooled to 1130ºF (610ºC) at 20ºF (11ºC) per hour, then air cooled.
This is a very time consuming operation, and as such it may be better to opt for a spheroidize anneal, heating to 1380ºF (750ºC), furnace cooling to 1300ºF (705ºC) then to 1050ºF (565ºC) at 5ºF (3ºC) per hour and air cooling. It may well be that the spheroidized structure gives better machinability than the lamellar pearlite structure.
It is always best, if in doubt, to consult with a heat treat provider.
The nominal normalizing temperature for 4337 grade is 1550ºF (840 ºC) but production experience may suggest a temperature either 50ºF (10ºC) above or below this temperature. When forgings are normalized before, say, hardening and tempering, the upper temperature range is used, whereas the lower range is used when normalizing is the final heat treatment.
The austenitizing temperature for hardening grade 4337 is 1500 – 1550ºF (815 – 845ºC). Parts should be oil quenched as hardenability is conducive to this.
If very high strength is required in the finished part then a stress relief should be used: otherwise tempering should be carried out to meet mechanical property requirements. Tempering in the range 400 – 790ºF (200 – 420ºC) may cause embrittlement and is not to be recommended.
This grade may also be hardened by nitriding or by flame or induction hardening.
A lamellar pearlite or spheroidized structure may be used for machinability reasons, depending upon the part’s section size and complexity and the amount of machining to be done. If there is any doubt it may be best to opt for the spheroidized structure.
Welding should be carried out in the annealed condition. Welding in the hardened and tempered, nitrided, flame or induction hardened conditions is not recommended. Parts should be preheated at 400 – 570ºF (200 – 300ºC), to be maintained during welding, slow cooled after welding in sand or ashes, and stress relieved if possible.