Forging of 4340 Alloy Steel
Preheat the steel 4340 first, heat up to 1150°C – 1200°C maximum
for forging, hold until temperature is uniform throughout the
Do not forge below 850 °C. 4340 has good forging characteristics
but care must be taken when cooling as the steel shows
susceptibility to cracking. Following forging operation the work
piece should be cooled as slowly as possible. And cooling in in
sand or dry lime is recommended etc.
AISI 4340 Steel Grade Heat Treatment
For pre-hardened steel stress relieving is achieved by heating
steel 4340 to between 500 to 550°C. Heat to 600 °C – 650 °C, hold
until temperature is uniform throughout the section, soak for 1
hour per 25 mm section, and cool in still air.
A full anneal may be done at 844°C (1550 F) followed by controlled
(furnace) cooling at a rate not faster than 10°C (50 F) per hour
down to 315°C (600 F). From 315°C 600 F it may be air cooled.
AISI 4340 alloy steel should be in the heat treated or normalized
and heat treated condition before tempering. The tempering
temperature for depends upon the strength level desired. For
strength levels in the 260 – 280 ksi range temper at 232°C (450 F).
For strength in the 125 – 200 ksi range temper at 510°C (950 F).
And don’t temper the 4340 steels if it is in the 220 – 260 ksi
strength range as tempering can result in degradation of impact
resistance for this level of strength.
Tempering should be avoided if possible within the range 250 °C –
450 °C due to temper brittleness.
- Flame or Induction Hardening
As mentioned above, pre-hardened and tempered 4340 steel bars or
plates can be further surface hardened by either the flame or
induction hardening methods resulting in a case hardness in excess
of Rc 50. AISI 4340 steel parts should be heated as quickly as
possible to the austenitic temperature range (830 °C – 860 °C) and
required case depth followed by an immediate oil or water
quenching, depending upon hardness required, workpiece size/shape
and quenching arrangements.
Following quenching to hand warm, tempering at 150°C – 200°C will
reduce stresses in the case with minimal effect on its hardness.
All de-carburised surface material must first be removed to ensure
Hardened and tempered 4340 alloy steel can also be nitrided, giving
a surface hardness of up to Rc 60. Heat to 500°C – 530°C and hold
for sufficient time (from 10 to 60 hours) to develop the depth of
case. Nitriding should be followed by slow cooling (no quench)
reducing the problem of distortion. The nitrided grade 4340
materials can therefore be machined to near final size, leaving a
small grinding allowance only. The tensile strength of the 4340
steel material core is usually not affected since the nitriding
temperature range is generally below the original tempering
Surface hardness achievable is 600 to 650HV.
Machining is best done with the alloy steel 4340 in the annealed or
normalized and tempered condition. It can be readily machined by
all conventional methods such as sawing, turning, drilling etc.
However in the high strength conditions of 200 ksi or greater the
machinability is only from 25% to 10% that of the alloy in the
Welding of steel 4340 in the hardened and tempered condition (as
normally supplied), is not recommended and should be avoided if at
all possible, because of the danger of quench cracking, as the
mechanical properties will be altered within the weld heat affected
If welding must be carried out, pre-heat to 200 to 300°C and
maintain this while welding. Immediately after welding stress
relieve at 550 to 650°C, prior to hardening and tempering.
If welding in the hardened and tempered condition is really
necessary, then the work piece, immediately on cooling to hand
warm, should be if possible stress relieved at 15 °C below the
original tempering temperature.