4340 Steel Round Bar Sheet Plate Heat Treatment and Process

4340 Steel Round Bar Sheet Plate Heat Treatment and Process
Stress Relieving
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.

Cold Working
AISI 4340 alloy steel can be cold worked using all conventional methods in the
annealed condition. It has high ductility.

Annealing
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.

Tempering
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.

4340 Steel Heat Treatment
AISI 4340 alloy steel is heat treated at 830°C (1525°F) followed by quenching
in oil.

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-carburized surface material must first be removed to ensure the best
results.

Nitriding
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 the 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 temperature employed.

Surface hardness achievable is 600 to 650HV.

Machinability
It can be machined using all conventional techniques. Machining process can be
performed in the annealed or normalized and tempered conditions.

Welding
Welding of steel 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 zone.

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.

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