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AISI 4340 steel
is a medium carbon, low alloy steel known for its toughness and
strength in relatively large sections. AISI 4340 is also one kind of
nickel chromium molybdenum steels. 4340 alloy steel is generally
supplied hardened and tempered in the tensile range of 930 – 1080 Mpa.
Pre hardened and tempered 4340 steels can be further surface hardened by
flame or induction hardening and by nitriding. The 4340 steel has good
shock and impact resistance as well as wear and abrasion resistance in
the hardened condition. AISI 4340 steel properties offer good ductility
in the annealed condition, allowing it to be bent or formed. Fusion and
resistance welding is also possible with our 4340 alloy steel. ASTM 4340
material is often utilized where other alloy steels do not have the
hardenability to give the strength required. For highly stressed parts
it is excellent choice. AISI 4340 alloy steel can also be machined by
all customary methods.

Due to availability the ASTM 4340 grade steel is often substituted
with European based standards 817M40/EN24 and 1.6511/36CrNiMo4 or Japan
based SNCM439 steel. You have the detailed data of 4340 steel below.

1. AISI 4340 Steel Specification and Relevant Standards

2. ASTM 4340 Steels And Equilvalents Chemical Composition

3. AISI Alloy 4140 Steel Mechanical Properties

Thermal Properties

4. 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 section.

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.

5. AISI 4340 Steel Grade Heat Treatment

• 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.

• 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.

• 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 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 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.

6. Machinability

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 annealed condition.

7. Welding

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 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.

8. Application of 4340 Steel

AISI
4340 steel is used in most industry sectors for applications requiring
higher tensile/yield strength than 4140 steel can provide.

Some typical applications such as:

• Aircraft Landing Gear

• Automotive,

• Oil and Gas Drilling,

• Forging,

• Warm and Cold Forming,

• Machine Building,

• Transfer Systems, like power transmission gears and shafts.

• General
  engineering industries and structural use applications, such as: heavy
  duty shafts, gears, axles, spindles, couplings, pins, chucks, molds etc.