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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.
CHEMICAL COMPOSITION
| Element | C | Mn | Si | P | S | Mo |
| 4140 | 0.38~0.43% | 0.75~1.0% | 0.15~0.30% | 0.035% | 0.04% | 0.15~0.25% |
| 4130 | 0.28~0.33% | 0.70~0.90% | 0.15~0.35% | 0.035% | 0.04% | 0.15~0.25% |
| 4340 | 0.38%~0.43% | 0.60~0.80% | 0.15~0.35% | 0.035% | 0.04% | 0.20~0.30% |
| 5140 | 0.38~0.48% | 0.70~0.9% | 0.15~0.35% | 0.035% | 0.04% | - |
HEAT TREATMENT OPTION
FABRICATION AND HEAT TREATMENT
Machinability
Long, gummy chips characterize this alloys machinability. It can be
machined in the annealed condition, however condition H1150M will
yield best results. Post machining solution treatment of parts will
be required prior to final hardening if machining in this
condition.
Welding
Successfully welded by common fusion and resistance methods, this alloy should not be joined by oxyacetylene welding. AWS E/ER630 filler metal is recommended if required.
Heat Treatment
CONDITION A--Soak at 1900 F (1038 C) for 30 minutes and cool below
60 F (16 C) for complete martensite transformation. CONDITION H
950- Treat Condition A material at 900 F(482 C) for 1 hour, air
cool.. CONDITION H925, H1025, H1075, H1100, H1150- Soak solution
treated material for 4 hours at specified temperature, air cool,
CONDITION H1150M- Soak solution treated material at 1400 F (760 C)
for 2 hours, air cool, then re-heat to 1150 F (620 C) for 4 hours
and air cool.
Forging
Soak for 1 hour at 2150 F (1177 C) prior to forging. Do not work
below 1850 F (1010 C). Post-work solution treatment is required
prior to final hardening.
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.
