AISI 4340, 1.6511, 36CrNiMo4, SNCM439
AISI 4340 is a type of low alloy
steel that can be heat treated and strengthened with 0.8% Cr, 0.2% Mo, and 1.8%
Ni alloy elements. In comparison to AISI 4140, AISI 4340 offers higher strength
and toughness, along with excellent fatigue, wear, and atmospheric corrosion
resistance. Typically provided in a hardened and tempered condition, AISI 4340
has a tensile strength ranging from 930-1080 Mpa and a hardness of 280-320HB.
The pre-hardened and tempered material can also be surface-hardened through
flame or induction hardening, as well as nitriding. This ultra-high strength
steel is classified as a medium-carbon, low-alloy steel, and has superior ductility,
toughness, creep resistance, and fatigue resistance compared to most other
steels. Depending on the heat treatment, the alloy can achieve high degrees of
hardenability, with HRC hardnesses ranging from 24 to 53.
WHAT IS AISI 4340 STEEL?
AISI 4340 Steel is a particular
type of steel alloy assigned prefix code by the AISI system. This letter prefix
is mainly used to denote the steelmaking process of the AISI 4340 Steel.
The grade number in the steel
alloys makes sure to differentiate their industrial applications.
4: The first
number ‘4’ indicates that the steel is ‘Molybdenum’ steel and Molybdenum
is most important alloy element for this steel as compared to other steel
series.
3:The second number’3′
means that three (3) elements are present, and they include nickel,
chromium, and molybdenum.
40: The last two
numbers ‘40’ indicates the amount of carbon present in the steel. For this
grade it has 0.40% carbon same as AISI 4140 steel.
Moreover, this particular number
is assigned to almost all the steel alloys by AISI to designate the different
steel compositions in its alloys.
Is AISI 4340 Stainless Steel?
Stainless steel has at least
10.5% Chromium, and the maximum carbon content is not more than 1.2%.The main
characteristics for these kind materials are good rust resistance and corrosion
resistance.
While AISI 4340 is defined as a
heat-treatable low-alloy steel containing 0.8% Cr, 0.2% Mo and 1.8% Ni as
strengthening alloying elements. Compared with Stainless steel, it has higher
strength and toughness, but its wear resistance and atmospheric corrosion
resistance are not as good as stainless steel.
Is AISI 4340 Steel
Corrosion Resistant?
AISI 4340 steel is a
medium-carbon nickel-chromium-molybdenum steel which has excellent
tensile strength, toughness and fatigue resistance.
AISI 4340 Steel is highly
resistant to atmospheric corrosion because of its combined alloy elements of
chromium, molybdenum, nickel and manganese which can not only improve the
strength and hardenability of steel, but also improve corrosion resistance.
The corrosion-resistant nature of
the AISI 4340 Steel is responsible for its use in the forged hydraulic
systems, and it is also used in many other machine appliances, including those
involved in the aerospace industry.
Is AISI 4340 Steel Magnetic?
AISI 4340 Steel is manufactured
with many basic properties, including high magnetic properties. Other than
magnetic properties, the AISI 4340 Steel is also equipped with durable
mechanical properties.
This steel allows very
substantial magnetic properties because a few percent of alloying elements are
present. The magnetization of the AISI 4340 steel is estimated at up to 21500
Gauss.
Can You Machine AISI 4340
Steel?
Yes, you can machine AISI 4340
Steel with the help of almost all types of conventional techniques. No excess
carbon contents are present, that’s why AISI 4340 steel can be machined easily.
This grade is easy to process,
depending on the size and complexity of the section and the amount of
processing to be carried out. The machining of AISI 4340 steel is carried out
under an assumed shape.
However, the recommended
machining process for AISI 4340 Steel is carried out under annealed, normalized
and tempered environments.
How is The Weldability of AISI
4340 Steel?
AISI 4340 Steel is recommended to
weld in the annealed condition, but welding in the quenched and tempered state
should be avoided as much as possible, because this will affect the mechanical
properties. It is not recommended to weld under nitriding, flame or induction
hardening conditions.
In addition to this, the
weldability of the AISI 4340 steel is carried out by preheating the material at
200 to 300 degrees. This high temperature for the welding is maintained to
ensuring the sustainability of the substrate. The welded parts should be cooled
slowly in the ashes or sand, and the stress should be relieved as much as
possible.
EQUIVALENT INTERNATIONAL GRADES
We can see the differences
between different national standards from the table below.
Chemical Composition
Standard |
Grade |
C |
Si |
Mn |
P |
S |
Cr |
Ni |
Mo |
ASTM A29 |
4340 |
0.38-0.43 |
0.15-0.35 |
0.6-0.8 |
≤ 0.035 |
≤ 0.04 |
0.7-0.9 |
1.65-2.0 |
0.2-0.3 |
|
36CrNiMo4 |
|
|
|
|
|
|
|
|
1.6511 |
|||||||||
|
EN24 |
|
|
|
|
|
|
|
|
817M40 |
|||||||||
JIS G4103 |
SNCM439 |
0.36-0.43 |
0.15-0.35 |
0.6-0.9 |
≤ 0.03 |
≤ 0.03 |
0.6-1.0 |
1.6-2.0 |
0.15-0.3 |
GB 3077 |
40CrNiMoA |
0.37-0.44 |
0.17-0.37 |
0.5-0.8 |
≤ 0.025 |
≤ 0.025 |
0.6-0.9 |
1.25-1.65 |
0.15-0.25 |
Physical Property
Density g/cm3 |
7.85 |
||||
Melting point °C |
1427 |
||||
Poisson's ratio |
0.27-0.30 |
||||
Machinability (AISI 1212 as 100%
machinability) |
50% |
||||
Thermal expansion co-efficient µm/m°C |
12.5 |
||||
Thermal conductivity W/(m.K) |
44.5 |
||||
Modulus of elasticity 10^3 N/mm^2 |
210 |
||||
Electric resistivity Ohm.mm2 /m |
0.19 |
||||
Specific heat capacity J/(kg.K) |
460 |
||||
|
100 ℃ |
200 ℃ |
300 ℃ |
400 ℃ |
500 ℃ |
205 |
195 |
185 |
175 |
165 |
|
|
100 ℃ |
200 ℃ |
300 ℃ |
400 ℃ |
500 ℃ |
11.1 |
12.1 |
12.9 |
13.5 |
13.9 |
4. Mechanical Property
Mechanical Condition |
T |
U |
V |
W |
X |
Y |
Z |
Ruling Section (mm) |
150 |
100 |
63 |
30 |
30 |
30 |
30 |
Tensile Strength Mpa |
850-1000 |
930-1080 |
1000-1150 |
1080-1230 |
1150-1300 |
1230-1380 |
>1550 |
Yield Strength, Mpa |
≥665 |
≥740 |
≥835 |
≥925 |
≥1005 |
≥1080 |
≥1125 |
Elongation % |
≥13 |
≥12 |
≥12 |
≥11 |
≥10 |
≥10 |
≥5 |
Izod Impact J |
≥54 |
≥47 |
≥47 |
≥41 |
≥34 |
≥24 |
≥10 |
Charpy Impact J |
≥50 |
≥42 |
≥42 |
≥35 |
≥28 |
≥20 |
≥9 |
Brinell Hardness HB |
248-302 |
269-331 |
293-352 |
311-375 |
341-401 |
363-429 |
>444 |
5.High Temperature Strength
For quenched and tempered heavy forgings |
|||||||
|
Yield strength MPa |
||||||
20 ℃ |
100 ℃ |
200 ℃ |
250 ℃ |
300 ℃ |
350℃ |
400℃ |
|
≤250 |
590 |
549 |
510 |
481 |
441 |
412 |
371 |
250-500 |
540 |
505 |
471 |
451 |
412 |
383 |
353 |
500-750 |
490 |
466 |
441 |
422 |
392 |
363 |
343 |
Forging
Forging temperature should be
carried out between 1150℃-1200℃,The lower
the forging-ending temperature ,the finer the grain size .hold suitable time
for the steel to be thoroughly heated before forge, but don’t forge below minimum forging temperature 850°C. AISI
4340 has good forging characteristics, but crack is easily occured when improper
cooling way after forged, so it should be cooled as slowly as
possible in still air or in sand after forged.
Normalizing
Normalizing is used to
refine the structure of forgings that might have cooled non-uniformly after forged,
and considered as a conditioning treatment before final heat
treatment. Normalizing temperature for AISI 4340 steel should be carried out
between 850℃-880℃. hold suitable time for the steel
to be thoroughly heated to complete the ferrite to austenite transformation.
Cool in still air.
Annealing
Full annealing is recommended for
AISI 4340 before machining, AISI 4340 should be carried our at a nominal
temperature of 830℃-850℃,hold suitable time for the steel
to be thoroughly heated, then furnace cooling to 610℃ at a rate of
11℃
per hour, finally air cooling.
Hardening
This heat treatment will obtain
martensite structure after quenched. It will increase the surface hardness and
strength.AISI 4340 should be carried out between 830℃-865℃,
hold suitable time for the steel to be thoroughly heated, soak for 10-15
minutes per 25 mm section, oil quench is recommended. Tempering should be
followed immediately after quenched.
HOW TO HARDEN AISI 4340 STEEL?
AISI 4340 alloy structural steel
belongs to gear steel which has high strength, toughness and outstanding
hardenability and anti-thermal stability.
AISI 4340 Steel is used for heavy
machinery high-load shafts, turbine shafts with a diameter greater than 250 mm,
helicopter rotor shafts, turbojet engine turbine shafts, blades, high-load
transmission parts, crankshafts, gears,etc.
But before we use them for above
applications, we need to harden the materials according to requirements. In the
actual production process,we often use water quench and oil quench to
harden AISI 4340 Steel.
Water Quench |
Oil Quench |
|
Quench Medium |
Water |
Oil |
Quench Temperature |
850~870℃ |
850~870℃ |
Quench Time |
Normal |
Longer |
Cooling ability |
Better |
Normal |
Crack Resistance |
Poor |
Normal |
Deformation |
Bigger |
Normal |
Hardness |
Higher |
Normal |
Hardenability |
Better |
Normal |
Tempering Following |
Immediately |
Immediately |
HARDNESS OF AISI 4340 STEEL
When we use AISI 4340 steel for applications,
we choose its high strength, high hardness and excellent toughness. Normally, AISI
4340 steel has a hardness below 229 HBW under annnealed conditon, 248-302HBW
under quenched and tempered condition. With suitable hardening process, it can
even achieve surface hardness up to 60HRC.
But can it be considered that the
higher the hardness, the better the performance? Of course not.
The higher the hardness, the
greater the strength, but the plasticity and toughness decrease. Especially in
the case of quenching and tempering conditon, in order to ensure the strength,
it is necessary to find a balance point between the strength and the toughness.
Tempering
AISI 4340 alloy steel should be
in the heat treated or normalized and heat-treated condition before tempering.
Tempering is usually carried out to relieve stresses from the
hardening process, but primarily to obtain the required hardness and
mechanical properties. The actual tempering temperature will be chosen to meet
the required properties.it is usually carried out at 450℃-
660℃,
hold until temperature is uniform throughout the section, soak for 1 hour per
25 mm of section, and cool in still air. Tempering between 250℃-450℃
is not avoided as tempering within this range will seriously reduce the impact
value, result in temper brittleness.
Application
AISI 4340 is often used in
preference to AISI 4140 at the higher strength levels because of its better
hardenability and improved CVN impact toughness.
Typical applications
include: Heavy-duty axles, shafts, heavy-duty gears, spindles, pins, studs,
collets, bolts, couplings, sprockets, pinions, torsion bars, connecting
rods, crow bars, conveyor parts, forged hydraulic, forged steel crankshafts etc.