AISI 4340 PROPERTIES

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
EN10250	36CrNiMo4	0.32-0.4	≤ 0.4	0.5-0.8	≤ 0.035	≤ 0.035	0.9-1.2	0.90-1.2	0.15-0.3
	1.6511
BS 970	EN24	0.36-0.44	0.1-0.4	0.45-0.7	≤ 0.035	≤ 0.04	1.0-1.4	1.3-1.7	0.2-0.35
	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
Modulus of elasticity 10^3 N/mm2	100 ℃	200 ℃	300 ℃	400 ℃	500 ℃
	205	195	185	175	165
Thermal expansion 10^6 m/(m.K)	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
Diameter mm	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.