AISI L6 | 1.2714 | SKT4

L6 is defined as L-Type, low alloy special purpose tool steel as per ASTM A681.

As a steel with a long history and reliable performance, the core advantage of L6 is its top toughness, which is comparable to S7 steel (impact-resistant tool steel) and even better in some cases, and much higher than many other tool steels (such as O1, A2, D2). This makes it an ideal choice for withstanding severe impact, bending or twisting without breaking, especially in the field of tool manufacturing and specific industrial molds (impact-resistant parts).

Through heat treatment, L6 can usually obtain good wear resistance and hardness (HRC 55-60) due to its relatively high carbon content, and a good balance between toughness and wear resistance can be achieved. In situations where higher toughness is required, some hardness can be sacrificed to obtain higher toughness.

After the working temperature of L6 steel exceeds 300°C, its hardness, strength and wear resistance will drop sharply. Therefore, you need to be careful to choose more suitable materials in high temperature and high wear scenarios, such as hot work tool steel H series: H10, H11, H13, H21.

CHEMICAL COMPOSITION：L6 & EQUIVALENT GRADE

Standard	Grade	C	Si	Mn	P	S	Cr	Ni	Mo	V
ASTM A681	L6	0.65-0.75	0.1-0.5	0.25-0.80	≤0.03	≤0.03	0.6-1.2	1.25-2.0	≤0.50	—
EN ISO 4957	55NiCrMoV7	0.50-0.60	0.1-0.4	0.60-0.90	≤0.03	≤0.02	0.8-1.2	1.50-1.8	0.35-0.55	0.05-0.15
DIN 17350	1.2714 56NiCrMoV7	0.50-0.60	0.1-0.4	0.65-0.95	≤0.03	≤0.03	1.0-1.2	1.50-1.8	0.45-0.55	0.07-0.12
JIS G4404	SKT4	0.50-0.60	0.1-0.4	0.60-0.90	≤0.03	≤0.02	0.8-1.2	1.50-1.8	0.35-0.55	0.05-0.15

L6 STEEL PHYSICAL PROPERTY

Density,kg/dm3	7.8
Specific heat capacity,J/g.K	0.46
Thermal conductivity ,W/m.K	35.9
Specific electric resist,Ohm mm2/m	0.3
Electrical conductivity,m/mm2	3.33
Modulus of elasticity long.,Gpa	215
Modulus of elasticity tang.,Gpa	82

L6 STEEL THERMAL PROPERTY

Temperature,°C	20-100	20-200	20-300	20-400	20-500	20-600	20-700
Thermal expansion,10^-6 m/(m*K)	12.5	13.1	13.4	13.9	14	14.3	14.5
Temperature,°C	20		350		700
Thermal Conductivity,W/(m*K)	35		39		36

L6 STEEL FORGING

Forging process	Temperature range	Operation requirements
Preheating	650-800°C	To avoid cracking caused by thermal stress, the heating rate should be slow (especially below 400°C) to ensure that the core of the steel also reaches a uniform temperature.
Initial Forging Temperature	1050-1150°C	The maximum temperature should not exceed 1200°C, otherwise there is a risk of overburning (oxidation and melting of grain boundaries, resulting in material scrap).
Final Forging Temperature	≥900℃	Forging below this temperature increases the risk of cracking and may result in a poor grain structure, compromising the steel's ultimate toughness (which is L6's most important property).
Cooling After Forging	Furnace cooling/sand cooling	L6 is sensitive to cooling rate. Rapid cooling (air cooling or even wind cooling) at high temperature after forging can easily produce hard and brittle martensite or bainite structure, resulting in high hardness and great risk of cracking.Full annealing must be performed after forging and slow cooling.

L6 STEEL HEAT TREATMENT

L6 Steel Annealing

Annealing is usually done after forging. Heat L6 steel to 790°C – 815°C and hold sufficiently (usually 2-4 hours/inch thickness). Cool very slowly in the furnace (<15°C / hour) to about 540°C and then air cool. Annealing hardness is HB 201-229.

Isothermal annealing (recommended): Heat to 790-815°C, hold and quickly transfer to a furnace at 650°C – 675°C, hold for 4-6 hours, then air cool. Isothermal annealing is more efficient and the structure of L6 steel is more uniform.

L6 Steel Stress Relieving

To eliminate residual stress caused by machining, welding or cold working, and reduce the risk of subsequent heat treatment deformation and cracking of L6 steel.

Heat L6 steel to 650°C – 675°C, keep sufficiently for 1-2 hours/inch thickness (at least 1 hour). Cool extremely slowly (<25°C / hour) to 540°C and then air cool. Avoid water or oil cooling in the 650-675°C range!

L6 Steel Quenching

L6 tool steel is a nickel-containing oil-quenched air-hardening tool steel known for its excellent toughness and impact resistance, especially in applications where high toughness is required (such as cold working dies, punches, shear blades, chisels, etc.). Its heat treatment process requires special attention to preheating and temperature control to maximize its toughness advantage and avoid cracking. The following are the standard heat treatment process steps and key points for L6 steel:

Preheating:
Critical! L6 steel has relatively poor thermal conductivity and contains high alloying elements (especially nickel and carbon). Directly entering the high-temperature austenitizing furnace will cause excessive thermal stress and easy cracking.
It is recommended to preheat and hold in stages at 400°C – 540°C and 760°C – 815°C
Austenitizing:Temperature range: 815°C – 870°C.
–Lower temperatures (such as 815-830°C): produce finer grains and obtain higher toughness, but may sacrifice a little hardness and hardenability.
-Higher temperatures (such as 855-870°C): improve hardness and hardenability, but the grains will be slightly coarser, the toughness will be slightly reduced, and the risk of cracking will increase.
Holding time: After reaching the austenitizing temperature, the holding time is calculated according to the effective thickness of the workpiece. The general rule is to hold the workpiece at temperature for 20-30 minutes per inch (25mm) of thickness. Ensure that the workpiece is fully and evenly austenitized. Avoid excessive holding time that will cause coarse grains and decarburization.
Quenching:
Quenching medium: L6 steel is usually quenched in oil.
-Quenching oil: It is the most commonly used and recommended choice, which can effectively control stress and deformation cracking risks while ensuring sufficient hardness.
-Hot oil (50-80°C): It can further reduce quenching stress and reduce deformation and cracking tendencies, especially for complex shapes or thin-walled workpieces. However, the hardness may be slightly lower than rapid oil quenching.
-Compressed air/forced air cooling: It can be used for very small cross-sections (<10mm) or extremely strict deformation requirements and slightly lower hardness. The hardness will be significantly lower than oil quenching.
-Water or brine quenching: Absolutely not recommended! It will lead to extremely high cracking risk.

L6 Steel Tempering

It must be done immediately! After quenching, the workpiece is in an unstable martensitic state with high hardness and extremely high internal stress. Tempering must be performed within 1 hour after quenching (the sooner the better) to prevent delayed cracking or cracking caused by improper stress release.

Temperature range: This is a key step that determines the final hardness and toughness. L6 steel is usually tempered in the range of 150°C – 540°C.
-Low temperature tempering (150-200°C): The highest hardness (HRC 58-62) is obtained, but the toughness is relatively low. Suitable for occasions with extremely high wear resistance requirements and small impact loads.
-Medium temperature tempering (315-425°C): The most commonly used range. A good balance between hardness and toughness (HRC 54-58). This is the first choice for most tools that require high toughness (such as punches, shears, chisels).
-High temperature tempering (480-540°C): High toughness is obtained, and the hardness drops to HRC 48-52. Suitable for tools subject to very high impact loads (e.g. heavy chisels, certain cold forging dies).
Holding time: At least 2 hours. For large cross-section workpieces (>50mm / 2 inches), longer times are required (e.g. 1 hour per inch of thickness). Double tempering is strongly recommended, cooling to room temperature after each holding before the next. The second tempering can further eliminate the residual austenite (part of the residual austenite is transformed into martensite during the first tempering cooling) and stress, stabilize the structure size, and improve toughness.
Cooling: After each tempering and heat preservation, cool to room temperature in still air.

Special note: When L6 steel is tempered in the temperature range of 260°C – 370°C, a significant secondary hardening peak (hardness recovery) will appear. Tempering in this range can obtain a hardness close to the peak value (HRC 58-60), but the toughness will drop significantly. Unless a specific application requires this peak hardness and can withstand lower toughness, tempering in this range is usually avoided. 400-450°C tempering is recommended to avoid the brittle zone and obtain an excellent combination of strength and toughness.

L6 Steel Mechanical Property with Q&T Condition

Quenching at 860 °C in oil	Tempering	100°C	150°C	200°C	250°C	300°C	350°C	400°C	450°C	500°C	550°C	600°C	650°C
	HB	634	615	595	577	543	512	482	468	442	409	390	—
	HRC	59	58	57	56	54	52	50	49	47	44	42	—
	R,N/mm 2	2420	2330	2240	2160	2010	1880	1760	1700	1580	1430	1340	—
Quenching at 860 °C in air	Tempering	100°C	150°C	200°C	250°C	300°C	350°C	400°C	450°C	500°C	550°C	600°C	650°C
	HB	560	—	512	—	482	—	442	421	400	371	336	301
	HRC	55	—	52	—	50	—	47	45	43	40	36	32
	R,N/mm 2	2070	—	1880	—	1760	—	1580	1480	1390	1250	1110	1010