SKT4 | Hot Work Tool Steel Manufacturer in China

SKT4 is a Ni‑Cr‑Mo hot work tool steel to JIS G 4404. Developed for medium and large forging dies, it has high toughness, deep hardenability and good thermal fatigue resistance. Its typical composition: 0.50–0.60% C, 1.40–1.80% Ni, 0.80–1.20% Cr. It keeps stable impact strength and uniform hardness in sections up to 300 mm thick. Molybdenum resists temper embrittlement and preserves toughness during slow high‑temperature cooling.

SKT4 steel stands out from standard hot work steels for excellent hardenability and consistent hardness in large cross-sections. Large SKT4 material blocks quenched at 820°C and tempered at 560°C show uniform hardness across the whole part. It maintains steady mechanical properties up to 600°C, around HB300 at 500°C. It is widely adopted for large forging dies, die blocks and hot pressing tools.

International equivalents: 1.2714 steel, AISI L6, GB 5CrNiMo, BS BH224/5, AFNOR 55NCDV7, GOST 5KhNM.

Supply Range of SKT4 Steel

1. Round Bar

Hot Rolled / Forged: Diameter Φ10mm – Φ500mm
Forged: Diameter Φ50mm – Φ400mm (larger sizes available upon request)
Length: 2000mm – 5800mm (customizable)
Tolerances: Compliant with JIS G 4404 or DIN EN ISO 4957
Surface conditions: Black skin, peeled, turned, or ground finish

Flat Bar / Plate

3. Forgings / Custom Shapes of SKT4

Measuring tool blanks, gauge blocks, die inserts, shear blade stock, forming dies, guide rails, and complex geometries – all manufactured from your prints
Full dimensional flexibility: thickness, width, length, and shape all tailored to batch order requirements
Maximum weight per piece: up to 10 tons
Sample Testing: 1–10 pieces available for evaluation before formal production

Keyspark Steel offers precision plate component machining of SKT4 steel, specializing in steel plate milling and precision hole finishing. Custom-made to your drawings.

We also provide our overseas customers with assembly services for various CNC machined parts.

Our team does rough and fine machining for all kinds of forging ring parts. We provided rough turning service like slewing bearing rings and gear rings to many European construction machinery partners. Keyspark Steel have strong custom ability and an engineer teams. We make products from your drawings and are very flexible.

Keyspark Steel also offers importing clients custom cold drawn steel round bars, steel square bars and hexagon steel bars in different sizes with cold drawing dies as supporting items. The material have tight tolerance, smooth surface and stable material performance. And also cold drawing dies as supporting items.

SKT4 Equivalent

Standard System	Country/Region	Equivalent Grade	Common Search Terms
JIS	Japan	SKT4	SKT4 steel, JIS SKT4, SKT4 tool steel
DIN / EN	Germany	1.2713, 55NiCrMoV6	1.2713 steel, DIN 1.2713, 55NiCrMoV6
AISI / ASTM	USA	L6, T61206	L6 tool steel, AISI L6
GB	China	5CrNiMo	5CrNiMo steel, 5CrNiMoV steel
BS	UK	BH224/5	BH224/5 tool steel
AFNOR	France	55NCDV7	55NCDV7 steel
GOST	Russia	5KhNM (5ХНМ)	5KhNM steel, 5ХНМ

SKT4 Composition

Standard	C	Si	Mn	P	S	Cr	Ni	Mo	V
JIS G 4404-2022	0.50-0.60	0.10-0.40	0.60-0.90	≤0.030	≤0.030	0.80-1.20	1.50-1.80	0.35-0.55	0.05-0.15

The high nickel content in SKT4 steel provides exceptional toughness and deep hardenability. Chromium and molybdenum enhance hardenability and temper resistance. Vanadium refines grain structure.

SKT4 Properties

MECHANICAL PROPERTY	METRIC VALUE	IMPERIAL VALUE
Hardness (Annealed)	≤ 248 HB	≤ 248 HB
Hardness (Quenched & Tempered)	53 – 58 HRC	53 – 58 HRC
Tensile Strength (σb)	≥ 646 MPa	≥ 93.7 ksi
Yield Strength (σ0.2)	≥ 513 MPa	≥ 74.4 ksi
Elongation (δ5)	≥ 14 %	≥ 14 %
Reduction of Area (ψ)	≥ 42 %	≥ 42 %
Impact Toughness (KV, 20°C, Longitudinal)	≥ 32 J	≥ 23.6 ft·lb

JIS SKT4 offers high toughness, good high-temperature strength, and excellent resistance to thermal fatigue, making it suitable for forging dies and hot-forming molds. The steel is typically supplied in the annealed condition with a maximum hardness of 248 HB. After oil quenching at 830–860°C and tempering, the working hardness can reach 42 HRC (standard) or 53–58 HRC (optimized grade). The tensile strength, yield strength, elongation, reduction of area, and impact toughness values are minimum guaranteed values from the JIS datasheet.

Delivery Condition of SKT4 Steel

Black Surface, Bright Surface, Turned Surface, Grinded Surface, Poblished surface.

JIS SKT4 Application

SKT4 is the go-to hot work tool steel for large-section forging dies and heavy-duty hot forming applications:

Forging Dies: Large and medium hammer forging dies, die blocks, forging press dies, die inserts for undercarriage links and automotive components.
Hot Extrusion Tools: Extrusion dies, mandrels, dummy blocks, containers for non-ferrous metal extrusion.
Hot Pressing Dies: Hot pressing molds, die-casting molds for aluminum and magnesium alloys.
Die Forging: Precision forging dies, die forging blocks, trim dies for large components.
Machine Components: Structural parts for aircraft and rockets operating at 400-500°C.
General Hot Work Tooling: Hot shear blades, piercers, hot gripper dies, and wear-resistant components for high-temperature service.

Heat Treatment & Processing of SKT4 Material

Heat Treatment

Soft Annealing: Heat to 760-780°C, hold adequately, then cool slowly in furnace at 10-20°C per hour. Resulting hardness ≤248 HB, optimized for batch machining.
Stress Relieving: Heat to approximately 650°C, hold for 1-2 hours, then slow cool. Recommended after rough machining to minimize distortion.
Preheating: Preheat in two stages: 350-450°C, then 650-750°C, to avoid thermal shock in large sections.
Austenitizing (Heating): 830-860°C for oil quenching. Soak time: 30 minutes after temperature equalization, plus additional time for large sections. Use the upper limit of the range for large dies and the lower limit for smaller dies.
Quenching (Cooling): Oil quench at 20-30°C. For large dies, pre-cooling treatment is recommended to avoid deformation. Quench to 150-180°C, then temper immediately. Hardness after quenching: 53-58 HRC.
Tempering: Double tempering is recommended to eliminate retained austenite and ensure dimensional stability. Temper at 530-600°C depending on target hardness. For large forging dies, tempering at 560-600°C provides an optimal balance of hardness and toughness.

Tempering Temp	As Quenched	400°C	500°C	530°C	560°C	600°C
Hardness (HRC)	53-58	48-52	44-48	42-46	38-42	34-38

For large forging dies of SKT4 tool steel requiring high toughness, temper at 560-600°C (34-42 HRC). For applications requiring higher wear resistance, temper at 530-560°C (38-46 HRC).

Processing Guide:

Forging: Heat uniformly to 1050-1100°C. Start forging at 1050°C, finish above 850°C. After forging, cool slowly in furnace or insulating media, then anneal. SKT4 steel is prone to white spot formation – strict control of smelting and post-forging cooling is essential.
Machining: Annealed SKT4 steel (≤248 HB) offers good machinability with carbide tools. Hardened parts require grinding with CBN wheels.
Welding: Not recommended for highly stressed die components. If necessary, preheat to 300-400°C and perform post-weld heat treatment.
Nitriding: Gas or plasma nitriding can be applied to enhance surface hardness and wear resistance for specialized applications.
Polishing: This grade offers adequate polishability for most forging die applications.
EDM: Suitable for wire EDM. Remove the recast layer after EDM and perform stress relief at 500-550°C.

Quality Control

Full-process control:

Raw material and manufacturer control
Test : Chemical composition, mechanical properties, UT(ultrasonic testing), specification, surface condition, straightness, etc.
Packing: Wooden case or pallet or VCI plastic film

Why Purchase SKT4 from Keyspark Steel?

Fully Customizable: Dimensions, heat treatment, and hardness – all adjustable per batch order.
Certified Quality: Each batch comes with MTC certificate (EN 10204 3.1/3.2) , full chemical and mechanical test report.
ESR Option: Premium quality with Electro-Slag Remelting available for critical applications.
Stable Production: Large forging and rolling capacity ensures consistent quality across orders.
Long-Term Supply: Reserved production slots for regular customers – no interruptions.
Neutral Packaging: Oiled, wrapped, and strapped.

FAQ of SKT4

Q1: What is SKT4?

A: SKT4 is a hot work alloy tool steel to Japanese JIS G 4404. It belongs to Mn-Cr-Mo-Ni tool steel for forging. Its hardness is 197–241 HB under annealed state. After quenching and tempering, working hardness reaches 53–58 HRC. It has high toughness, high strength and good thermal shock resistance. It fits hammer dies under heavy impact load.

Q2: What role does nickel play in SKT4?

A: Nickel content in SKT4 is about 1.40–1.80%. It is the key element different from other hot work tool steels. Nickel greatly improves hardenability and impact toughness. Steel with 300 mm thick section gets even hardness across the part after oil quenching at 820°C. Nickel also lowers ductile-brittle transition temperature. The steel does not crack easily under repeated impact load.

Q3: How does the toughness of SKT4 compare with its wear resistance?

A: SKT4 is designed to prioritize toughness. Its carbon content is only 0.50–0.60%. The alloy level is low and carbide amount is limited. So its wear resistance is medium, but its impact toughness stands out among hot work tool steels. It keeps hardness around HB 300 at 500°C. It works well for heavy impact conditions instead of heavy abrasive wear.

Q4: What thickness of SKT4 can achieve uniform hardness after quenching?

A: SKT4 has excellent hardenability. A large steel block of 300mm×400mm×300mm gets nearly even hardness on all sections after oil quenching at 820°C and tempering at 560°C. This feature makes it ideal for medium and large hammer forging dies over 400 mm in size. Ordinary cold work tool steel cannot reach this performance.

Q5: What is the main difference between SKT4 and SKD61?

A: They serve different purposes. SKT4 contains nickel and has better toughness with lower carbon content. It fits large hammer forging dies under impact load. SKD61 has no nickel, with higher Cr and Mo content. It has better high-temperature strength and thermal fatigue resistance. It fits die casting dies and hot extrusion dies. SKD61 is better in thermal shock and thermal fatigue, but SKT4’s toughness cannot be replaced in hammer forging work.

Q6: Why is salt bath quenching preferred over oil quenching for SKT4 forging dies?

A: Salt bath quenching removes the vapor film formed in oil quenching. It uses direct convection cooling for more uniform temperature drop and smaller deformation. With salt bath quenching plus tempering at 400°C, SKT4 reaches max hardness of 46.1 HRC and minimum wear rate of 6.98×10⁻⁵ mm³/kg·m. After 6000 forging cycles, the die has no visible defects. Its service life is much longer than parts made by oil quenching.