5CrNiMo steel is a traditional low-alloy hot-work tool steel that has been widely used in various applications, particularly for forging dies. It has excellent hardenability, good toughness, and moderate strength at elevated temperatures, though it has some limitations in very high-temperature or high-wear applications. 5CrNiMo is a grade under China’s GB/T standard, with equivalent grades including American AISI L6, Japanese JIS SKT4, and German DIN W-Nr. 1.2713 and 1.2714.

1. Chemical Composition of 5CrNiMo Steel(GB / T 1299—2000)

C	Si	Mn	Kr	Ni	Mo	P	S
0.50-0.60	≤0,40	0.50-0.80	0.50-0.80	1.40-1.80	0.15-0.30	≤0,030	≤0,030

2. Physical Properties of 5CrNiMo

2.1 Critical Temperature of 5CrNiMo

Punkt krytyczny	Ac1	Ac3	Ar1	Ar3	SM
Temperature/ °C	730	780	610	640	230

2.2 Linear Expansion Coefficient of 5CrNiMo

Temperatura / °C	100 ~ 250	250 ~ 300	350 ~ 600	600 ~ 700
Linear Expansion Coefficient α / x10⁻⁶ °C⁻¹	12.55	14.1	14.2	15.0

2.3 Elastic Modulus of 5CrNiMo

Temperatura / °C	Room Temp	100	200	300	400	500
E / MPa	209 720	206 780	203 840	198 940	193 060	186 200

2.4 Shear Modulus of 5CrNiMo

Temperatura / °C	Room Temp	100	200	300	400	500
G / MPa	83 300	82 320	80 360	78 400	75 460	73 500

2.5 Thermal Conductivity

Temperatura / °C	Room Temp	100	200	300	400	500
λ / [W / (m · K)]	0.44	0.44	0.43	0.43	0.39	0.35

2.6 Specific Heat Capacity

Temperatura / °C	Room Temp ~ 100	Room Temp ~ 200	Room Temp ~ 300	Room Temp ~ 400	Room Temp ~ 500
cₚ (20°C) / [J / (kg · K)]	0.484	0.497	0.509	0.531	0.552

2.7 Density is 7.804 g/cm³.

3. Hot Forging Process Specification of 5CrNiMo

Przedmiot	Temperatura ogrzewania / °C	Początkowa temperatura kucia / °C	Końcowa temperatura kucia / °C	Metoda chłodzenia
Sztabka stali	1140 ~ 1180	1100 ~ 1150	800 ~ 880	Powolne chłodzenie (chłodzenie w dołku lub piasku)
Kęsy stalowe	1100 ~ 1150	1050 ~ 1100	800 ~ 850	Powolne chłodzenie (chłodzenie w dołku lub piasku)

Notatka: 5CrNiMo steel can be hardened by cooling in air, but it is prone to forming white spots, so it should be cooled slowly after forging. For large forgings, they must be held in a furnace at 600°C until the temperature is uniform, then slowly cooled to 150-200°C, and finally cooled in air. For larger forgings, tempering should be carried out immediately after cooling to 150-200°C.

4. Heat Treatment of 5CrNiMo

4.1 Podgrzewanie wstępne

① Post-Forging Annealing Options

Preliminary Heat Treatment Option	Parametry procesu
Wyżarzanie po kuciu	Heat to 760–780°C, hold for 4–6 hours, then cool the furnace to below 500°C before unloading for air cooling. After annealing, the hardness ranges from 197 to 241 HBW, with a microstructure of pearlite + ferrite.
Wyżarzanie izotermiczne po kuciu	Heating temperature: 850–870°C, held for 4–6 hours; Isothermal temperature: 680°C, held for 4–6 hours; Cooled in the furnace to below 500°C, then removed for air cooling; Post-annealing hardness: 197–241 HBW; Microstructure: pearlite + ferrite.
Forging Die Renewal Annealing	Heat to 710–730°C, hold for 4–6 hours, then cool in the furnace to below 500°C before removing for air cooling. Post-annealing hardness: 197–241 HBW.

② Microstructure and Hardness Before and After Annealing

Średnica wcięcia / mm	HBW	Mikrostruktura (przed wyżarzaniem)	Mikrostruktura (po wyżarzaniu)
3.9 ~ 4.3	241 ~ 197	Troostyt + Martenzyt	Pearlite + Ferrite

4.2 Hartowanie

① Recommended Quenching Process Specification

Temperatura hartowania/°C	Środek chłodzący do hartowania	Temperatura czynnika chłodzącego hartującego/°C	Continued	Twardość HRC
830 ~ 860	Olej	20 ~ 60	Temper immediately after cooling to 150 ~ 180°C	53 ~ 58

Notatka:

• For large molds, use the upper limit for quenching heating temperature; for small molds (side length below 200–300 mm), use the lower limit.
• To minimize stress and deformation during quenching, after heating to 830–860°C, first pre-cool in air to 750–780°C, then oil-quench to 150–180°C. Remove and immediately temper.
• For large molds, slowly heat to 600–650°C and hold for 1–1.5 hours. Only increase furnace temperature after thorough heating. To improve heating quality, place workpieces on 60–100mm thick backing plates during heating.
• Module quenching at low oil immersion temperatures increases cracking risk. To prevent cracking, actual practice often involves oil immersion around 200°C. While this produces a martensitic surface layer, the core remains in the austenitic state. To extend service life, isothermal quenching can be employed.

② Isothermal Quenching Process

Process Type	Quenching Temp. (°C)	Medium chłodzące	Medium Temp. (°C)	Isothermal Temp. & Time	Quenching Hardness (HRC)	Microstructure after Quenching
Isothermal Quenching	830 – 860	Olej	20 – 60	Cool mold surface to 150-200°C, then hold in a 280-300°C bath for 2-3h	–	Small amount of bainite + lower bainite + retained austenite. After tempering, bainite transforms to lower bainite.
High-Temp Quenching	890 – 910	Olej	–	–	61.5	Lath martensite + retained austenite (approx. 9.2% by volume), grain size 7-8.

Uwagi:

• Isothermal Quenching: This process reduces the tendency for the mold to crack and improves its service life.
• High-Temperature Quenching: Tempering temperature is 420-550°C, tempered twice, resulting in a hardness of 38-47 HRC.

4.3 Hartowanie

① Zalecane specyfikacje procesu hartowania

Plan	Cel hartowania	Forging Die Specification	Temperatura odpuszczania (°C)	Sprzęt grzewczy	Twardość (HRC)
I	Eliminate stress, stabilize structure and size	Small	490 ~ 510	Gas furnace or Electric furnace	44 ~ 47
		Średni	520 ~ 540		38 ~ 42
		Large	560 ~ 580		34 ~ 37
II		Dovetail (Medium)	620 ~ 640		34 ~ 37
		Dovetail (Small)	640 ~ 660		30 ~ 35

Notatka: After tempering, oil quenching should be performed to prevent the development of temper brittleness. To relieve stresses generated during oil quenching, a second tempering at 160–180°C may be conducted. Large forging dies must not be cooled to room temperature in oil during either quenching or tempering, as this may cause cracking.

② Effect of Quenching and Tempering Temperatures on Impact Toughness

	Tempering Temp. (°C)	300	350	400	450	500
Quenching Temp. (°C)840	The values in the table represent impact toughness, a_k / (J/cm²).	21	25	29	35	45
Quenching Temp. (°C)950		19	20	23	25	35
Quenching Temp. (°C)1000		13	16	20	23	30

Notatka: When the 5CrNiMo steel module is tempered at 380-450°C, the supercooled austenite in the core transforms into an upper bainite structure, resulting in extremely poor impact toughness.

③ Effect of Quenching and Tempering Temperatures on Hardness of 5CrNiMo

	Tempering Temp. (°C)	300	350	400	450	500	550	600
Quenching Temp. (°C)850	Twardość HRC	52	50	48	45	41	38	32
Quenching Temp. (°C)900		52	50	48	45	41	38	32
Quenching Temp. (°C)950		53	51	49	46	42	39	33
Quenching Temp. (°C)1000		54	52	50	47	43	40	34

5. Właściwości mechaniczne

5CrNiMo steel is a traditional hot-forging die steel, exhibiting good plasticity and toughness alongside adequate strength and wear resistance. It is insensitive to size effects, with room-temperature mechanical properties nearly identical to those at temperatures ranging from 500 to 600°C. When heated to 500°C, it still maintains a hardness of approximately 300 HBW. Due to its low content of carbide-forming elements, the secondary hardening effect is weak, resulting in poor thermal stability and low high-temperature strength.

5.1 High-Temperature Hardness of 5CrNiMo

Temperatura (°C)	300	450	600	650	710	750
Hardness (HV)	383	351 ~ 354	254 ~ 274	201 ~ 203	147 ~ 154	71.3 ~ 72.7

5.2 High-Temperature Impact Toughness of 5CrNiMo

Temperatura (°C)	300	600	650	700
Impact Absorption Energy (KU/J)	48.4	36.4	36.3	70.3

5.3 Thermal Fatigue Performance

Test Method & State	20°C ←→ 650°C (1000 cycles)	20°C ←→ 750°C (1000 cycles)
Level¹	13.4	20.0

¹The higher the level, the worse the thermal fatigue performance.

5.4 Thermal Wear Performance

Number of Revolutions	300	600	900	1200	1500	2500
Weight Loss / mg	1.00	2.43	3.27	4.2	4.97	5.97

Notatka: Using the S. R. Tittagala method, at a temperature of 800 ~ 850°C and a pressure of 784~850N.

5.5 Surface mechanical properties and core mechanical properties

Part	Location	RM/ MPa(Tensile Strength)	Rel/ MPa(Yield Strength)	A (%)(Elongation)	Z (%)(Reduction of Area)	KU/J(Impact Energy)
Room Temp	Surface	1310	–	–	41	31.3
Room Temp	Core	1020	761	16	42	14.1
300°C	Surface	1214	–	–	58	48.4
300°C	Core	1011	833	26	62	59.2

Notatka: During quenching of large-section workpieces, differing cooling rates between the surface layer and the core result in distinct mechanical properties. This phenomenon in steel is termed the mass effect, also known as the size effect.

6. Zastosowania

5CrNiMo steel, due to its excellent hardenability, exhibits nearly uniform hardness across its cross-section when large blocks (300mm × 400mm × 300mm) undergo quenching in oil at 820°C followed by tempering at 560°C. Primarily used for manufacturing various small and medium-sized forging dies with side lengths ≤300mm and operating temperatures below 500°C. It is more suitable for producing large and medium-sized modules in mass production, and can also be used to fabricate complex-shaped casting mold components.