4130 Alloy Steel: Properties and Applications

4130 alloy steel is classified as a medium-carbon, low-alloy steel under the AISI system. Its designation highlights its primary alloying elements: chromium (approx 1%) and molybdenum (approx 0.20%). This versatile steel offers a solid combination of strength, toughness, and weldability.

1. 4130 alloy steel Chemical Composition

The typical chemical composition of 4130 steel (by weight percentage) is:

• Carbon (C):28-0.33% (Provides a balance of strength and ductility)
• Manganese (Mn):40-0.60% (Contributes to hardenability and strength)
• Silicon (Si):20-0.35% (Acts as a deoxidizer and can increase strength)
• Chromium (Cr):80-1.10% (Improves hardenability, corrosion resistance, and wear resistance)
• Molybdenum (Mo):15-0.25% (Enhances hardenability and high-temperature strength and resists temper embrittlement)
• Phosphorus (P) & Sulfur (S): Kept low (typically max 0.030-0.040% each)
• Copper (Cu): May be present (up to 0.35% in some specifications)

2. Hardenability Characteristics

4130 is a water-hardening alloy steel with low to intermediate hardenability. This means that achieving full hardness through quenching requires careful consideration of section thickness. Thinner sections or more aggressive quenching mediums (like water) are generally needed than steels with higher hardenability. The ideal critical diameter (DI) illustrates this; for example, a specific heat with 0.29% C, 1.02% Cr, and 0.15% Mo calculated a DI of 68.3 mm (2.69 in.).

3. Heat Treatment of 4130 Alloy Steel

AISI 4130 is a highly versatile medium-carbon, low-alloy steel known for its strength, toughness, and weldability. Its key alloying elements are chromium and molybdenum. To fully leverage the capabilities of 4130 steel and tailor its mechanical properties for specific applications, precise heat treatment is essential. The response 4130 to various thermal cycles allows for a broad spectrum of performance characteristics.

3.1 Common Heat Treatment Processes for 4130

Based on extensive industry experience and technical data, Aobo Steel outlines the primary heat treatments applicable to 4130 steel:

3.1.1 Normalizing

• Objective: To refine the grain structure of the steel, improving uniformity and enhancing machinability after forging or rolling.
• Process: Heat the steel uniformly to a temperature between 870°C and 925°C (1600°F to 1700°F). This range is approximately 55°C to 85°C (100°F to 150°F) above the upper critical temperature (Ac3), ensuring complete transformation to austenite. Hold at temperature for adequate time to ensure heat penetration, typically 1 hour per 25mm (1 inch) of maximum thickness, with a minimum hold time often specified. Cool in still air to room temperature.
• Post-Normalizing: It is common practice to temper normalized 4130 steel at temperatures of 480°C (900°F) or higher to achieve specific yield strength requirements.

3.1.2 Annealing

• Objective: To produce the softest possible condition for 4130 steel, maximizing ductility for operations such as cold forming or complex machining.
• Process: Heat the steel to a temperature between 830°C and 870°C (1525°F to 1600°F). Hold at temperature for a duration dependent on section size or furnace load. Crucially, cool very slowly within the furnace, typically at a rate not exceeding 15°C per hour (30°F per hour), down to approximately 480°C (900°F). Below this temperature, cooling can continue in air. This controlled slow cooling promotes the formation of a soft, coarse pearlite microstructure.

3.1.3 Quenching and Tempering (Hardening)

• Objective: To develop high strength, hardness, and toughness, the most common condition for structural applications of 4130.
• Austenitizing: Heat the steel uniformly to the appropriate austenitizing temperature. This is typically between 855°C and 865°C (1575°F to 1600°F), depending on the exact composition and section size. Hold at temperature long enough for complete austenitization.
• Quenching: Rapidly cool the steel from the austenitizing temperature. Due to 4130’s low-to-intermediate hardenability, the quenching medium (water, oil, or polymer) must be selected carefully based on the component’s section thickness and the desired final properties. Water provides faster cooling and higher hardness in thinner sections but increases distortion risk. Oil quenching is common for moderate sections. Achieving the critical cooling rate, especially through the transformation range around 540°C (1000°F), is vital for obtaining a fully martensitic structure, particularly in thicker sections.
• Tempering: Reheat the quenched (hardened) steel to a specific temperature below the lower critical temperature (Ac1). Tempering temperatures for 4130 typically range from 205°C to 705°C (400°F to 1300°F). Hold at the tempering temperature (usually at least 2 hours), then cool (typically in air). Tempering reduces the brittleness of the as-quenched martensite and establishes the final balance of hardness, strength, and toughness. Lower tempering temperatures yield higher strength and hardness, while higher temperatures increase ductility and toughness at the expense of strength.

3.2 Hardenability Considerations

The hardenability of 4130 steel is a critical factor. It describes the steel’s ability to harden through its cross-section during quenching.

• 4130 has low-to-intermediate hardenability compared to higher-alloy steels.
• Section Thickness: The achievable hardness and the hardening depth highly depend on the component’s size. Thicker sections cool more slowly, especially at the core, potentially resulting in a less uniform microstructure and lower core hardness than the surface.
• Ideal Critical Diameter (DI): This calculated value (e.g., ~68 mm or ~2.7 inches for a typical composition under ideal conditions) indicates the theoretical maximum diameter that can be hardened through to the center under a perfect quench. Practical results with less severe quenches (oil, polymer) will achieve hardening to shallower depths.
• Quench Severity: Careful selection of the quenching process is vital to achieve desired properties without causing cracking or excessive distortion, particularly when compared to higher carbon steels like 4140, which have greater hardenability but also a higher risk of quench cracking.

3.3 Stress Relieving

• Objective: To reduce internal stresses induced by manufacturing processes like heavy machining, cold forming, or welding, often performed before final hardening or on normalized/annealed parts.
• Process: Heat the steel uniformly to a temperature typically between 650°C and 675°C (1200°F to 1250°F). Hold for sufficient time (e.g., 1 hour per inch of thickness), followed by slow cooling (usually in furnace or air).
• Important Note: If stress relieving is performed after quenching and tempering, the stress-relieving temperature must be kept below the original tempering temperature (usually at least 15°C or 25°F lower) to avoid negatively impacting the previously established mechanical properties.

4. Mechanical Properties Overview

The final mechanical properties of 4130 steel depend directly on the heat treatment applied. Quenched and tempered conditions offer a wide range of achievable tensile strength, yield strength, elongation, and hardness values.

It’s important to consider the mass effect: larger cross-sections cool slower during quenching, resulting in lower hardness and strength in the core compared to the surface or compared to smaller sections receiving the same heat treatment.

Typical mechanical properties of heat-treated 4130 steel

Tempering Temperature		Tensile Strength		Yield Strength		Elongation in 50 mm (2 in.), %	Reduction in Area, %	Hardness, HB	Izod Impact Energy
°C	°F	MPa	ksi	MPa	ksi				J	ft-lb
Water quenched and tempered
205	400	1765	256	1520	220	10	33	475	18	13
260	500	1670	242	1430	208	11.5	37	455	14	10
315	600	1570	228	1340	195	13	41	425	14	10
370	700	1475	214	1250	182	15	45	400	20	15
425	800	1380	200	1170	170	16.5	49	375	34	25
540	1000	1170	170	1000	145	20	56	325	81	60
650	1200	965	140	830	120	22	63	270	135	100
Oil quenched and tempered
205	400	1550	225	1340	195	11	38	450	—	—
260	500	1500	218	1275	185	11.5	40	440	—	—
315	600	1420	206	1210	175	12.5	43	418	—	—
370	700	1320	192	1120	162	14.5	48	385	—	—

Effects of mass on typical properties of heat-treated 4130 steel

Bar size	Tensile strength	Yield strength	Elongation in 50 mm (2 in.), %	Reduction in area, %	Surface hardness, HB
mm	MPa	ksi	MPa	ksi	%
25	1040	151	880	128	18
50	740	107	570	83	20
75	710	103	540	78	22

Source: ASM HANDBOOK

5. Common Applications

Thanks to its reliable performance profile, 4130 steel is used across various industries:

• Automotive components (e.g., axles)
• Structural parts requiring good strength and toughness
• Shafts, gears, and bearings needing good fatigue and wear resistance (often in tempered conditions)
• Applications requiring high toughness (in spheroidized conditions)
• Electron beam hardening processes

While not classified as a primary tool steel, its versatility makes it a common material in toolrooms.

6. International Standards Equivalents

4130 steel corresponds to designations in several international standards:

• ASTM: A322, A29/A29M
• SAE: J404
• JIS (Japan): SCM 425, SCM 430