42CrMo4 Steel Technical Overview

42CrMo4 Steel Technical Overview: As a widely used alloy structural steel, 42CrMo4 (also known under the EN standard 1.7225) is valued for its strength, toughness, and hardenability. With our 20+ years of forging experience, we at Aobo Steel understand the critical role this material plays in demanding engineering applications. Here’s a straightforward look at its technical profile.

1. 42CrMo4 Steel Chemical Composition

The performance of 42CrMo4 starts with its specific chemical makeup. While slight variations exist between standards, the typical composition generally falls within these ranges:

• Carbon (C): 38% – 0.45% (Some standards may specify 0.38% – 0.42%)
• Silicon (Si):≤ 0.40% (Often specified as 0.10%- 0.40%)
• Manganese (Mn): 60% – 0.90% (Sometimes 0.50% – 0.80%)
• Chromium (Cr): 90% – 1.20%
• Molybdenum (Mo): 15% – 0.30% (ASTM standards often cite 0.15% – 0.25%)
• Phosphorus (P):≤ 0.035% (Some tighter specifications require ≤ 0.025%)
• Sulfur (S):≤ 0.035% (Some tighter specifications require ≤ 0.025%)

Note: The variant 42CrMoS4 includes a controlled sulfur addition (typically 0.020% – 0.040%) specifically to improve machinability. Always confirm the exact specification required for your application.

2. 42CrMo4 Steel Equivalent Standards

42CrMo4 is recognized internationally under various designations. Understanding these equivalents is crucial for sourcing and specification:

• EN: 42CrMo4 (Material No.: 1.7225)
• ISO: 42CrMo4
• ASTM/AISI (USA): Commonly compared to 4140 (G41400) and 4142 (G41420)
• JIS (Japan): SCM440 / SCM445
• GOST (Russia): 38XM
• DIN (Germany): 42CrMo4
• BS (UK):  42CrMo4 / 708M40
• NF (France): 42CrMo4
• GB/T (China): 42CrMo (A30422) / 42CrMoH (A30425)

Aobo Steel can supply 42CrMo4 according to your required standard.

3. Heat Treatment of 42CrMo4 Steel

Understanding the heat treatment options for 42CrMo4 alloy steel is essential for achieving the desired mechanical properties for specific applications. Aobo Steel has extensive experience with this material, and this guide outlines the common heat treatment processes.

3.1 Initial Supply Conditions

42CrMo4 steel is typically supplied in specific conditions that influence subsequent processing. Understanding these is the first step.

3.1.1 Annealed Condition

Heat treatment is typically performed between 830-850°C, followed by slow furnace cooling. The holding time depends on section thickness (e.g., 3-4 hours for standard sections). This process results in a softer microstructure (max hardness ≤217 HBW), ideal for machinability prior to further treatments.

3.1.2 Normalized Condition

Normalizing involves heating to 870-880°C for 1-2 hours, followed by air cooling. This refines the grain structure, creating a uniform base for further heat treatment. Hardness is typically ≤255 HBW. Occasionally, tempering at or above 480°C (900°F) follows normalizing to reduce yield strength if required.

3.2 Hardening and Tempering (Quenching & Tempering – Q&T)

For applications demanding higher strength and hardness, Quenching and Tempering (Q&T) is the standard process applied to 42CrMo4.

3.2.1 Hardening (Quenching)

This step involves heating 42CrMo4 to its austenitizing temperature, generally 850-880°C. Adequate soaking time at this temperature is crucial for a homogenous austenitic structure. Subsequently, rapid cooling (quenching) transforms austenite into hard martensite.

Oil quenching is standard for 42CrMo4, balancing the cooling rate with minimal distortion risk, particularly for small to medium sections. Water quenching can be used for thicker sections to achieve through-hardening but increases the risk of cracking due to greater thermal shock. Similar grades like 4140 are also typically oil quenched after austenitizing (845-925°C / 1550-1700°F).

3.2.2 Tempering

As-quenched martensite is hard but generally too brittle for engineering use. Tempering is essential to improve toughness. This involves reheating the quenched steel to a specific temperature below Ac1 (typically 450-650°C), holding for a set duration (e.g., 2 hours), and then cooling (often in air or oil).

The chosen tempering temperature directly controls the final balance of mechanical properties:

• Low Temperatures (e.g., 450°C): These temperatures maximize hardness and strength, making them suitable for high-wear-resistance applications.
• Intermediate Temperatures (e.g., 500-600°C): Offer a balanced combination of high strength and good toughness, common for components like gears and shafts. Tempering at 580°C after oil quenching from 850°C can achieve ≥1100 MPa tensile strength and ≥950 MPa yield strength.
• High Temperatures (e.g., 650°C): Yield lower strength and hardness but significantly enhance ductility and impact toughness.

Note: Tempering in the 230-370°C (450-700°F) range should generally be avoided for 42CrMo4, similar to 4140 steel, due to the risk of ‘blue brittleness,’ which can reduce toughness.

3.3 Surface Hardening Techniques

Specific applications may benefit from surface hardening to improve wear resistance while maintaining core toughness.

3.2.3 Induction Hardening

This process rapidly heats the surface layer using an electromagnetic field to the austenitizing temperature, followed by immediate quenching (often in an emulsion). A low-temperature temper (e.g., 150-180°C) can follow to relieve stress without significantly lowering surface hardness (potentially ≥56 HRC). Successful induction hardening of 42CrMo4 can achieve surface hardness around 670 Hv.

3.2.4 Nitriding and Boronizing

For superior surface hardness and wear resistance, processes like nitriding or boronizing can be applied after the primary Q&T treatment. These thermochemical treatments modify the surface composition to create extremely hard layers.

4. 42CrMo4 Steel Mechanical Properties (Quenched and Tempered Condition)

In the quenched and tempered (Q+T) state, 42CrMo4 delivers excellent mechanical performance. Typical values after oil quenching (850°C) and tempering (580°C) can include:

• Tensile Strength (Rm): ≥ 1100 MPa
• Yield Strength (ReL): ≥ 950 MPa
• Elongation (A): ≥ 12%
• Impact Toughness (KV/KU or aK): Values depend on test method, but good toughness is characteristic (e.g., ≥ 80 J/cm²).

Remember, final properties depend directly on the specific heat treatment parameters used.

5. Common Applications

The combination of strength, toughness, and wear resistance makes 42CrMo4 suitable for a wide range of demanding components:

• Machine Building: Highly stressed parts like gears, shafts, axles, connecting rods, crankshafts, and high-strength bolts.
• Tooling: Large and medium-sized plastic molds, mold frames, forging dies (depending on application demands), cold heading dies.
• Other: Large volume gas cylinders and components requiring high fatigue strength.