34CrMo4​ is a medium-carbon chromium-molybdenum alloy steel​ widely used for high-stress, wear-resistant, and high-temperature components. It belongs to the EN 10083-1 standard​ (European Norm for steel for quenching and tempering) and is also covered in DIN standards.

Here's a breakdown of its key properties and characteristics:

Composition (Typical Weight %, EN 10083-1):​

Carbon (C):​ 0.30 - 0.37% (Provides strength and hardenability)

Silicon (Si):​ 0.10 - 0.40% (Deoxidizer, strengthens ferrite)

Manganese (Mn):​ 0.60 - 0.90% (Improves hardenability and strength)

Phosphorus (P):​ ≤ 0.025% (Impurity - reduces ductility/toughness)

Sulfur (S):​ ≤ 0.025% (Impurity - reduces ductility/toughness, improves machinability slightly)

Chromium (Cr):​ 0.90 - 1.20% (Strongly increases hardenability, corrosion/oxidation resistance, wear resistance, forms stable carbides)

Molybdenum (Mo):​ 0.15 - 0.30% (Increases hardenability (deep hardening), strength at elevated temperatures, creep resistance, reduces susceptibility to temper embrittlement)

Heat Treatment:​

Primary Process:​ Quenching & Tempering (Q+T) - Mandatory.​ The alloying elements require this treatment to achieve its characteristic high strength and toughness.

Quenching:​ Heated to approx. 850°C (1562°F), then rapidly cooled (oil or water), forming hard martensite.

Tempering:​ Reheated to a temperature typically between 450°C and 650°C (842°F - 1202°F), held, then cooled. This transforms martensite into tempered martensite, drastically improving toughness and ductility while retaining significant strength.

Properties are highly dependent on the Tempering Temperature:Lower tempering → higher strength, lower toughness. Higher tempering → lower strength, higher toughness.

Mechanical Properties (After Q&T - Values vary significantly with tempering temp and section size):​

Tensile Strength (Rm):​ Typically 700 - 1100 MPa​ (100 - 160 ksi). Commonly tempered for strength levels around 800-900 MPa or 1000-1100 MPa.

Yield Strength (Rp0.2):​ Typically 550 - 850 MPa​ (80 - 125 ksi). Good yield-to-tensile ratio.

Elongation at Break (A₅):​ Typically 10% - 18%​. Ductility increases with higher tempering temperatures.

Hardness:​ Typically 24 - 38 HRC​ in its standard strength grades. Can be tempered for hardness up to ~42 HRC.

Impact Toughness (KV at 20°C):​ Typically 40 - 100 J​. Good toughness at room temperature, significantly better than comparable carbon steels.

Fatigue Strength:​ Very good.​ High endurance limit (approx. 50-55% of tensile strength). Tempering above 550°C improves it.

Wear Resistance:​ Good, especially in hardened states.

Key Advantages & Characteristics:​

Excellent Strength/Toughness Balance:​ Achieved through proper quenching & tempering.

Deep Hardenability:​ Chromium and Molybdenum allow it to achieve high strength through thicker sections (up to ~80-100mm / 3-4 inches effectively).

Good Fatigue Strength:​ Critical for dynamic/cyclic loading.

Good Resistance to Temper Embrittlement:​ Molybdenum content helps prevent embrittlement at critical tempering temperature ranges (around 375°C / 700°F).

Good Elevated Temperature Properties:​ Retains strength and resists softening better than plain carbon steels up to approx. 500°C (932°F). Mo improves creep resistance.

Good Wear Resistance:​ Especially in hardened condition (Cr carbides).

Applications (Typical):​ Used where high static strength, good fatigue strength, toughness, and wear resistance are required, often in moving components:

Power Transmission:​ Gear shafts, pinions, connecting rods, studs, bolts (high-strength fasteners).

General Engineering:​ Machine parts, crankshafts, axles, spindles, heavy-duty couplings.

Vehicles:​ Transmission components, drive shafts (especially heavy trucks), steering components.

Tooling:​ Backing plates, bases for large tools/dies, non-cutting tool parts.

Agriculture/Mining Equipment:​ High-stress pins, shafts.

Fasteners:​ High-strength bolts (e.g., ASTM A320 Grade L7/B7 - where 34CrMo4 is a common base material).

Machinability & Weldability:​

Machinability:​ Fair to difficult​ in annealed condition (~70% of free-cutting steel). Requires harder, positive rake tools. Grinding is preferred for finished hardened parts. Significantly more difficult in hardened condition.

Weldability:​ Fair​, but requires strict control​ due to high hardenability (risk of cold cracking/HAZ hardening).

Requires preheating​ (200-300°C / 392-572°F) and post-weld heat treatment​ (stress relieving, typically at 600-650°C / 1112-1202°F) is often mandatory to restore toughness and relieve stresses.

Low-hydrogen welding processes (SMAW, GTAW, GMAW with low-H electrodes/wire) are essential.

Matching or overmatching filler metals are used (e.g., high-strength CrMo wires).

Surface Treatment:​ Excellent candidate for surface hardening​ processes like induction hardening​ or nitriding​ to achieve extremely hard, wear-resistant surfaces while maintaining a tough core.

Comparable Standards:​

ISO:​ 34CrMo4 (identical designation).

Germany:​ DIN 34CrMo4.

UK:​ EN 19 (Old BS designation, very similar).

USA:​ AISI/SAE 4130​ (Very close composition), AISI/SAE 4140​ (Slightly higher carbon 0.38-0.43%, thus slightly stronger/harder), ASTM A320 Grade L7/B7​ (for fastener grades).

Japan:​ SCM430 (Similar to 4130), SCM440 (Similar to 4140).

In Summary:​

34CrMo4 is a versatile, high-performance chromium-molybdenum alloy steel​ that achieves an excellent balance of strength, toughness, and fatigue resistance through quenching and tempering.​ Its deep hardenability and resistance to temper embrittlement make it a popular choice for critical, high-stress engineering components like axles, shafts, gears, high-strength fasteners, and parts operating at moderately elevated temperatures. Care is needed during welding due to its hardenability.