Optimizing Heat Treatment for 21CrMoV5-7 Steel Forged Piston Heads
10 May 2025 66 Blog

Optimizing Heat Treatment for 21CrMoV5-7 Steel Forged Piston Heads

Dr. Robert Anderson

Dr. Robert Anderson

Senior Materials Scientist

Optimizing Heat Treatment for 21CrMoV5-7 Steel Forged Piston Heads: A Comprehensive Guide

Introduction

21CrMoV5-7 steel (also known as 1.7709, ISO 21CrMoV5-7, EN 21CrMoV5-7) is a high-strength, heat-resistant martensitic steel widely used in forged piston heads, high-temperature bolts, and turbine components due to its excellent mechanical properties, creep resistance, and fatigue strength.

This article provides a detailed analysis of the heat treatment process for 21CrMoV5-7 forged piston heads, including:

  • Optimal quenching & tempering parameters
  • Microstructural evolution & mechanical properties
  • Comparison with equivalent steel grades (ASTM, DIN, JIS, GB standards)
  • Industrial applications & failure prevention strategies

Chemical Composition & Equivalent Grades

1. Chemical Composition (wt%) of 21CrMoV5-7

Element 21CrMoV5-7 (1.7709, DIN 17240) ISO 21CrMoV5-7 ASTM Equivalent (Approx.)
C 0.17–0.25 0.17–0.25 AISI 4140 (modified)
Si ≤0.40 ≤0.04 –
Mn 0.40–0.80 0.40–0.80 –
Cr 1.20–1.50 1.20–1.50 –
Mo 0.55–0.80 0.65–0.80 –
V 0.20–0.35 – –
Ni ≤0.60 ≤0.60 –

*Sources: *

2. Equivalent Steel Grades Across Standards

Country/Standard Equivalent Grade Key Differences
Germany (DIN) 1.7709 Same as 21CrMoV5-7
Europe (EN) EN 21CrMoV5-7 Similar composition
USA (ASTM) AISI 4140 (modified) Lower V, higher Cr
China (GB) 25Cr2MoV Higher C, lower Ni
Japan (JIS) SNCM439 Similar but with Ni

Optimal Heat Treatment for 21CrMoV5-7 Forged Piston Heads

1. Recommended Heat Treatment Process

Stage Temperature Range Cooling Method Objective
Austenitizing (Quenching) 890–940°C (1634–1724°F) Oil quenching Full martensitic transformation
Tempering 650–720°C (1202–1328°F) Air cooling Stress relief, toughness improvement

Key Findings from Research:

  • 910°C oil quenching + 650°C tempering yields the best balance of strength & toughness:
    • Tensile strength: 1010 MPa
    • Yield strength: 900 MPa
    • Elongation: 16%
    • Impact energy: 45–50 J
  • Lower tempering (<600°C) retains higher strength but reduces ductility.
  • Higher tempering (>700°C) improves toughness but lowers hardness .

2. Microstructural Evolution

  • As-quenched structure: Lath martensite + minor retained austenite.
  • After tempering: Tempered martensite (sorbitic structure) with fine carbides (Mo₂C, V₄C₃) .
  • Excessive tempering (>720°C) leads to coarse carbide precipitation, reducing fatigue resistance.

Mechanical Properties After Heat Treatment

Property 21CrMoV5-7 (910°C Quench + 650°C Tempering) AISI 4140 (Comparable) 25Cr2MoV (China GB)
Tensile Strength (MPa) 1010 850–1000 950–1100
Yield Strength (MPa) 900 655–850 800–950
Elongation (%) 16 15–20 12–15
Impact Energy (J) 45–50 40–60 35–45
Hardness (HRC) 28–32 28–32 30–35

*Sources: *

Industrial Applications & Case Studies

1. Piston Heads in High-Performance Engines

  • 21CrMoV5-7 piston heads outperform 42CrMo4 in thermal fatigue resistance .
  • No thermal cracking observed at 600°C service temperature due to Mo/V stabilization.

2. High-Temperature Bolts & Fasteners

  • Used in power plants & aerospace due to creep resistance (up to 580°C).
  • ESR-refined 21CrMoV5-7 shows better fatigue life than conventional grades .

Comparison with Other Heat-Resistant Steels

Steel Grade Key Features Best For Limitations
21CrMoV5-7 (1.7709) Balanced strength-toughness, high creep resistance Piston heads, turbine bolts Limited weldability
AISI 4140 Lower cost, good hardenability General machinery parts Lower heat resistance
X20CrMoV12-1 (DIN) Superior creep strength (>600°C) Steam turbine blades Poor machinability
SNCM439 (JIS) Higher Ni content, better toughness Aerospace components Expensive

Conclusion & Recommendations

1. Best Practices for Heat Treatment

  • Quench at 910°C for full austenitization.
  • Temper at 650°C for optimal strength-toughness balance.
  • Avoid excessive tempering (>720°C) to prevent over-softening.

2. Future Trends

    <616>AI-controlled heat treatment for real-time property adjustment.
  • Additive manufacturing of topology-optimized piston heads.

For detailed material specifications, refer to DIN 17240 or ASTM A29.

This guide combines metallurgical research, industrial case studies, and global material standards to optimize 21CrMoV5-7 steel heat treatment for high-performance applications.

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