SAE 4320 alloy steel

Introduction

We manufacture forging ingots, billets, and blooms in alloy steel specifically for SAE 4320, developed for demanding case hardening applications in automotive and engineering industries.

SAE 4320 is a low carbon nickel–chromium–molybdenum alloy steel designed to provide a hard, wear-resistant surface combined with excellent core toughness. This combination makes it ideal for components subjected to high surface stress, impact loads, and cyclic fatigue.

Nickel improves toughness and fatigue resistance, chromium enhances hardenability and wear resistance, and molybdenum increases strength and resistance to softening during tempering. SAE 4320 offers good machinability in the annealed condition and maintains dimensional stability after heat treatment, making it suitable for forging and precision machining processes.

Chemical composition

Element	Content (%)	Function / effect
Carbon (c)	0.17 – 0.23	Enables case hardening with a tough core
Manganese (mn)	0.40 – 0.70	Improves hardenability and strength
Silicon (si)	Max 0.35	Enhances strength and elasticity
Chromium (cr)	0.45 – 0.80	Improves hardenability and wear resistance
Nickel (ni)	1.65 – 2.00	Increases toughness and fatigue strength
Molybdenum (mo)	0.20 – 0.30	Improves strength and temper resistance
Phosphorus (p)	Max 0.035	Controlled impurity; improves toughness
Sulphur (s)	Max 0.035	Controlled impurity; aids machinability

Mechanical properties (typical, depending on condition & heat treatment)

• Core tensile strength: 850 – 1050 MPa
• Core yield strength: 650 – 800 MPa
• Case hardness: 58 – 62 hrc
• Core hardness: 30 – 40 hrc
• Impact toughness: very good
• Fatigue resistance: excellent

Heat treatment

• Annealing: softens the steel to improve machinability before carburizing.
• Carburizing: carbon enrichment at elevated temperature to develop a hard surface layer.
• Quenching: rapid cooling to form a hard martensitic case.
• Tempering: relieves internal stresses and improves toughness while maintaining surface hardness.

Heat treatment parameters can be customized to achieve the required case depth and core strength based on specific application requirements.

Key features and benefits

• Excellent case hardening characteristics
• High surface hardness with tough and ductile core
• Superior fatigue and impact resistance
• Good machinability in annealed condition
• Dimensional stability after heat treatment
• Suitable for forging and precision machining

Typical applications

• Gears and pinions
• Transmission and gearbox components
• Camshafts and crankshaft components
• Automotive drivetrain and differential parts
• Heavy-duty engineering components

Equivalent steel grades

SAE 4320 has several close international equivalents, including:

• En 10084 – 18CrNimo7-6 (approximate)
• Din 1.6587 (approximate)
• Bs 970 – 4320m

(exact equivalence may vary depending on standard and heat treatment condition.)

Why choose SAE 4320 alloy steel?

SAE 4320 is an excellent choice for components requiring a hard, wear-resistant surface and high core toughness. Its balanced alloying ensures reliable performance under heavy loads, improved fatigue life, and long service life in critical applications.

Comparison table for similar grades

Grade	Carbon (%)	Key applications	Heat treatment	Comments
SAE 4320	0.17 – 0.23	Gears, shafts, drivetrain parts	Case hardening	High toughness and fatigue strength
SAE 8620	0.18 – 0.23	Automotive gears	Case hardening	Widely used carburizing steel
20CrNimo	0.17 – 0.25	Heavy-duty gears	Case hardening	Higher strength due to mo
En353	0.17 – 0.22	Gear and pinion parts	Case hardening	Common european grade

Summary

SAE 4320 is a premium nickel-chromium-molybdenum alloy steel optimized for case hardening applications. It delivers outstanding toughness, wear resistance, and durability for automotive and engineering components operating under severe conditions.