AISI 4320 Alloy Steel (UNS G43200)

Topics Covered

Introduction
Chemical Composition
Physical Properties
Mechanical Properties
Thermal Properties
Other Designations
Fabrication and Heat Treatment
     Machinability
     Forming
     Welding
     Heat Treatment
     Forging
     Hot Working
     Cold Working
     Annealing
     Tempering
     Hardening


Introduction

Alloy steels comprise different kinds of steels having compositions exceeding the limitations of B, C, Mn, Mo, Ni, Si, Cr, and Va set for carbon steels. They are designated by AISI four-digit numbers.

AISI 4320 alloy steel has high toughness and strength. It contains low carbon content to achieve good welding characteristics.

Chemical Composition

The following table shows the chemical composition of AISI 4320 alloy steel.

Element Content (%)
Iron, Fe 95.855 - 96.98
Nickel, Ni 1.65 - 2.00
Manganese, Mn 0.450 - 0.650
Chromium, Cr 0.400 - 0.600
Molybdenum, Mo 0.200 - 0.300
Carbon, C 0.170 - 0.220
Silicon, Si 0.150 - 0.300
Sulfur, S  0.0400
Phosphorous, P  0.0350

Physical Properties

The physical properties of AISI 4320 alloy steel are highlighted in the following table.

Properties Metric Imperial
Density 7.85 g/cm3 0.284 lb/in³
Melting point 1422°C 2590°F

Mechanical Properties

The mechanical properties of annealed AISI 4320 alloy steel are displayed in the following table.

Properties Metric Imperial
Tensile strength 580 MPa 84100 psi
Yield strength 425 MPa 61600 psi
Bulk modulus (typical for steel) 140 GPa 20300 ksi
Shear modulus (typical for steel) 80 GPa 11600 ksi
Elastic modulus 190-210 GPa 27557-30458 ksi
Poisson's ratio 0.27-0.30 0.27-0.30
Elongation at break (in 50 mm) 29% 29%
Reduction of area 58% 58%
Hardness, Brinell 163 163
Hardness, Knoop (converted from Brinell hardness) 184 184
Hardness, Rockwell B (converted from Brinell hardness) 84 84
Hardness, Vickers (converted from Brinell hardness) 170 170
Machinability (annealed and cold drawn. Based on 100 machinability for AISI 1212 steel) 60 60

Thermal Properties

The following table outlines the thermal properties of AISI 4320 alloy steel.

Properties Metric Imperial
Thermal expansion co-efficient (@ -18-95°C/-0.400-203°F, condition of specimen unknown) 11.3 µm/m°C 6.28 µin/in°F
Thermal conductivity (typical steel) 44.5 W/mK 309 BTU in/hr.ft².°F

Other Designations

Other designations that are equivalent to AISI 4320 alloy steel include:

  • ASTM A29 (4320)
  • ASTM A322 (4320)
  • ASTM A331 (4320)
  • ASTM A519 (4320)
  • ASTM A535 (4320)
  • ASTM A752 (4320)
  • SAE J1397 (4320)
  • SAE J404 (4320)
  • SAE J412 (4320)

Fabrication and Heat Treatment

Machinability

AISI 4320 alloy steel has good machinability in the annealed condition. However, it is difficult to machine this steel if it is hardened by heat treatment.

Forming

AISI 4320 alloy steel has good forming characteristic in the annealed condition.

Welding

AISI 4320 alloy steel has good weldability.

Heat Treatment

AISI 4320 alloy steel is normalized at 872°C (1650°F) before heat treatment. It is then heat treated at 816°C (1500°F) followed by quenching in oil and tempering to achieve desired hardness.

Forging

AISI 4320 alloy steel is forged at 1038 to 1205°C (1900 to 2200°F)

Hot Working

AISI 4320 alloy steel can be hot worked at 872 to 1205°C (1600 to 1900°F)

Cold Working

AISI 4320 alloy steel can be cold worked using by conventional methods in the annealed condition.

Annealing

AISI 4320 alloy steel is annealed at 872°C (1600°F) followed by slowly cooling in the furnace.

Tempering

AISI 4320 alloy steel can be tempered at 205 to 705°C (400 to 1300°F).

Hardening

AISI 4320 alloy steel can be hardened by cold working, or by teat treatment and tempering.

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