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"Mechanical properties of 4340 steel"


Q. Sir,

I am doing a presentation on 4340 steel, and I would like to know the effect of the different cooling rates and microstructures on the mechanical properties of the 4340 steel.

Syed Midhatullah Quadri
Uni of Southern Queensland - Queensland, Toowoomba, Australia


A. Syed, if you'll take a look at the T-T-T curve you will see that you can you can get martensite, pearlite, or bainite with relative ease with 4340 -- that's one of the beauties of the alloy. I'm certain your presentation will cover the differences between these structures, and how they relate to the mechanical properties. Remember that 4340 is a 0.40% carbon steel, and carbon is the prime alloying ingredient with respect to properties. The nickel, chromium, and molybdenum merely effect the cooling rate necessary to get those aforementioned structures.

Most libraries in the States have a copy of "The Making, Shaping and Treating of Steel" [affil. link to book on Amazon], and you can also look for "Optical Microscopy of Carbon Steels" [affil. link to book on Amazon] by Samuels, or just about any book or article by George Krauss. These should give fine insight into the wonderfully complex world of alloy steel heat treatment.

Good luck, and have fun!

lee gearhart
Lee Gearhart
metallurgist - E. Aurora, New York


A. Dear sir, If you look at CCt diagram of 4340 ASE and superimpose cooling curve on it you can see that if the rate of cooling increases the cooling cure passes through the martensite region and it means that the faster you cool your steel the harder you will have! For more info., see the AC3 software.

Hatef khaledi alidoosti
IUST (university) - Iran


Q. I want CCT diagram for 4340, cooling rate include mechanical properties.

Mohammadreza Arjmand
Sharif university - Karaj, Tehran, Iran

September 1, 2011

Q. Can 4340 be used in place of 4140 P110 in a normal machining situation? No extreme circumstances.

Mike Finney
Inspector - Ft. Worth, Texas, U.S.A.

April 21, 2014

Q. Dear Sir,
How good is 4340 STL in handling fatigue loading in parts for fighter Jets?

Farhan Ahmad
- Spartanburg, South Carolina, USA

May 9, 2014

A. Hello Farhan!

Using 4340 in fatigue limited applications is a common practice. The key is having a properly designed, properly manufactured, and properly heat treated part, to maximize the benefit you get from this alloy.

lee gearhart
Lee Gearhart
metallurgist - E. Aurora, New York

February 11, 2015

Q. Sir.I want to know the other mechanical properties of 4340 steel, that has a UTS of 1230 MPa, Yield strength 1080 MPa and shear strength 740 MPa.

Preethi Rajkumar
Student - india

May 6, 2015

A. Preethi,
The 4340 material as described does not match my data tables for annealed 4340 so you may be referring to a tempered batch. Nonetheless, here is some information on annealed 4340: Bulk modulus 140 GPa; Shear modulus 80 GPa; Elastic modulus 190-210 GPa; Poisson's ratio .27-.3; Elongation at break 22%; Reduction of area 50%; Vickers hardness 228.

blake kneedler
Blake Kneedler
Feather Hollow Eng. - Stockton, California

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