Atlas 4340 is a nominally 1.8 percent Nickel Chromium Molybdenum low alloy steel of high hardenability, with high strength and toughness in relatively large sections achieved with a "quench-and-temper" heat treatment. A contemporary alternative to 4340 is Atlas 6582 through-hardening steel that has improved through-hardening characteristics and offers superior impact properties to 4340 at equivalent strength levels. 

Related Specifications
Atlas 4340 low-alloy steel bar complies with grade AS1444, Grade 4340 and/or ASTM A434(A29) Grade 4340. Atlas 4340 is an alternative to Atlas 6582 through-hardening steel and to JIS SCNM447. 

Chemical Composition 

Conditions of Supply – Mechanical Properties
Atlas 4340 is supplied in the hardened and tempered condition. The level of hardness is selected to give useful strength while still maintaining the ability to machine the material into finished components. 

Conditions of Supply – Surface finish and Machining Allowance
Atlas 4340 Black is supplied with +/- tolerance according to DIN 1013 or better. Atlas 4340 Bright is supplied cold drawn with h10 tolerance for bars up to 25mm diameter. Bars with diameters 25 to 75mm are supplied cold drawn to h10 or peeled to k12. Bars 75 to 210 mm are supplied peeled to k12 and sizes 210mm and over are peeled to a - 0/+2mm tolerance. 

Machining allowances for Atlas 4340 round bar (mm on diameter) 

Heat Treatment

Hardening
Heat components slowly to 850°C, hold until uniform then quench vigorously in oil (or polymer with an equivalent quench rate) to between 80 and 110°C.
Tempering
While still warm, re-heat to 450 to 650°C, hold one hour per 25mm of section (2 hours minimum) and cool in air. Select tempering temperatures according to the required mechanical properties – refer to tempering curve.
Stress relieving
For pre-hardened steel stress relieving is achieved by heating to between 500 to 550°C. Retreated bars or forgings heat to 25°C below tempering temperature. Annealed components, 600 to 650°C. Hold in this temperature range for 1 to 2 hours, furnace cool to 450°C, then air cool. 

Surface Hardening
Nitriding
Nitride at approximately 510°C for 10 to 60 hours, depending upon required case. Surface hardness achievable is 600 to 650HV.
Induction or Flame Hardening
Atlas 4340 can be surface hardened to 58HRC (water quench) or surface hardened to between 53 to 55HRC (oil quench). Tempering at 200°C.

Welding
Welding is not recommended because of the danger of quench cracking. If welding must be carried out, pre-heat to 200 to 300°C and maintain this while welding. Immediately after welding stress relieve at 550 to 650°C.

Applications of Atlas 4340
Parts exposed to high permanent and fluctuating stresses. Applications where excellent fatigue and toughness properties of the material are required. Typical components include gears, planetary gears, axles, pinions, shafts, bushes and sleeves.