SAE 4340 Flat Bars

SAE 4340 flat bar is hot rolled rectangular bar in the nickel chromium molybdenum through-hardening alloy steel, the SAE designation of the EN 24 recipe and the same steel European standards call 34CrNiMo6, material number 1.6582. It carries about 0.4 percent carbon with nickel, chromium and molybdenum added, and those additions raise hardenability, the depth to which a quench turns the section hard. A flat of this grade therefore reaches strength at its core as well as at its rolled faces, where SAE 4140, which has the chromium and molybdenum but no nickel, falls away towards the middle of a thick section. On thin flats, or at lower strengths, 4140 is the plainer choice. The steel is magnetic and has no corrosion resistance.

The flat is rolled 20 to 150 mm wide and 5 to 50 mm thick in lengths to 6.4 metres, and is supplied annealed, normalised, or hardened and tempered to a BS 970 condition such as T, U or V. Pickling is a stainless finishing step and does not apply here, so the rolling scale comes off by machining or blasting. Annealed flat machines predictably; in the harder conditions the cut is slower and tooling wears faster, so parts are often roughed soft and finished after tempering. Welding needs preheat and a temper afterwards, because the carbon and alloy content leaves the weld zone prone to cracking. Flats in this grade go into machine tool levers and links, die holders and bolsters, gear and sprocket blanks, and lifting and earthmoving linkage plates. Laxcon Steels rolls the flat to order.

Composition limits in weight percent are carbon 0.38 to 0.43, manganese 0.6 to 0.8, sulphur 0.04 maximum, phosphorus 0.035 maximum, silicon 0.15 to 0.35, chromium 0.7 to 0.9, nickel 1.65 to 2.0 and molybdenum 0.2 to 0.3.