AISI 4340 steel heat treatment guide

     Performance: the AISI 4340 steel is a high hardenability steel, AISI 4340 steel heat treatment hardenability than any kind of steel in the AISI standard of the higher. When the elements are concentrated, hardenability is within the allowable range, the upper curve of hardenability essentially become a straight line, this indicates that the AISI 4340 steel need air quenching in thinner area. In accordance with the precise carbon content, when hardening, Rockwell hardness in the 54-59 range. Due to high hardenability, AISI 4340 steel using traditional welding methods are not suitable, although it can use this old methods, such as electron beam welding. AISI 4340 steel can be forged effortlessly, although its thermal performance than carbon or low-alloy steel is higher, the need to use high power forging machine. The mechanical processing is not very good.
     Forging: heated to 1230℃. After the temperature dropped to 900℃ to stop forging. Cool slowly to prevent cracks.
     Recommended AISI 4340 steel heat treatment operations:
     Normalizing: heating to 870℃. Cooling in air to 565 degrees; or heated to 830 degrees, rapidly cooled to 650 ℃, and hold 8 hours. In order to get the ball structure and heated to 750℃, rapidly cooled to 705℃, and then at a rate of not more than 3℃ per hour was cooled to 650℃, and hold 12 hours. Ball organizational structure is usually easy machining and heat treatment.
     Quenching: 845 degrees at the time, the steel organization becomes austenitic, then oil quenched. In the casting the thin area can be air quenching.
     Tempering: with all high hardenability steel the same, alloy steel 4340 will produce quenching cracks. In the casting to ambient temperature before (38-50℃), they should be placed in tempering furnace. Tempering temperature depends on the desired hardness or comprehensive mechanical properties.
     Nitriding: through nitriding to produce high surface hardness and enhance the anti fatigue performance.

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