Heat treatment and process of 4340 steel round bar&plate

AISI 4340 alloy steel can be cold worked in the annealed condition using all conventional methods. It is highly malleable.

Annealing & Temper

Full annealing may be done at 844°C (1550 F), followed by controlled (furnace) cooling to 315°C (600 F) at a rate not to exceed 10°C (50 F) per hour. It can be air-cooled from 315°C to 600F.

AISI 4340 alloy steel shall be in the heat-treated or normalized heat-treated condition before tempering. The tempering temperature depends on the desired strength level. For strength levels in the 260 – 280 ksi range, temper at 232°C (450 F). Temper at 510°C (950 F) for strengths in the range of 125 – 200 ksi. Do not temper 4340 steel if it is in the 220 – 260 ksi strength range as tempering will result in reduced impact resistance at that strength level. Tempering in the range of 250 °C – 450 °C should be avoided where possible due to temper brittleness.

4340 steel heat treatment

AISI 4340 alloy steel is heat treated at 830°C (1525°F) and quenched in oil.

*Machinability

It can be processed using all conventional techniques. Processing can be carried out in the annealed or normalized and tempered condition.

*Welding

Welding of steels in the quenched and tempered condition (commonly supplied) is not recommended and should be avoided if possible due to the risk of quench cracking as the mechanical properties change in the weld heat-affected zone.

If soldering is necessary, preheat to 200 to 300°C and maintain this temperature while soldering. Stress is relieved immediately after welding at 550 to 650°C, followed by quenching and tempering.

If it is indeed necessary to weld in the quenched and tempered condition, the workpieces should be stress relieved as far as possible at a temperature 15°C below the original tempering temperature, after cooling immediately to hand temperature.