40CrNiMoA/SAE 4340 Alloy Steel Round Bar with Polished Surface and OD 6-630mm for High Strength Applications

GB 40CrNiMoA BS S535/AMS 6350 AISI 4340 Round Bar JIS SNCM439

Preheat the steel 4340 first, heat up to 1150°C - 1200°C maximum for forging, hold until temperature is uniform throughout the section. Do not forge below 850°C. 4340 has good forging characteristics but care must be taken when cooling as the steel shows susceptibility to cracking. Following forging operation, the work piece should be cooled as slowly as possible. Cooling in sand or dry lime is recommended.

For pre-hardened steel, stress relieving is achieved by heating steel 4340 to between 500-550°C. Heat to 600°C-650°C, hold until temperature is uniform throughout the section, soak for 1 hour per 25 mm section, and cool in still air.

A full anneal may be done at 844°C (1550°F) followed by controlled (furnace) cooling at a rate not faster than 10°C (50°F) per hour down to 315°C (600°F). From 315°C (600°F) it may be air cooled.

AISI 4340 alloy steel should be in the heat treated or normalized and heat treated condition before tempering. The tempering temperature depends upon the strength level desired. For strength levels in the 260-280 ksi range, temper at 232°C (450°F). For strength in the 125-200 ksi range, temper at 510°C (950°F). Avoid tempering within the range 250°C-450°C due to temper brittleness.

Pre-hardened and tempered 4340 steel bars or plates can be further surface hardened by either flame or induction hardening methods, resulting in a case hardness in excess of Rc 50. AISI 4340 steel parts should be heated as quickly as possible to the austenitic temperature range (830°C-860°C) followed by immediate oil or water quenching. Following quenching to hand warm, tempering at 150°C-200°C will reduce stresses in the case with minimal effect on its hardness.

Hardened and tempered 4340 alloy steel can also be nitrided, giving a surface hardness of up to Rc 60. Heat to 500°C-530°C and hold for sufficient time (from 10 to 60 hours) to develop the depth of case. Surface hardness achievable is 600 to 650HV.

Machining is best done with the alloy steel 4340 in the annealed or normalized and tempered condition. It can be readily machined by all conventional methods such as sawing, turning, and drilling. However, in high strength conditions of 200 ksi or greater, machinability is only 25% to 10% that of the alloy in the annealed condition.

Welding of steel 4340 in the hardened and tempered condition (as normally supplied) is not recommended and should be avoided if possible, because of the danger of quench cracking. If welding must be carried out, pre-heat to 200-300°C and maintain this while welding. Immediately after welding, stress relieve at 550-650°C prior to hardening and tempering.

AISI 4340 steel is used in most industry sectors for applications requiring higher tensile/yield strength than 4140 steel can provide. Typical applications include:

• Aircraft Landing Gear
• Automotive Components
• Oil and Gas Drilling Equipment
• Forging Applications
• Warm and Cold Forming
• Machine Building
• Transfer Systems (power transmission gears and shafts)
• General engineering industries and structural use applications
• Heavy duty shafts, gears, axles, spindles, couplings, pins, chucks, molds

Widely used for transmission systems, motors of tractors and middle-small automobiles. Alternative to 12CrNi3 for making higher class carburization and cyanide carburizing parts such as oil drilling claws and cones for mining. Commonly used for carburized gears, pinions, shafts, bearings, piston rings, ring gears, crankshafts, etc.

Our steel materials are available in different sizes and conditions specially catered to our customers' needs. They come in round bars, flat bars, plates and blocks. The conditions of the materials vary from pre-hardened, hot rolled, annealed, quenched and tempered to surface conditions such as black, pre-machined, grinding and polish.