ASTM 668 vs AISI 4340: Hardness Comparison

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In summary, ASTM 668 is a standard that covers a specific form and heat treatment schedule of AISI 4340 steel, as well as potentially other grades of steel. AISI 4340 has a composition of 0.4% carbon and its hardness is typically limited to 35 HRC, while ASTM 668 allows for hardness values up to 38 HRC. The final microstructure of AISI 4340 can be affected by temperature and degree of work. There are also other standards that apply to AISI 4340, depending on its geometry, heat treatment, and amount of cold work. The ductility and hardness of AISI 4340 can vary, depending on the amount of carbon and cold work present
  • #1
manmeet123
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Please can anyone help me to understand, is ASTM 668 is equivalent with AISI 4340.
In AISI 4340 Hardness values are restricted upto 35 HRC , whereas ASTM 668 hardness values are upto 38 HRC.
waiting for replies. Please help me out.
 
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  • #2
ASTM is a standard which covers a particular form (and heat treatment schedule) of 4340, and perhaps other grades of steel.
http://www.astm.org/Standards/A668.htm

AISI 4340 is a particular grade with a particular composition with nominally 0.4% C (0.38-0.43) - Ref: http://www.suppliersonline.com/propertypages/4340.asp
Note the ranges of heat treatment and forging temperatures. The temperature and degree of work will affect final microstructure.
Note the many standards that apply to 4340, but each may represent a different geometry, e.g, wire, rod, plate, bar, strip, . . . , final heat treatment and degree of cold work.

Usually there is a range of ductility and hardness, part of which is dependent on the amount of carbon, and the amount of cold-work (dislocation density) remaining.

I'll try to find more information, but I don't have the ASTM 1.06 Standards book.
 

1. What is the difference between ASTM 668 and AISI 4340?

ASTM 668 and AISI 4340 are two different standards used to specify the chemical composition and mechanical properties of steel. ASTM 668 is a specification for carbon and low-alloy steel bars, while AISI 4340 is a specification for nickel-chromium-molybdenum alloy steel.

2. Which one is harder, ASTM 668 or AISI 4340?

Generally, AISI 4340 has a higher hardness compared to ASTM 668 due to its higher alloy content. However, the exact hardness of each steel can vary depending on the heat treatment and processing it has undergone.

3. What are the typical hardness values for ASTM 668 and AISI 4340?

The typical hardness values for ASTM 668 range from 180 to 220 Brinell hardness (HB), while AISI 4340 typically has a hardness range of 217 to 255 HB. However, these values are not standardized and can vary depending on the specific application and processing methods.

4. Which steel is more suitable for high-stress applications?

AISI 4340 is generally preferred for high-stress applications due to its higher strength and toughness compared to ASTM 668. Its nickel-chromium-molybdenum alloy composition provides excellent strength and wear resistance, making it suitable for demanding applications in industries such as aerospace and oil and gas.

5. Can ASTM 668 and AISI 4340 be used interchangeably?

No, ASTM 668 and AISI 4340 are not interchangeable as they have different chemical compositions and mechanical properties. It is important to use the appropriate steel for the intended application to ensure optimal performance and safety.

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