ASTM 668 vs AISI 4340: Hardness Comparison

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SUMMARY

ASTM 668 is not equivalent to AISI 4340; rather, it represents a specific standard for a form of AISI 4340 steel, which has a hardness limit of 35 HRC compared to ASTM 668's limit of 38 HRC. AISI 4340 contains approximately 0.4% carbon and its properties can vary significantly based on heat treatment and forging temperatures. The final microstructure, ductility, and hardness are influenced by the carbon content and the degree of cold work applied to the steel.

PREREQUISITES
  • Understanding of steel grades and compositions, specifically AISI 4340.
  • Knowledge of hardness measurement scales, particularly HRC.
  • Familiarity with heat treatment processes and their effects on steel properties.
  • Awareness of ASTM standards and their applications in material specifications.
NEXT STEPS
  • Research the differences between ASTM 668 and AISI 4340 specifications.
  • Learn about the heat treatment processes for AISI 4340 steel.
  • Explore the impact of carbon content on the mechanical properties of steel.
  • Investigate the various forms and applications of AISI 4340 in engineering.
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Materials engineers, metallurgists, and professionals involved in steel manufacturing and quality control will benefit from this discussion.

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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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.
 

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