This begs the question, whats the difference between SD421 4140 steel rods, and 58-62 370/389 1140 steel rods.
They are very close. There's some chromium, molybdenum, silicon added to the SD421 4140 rod. But they are within a whisker of each other.
SAE-AISI 1140 Steel vs. SAE-AISI 4140 Steel ::
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