IN GENERAL: Any steel connecting rod will have a heat treatment at some point. As these are 4340 rods I am 100% sure that they, like any 4340 rod, will have been heat treated in a very specific way. When one gets out of the straight carbon steels and into the stronger alloys the heat treat can increase strength maybe 30%. Also each designer will have his own targets for strength and ductility.

Here are temps to achieve different colors. This is by uniform heating and then rapid cooling. This is different from the colors that happen when the steel is heated and held at a specific temp, like "cherry red heat".

1.HEAT COLOURING OF IRON AND STEEL
yellow/228C
brown/254C
purple red/265C
light blue/264C
dark blue/293C

Here is a normal heat treat for 4340:PROCESSING TEMPERATURES
Typical

Forge
2200°F MAX

Normalize
1550/1650°F

Case Harden
1475/1550°F

Anneal
1475/1575°F

Temper
1200°F & 400°F

I started out thinking use them. But, 400F is around 200C so even the least colored rods have been heated past the last tempering temperature. For any kind of high performance work I'd sell them as-is on Ebay and buy new ones.

R.