Im about to buy my frame tube. Ive worked out the diameter i want and the length. Ive found a supplier of CDS tube (now called CFS and often called DOM by the Americans) - but theyve thrown an option into the can of worms. Do I want it BK (as drawn - work hardened) or NBK (normalised - back to its cast condition which is a bit softer). NBK is by the way the spec for hydraulic pipe.
Well, what do I want? does it effect 1) bendability, 2) weldability, 3) vehicle strength and durability?
Wikipedia says......
"Normalizing: Carbon steel is heated to approximately 55 °C above Ac3 or Acm for 1 hour; this assures the steel completely transforms to austenite. The steel is then air cooled, which is a cooling rate of approximately 38 °C (100 °F) per minute. This results in a fine pearlitic structure, and a more uniform structure. Normalized steel has a higher strength than annealed steel; it has a relatively high strength and ductility"
Already referenced elsewhere in this blog; Miller Welds ( http://www.millerwelds.com/education/articles/articles70.html ) says.....
"Drawn-over mandrel tubing is manufactured to ASTM Specification A513 Type 5, and frame fabricators often chose SAE Steel Grades 1018 and 1020. These have a yield strength of 70 ksi or 483 N/mm2"
Now i did metallurgy as part of my first degree, but i suspect i was sitting at the back designing jazz club ties with Hughie rather than listening to the lecture that talked about "yield strength" - and now my head is in a spin
Its amazing how many manfacturers of motorbikes claim that their machines are made of the "highest grade steel" - but none of them can quote what that actually is - buyer beware!
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