I was surfing and came upon this thread from March. The discussion was interesting, but while there was some dicussion of weathering steels such as the COR-TEN types, there really wasn't any mention of copper strengthened medium strength (up to 100ksi) structural steels.
This link:
takes you to a brochure for ISG steel company's medium strength copper strengthened steels. There are 5 grades listed (3 actually with a couple of variations for different thickness ranges). The HPS grade (spartan V) is a 100ksi yield strength bridge weathering steel, the others are 80 and 100 ksi yield strength ship hull steels. None of these steels are "stainless steels".
All of these steels have copper precipitation hardening as their primary strengthening mechanism. The amount of copper far exceeds that needed for weathering. The ship steels aren't considered weathering since that mechanism doesn't work in chloride containing water.
The copper was added specifically to replace carbon for the purpose of significantly improving weldability. While copper is an expensive alloy replacement for carbon strengthening, the improved weldability more than makes up for the increased alloy cost.
The corresponding steels that were replaced had approximately 0.12% to
0.18% (or slightly more) carbon and were quench and temper martensitic strengthened steels. The copper versions have up to 1.75% copper and more complex strengthening mechanisms, but are solution treated, quenched, and aged with most of the strength from copper precipitation. The temperatures and cooling rates for each step are about the same for the old and new alloys, but the metallurgical reactions are differerent, hence the different names. Also the effects of each step are different. The carbon containing steels are very hard and brittle after quenching and then moderate their strength and greatly increase their impact and fracture toughness during tempering. The copper steels are not very hard after quenching, and the strength increases during aging. This is one of the features that makes them more weldable - the weld HAZ hardness doesn't spike like it does in the carbon martensite steels.The brochure gives some good tables and charts showing how these steels respond to heat treatment.
By the way, the color of the steel looks like steel at this level of copper.
Tom (not associated with ISG)