Blue Brittle Range 1

[Vattappilly]

I ran into an Iron-Iron carbide diagram that talks about "Blue Brittle
Range" (300-700F)
They prohibit peening or working of steels within this range.

Does this mean that I have to stay away from that range if I do any cold
working on low carbon steel, Such as SA-36 or SA-106 Gr. B.

 

[metengr]

No. For carbon steels like ASME SA-36 and SA-106 Grade B, these steels are produced to an ASME specification that results in a metal structure (after the required heat treatment) that will not exhibit blue brittle behavior, even if re-heated to between 350 to 600 deg F in service.

Typically, blue brittle behavior occurs after certain carbon and low alloy steels have been subjected to a different heat treatment - hardening (heated above 1350 deg F and liquid quenched) and tempering (reheated) within the range you stated (300 to 650 deg F). Precipitation of certain carbides within the tempering temperature range (when the steel is heated in air it develops a blue color from surface oxidation) results in increased strength but reduced ductility and toughness (thus, blue brittle behavior). Your SA grades of steel are not supplied in the hardened and tempered condition.

In your case, as long as the SA-36 and Sa-106 Grade B steels have not been subjected to hardening and tempering heat treatment after fabrication, they should be fine.

 

[metengr]

*CORRECTION* on blue brittleness (not having my official reference material handy during my first post). I did some research and here is the actual definition of "blue brittleness" behavior in steel - the effect of increased strength and reduced ductility is caused by "dynamic strain aging" (DSA) of the steel between 450 to 700 deg F (source; Physical Metallurgy Handbook, page 4.33). Second source is Metals Handbook Desk Edition, page 289.

At a strain rate of 10^-3m/ms and within the temperature range of 450-700 deg F, certain steels will exhibit high strength with reduced tensile ductility and reduced notch impact resistance. Carbide-and nitride forming elements are usually added to prevent blue brittleness in certain steels. Blue brittleness can be eliminated if elements tie up nitrogen, such as aluminum or titanium. Keeping the nitrogen content as low as possible is beneficial.

Sorry about the confusion.

Regarding your SA steels, you should still be at a low risk for blue brittleness, especially if aluminum was added to deoxidize the steel.