Test for AlN embrittlement in steel castings - ASTM A703/E340 1

[kclim]

Folks,

I have a 0.5 Cr 0.5 Mo 0.25 V steel casting which has been in service for some time. It has become brittle.

I have performed an HCl macroetch on a sample of the steel (as per astm standards quoted in subject line), and it indicates that the steel has embrittled ('highly networked structure'). At first I put this down to aluminium nitride embrittlement.

However, I did note in that in A703, a network structure suggests "aluminium nitrides, OR other constituents prone toward precipitating and the grain boundaries" would give a positive indication. A big 'or' in my case.

Was wondering if anyone had any experience regarding this macro-etch test, and whether the results would be expected to be different whether it was aluminium embrittlement, or temper embrittlement (S, P, Sn, As at grain boundaries). If the etch isn't selective, is there another test possible to distinguish the two? (Auger spectroscopy?)

Thanks in advance

 

[metengr]

I have a 0.5 Cr 0.5 Mo 0.25 V steel casting which has been in service for some time. It has become brittle.

How did you confirm this? Did you run CVN impact tests? Have you run a complete chemcial analysis?

I would not run and jump to any conclusions unless you answer yes to the above questions. The Cr-Mo steels can suffer from thermal embrittlement resulting in a loss of toughness and even ductility related to tramp elements and not AlN.

 

[kclim]

Brittleness was suggested by component cracking and confirmed by testing.

CVN tests were conducted - 4 J at room temp. A series of charpys were conducted at various elevated temperatures, FATT appears to be above 150C.

Complete chemical analysis was performed, the aluminium content was the only element that really stood out (0.1% by weight).

All the other impurity elements were within spec, although the spec is quite old (S, P limits were 0.03%, my understanding is that nowadays, limits are about a factor of 10 lower).

So we know the material is embrittled, and is likely due to grain boundary impurities as indicated by the macroetch - we're looking to take the next step to determine what these impurities are.

 

[metengr]

Did you find any other tramp elements in the chemical analysis results, like As, Sb, Sn? The P will significantly enhance susceptibility to thermal embrittlement for Cr-Mo-V alloys, cast or wrought. What you are reporting is no different than what I have seen in forged high pressure and intermediate pressure steam turbine rotor steels.

Did you evaluate the fracture surfaces of the CVN impact samples using an SEM? If not, I would start there and evaluate the constituents along the grain boundaries (I am assuming intergranular fracture at the reported CVN values).

 

[kclim]

Did you evaluate the fracture surfaces of the CVN impact samples using an SEM? If not, I would start there and evaluate the constituents along the grain boundaries (I am assuming intergranular fracture at the reported CVN values).

We've attempted to, but we didn't get any definitive results (i.e., couldn't see P, or Al for that matter). However, literature suggests that the impurities are highly localised to the fracture surface (order of angstroms), so EDAX would be relatively insensitive to it.

Are there any other options for distinguishing the two embrittling mechanisms (temper & AlN)? I've heard Picral etchant is quite sensitive to P segregation, and it may be possible to heat treat to affect one but not the other.