High Temperature Oxidation
High Temperature Oxidation
(OP)
Does anyone have any data on high temperature oxidation (900 F) of Cr-Mo steel as a function of microstructure (martensite/tempered martensite/pearlite)? I have a dissimilar metal weld that has failed (classic DMW creep along the fusion line) but also has a slight flare in the OD profile leading up to the fusion line on the ferritic side (which also shows significant oxidation/exfoliation from sitting in the penthouse environment for 30 years). The flare starts where the heat affected zone ends.
I'm thinking the flare is merely due to more oxidation of the tube away from the HAZ and the HAZ has lost less material, but I can find no published data to support this. Any thoughts?
I'm thinking the flare is merely due to more oxidation of the tube away from the HAZ and the HAZ has lost less material, but I can find no published data to support this. Any thoughts?





RE: High Temperature Oxidation
RE: High Temperature Oxidation
I have not see any published data on high temperature oxidation characteristics related to microstructure for a given alloy content.
How close is the DMW failure to the high crown seal or penthouse floor?
RE: High Temperature Oxidation
As you go thru boiler start-up and shut-down cycles, exfoliation of the oxide exposes fresh tube material, and the process begins again. It may well be that the microstructure of the HAZ is not as prone to intergranular oxidation. This is only speculation on my part.
RE: High Temperature Oxidation
metengr (second comment): You raise a possibility. It occurs to me also that perhaps the martensitic HAZ, having a higher strength, might be less susceptible to the flexing needed to exfoliate the oxide. On the other hand, wouldn't the modulus of elasticity would remain the same regardless of it being pearlite or martensite? so it shouldn't matter.
Puzzles like this keep reminding me why I got into failure analysis.
RE: High Temperature Oxidation
At the operating temperature of the boiler tube you will not be in the elastic regime. The tempered martensite will have a different creep rate and strength level in comparison to the unaffected tube material (away from the DMW).
I've got one for you along the same lines. DMW failure from a supercritical boiler in the high crown seal (furnace side). Confirmed the filler metal is Inconel. The crack was along the toe of the weld - except the tube only lasted 8 years. Found three more DMW's during a recent tube repair. Looks to me like fatigue versus classic creep failure along the weld fusion zone on the T22 tube side.
RE: High Temperature Oxidation
This will cause a flaring even if oxidation of the carbon steel is uniform, as would normally be expected.
RE: High Temperature Oxidation
mcguire: Would your mechanism apply to both the ID and the OD? In my case, this is an actual increase in wall thickness.
RE: High Temperature Oxidation
The flaring you discussed may be related to the Cr-Mo creep relieving to reach the same hot OD as the SS. Although the SS might have a lower yield stress, the Cr-Mo has a much lower creep strength.
Goodlong term availability of the DMW's has ben found by using a Nickel based weld filler ( incolnel) , but using a very wide weld widht and applying the weld using an automatic orbiatl welder. It had been found that if the weld was applied manually, the welder would decrease the amps to limit weld splatter, but would then cause insufficient weld penetration.
RE: High Temperature Oxidation
Yes, the differential thermal expansion would cause the ferritic steel, which has much lower yield strength at high temperature, to grow in the thickness direction also.