9Cr heat treatment for heater tubes 1

[kevlar49]

Is there a preference for the heat treatment applied to 9Cr heater tubes in an FCC (Fluid Catalytic Cracking) furnace? ASTM A234 states that tubes shall receive one of three heat treatments: full anneal, isothermal anneal, or normalized and tempered.

Operating condition will be >1000 F. Sulfur may double to 2wt%, TAN is usually <1.0.

I am requesting stress relieving at 1300-1400 F for all welds and weld repairs and hardness.

 

[kevlar49]

One other thing. I am requesting that if liquid quenching is done that it be oil and not water to avoid quench cracks. Any comments?

 

[metengr]

kevlar49;

I just have one question, why did you specify SA 234, when this specification deals with fittings, not tubes?

Did you mean SA 213 T9?

 

[metengr]

.....Assuming that you meant SA 213 T9, and this is for existing tubing in the furnace, and not new (if this is new you better review SA 213), any weld repair to this material requires a local post weld heat treatment. Your PWHT temperature range is acceptable.

Regarding new tubes under this specification, air cooling with air movers is more than adequate because of the hardenability of this material. Oil quenching is not necessary. Second, the specified maximum hardness for Grade T9 in SA 213 is stated to be 179 BHN (or 89 HRb).

 

[kevlar49]

A234 for potential return ells. Could specify either A335 for pipe or A213 for tube.

 

[kevlar49]

For retube of furnace, so specifying new tubes.

 

[kevlar49]

Thanks metengr. What I meant to say is does the type of heat treatment enhance or decrease creep/corrosion properties significantly?

 

[metengr]

kevlar49;
Ok, thanks for the clarification. For the return elbows of WP9, I see that liquid quenching is permitted as agreed upon between the purchaser and the vendor. Just curious, how thick is the wall for this fitting that you want liquid quenching? Is the liquid quenching specified for the new tube/pipe material as well?

The maximum hardness specified under SA 234 is 217 BHN. In compariosn to SA 213, SA 335 is a seamless pipe specification, no hardness is specified for P9, only P91,92 and 122.

 

[kevlar49]

Original 5Cr tubes were 0.375" in the radiant; not sure what the fittings would end up being.

Liquid quenching is not being specified per se; I am just restricting the ways that the manufacturer quenches. I am just trying to avoid quench cracks. Good point: hardenability may obviate liquid quenching.

 

[metengr]

kevlar49;
Sorry for the delayed posts here.

What I meant to say is does the type of heat treatment enhance or decrease creep/corrosion properties significantly?

My preference and from the creep data I have seen for Cr-Mo boiler tubes (not Grade 91 or the enhanced ferritics) is that the most stable structure is that obtained from isothermal anneal or full anneal. Having a stable, well distributed, chromium carbide structure in ferrite for your basic Cr-Mo alloys seems to bolster creep strength, in comparison to normalize and temper.

Watch Note 3 if you specify SA 335, if you temper, make sure that the temping temperature is 100 deg F above the expected service temperature for the material (if at or below 1100 deg F).

 

[kevlar49]

No worries about the delayed post. I am grateful for the advice.

Thanks! I noticed that note too. Why doesn't it appear in A335 I wonder?

 

[metengr]

kevlar49;
Ah, you have discovered the difference between ASME B&PV Code SA Specifications and published ASTM specifications, and why ASME B&PV Code, Section II endorses their own specification (editorial changes and other changes) for boiler and pressure vessel code work.

To answer your question; more conservative requirements regarding creep strength when it comes to boiler external piping and to boiler piping under ASME Section I.

 

[kevlar49]

last post I meant why don't I see note 3 in A213 as well.

Do I understand your last comment correctly? A213 should be more conservative than A335 since it is for boiler service?

 

[metengr]

No. SA 335 being a pipe specification would most likely be used outside of the boiler setting with higher risk of exposure to personal injury concerning defects and creep stregnth (thus a more conservative approach needs to be considered regarding fabrication) versus boiler tubing, which is normally within a furnace setting.

 

[kevlar49]

Why is there a restriction on vanadium in A234? Is it to reduce delayed hydrogen cracking?

 

[metengr]

Reduce reheat cracking susceptibility related to welding.

 

[kevlar49]

Our internal specification allows for either MIG or TIG root pass. Don't have experience with the difference in quality. Would you expect TIG to be better?

 

[metengr]

kevlar49;
I would use GTAW for the root and hot pass in tubes and pipe. GMAW (MIG) can be used, but requires welder skill and I have seen too many problems with LOF defects.

 

[0707]

These P9 tubes are normaly hot finished normalized end tempered.

I would recommend stress relive welds at 720ºC plus or minus 20ºC during 1 hours minimum, followed of a cooling rate of 100ºC/hour until 300ºC, then slow cooling under ceramic blankets. At the end hardness welds should be less than 240HV.

First root passes, should be TIG restant can be SMAW. Alternativly all the well can be TIG.

Regards

Luis

 

[metengr]

Luis;
Yes, normalize and temper is normally supplied to most customers. However, for Cr-Mo tubes an annealed tube metal structure affords even better creep strength. Keep in mind that tempering does not result in the most stable of microstructures for elevated temperature service. The creep rupture data I have seen for these materials confirms this view.