Fracture Toughness SA-213 T11 (1.25 Cr 1/2 Mo)

[InTheField]

Three part question:

1. Is there an index or list that cross references AISI materials with ASME Boiler tube materials.

2. Does anyone have the fracture toughness modulus for SA 213 T11 (I did a ballpark estimate by using leak before break criteria - but I don't trust the value).

3. I could also use the value of Kth (fracture toughness threshold value for fatigue analysis) - or even better yet - if someone could point me to a load vs. CGD (crack gage displacement) graph.

I guess if all else fails, I could use a charpy test to estimate Kic for a BCC mtl.

Thanks To All (oh the things I get myself into in my free time . . .)

 

[metengr]

InTheField;
Yes, to your first question. A majority of the ferrous and nonferrous materials that are accepted for use by ASME B&PV Code have a UNS No that can be used to cross reference certain Grades of Steel listed by AISI or other Standards. The web site below contains some background information on the development of the Unified Numbering System;

http://www.astm.org/DATABASE.CART/ALLPRODUCTS/SNEWS/MAY_2002/cobb_may02.html

The book below can be used to cross reference UNS No with ASTM, JAE and AISI grades of metals;

Metals & Alloys in the Unified Numbering System, 10th Edition
Published September 2004, Paperbound, ISBN: 0-7680-1488-3

I was curious as to the reason you need fracture toughness data for SA 213 T11 material. Typically, the Cr-Mo materials supplied in tube or pipe grades are used in moderate to elevated temperature service so fracture toughness is not something that is commonly needed in design.

Most likely, you will need to either have actual fracture toughness testing performed on this material or you can have Charpy impact testing performed on this material (cheaper cost), and convert to an equivalent KIc fracture toughness value. However, I would not use an equivalent KIc for design purposes.

 

[Carburize]

Check - ASM Vol 19 Fatigue and Fracture" for toughness and fatigue crack growth data. Not sure if it has the specific alloy but you may find something close enough to confirm you assumptions.

 

[InTheField]

Thanks for the post metengr. Basically, I am doing my own failure analysis for a leak that occured about six months ago. The tube leaked at a weld just below an A type connection at a tube to header weld.

The tube extends through the roof into the convection pass of the boiler. There is a temperature difference between the header and roof (saturation vs. superheat temp) and thus differential expansion. The simple anwser was that the boiler components are 33 years old and overdue for replacement - but I was curious to see what a failure analysis would give - I would not look at designing the component in this method but it is an interesting exercise to go through.

Anyway - I am getting closer to the solution now - Cheers.

 

[metengr]

InTheField;
Thanks for the follow-up information. According to your post, what you really need to be concerned with is creep-fatigue crack propagation because most likely from the scenario you described the mode of crack propagation is creep-fatigue. The fracture toughness would only come into play during conditions where that cracked tube is subjected to a pressure test with water, where the water temperature is below the ductile to brittle transition temperature of the T11 tube. Under normal operating conditions, the tube metal will contain much higher toughness in comparison to room temperature pressure testing.

Under normal service conditions, the crack will propagate until the remaining ligament thickness can no longer sustain internal pressure or thermal/mechanical bending stresses in the tube drive the crack thru wall.

 

[InTheField]

I am starting with the fatigue analysis to start with - the service temperature is not quite 1/3 of the melting temp - so it is not quite in the creep range - that is not to say that there is none. I was looking at a cumulative creep-fatigue solution following the FM approach.

Any chance of sperodization with a T11 material? I would doubt it because of the chrome - but I thought I would ask. If I remember correctly the carbon would be in the right range (up to AISI 1050 or so for spheriodization?). ANyway, that all for now.

 

[metengr]

Yes, spheroidization is very common with the Cr-Mo low alloy steels in superheater or reheater boiler tube applications. Spheroidization has nothing to do with chromium content, this is a damage mechanism from long term exposure to elevated temperature service. You might be thinking about Graphitization. In general, carbides love to end up as spheroids.

 

[stanweld]

A description of the crack morphology would help. Did the crack follow the fusion line (type 3A creep cracking associated with carbon mismatch at the fusion line); in the HAZ (Type 4A creep cracking; or initiate at a minor change in geometry "faceted bore cracking" generally associated with upshock cycling temperature?

 

[InTheField]

Unfortunately stanweld I cannot answer that question yet. The tubes were repaired in place. No samples were taken. I may yet obtain a sample when the elements are replaced (possibly this spring).

 

[stanweld]

Another item to check. Have the number of start/stop cycles increased in the past 5 to 6 years?

 

[InTheField]

No real change in cyclic loading - if anyting there has been a decrease. I completed a stress life analysis using material properties at elevated temperatures. My theoretical calcs have come within two years of the actual failure (over a 35 year span) - I am fairly satisfied with the results. I can chalk up the difference between practical and theory with the mean probability of failure for stress life analysis.

One more thought - if anyone has an RT NDT temp for sa 213 t11 i could take the fracture mechanics approach further. Although I would still need a loading vs. crack gage displacement test to be accurate - although this would be taking things a bit far.

Cheers to all the help!