PHOTOMICROGRAPH OF A216-WCB

[valvesarefun]

Looking for some feedback on the attached file. Both pictures are from the same heat treatment lot, and the same parent material, and yet the microstructures differ quite a bit in appearance. Does anyone have any input as to why this might occur, and if this type of variation between samples of a particular heat lot is common.

Thanks in advance!

 

[metengr]

If what you state is correct, it looks like from a low magnification view the microstructure of the valve body did not have suitable time for complete austenization treatment (austenite transformation after the casting was poured) prior to cooling in air or other medium, resulting in the inconsistent grain size and phase constituents.

 

[valvesarefun]

Metengr,

Thank you for your reply. Obviously i am not an expert in these matters, however am I correct in my analysis when i say that the top picture is a prototypical example of what a WCB casting should look like, while the bottom sample represents an obviously less than optimal casting? What detrimental effect, might this have on the casting? I forgot to mention that this is a body of a valve intended for pipeline service oil and gas pipeline service.

Best regards,

VRF

 

[metengr]

valvesarefun;
If the microstructure of the body is indeed represented by the micrograph you provided, the mechanical properties may not be consistent as compared with the microstructure depicted in the tailpiece microgrpah. If you review SA 216, the WCB castings can be supplied either annealed, normalized or normalized and tempered.

I have no further information to provide because you have not stated what heat treatment was performed on the castings. I would go back and review what heat treatment was specified and documentation associated with the heat treatment.

Second, you may want to sample one the of castings having this microstructure and remove tensile specimens for testing to evaluate properties in accordance with SA 216.

 

[valvesarefun]

Metengr,

Both pieces were certified "normalized @ 920C for 3 hours"

Mechanical properties were also evaluated in this examination.

In both of the tensile tests, the samples taken from the body were weaker than those taken from the tailpiece. However the difference doesnt appear to me to be appreciable (1-2% decrease in TS). All the mechanical properties still fall within the respective limits of WCB.

 

[arunmrao]

valvesarefun(too long a handle,can you make it short please)

Your response is on expected lines and not at all surprising. WCB grade has a wide range of acceptable limits and sure enough they would satisfy. What might have happened is that the body casting was given an early shakeout,and subsequent heat treatment soaking time was not adequate for the carbides to dissolve and form new grains,thus you have got different microstructures but acceptable mechanical properties.

If you have the time and facility available,try Charpy test on two sample,they would differ in my opinion,though such a test is not mandated by the specification.

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[valvesarefun]

Arunmrao,

Sorry for the long handle, all the good names were taken!

Just call me VRF

So what you are saying is that if a casting is removed from the mold too soon (before complete cooling) it may require a longer heat treatment time for fully normalize?

Also, if we are to perform a charpy test, what temperature is recommended? My experience with charpy tests is limited to low temperature materials like LCB where the test is preformed at -50F, but I don’t think that would apply in this case.

 

[arunmrao]

VRF,

My suggestion for charpy test is only academic ,yes I do confirm that an early shakeout is the cause of your problem.

If a valve casting does not qualify for LCB grade do you accept it as WCB casting without scrapping the casting?

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[valvesarefun]

Arunmrao,

No that is not what I am saying.

I only mention LCB because that is the only time we preform charpy tests. With LCB the charpy exam is preformed at low temprature, however because you suggested a charpy test for this WCB casting, I was wondering what temprature we should preform the test at.

Should the WCB casting be impact tested at low temprature or ambient temprature?

 

[arunmrao]

A normally processed wcb charpy test specimen will fail at low temperature,if you can try and test ambient temperature,perhaps you might get an answer. I hope,I am not confusing you unnecessarily.

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[valvesarefun]

Arunmrao,

I understand what you are saying, and I understand that the WCB specimine would "fail" under cold tempratures.

The point being not really pass/fail but you are saying that there would be a difference between the two samples caused by the different microstructure of the steel.

 

[arunmrao]

Please test and let us know,if my comments are valid or not.

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year