Fine Grain Practice

[bmoorthy]

Hello All

This is in connection with Carbon steel plates.

Some specification calls for fine grain steel and specify that the grain size is to be checked as per E 112.

Now the question is

1)Is it OK to take a sample of plate and conduct MICRO Examination and determine the grain size and conclude that the plate is fine grain plate

Or

Is it necessary to carburize the coupon (McQuaid Ehn Test)and then check the austenitic grain size and then conclude that the plate is fine grain

2) Is it possible to take a fine grain (Fine grain Melting Practice material) to be subsequently heat treated and get a corse grain (By annealing). That is Austenitic Grain Size is Fine Grain (By Fine Grain Melting Practice) and have a ferritic coarse size by Annealing?

 

[metengr]

Answer to Question 1).
If the carbon steel plates are supplied under an ASTM/ASME specification, refer to ASTM/ASME A-20/SA-20, which is a Specification for General Requirements for Steel Plates for Pressure Vessels.

Under Metallurgical Structure in Specification ASME SA-20;

8.2 Fine Austenitic Grain Size;
If Al is used to provide for grain refinement, no further testing is required PROVIDED the product heat analysis contains at least 0.020% total Al or 0.015% acid soluble aluminum.

If Al was not used for grain refinement, the steel shall have a carburized grain size of 5 or higher (finer) as determined by the McQuain-Ehn test in accordance with Methods ASTM E112 Plate IV. One test per heat.


Answer to 2). It depends on the type of carbon steel plate. If carbon steel plates are supplied as fine grained with aluminum as the grain refiner, you will not develop coarse grains. Aluminum used for grain refinement forms oxides and nitrides that inhibit grain growth during processing.

 

[TVP]

One thing to keep in mind is the difference between austenitic or prior austenite grain size and ferritic grain size.

 

[bmoorthy]

So if the coupon is carburized as specified in SA 20, under micro examination what grain do we observe. Is it Austenitic impression or is it ferritic grain?

 

[metengr]

It is the prior austenitic grain size that you will be evaluating.

 

[stanweld]

bimoorthy,
In answering your question No. 2, ferrite grain coarsening will occur with subsequent annealing or heating slightly below the upper critical temperature. One would have to hold at temperature for a long, long time to coarsen the grain to that af a "Coarse Grain Structure" though. I have typically seen only a coarsening of about one size during annealing. While greater coarsening was seen when fine grain melting was done through additions of niobium or vanadium, coarsening was also observed in steels that were fine grain melted with aluminum.

 

[metengr]

Stanweld mentions an interesting point. The data that I have from "Physical Metallurgy Handbook" by Sinha shows a step change in grain coarsening at a minimum temperature of 1925 deg F for carbon steel treated with aluminum. This is well above normal process temperatures. Keep in mind that carbon steels using aluminum for fine grain practice have been reported to show an increase in susceptibility to graphitization upon long term exposure to elevated temperature service.

 

[bmoorthy]

Thanks a lot for the inputs.

Now from the point of view of acceptance or rejection of the material

1) The material ordered is SA 516 Gr 60. Thickness 50mm (Normalized). Certificate shows Normalised but does not indicate any thing about melting practice and grain size.

2) Is it mandatory for the certificate to show that the material has under gone fine grain practice melting or should it show the grain size. Does it not automatically implied that the material has under gone fine grain practice melting process by the virtue of the material being SA 516?

3) Follow up question, Let us say the melting practice is not known, by normalizing does it automatically imply that the material is fine grain now.

4) Does the ferritic grain (Simply seeing under microscope with out carburizing) any indicator of the required Austenitic fine grain. That is if the plate is normalized and then i evaluate the grain size (With out carburizing, when we see Ferritic grain), does it meet the intent of the requirement of fine grain practice of the code. After all we are interested in the properties of the material which we are using and not about the Austenitic history?

 

[metengr]

Your first statement is key regarding acceptance or rejection. Please note SA-516 states and I quote;

"The steel shall be killed and shall conform to the fine grain practice of SA-20"

If you are questioning the source of the steel plates this is a different matter, and concerns traceability and nonconformance. If you had ordered the plates direct then they should have been ordered with a Material Test Report in accordance with requirement 3) in SA-516 and the requirements in SA-20 on the original PO.

I suspect what you have is a certificate of conformance??? If you have this, you should be able to go back and obtain a Material Test Report from the mill, this will answer your questions.

 

[metengr]

Follow-up to answers for Question 3 and 4;

3). No. Normalizing is a heat treatment process and does not equate with fine grain practice. Fine grain practice begins with deliberate alloy additions during steel making.

4). No.

Please note, if you intend to use these plates in a pressure vessel, you need to go back to the original code of construction (ASME or other) to re-certify the plates. See ASME B&PV Code, Section VIII, Div 1, Part UG.