API 6A TC vs. QTC 3

[salmon2]

Can anyone tell me what is the difference between TC and QTC in API 6A? They seem awfully similar to me. Basically PSL 1 requires usage of TC and PSL 2-4 requires QTC. Your any comments are highly appreciated.

 

[texag]

salmon2,

A sacrificial production part is allowed to be used for extracting a TC but is not allowed for a QTC. A QTC must be extracted from a trepanned core or prolongation removed from the production part.

This would appear to be consistent with the elevated quality requirements of PSL2-PSL4 since the coupon must be extracted from the part that will go into service rather than a sacrificial part.

Will

 

[salmon2]

Texag, thank you so much for your comment I have been waiting so long for anyone's input. What you said make sense to me. But still I have two more questions:

1) what is the difference between a sacrificial production part and production part? The only difference I can think of is maybe length?

2) When will TC or QTC be cut? Before or after heat treatment? My understanding is that it is before heat treatment and then TC or QTC will go through the same thermal cycles with production. Am I right? In that case, there is really no difference between TC or QTC, or between sacrificial part and production part except possible size difference between TC or QTC.

Thanks a lot again.

Salmon2

 

[blacksmith37]

A "sacrifical" part is a standard production part from which test coupons are cut AFTER all heat-treatment. After HT is essential so the mass affect on HT is shown in the test coupon/results.

 

[redpicker]

The TC (Test Coupon) or QTC (Qualification Test Coupon) is the piece of metal from which the test sepcimens are machined. It can be either a (sacrificial) production part, a prolongation on a production part, or a separate coupon with a simple geometry used to qualify parts with more complex geometries.

You have to follow the rules for the PSL you are producing. The main difference in the TC and QTC is that the TC is used for PSL 1 and the QTC is used in PSL 2-4. There are other specific differences, such as for remelt grades, the PSL 1 TC is only needed per each master heat while for PSL 2-4, you need a seperate QTC for each remelt ingot. There may be other differences, I am not sure.

A production part is a sacrifical production part when it is destroyed in obtaining the test specimens. For some components, it is easier to just produce some of the produciton parts with prolongations, say parts that are produced out of a thick wall tube that are cut to 2.0 foot lengths prior to heat treatment. It is easy so cut some of them at 2.75 feet so that the end can be cut from one of the longer ones to obtain the test secimens. Other parts, such as close die forgings, it is just simpler to cut up one of the parts to get your specimens. The rules are written to permit the manufacturer the maximum flexability in his manufacturing process, but still have clearly defined, auditable requirements for obtaining the test specimens.

The test specimens are removed from the TC (or QTC) after heat treatment. Obviously, if the TC (QTC) is a prolongation, the point is that the prolongation is not removed until after heat treatment. If the TC (QTC) is a separate coupon produced to a simple geometry to represent the heat treat response of a more complex geometry, then it is (obviously) separate.

I am not sure, but I think the reason TC is used for PSL 1 and QTC for PSL 2-4 is that the original 6A requirements were what is now PSL 1, with PSL 2-4 added as the sepcification evolved. It appears to me that using QTC for PSL 2-4 allowed them to place additional requirements on how the test specimens were obtained for these new levels without affecting the original product level requirements. This is only a guess, however, and you would need to find someone who worked on API sub-committee 6 for a more accurate explanation.

rp

 

[salmon2]

Thank you all for the input, especially redpicker's comprehensive discussion, very informative. But I have to say I am still a bit confused regarding the difference between TC and QTC. For example, I have seen enough MTRs saying that a 4''X4''X8'' QTC was used for testing, but why it is called QTC instead of TC? Is it simply because it is for PSL2+?

So far after I got all the input (mainly based on redpicker's), my understanding is:

Option 1) sacrificial part, a whole finished part, no "coupon" involved any more. Then use 'equivalent round' (ER) method to define the location to cut tensile and Charpy testing specimen.

Option 2) prolongation, an extension of a finished part, good for ball or tube obviously. Need to use ER method to judge the dimension of prolongation if cross section is not uniform. Again no TC or QTC involved.

Option 3) TC or QTC, an actual coupon with simple geometry based on ER method, which is cut before heat treatment and thermally processed along with production. Here is the question I still have: what is the difference between TC or QTC? I know there may be some size difference. For example, QTC does not have to be larger than 5.0'' Dia according to API 6A and above 4''X4''X8'' is ER 5.0'' for whatever sizes. By the way, this (ER 5.0'' for any diameters) sounds unreasonable to me but it is what it is in API 6A. I don't have API 6A with me at home, but I remember TC does not have this ER 5.0'' max and may have something different. This is only difference between TC and QTC I can see.

 

[redpicker]

Option 1) sacrificial part, a whole finished part, no "coupon" involved any more. Then use 'equivalent round' (ER) method to define the location to cut tensile and Charpy testing specimen.

Not quite. With this option, the sacrificial part IS the TC or QTC.

The sacrificial part can be a whole part blank. That is, the part machined to the blanking dimensions used prior to heat treatment. In addiiton, the ER method is not directly used to define the test specimen locations. The test specimens are to be machined from inside the 1/4 T envelope of the thickest section. Only for cases where the ER of the thickest section exceeds the maximum required for the applicable PSL does the ER method come into play when using a sacrificial part.

Option 2) prolongation, an extension of a finished part, good for ball {bar?} or tube obviously. Need to use ER method to judge the dimension of prolongation if cross section is not uniform. Again no TC or QTC involved.

Correct, except that there is a TC (QTC) involved; the prolongation is the TC (QTC).

Option 3) TC or QTC, an actual coupon with simple geometry based on ER method, which is cut before heat treatment and thermally processed along with production. Here is the question I still have: what is the difference between TC or QTC? I know there may be some size difference. For example, QTC does not have to be larger than 5.0'' Dia according to API 6A and above 4''X4''X8'' is ER 5.0'' for whatever sizes. By the way, this (ER 5.0'' for any diameters) sounds unreasonable to me but it is what it is in API 6A. I don't have API 6A with me at home, but I remember TC does not have this ER 5.0'' max and may have something different. This is only difference between TC and QTC I can see.

As I said in my original post, I can only guess as to the reason for using TC or QTC for the different PSLs. I was involved with 6A years ago, and it has changed substantially since then.

It may help you to understand that these methods were developed to standardize the testing methods for valve bodies used in wellhead assemblies, mainaly spools and casing heads. Typically forgings, they can weigh several hundreds of pounds each. It is not resonable (or necessary) to cut up actual parts to verify mechanical properties, and seperately forged test bars are typically used. These methods evolved over the years to insure all manufacturers were using the same qualification methods. They had to be modified so they could be applied to other products and manufacturing methods.

If you still have questions, contact API (

http://www.API.org).

There is a "Contact Us" link on the main page that will allow you to send a message. Be sure to choose "Standards" as the section name and identify the specific standard (6A) and applicable sections you have questions on and they will direct your question to the sub-committee that maintains the standard. Doing this actually helps API maintain the standards as it lets them know what sections need clarification.

rp