IMPACT TESTING 6

[kandula1]

hi every one,
I am studing ucs-66 and have several questions regarding IMPACT TESTING.My questions are

why should we do Impact testing?
By doing the test what the resullt has to do with MDMT

 

[vesselfab]

question 1 Impact testing is to prove suitability for lower temperatures (MDMT)

question 3 see answer above

 

[vesselfab]

should have said "ductility" at lower temps

 

[kandula1]

Thanks vesselfab i am a new trainee engineer and i have a case where my int pressure is 3000 psi with a thickness of 2.3440 sa-106-b pipe and my MDMT required is 20F so till how much temperature do i need to impact test the pipe and also i couldnt relate what impact testing is to do with mdmt?

 

[metengr]

why should we do Impact testing?

Why? Because most ferritic materials used in pressure vessel construction exhibit notch toughness sensitivity to low temperatures. This means that if steel does not possess sufficient notch toughness behavior, existing cracks or fabrication flaws can result in sudden (catastrophic) failure from brittle fracture.

By doing the test what the resullt has to do with MDMT

This is a relatively simple, yet important, test that measures the amount of energy necessary to fracture a metal with a notch machined into it. This is a standard test called the Charpy V-notch (CVN) impact that enables a pressure vessel designer to know the limitation of metals at lower temperature service. Lower energy for failure means brittle fracture conditions.

Actually, very good questions for someone unfamiliar with materials and their use in low temperature service.

 

[kandula1]

Thanks for the reply
In order to increase the MDMT from -20 to 20F we need to do impact testing what helps or what changes the temp to 20f after impact tested using CVN.I couldnt relate this.Is CVN test dependent or independent of temperature?

 

[DVWE]

i couldnt relate what impact testing is to do with mdmt?

Impact tests are used to determine your MDMT of the vessel. If there is a minimum requirement of absorbed energy (ft-lb) at a minimum temperature, your tests will determine whether or not the material you have selected will suffice the requirement. You should also read UG-84 and become familiar with Fig. UG-84.1 to determine the minimum required absorbed energy at your temperature (20 F).

 

[metengr]

kandula1;
Your post above is confusing to me. The objective is to conduct impact testing to assure that the steel you select has adequate notch toughness at the lowest design temperature. The steel can be used at service temperatures above the lowest test temperature because as temperature increases so does notch toughness.

In reality most designers would rather try to go from 20 deg F to -20 deg F instead of the other way around.

Based on your questions, I would recommend you visit this web site for information on notch toughness testing of steels. There are some good articles for you and they are free

http://steel.keytometals.com/default.aspx?ID=Articles#p10

 

[metengr]

Impact tests are used to determine your MDMT of the vessel.

No, this is not really a correct statement for PV Code design. The MDMT is selected by the Design Engineer based on process function and the steel is selected for use at the MDMT. Not the other way around.

 

[DVWE]

Yes, but the impact properties of the steel must be known for the MDMT chosen, and there is only one way to find out what they are.

 

[vesselfab]

no that is not correct

the mdmt is a design physical requirement

the impact test is to prove material chosen is suitable for the temperature.

 

[Ballbearing1]

Hello guys,
I am a CWI who is trying to learn more about impact testing so I will offer my thoughts (which may help kandula 1)and if you could please correct me if I am wrong it would be greatly appreciated.
1 Request for design of vessel, piping etc goes to the design engineer.
2 Code of design, fabrication etc is nominated.
3 Material for vessel, piping etc is selected based on service conditions.
4 MDMT is calculated for the vessel, piping etc based on service conditions.
4 Applicable Code is reviewed to ascertain if impact testing is required based on material type, MDMT and material thickness.
5 If impact testing is not required by the applicable code the material is acceptable to use "as is".
6 If impact testing is required by the applicable code testing shall be done in accordance with the code requirements and the acceptance criteria will be as per the code.
7 If the material passes impacts it is deemed acceptable to use for low temperature service but if it fails then you are back to square one and must select a new material type and/or thickness

Am I on the right track or have I got some of the steps around the wrong way ?
Regards,
BB

 

[DVWE]

BB,

Yes. And at step 7 you can either change material, or raise the MDMT.

 

[DVWE]

the impact test is to prove material chosen is suitable for the temperature./

That is exactly true. And if the tests fail, then your MDMT is incorrect.

Let me go back to my original post. The question was, how does impact testing relate to the MDMT, and not how is MDMT determined. OK??

 

[Ballbearing1]

DVWE,
Now I am confused.
If you have service conditions where temperatures range from -35 degrees C to +50 degrees C then surely your MDMT would have to be at least -35 degrees C.
How can you change the MDMT if you fail the impacts?
I thought you would have to change material type / thickness to try and achieve the required results at -35 degrees C.
Regards,
BB

 

[vesselfab]

sometimes -20*F is the default when not really required. Some combinations of alloys and/or thickness will not make that. So the MDMT is raised to a more reasonable level.

 

[DVWE]

BB,

Raising the MDMT is just one option. If it can't be done due to service conditions, then it can't be done and the material has to be changed. Or, even change the heat treat conditions of the original material. For example change from 516 Gr 70 as rolled to 516 Gr 70 Normalized.

 

[Shmulik]

http://www.absa.ca/Forms/AB-511 Impact Testing Enigma.pdf