Is Impact test done only for Low temperature suitability? 5

[bmoorthy]

I know of atleast 3 parameters that gets checked in Impact testing when tested at low temperature.

1) Absorbed Energy (For Ferritic and non Austenitic steels when they are subjected to Low temperature)(2) Lateral Expansion (In case of SS) (3) Percentage shear (In case of Sub Sea pipelines (In addition to Absorbed energy)

A) Are there High temperature Impact, for example would some one conduct impact test at + 100 Deg C (If that is being done, what would have been the probable objective of the specifier)

B) What are the other circumstance under which one would ask for Impact test?

C) Are the results of Impact test done only to determine the suitability of material for low temperature application (or) whether Impact tests are done for other reasons (Like checking the corrosion resistance or resistance to fatigue or resistance to creep etc)

 

[SJones]

A)
Some industries may wish to undertake high temperature impact testing for various reasons. Looking at ASME VIII Div 1 UCS-66, you will note that the highest temperature that would demand toughness testing under curve A is around 48 deg C..

B)
Pipeline fracture arrest requirements. Quality control requirements. These amongst numerous other reasons.

C)
See B above.
and
Background to requirements for the prevention of brittle fracture in the European standards for unfired pressure vessels (prEN 13445) and metallic industrial piping (prEN 13480)
Paper published in International Journal of Pressure Vessels and Piping, vol.78, no.6. June 2001. pp.391-399.
C S Wiesner, S J Garwood, R Sandström, D M Street and K J Coulson


Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdoweb/

 

[metengr]

(a) Higher temperature impact tests can be effectively used to evaluate the extent of formation of intermetallic compounds that can occur in certain alloy systems upon exposure to elevated temperature service. The higher the volume fraction of intermetallic compounds, the impact toughness should decrease.

 

[redpicker]

Just to add to what Steve has posted,

A) In general, for low alloy steels, impact tests performed at lower temperatures indicate acceptable impact strength at higher temperatures. There are some ISO specifications written, in fact, that explicitly state that impact tests performed at lower temperatures than those specified are acceptable provided the absorbed energy meets the requirement of the higher temperature.

A) (cont.) For austenitic stainless steels, low temperature impact testing is not informative. This is because the FCC structure of austenite does not exhibit a temperature dependent ductile to brittle transition. The only time I have seen impact testing required for austenitic SS is when the people requiring the test do not understand what the Chapry test tells us.

A) (cont. II) For other materials, there may be mechanisms that could come into play at higer temperatures, but I am not aware of any. There are some temperature-related embrittling mechanisms with carbon steels, but these are usually identified by testing after cooling to room tempeature after the high temperature exposure.

B) As stated, there are other reasons to require an impact test other than low temperature exposure. Mostly, these are insure adequate toughness after a hardening opeartion or some other operation that could cause potental embrittling (welding, high temperature exposure, etc...). Note the Impact test is useless to detect Hydrogen Embrittlement.

C) As mentioned above, yes. But, in my opinion, it is way overused. There is a general belief that impact strength can be related to fatigue resistance, but no strong relation has been identified by researchers. It could be used, I suppose, to qualify corrosion resistance by detecting a mechanism that caused both a reduction in impact resistance and corrosion resistance. If corrosion resistance was of prime concern, however, I would think the user would be better served in performing a corrosion test. There is a great tendency to want to correlate fracture toughness (KIC, CTOD, for example) to Charpy testing, since the Charpy test is much cheaper to perform, but there are really too many factors affecting the Charpy test results other than the fracture toughness properties of the material for a useful correlation to be made.

When in doubt, it seems that many designers will arbitrarliy increase the Charpy impact reqruiement for components that do not fail as a result of either impact loading or a lack of toughness in the belief (hope?) that the increased Charpy results will give them improved performance (often I see this in regard to fatigue performance). The end result is increased cost and longer lead times for no tangible benifit.

rp

 

[GRoberts]

I may be showing my ignorance here, but why would low temperature impact tests for austentic stainless steel used for cryogenic service not be informative. Does it not indicate if the material, especially welds/HAZs have too much ferrite for good low temperature toughness?

 

[strider6]

Impact test are used to assess the thougness of a materials, and usually the thougness of a material decrease with temperature. I've never seen impact test done to assess corrosion resistance or cracking resistance or other mechanisms.
This is my experience.

S.

http://www.corrosionist.com

 

[metengr]

GRoberts;
You bring up an excellent point, and the ASME B&PV Code supports that point for service temepratures below -320 deg F.

 

[redpicker]

Yes, GRoberts, as metengr notes, this is an excellent point.

My comment was mainly directed at the fact that FCC materials do not exhibit a ductile to brittle transition. Obviously, if ferrite exists, it isn't entirely FCC, so my reasoning falls apart.

I love learning new things!

rp

 

[cnuk]

A related question. I thought I read somewhere (ASME BPVC?) that most plain carbon steels do not require impact testing if the operating temperature is above -29degC? The reason I ask is that I am designing a device that operated on an oilwell and is made of ASTM A36 plate and CSA G40.21 44W HSS tubing. The device needs to be rated to -40 degC. I think all the material requires impact testing/rating if we want to say the device is rated to -40 degC but others do not. Their argument is that the likelihood of impact is low. My argument is that if the device is operating at -40 degC everything should have impact testing. Perhaps you can share your experience.

Thank You

 

[metengr]

cnuk;
Comment before answering your question; please do not add on to existing threads. This is really a separate question and should be under a new posting.

Ok, getting that out of the way, the answer to your question is simple, please review ASME Section VIII, Div 1, Part UCS-66. All of the information regarding exemptions for impact testing, and minimum design metal temperature is covered.