Lower impact temperatures for higher thickness of Base material 2

[sayee1]

I have come across quite a number of specifications which specify a lower impact temperature for the materials as the thickness increases. For eg for an offshore comp-lex using S355 grade of material upto 45mm, the testing of the material and welds is required @-20 Deg C (The complex is basically for warm seas). However for the same complex materials S355 45mm to 65mm thick need impact at -40 Deg C and the welds for these higher thk need impact @-30 Deg C. I have also encountered this on pipelines where some clients had specified Design temp-10 for thicknesses upto 20mm and DT-20 Deg C for thicknesses above 20. What is the logic metallurgically in asking for properties for higher thickness at a lower temperaTURE?

Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668
email: sayee_prasad@yahoo.com
The black holes of nature are the most perfect macroscopic objects there are in the universe: the only elements in their construction are our concepts of space and time.

 

[stanweld]

Basically, these requirements are based on the effect of base metal thickness relative to the geometry of the Charpy test specimen. Some of the "logic" may be seen in the reduced temperature requirements when using subsize Charpy specimens long established in ASME B31.3 and ASME VIII.

In the past, we often specified increased minimum absorbed energy values (40-70 ft-lbs) as thickness increased beyond about 100 mm, which was the approximate thickness established for the geometry induced plain stress/plain strain transference in low carbon steel plate. Owner/user engineers liked this so much that they started lowering the thickness requirements to 75 mm and then to about 50 mm. Whether these requirements are really valid for your design conditions are open to debate.

Similarly, engineers chose to reduce the test temperature and maintain the same relatively low 15 to 20 ft-lb.minimum absorbed energy.

 

[arunmrao]

When the thckness of thebase material is increased plane strain conditions are set up instead of plane stress conditions.Hence your observation.

 

[grampi1]

First one must realize that Charpy is not a valid fracture mechanics criterion. Code Charpy values are not a guarantee of leak before break or of even preventing brittle fracture. They simply represent a level of toughness that has a low probability of allowing a failure if all the other rules of that Code are followed.

Fracture mechanics tells us that crack growth is a function of material toughness, flaw size and applied loading. Codes that allow comparatively high loadings should require more toughness. Codes that allow comparatively high strength materials, since loading is usually a percentage of the strength, should require more toughness.

So if your building something where 100's have already been built to your Code and they give reasonable service, Charpy requirements are a good way to assure your item will be as reliable as those that came before it.

When one tries something truly new, do not rely on Code rules. Do fracture mechanics.

 

[husni]

With the same process condition, that section will experience lower temperature due to metal will conduct energy (heat) faster than surrounding. Through thickness heat gradient will result thicker section will experience lower temperature at the inner surface.

As for offshore pipeline, closer to the platform thicker section due to higher safety factor and lower temperature due to condition during blow down (normally gas, will adsorb energy after the compressor ), area closer to the source (platform) will experience lower temperature and higher temperature when it goes further.

 

[priyasachin]

Dear friend,

There are two aspects involved in this. Firstly, thickness. The very fact that we have gone in for thicker material is expected to see higher pressure. Since the size of the impact specimen and its location in the section is same, only aspect that varies is the centre-line of the impact specimen as the section size changes. Thicker the material lower would be the location. Further thicker material will undergo comparatively lesser reduction than the thin ones, and at the piont of referance would have cooled slower than the lower thickness member.

The impact test is an index which does not fit in in any design calculation. The actual value only indicates the availability of the degree of toughness. So when one goes in for material of varying thickensses, to enhance the confidence level, some opts for lower testing temperatures, while some for higher impact values at the same test temperature level.
Regards,
MRCN

 

[sayee1]

Stanweld,
In a subsize specimen, we use a lower temperature as you rightly pointed out! But a subsize specimen is of lower cross sectional area that a normal size specimen. So why lower the temperature in that case. Some specs ask for a reduction in the impact values but a number of specifications have specified same impact values@lower temperature for a subsize specimen. So whats the logic for reduction in temperature for subsize specimens?
And now that you draw an anal;ogy from subsize specimen to the lower impact testing for higher thickness, aren't the two contrary to each other. I am totally confused now!!ANy metallurgist with some help, Kenvlach, Maui, TVP, Metalguy.......Please help!

Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668
email: sayee_prasad@yahoo.com
The black holes of nature are the most perfect macroscopic objects there are in the universe: the only elements in their construction are our concepts of space and time.

 

[kenvlach]

I pretty much agree with all preceding responses.

Metallurgically speaking, the smaller section material will generally have a higher fracture toughness (lower DBTT or higher energy at the same T) due to a smaller grain and defect size. This is the result of more rapid cooling and greater deformation during the initial fabrication, as has been mentioned.

So, the answer may be that the owner/specifier/financer wants greater confidence in non-brittle behavior of larger structures, where failure may be more serious in terms of life or money.

 

[stanweld]

To sayeeprasader:
I use the word "logic" somewhat tongue-in-cheek for the temperature reductions required by the Codes mentioned. As a former member of Code bodies, changing code requirements once established can be exceedingly difficult, especially when the involved Codes have been incorporated into Government laws & regulations. Many of these requirements were based on the work of Puzak and Pulliny in the '40s and '50s and not modern day fracture mechanics. Never-the- less, it is well understood that the thickness of the test specimen versus the thickness of the actual part is relevant to the toughness of the material. One of the methods proposed to counteract this phenomenon was to lower the test temperature. The test temperature reductions were often based on emperical information provided by steel manufacturers as to what they could guarantee without downgrading product (loss of money).

Per ASME VIII and B31.3, temperature reduction is only required when the the largest obtainable Charpy V-notch specimen has a width along the notch that is less than 80% of the material nominal thickness or when the material thickness is > 10 mm when largets possible Charpy V-notch test specimen has a width along the notch less than 8 mm. SA-370 modifies (reduces) the absobed impact energy requiremnents for the subsize specimens.