Impact Requirements For Low Temperature Applications

The rules in ASME 8 Div 1 Section UCS 66 provide a good guide to reducing the risk of brittle fracture.

Materials are divided into 4 groups: A) high  risk to D) low risk:-

A)    Materials with low toughness or materials that don’t fit into the other groups.
B)    Most materials fit into this group

C)    Low strength materials; such as an SA516 Grade 60, note: grade 70 goes in B
D)    Tough fine grain steels, all normalised grades of SA516.

If in doubt assume A

As thickness is increases so does the risk of brittle fracture; therefore the temperature before impacts tests are required must get warmer.  This is the basis of Fig UCS 66, which plots each material group between MDMT (Minimum Design Metal Temperature) and thickness. If the MDMT falls below the appropriate material curve (A to D), Impact testing is mandatory. 

However if the item is not highly stressed there is less risk of brittle fracture, Fig UCS66.1 permits the impact test temperature to be raised if the full material thickness is not required.  If the resulting test temperature exceeds the values in UCS 66, impact testing is not required.

Residual stresses are present in all welded structures and can be up to yield point in magnitude.  Whilst these stresses will not cause failure by yielding, they could drive a brittle fracture.  Therefore stress relief is mandatory on thick material that is prone to brittle fracture (UCS 56).  Allowances for stress relief is built into the code rules, but stress relief of any item when its not required will permit an increase in the impact test exemption temperature (UCS68c).

Welding defects can also cause brittle fracture; therefore when material thickness exceeds a certain limit, full radiography is required. (Table UCS 57).  The toughness of weld metal must also be demonstrated by production test plates, which are mandatory if impact testing is required.

As the strength of the material increases so does the risk of brittle fracture, particularly as thickness increases.  Therefore higher absorbed energy is required from strong materials above a certain thickness. (Fig UG 84.1).

 
The following two programs compute the impact requirements of Fig UCS 66 and Fig UCS 66.1.

Fig UCS 66 Impact test exemption curves
Enter thickness described in Fig UCS 66.3, which must be between 0.25 inch and 6 inch, and click the calculate button.  If the MDMT is below the calculated values, impact tests are required, unless except by UCS66.1.


 

  Enter Thickness     mm 

  Impact test exemption temperature

  A =    0C                     0 F
  B =    0C                    0 F
  C =    0C                    0 F
  D =    0C                    0 F
 

FIG UCS-66.1 Reduction in Min Design Metal Temp Without Impact Testing
Enter either ratio defined below and click the calculate button.  The impact test temperature can be made warmer by the calculated temperature difference.  If the new test temperature exceeds the figures calculated above, impact tests are not required.

Example if impact tests are carried out at -10C and the Ratio = 0.5, the MDMT can be as low as -42C (32C below -10C,)


Ratio =                         Required calculated thickness x E*                          
                   Actual thickness less corrosion and any thinning allowances


Ratio =     Calculated Stress NOT including joint factor x E*
                        Design Stress x E {Actual Joint Factor}

Note:

  • The required thickness is the minimum thickness calculated by code rules, including a joint factor, but excluding any thinning or corrosion allowance.

  • E* is the joint factor, if the actual joint factor is less than 0.8 then E* = 0.8.

  • Impact testing is required if the MDMT is below: -55°F, -48°C unless the above ratio is 0.35 or less, then its -155°F, -104°C.

  • The above reductions can’t be applied to an MDMT colder than -155°F, -104°C




  Enter Ratio         

            Temperature Reduction

            0C                     0F



 


UCS66 and UG84  MUST be read very carefully before any decision is made concerning impact testing, as both sections contain many exceptions and exemptions.
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