Re: Cracking [text][bcc][faked-from][bayes]

From: <Balakrishnan>
Date: Mon May 17 2004 - 16:05:00 EDT


Both SSC and HIC are two types of metal failures   involved in Pipes with Hydrocarbon service , where there are substancial amount of H2S and Hydrogen ion concentration.

Sulphide Stress Cracking

  is a Brittle Failure by Cracking
under the combined action of Tensile stress and Corrosion in the presence of Water and H2S.

NACE standard MR-0175 says:

Sulphide Stress Cracking (SSC) is affected by following factors: 1) Metal Chemical composition, Strength, heat Treatment (Hardness) and microstructure. 

2) Hydrogen Iron Concentration (PH) of the environment
3) H2S Concentration and total pressure
4) .Total tensile stress (Applied + Residual)
5)Temperature
6) Time.

Hence,
Any Material has got a limiting value of Hardness number (HRC 22 Maximum; even Stainless Steels have to meet this requirement)   if it is to be used under SSC -prone environments. Hydrogen Induced Cracking.

Paltes and simillar rolled items which are used as the raw material for Pipe and Pipe fittings have
Fiber structure along the Rolled Directions.

Under Service Conditions with high Concentration of Hydrogen Ions, there is a chance of
Cracks forming along these fibers and many such Cracks running parallel may Join
among themselves forming a series of Cracks.

Inorder to avoid this possibility and to ensure that there won't be any such Crack for a given material,
we may have to test the material for HIC, before it is used in such Environments.

HIC requirements are as given below:

  1. Material shall be either HIC tested as per TM-0284 OR shall satisfy the following chemical composition

Carbon equivalent calculated as CE=C+ Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 shall be 0.43% maximum for all items

  1. Max. Mangenese 1.35%
  2. Max. sulphur 0.005%
  3. Max. Calcium 0.0065%

HIC testing, as per TM-0284 shall be conducted using Either Solution A
OR
  synthetic sea water solution-B (Well explained in TM-0284)

and the acceptance criteria are as follows a) CLR -15% max
  b) CTR -8% max
c) CSR -1.5% max and surface blistering shall be acceptable

See, the Requirements which we have asked for ASTM A516 Grade 70 Plates (Fabricating SAW Pipes) :

  C.S Plates will be HIC tested as per NACE TM 0284 using test solution A. Acceptance norms for

     HIC test results shall be as follows : • Maximum individual crack length in any section not to exceed 5 mm • Crack Length (CLR) : not to exceed 15% • Crack Thickness Ratio (CTR) : not to exceed 5% • Crack Sensitivity Ratio (CSR) : not to exceed 1.5%

   Above criteria shall apply to average ratios for 3 examined sections from one specimen.

   Surface blistering is acceptable.

In case the material is acceptable as per the above requirements, there is no harm in using such
materials.

Regards,
Balakrishnan

Received on Mon May 17 16:05:00 2004

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