Castings in foundry

Understanding end user valve approvals. Part 11g - Castings

Barrie Kirkman - 10 March 2016

The end user visit continues at the foundry. We now move onto pilot castings and production castings.

About the author

Mr Barrie Kirkman
Barrie Kirkman, BSc CEng MIMechE, writes a regular column for Valve World, bringing his own personal views from inside the valve business.
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Pilot castings & production castings

Current foundry practice on pilot and production castings varies considerably on when to or when not to pour “Pilot” castings. Also the “quality control process” ranges from zero to acceptable. This article challenges the foundries to improve to meet the latest international standards. End users are now mobilising to enforce these standards.

Pilot castings

Pilot Castings

A simple check list for end users given below should be a minimum for the need of pilot castings. Auditors should use this as a template perhaps rating the foundries compliance?

  • For first use of a pattern at a foundry
  • For a change in the pattern design
  • For a change in brand name
  • For a change of material (group)
  • For a change of foundry
  • For a change in the casting process

Each pilot casting should meet the minimum requirements;

  • Traceability of the raw materials
  • Acceptable and traceable heat treatment
  • Acceptable NDT of the casting including radiography of critical areas
    • as defined in ANSI B16.34
    • surface detection by Dye Penetrant / Magnetic particle examination
  • Visual examination of internal and external surface to MSS-SP-55
  • Dimensional checks
  • No weld repairs permitted

Pilot test failed
The end user should establish first time success rate for pilot castings. Normally investment castings tend to be higher than sand castings. Typically 85% and 65% respectively are stated by the foundry.

The above seems simple enough to follow but in reality foundries try to cut corners by passing the above. One classic example is modifications to the pattern, say, due to improvements needed to reduce failure rates during production. Another is to extend, say, WCB materials to WC6 or above. Both examples are without re-pilot testing.

Foundries
The change in brand name always gets strongly challenged. In these instances scrutiny of historic pilots should be done to ensure all is in order. At least a sample pilot casting should be done that will re-confirm the foundries casting quality and give the end user a direct comparison with the brand change.

Production castings

Production castings

The intent of production castings is to monitor quality via selection of pressure containing castings and undertake

  • NDT of the casting including radiography of critical areas as defined in ANSI B16.34
  • Surface detection by Dye Penetrant / Magnetic particle examination
  • Visual examination of internal and external surface to MSS-SP-55
  • Dimensional checks
The key discussion points are;
  • What numbers / percentage should be tested?
  • What happens if the castings are in non-compliance?

Typical findings

During the end user audit questions concerning the foundry’s normal practice should be raised and available data reviewed.

Often there is no data…..as no production testing is undertaken.

  • Broad statements are often used as the pilot casting level for pilot is 2 where production valves are level 3 or 4.
  • Statements that NDT are claimed but without documentary evidence.

When the end user finds themselves in the above situation one of caution is recommended. Have any other clients undertaken any such NDT? The end user always has the option to test the completed valves to API 591.

Some good examples of production monitoring in the market are as follows:

Production quality
The recently issued MS-SP147 recommends NDE 1% for 2” 150 class castings with a range up to 10% for castings greater than 24” 1500 class. Any failures are to:
  • Be properly repaired to acceptable standards*
  • And the foundry should review the acceptability of the patterns and casting process and if they need to be revised then re-pilot testing is required.

The percentage selection of the castings should take into account the heat numbers to get a broad spectrum of the foundry’s quality.

* The end user should review the foundry’s Casting Repair Procedure to ensure it’s acceptable. If the foundry allows full thickness weld repairs then it is recommended this be rejected. Allowable excavation for defects, NDT and heat treatment etc. should be checked.

Another generic method seen is based upon numerical statistical testing with up to 3 of batch samples from each heat with acceptable percentage pass / failures. If the failures exceed the allowable for batches 1 and 2 batch samples a third final 100 percent of the heat are tested. Further failure condemns all the heat castings and a non-compliance is issued. Otherwise the valves are repaired. For example;

  • A first 2% batch sample of all sizes with minimum numbers per heat are tested.
  • An acceptable pass rate is selected such as 90% with a subsequent failure rate of 10%. (Some foundries may select a higher percentage, others much lower)
  • If the actual pass rate is greater than 90% (failure rate less than 10%) then the 2% test batch is accepted with the failure valves being properly repaired and put back into production. No action on the patterns or casting process is done.
  • If the actual pass rate is less than 90% (failure rate greater than 10%) then a second 2% sample size from the heat is tested.
  • If the actual pass rate for the second batch sample is greater than 90% (failure rate less than 10%) then the 2% test batch is accepted with the failure valves from sample 1 and sample 2 being properly repaired and put back into production. No action on the patterns or casting process is done.
  • If the actual pass rate for the second batch sample is less than 90% (failure rate greater than 10%) then a third final 100% of the heat castings are tested.
  • If the actual pass rate for the third batch sample is greater than 90% (failure rate less than 10%) then the 2% test batch is accepted with the failure valves from sample 1, 2 and 3 being properly repaired and put back into production. No action on the patterns or casting process is done.
  • If the actual pass rate for the third batch sample is less than 90% (failure rate greater than 10%) then all the heat castings are condemned. A formal Non-Conformance is issued for corrective action. A full investigation is undertaken to determine the root cause.

Where the foundry offers pilot and production casting information the end user should ensure all is in order and is sufficient to gain confidence in the castings quality. Though I recommend MS-SP147 as a basis, if the foundry has a proven process these should be reviewed on a one by one basis.

Once again I appear to have let my “pen” run away with me. It’s amazing how much there is to share on castings which, by the way, is really just an introduction.

A quick thank you to the various readers who have taken the time to comment on previous articles!

Thank you for your continued support. Please contact me if you have any questions or different views. They are most welcome.

Barrie Kirkman, BSc CEng MIMechE, writes a regular column for Valve World, bringing his own personal views from inside the valve business.

Barrie can be reached on: barriekirkman@ntlworld.com.

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