Some of the advantages of Sand Casting vs. Investment Casting are:
Sand castings are less expensive in small quantities (less than 100), ferrous and non-ferrous metals may be cast, one can possibly cast very large parts, and the tooling is least expensive in sand casting.
Whereas Investment castings provide close dimensional tolerances due to the reason that investment castings can form complex shapes and are used to produce casting for equipments with fine detail, further intricate core section and thin walls are easy to make in investment cast, and it is possible to cast ferrous and non-ferrous metals, and one can achieve excellent finish in investment casting (as low as 64-125 RMS).
Some of the disadvantages of Sand Casting Vs. Investment Casting are:
Sand castings are inferior in dimensional accuracy hence process requires larger tolerances. Castings occasionally exceed calculated weight, and surface finish of metal castings is usually higher than 125RMS.
Whereas Investment casting are expensive due to tooling required, and they cost higher than sand casting.
Use of Sand castings is common when strength /weight ration permits are required and tolerance, surface finish, and low machining cost does not war- rant a more expensive process.
Whereas use of Investment castings is done when complexity precludes use of sand castings, the process cost can be justified through savings in machining and weight savings justified increased cost.
Foundries and valve manufacturers work in a few different models i.e. most of the valve manufacturers used third party foundries, some as Joint Venture Foundry, a few have vertically integrated with their own foundry and also use other third party foundries and some use only their own foundry.
Manufacturers operate their own approval methods restricting to a few foundries, where as others are less strict to the other extreme that the foundries are chosen based on ISO9001 certification and commercial reasons.
In an ISO 9001 foundry, an auditor looks for:
• ISO Certification
• Valve manufacturer assessment data of the foundry, including pilot and casting production quality monitoring.
• Heat treatment specification to review heating/cooling rates etc.
• Review heat treatment charts against the specification.
• Production casting analysis of defect/ failures and continuous improvement.
• The valve manufacturer QA/QC control of the foundry.
• Last but not least – ask to see an original foundry material certificate.
The above information often reveals discrepancies such as heating rates, hold temperatures for the actual thickness etc.
As an auditor, one should ask the goods-in inspector the following:
• Inspector should run through precisely the acceptance criteria they follow. They should be familiar with ANSI/ MSS-SP 55 “Quality Standard for Steel Castings for Valves, Flanges, Fittings and Other Piping Components – Visual Method for Evaluation of Surface Irregularities”
• Are any castings rejected? If so for what reason and for which foundry?
• Is any statistical analysis of rejects etc., undertaken?
The castings should have been through due control at the foundry such that rejects should be very low at the valve manufacturer. Actually zero but often they are not. Goods-in is used to control the casting quality which is most ineffective. It’s a common practice in some parts of the world. Visibly the goods-in area looks impressive with copies of MSS-SP55 displayed on the walls to give guidance to inspectors on surface finishes.
This is a warning bell as the inspectors should have been suitably trained to MSS-SP-55.
Also sample inspection is employed with no process to increase inspection levels when castings are rejected. Too often poor castings are accepted that are clearly not meeting the surface requirements. Poor weld repairs seen with visible surface defects/cracks. As if, the foundry itself has no chance of meeting these requirements. Sometime, the casting shop advises inspector that the casting is urgently required for production so they are accepted. So the inspector “Accepts the castings.” Color markings, hard stamp or simple paper acceptance are used. Where hard stamp is used correctly quality control tends to be good. Identification of the inspector gives ownership and full traceability.
As a “rule of thumb” if the visual inspection of the casting reveals unacceptable surfaces/defects/ high level of weld repairs then the quality of the casting internally would be suspect. Machining of flange surfaces often reveal issues so the auditor continues to follow the manufacturing process.
Whenever an auditor experiences any of the above findings then qualification of the valve manufacturer’s sub sup- plier foundry is highly unlikely. It may also jeopardize approval of the valve manufacturer.
Further, API committee decided to producer numerous codes on casting quality, these are created by Com. 20 “Supply Chain Management” at API.
- 20A, Castings
- 20B, Open Die Forgings
- 20C Closed Die Forgings
- 20D, NDE for API Products
- 20E, Alloy & Carbon Steel Bolting
- 20F, Corrosion Resistant Bolting
- 20G, Qualification of Welding Suppliers
- 20H, Heat Treatment
- 20x, Source Inspector Certification
ASME B16.34 addresses the Casting requirements per clause 5.1.3 and 8.3, etc.
- MSS-SP-55 is a code on “Quality Standard for Steel Castings for Valves, Flanges, Fittings and Other Piping Components – Visual Method for Evaluation of Surface Irregularities”
- MSS-SP-93 is a code on “Quality Standard for Steel Castings and Forgings for Valves, Flanges, Fittings and Other Piping Components – Liquid Penetrant Examination Method”
- MSS-SP-94 is a code on “Quality Standard for Ferritic and Martensitic Steel Castings for Valves, Flanges, Fittings, and other Piping Components – Ultrasonic Examination Method”
- MSS-SP-147 is a code on “Quality Standard for Steel Castings Used in Standard Class Steel Valves-Sampling Method for Evaluating Casting Quality”
A few discussion questions with a Foundry with response as below:
a) How to identify porosity and voids?
Answer: Surface porosity can be found either visually or with MT. MSS-SP55 standard – it has a large number of pictures as well as whether the defects found would be acceptable or reject- able. Internal porosity / voids are found using UT or RT – the standards for this are MSS SP 94 for UT and MSS SP 54 for RT. We can also look at ASME section XIII for NDT testing.
b) What porosity and voids are considered detrimental?
Answer: This would depend entirely on what the castings are used for – load bearing components are normally tested to API 8C (or similar) with a level 1 for critical areas and level 2 for non- critical. Castings for heaters are level 3 or 4 again depending on if it is a critical area or not. You will never find a casting that would be 100% level 1 in the entire part (unless they are small castings and possibly investment castings)
c) How are they fixed and reinspected?
Answer: This depends on what standard you are testing to. Each ASTM standard has this spelled out in one of the sections (blend out the defect or weld it up) – you would have to check the standard that the castings were made to, to determine any repair procedures.
d) What documentation is required for cosmetic and other than cosmetic repairs?
Answer: Some customers require a weld map (pictures of where the defect was in relation to the casting) but generally if the cavity of the weld is greater than 20% of the wall thickness, you will have to RT that area and the report from the testing company would be sufficient.
e) What processes and techniques are allowed for repairs at foundry and at machine shop?
Answer: Castings in a foundry are usually welded using stick welding and then tempered to relieve the stresses, on a machined surface sometimes they will use MIG to keep the heat to a minimum depending on the size of repair necessary ( all depends on what was found and marked up as a repair).
How to Identify a defect in Castings
The most common Non Destructive Evaluation (NDE) methods to identify flaws, like porosity and voids, in a casting are: visual inspection, Radiographic Testing (RT), Ultrasonic Testing (UT), Liquid Penetrant Examination (LP), Magnetic Particles Examination (MP).
The method to be used is to be agreed with the Client and included in the contract agreement.
ASME B16.34 and ASTM standards provide a reference on how the tests have to be performed (e.g. for RT: ASTM E 94 and ASTM E 142; for MP: ASTM E 709; etc.).
The most accurate method is the RT. This method is also the most expensive and the one requiring more time (with potential impact on the delivery time).
The UT is very simple but its effectiveness is highly related to the ability of the technician to interpret the test results. The UT is not applicable on austenitic steel castings.
The repair acceptability and their classification depend on the contractual agreements between the casting sup- plier and the buyer (Client).
The ASME B16.34 standard provides a guide- line for acceptance criteria for the flaws.
Manufacturers internally classify repairs as:
• “Major repairs”: defects whose cavity originated by the total defect removal is deeper either than 25 mm or than the 20% of the nominal thickness (the smaller between the two parameters is the valid one) or with the removed surface wider than about 65 cm2.
• “Minor repairs”: defects whose cavity originated by the total defect removal has a depth smaller than 20% but higher than 10% of the nominal thickness, up to a maximum of 16 mm with an area smaller than 45 cm2.
• “Cosmetic repairs”: defects whose cavity originated by the total defect removal has a depth smaller than 10% of the nominal thickness, up to a maximum of 8 mm and with an area smaller than 25 cm2.
How to Repair
The repair entails 5 steps:
1. Removal of the defect;
2.Repair by welding
; 3.Post-weld heat treatment; 4.Inspection of the repaired areas; and 5.Document
1. Removal of the defect
The defect should be removed by grinding, chipping or arc-air. The cavity obtained by removing the defect has to be smooth, without edges and without areas with excess of carbon content (in case of removal by arc-air). The complete removal of the defects shall be confirmed by an inspection, for example by LP or MP.
2. Repair by welding
The repair of the defect shall comply with the applicable welding procedure specification approved by the customer (WPS) and shall be made by welding operators qualified in accordance with the ASME code (section IX).
3. Post-weld heat treatment
Castings with cosmetic defects shall not undergo any heat treatment after the welding repair. All the parts with minor or major re- pairs (not covering the 100% of the wall thickness or having an extension smaller than 200 cm2) shall undergo a Stress Relieving Heat Treatment.
All the parts with major repairs covering the 100% of the thickness or having an extension greater than 200 cm2, shall undergo the complete heat treatment: this heat treatment shall be done in accordance to the applicable ASTM specification.
4. Inspection on repaired areas
After the final heat treatment, all the repaired areas shall be inspected again at least with the same NDE that initially showed the defect.
If other defects should result from this inspection, the procedures above shall be repeated until only acceptable defects are shown.
After final valve assembling, the valve shall be hydro tested according to the applicable standards (API, BS or internal procedure): acceptance criteria shall be per API, BS or Client’s requirements.
Gas test may be carried out as contractually agreed with the customer.
For cosmetic repairs, usually no document is shared with the Client because no thermal treatment after the repair is needed therefore the repair does not affect the mechanical properties of the casting.
For minor and major repairs, if indicated in the contractual documents, the description of the defects, the used WPDs and PQRs, and the heat treatment diagrams shall be shared with the Client.
About the Authors
Barry Messer is Technical Director and Senior Fellow with Fluor Corp. and also manages the Fluor Canada Ltd Metallurgy and Welding Engineering Group in Calgary, Alberta. He is a Director with the Canadian Welding Bureau. Barry has over 35 years experience in metallurgy, welding, materials selection and NDE development. He is regularly involved in the analysis and mitigation of fabrication and in-service failures for the chemical, petroleum, power, pipeline and mining industries. Barry is an active member of NACE and ASME.
Gobind Khiani, M.Eng., P.Eng. has served in engineering and project management roles for both Operating and Engineering, Procurement and Construction (EPC) companies. He has a bachelor’s degree from the University of Pune, India and Masters from the University of Calgary, Alberta. Currently he is Global Valve SME with Fluor Canada Ltd at Piping Engineering Group. He is a Chairman of Calgary Branch Executive Committee at APEGA and Valve Users Group. Gobind has experience in Piping/Pipelines, Valves, Surge/Control Valve Selection and Sizing, Modularization in Valves, Safety Integrity of Piping Systems, Fugitive Emissions, and material selection. Gobind is an active member of API, ISO and CSA.