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PART THREE

Pinhole Defects in a Casting: Causes and Corrective ActionPinholes, often in a casting, are referred as surface blowholes. They occur sporadically and over large areas and can affect all cast components. The pinholes are not visible to the naked eye even if they are primarily found on the outside of the cast piece.

Pinholes can appear in different forms. Examples include spherical blisters with a bare metal surface, covered with small graphite skins or larger, irregularly shaped cavities accompanied by slags or occurrences of oxidation.

The main problem of pinholes is based on the specific properties of the metal as well as the mold material system and the difficulty to detect them.

This article reviews the formation of pinhole and techniques to identify pinholes.

By Gobind Khiani & Barry Messer – Fluor Canada

Formation of PinholesPinholes are formed in the following ways:

• Reaction of the water vapor with the accompanying elements. Metal oxides and atomic hydrogen form, which diffuse into the liquid metal. Nitrogen-hydrogen compounds are split in a similar way and also diffuse into the liquid metal.

• Micro gas bubbles form due to the re-action between the metal oxides and the carbon of the melt.

• Hydrogen and possibly nitrogen dif-fuse into the micro gas bubbles and enlarge the bubbles.

• Excessive hydrogen content in the melt can occur due to hydrogen car-riers. Carriers include moist charge materials due to fine-grained, unpro-tected ferro-alloys that often absorb water; highly rusty feedstock; oils and emulsions that give off hydrocar-bons; and the influence of increased air humidity itself.

• Excessive nitrogen content in the melt can be introduced through nitro-gen carriers such as scrap steel.

• Too much nitrogen in the molding sand with excessive moisture content.

• An unfavorable gas atmosphere in the mold cavity, whose causes should be sought in the type and amount of

materials that form lustrous carbon.

• Too much nitrogen in core sands or too high nitrogen-hydrogen compounds in the core molding material binders.

Mechanism of PinholesIn a reducing atmosphere, ammonia breaks down according to the equation:

2NH3 + N2 + 3H2

The ammonia is almost fully dissociated at 600°C and under atmospheric pres-sure. The gas volume doubles in this process, and 1 mole of nitrogen and 3 moles of hydrogen emerge from 2 moles of ammonia. This hydrogen gas can react with the precipitated carbon monoxide according to the following equation:

CO + H2 + H2 O + C

Figure A. MSS SP 55 - Gas Porosity.

Figure B. The surface involved shows NO INDICATION at all, even using a magnifying glass (see Figure C).

Figure C. This method is not effective for the detection of any pinhole.

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The bentonite-bonded mold material is not sufficiently conditioned, which leads to the presence of free water that is not bound in the bentonite and that increases the risk of hydrogen form-ing. There are other aspects besides the composition of the iron and the proper-ties of the mold material that influence the formation of pinholes.

If the slag is not skimmed completely, slag inclusions may possibly act as seeds for the formation of gas bubbles. Even if the slag has been removed, cast iron with nodular graphite contains nu-merous oxides that can contribute to the formation of pinholes. The slag that develops during the magnesium treat-ment also has an impact, even if it is not clear whether it causes a surface defect in a purely mechanical way or pro-motes the formation of gas. The casting and gating techniques can also exert influence on the formation of pinholes. A gating system with a non-turbulent flow and short casting runs reduces the pinhole tendency. Cold drops can also be a cause for pinholes. They are oxi-dized and then enveloped by the cast-ing flow. A reaction in which CO forms can occur here and leads to the forma-tion of gas bubbles.

How to Identify a Pinhole in a Valve Casting and Possible Corrective Action – a Real CaseWhen the root cause of the leakage is clearly a pinhole on the valve body, a deep root cause analysis for the reason of the pinhole is conducted.

• Perform a visual examination for gas porosity. A pinhole, during visual in-spection, would be at acceptable as shown in Figure A.

• Magnetic particle examination ac-cepts all indications of nonlinear imperfections which have any di-mension less than 13 mm. A pinhole, during MT examination, with dimen-sion less than 13 mm, is considered as acceptable.

• The best method to identify a pinhole is to subject the casting to a pressure test. Test pressure at 1.5 times the maximum allowable working pres-sure at room temperature.

However, also in this case, no leakage can be visible, due to one of the following:

• the pinhole is so tiny that it leaves and passes nothing

• the pinhole is not yet open to the sur-face (not pass-through)

In both these cases, when the test is repeated two or more times, the hid-den defect affected area could be over stressed and consequently an unac-ceptable leakage could be detected.

NOTE: All the above referenced tests (NDE + pressure test) should be per-formed with satisfactory results and be witnessed by final client appoint-ed inspectors.

Techniques Used by Valve Manufacturers to Identify Porosity in a Casting Include: • Foundry casting technique check• Visual examination• Dye penetrant examination• Hardness test • Pressure shell test

The article examines each technique and its ability to identify pinholes.

Foundry Casting TechniqueFoundry casting pattern is qualified by radiographic examination of the initial casting. Radiography examination is performed in accordance with client specifications and ASME B16.34, para-graph 8.3.1.1 and should include all crit-ical areas with satisfactory results.

Visual ExaminationA visual examination of involved area is performed according to client specifica-tions and MSS SP 55 Figure A using an artificial light (1500 lux).

Dye Penetrant Examination Dye penetrant examination is per-formed on the involved area according to client specification and MSS SP 93.

As it can be seen on Figure G, the result is perfectly in compliance with the applica-ble specification MSS SP 93 acceptability criteria, which is 0.5 in. (13 mm) diameter for materials over 0.5 in. (13mm) thick (castings rounded indication).

The detected external indication is about 4 mm and the related internal one is less than 1 mm.

Figure D. Dye penetrant application on external involved area.

Figure E. Dye penetrant application on internal involved area.

Figure F. Results on the external involved area: rounded indication of 4mm.

Figure G. Results on the internal involved area: rounded indication less than 1 mm.

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ABOUT THE AUTHORS

Barry Messer is Technical Director and Senior Fellow with Fluor Corp and also manages the Fluor Canada Ltd Metal-lurgy 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 EPC companies. He has a bachelor’s degree from the University of Pune in India and a Master of Engineering from the Uni-versity of Calgary in Alberta, Canada. Currently, he is the Fellow Piping Valves with Fluor in the piping materials en-gineering group. He is a chairman of Calgary Branch Execu-

tive Committee at the Association of Professional Engineers and Geoscientists of Alberta and Valve Users Group and Vice Chairman of International Standards Organization, representing Canada.

Please see Part One in February, Part two in April 2017 issue of Valve World Americas Journal.

REFERENCES: • Ask Chemical GmbH

• Alessandra Spagnolo, Quality Manager - Orion Valves Italy

• Manufacturer Standards Society and Standard Practices, USA

• Pictures courtesy Orion Valves Italy

Therefore this method is not effective for the detection of any pinhole.

Hardness Test A hardness test is performed and the measured values are in the range of 210 to 238 HB.

If some welding repair had been not properly done on these area (so a pos-sible cause of pinhole), there would be a difference of hardness far superior to that found.

Hydrostatic Pressure Shell Test A hydrostatic pressure shell test is per-formed on the valve according to client specification and API 598:

• Test pressure: 78 bar

The fact that the leakage started after numerous pressure cycles reinforces that, when the valve was first tested in-ternally and witnessed by the appoint-ed inspector, the pinhole was not yet open to the surface (not pass-through).

Therefore, when the test is repeated two or more times, the hidden defect becomes over stressed and an unac-ceptable leakage can be detected.

Corrective Action: After the root cause analysis and the rele-vant inspections, the following is affirmed:

a. The pinhole location is not in a criti-cal area.

b. NDT examination (visual and dye pen-etrant) results in compliance with accep-tance criteria within client specification, MSS SP 55 figure A and MSS SP 93.

c. As per hardness inspection, no weld repair is performed on the pinhole area.

d. Hydrostatic valve shell tests with passing leaking.

ConclusionThis defect (pinhole) is not the evidence of systematic defect, but is identified as an occasional defect.

Consequently all the involved process-es are fully in line with the procedures and standards.

To detect this type of occasional de-fects, which in most cases are high-lighted only after many cycles of pres-sure, an internal pressure shell test with at least five cycles of two minutes each are conducted in order to simulate the process cycling and increase the ability to detect these types of defects before the official testing.

Figure J. During the second cycle, a drop starts to form on the involved area after about 10 minutes.

Figure H

Figure I. Holding time: 5 minutes (first cycle), 15 minutes (second cycle).

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