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The Casting Quality

Question

Winte r 2009VOLUME 21 , NO . 1

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V alve manufacturers frequentlyencounter the words, “No Chi-nese castings” in customer specifi-cations. Why do end users insist onthis restriction? Those who wereable to attend the recent ValveWorld conference (held Nov. 4-6,2008 in Maastricht, The Nether-lands), where several sessionsfocused on end-user problems, canattest that material quality is clear-ly a problem. And the perception isthat the problem exists because ofwhere the castings are made. Thisarticle reveals prevalent types ofcasting defects and identifies thedeficiencies that enable them tooccur.

The valve manufacturing com-munity bears a large part of theblame for end-users feeling thatthey need to restrict sources of sup-ply. End users should not have tocare about where castings arepoured and they should not feel theyneed to impose this restriction.

PROOF OF QUALITY?At the forefront of this issue is thefact that the quality of a castingcannot be attributed to the existenceof a certificate claiming confor-mance to a standard. Too often weobserve blind acceptance of certifi-cates as verification of quality. Butan ISO 9000 certification simplymeans the foundry has a qualitymanagement system for its process-es. The certificate does not prove orverify technical competence. Mate-rial standards such as ASTM typi-cally list sizeable ranges for chem-istry, mechanical properties, andtimes and temperatures for heattreatment. However, many of the

HOW DO YOU ANSWER THE

CASTING QUALITYQUESTION?

limits in material standards are minimums, and how often do you expect to meet allthe requirements if you just meet the minimum? Should we really trust CertifiedMaterial Test Reports since these reports are generated from a test coupon—a sam-ple that is not typically representative of the actual casting size?

Simply relying on the existence of certificates as proof of quality exposes theinexperienced to inevitable failure.

As global sourcing becomes the norm, the need for quality beyond the standardsbecomes increasingly important. But consistently good quality can only be obtainedby going beyond certifications and standards to impose tighter limits to ranges inchemistry, more specific requirements for time and temperature during heat treat-ment, direct foundry visits and routine material auditing.

The following are a few examples of poor casting quality found at end-user facili-ties that damaged the reputation of the valve supplier:

ChemistryThe high cost of alloys drives foundries to raise residual elements to a maximum andminimize use of higher cost elements. Even though the material meets standards, itstill may not be adequate for its purpose. High residual elements combined may act asan alloy with unknown effects. The surge in alloy costs over the last few years hascaused some foundries to go below the limits of the standards and minimize the addi-tion of costly key elements. You should question whether your stainless-steel castingscontain the proper amounts of chrome and nickel. Do you believe they do because the

AS GLOBAL SOURCING OF MATERIALBECOMESTHE NORM,THE NEED FORQUALITY BEYONDTHE STANDARDSTAKESON INCREASING IMPORTANCE.HOWEVER,WE CAN ONLY ENSURE CONSISTENTLYGOOD QUALITY BY GOING BEYONDCERTIFICATIONS AND STANDARDSTOIMPOSETIGHTER LIMITSTO RANGES INCHEMISTRY,MORE SPECIFIC REQUIRE-MENTS FORTIME ANDTEMPERATUREDURING HEATTREATMENT,DIRECTFOUNDRY VISITS AND ROUTINEMATERIAL AUDITING.BY LES PELKEY

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test reports say so, or have you confirmedthe test reports are accurate and repre-sent the actual casting that will be used?

Heat treatmentRecommended heat treatments may pro-vide guidance for general corrosionresistance or certain mechanical proper-ties, but they do not guarantee the high-est quality. For example, proper solutionannealing of cast austenitic stainless steelrequires the correct chemistry holding atsufficient temperature and time followedby a rapid quench. If done properly, theresulting microstructure would appear asshown in Figure 1, contrastingwith Fig-ures 2 and 3, which do not exhibit the

appropriate microstructure.How often do you verify the quality

of your cast austenitic stainless steel?Figure 4 shows a CF8M valve body thatwas not quenched at an acceptable rate.Note the severe intergranular corrosionthat occurred in just three weeks ofservice in nitric acid.

Repair welding:competence and procedureIt is impractical to expect 100% perfectcastings every time; defects areinevitable. Repair of defects at foundriesis commonplace, but it is often over-looked. I strongly recommend that youvisit the weld repair room whenever visit-

ing a foundry. Are the welders routinelyqualified to any standard? Do they haveweld procedures to control their process-es? Do they follow those procedures? Isthe equipment in good shape and appro-priate for the types of repair being done?Are the filler metals the correct ones andare they stored properly? Are defectsfully excavated before welding? Thisplenitude of questions, when left unan-swered, puts you at high risk for lowquality. See Figure 5.

Stainless-steel passivationThere are many ways for free iron toembed into the surface of stainless steel.In sand castings, reuse of sand previous-

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Figure 3

Figure 4

Figure 1 Figure 2

Figure 5

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ly used for carbon steel, cleaning ofstainless castings with carbon steel shotor carbon steel wire brushes are exam-ples. If no steps are taken to check forand remove the free iron, your stainlesssteel casting can rust on the exterior.Passivation is one way to remove thisfree iron. See Figure 6.

Porosity and shrinkPorosity and shrink are both voids inthe casting caused by inappropriaterigging or sloppy pouring of moltenmetal into the mold. When discovered,they can usually be eliminated byimproving the process. The problem is:how do you discover them? Radiogra-phy can determine the existence ofsuch defects but it has its limitationsand is too costly to invoke for everycasting. Very small porosity may notbe visible in a radiograph and may notleak through the wall during a shortduration shell test with water. Howev-er, it will leak through the wall whentested with gas. What steps do you taketo limit your risk of porosity? See Fig-ures 7 and 8.

Pattern cleanlinessAny debris within the ladle, mold orshell can become entrained within the

molten metal as it fills the cavity. Whenvisiting a foundry, watch to see how per-sonnel clean the molds and equipmenthandling of the molten metal. But evenif an effective process is in place, willthe mold remain clean until it is used?Figure 9 shows an investment castingwhere a piece of the shell nearly projectsthrough the wall of the casting. Whiledifficult to discern from the picture, afoundry weld repair was done over thetop of this defect. This illustration fur-ther stresses the points on weld repair.

Heat treatment furnace loadingHere are two easy steps for checkingheat treatment if you periodically visitthe foundry: Before castings are placedin the oven, are they neatly stacked sothey receive uniform heating through-out the lot? Are test coupons placed sothey represent all of what is heat treat-ed or are they at the edge of the load,far away from the castings? See Fig-ures 10 and 11.

WORDS OF CAUTIONI have a large collection of picturesdepicting casting defects and failuresaccumulated over many years. Theimages shown here are only a few ofthe common problems that can

occur—there are many more potentialproblems with equally damagingresults. Pictures used in this articlerepresent products from severalcompanies and were selected to avoididentification of any particularmanufacturer or foundry. However, afew casting failures this year inEurope were openly broadcast througha notified body in the European Union.The claim was that the castingsviolated the Pressure EquipmentDirective, and the specific valvemanufacturers, not the foundries, werelisted in the notice. This notice isopenly available on the Internet andmore than 900 valves had to bereplaced.

My last two questions to you arethese: Have you taken sufficient steps toensure this cannot happen to your com-pany? Are your castings of the highestquality? VM

LES PELKEY is a materials engineering specialistfor MetsoAutomation USA (www.metso.com).Reach the author at les.pelkey@jamesbury.com.

Metso North America Corporate Office44 Bearfoot RoadNorthborough,Massachusetts, 01532 U.S.ATel: 1 (508) 852-0200Fax: 1 (508) 393-0978www.metsoautomation.com

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Figure 7 Figure 8

Figure 10 Figure 11Figure 9

Figure 6

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