CONTROLVALVES I Materialof Gonstruction By Arrwish Chopra CEO - AnergyInstrwnents Put. Ltd., Foridrtbad

anycontrol valve specifications call for valvebody made iron', but actually it is not a single material, it is a family of of brass,gunmetal or bronzefor sizes50mm diameter materialswhose major constituent is iron with varying amounts and below,and castiron for sizes65mm diameter and of carbon and silicon. Cast irons are natural composite materials above. These specificationsseem to have been in usefor over whose properties are determined by their microstructures' half-a-century and show a bias towardscopper alloys over cast Cyey lron - Grey iron is an iron alloy characterizedby iron for small sizedvalves. its relatively high carbon content, usually 7o/oto 4o/o' When The reasonsfor this bias could be many, the easiestbeing molten iron solidifiessome of the carbonprecipitates as graphite, the better corrosion resistanceof copper alloys,but that seems forming tiny, irregular flakeswithin the crystal sffucture of the unlikely in HVAC systemswhere most of the piping itself is of metal. The presenceof graphite flakesgive grey iron excellent mild-steel,prone to corrosion. casting,machining and self-lubricating properties,but distrupt The intent of this article is to go through the characteristics the crystal structure providing a nucleation point for cracks, and physical propertiesof someof the copper and iron alloys to leading to grey iron's characteristic brittleness. find out other reasonsfor this bias and its relevance today. Ductile lrut -lnductile iron the graphite is in the form of Gopper Alloys sphericalnodules rather than flakes,thuinhibitlng the creation Copper was the first metal used by man and it was soon of cracksand providing enhancedductility' It is alsoknown as found that its propertiescould be improved by alloying' Brass, S.G. iron or nodular iron. It was invented around 1948. gunmetal and bronze are all alloys of copper with different Its high tensile strength, ductility and easeof casting for applications where use of percentages' of zinc, tin and lreadas alloying elements' makes it material of choice Brurie- Bronzewas the fiist copper alloy usedby man. Early steel is expensive. It has replaced many components in the bronze was an alloy of copper and arsenic that occurs together automotive industry. naturally. Later on, arsenicfell out of favour for its toxicity and was In HVAC context, ductile iron can replace copper alloy replacedby tin. Addition of tin makesthe alloy harder and more valve bodies without increasing the wall thickness and corrosion resistant then the virgin metal, thus more useful. component weight. It can alsobe usedto replacethe grey iron Bronzewas widely useduntil iron becamecheaper and more valve bodies with increasedstrength and pressurerating' plentiful. Bronze is harder and tougher than any other copper To summarise,all copper alloys have excellent resistance alloy. In HVAC context bronze is more suitable for valves for to corrosion but eachhas its distinct advantages.Brass is most high-pressurehot water and steam applications' suitable for forged and die-casted valve bbdies, gunmetal is Brcss - Brassesare copper alloys in which the principal more suitable for sand casting and bronze is more suitable for alloying element is zinc. Their properties depend primarily high pressureand temperatureapplications. upon the proportion of zinc present. Most commonly Grey iron can be usedfor casting all kinds of valve bodies, used brassescontain 30Vo . 40o/oof zinc. These brassesare except of very imall sizeswith thin sections,whereas Ductile/ characterizedby their ductility at room temperature, and can Nodular/S.G. iron overcomesthis weaknessand can be usedto. be extensivelydeformed by rolling, drawing,bending, spinning, producecomponents from lessthan 20 gramsto more than 200 deep-drawingand forging. tons of section sizesas small as 2mm to more than 500mm. The combination of ductility, machinability and strength Going back to the specifications, the other most likely makesthis material most suitable for small valve bodies,with reasonsfor biasfavoring copper alloys could have been the easeof thin wall sections,which are either forged or die-casted. manufacturing,possibility of usingthin wall sections,thus reducing price differential. Qunmetal. It is a copper alloy containing both zinc and component weight, and not very high tin asalloying elements. It wasdeveloped by British admiralty But in today's scenario when, copper alloys have become for ordnanceparts by adding zinc to bronze,to improve casting much more expensive,improved foundry practiceshave made characteristics. it possible and easierto cast small components of grey iron, Gunmetal has good casting characteristics,particularly as and development of ductile iron has made it possibleto cast a sand casting. It is most suited for sand castedvalve bodies components with thin sections,it is time to rewrite these wherehigh strength,pressure tightness and corrosionresistance specificationsand remove the unnecessarybias towardscopper are important. alloys. lrcn Alloys Small valves of forgedbrass are likely to remain popular for 'cast 'grey Cast lron - The terms iron' generally refers to the easeof manufacturing, thus low overall cost. {'

134 AirConditioningandRefrigerationJournal* January'March2009