In comparison to castings, the structure of forgings ensures wall thickness and dimensional tolerances are uniform. High impact closed die press forces and a segregation free billet guarantees superior strength, uniformity, integrity and density. Casting issues include inter granular corrosion, stress weakness, fatigue, cracks, porosity, sand pockets, weld repairs, scars, etc. Another benefit of forgings is the directional flow lines structure. Due to quality concerns brought about by low cost country valve manufacturing we now often see customers first specify a forging rather than a casting, then specify it must be a European forging and an approved brand, yet they then still ask for 100% MPI and UT on common A105 valves increasing the cost exponentially. Using good quality forging can reduce the need for NDT. Of course UT and MPI is a good idea to provide reassurance of the quality, especially for castings in larger sizes and higher classes.
Valves sold as forgings in low cost countries can often be not hot forged but are machined from bar. The reason is, labour is cheaper and Chinese plants don’t own sets of dies for every size & class. (This is another reason Chinese plants pop up “out of nowhere” and can make every size & class). The use of bar is allowed in smaller sizes but beware of equivalent bar that is not a forging quality despite the certificate stating A105 or 316, etc. Ensure cold forging is carried out where required.
Cast can be as good as is needed
However, if cast valves are made properly they can and have worked equally well in all services and usually at a much lower cost than forged valves. People forget in the seventies and eighties, all valves including critical trunnion ball valves for instance were two-piece body castings. WKM, KTM, Kitz, Richards Australia, Saut Du Tan & Cameron manufactured 150 to 600 class up to 750NB (30″), 900 class to 400NB (16″) and 1500 class to 300NB (12″). Then along came the TK (Scotland) and Grove (Italy) forged valves which have been widely copied ever since. Whilst gate, globe & check valves are made in cast in all sizes & classes, there is some ‘casting snobbery’ in the area of trunnion mounted ball valves (of course many users have experienced quality issues with castings from low cost manufacturing countries).
For both forged and cast valves the process begins with molten metal being poured into a mould. Shrinkage is a defect commonly shared by castings and forgings. Forging defects occur less frequently and can include seams, laps, inclusions, cracks, etc. Sand casting defects are much more common and can consist of tears, sand pockets, pin holes, porosity, inclusions, etc. Lost wax process investment castings are better but for higher pressures, ‘resin’ sand cast can be better. For critical Alloy & Stainless steel materials, silica solution castings are superior and so it goes on. Then again the way the foundry makes the casting is also a critical factor. As always buy a brand name you trust.
Another advantage of cast valves is that they can be much more easily produced in more complex designs than forged valves.
Forged trunnion ball valves (for instance) are usually in 3 pieces, particularly the larges sizes to reduce costs as larger forgings are exponentially dearer. This means there is either an additional flanged connection that can be an extra potential leak path.
Castings don’t have a good reputation due to the recent emergence of low cost, low quality casting foundries. Also ASTM casting specifications such as ASME section VIII are less strict in relation to inspection, composition and heat treatment requirements. Without the knowledge to implement tighter operations some foundries are without the will or too price focused to ensure all requirements are met. The risks can be reduced if NDT is performed.
However, a more cost-effective way is to deal with valve suppliers who already control the quality of the products they produce and have a long and successful track record. In either case, the decision to go with cast or forged valves depends on several factors and unfortunately cost is usually the determining one. For critical valves with thinner walls, such as ball valves in higher classes but even gate, globe and check valves in larger sizes & higher classes, consider a higher quality NDT tested casting or forging. In WCB castings, many manufacturers use WCC in lieu as it has a better “bind” and LCC is better again for the same reason (aside from wider temperature) and of course it is Charpy Impact tested.
The article is from http://globalsupplyline.com.au
