Valve World Magazine September 2024 | Page 34

MATERIALS

gas industry . Equivalent cast versions of these alloys were then developed as valves and pumps were required . These are known as CK3MCuN and CN3MN respectively but the materials are more commonly referred to as ‘ 6Mo ’ grades as they contain at least 6 % molybdenum . The cast versions of these grades tend to be problematic , partly because there is often no secondary argon oxygen decarburisation ( AOD ) stage in foundries unlike large stainless steel mills which manufacture wrought products . These materials are difficult to cast in thick sections , they are prone to hot cracking , macro-segregation , micro-segregation and secondary phase precipitation . Equally , they can also experience grain growth during solution heat treatment . There is another issue regarding the mechanical properties . Both the minimum yield and tensile strength of cast 6Mo grades are significantly lower than in the wrought versions , restricting their suitability for applications involving high pressure and the design parameters in terms of system weight .

Bucking the trend

The initial success of superduplex stainless steels was partly due to the demand for better cast materials which could compete with 6Mo grades on corrosion resistance but also possessed higher strength properties , enabling savings for piping systems in terms of weight , size and cost . The original 25Cr superduplex – which became the stainless steel cast grade 6A – was invented around the same time as 6Mo but as a cast material for pumps and valves . Its sudden popularity then created the need for a wrought version for pipes , fittings and flanges . It is a unique example of an alloy developed specifically for improved castability and in-service performance in cast products as opposed to simply modifying materials that were designed for another purpose entirely . The metallurgist behind the superduplex

Pouring N ’ GENIUS 326L35M4N into cast blocks for testing

invention has since adopted this same principle more than 40 years later – although this time for a whole family of grades .

Breaking new ground

Dr Ces Roscoe , who pioneered the first superduplex grade whilst at pump manufacturer Mather + Platt in 1983 , has invented and patented an entire series of high strength austenitic stainless steels tailored for cast product forms as well as wrought . Grades in the N ’ GENIUS Series possess all the typical benefits of conventional austenitic stainless steels including excellent ductility and toughness at ambient , sub-zero and cryogenic temperatures . But with vastly superior strength and corrosion resistance . It provides engineers with an extensive choice of high-performance , high strength austenitic stainless steels including some materials which are cost-effective alternatives to certain nickel alloys . Dr Roscoe , CEO of N ’ GENIUS Materials Technology , said : “ The 6Mo grades and superduplex have been well utilised over many decades and will continue to be in the future . But like with any material , there is a limit to their technical capabilities .” “ The industries they were originally intended for use in such as the oil and gas sector now often require piping systems with better corrosion resistance beyond the normal range for these grades .” “ Examples of this are sour gas developments in Middle East countries including Qatar , Saudi Arabia and UAE which can contain high levels of CO 2

, H 2

S and chlorides .” “ 6Mo also lacks the mechanical strength properties required for high pressure service . So there is clearly a need for more high-performance stainless steels that have these features and some of the N ’ GENIUS high strength austenitic grades are the perfect solution .”

A world first in castings

Stainless steel manufacturers have already started noticing this trend . It ’ s one of

N ' GENIUS™ CAST ALLOY COMPARISON TO 6Mo CAST GRADES

* ALL CAST MATERIALS SOLUTION HEAT TREATED AND WATER QUENCHED ACCORDING TO ASTM INTERNATIONAL STANDARDS ** ASTM G48 TEST

Table : Cast N ’ GENIUS 326L35M4N has vastly improved strength and corrosion resistance to cast 6Mo

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