Offshore

With expertise in high performance and safety critical bolting applications for harsh environments, BUMAX^® not only offers the marine sector high performance stainless steel fasteners, but has developed a complete range of high corrosion resistant stainless steel fastener grades to overcome many of the technical challenges that come with a harsh environment, including its latest additions BUMAX^® DX 149, BUMAX^® NITRO 109 and BUMAX^® SAN35.

With its new fastener grades, BUMAX is presenting technically viable and cost-effective alternatives to traditionally more expensive nickel and titanium alloy grades.

Nimeka de Silva, head of marketing at BUMAX AB, explains: “Possibly the greatest technical challenge in the industry is that of corrosion – thanks to seawater. As a result, stainless steel materials are widely used in the marine industry due to their high resilience against general corrosion. However, stainless steels can still be at risk from other forms of corrosion, including pitting, crevice corrosion and stress cracking
in seawater.”

There are four main families of stainless steel material and well over a hundred specific grades available on the market, all with varying mechanical properties and corrosion resistance capabilities in seawater. One method of determining a stainless steel’s corrosion resistance in seawater is the Pitting Resistance Equivalent Number (PREN). The PREN is based around three elements within the material’s chemical composition, namely Chromium (Cr), Molybdenum (Mo) and Nitrogen (N) and the most used version of the PREN formula is: 

PREN = %Cr + 3.3 x %Mo + 16 x %N

With this in mind, BUMAX developed a complete range of high corrosion resistant stainless steel fastener product grades including the BUMAX^® 88 and BUMAX^® 109, an A4/316L material with high strength equivalent to class 8.8 and 10.9. Despite being an A4/316L material, the particular grade of material used for BUMAX^® 88 and BUMAX^® 109 guarantees a higher minimum level of molybdenum content than standard A4/316 materials. 

Molybdenum is a key element in a stainless steel material’s ability to resist pitting corrosion. As a result, BUMAX^® 88 and BUMAX^® 109 have a PREN of approximately 27, whereas generic A4/316L materials will have a PREN of 25. Nimeka underlines: “In ‘real life’ applications, this difference in PREN can mean many more years in service before the first signs of corrosion. BUMAX^® 88 and BUMAX^® 109 can therefore offer an intermediate solution, between lower performance generic A4/316 stainless fasteners and the prospect of having to upgrade to more expensive higher alloy fasteners.” 

Thanks to this superior performance, BUMAX^® 88 and BUMAX^® 109 have established a significant base of loyal OEM customers in the marine industry, including several with decades long experience of using BUMAX^® 88 in critical applications.

Furthermore, for marine applications with an even higher demand on mechanical properties BUMAX^® DX 129 is available, in diameters M3 – M20. BUMAX^® DX 129 overcomes the traditional challenges of trying to reliably and cost-effectively combine both high strength and corrosion resistance. In addition, BUMAX^® DX 129 is a duplex stainless material grade developed to be an especially high performing all rounder, with corrosion resistance considerably higher than A4/316L (PREN of minimum 34), very high strength (equivalent to class 12.9) and better ductility (approximately 33% greater than 12.9 carbon steel). 

More recently new generation grades have been developed and added to the BUMAX^® range, including the BUMAX^® DX 149, which pushes the mechanical boundaries of BUMAX^® DX 129 even further, with tensile strengths of a minimum 1,400 MPa and a yield strength of a minimum 1,260 MPa, while still retaining excellent ductility. BUMAX^® DX 149 can be manufactured in diameters M3 – M20.

Another new addition, BUMAX^® NITRO 109, was developed specifically for shipbuilding and offshore structural applications, to deliver very high mechanical strength on large diameter bolts. Strength equivalent to class 10.9 can be manufactured to order in diameters from M30 to M36 and class 100 for M42 diameters. NITRO 109 is an austenitic stainless material grade with high corrosion resistance (PREN of 34), similar to that of duplex stainless steel.  

The latest addition to the BUMAX^® range of marine grade products is BUMAX^® SAN35. This particular product grade is based on an advanced stainless alloy that combines the best features of super-austenitic and nickel alloy grades, surpassing the performance of super-duplex, super-austenitic and even some nickel alloy materials. SAN35 offers high mechanical strengths and super high resistance to pitting, crevice corrosion and stress corrosion cracking in seawater (PREN of 52), even at elevated temperatures.