A number of our CNC machining customers have started specifying SAF Stainless Steels for their custom parts. So we thought we would tell you more about this type of stainless steel. Please read on.
Sandvik Austenite-Ferrite (SAF) stainless steel is a trademark by the Swiss industrial leader, Sandvik. Sandvik is an engineering group that develops material technology in advancing stainless steel and special alloys, tooling systems for industrial metal-cutting, and equipment and service solutions in mining and construction industries.
Ocean Monarch Oil Platform - Bass Strait Australia
What are the SAF stainless steel grades?
The SAF grade series has two categories: the duplex stainless steels and the hyper-duplex stainless steels.
Duplex stainless steels have a combined chemical composition of a balance phased austenite and ferrite mixture. This combination overcomes the technical weaknesses found in standard austenitic steels such as 304, (low resistance to stress corrosion cracking) and ferritic steels like 430 (difficulty in weldability and poor toughness in low temperatures).
Duplex stainless steel grades:
Sandvik SAF 2205 (Plate sheets, wirelines, billets, bars, strip steel)
Sandvik SAF 2507 (Plate sheets, wirelines, billets, bars, strip steel)
Sandvik SAF 2304 (Seamless tube and pipes)
Sandvik SAF 2906 (Seamless tube and pipes)
Sandvik SAF 31803 (Seamless tube and pipes)
Hyper-Duplex (HD) stainless steels are a new generation of duplex steels developed to exhibit both the highest levels of corrosion pitting resistance as well as mechanical strength within the SAF modern duplex family.
Hyper-Duplex (HD) stainless steel grades:
Sandvik SAF 3207 HD (Strip steel, Seamless tube and pipes)
Sandvik SAF 2707 HD (Seamless tube and pipes)
What are the advantages of SAF?
1. Pitting corrosion resistance
Hyper-duplex stainless steels have high critical pitting temperatures (CPT) and critical crevice corrosion temperatures (CCT). SAF 2707 HD has a well-balanced composition of chromium, nitrogen, and molybdenum and has an average Pitting Resistance equivalent (PRE) number of 49 which is relatively high in comparison to other duplex grades (steel grades with a PRE number of 32 are accepted for seawater applications). This makes SAF HD stainless steels excellent in harsh, corrosive, chloride-containing environments.
SCC is a form of corrosion where the material is both exposed to a corrodent and sustained tensile stress. It is encountered in many industrial processes and it leads to rapid material failure. The advanced dual-phase structure as in Sandvik SAF 3207 HD, provides a solution as it performs superior in SCC resistance in severe chloride environments at elevated temperatures. This results in an enhanced performance in comparison to austenitic stainless steels (304 and 316).
3. Superior tensile strength and ductility
SAF is developed to have high tensile strength, enabling it to withstand rough conditions under high subsea pressures without compromising ductility. This is due to the balanced composition of fine-grained austenitic and ferritic phases in the material. With this high mechanical property though, fabricating and machining SAF steels can pose a challenge. Modern, stable cutting CNC machines are recommended in order to overcome the difficulty in chip breaking and vibration.
4. High impact toughness
Impact toughness is important in considering the selection of steel grades in the duplex family. It is the energy required to fracture a material at high velocity through impact testing. SAF hyper-duplex and SAF super duplex grades have high strength and impact toughness when subjected to impact testing. With this, the machinability requirements of duplex steels are different in comparison to carbon steels or ferritic steels. Generous cutting fluids are required from CNC machines in order to dissipate heat and improve dimensional stability.
5. High fatigue strength
A material’s fatigue can be defined as the localised, progressive permanent microstructure change in a material under fluctuating stress. SAF duplex stainless steels show high fatigue strength when subjected to high fatigue cycles and corrosion fatigue in artificial seawater tubes that test for crack growth. This means that SAF stainless steels perform well in the conditions required in offshore oil and gas applications.
SAF stainless steel considerations
SAF duplex and hyper-duplex stainless-steel grades are developed to withstand variable service conditions and are expected to overcome non-stable operating conditions. SAF steels have extra-high strength, high ductility, improved corrosion resistance, and are reliable in lowering life cycle costs.
These advantages, however, do make it difficult when it comes to machinability and fabrication. The weldability of SAF steels can be challenging, that’s why particular welding fillers and consumables are also developed by Sandvik. Welding guides specific for SAF steels are also provided with the details in welding techniques, temperature, and preparation.
When it comes to machining, CNC machines must be prepared to have ample coolant, updated inserts, and stability when machining duplex steels. It’s recommended that you choose a professional CNC machining service which has specific experience in working with SAF stainless steel. This will result in less material wastage and a higher quality end product.
At Challenge Engineering, in Sydney Australia, we machine custom metal and plastic components, and pride ourselves on great customer service. We can manufacture and deliver anywhere in Australia. Contact us and find out how we can assist your business. Sign up to Challenge Engineering’s blog, send through a Request A Quote form, email us or phone (02) 9632 0010.