Duplex UNS S32760
32760 was originally developed for use in the maritime environment it is recognised for its excellent resistance to both chloride and sulphide corrosion. It combines this with high yield strength and hardness; it is reasonably ductile and does not become brittle at sub-zero temperature.
These qualities are due to the chemical composition and carefully controlling the temperature during the production process. This results a a steel composed of a mix of austenitic ferritic forms - betweeen 40 & 50% ferrite in the annealed form.
Duplex S32760 is a super duplex stainless steel, so-called due to its higher Chromium and Molybdenum content. It also contains both Tungsten (W) and Copper (Cu).
Tungsten is particularly useful in contributing to hardness at high temperatures but this is of little benefit in duplex steels which are generally limited to temperatures below 300oC due to the effect of higher temperatures on their duplex structure. The main function tungsten fulfils is to promote finer grain sizes thereby contributing to the corrosion resistance.
The addition of copper further contributes to the corrosion resistance.
S32760 has high impact strength and shows no ductile-brittle transformation as temperatures drop. It does, however, show a slight reduction in impact strength at lower temperatures.
Having originally been developed for seawater resistant pumps in the North Sea oil and gas industry its combination of hardness and corrosion resistance has seen its use spread to many industries where hostile environments combine with temperatures up to 300oC are found.
Its high strength and hardness coupled with its excellent corrosion resistance make useful in:
- Oil & Gas industry, particularly offshore
- It is widely used pipes, pumps, valves and wellhead equipment
- Subsea equipment
- Process industry components in nitric acid, dioxide, polypropylene & PVC production, caustic evaporators
- Equipment handling organic & fatty acids
- Marine industry and shipbuilding propellers, shafts, rudders, shaft seals, pumps, bolts, fasteners, valves, instrumentation
- Oil and chemical tankers
- Oil & Gas industry pumps, valves, pipe, vessels, wellhead equipment, subsea equipment
- Pollution control systems, fans and pumps, wet scrubbers, incinerators, flue gas desulphurisation (FGD)
- Pulp & paper industry components such as black liquor heater tubes, digester blow valves, rotary feed valves,
- Food Industry processing plant
- Fertiliser production ( wet phosphoric acid)
- Sewage treatment
- Seawater cooling and desalination plants
- Propellor shafts
It is particularly useful in the last of these due to its resistance to cavitation erosion.
Super Duplex S32750 offers high tensile and yield strength, and a high degree of hardness, but its outstanding quality is its corrosion resistance. It is also ductile and impact resistant at both ambient and sub-zero temperatures (but not cryogenic).
S32760 has a yield strength of 551.6MPa and Ultimate Tensile strength of 751.5MPa. This declines slightly as the temperature rises dropping to 400 and 648.1 at 250oC more than two and a half times that of 316 (215MPa).
Brinell hardness is 270 as compared with 123 for 316. This offers significant advantages where hardness is required it also makes machining more difficult. sharp tools and a 20% slower cutting rate is advised. This hardness contributes to corrosion resistance by helping to resist surface corrosion.
As noted above the corrosion resistance of S32760 is probably its major benefit over other similar steels.
Resists corrosion by a wide range of generally highly corrosive chemicals
It has excellent resistance to pitting corrosion (PREN >40)
- Excellent corrosion resistance in a wide variety of corrosive chemicals
- Pitting Resistance Equivalent number (PREN) of >40
- Outstanding resistance to pitting and crevice corrosion in seawater and other chloride containing environments
- Critical Pitting Temperature exceeding 50°C
- High strength compared to austenitic and 22%Cr duplex stainless steels
- Excellent ductility and impact strength at both ambient and sub-zero temperatures
- High resistance to abrasion, erosion and cavitation erosion
- Excellent resistance to stress corrosion cracking in chloride containing environments
Duplex 2205 resists oxidation at higher temperatures however it will be embrittled if held a temperature above 300oC for even a short period. Should this occur it can be corrected by full solution annealing. this requires heating to between 1020oC and 1100oC and cooling rapidly.
One of the major benefits of Duplex 2205 can be cost. Due to its greater yield strength, significantly thinner sections can be used to achieve the same strength. This offers weight savings which can also impact other areas of any fabrication. The consequence is that although the cost per kilo of 2205 is higher its use can significantly reduce the cost of a project.
A common failing is to switch to 2205 for its superior corrosion resistance or hardness and fail to take advantage of its superior strength.
The primary limitations of Duplex S32760 are its restricted operating temperature ranges.
Above 300oC brittle microconstituents are precipitated significantly harming its corrosion resistance and at temperatures above this, its duplex structure is compromised. This can be resolved by annealing at a minimum temperature of 1100oC and quenching rapidly.
The qualities of high tensile and yield strength mean that bending and forming are difficult. Cold bending will demand higher capacity equipment than austenitic steels and the temperature constraints make it unsuitable for hot bending. However, if the design has been done with an understanding of the higher tensile strength the bending forces required will be similar to those of an austenitic component with the same strength.
Welding must be done with care to ensure the surrounding material is not overheated. Manual metal-arc welding with covered electrodes or gas-shielded arc welding are the recommended approaches. Preheating should be avoided and post-weld heat treatment is unnecessary.
Oxyacetylene welding is not recommended due to the possibility of carbon pick up in the weld area.
The main constituents of Super Duplex S32760 stainless steel - other than iron - are Chromium and Nickel. However, it is the addition of Molybdenum (Mo), Tungsten (W), Copper (Cu) and Nitrogen (N) that provides the increased corrosion resistance.
32760 contains 24 - 26% Chromium (Cr). Chromium is the essential chemical in all stainless steel and it is that which forms the thin passive layer that makes the metal "stainless". It also, particularly at these percentages, increases the yield strength
The 6.0 - 8.0% Nickel (Ni) contributes to its corrosion resistance, hardness and notch toughness. The Tungsten encourages the formation of small grains, reducing the problem of intergranular corrosion. It also forms carbides reducing the formation of chromium carbide and increasing the amount of free chromium. Nitrogen contributes to the hardness of the steel. The copper improves the corrosion resistance. The precise interaction of copper is not well understood but it is the copper that makes it inadvisable not to weld it with an oxidizing oxy-acetylene flame as it can result in preferential oxidation of iron and Cu enrichment.
A number of other chemicals may be present but these are expressed as maximum permitted levels.
- Special Sections
- Flat Bar
- Round Bar
- Hollow Bar
- i Beam
- U Channel