By John Tarboton, Manager: Fabrication, Welding and Technical, Southern Africa Stainless Steel Development Association
Stainless steels have traditionally been specified in applications where the primary requirement is for corrosion resistance. However, since their invention over 100 years ago, stainless steels have also been recognised for other attributes such as durability, versatility, quality, sustainability, hygiene and aesthetic appeal. It is this combination of properties that has seen stainless steels become the material of choice in a wide variety of uses, from the utensils and kitchenware used to prepare food; in a range of applications in the transport industry; as process equipment in the food and beverage industry; for the manufacture of pharmaceutical products; in the medical field; through to very demanding applications in the chemical processing and power generation industries.
Widely accepted and being used to an increasing extent in architectural applications, stainless steels are now becoming more widely utilised in traditional structural applications, such utilisation often being driven by other features of stainless steel such as high strength, improved fire resistance properties and good impact resistance. Evidence of this trend can be seen with the introduction of stainless steels in structural design codes in South Africa, the United States, Australia/New Zealand, and Europe.
As a material group, stainless steels are made up of some 200 different primary grades with variants within individual grades adding to the mix. However, for structural usage in South Africa, there are three groups of stainless steel grades that would cover the vast majority of applications. These are the utility ferritic stainless steels, the austenitic stainless steels and the duplex stainless steels.
Utility Ferritic Stainless Steels
There are a number of utility ferritic stainless steels, but perhaps the most well-known and specified grade is 3CR12. 3CR12 was invented by Columbus Stainless in 1977. This steel is now certifiable to ASTM A240, UNS types S41003, S40977 or S40975. It is a 12% chromium steel, with the full chemical composition shown in Table 1. It has an atmospheric corrosion resistance of 150 times that of carbon steel and 30 times that of the zinc coating of galvanized steel. Even though the corrosion rate is extremely low (<2mm/yr), even in the most severe marine environments, it can form a brown patina. Thus, if aesthetics are important, consideration should be given to coating 3CR12 or to using one of the higher alloyed stainless steels, as detailed below. In marine applications, some light pitting of 3CR12 is also possible, but in the CSIR atmospheric exposure programme, the maximum pit depth that was ever observed was 0.25mm. The main conclusion from this programme was that, from a corrosion resistance point of view, 3CR12 is suitable for use in any atmospheric environment in South Africa.
3CR12 has a minimum 0.2% Proof Stress of 300MPa, a minimum elongation of 20%, as shown in Table 2, and is tough even after welding and at sub-zero temperatures, down to about -30°C. Although not yet a commercial option, recent trial developments for structural applications, based on earlier laboratory research with this grade, has resulted in an enhanced minimum 0.2% Proof Stress of 460MPa being obtained while still retaining the minimum elongation requirement of 20%.
Austenitic Stainless Steels
There are two main grades of austenitic stainless steels that are important for structural applications, namely AISI 304 and 316, and the choice is dependent on the corrosion resistance required. These two steels have a minimum 0.2% Proof Stress of 205MPa, as shown in Table 2, but they have outstanding ductility with a minimum elongation of 40%. They are also exceptionally tough, even when welded and remain tough, down to liquid nitrogen temperatures (-197°C) or lower.
AISI 304 stainless steel (UNS type S30400) is the most widely used stainless steel, commonly referred to as 18/8 or 18/10, both referring to the same nominal composition of 18% chromium and 8% nickel, as shown in Table 1. It retains its aesthetic appearance in most atmospheres, except those within 20km of the coastline or in severely polluted environments. If aesthetics are important in these environments, then 316 (UNS S31600) should be specified. This grade is similar to 304, except that it
has a 2% molybdenum addition. This molybdenum increases the pitting resistance and the steel is thus suitable for marine and severely polluted atmospheric environments.
Duplex Stainless Steels
Duplex stainless steels are roughly half austenitic and half ferritic in their microstructures. They have a minimum 0.2% Proof Stress of at least double that of austenitic stainless steels, such as 304 or 316, while maintaining good ductility with a minimum elongation of 25%, as shown in Table 2. They are tough, even when welded and this toughness is retained down to at leas-50°C.
There are three types of duplex stainless steels that are relevant to the structural industry. Firstly, there are the lean duplex stainless steels. ‘Lean’ refers to the alloy composition, with these steels being designed to maximise cost competitiveness
compared to the traditional duplex grades. The lean duplex stainless steels are still able to achieve a corrosion resistance similar to 304, with grades such as 2001 and LDX 2101®. 2001 (UNS type S32001) has a nominal composition of 20% chromium and 1% nickel, while LDX 2101® (UNS type S32101) has a nominal composition of 21% chromium and 1% nickel, as shown in Table 1.
Then there is 2304 (UNS S32304), also a lean duplex stainless steel, which has a nominal composition of 23% chromium and 4% nickel. It has general corrosion resistance and pitting resistance that is even better than 316 and is thus suitable for marine atmospheric environments.
Finally, 2205 (UNS32205) has a nominal composition of 22% chromium, 5% nickel and 3% molybdenum. It is a standard duplex stainless steel with very good general and pitting corrosion resistance. 2205 is suitable for even the most aggressive marine environment, except for seawater immersion applications.
Structural applications can take advantage of the high strength of duplex stainless steels and down-gauging becomes possible, depending on buckling and deflection constraints. In practice, this means that the cost of a structure made from duplex stainless steel can be significantly lower than if the structure had been made from an austenitic stainless steel of equivalent corrosion resistance.
Figure 1 shows which stainless steels would be selected for structural applications, depending on the environment, assuming aesthetics are important and no staining is acceptable. If aesthetics are not important, 3CR12 is suitable for all environments, from a corrosion resistance point of view. An indicative pricing is given, per ton, relative to galvanized steel.
Conclusion
3CR12 is suitable for any atmospheric environments, from a corrosion resistance point of view, provided that aesthetics are not important. For inland atmospheres, 304, 2001 or LDX® 2101 can be specified. Although the cost per ton of these two lean duplexes is more than 304, the down-gauging that is possible due to their high strength means that the cost of the structure can be lower. For marine applications, 316 or 2304 can be specified, with 2304 having significant cost benefits. If there is any danger of sea spray or splashing, then 2205 can be specified.
