How can you make products of superior quality and performance? How can we make products of comparable quality at a lower cost? These questions are on everyone's mind, from the product designer to the manufacturing manager to the buyer, and they are also important for product manufacturers to maintain a competitive edge in the marketplace. In the field of nickel-containing stainless steels, different alloys have different characteristics. When it comes to stainless steels, how does a material selector choose the right type of alloy?
Duplex Stainless Steels
Duplex (austenitic-ferritic) stainless steels have much higher yield and tensile strengths than austenitic or ferritic stainless steels. Therefore, the use of higher strength duplex series materials can reduce the wall thickness of storage tanks or pressure vessels, which has led to market interest. However, the higher ductile strength of duplex stainless steel has an impact on the formability of the product. In addition, duplex stainless steel is different from common austenitic alloys, which can also create some difficulties in the welding process. There can also be significant differences in welding between different sub-series of duplex stainless steels, such as economy, standard and super duplex stainless steels.
The temperature at which the application is expected to take place is also important as there are a number of constraints on the use of duplex alloys. The ferrite phase can become brittle at low temperatures and can also produce harmful microstructural changes at temperatures as low as 270°C. Each sub-series includes many different types of alloys, which leads to many difficulties for manufacturers when sourcing complete kits in different sizes and product forms. Since the nitrogen alloying of duplex stainless steels, their market share has increased significantly. However, over the past 10 years, the market share of duplex stainless steel has hovered around 1% due to the complexity of the process in which the product is used.
200 Series Stainless Steels
The fully austenitic 200 series stainless steel has been in development for over 70 years. Due to the high manganese content of this alloy, the nickel content can be suitably reduced, but not eliminated completely. Today, most 200 series products are available in low chromium alloys, where the chromium content is typically 16 to 12 per cent. In contrast, 304 stainless steel, with a minimum chromium content of 17.5 per cent, offers higher corrosion resistance. If a product requires only minimal corrosion resistance, then 200 series can be used. 200 series stainless steels harden rapidly during cold forming, sometimes producing cold cracks after a period of cooling, so copper is often added to these stainless steels to reduce the rate of work hardening. Therefore, when selecting low chromium 200 series stainless steels, consideration should be given to ensuring that they have adequate corrosion resistance and suitable machinability.
Ferritic Stainless Steels
The unique properties of ferritic stainless steels were discovered more than 110 years ago. The chromium content of this family of stainless steels ranges from an absolute minimum of 10.5% (the definition of stainless steel) to nearly 30%. Ferritic stainless steels are most commonly used for plate thicknesses ≤ 4mm, as extreme grain growth is a problem in the medium to heavy plate thickness range, especially during welding. As with duplex stainless steels, there are constraints on the use of ferritic phases at both low and high temperatures. Initially, low cost was the main attraction of the ferritic series to material selectors, as they are nickel-free or have very low nickel content. However, their production costs are usually higher than those of austenitic stainless steels and limitations in terms of thickness and use at different temperatures, as well as weldability, have restricted the development of the ferritic series. In fact, the percentage of ferritic stainless steel series in total production has fallen by about 30 per cent since 2010. However, in applications such as thin-walled heat exchanger tubes, ferritic stainless steel's lower thermal expansion and higher thermal conductivity play an important role.
