Alternative stainless steel

This article is the second of a two-part series outlining new and emerging stainless steel grades which may be considered as alternatives to the more traditional and widely known varieties. The first article was included in the September newsletter and is available in the feature articles section. This article describes the ferritic and duplex grades.

The growing demand from China and the rest of the developing world has driven up the price of the alloying elements in stainless steels. The relative cost of different grade groups of stainless steels has also changed, depending on the content of the more expensive alloying elements, particularly nickel and molybdenum.

In the last issue, we described the austenitic 200 series group, one of the alternative groups to the austenitic 300 series that traditionally dominate the market. This article describes the other two alternative groups, ferritic and duplex grades.

Ferritic 400 series

These stainless steels have the ferritic structure also found in carbon steels. They do not contain the nickel addition used to stabilise austenite in 300 series grades. The quality of ferritic grades has advanced with modern steelmaking equipment and, after several generations of ferritic grades, a number of technical limitations have been overcome.

Toughness is the remaining limitation that has not been overcome. All ferritic grades show the ductile to brittle fracture transition well known from carbon steels. Unlike the carbon steels, there is no phase transformation when heated during welding, and hence the grain size of the HAZ can be high. This limits the toughness of the ferritic stainless steels, and with a few exceptions they are used at up to about 3 mm thickness, where the toughness transition temperature after welding is adequate.

There are ferritic grades with 10.5-30% chromium, and many also contain molybdenum. The ferritic grades have the corrosion resistance their chromium and molybdenum contents give them, and in addition they are very resistant to stress corrosion cracking. Later generations of ferritics are not susceptible to sensitisation and intergranular corrosion.

The ease of fabrication of ferritic grades, which behave in a similar way to carbon steel, has seen them used to replace competing materials and grow the market for stainless steels. A recent publication of the International Stainless Steel Forum 'The Ferritic Solution - The Essential Guide To Ferritic Stainless Steels' (available from ASSDA) has several examples.

Mechanical and physical properties

Yield strength is a little higher than that of the austenitic grades, and tensile strength a little lower. Ductility is about half that of the austenitics, and is similar to carbon steel.

Ferritic grades cannot be strengthened by heat treatment, and since their work hardening is weak they are rarely strengthened by cold work. Ferritic stainless steel work hardens in a similar way to carbon steel, which can be an advantage, particularly in fabrication where experience and settings gained with carbon steel can be applied to ferritic stainless steels with few modifications.

Ferritic grades are ferromagnetic, and have much lower thermal expansion and higher heat conductivity than austenitic grades.

Attributes

First-generation ferritic stainless steels are usually used unwelded, as they have high carbon (~0.05%), which causes the formation of brittle films of low corrosion resistance on HAZ grain boundaries. Grade 430 is the most widely used of this group: it has enough corrosion resistance for indoor applications such as food preparation and display equipment, but is rarely fusion welded. Grade 430 is usually used with a bright annealed (BA) finish: finishes in ferritic grades are generally brighter than their austenitic equivalent. Large amounts of first-generation ferritic grades, with molybdenum added for extra corrosion resistance, are used for automotive trim.

Second-generation ferritic stainless steels have lower levels of carbon and nitrogen, and have titanium and/or niobium added to combine with what's left. This makes the grades more weldable, and the first second-generation ferritic grade developed, 409, is now widely used in automotive muffler systems. The current production of 409 in the US rivals the tonnage of the most popular stainless steel, 304. Welds in second-generation grades are tough at room temperature up to about 2 mm thickness, and do not suffer from sensitisation or stress corrosion cracking. There are titanium treated versions of 430, widely used in whitegoods such as welded washing machine drums.

Third-generation ferritic grades have even lower carbon, nitrogen, titanium and/or niobium additions, with higher contents of the corrosion-resisting elements chromium and molybdenum. The most common grade of the group, 444, is used for challenging applications such as heat exchangers and hot water tanks.

Fourth-, or new-generation grades, are further refined using vacuum equipment to achieve better toughness and weldability, and better surface quality. They are often used in applications where austenitic grades fail by chloride stress corrosion cracking or pitting corrosion, and they are increasingly being used in many applications to replace the common austenitic grades.

Limitations

The limited toughness of ferritic grades has been noted, and they are rarely used in structural applications.

A further limitation is the tendency of ferritic stainless grades to suffer 475°C embrittlement and phase formation more quickly than austenitic grades, which limits their use to about 350°C in the higher chromium grades. However, large tonnages of the lower chromium grades are used in automotive muffler systems at higher temperatures without problems.

Applications

The largest tonnage of ferritic grades is used in automotive muffler systems, and there are also significant uses in automotive trim, commercial catering equipment and indoor decorative applications. The higher alloyed later-generation grades give outstanding performance in heat exchanger and piping systems for chloride-containing aqueous solutions and seawater, where stress corrosion cracking of austenitic grades can be a problem. The ferritics are also ideally suited to roll forming to roofing, walling and rainwater goods.

Duplex grades

These grades consist of an intimate mixture of about equal amounts of austenite and ferrite. About half of the amount of nickel needed to be fully austenitic at the chromium content is added in most of the grades. A newer grade, LDX 2101, follows the approach of the 200 series austenitics by using manganese instead of most of the nickel.

There are grades within the duplex group with a range of different corrosion resistances, depending on the chromium and molybdenum contents. The duplex grades tend to use more chromium and less molybdenum than an austenitic grade of similar corrosion resistance - a more economical balance.

As chromium is increased in the austenitic 300 grades to improve corrosion resistance, more nickel must be added, making high chromium austenitic grades expensive. The more corrosion-resistant duplex grades, containing less nickel and a better balance of chromium and molybdenum, have penetrated the market to a greater extent than the leaner alloys, and 2205 has become the most common alloy where the corrosion resistance of grade 316 is inadequate.

The duplex grades are much more resistant to stress corrosion cracking than the austenitic grades, and they are effectively immune in potable water. They are also less prone to sensitisation than austenitic grades, although not immune.

Mechanical and physical properties

Duplex grades have about twice the tensile strength and 50% higher yield strength than austenitic grades. The ductility is about half, but is still high enough to give good formability, with work-hardening behaviour similar to that of carbon steels. Unwelded, duplex grades are tough to low temperatures (-50 to -100°C), and they can often be welded to give transition temperatures well below 0°C.

Duplex grades cannot be strengthened by heat treatment, and since their work hardening is weak they are rarely strengthened by cold work.

Duplex grades are ferromagnetic, and have lower thermal expansion and higher heat conductivity than austenitic grades.

Attributes

Their much higher strength than austenitic grades often allows duplex grades to be down-gauged to thinner material, with good savings in costs.

The higher strength can be a handicap if the opportunity of down-gauging is not taken, as forming loads are high and may be beyond the capability of the equipment. Many of the uses of duplex grades are at thicker gauges (greater than ~1.2 mm), where the savings of down-gauging can be achieved without getting to the lighter sheet metal gauges that fabricators can find difficult to weld.

Welding duplex grades requires more control of welding parameters, particularly heat input and interpass temperature, but pre-heat, post-heat and post-weld heat treatment are not required and weldability is considered good.

Limitations

The high alloy content of most duplex grades makes them susceptible to embrittlement from the formation of intermetallic phases after extended service at high temperatures. Corrosion resistance is also reduced. Service temperatures are generally limited to less than about 300°C.

Applications

The higher strength of the duplex grades makes them suitable for large tanks, and savings of 40% or more in material costs can be achieved. They are also used for heat exchangers and chemical equipment, often where chloride stress corrosion cracking has limited the life of austenitic grades.