Stainless steels are often selected for their corrosion resistance, but they are often also construction materials. Mechanical properties such as strength, high-temperature strength, ductility, and toughness are therefore also important considerations.
Room temperature mechanical properties
In terms of mechanical properties, stainless steels show similar properties within each group: austenitic, ferritic, duplex, and martensitic.
Austenitic stainless steels generally have a relatively low proof strength and are characterized by strong work hardening. Their strength increases with increasing levels of carbon, nitrogen and, to a certain extent, molybdenum. It should however be noted that carbon increases the risk of sensitization, which has a detrimental effect on corrosion resistance. Austenitic steels exhibit very high ductility; they have a high elongation to fracture and are very tough, also at low temperatures.
Ferritic stainless steels have relatively low proof strength and work hardening is limited. Their strength increases with increasing carbon content, but the effect of chromium content is negligible.
However, ductility decreases at high chromium levels and good ductility requires very low levels of carbon and nitrogen.
Duplex stainless steels have a high proof strength and work hardening is limited. Modern duplex grades such as LDX 2101 are alloyed with nitrogen, which results in a high strength. Increased ferrite content will, within limits, also increase the strength of duplex steels.
Martensitic stainless steels are characterized by high strength in the quenched and tempered condition and are strongly affected by the heat-treatment cycle. These steels are therefore usually used in a quenched and tempered condition. A martensitic grade can have a high ultimate tensile strength (above 1,000 MPa) and a low elongation to fracture (less than 10%) depending on the heat treatment conditions.
The toughness of steel is its ability to absorb energy in the plastic range. The toughness of the different types of stainless steels shows considerable variation, ranging from excellent toughness at all temperatures for the austenitic steels to the relatively brittle behavior of the martensitic steels.
Toughness is dependent on temperature and generally increases with increasing temperature. There is a fundamental difference at low temperatures between austenitic steels (high toughness) and martensitic, ferritic (less toughness), and duplex steels (intermediate toughness). The martensitic, ferritic, and duplex steels are characterized by a transition in toughness, from tough to brittle behavior at a certain temperature range, called the transition temperature. For ferritic steel, the transition temperature increases with increasing carbon and nitrogen content – the steel becomes brittle at successively higher temperatures.
Martensitic stainless steels have transition temperatures around, or slightly below, room temperature, while those for the ferritic and duplex steels are in the range 0 to –60 °C, with the ferritic steels in the upper part of this range. Austenitic steels do not exhibit any toughness transition but have excellent toughness for a large temperature range.
Mechanical properties at cryogenic temperatures
Austenitic grades also have excellent toughness properties at low temperatures and are usually the only material solution for very low (cryogenic) temperature applications. Ferritic, duplex, and martensitic grades are generally not suitable for low temperature applications due to their brittle behavior at very low temperature.
High temperature mechanical properties
Creep is the time-dependent slow plastic deformation of metals under a constant stress. Most austenitic steels have lower strength than the other stainless steel grades in the temperature range up to about 500 °C. In terms of creep strength, austenitic stainless steels are superior to ferritic grades and the other types of stainless steel.
Ferritic steels have relatively high strength up to 500 °C, but their creep strength, which is usually the determining factor at temperatures above 500 °C, is low. The normal upper service temperature is limited by the risk of embrittlement at temperatures above 250 °C. However, due to the good resistance of chromium steels to high temperature sulfidation and oxidation, a few high chromium grades are used in the creep range. In these cases, special care is taken to ensure that the load is kept to a minimum.
Duplex steels behave in the same way as ferritic steels but have higher strength at low temperatures. Their upper service temperature limit is normally 250 °C due to the risk of embrittlement at higher temperatures.
Fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. Fatigue occurs when a material is subjected to repeated loading and unloading. Microscopic cracks will begin to form if the loads are above a certain threshold. Eventually a crack will reach a critical size, and the structure will suddenly fracture due to rapid crack propagation. The shape of the structure significantly affect fatigue life: welds, square holes, or sharp corners will lead to elevated local stresses where fatigue cracks can initiate. Round holes and smooth transitions or fillets are therefore important in order to increase the fatigue strength of a structure.
High temperature fatigue
At elevated temperatures there are other fatigue types that are more common life-limiting factors. Temperature gradients during start-ups and shut-downs, and major changes in service conditions can lead to the generation of thermal and/or mechanical stresses and strains, which will lead to cracking and ultimately failure of the temperature cycled components.
Although the number of cycles is not very great, the strain range in each cycle can (at least locally) be large enough to cause failure in a limited number of cycles. This strain-controlled fatigue type is often referred to as low-cycle fatigue (LCF), while the common stress-controlled fatigue is called high-cycle fatigue (HCF).
Hardness is a measure of how resistant a material is to a permanent shape change when a force is applied. For metals, indentation hardness is typically used. It measures the resistance to deformation due to a constant compression load from a sharp object normal to the metal surface.
Proof strength, Rp0.2
The engineering stress level that gives 0.2% permanent engineering strain after loading up to Rp0.2. This is defined as the start of plastic deformation for stainless steel. The proof strength at 1% (Rp1.0) is also commonly used for the austenitic grades.
In the elastic region an imposed strain is fully recovered upon unloading, while in the plastic region, only the elastic part of the strain is recovered.