Typical chemical composition is given in the table below. The chemical composition is given as % by weight.

Whilst Outokumpu high temperature steels are mainly optimised for oxidation and high temperature corrosion resistance, they also have good mechanical properties, partly due to their austenitic structure and partly due to certain alloying elements. Design values are usually based on minimum proof strength values for constructions used at temperatures up to around 550°C. For higher temperatures, mean creep strength values are used.

¹⁾Elongation according to EN standard:
A₈₀ for thickness below 3 mm.
A for thickness = 3 mm.
Elongation according to ASTM standard A_2” or A₅₀.

Aqueous corrosion

Since most high-temperature materials are optimised with regard to strength and corrosion resistance at elevated temperatures, their resistance to electrochemical low-temperature corrosion may be less satisfactory. Components made of high-temperature material should therefore be designed and operated so that acid condensates are not formed, or at least so that any such condensates are drained away.

The resistance of a material to high-temperature corrosion is in many cases dependent on its ability to form a protective oxide layer. In a reducing atmosphere, when such a layer cannot be created (or maintained), the corrosion resistance of the material will be determined by the alloy content of the material.

When a material is exposed to an oxidising environment at elevated temperatures, a more or less protective oxide layer will be formed on its surface. Even if oxidation is seldom the primary cause of high-temperature corrosion failures, the oxidation behaviour is important, because the properties of the oxide layer will determine the resistance to attack by other aggressive elements in the environment.The oxide growth rate increases with increasing temperature until therate of oxidation becomes unacceptably high or until the oxide layer begins to crack and spall off, i.e. the scaling temperature is reached. The alloying elements that are most beneficial for oxidation resistance are chromium, silicon, and aluminium. A positive effect has also been achieved with small additions of so-called (re)active elements, e.g. ttrium, hafnium, rare earth metals (REM, e.g. Ce and La). These affect the oxide growth so that the formed layer will be thinner, tougher, and more adherent and thus more protective.

Various sulphur compounds are often present in flue gases and other process gases. As a rule, they have a very detrimental effect on the useful life of the exposed components. Sulphides can nucleate and grow due to kinetic effects even under conditions where only oxides would form from a thermodynamic point of view. In existing oxide layers, attacks can occur in pores and cracks. It is therefore essential that the material is able to form a thin, tough, and adherent oxide layer. This requires a high chromium content and preferably also additions of silicon, aluminium, and/or reactive elements.

PRE Pitting Resistant Equivalent calculated using the formula: PRE = %Cr + 3.3 x %Mo + 16 x %N
CPT Corrosion Pitting Temperature as measured in the Avesta Cell (ASTM G 150), in a 1M NaCl solution (35,000 ppm or mg/l chloride ions).
CCT Critical Crevice Corrosion Temperature is the critical crevice corrosion temperature which is obtained by laboratory tests according to ASTM G 48 Method F

For more information see Outokumpu Corrosion Handbook or contact Outokumpu.

Data according to EN 10088

*) Austenitic stainless steel grades may be magnetizable to a certain degree after cold deformation, e.g. in temper rolled condition.

Like other austenitic steels, heat-resistant steels can also be formed in cold condition. However, as a result of their relatively high nitrogen content, the mechanical strength of certain steels is higher and consequently greater deformation forces will be required.
Machining
The relatively high hardness of austenitic steels and their ability to strain harden must be taken into consideration
in connection with machining. For more detailed data on machining, please contact Outokumpu, Avesta Research Centre.
Welding
High temperature constructions are frequently exposed to thermal fatigue due to variations of temperature. For this reason it is very important to design the welded joint without notches. Furthermore it is important that welds have oxidation resistance and creep strength compatible with the parent material.
To ensure weld metal properties (e.g. strength, corrosion resistance) equivalent to those of the parent metal, a filler metal with a matching composition should preferably be used. Recommended filler metal is 25 20. In some cases, however, a differing composition may improve e.g. weldability or structural stability. Gas shielded welding has resulted in the best creep properties for welds. 4845 is a fully austenitic steel and susceptible to hot cracking. Due to this, the heat input should be limited to maximum 1,0 kJ/mm. For this reason SAW should be avoided. The use of filler and a basic flux/coating will reduce the risk of hot cracking.
When welding heat resistant stainless steel to carbon steels, 23Cr 12Ni filler can be used. Nickel-based filler may be a better alternative if there is a high risk of loss of strength in the HAZ of the carbon steel. The reason is that if carbon in the construction steel may diffuse into the low carbon weld metal, the HAZ in the carbon steel well lose strength.
Repair welding of exposed and damaged high temperature equipment is easily performed with MMA. Before welding, it is important to remove all magnetic areas close to the weld joint since these may contain embrittling phases. Machining or grinding is suitable methods.

More detailed information concerning welding procedures can be obtained from the Outokumpu Welding Handbook, available from our sales offices.

The most commonly used international product standards are given in the table below.