Most decisionsabout which steel to use are based on a combination of the following factors:
1.What is the corrosive environment?
Atmospheric, water, concentration ofparticular chemicals, chloride content, presence of acid.
2.What is the temperature of operation?
High temperatures usually accelerate corrosion rates and thereforeindicate a higher grade. Low temperatures will require a tough austeniticsteel.
3.What strength is required?
Higher strength can be obtained from theaustenitic, duplex, martensitic and PH steels. Other processes such as weldingand forming often influence which of these is most suitable. For example, highstrength austenitic steels produced by work hardening would not be suitablewhere welding was necessary as the process would soften the steel.
4.What welding will be carried out?
Austenitic steels are generally moreweldable than the other types. Ferritic steels are weldable in thin sections.Duplex steels require more care than austenitic steels but are now regarded asfully weldable. Martensitic and PH grades are less weldable.
5.What degree of forming is required tomake the component?
Austenitic steels are the most formableof all the types being able to undergo a high degree of deep drawing or stretchforming. Generally, ferritic steels are not as formable but can still becapable of producing quite intricate shapes. Duplex, martensitic and PH gradesare not particularly formable.
6.What product form is required?
Not all grades are available in allproduct forms and sizes, for example sheet, bar, tube. In general, theaustenitic steels are available in all product forms over a wide range ofdimensions. Ferritics are more likely to be in sheet form than bar. Formartensitic steels, the reverse is true.
7.What are the customer’s expectationsof the performance of the material?
This is an important consideration oftenmissed in the selection process. Particularly, what are the aestheticrequirements as compared to the structural requirements? Design life issometimes specified but is very difficult to guarantee.
There may also be special requirementssuch as non-magnetic properties to take into account.It must also be borne inmind that steel type alone is not the only factor in material selection.Surface finish is at least as important in many applications, particularlywhere there is a strong aesthetic component. See Importance of Surface Finish.Availability. There may be aperfectly correct technical choice of material which cannot be implementedbecause it is not available in the time required.
Cost. Sometimes the correct technicaloption is not finally chosen on cost grounds alone. However, it is important toassess cost on the correct basis. Many stainless steel applications are shownto be advantageous on a life cycle cost basis rather than initial cost. See Life CycleCosting.
The final choice will almost certainlybe in the hands of a specialist but their task can be helped by gathering asmuch information about the above factors. Missing information is sometimes thedifference between a successful and unsuccessful application. See also Generalprinciples for selection of stainless steels.