Austenitic stainless steels are often chosen for: refrigeration plants; the processing, storage and transportation of liquefied gases; and structural applications at very low temperatures.
Carbon steels and ferritic, martensitic and duplex stainless steels undergo a transition from ductile to brittle failure as the temperature decreases. Although the strength of these materials is unaffected or may increase as the temperature decreases, they may fail without warning, particularly under impact loads. This lack of toughness restricts their use to temperatures above the ductile to brittle transition temperatures (DBTT).
The exact temperature at which this transition from ductile to brittle failure occurs in ferritic steels is determined by a variety of factors, including the composition, the grain size and impurity levels.
Wide variations in DBTT can occur. The common ferritic stainless steels generally have DBTTs a little below 0ºC.
Austenitic materials do not undergo brittle failure, so can be used in structural applications at temperatures down to -276ºC. Austenitic stainless steels are often used, and easily retain their ductility, even when welded.
Metals designed for elevated temperature applications must survive corrosion arising from the high temperature environment and the effect the high temperature will have on the strength of the metal.
Stainless steels may be used at high temperatures for applications which are non load bearing (eg. furnace liners) or load-bearing (eg. furnace structures, pressure vessels). The maximum service temperature is usually limited by oxidation properties for non load-bearing applications, or by creep behaviour where the steel is load-bearing. Creep conditions are more severe and maximum service temperatures are lower.
Precipitation reactions at elevated temperatures, which can reduce corrosion resistance and/or toughness, may also need consideration when choosing a stainless steel grade.
Stainless steels are particularly resistant to oxidation effects. The chromium that provides the passive corrosion resistant film at room temperature also helps to resist oxidation at elevated temperatures. The generally accepted temperatures for the use of stainless steels in air are given in Table 5 on page 12.
This table is based on an oxidation rate of 10 milligrams per square centimetre (10 mg/cm2) in 1000 hours. The variation with intermittent temperatures is related to the different expansion coefficients of the base metal and the scale. The nickel content of the austenitic steels provides a higher spalling resistance and hence this effect is not quite as significant with these steels.
These values should only be taken as a guide. The effect of minor variations in temperature cycling and atmosphere composition, particularly sulphur bearing gases, can have a marked effect on high temperature corrosion resistance.
Final selection should be based on tests carried out under the conditions that apply in the actual environment.
The strength of a metal at room temperature does not accurately reflect the strength at high temperatures.
At elevated temperatures, time and temperature both affect the strength of the metal. The effect of time introduces creep (the failure of a metal at a load below its room temperature strength).
The creep strength of a metal can be expressed in two ways:
Although not classified as creep resisting materials, stainless steels all have relatively high creep strengths, particularly the “H” or higher carbon grades, and are often used at intermediate temperatures because of this.
Both the ferritic and duplex steels can give problems at temperatures from 370 to 540ºC. There is a change in structure of the ferrite that gives an increase in hardness but a major decrease in room temperature toughness. The change is sometimes referred to as 475ºC embrittlement.
Ferritic steels containing higher chromium and duplex stainless, particularly those with molybdenum, can also be prone to the formation of another embrittling phase, sigma, if they are held for prolonged periods above approximately 560ºC. Some austenitic grades can also suffer from sigma phase embrittlement.
A similar problem with a further embrittling phase, chi, is only found in the molybdenum containing ferritic stainless steels. Austenitic grades will suffer carbide precipitation during service for extended periods in the temperature range 425 to 875ºC, unless stabilised with niobium or titanium (e.g. grades 321 and 316Ti).
Carbide precipitation may reduce toughness but, in practice, reduced resistance to intergranular corrosion in sulphur dioxide, chloride or related environments at room temperature may be more important. L grades, with carbon restricted to ≤ 0.03% will give better performance in this respect than the standard grade or H grade.
Martensitic and precipitation hardening steels are also unsuitable at higher temperatures. This is predominantly related to the effect the increased temperature has on tempering the steel.
Similarly, the austenitic steels that have been strengthened by cold working cannot be held at high temperatures without a loss of strength due to the annealing effect on the cold worked structure.