Introduction to Stainless Steel
The commonest austenitic steel is so-called 18/8 containing around 18% Cr and 8% Ni. It has the lowest nickel content concomitant with a fully austenitic structure. However in some circumstances, e.g. after deformation, or if the carbon content is very low, it may partially transform to martensite at room temperature. Greater stability towards the formation of martensite is achieved by increasing the nickel content, as illustrated in the 301 to 310 types of steel. 18/8 stainless steel owes its wide application to its excellent general resistance to corrosive environments. However, this is substantially improved by increasing the nickel content, and increasing the chromium gives greater resistance to intergranular corrosion.
Austenitic steels are prone to stress corrosion cracking, particularly in the presence of chloride ions where a few ppm can sometimes prove disastrous. This is a type of failure which occurs in some corrosive environments under small stresses, either deliberately applied or as a result of residual stresses in fabricated material. In austenitic steels it occurs as transgranular cracks which are most easily developed in hot chloride solutions. Stress corrosion cracking is very substantially reduced in high nickel austenitic alloys.
Type 316 steel contains 2-4% molybdenum, which gives a substantial improvement in general corrosion resistance, particularly in resistance to pitting corrosion, which can be defined as local penetrations of the corrosion resistant films and which occurs typically in chloride solutions. Recently, some resistant grades with as much as 6.5% Mo have been developed, but the chromium must be increased to 20% and the nickel to 24% to maintain an austenitic structure.
Corrosion along the grain boundaries can be a serious problem, particularly when a high temperature treatment such as welding allows precipitation of Cr23C6 in these regions. This type of intergranular corrosion is sometimes referred to as weld-decay. To combat this effect some grades of austenitic steel, e.g. 304 and 316, are made with carbon contents of less than 0.03% and designated 304L and 316L. Alternatively, niobium or titanium is added in excess of the stoichiometric amount to combine with carbon, as in types 321 and 347.
The austenitic steels so far referred to are not very strong materials. Typically their 0.2% proof stress is about 250 MPa and the tensile strength between 500 and 600 MPa, showing that these steels have substantial capacity for work hardening, which makes working more difficult than in the case of mild steel. However, austenitic steels possess very good ductility with elongations of about 50% in tensile tests.
The Cr/Ni austenitic steels are also very resistant to high temperature oxidation because of the protective surface film, but the usual grades have low strengths at elevated temperatures. Those steels stabilized with Ti and Nb, types 321 and 347, can be heat treated to produce a fine dispersion of TiC or NbC which interacts with dislocations generated during creep. One of the most commonly used alloys is 25Cr20Ni with additions of titanium or niobium which possesses good creep strength at temperatures as high as 700°C.
To achieve the best high temperature creep properties, it is necessary first to raise the room temperature strength to higher levels. This can be done by precipitation hardening heat treatments on steels of suitable composition to allow the precipitation of intermetallic phases, in particular Ni3(Al Ti).
The importance of controlling the γ-loop in achieving stable austenitic steels was emphasized. Between the austenite and δ-ferrite phase fields there is a restricted (α+γ) region which can be used to obtain two-phase or duplex structures in stainless steels. The structures are produced by having the correct balance between α-forming elements (Mo, Ti, Nb, Si, Al) and the γ-forming elements (Ni, Mn, C and N). To achieve a duplex structure, it is normally necessary to increase the chromium content to above 20%. However the exact proportions of α+γ are determined by the heat treatment. It is clear from consideration of the γ-loop section of the equilibrium diagram, that holding in the range 1000-1300°C will cause the ferrite content to vary over wide limits.
The usual treatment is carried out between 1050 and 1150°C, when the ferrite content is not very sensitive to the subsequent cooling rate The duplex steels are stronger than the simple austenitic steels, partly as a result of the two-phase structure and also because this also leads normally to a refinement of the grain size. Indeed, by suitable thermo mechanical treatment between 900°C and 1000°C, it is possible to obtain very fine micro duplex structures which can exhibit super plasticity, i.e. very high ductilities at high temperatures, for strain rates less than a critical value.
A further advantage is that duplex stainless steels are resistant to solidification cracking, particularly that associated with welding. While the presence of δ-ferrite may have an adverse effect on corrosion resistance in some circumstances, it does improve the resistance of the steel to transgranular stress corrosion cracking as the ferrite phase is immune to this type of failure.
There is another important group of stainless steels which are essentially ferritic in structure. They contain between 17 and 30% chromium and, by dispensing with the austenite stabilizing element nickel, possess considerable economic advantage. These steels, particularly at the higher chromium levels, have excellent corrosion resistance in many environments and are completely free from stress corrosion.
These steels do have some limitations, particularly those with higher chromium contents, where there can be a marked tendency to embrittlement. This arises partly from the interstitial elements carbon and nitrogen, e.g. a 25% Cr steel will normally be brittle at room temperature if the carbon content exceeds 0.03%. An additional factor is that the absence of a phase change makes it more difficult to refine the ferrite grain size, which can become very coarse after high temperature treatment such as welding. This raise still furthers the ductile/brittle transition temperature, already high as a result of the presence of interstitial elements. Fortunately, modern steel making methods such as argon-oxygen refining can bring the interstitial contents below 0.03%, while electron beam vacuum melting can do better still.
The ferritic stainless steels are somewhat stronger than austenitic stainless steels, the yield stresses being in the range 300-400 MPa, but they work harden less so the tensile strengths are similar, being between 500 and 600 MPa. However, ferritic stainless steels, in general, are not as readily deep drawn as austenitic alloys because of the overall lower ductility.
However, they are suitable for other deformation processes such as spinning and cold forging.
Welding cause’s problems due to excessive grain growth in the heat affected zone but, recently, new low-interstitial alloys containing titanium or niobium have been shown to be readily weld able. The higher chromium ferritic alloys have excellent corrosion resistance, particularly if 1-2% molybdenum is present.
Finally, there are two phenomena which may adversely affect the behavior of ferritic stainless steels. Firstly, chromium-rich ferrites when heated between 400 and 500°C develop a type of embrittlement, the origins of which are still in doubt.
The most likely cause is the precipitation of a very fine coherent chromium-rich phase arising from the miscibility gap in the Fe-Cr system, probably by a spinodal type of decomposition. This phenomenon becomes more pronounced with increasing chromium content, as does a second phenomenon, the formation of sigma phase. The latter phase occurs more readily in chromium-rich ferrite than in austenite, and can be detected below 600°C. As in austenite, the presence of sigma phase can lead to marked embrittlement.
Some austenitic steels are often close to transformation, in that the Ms temperature may be just below room temperature. This is certainly true for low-carbon 18Cr8Ni austenitic steel, which can undergo a martensitic transformation by cooling in liquid nitrogen or by less severe refrigeration. The application of plastic deformation at room temperature can also lead to formation of martensite in metastable austenitic steels, a transformation of particular significance when working operations are contemplated.
In general, the higher the alloying element content the lower the Ms and Md temperatures and it is possible to obtain an approximate Ms temperature using empirical equations. Useful data concerning the Md temperature are also available in which an arbitrary amount of deformation has to be specified. The martensite formed in Cr-Ni austenitic steels either by refrigeration or by plastic deformation is similar to that obtained in related steels possessing an Ms above room temperature.
Manganese can be substituted for nickel in austenitic steels, but the Cr-Mn solid solution then has much lower stacking fault energy. This means that the fee solid solution is energetically closer to an alternative close-packed hexagonal structure, and that the dislocations will tend to dissociate to form broader stacking faults than is the case with Cr- Ni austenite. Manganese on its own can stabilize austenite at room temperature provided sufficient carbon is in solid solution. The best example of this type of alloy is the Hadfield’s manganese steel with 12 % Mn, 1.2 % carbon which exists in the austenitic condition at room temperature and even after extensive deformation does not form martensite.
However, if the carbon content is lowered to 0.8%, then Md is above room temperature and transformation is possible in the absence of deformation at 77°K. Both ε and α’ martensites have been detected in manganese steels. Alloys of the Hadfield’s type have long been used in wear resistance applications, e.g. grinding balls, railway points, excavating shovels, and it has often been assumed that partial transformation to martensite was responsible for the excellent wear resistance and toughness required. However, it is likely that the very substantial work hardening characteristics of the austenitic matrix are more significant in this case.
In general, f.c.c metals exhibit higher work hardening rates than b.c.c metals because of the more stable dislocation interactions possible in the f.c.c structure. This results in the broad distinction between the higher work hardening of austenitic steels and the lower rate of ferritic steels, particularly well exemplified by a comparison of ferritic stainless steels with austenitic stainless steels.
The advantages obtainable from the easily fabricated austenitic steels led naturally to the development of controlled transformation stainless steels, where the required high strength level was obtained after fabrication, either by use of refrigeration to take the steel below its Ms temperature, or by low temperature heat treatment to destabilize the austenite. Clearly the Ms - Mf range has to be adjusted by alloying so that the Ms is just below room temperature. The Mr is normally about 120°C lower, so that refrigeration in the range -75 to -120°C should result in almost complete transformation to martensite.
Alternatively, heat treatment of the austenite can be carried out at 700°C to allow precipitation of M23C6 mainly at the grain boundaries. This reduces the carbon content of the matrix and raises the Ms so that, on subsequent cooling to room temperature, the austenite will transform to martensite. Further heat treatment is then necessary to give improved ductility and a high proof stress; this is achieved by tempering in the range 400- 450°C.
Another group of steels has been developed to exploit the properties obtained when the martensite reaction occurs during low temperature plastic deformation. These steels, which are called transformation induced plasticity (TRIP) steels, exhibit the expected increases in work hardening rate and a marked increase in uniform ductility prior to necking. Essentially the principle is the same as that employed in controlled transformation steels, but plastic deformation is used to form martensite and the approach is broader as far as the thermomechanical treatment is concerned.
In one process, the composition of the steel is balanced to produce an Md temperature above room temperature. The steel is then heavily deformed (80%) above the Md temperature, usually in the range 250-550°C, which results in austenite which remains stable at room temperature. Subsequent tensile testing at room temperature gives high strength levels combined with extensive ductility as a direct result of the martensitic transformation which takes place during the test.
For example, a steel containing 0.3% C, 2% Mn, 2% Si, 9% Cr, 8.5% Ni, 4% Mo after 80% reduction at 475°C gives the following properties at room temperature:
Higher strength levels (proof stress ~2000 MNm2) with ductilities between 20-25% can be obtained by adding strong carbide forming elements such as vanadium and titanium, and by causing the Md temperature to be below room temperature. As in the earlier treatment, severe thermostatically treatments in the range 250-550°C are then used to deform the austenite and dispersion strengthen it with fine alloy carbides. The Md temperature is, as a result, raised to above room temperature so that, on mechanical testing, transformation to martensite takes place, giving excellent combinations of strength and ductility as well as substantial improvements in fracture toughness.
• 0.2% Proof stress | 1430 MPa |
• Tensile strength | 1500 MPa |
• Elongation | 50 % |
Post a Comment
0Comments