Stainless steels could be divided into five categories: 1. Ferritic Stainless Steels 2. Martensitic Stainless Steels 3. Austenitic Stainless Steels 4. Duplex Stainless Steels 5. Precipitation Hardenable Stainless Steels.
Type # 1. Ferritic Stainless Steels:
These are iron-chromium alloys having chromium from 11.5 to 27% but usually between 17 to 26%. These alloys are ferritic in structure up to the melting point.These may contain small additions of manganese, silicon, nickel, aluminium, molybdenum and titanium.
The carbon content is kept as low as economically feasible (0.08 to 0.2%) to improve toughness and reduce sensitization. Newer steel refining methods are used. Argon-oxygen (AOD) refines the carbon below 0.03%. Other processes such as VOD, VIM are also used with same aim. Electron beam vacuum melting can reduce the carbon further (0.002%). As the carbon content of the ferritic steels is low, these are not high strength steels.
The yield strength in annealed state could be 275-415 MPa, and as these steels work-harden less, so the tensile strengths lie between 500-600 MPa. Molybdenum may be added to increase the corrosion resistance particulary pitting and local corrosion. Niobium and titanium are added to steels to stablise against intergranular corrosion. Table 12.18 gives composition of some standard ferritic steels, whereas table 12.19 gives their properties in annealed state.
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The following simple relationship holds good to know the ferritic type of stainless steel:
(Cr% – 17×C) > 12.7 … (12.3)
Type 430 is a general purpose grade used in chemical industry (where weldability is not required)
Advantages of Ferritic Stainless Steels:
1. As expensive nickel is not added, ferritic stainless steels are much cheaper than austenitic stainless steels and have replaced them for domestic, atmosphere and catering-type atmospheres such as for washing machine tubs, sinks, drainers, hollowware’s and in chemical and food industries, ducts, screens, roofing’s; for handling fresh water (Type 444), even sea water for condenser tubing’s (SEA-CURE, AL-4-2, Al 20-4); heat exchangers (E Brite).
2. The most important useful property of these steels is corrosion resistance which increases with the increase of chromium content, i.e., the increase of chromium increases the range of conditions in which the passive film (Cr2CO3) is stable. Molybdenum increases the stability of this film. Ferritic steels are highly resistant to rural and urban atmospheres.
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3. One of the major advantages of ferritic steels over austenitic stainless steels is their complete freedom from stress corrosion. Thus, these steels are used in chemical plants as these have immunity to chloride stress-corrosion cracking.
4. Ferritic stainless steels have reasonable cold-formability. Simple designs could be easily made from them.
5. Ferritic stainless steels have in general excellent hot-ductility.
6. Chromium-rich ferritic steels have good oxidation resistance at high temperatures to be used as heat-resisting grades in the making of furnace components.
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7. Good machinability, higher thermal conductivity, lower thermal expansion than austenitic stainless steels.
8. These are magnetic in nature.
Disadvantages of Ferritic Stainless Steels:
1. Ferritic stainless steels get corroded in chloride and sulphur dioxide containing industrial and marine atmospheres. The 17% Cr-ferritic steels are less corrosion resistant than austenitic stainless steels in reducing atmospheres.
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2. As no phase change occurs on heating in fully ferritic stainless steels, grain refinement is difficult. Grain growth is rapid on heating in such a single phase microstructure as diffusion too is faster in BCC structure. Ferritic stainless steels start coarsening rapidly at 600°C compared to 900°C for austenitic stainless steels. Thus, after a high temperature treatment such as welding, the grains become very coarse and are difficult to be refined. It is a major drawback.
3. As ferritic stainless steels have BCC structure, they show a ductile to brittle transition, and this impact-transition temperature is considerably higher than that for mild steel due to embrittlement effect of chromium dissolved in ferrite. A 25% chromium steel will be brittle even at room temperature if the carbon content exceeds 0.03%. As ferrite grains cannot be refined, the presence of coarse grains further raises the ductile to brittle transition temperature.
4. As ferritic stainless steels have BCC structure, these steels have lower general ductility and higher yield strength than austenitic stainless steels. The ferritic steels are inferior where stretch-forming is required such as deep drawing for applications such kitchen sinks, domestic catering utensils, although ferritic steels can be readily coined, embossed, cold forged, or spinning can be done easily.
5. Ferritic stainless steels show stretcher strains during drawing or stretching. A more serious problem common in ferritic stainless steels is ‘ridging, or roping’, which is not due to yield point phenomenon but to a crystallographic textural effect. Rope marks are depressions on one side of the sheet, matching an elevation on the other side; the thickness remains constant and the markings are parallel to the rolling direction. These mar the appearance of parts formed from sheet.
6. Apart from the natural brittleness of ferrite stainless steels, two other embrittlement effects occur. Chromium-rich ferritic stainless steels having 17% to 25% chromium suffer from ‘475°C embrittlement’, i.e., when the steels are in temperature range of 400°C to 500°C, embrittlement decreases the general ductility with drastic drop in impact properties. This probably is due to the precipitation of very fine coherent chromium-rich, alpha prime particles arising from the miscibility gap in Fe-Cr system.
This phenomenon is more pronounced with the increase of chromium content. This is also true of the formation of sigma phase, the second cause of embrittlement in ferritic as well as in austenitic stainless steels. Its formation occurs in between 500 and 900°C. This intermetallic phase is a hard brittle precipitate in the matrix and has low toughness and is bad to corrosion resistance. But it is a very sluggish reaction.
7. Ferritic stainless steels suffer from intergranular corrosion in the heat-affected zone of the weld as in austenitic stainless steels due to the precipitation of chromium carbides. Sensitisation occurs when steels heated above 900°C, and cooled slowly. It occurs rapidly in ferritic than in austenitic steels as diffusion is faster due to its BCC structure. Sensitisation is removed by annealing in range 650-850°C to allow chromium to diffuse back to depleted regions or stabilisation can be done by Ti or Nb.
Welding of ferritic stainless steels also causes problems due to excessive grain growth in the heat-affected zone. Grain refinement is difficult in these steels due to the absence of phase change in these steels. Arc welding (inert-gas TIG and MIG processes), plasma and electron-beam welding give good results. New low-interstitial alloys containing titanium, or niobium have been seen to be readily weldable.
Type # 2. Martensitic Stainless Steels:
These are the heat treatable type of stainless steels, and normally contain 12-17% chromium and 0.10 to 1.20% carbon.
The following relationship tells the composition in which austenite to martensite transformation occurs:
% Cr – 17 (% C) ≤ 12.7 …(12.4)
These steels are austenitic at temperatures of 950-1000°C, but transform to martensite on cooling, i.e., these steels can be hardened and tempered to obtain yield strength of 550-1860 MPa. As the chromium content increases, the hardenability increases to make some of these steels as air- hardening even in large sections. Such steels have problems in making them soft for machining and fabrication purposes.
Martensitic stainless steels can be broadly divided into two categories:
i. Low Carbon High Strength Martensitic Stainless Steels:
As these steels are primarily high strength structural steels, and thus should possess good weldability, formability and impact toughness. These steels find applications in petro-chemical and chemical plant construction, gas-turbine engines, turbine blades, electrical generation plant, compressors and discs and a variety of aircraft structural and engine applications, propeller shafts in ships sailing in fresh water.
The carbon content of these steels is kept low (≈ 0.10%) to make the steels have good weldability, formability and impact strength. Such steels are quenched in oil, or air from around 1050°C (when it is fully austenitic), and then tempered. The tempering temperature is kept low for high tensile as well as yield strength with low toughness. The steels are tempered at high temperature to obtain higher toughness.
New low carbon high strength martensitic steels have been developed. The carbon is kept low ≈ 0.1%. With such low carbon, the tensile strength of martensite is 1300 MN/m2. As it should not decrease much with tempering, alloying elements should be added in steel to obtain maximum tempering resistance The alloy steels should be fully austenite at around 1050°C, which should be made to transform to fully martensitic structure prior to tempering.
The content of alloying elements should be chosen to have this structure, referring to Schaeffler diagram. The addition of alloying elements should not depress Ms temperature to below room temperature to avoid distortion of parts and to avoid getting retained austenite. Also, elements should be added in large amount which lower.
A1 temperature as the steel has to be tempered at the highest tempering temperature to obtain maximum toughness without the reformation of austenite. Tempering range of 440 to 540°C is avoided at it causes reduction in impact strength.
ii. High Carbon High Hardness Martensitic Steels:
The strength and hardness of the martensitic stainless steels can be increased by increasing the carbon content of the steels, but it is at the expense of weldability, toughness and even corrosion resistance. Increased carbon increases the amount of the carbides, and thus higher austenitising temperatures have to be used to dissolve them, which gives lower impact properties.
For Example:
(i) Cutlery stainless steel has 0.3%C and 12% chromium and attains a hardness of 400 VPN after hardening and tempering, but precipitation of Cr23C6, takes place at austenitic (original) grain boundaries, which causes pitting commonly seen in stainless steel knives with brazed handles. This steel is also used for making gears, stainless steel bearings, needle valves and components for high temperature applications.
(ii) Stainless steel razor blades contain 0.6 to 0.7%C and 16-18% Cr.
(iii) Surgical Stainless steel- 0.95-1.20%C; 16-17% Cr. The hardness after tempering is 600-700 VPN.
The steel is used for surgical implements, scalpels, coal hammers; ball bearings for high temperature applications.
Type # 3. Austenitic Stainless Steels:
Stainless steels having 16-25% chromium and sufficient amount of austenite-stabilising elements like nickel, manganese, or nitrogen, so that the steels are austenitic at room temperature, are called austenitic stainless steels. Simple austenitic AISI 300 series has nickel as the austenite stabilising element, and normally the compositions have 16-25% Cr, 8-20% Ni and 0.03- 0.10%C. When nickel is partly or fully replaced by cheaper manganese.
As the nickel content increases in steel, the amount of austenite present at the solution-treatment temperature increases, and the Ms temperature is lowered, so that at 8% nickel, the Ms temperature is just below room temperature.
Thus, stainless steel having 0.1 %C, 18% Cr, 8% Ni as illustrated in Fig. 12.4, when heated for solution treatment between 1050-1100°C dissolves all the carbon in austenite, and the rapid cooling from this temperature range results in supersaturated austenite solid solution at room temperature.
This rapid cooling step is of utmost importance as slow cooling, or reheating within the range 550-800°C causes the precipitation of chromium carbide, Cr23C6, even when the carbon is below < 0.05%.
This structure is not fully austenitic but has carbides which cause intergranular corrosion. As Ms temperature normally is just below room temperature, austenite may transform to martensite either by sub- zero treatment, or during cold working. Schaeffler diagram is commonly but carefully used to choose the composition.
Characteristics of Austenitic Stainless Steels:
1. As austenitic stainless steels have FCC structure:
(a) They are non-magnetic in nature.
(b) They are tough even at low temperatures as there is no ductile to brittle transition temperature.
(c) They have good ductility with elongation of about 50% in tensile tests.
2. Austenitic stainless steels find wide applications due to excellent corrosion resistance in normal atmospheres and a wide range of corrosive media. Chlorides in marine atmospheres decrease the corrosion resistance as ions penetrate the passive film and cause pitting corrosion (Pitting is a form of very localized corrosion in atmospheres having very aggressive ions.
Type 316 steels containing 2-3% molybdenum show better general corrosion resistance as well as in chloride medium to pitting corrosion. New steels having up to 6.5% Mo with chromium 20% and nickel 24% show excellent pitting resistance).
3. The Cr/Ni austenitic steels are also very resistant to high temperature oxidation because of the protective surface film.
4. Austenitic stainless steels are prone to stress corrosion cracking in presence of chloride ions even when present in small amount of few ppm. This failure can occur in mild corrosive atmosphere in the presence of small stresses (which are deliberately applied, or even the residual stresses present in fabricated materials).
The fracture is transgranular with little or no plastic deformation but more easily developed in hot chloride solution. This corrosion can be substantially reduced in high nickel (> 30% Ni) austenitic alloys, or reduce the stress as well as eliminate chloride ions from the atmosphere.
5. As these steels are single phase FCC materials, these are not very strong materials.
These steels can be strengthened by:
(a) Cold Working:
The work- hardening rate of these steels is very high because of low stacking fault energy of 0.002 J/m2. The cold working increases the low yield strength of 240 MPa to 1035 MPa and the tensile strength of 585 MPa gets doubled, when cold worked by 60%. But cold working produces only thin flat rolled products or wires, and one of the main disadvantages is that strength is lost at temperatures above 600°C and also in the heat-affected zone of a weld.
(b) Solid Solution Strengthening:
Substitutional solutes show little increase of strength of the steels. Interstitial solutes are very effective. As an increase in carbon content also causes deleterious effects due to the precipitation of chromium carbides, the amount of nitrogen can be increased but it requires simultaneous presence of elements like manganese. This has resulted in the development of AISI 200 series of Cr-Mn-Ni-N steel (table 12.23).
6. To improve the deep-drawability of austenitic stainless steels, the formation of martensite should be inhibited. This requires the use of elevated deep drawing temperatures (above Md temperature), and also by the use of more stable austenitic steel produced by the control of composition. Warm working at temperatures between 700 and 900°C can be done. This process can be made more useful by adding small amount of Nb to the steel to inhibit recrystallisation by the precipitation of niobium carbide or nitride.
The steel can be reheated to about 850°C after rolling. The finely dispersed carbides, or nitrides block the motion of creep dislocations. Thus, austenitic stainless steels could be used from – 196°C to 800°C. An alloy having 25% Cr and 20% Ni with additions of titanium, or niobium has good creep strength at high temperature of 700°C.
7. As these steels are single phase materials, these can be easily welded. But corrosion along grain boundaries-intergranular corrosion-occurs due to the precipitation of Cr23C6, in these regions-called weld decay, Steel 304 and 316 have been made to have carbon less than 0,03% and are designated as 304L and 316 L to take care of weld decay. In steels, 321 and 347, niobium or titanium is added (to combine with all the carbon) in excess of the stoichiometric amounts to take care of weld decay. It has been seen that stabilisation is rarely entirely effective, and it is better to use steels of lower carbon content than 0.03%.
8. As austenitic stainless steels have high nickel content, these are expensive steels. But as have best corrosion resistance, these are used for general purpose, house-hold utensils, and structural purposes. Stabilised and molybdenum grades find applications in chemical industry, and welding purposes. High chromium grades have high oxidation and scaling resistance to find applications in steam pipes, boiler tubes, radiant super-heater tubes, furnace parts, etc.
9. Austenitic stainless steels don’t have 475°C embrittlement, but these steels on heating at 700-950°C show reduced ductility and toughness due to the formation of brittle intermetallic compound called sigma-phase, the amount of which increases with the increase of chromium content of steels, and presence of elements like molybdenum, titanium and silicon. Sigma phase causes brittleness when the steel is at lower temperatures. Manganese helps to reduce the formation of sigma- phase.
Alloys 904L, A1-6X and 254 SMo have been developed for sea water uses to avoid pitting, crevice, etc. Nitrogen substituting costly nickel suppresses the occurrence of sigma-phase precipitation and improves corrosion resistance. High Mo stainless steels are used for brackish, or sea water cooling heat exchangers/condenser tube applications for power stations and as material for off-shore platform process piping, and other industries such as paper and pulp, etc.
Type # 4. Duplex (Ferritic-Austenitic) Stainless Steels:
These stainless steels contain ferrite and austenite in microstructure (and thus, combine the toughness and weldability of austenite with strengths and resistance to localised corrosion of ferrite), the exact proportion of the phases is controlled by the heat treatment.
However, the duplex structure is obtained by making a proper balance between chromium equivalent elements and the nickel equivalent elements as per figure 12.5. Normally the chromium content is 23-30%; Ni = 2.5-7% and some titanium or molybdenum.
Characteristics:
I. Duplex steels are stronger than the simple austenitic steels:
1. A two phase structure is stronger than single phase. An increase of 1% delta-ferrite increases the tensile strength by ~ 2.2 MN/m2, and the 0.2% proof stress by ~ 2.5 MN/m2. The ultimate tensile strength at room temperature rises to a maximum at about 70 to 80% ferrite and then, decreases.
The reason is the higher rate of work hardening of austenite, which though has lower yield strength but higher tensile strength than ferrite. Stainless steel AISI 329 (26% Cr. 4% Ni, 1.5% Mo) attains a minimum yield strength of 435 MPa Steel IN 744 (0.05% C. 26% Cr, 6.5% Ni. 0.3% Ti) attains a 0.2%- proof stress of 570 MN/m2, and tensile strength of 740 MN/m2 with % elongation of 24%.
2. The presence of delta-ferrite causes grain-refinement of the austenite, which produces additional strengthening.
3. Further refinement in grain size is obtained by using suitable thermo-mechanical treatment, say, controlled rolling treatment by hot working in range of 900-950°C, or even lower temperatures. This results in a very fine dispersion of ferrite and austenite grains-called micro-duplex steels-having grain size 3 to 10 µm. Steels in such a state exhibit super-plasticity, i.e., very high ductilities of about 500% at high temperature of near 950°C.
II. Duplex steels have good corrosion resistance similar to austenitic stainless steels.
III. Duplex steels have freedom from transgranular stress corrosion cracking, as the ferrite phase is immune to this type of failure.
IV. These steels have good weldability, but micro-duplex structure is destroyed in heat-affected zone, which decreases the strength as well as stress-corrosion resistance. However, duplex steels don’t suffer from intergranular corrosion.
V. Due to presence of ferrite, duplex steels also have ductile to brittle transition temperature.
VI. These steels suffer from both types of embrittlement effects- 475°C embrittlement as well as due to the formation of sigma phase.
Some more compositions of Duplex steels are given above. Nitrogen is added to good great advantage as it reduces partitioning of chromium, and also enhances pitting resistance of austenitic phase. It also provides superior resistance to stress corrosion cracking particularly in chloride and sulphide bearing aqueous solutions.
Nitrogen also reduces deleterious effects of welding. Such steels, SAF 2205, or AF 22, finds wide applications for tubing in corrosive oil and gas wells. Ferralium 255 finds applications in handling brackish and salt waters.
5. Precipitation Hardenable Stainless Steels:
Precipitation hardenable stainless steels offer attractive combinations of properties, but these are quite expensive and quite often may be difficult to hot-process. As the steels may be required to be vacuum melted, these alloys are restricted for use to high strength to weight ratio applications, particularly for high temperatures such as the high-temperature power plant steels.
Age hardening takes place due to coherency strains and general dispersion-strengthening. Thus, its effects can be increased by increasing the volume fraction of the precipitate or by intensifying the coherency strains by increasing the misfit between the zones and the matrix. Also the rate of overageing should be minimised.
The matrix in precipitation hardenable stainless steels could be:
1. Austenite, which additionally offers being non-magnetic needed for some applications. Such a steel, at least, has 10% Ni or, nickel + manganese, so that even after precipitation of intermetallic compounds, there is enough nickel, or nickel + manganese to keep the steel austenitic. Ni is normally 20%. The strengthening elements which have been used could be P, Mo, Cu, Nb, or Ti, but commonly, it is Ti and Al.
The alloys are solutionised at about 1200°C, and then aged at 700-800°C to result in yield strength of over 700 MPa. As the ageing temperature is quite high, these steels could be used for high temperature applications. The precipitating intermetallic phase is Ni, (Al Ti). Table 12.24 gives composition and properties of four such steels.
2. Martensite, in which lath martensite is obtained first, which provides some strength, but provides an abundance of nucleation sites for the precipitation of intermetallic compounds. Carbon is kept low (< 0.05%) to have good toughness. Nickel is lowered to between 4-7% and composition adjusted, so that the Ms temperature is between 125-250°C. The precipitate forming elements are Cu, Mo, Al, Ti, Nb, and N. The common precipitates are Ni3Ti, Ni3Al, NiAl, etc. After quenching, the steel is aged at 400-500°C.
The problem with such steels is that at the maximum ageing (tempering) temperature of around – 500°C, maximum toughness cannot be obtained, and the higher temperatures shall result in overageing to cause loss of strength. However by increasing molybdenum to 4% and with addition of cobalt, intermetallic compounds based on Mo- Cr-Co phase precipitate while being aged at 600-650°C to develop maximum hardness.
The carbon content is kept low (0.03-0.4%) as it does not take part in precipitation and also helps to have high ductility and toughness. Instead of molybdenum, the element tungsten results in better impact properties. Table 12.25 gives composition and properties of some such steels.
17-4PH: 0.07C 15.5-17.5Cr ……. 3/5 Ni, 1 Mn, 1 Si; Cu 3-5; (Nb + Ta) 0.15-0.45
PH 13-8: Mo 0.05C 12.75-13.25Cr, 2-2.5Mo-0.01N 7.5/8.5 Ni; Mn = .10; Si 0.1 Al – .9-1.35
Referring to Fe-Cr phase diagram (Fig. 12.2), it is clear that chromium stabilises ferrite, leading to the formation of closed-gamma loop with maximum point at 12.7% chromium at about 1000°C, i.e., when the chromium content is more than 12.7% in iron, ferrite phase becomes stable over the entire temperature range up to the melting point. In the presence of carbon, or nitrogen, which is strong austenite stabilisers, the solubility of chromium increases in austenite, i.e., expands the gamma loop.
Thus, in presence of carbon, complete ferritic stainless steel is obtained when Cr % – 17 times % C > 12.7. If this is less than 12.7%, then, it is martensitic stainless steel as martensite forms on cooling the high temperature austenite to room temperature.
Nickel stabilises austenite (gamma-phase), so that austenite is present at 30% Ni, or more from about 500°C to about 1450°C. When γ-phase is cooled, its transformation is very sluggish, so that alloys may be easily undercooled without transformation.
Because of sluggishness of reactions, the phase diagram is not able to predict the structure in an alloy even in Fe-Cr-Ni system. When nickel is added to a low carbon iron-18% Cr alloy the field of γ-phase is expanded until at about 8% Ni, the y-phase persists to room temperature leading to very well-known group of austenitic steels based on 18% Cr and 8% Ni. This particular composition is important as minimum nickel content is required to retain the austenite at room temperature.
Nickel content has to be increased to retain austenite at room temperature when chromium is less, or more than 18%. A stable austenite is defined as one in which the Ms temperature is lower than room temperature, which is true for 18/8 steel, but cooling below it may lead to the formation of martensite.
For example, the carbide dissolves in austenite in steel when heated to above 1100-1050°C, and quenching from this temperature avoids the formation of carbide to result in only austenite at room temperature, but if heated again in the range of 550-750°C, the carbide precipitates preferentially at the grain boundaries.
This precipitation renders the steel susceptible to intergranular corrosion as it depletes chromium from the grains. Addition of nickel to obtain austenitic steel yields a steel of higher ductility and impact strength. Addition of nickel helps to increase the corrosion resistance of steel against weakly oxidising acids, or even reducing acids as well as neutral chloride solutions. Ni imparts ease-of weldability.
Manganese also stabilises austenite but is half as effective as nickel. It may be added (1-2%) to partly substitute the expensive nickel in austenitic stainless steels. Normally steel having 12-15% chromium needs 12-15% manganese to become fully austenitic provided the carbon content is kept low. Manganese increases, the hot-workability of austenitic steels without reducing the corrosion resistance of the steels.
Nitrogen too, is a strong and cheap austenite stabliser (increases strength and hardness), and also helps to reduce intergranular corrosion and pitting corrosion. Nitrogen up to 0.25% may be present in austenitic stainless steels. As it is a very effective solid solution strengthener, it increases the proof stress of the steels.
The presence of molybdenum in stainless steels improves the resistance to sulphuric, sulphurous, organic acids as well as to halogen salts. Molybdenum increases resistance to pitting in sea water, and also crevice corrosion. Niobium and, or titanium takes care of carbon and nitrogen in steels.
Some other elements are added to improve the machinability (sulphur, selenium, and lead), oxidation resistance (aluminium, silicon), processing characteristics (vanadium, zirconium, rare earths, and boron), and resistance to sulphuric acid (copper).
It is very common to represent the effect of alloying elements on the microstructure of the stainless steels by Schaeffler diagram (Fig. 12.5). This diagram illustrates the phases present at room temperature in such steels as a function of nickel equivalent and chromium equivalent.
Nickel equivalent is empirically determined by austenite-stabilising elements such as:
Ni equivalent = Ni + Co + 0.5 (Mn) + 0.3 (Cu) + 25 (N) + 30 (C) …(12.1)
and chromium equivalent is obtained by ferrite-stabilising elements in steel as:
Cr equivalent = Cr + 2 (Si) + 1.5 (Mo) + 5 (V) + 5.5 (Al) + 1.75 (Nb) + 1.5 (Ti) + 0.75 (W) …(12.2)
Although this diagram has been originally intended for welding, but helps to estimate the phases present in stainless steels at room temperature.
Oxidation Resistance:
When a clean surface of stainless steel is exposed to an oxidising atmosphere, chromium having more affinity for oxygen than iron has gets selectively oxidised to form a thin, passive, impervious and transparent oxide film of Cr2O3, if chromium is more than 11.5%. This film protects the steel from further oxidation.
As the temperature increases, thick scale rich in Cr2CO3 forms which depletes chromium from the neighbouring region. To prevent oxidation of iron, increased amount of chromium should be present in the original Steel to maintain this Cr2O3 film as illustrated in Fig. 12.6, i.e., higher is the chromium content in steel, higher is the resistance to oxidation. The presence of nickel increases the maximum service temperature of the stainless steels.
Welding Characteristics:
The heat-affected zone during and slow cooling after welding attains a temperature in the range of 500-800°C, when chromium carbide (Cr23C6) form mainly at the grain boundaries even when the carbon content of the steel is < 0.05%. These precipitates not only decrease the low temperature ductility, but cause more serious troubles by depleting the region of chromium, which then becomes more prone to corrosive attack to cause intergranular corrosion to lead to disintegration of the stainless steels. This is commonly called ‘weld decay’, or ‘sensitization’.
Some of the methods of de-sensitization or elimination of the formation of Cr23C6, or ‘stabilisation of steels’ are:
1. Use stainless steels of very low carbon content (< 0.02%), so that there is not much carbon for chromium to form chromium carbide. Such as steels like, 304 L and 316 L.
2. Heat the welded steel to 950°-1100°C (so that the carbides dissolve again) and cool rapidly (to avoid their re-precipitation).
3. Use stainless steel having strong carbide forming elements (than chromium) like niobium, titanium. These elements form their carbides without depleting the steel of chromium. Some such stabilised grades are 321, 347.
4. Addition of molybdenum to steel increases the sensitisation time, so that in a shorter time than this Cr23C6 does not form.
5. ‘Healing Treatment’ is annealing of welded part at about 900°C to allow chromium to diffuse back into depleted regions.