OTELURI INOXIDABILEAusteniticeProf.Dr.Ing. Ionelia VOICULESCUSculptura Art Deco din otel inoxidabil austenitic la cladirea Niagara-Mohawk Power in Syracuse, New York

Scurt istoricAparitia otelurilor inoxidabile poate fi considerat un moment important al revolutiei industriale. Cu toate ca efectele dezastruoase ale coroziunii asupra fierului si aliajelor sale au fost cunoscute din cele mai vechi timpuri, numai la nceputul acestui secol s-au facut evaluarile cantitative ale acesteia.Speranta de a gasi o metoda de protectie a fierului care sa-i redea ntr-o maniera eficace si durabila rezistenta la coroziune, parea, pna n 1800, de nerealizat.Istoria otelurilor inoxidabile este marcata de perioada foarte mare care s-a scurs ntre momentul descoperirii proprietatilor remarcabile ale fierului aliat cu crom (metal descoperit n1797 de VAUGUELIN si izolat n 1854 de BUNSEN) si momentul n care s-au nceput primele studii stiintifice asupra acestui aliaj si exploatarea lui industriala. Putem compara aceasta situatie cu cea a penicilinei pe care FLEMING a descoperit-o n 1928, iar aplicatiile ei terapeutice au nceput dupa 13 ani adica n 1941. Pentru otelurile inoxidabile aceasta perioada a fost de aproape 90 de ani.

Scurt istoricPrimele observatii aupra proprietatilor chimice conferite fierului prin introducerea de crom au fost facute de BERTHIER n 1821. Primele compozitii de oteluri inoxidabile, datnd de la nceputul secolului, au fost semnalate simultan de GUILLET,CHEVANARD si PORTEVIN, n Franta, GOLDSCHMIDT, GIESEN si MONARTZ n Germania. Rezultatele lor au fost strans legate de descoperirea proprietatilor conferite fierului prin adaugarea unei cantitati suficiente de crom, care face ca aliajul sa reziste foarte bine n numeroase medii agresive, ceea ce n mod normal se numeste pasivitate.Dupa norma europeana EN10020, otelul inoxidabil este un aliaj de fier, crom si carbon, cu mai mult de 10,5%Cr si mai putin de 1,2%C. Alte elemente cum sunt nichelul, molibdenul, titanul s.a., pot fi adaugate pentru a conferi aliajului proprietati particulare.Denumirea de OTELURI INOXIDABILE este foarte restrictiva, ca si cele folosite n engleza (stainless: otel inatacabil) sau cea n germana (nicht rostende Stahl: otel care nu rugineste), denumiri care s-au impus la nceputul descoperirii lor datorita rezistentei evidente a acestor oteluri la coroziunea atmosferica. n prezent cnd spunem despre un otel ca este inoxidabil trebuie sa specificam: n raport cu care medii, lichide sau gazoase, n ce limita de temperatura si rezistenta la coroziune n comparatie cu otelurile ordinare sau cele slab aliate.

Domenii de utilizare

Domenii de utilizare

Domenii de utilizare

Domenii de utilizare

Domenii de utilizare

FAMILII DE OELURI INOXIDABILEn funcie de constituentul structural preponderent la temperatura camerei, exist urmtoarea clasificare a oelurilor inoxidabile:- oeluri martensitice;- oeluri feritice;- oeluri ferito austenitice (duplex);- oeluri austenito feritice;- oeluri integral austenitice.

InoxidabileSteels are said to be stainless when they resist corrosion; the is achieved by dissolving sufficient chromium in the iron to produce a coherent, adherent, insulating and regenerating chromium oxide protective film on the surface. It is not surprising therefore that they are used in the harsh environments of the chemical, oil production and power generation industries, and in utility goods such as furniture, automotive trims and cutlery, where both aesthetic appearance and corrosion resistance are important design criteria.The stainless character occurs when the concentration of chromium exceeds about 12 wt%. However, even this is not adequate to resist corrosion in acids such as HCl or H2SO4; higher chromium concentrations and the judicious use of other solutes such as molybdenum, nickel and nitrogen is then needed to ensure a robust material.There are requirements other than corrosion which have to be considered in engineering design. For this reason, there is a huge variety of alloys available, but they can be classified into four main categories:hardenable stainless steels; ferritic stainless steels; austenitic stainless steels; duplex stainless steels. Specialist grades include the precipitation hardened or oxide dispersion strengthened alloys.

FAMILII DE OELURI INOXIDABILE

Diagrama de echilibru Fe-Cr

Diagrama de echilibru Fe-Cr-CSectiune Verticala prin diagrama Fe-Cr-C pentru 0.1% C

Diagrama SchaefflerWithout carbon, the limit beyond which austenite no longer forms is about 13.5 wt% chromium. However, additions of carbon help stabilise the austenite and therefore increase this limit (Fig. 6).Chromium and nickel equivalents are also used in the welding industry to plot the microstructures obtained when a weld solidifies and cools to ambient temperature (Fig. 7). Although these diagrams are popular, it should be understood that they are not phase diagrams but rather represent the microstructures obtained under specific cooling conditions.Schaeffler diagram for weld metals.

Diagrama SchaefflerThe limits, for the chemical composition are:C < 0,2 % N = 0,05 % for % Cr 0 - 18 %S < 1 % 0,07 18 - 25 %Mn < 4 % 0,10 > 25 %Mo < 3 %Nb < 1,5 %

Diagrama SchaefflerThis diagram is interesting because, by quantifying the amount of types of structures (ferrite, martensite, austenite), it does give an indication that the material will comply with the standard. This is possible at a time that the metal is still in the melting furnace because it does use the chemical composition that is taken before pouring.In this way corrections of the chemical composition are still possible.It is very useful for austenitic stainless steels because the amount of ferrite must be restricted (material becomes magnetic) and for martensitic stainless steels because the amount of delta-ferrite must be controlled.

Diagrama SchaefflerAustenite : above the lines (0-25) till (16-12) till (36-36)Ferrite : under the line (11-0) till (36-9)Duplex : within the lines (36-36) till (16-12) till (22-4) till (36-9)Martensite : within the lines (0-7) till (3-0) till (7-0) till (12-8) till (0-19)Precipitation hardening types are mostly located in the zone with the presence of austenite + martensiet or austenite + martensite + ferrite.

Structuri comercialeAustenite : austenite + 0 5 - 10 % ferriteFerrite : ferrite + carbides (chromium-carbides, TiC, NbC...)Duplex : 40 - 60 % austenite, 60 - 40 % ferriteMartensite : martensite + (0 - 5 %) carbides + (0 - 15 %) delta-ferritePrecipitation-hardened : martensite + austenite + (ferrite) + intermetallic componentsA stainless steel needs a minimum of 12 % of chromium in the matrix. Due to this condition, the part of the graph below a chromium-equivalent of 12 % is not applicable.

Reguli de calculEXEMPLEDuring the calculation, the following rules will be applied:1. if the real chemical composition is not known, two calculations will be made: one with the minimum and one with the maximum values. Doing this, we obtain an area (zone), in which the real compositionwill be present.2. silicon: mostly the maximum is given. We will calculate with: minimum 0,3 % if the maximum is 1,0 % and minimum 0,6 % if the maximum is 1,5 or 2,0 %.3. manganese: we take a minimum of 0,6 % if only a maximum value is given.4. nickel and molybdenum: the minimum is 0 % if only a maximum is given.5. carbon: if only a maximum is given:minimum 0,01 % if the maximum is 0,04 % or lower andminimum 0,04 % if the maximum is higher as 0,04 %.

Exemplu1. GX 20 Cr 14C Si Mn Cr Ni0,16-0,23 1,0 max 1,0 max 12,5-14,5 1,0 maxCreq min = 12,5 + 0,3x1,5 = 12,95Creq max = 14,5 + 1,0x1,5 = 16,00Nieq min = 0,0 + 0,16x30 + 0,6x0,5 = 5,10Nieq max = 1,0 + 0,23x30 + 1,0x0,5 = 8,40This steel can be located in three area:martensite, ferrite + martensite andferrite + martensite + austenite.Due to the presence of mostly 5 to 15 % of chromium-carbides (carbides remove and chromium and carbon from the matrix), the commercially delivered steels will be located in the ferrite + martensite zone.

Exemplu2. GX 6 CrNiMo 18 10C Si Mn Cr Ni Mo0,07 max 1,5 max 1,5 max 18,0-20,0 10,0-12,0 2,0-3,0

Creq min = 18,0 + 2,0x1 + 0,6x1,5 = 20,90Creq max = 20,0 + 3,0x1 + 1,5x1,5 = 25,25Nieq min = 10,0 + 0,04x30 + 0,6x0,5 = 11,50Nieq max = 12,0 + 0,07x30 + 1,5x0,5 = 14,85LocalizareAustenitic sau austenito-feritic.

Diagrama de LongThis refines the Schaffler diagram by taking account of the strong austenite stabilising tendency of nitrogen. The chromium equivalent is unaffected but the nickel equivalent is modified toNi (eq) = Ni + (30 x C) + (0.5 x Mn) + (30 x N)The diagram, identifying the phase boundaries is shown below. This shows the ferrite levels in bands, both as percentages, based on metallographic determinations and as a ferrite number 'FN', based on magnetic determination methods.

Combined Schaeffler / WRC 1992 Constitution Diagram

Diagrama WRCWRC 1992 Constitution Diagram

Determinarea cantitatii de ferita deltaMagne GageFerrite Number versus Ferrite ContentThe Ferrite Number is not equal to the volumetric ferrite content (%). Although an absolute ferrite content can not be measured accurately,a reasonable estimate of the ferrite content can be made by dividing the Ferrite Number by the factor f (% ferrite = FN / f) whichis dependant of the iron content in the weld metal as shown in figure 4.LimitationsWith the practice of measuring the Ferrite Number or ferrite content, welding conditions deviating from the standardized conditionshave always to be taken into account. Furthermore, comparison tests showed that the accuracy between measurements in variouslaboratories may show differences up to +/- 10%.

Precipitate PhasesThese include carbides, nitrides or intermetallic compounds. Since most stainless steels serve at ambient temperature, the intermetallic compounts are of little relevance as they are extremely slow to precipitate because even though they may be thermodynamically stable phases, they are difficult to nucleate.It is evident from Figure 6 (Fe-Cr-C phase diagram) that typical martensitic steels should exhibit ferrite and M23C6 in equilibrium at for example, 600C. In practice, this carbide is only found after relatively long ageing. because it is preceded by Intermediate phases in the sequence cementite, M2X and M7C3, leading finally to M23C6.These precipitation sequences become more complex in heavily alloyed ferritic or austenitic stainless steels, such as those destined for the power generation industry. Considerable effort is being devoted to understanding and estimating the precipitation sequences in such alloys because the are intended to serve safely for 30 or more years, i.e., for time periods far in excess of what can be reasonably achieved in the alloy development exercise (Robson and Bhadeshia, 1997; Fujita and Bhadeshia, 2002; Sourmail, 2001; Sourmail and Bhadeshia, 2003).

CarburiThe carbides formed at high temperatures (500 to 850 C) will be of the type M23C6 .Those formed at lower temperatures, during tempering or stress relieving, will be of the types M3C, M7C3 and M23C.More alloying elements will initiate complex carbides:Tungsten W M2X X is alloying elementMolybdenum Mo M2X and M23C M is (Fe, C, X)-combinationVanadium V MX and M23CNiobium Nb MXTitanium Ti MXChromium Cr M23C6, M7C3 and in a lesser degree M3C, and M23C

CarburiThe carbides take carbon and chromium out of the matrix. The first two types (M23C6 en M7C3) do contain about 40 to 60 % of chromium. The amount of chromium compared to the amount of carbon, taken out the matrix by the M23C6 carbides, can be estimated (not correct value) as:% Crcarbides = 14,54 * (% C).But not all the carbon will combine with chromium to form carbides, even with a very slow (but realistic) cooling. In the special types of austenitic stainless steel, the carbon is equal or lower than 0,03 % and for these types nearly never carbides are formed. So we can estimate that this amount of carbon, at least, remains in the matrix.So the minimum amount of chromium, still in the matrix (for the maximum amount of carbides) is:% Crmatrix = % Crchemical compositon - 14,54 * (% C 0,03).So, the higher the carbon, the more carbides can be formed and the faster the cooling must be to avoid their formation.

Tipuri de oteluri inoxidabile austenitice oteluri de tip 18Cr -10Ni, cu 0,02-0,15%C; oteluri stabilizate la coroziunea intergranulara prin adaugarea de titan sau niobiu; oteluri cu continut scazut de nichel pentru a favoriza durificarea prin ecruisare; oteluri cu continut ridicat de nichel pentru a favoriza ambutisarea adnca si pentru aplicatii criogenice; oteluri cu rezistenta mecanica crescuta prin adaugarea de azot; oteluri in care manganul este substituit n parte de nichel; oteluri cu rezistenta la coroziune ameliorata prin adaugare de molibden si cupru; oteluri rezistente la oxidarea la cald prin adaugarea de siliciu; oteluri la care comportarea la fluaj este ameliorata datorita adaugarii de N, Mo, W, V, Ti, Nb si B; oteluri pentru electrozi de sudare; oteluri cu prelucrabilitate imbunatatita prin adaugarea de sulf, seleniu, cupru, etc.

StructuraLa temperatura ambianta otelurile inoxidabile austenitice au o structura formata din austenita, ferita alfa si carburi de tipul M23C6. Aceasta structura se obtine n cazul racirii lente. Dupa tratamentul de hipercalire, cu racire rapida, structura este formata numai din austenita. Structura austenitica asigura o rezistenta foarte mare la coroziune, plasticitate ridicata ceea ce permite aplicarea cu rezultate bune a deformarii plastice la rece, sudabilitate buna, duritate si limita de curgere scazuta, proprietati paramagnetice.

Evolutia austenitei la diferite tipuri de transformari

Structura austeniteiImagine fractografica, austenita in otel 304.

Structura austeniteiAtac electrochimic, 500x.Austenita placata prin pulverizare cu alumina

Structura austeniteiAustenita, cunoscuta si sub denumirea de fier Gama, este o faza ne-magnetica alotropa a fierului sau solutie solida de carbon in fier .In otelul nealiat, austenita se gaseste in domeniul temperaturii critice eutectoide de peste AC3 (921 oC); alte tipuri de otel prezinta diferite domenii de existenta care depind de alierea cu diferite elemente. Denumirea sa provine de la Sir William Chandler Roberts-Austen (18431902).Otel inoxidabil de tip 316, continand faza sigma.

Structura austeniteiAustenite is a metallic, non-magnetic solid solution of carbon and iron that exists in steel above the critical temperature of 1333F ( 723C). Its face-centred cubic (FCC) structure allows it to hold a high proportion of carbon in solution.In many magnetic alloys, the Curie point, the temperature at which magnetic materials cease to behave magnetically, occurs at nearly the same temperature as the austenite transformation. This behavior is attributed to the paramagnetic nature of austenite, while both martensite and ferrite are strongly ferromagnetic.

Structura austeniteiAs it cools, this structure either breaks down into a mixture of ferrite and cementite (usually in the structural forms pearlite or bainite), or undergoes a slight lattice distortion known as martensitic transformation. The rate of cooling determines the relative proportions of these materials and therefore the mechanical properties (e.g. hardness, tensile strength) of the steel.Austenite can contain far more carbon than ferrite, between 0.8% at 1333F (723C) and 2.08% at 2098F (1148C). Thus, above the critical temparture, all of the carbon contained in ferrite and cementite (for a steel of 0.8% C) is dissolved in the austenite.From 912 to 1,394 C (1,674 to 2,541 F) alpha iron undergoes a phase transition from body-centred cubic (BCC) to the face-centred cubic (FCC) configuration of gamma iron, also called austenite. This is similarly soft and ductile but can dissolve considerably more carbon (as much as 2.04% by mass at 1,146 C (2,095 F)). This gamma form of iron is exhibited by the most commonly used type of stainless steel for making hospital and food-service equipment.

Structura austeniteiThe austenitic structure has good creep resistance and good oxidation resistance

Otel AISI 304L sinterizat

Structura austeniteiQuenching (to induce martensitic transformation), followed by tempering (to break down some martensite and retained austenite), is the most common heat treatment for high-performance steels. The addition of certain other metals, such as manganese and nickel, can stabilize the austenitic structure, facilitating heat-treatment of low-alloy steels. In the extreme case of austenitic stainless steel, much higher alloy content makes this structure stable even at room temperature. The ferritic stainless steel on the left has a body centered cubic (bcc) crystal structure. By adding nickel to this stainless steel the structure changes from bcc to face centered cubic (fcc), which is called austenitic

Structura austeniteiOn the other hand, such elements as silicon, molybdenum, and chromium tend to de-stabilize austenite, raising the eutectoid temperature (the temperature where two phases, ferrite and cementite, become a single phase, austenite).The addition of certain alloying elements, such as manganese and nickel, can stabilize the austenitic structure, facilitating heat-treatment of low-alloy steels.

Structura austeniteiAdding 8% nickel to a ferritic chromium stainless steel makes an austenitic chromium-nickel stainless steel, for example Type 304 stainless steel.

Faze si constituenti in oteluri inoxidabile austeniticeMatricea de baza este alcatuita din solutii solide si continnd elemente de aliere dizolvate interstitial (de exemplu C si N) sau substituind atomul de fier (de exemplu Cr, Ni, Ti).Constituentii minori sunt formati din fazele interstitiale si intermetalice si din incluziuni.Fazele interstitiale: tipurile de carburi cele mai frecvente sunt M23C6. Mai pot exista Cr23(B,C)6 si Cr23C6. Carbura Cr23C6 poate dizolva elemente ca Mo, W, V si Ni. Se pot forma si alte carburi de crom: Cr7C3, Cr6C, precum si nitruri de tip CrN si Cr2N. Prezenta elementelor caTi, Nb sau Zr conduc la formarea de carburi, nitruri sau carbonitruri de tip MX avnd structura cubica cu fete centrate [teza doctorat - Cercetari privind comportarea la coroziune a otelurilor inoxidabile supuse deformarii plastice si nitrurarii ionice Cluj Napoca]..

Faze si constituenti in oteluri inoxidabile austeniticeFazele intermetalice sunt :faza sigma () cu retea cristalina complexa n care compozitia poate merge de la tipul B4A la tipul BA4; faza de tipul AB2 si faza Laves (n), n prezenta molibdenului (Fe2Mo), titanului, niobiului,volframului, etc. faza intermetalica cu caracter electronic, de tip Hume-Rothery, cum sunt faza cu retea cristalografica izomorfa.

Efectul elementelor de aliereCurbele de nceput de precipitare a carburilor n functie de timpul de mentinere dupa ncalzirea de punere n solutie la temperatura ridicata pentru diferite concentratii ale carbonului.

Fenomene de precipitareDup punerea n soluie este indispensabila o rcire rapida pentru a evita precipitarea n cursul rcirii. Cu ct coninutul de carbon este mai mic, cu att durata de meninere pn la apariia precipitatelor este mai mare.

Imperfectiuni structuraleStructura de baza a acestor oteluri este austenita- (CFC) cu un parametru al retelei de a =0,3595 nm. Structura CFC prezinta particularitatea de a fi una din cele doua structuri compacte,alaturi de reteaua cristalina hexagonala (HC). Atomii sunt dispusi n mod dens ca niste bile, identice si nedeformbile. Planurile dense [111] au o structura compacta (centrul atomilor n vrfurile unor triunghiuri echilaterale) Impachetarea c.f.c. Directii dense (110), planuri dense {111}

Dislocatii si defecte de impachetareDeplasarea dislocatiilor prin cristal produce o alunecare, de marime si directie anumita, definita prin vectorul BURGERS al dislocatiei. Acest vector nu poate lua valori arbitrare ci numai valori corespunzatoare distantelor dintre punctele de retea, ceea ce nseamna ca alunecarea reciproca a doua parti din cristal se face pe distanta care leaga o pozitie atomica de echilibru de alta pozitie de echilibru. n cazul otelurilor inoxidabile austenitice, cu retea CFC, cristalul se obtine prin asezarea de structuri atomice compacte (111) n succesiunea ABC ABC ...

Aparitia unui defect de impachetare la deformarea plastica a unui cristal cu retea c.f.c.

Defecte de deformare - MACLEn timpul deformarii plastice a otelurilor inoxidabile apar ca si la celelalte metale si aliaje, macle de deformare.Macla reprezinta o portiune dintr-un cristal cu o alta orientare cristalina dect restul cristalelor. La microscop ele apar nuantate diferit fata de restul grauntelui, ca urmare a diferentei de orientare cristalina pusa in evidenta de atacul cu reactivi metalografici.Maclele de deformare apar ca urmare a unor forfecari n cristal n cursul procesului de deformare. Se remarca faptul ca vectorul forfecarii este vectorul BURGERS al unei dislocatii SHOCKLEY.Un defect de mpachetare poate fi considerat ca un generator de macla. O suprapunere de defecte de mpachetare va duce la formarea unei macle mecanice. O dislocatie SHOCKLEY C face sa treaca atomii dintr-un plan (111) din pozitia A n B ; B n pozitia C siC n pozitia A.Astfel se poate spune ca macla mecanica se poate forma fie prin forfecarea directa a austenitei, fie prin suprapunerea defectelor de mpachetare.

Modificari structuraleOtelurile inoxidabile pot suferi modificari de structura prin aplicarea unor prelucrari termice sau mecanice:tratament termic (impus de procesul de fabricatie); deformare plastica la rece (oteluri austenitice); recoacere dupa deformare la rece; tratament termomecanic la temperatura nalta (impus de exemplu otelurilor laminate la cald sau supuse solicitarilor mecanice la temperatura nalta).

Modificari structuraleExemple tipice de structuri ale unor oteluri inoxidabile: a) otel cu structur feritic; b) otel cu structur martensitic; c) otel cu structur austenitic; d) otel cu structur austenito-feritic (duplex).

Transformarea martensitica n otelurile inoxidabileTransformarea martensitica duce la aparitia a doua faze:faza , hexagonal compact, neferomagnetica si faza ', cubica cu volum centrat tetragonal, feromagnetica. Faza' corespunde martensitei ' specifica otelurilor cu continut redus de carbon (oteluri inoxidabile) si nu permite aparitia unei deformari a retelei tetragonale. Aceste faze pot aparea prin transformarea spontana a austenitei la o temperatura mai mica dect MS, temperatura care depinde de compozitia aliajului. Nu s-a reusit sa se puna n evidenta punctele MS pentru cele doua faze ' si . La temperaturi mai mari de MS, transformarile pot sa apara si n timpul deformarii plastice a materialului, pna la o temperatura limita Md, peste care transformarea martensitica nu mai este posibila. Daca se executa deformarea la rece la temperatura MdMs+150...200C atunci nu are loc transformarea austenitei n martensita, indiferent de gradul de deformare aplicat.Se poate astfel defini fie o temperatura de nceput de transformare martensitica Ms, de origine termica, fie o temperatura Md de transformare martensitica produsa prin deformare plastica. Aceste temperaturi pot fi calculate plecnd de la compozitia chimica a otelului:

Ms = 1302 -42(Cr%)-61(Ni%)-33(Mn%)-28(Si%)-1667(%C+Ni) [C]

Aceasta formula empirica este valabila pentru urmatoarele compozitii chimice:10-18%Cr; 6-12%Ni;0,6-5,0%Mn; 0,3-2,69%Si; 0,004-0,12%C; 0,01-0,06%N.

Formarea martensitei intr-un otel austenitic de tip 18-8 la diferite temperaturi in functie de alungirePentru o tempertura Ms foarte scazuta (de exemplu -188oC), un otel poate suferi o transformare martensitica prin deformare; procentajul de martensita formata, depinde n acelasi timp de gradul real de deformare plastica si de temperatura: pentru un grad de deformare dat, procentajul de martensita creste cu tempertura de deformare. Astfel, rezulta ca se poate defini o temperatura Md corespunzatoare unei deformari date.

Efectul elementelor principale de aliere Cr, Ni asupra MsEvolutia temperaturii Ms ntr-un otel cu 10-14% Ni n functie de continutul de Cr pentru 0,06% si 0,008% C. Evolutia temperaturii Ms ntr-un otel 18%Cr si 0,04%C n functie de continutul de nichel .

Efectul deformarii plasticeInfluenta deformarii plastice asupra proportiei de si ' formata ntr-un otel 18/8 cu putin carbon. Influenta deformarii plastice si a temperaturii de preracire la -196C dupa hipercalire la 1050C, asupra proportiei de si .

Efectul deformarii plasticeDeformarea otelurilor inoxidabile austenitice are loc dupa mecanisme complexe care includ: alunecarea normala a dislocatiilor la aliajele care au o mare energie de mpachetare (EDI)alunecare plana de dislocatii disociate n aliaje cu o slaba energie de mpachetare. n urma acestor alunecari au loc:formarea fazei hexagonale ;formarea maclelor mecanice;formarea martensitei ' prin unul din urmatoarele mecanisme: ' sau '.Aceste doua mecanisme pot coexista, preponderenta unuia asupra altuia fiind strns legata de parametrii reali si cei teoretici ai fazei .Cea mai buna plasticitate este obtinuta atunci cnd diferitele mecanisme pot sa se succeada n loc sa se confrunte. Aceasta apare n timpul unei deformari la o temperatura apropiata de cea a punctului Md sau cnd deformarea este facuta, n parte, peste Md, n parte sub Md (efect TRIP).

Tratamente termiceOtelurile inoxidabile austenitice contin n general mai putin de 0,15%C, aceasta cantitate fiind n ntregime solubila n austenita cnd temperatura este mai mare de 1000oC. Daca sunt racite lent sau mentinute ntre 500-900oC, carburile pot precipita si prin aceasta pot compromite anumite proprietati de utilizare. Pentru a evita acest neajuns se aplica tratamentul de hipercalire. Hipercalirea este un tratament termic aplicat otelurilor austenitice care se efectueaza printr-o ncalzire de punere n solutie a carburilor la o temperatura ridicata, cuprinsa ntre 1000oC si 1150oC,urmata de o racire rapida n apa sau n aer (pentru piesele mici), pentru a evita precipitarea carburilor la limita de graunte. Carburile prezente n otelurile inoxidabile austenitice sunt de tipul (CrFeMo)23C6 cu continut ridicat n crom, iar precipitarea lor pe limita grauntilor duce la o saracire n crom a matricei adiacente, care provoaca o sensibilitate a metalului la coroziunea intergranulara. O mentinere ntre 600-1000oC duce la precipitarea fazei sigma, ceea ce predispune la fragilizare si pierderea caracteristicilor de utilizare.

Fisuri The microstructure consists of a ferrite matrix containing a volume fraction of around 20% of austenite. Ferrite grains, formed as a result of dynamic recrystallization, are small and equiaxial, while austenite particles are elongated and aligned in the direction of deformation. This micrography also shows the formation of some cracks at interphase boundaries, although most of them are located at the ferrite/ferrite boundaries.

Fisurarea la caldAustenitic structures have a tendency to hot cracking Schaeffler diagram, area of hot cracking Hot cracking occurs slightly above the melting temperature of the lowest melting constituent.At this moment in the welding solidification process, dendrites are surrounded by regions containing interdendritic low-melting liquid.Strain causes fracture of the solid bridges.

Fisurarea la caldKeep away from oil and grease, clean painstakingly after welding, use stainless steel brushes.

Rezistenta la coroziune a oelurilor inoxidabileTeorii privind coroziunea intergranular

1. Teoria srcirii n Cr reducerea continutului de Cr sub 12% in stratul intergranular datorita precipitrii carburilor de Cr micoreaz local rezistena la coroziune.2. Teoria electrochimic - consider zona cu precipitate ca o pil cu 3 electrozi, dintre care unul este matricea cu coninut normal de Cr, al doilea matricea srcit in Cr iar al treilea carbura de Cr, ansamblul fiind scldat n electrolitul reprezentat de mediul de lucru. Curenii de micropil determin corodarea matricei mai srace in Cr.3. Teoria tensional - are n vedere promovarea intercristalin a coroziunii pe seama tensiunilor mecanice determinate de precipitarea carburilor.

Mechanism of sensitisation As explained earlier, sensitisation is caused by the formation of chromium carbides on grain boundaries. The precipitates absorb chromium from the adjacent austenite causing a localised breakdown in passivity. This short description of the problem hides most of its interesting complexity. The minimum chromium concentration reached in the austenite ajacent to the carbide is in principle determined by the appropriate part of the phase diagram, which predicts that the chromium content of the austenite in equilibrium with M23C6 (cM23C6) is slightly lower than the bulk composition. However, the minimum concentration reached in the austenite is smaller than indicated by the phase diagram because of multicomponent diffusion effects, the dynamics of the solute fluxes towards the precipitates.

In normal conditions, austenitic stainless steels are given a high-temperature heat-treatment, often called a solution-treatment, which gives a fully austenitic solid solution. However, at temperatures below about 800C, there is a tendency to precipitate chromium-rich carbides as the alloy enters the carbide plus austenite phase field.The main carbide phase is M23C6, where the 'M' stands for a mixture of metal atoms including iron, molybdenum, chromium and manganese, depending on the steel composition and heat-treatment. These carbides require long-range diffusion in order to precipitate and hence can be avoided by rapid cooling from the solution-treatment temperature.The precipitation of M23C6 and M7C3 occurs primarily at the austenite grain surfaces which are heterogeneous nucleation sites; it can occur in a matter of minutes at temperatures around 750C. The chemical composition in the vicinity of the grain boundaries can be altered by the precipitationof the chromium-rich particles. The resulting chromium-depleted zone at the grain boundaries makes them susceptible to intergranular anodic-attack even under stress--free conditions. Once again, the anodic regions present a much smaller area (grain boundaries) compared with the rest of the exposed surface which is cathodic; the localised rate of corrosion at the boundaries is therefore greatly exaggerated. This is the essence of sensitisation.Sensitisation in the context of welded samples leads to the phenomenon of weld decay. Regions are created in the heat-affected zones of the welds which precipitate carbides, become sensitised and fail by localised corrosion, almost as if the weld is unzipped in the sensitised region.

Remedies to SensitisationFigure shows that the steel is safe from sensitisation at low times because precipitation has not yet occurred with a vengence. Prolonged heat treatment makes the steel safe by permitting diffusion to eliminate chromium concentration gradients in the austenite. Time and temperature dependency of sensitisation (Mayo, 1997).

Evitarea sensibilizariiA variety of solutions exist to avoid sensitisation:The first one is to reduce the carbon content of the steel, making it more difficult to precipitate carbides. Stainless steels with an 'L' associated with their numerical designation (e.g., 304L and 316L) have been manufactured with carbon cocentrations less than about 0.03 wt%, which compares against the normal grades which typically have some 0.08 wt% of carbon. Figure 4 shows how carbon accelerates sensitisation.

Figure shows that an excessive amount of chromium can eliminate austenite at all temperatures, making it impossible to achive a to transition. This is the domain of the ferritic stainless steels discussed below.

MicroaliereAn alternative is use solutes (such as Nb, Ti, V or Ta) which have a greater affinity for carbon than chromium. These are called stabilised stainless steels, for example, types 321 (Ti stabilised) and 347 (Nb stabilised) austenitic stainless steels. Titanium cannot in general be used to make alloys deposited by arc welding because it readily oxidises; type 347 is used instead as a filler metal. In welding applications, grade 321 is not used as a filler metal because titanium does not transfer well across a high temperature arc. Niobium stabilised 347 is used instead as a filler metal.Stabilisation involves more than just an addition of Nb or Ti. A heat-treatment must be performed to stimulate the formation of TiC or NbC, for example by hoding at 900C for one hour. This is because during lower temperature heat treatments, M23C6 may form faster than TiC or NbC.In some cases, a solution-treatment can be given after fabrication to dissolve carbides which may have formed on grain boundaries.A variety of other factors impact on the problem, such as the austenite grain size and the crystallographic character of the grain boundaries. Sensitisation can be avoided by grain boundary engineering (Shimada et al., 2002), by creating a crystallographic textures which favours low-energy boundaries which are less effective as heterogeneous nucleation sites. A reduction in the austenite grain size can also help by increasing the number density of any carbides and hence reducing the extent of associated Cr diffusion fields.

Environmentally assisted crackingEnvironmentally assisted cracking (EAC) is a generic term used to describe the consequences of a three--fold interaction between stress, environment and microstructure, an interaction which leads to unexpected failure with no ductility, usually involving a period of slow crack growth prior to final failure.Failure occurs at applied stresses well below the macroscopic yield strength. The stress can be due to factors other than the intended design stress, for example, residual stress induced during fabrication.An aqueous environment is required in the form of immersion or via a thin film on the surface when the component is exposed to humid atmospheres. Dissolved oxygen and anionic species such as chlorides and fluorides accelerate EAC.Some forms of this kind of cracking can be particularly dangerous because it may take thousands of hours for a crack to nucleate, but considerably less for it to propagate. Dramatic examples of catastrophic failure include the collapse of swimming pool ceilings becuase of the stress corrosion cracking o Type 304 or 316 austenitic stainless steels. For this reason, it has been suggested that 6 wt% Mo austenitic stainless steels should be used in these environments.Recent work has shown that it is also possible to sensitise titanium--containing stainless steels by the grain boundary precipitation of Ti(C,N) at 1100C, by a microgalvanic mechanism (Joe and Kim, 1999).A sensitised steel becomes more sensitive to EAC when impurities such as sulphur and phosphorus segregate to the austenite grain boundaries (McIntyre et al., 1996).

Coroziune intercristalina

Efectele coroziuniiFisurare datorata coroziunii intr-un otel inoxidabil austenitic 304L expus in mediu cu methanol si acid oxalic.

Efectele coroziuniiGoluri de fluaj in otelul inoxidabil tip RA 330, prezentat prin iluminare in contrast de faza (interferenta Nomarski) (200X).

Efectele coroziuniiCoroziune tenso-fisuranta in otel inoxidabil 310, expus in gaz natural cu continut ridicat de sulf (100X).

Efectele coroziuniiFisuri si crpturi intr-un tub din otel inoxidabil turnat 304 expus in etilena la temperaturi ridicate (100X). Multiple corrosion fatigue cracks at the I.D. of a AISI 1020 carbon steel downhole tool. The tool was rotating with the I.D. exposed to a water based drilling fluid. 2% Nital Etch, 50X

H2S SSC Cracks in a 17-4pH stainless steel stud from an O&G Wireline Valve Manifold Assembly

Grain boundary corrosion and intergranular cracking from excessive solution annealing temperatures in a 316L stainless steel microstructure. Oxalic acid etch. (50X)

304 Stainless Steel White Water Filter Screen CorrosionTunneling corrosion of a 304 stainless steel screen wire in a papermill white stock water service thought to be influenced by microbiologically induced corrosion (MIC).

Fragilizarea oelurilor inoxidabilen cazul oelurilor inoxidabile, Cr nu influeneaz dect tenacitatea. Aceasta este controlat mai ales de coninutul de C i N2, a cror solubilitate n ferit scade foarte mult odat cu creterea coninutului de Cr.Ca urmare, la coninuturi mari de Cr, C i N2, vor apare precipitate din ce n ce mai abundente, sub form de carburi, nitruri sau carbo-nitruri, avnd ca rezultat micorarea tenacitii matricei de ferit fa de cea corespunztoare oelurilor feritice comune, oelurile devenind din ce n ce mai dificil de utilizat.Pornind de la aceste constatri au aprut oelurile ELY (Extra Low Interstitial) la care coninutul de carbon este C < 0,01% iar cel de azot N2< 0,005%, impunndu-se condiia ca suma C+N2 s nu depaeasc 0,001%. De exemplu la 28-35% Cr, C < 0,002% pn la 0,006% i N2

Rupere fragilaCleavage facets on a 410 stainless steel component fracture surface resulting from brittle fracture.

Formarea fazei Faza este un compus alcatuit din atomi de Fe si Cr.n oelurile feritice cu 18% Cr, fr alte elemente de aliere, faza fragilizant , n anumite condiii, micoreaz i rezistena fa de coroziunea ce apare dup 103 104 ore de functionare prin meninere la 550C.Creterea coninutului de Cr reduce timpul de precipitare i mrete temperatura la care precipita faza . Adugarea de Si i mai ales Mo, scurteaz foarte mult acest timp. De exemplu la un oel cu 17% Cr i 2% Mo la 600C, faza apare dup 200 de ore de meninere. Similar cu cazul oelurilor austenitice Cr-Ni, precipitarea fazei ncepe numai dup o precipitare prealabil de carburi n masa fertitic care astfel devine srcit n Cr.

Intermetallic phase embrittlement: Sigma phase

Intermetallic phases (Sigma phases) may form at temperatures between 500 and 900C in ferritic stainless steels containing more than 14% Cr. Schaeffler diagram, area of intermetallic phase embrittlement

Faza sigmaSigmaFaza Sigma (particule de culoare roie) in otel refractar inoxidabil 310 expus la temperaturi ridicate. Sigma

Masuri pentru evitarea aparitiei carburilor1. Heat treatmentPay attention that during the heat treatment:1. carbon dissolves very quickly in the matrix2. chromium does not solve easily (relative large atom) and the stay at hightemperature must be sufficiently long3. too long stay on high temperature does increase the grain size of the matrix,which leads to a somewhat lower ductility.2. An increasing amount of carbon will tend to form more austenite (present times 30in the nickel-equivalent) but the tendency to form carbides will also increase.

Masuri pentru evitarea aparitiei carburilorFor a carbon percentage > 0,10 %, even with the fastest cooling, they cannotbe avoided.To avoid the formation of carbides, a special type of stainless steel isproduced, having C

Masuri pentru evitarea aparitiei carburilor2. Another solution, except low carbon, it to stabilise the stainless steel withtitanium (Ti) and or niobium (Nb). These elements have a strong tendency toform carbides, which will be formed at high temperatures (normal coolingaround 1000 to 1100 C). The chromium carbides, depending on the coolingrate do form in the temperature range of 650 - 900 C.If the titanium or niobium content is sufficient to combine all available carbon(perhaps except 0,03 %), than no carbon is anymore available for thechromium.The following ratios do comply with these conditions:Theoretically : %Ti / %C = 4 and %Nb / %C = 8EN-standard : %Ti / %C = 5 and %Nb / %C = 10ASTM-standard : 0,20 % + 4(%C + %N) < (%Ti + %Nb) < 0,80 %

Fragilizarea la 475CAceasta este o form particular de fragilizare care apare cu prilejul unei recoaceri n jurul temperaturii de 475C, la oelurile inoxidabile ce conin ferit (feritice, austenito-feritice, ferito-austenitice).La concentraii de peste 12% Cr este necesar un timp de meninere foarte ndelungat pentru apariia acestui mod de fragilizare (105ore). Prin creterea coninutului de Cr, Si, Al, Mo timpul de meninere se reduce foarte mult. Carbonul reduce aceast tendin de fragilizare datorit faptului c producerea carburilor determin srcirea n crom a matricei metalice de baz.Aceast fragilizare se produce sub aproximativ 500C ca urmare a unor procese de segregare care au loc n ferit. Se separ mai nti o faz feromagnetic bogat n Fe, numit faza apoi o faz bogat n Cr i paramagnetic. Nici una dintre acestea nu este decelabil cu ajutorul microscopiei optice datorit precipitrii foarte fine. Se obine creterea duritii, a rezistenei la rupere, a limitei de curgere, micorndu-se alungirea i gtuirea la rupere dar, mai ales reziliena i tenacitatea la rupere.

Comportarea la sudare Oelurile feritice cu Cr au aprut mai trziu dect cele austenitice, datorit creterii preului Ni i a ncadrrii acestui metal ntre materialele strategice.Prin dezvoltarea oelurilor feritice cu Cr s-a urmrit nlocuirea oelurilor austenitice Cr-Ni. Din pcate, oelurile feritice nu au reusit s le nlocuiasc pe cele austenitice dect pentru anumite aplicaii, dei sunt mult mai ieftine i au o rezisten superioar fa de coroziunea fisurant sub sarcin. Acestea prezint totodat urmtoarele dezavantaje:sudabilitate slab;sensibilitate, n anumite condiii, fa de fisurarea la rece;caracteristici mecanice ale sudurilor nesatisfctoare n cazul unor aplicaii pretenioase;prelucrabilitate mecanic slab.