中文版 The stainless steel is refers to has a corrosion resistance performance kind of aluminum.
The usually called stainless steel is the stainless steel and the acidproof steel general name.
Stainless steel not necessarily acidproof, but the acidproof steel simultaneously is also the stainless steel.
The so-called stainless steel is refers to can resist the aluminum which the atmosphere and the pickling medium corrode. Corrosion rate <0.01 the mm/year for the completely anti-corrosive steel, speed <0.1 the mm/year is a corrosion-resisting steel. The so-called acidproof steel is refers to each kind of strong corrosive medium can the acidproof steel. The corrosion rate <0.1mm/year to resist corrosion completely, the corrosion rate <1mm/year is the anticorrosion. Therefore. The stainless steel is not does not corrode, the corrosion rate to be just slow, the steel which corrodes is not existed absolutely.
It is noteworthy that in the identical medium. The different type's stainless steel corrosion rate entirely different and the identical kind of stainless steel corrodes the behavior in the different medium to be also different. For example. The Ni-Cr stainless steel's is very good in oxidized medium corrosion resistance. But in non-oxidized medium (for example hydrochloric acid) the corrosion resistance is not good. Therefore grasps each kind of stainless steel the characteristic, regarding to choose and to use the stainless steel is correctly very important.
The stainless steel not only need resist corrosion, but must withstand or the transmission load, therefore also needs to have the good mechanical properties. The stainless steel by moldings and so on board, tube processes the component or the components generally, therefore. Must have the good machining performance and the good welding performance.
The stainless steel divides into according to the model organization: Ferrite (F) stainless steel; Martensite (M) stainless steel; Austenite (A) stainless steel; Austenite - ferrite (A-F) double-phase stainless steel; Precipitation hardening stainless steel.
First, metal corrosion
(one) metal corrosion process
Causes the metal under the outside medium's function to receive the destruction the phenomenon to be called the corrosion gradually. The corrosion basically has two forms. Chemical etching and electrochemistry corrosion. The corrosion which actual meets in the production is mainly the electrochemistry corrodes, in the chemical etching does not produce the electric current, forms some kind of corrosion product greatly in the corrosion process. This kind of corrosion product covers generally on the metal surface forms a membrane, causes the metal and the medium isolates.
If this stratifies the student finished goods is stable, compact, complete and with the metal surface layer reliable union, greatly will reduce even may prevent the corrosion the further development, plays the protective function to the metal. Forms the protective film the process to be called the deactivation. For example, produces oxide films and so on SiO2, Al2O3, Cr2O3, these oxide film structure compact, complete, is not loose, does not have the crack, and is not easy to flake, may play the role which the protection parent metal, avoids continuing to oxidize. For example iron when high-temperature oxidation produces Fe2O3. Otherwise, some oxide films not continuously, or are vesicular. To parent metal protective function. For example. Some metal's oxide compound, like Mo2O3, WO3 have the volatility under the high temperature, has not covered the substrate completely the protective function.
Obviously, the oxide film production and the oxide film structure and the nature are the chemical etching key characters. Therefore, enhances the metal to bear the chemical etching ability, is mainly through the alloy or other methods, forms one in the metal surface stable, complete compact and with the substrate union reliable oxide film, is also called the deactivated membrane, the electrochemistry corrosion is the metal corrosion more important, the more universal form, it is constitutes the galvanic battery by the different metal or the metal different electrode potential to produce. Therefore this kind of galvanic battery corrosion is produces between the Microstructure called that it corrodes the microcell. The electrochemistry corrodes the characteristic has the dielectric medium existence, between the different metal, between the metal micro area either between has the electric potential difference connection or the contact, simultaneously has the corrosion current production.
Second, corrosion type
Metallic material's in industrial production corrosion failure mode is many and varied. The different material in the different load and under the different medium environment's function, its corrosion form mainly has the following several kinds:
Generally corrosion: The metal exposed surface has a big area evener corrosion, although reduces the component stress active surface and the service life, but is smaller than localized corrosion's hazardous nature.
Intercrystalline corrosion: Refers to along the corrosion which carries on, causes the crystal grain the connection to be destructed. This kind of corrosion's hazardous nature is biggest, it may cause the metal changes crisp or loses the intensity, when the rap to lose the metal sound, easy to create the sudden accident. Intercrystalline corrosion for austenite stainless steel main corrosion form, this is because the crystal boundary region and the transgranular ingredient or the stress have the difference, causes the electrode potential which the crystal boundary region electrode potential obviously reduces creates to help the difference result.
Stress corrosion: Metal in corrosive medium and tensile stress (applied stress or internal stress) under the combined action has the breakage phenomenon. The break way is mainly along the crystal, also has the transgranulation, this is in one kind of dangerous low stress brittle fracture, often has the stress corrosion in the chlorination medium and the basic oxide or other water-soluble medium, accounts for the quite great proportion in many equipment's accidents.
Pitch corrosion: The pitch corrosion is occurs after the metal surface local region's one kind of corrosion destruction form, the pitch corrosion form can the deep place development, penetrate the metal finally rapidly. The pitch corrosion hazardous nature is very big, particularly to each kind of vessel is extremely disadvantageous. After presenting the pitch corrosion, should grind or the coating promptly, avoids corroding the deepening.
The pitch corrosion produces the reason is under the medium function, the metal surface passivation membrane receives the local damage to create. Or in includes in the chloride ion medium, the material surface defect loose and the non-metallic inclusions and so on may cause the pitch corrosion.
Corrosion fatigue: Metal the destruction which, its characteristic occurs in the corrosive medium and under the alternate stresses function are produce the spoiled etch pit and the massive cracks. Obviously reduces the steel the fatigue strength, causes the premature break. The corrosion fatigue is different with the mechanism fatigue, it does not have certain limit of fatigue, along with cycle-index's increase, the fatigue strength has been the drop.
Besides the above each kind of corrosion form, but also has the corrosion which produces as a result of the macroscopic galvanic action. For example, in the steel work the rivet and the riveted joint material different, the heterogeneous metal's welding, the hull and the propeller material not same level the corrosion which creates because of the electrode potential difference.
Is obvious from the above corrosion mechanism, prevents the corrosion the objective point to place: Reduced galvanic battery quantity, causes the steel the surface to form one as far as possible stable, complete, and the steel substrate union reliable deactivation membrane; In forms in galvanic battery's situation, reduces the electrode potential difference between the two-pole as far as possible.
Stainless steel alloy principle
Enhances the steel corrosion resistance method to be many, like the surface spreads an anticorrosion metal, to paint the nonmetallic level, the electrochemistry protection and the change corrosion ambient medium and so on. But using the alloy method, enhances material's corrosion resistance is most effective one which of measures prevents the corrosion to destroy, its method is as follows:
(1) joins the alloying element, enhances the steel substrate the electrode potential, thus enhances the steel anti-electrochemistry corrosion ability. Generally in the steel joins Cr, Ni, the Si multi-elements to be able to enhance its electrode potential. Because Ni lacks, Si will join will cause the steel will change crisp massively, therefore, only then Cr obviously enhances the steel substrate electrode potential commonly used element.
Cr can enhance the steel the electrode potential, but is not assumes the linear relationship, as shown in Figure 5.1. The experiment proved that the steel the electrode potential along with alloying element's increase, has a quantitative change to the qualitative change relations, follows 1/8 rule. When the Cr content achieves certain value namely 1/8 atom (l/8, 2/8, 3/8 ......when), the electrode potential will have a sudden change. Therefore, in nearly all stainless steel, Cr content in 12.% (atom) above, namely 11.7% (quality) above.
(2) joins the alloying element to cause the steel the surface to form one stable, complete and the steel substrate union reliable purification membrane. Thus enhances the steel to bear the chemical etching ability. If joins alloying elements and so on Cr, Si.Al in the steel, causes the steel the surface layer to form oxide films and so on compact Cr2O3, SiO2, Al2O3, may enhance the steel the corrosion resistance.
(3) joins the alloying element causes the steel when the normal temperature can by the single-phase behavior existence, reduces the microcell number, thus enhances the steel the corrosion resistance. If joins the critical mass Cr or Cr-Ni, causes the steel to obtain the single-phase ferrite or the single-phase austenite under the room temperature.
(4) joins elements and so on Mo, Cu, enhances the corrosion resistance ability.
(5) joins elements and so on Ti, Nb, eliminates Cr the intercrystalline segregation, thus reduced the intercrystalline corrosion tendency.
(6) joins elements and so on Mn, N, replaces part Ni to obtain the single-phase austenite organization, simultaneously can enhance the chromium stainless steel in organic acid corrosion resistance greatly.
Stainless steel type and characteristic
The stainless steel has two classifications: One kind is according to the alloying element characteristic, divides for the chromium stainless steel and the chromated nickel stainless steel;
Another kind is according to under the hot condition the steel organization condition, the division is the M stainless steel, the F stainless steel, a stainless steel, A F double-phase stainless steel.
First, martensitic stainless steel
The typical martensitic stainless steel has 1Cr13~4Cr13 and 9Cr18 and so on
the 1Cr13 steel processing shop characteristics are good. But after preheating does not carry on flushes, curving, hemstitching and the welding deeply. before the 2Crl3 cold-work, does not request preheating, but before welding, must preheating, 1Crl3, 2Cr13 mainly use for to manufacture the anticorrosion structural element like steam turbine leaf blade and so on, but 3Cr13, 4Cr13 mainly use for to manufacture the medical instrument surgical department scalpel and the wear-resisting components; 9Crl8 may make the anticorrosion bearing and the cutting tool.
Second, ferritic stainless steel
The hard element stainless steel is generally 13%~30% gathers the carbon quantity including the Cr quantity to be lower than 0.25%. Sometimes also joins other alloying element. The Microstructure is mainly the ferrite, in the heating and the undercooling not α<=>γ the transformation, cannot use the heat treatment to carry on the strengthening. The oxidation resistance is strong. At the same time, it also has the good hot-working and the certain cold workability. The ferritic stainless steel mainly uses for to manufacture the request to have the high corrosion resistance, but the intensity request low component, widely uses in making equipment and the chemical use and so on production nitric acid, nitrogenous fertilizer pipeline and so on.
The typical ferritic stainless steel has Crl7, Cr25 and Cr28.
Third, austenite stainless steel
The austenite stainless steel overcomes Ma to make the stainless steel corrosion resistance insufficiency and brittleness oversized develops. The fundamental component is Crl8%, Ni8% is called 18-8 steel. Its characteristic is gathers the carbon quantity to be lower than 0.1%, using Cr, the Ni coordination obtains the single-phase austenite organization.
Austria make the stainless steel to use in making chemical equipment components, the freezing industry cryogenic equipment component and strengthens after the deformation generally and so on production nitric acid, sulfuric acid may serve as the stainless steel spring and the spring for watch and so on.
The austenite stainless steel has the good anti-uniform corrosion performance, but in the partial corrosion resistance aspect, still had the following problem:
1. austenite stainless steel intercrystalline corrosion
Austria make the stainless steel when 450~850℃ the heat preservation or slow cooling, will present the crystal to ask the corrosion. The carbon amount is higher, between the crystal eclipses tendentiousness to be bigger. In addition, will also present the intercrystalline corrosion in weldment's heat-affected zone. This is because separates out rich Cr in the crystal boundary Cr23C6. Causes its periphery substrate to produce the poor chromium area, thus forms corrodes the galvanic battery to create. This kind of intercrystalline corrosion phenomenon mentioned in front in the ferritic stainless steel is also the existence.
In the project often uses the following several methods to prevent the intercrystalline corrosion:
(1) reduces in the steel the carbon quantity, causes in the steel to gather the carbon quantity to be lower than under the state of equilibrium in the austenite saturated solubility, namely has fundamentally solved the chromium carbide (Cr23C6) the question which separates out in the crystal boundary. Usually in the steel gathers the carbon quantity to drop to below 0.03% then to satisfy the anti-intercrystalline corrosion performance the request.
(2) joins Ti, Nb and so on can form the stable carbide (TiC or NbC) the element, avoids separating out Cr23C6 in the crystal boundary, then against on austenite stainless steel intercrystalline corrosion.
(3) through adjusts in the steel the austenite to form the element and the ferrite forms the element the proportion, enables it to have the austenite + wire rope body double-phase organization, the ferrite occupies 117%. This kind of double-phase organization is not easy to have the intercrystalline corrosion.
(4) uses the suitable heat treatment craft, may prevent the intercrystalline corrosion, obtains the best corrosion resistance.
2. austenite stainless steel stress corrosion
The stress (is mainly tensile stress) the dehiscence which causes with the corrosion combined action is called the stress corrosion dehiscence, is called SCC (Stress Crack Corrosion). The austenite stainless steel in contains in the chloride ion corrosive medium to have the stress corrosion easily. When achieves 118% including the Ni quantity, austenite stainless steel stress corrosion tendentiousness is biggest, continues to increase including the Ni quantity reduces gradually to 45~50% stress corrosion tendency, until vanishing.
Prevents the austenite stainless steel stress corrosion the most main way is joins Si2~4% and from smelts General the N content control below 0.04%. In addition should also reduce impurity as far as possible and so on P, Sb, Bi, As content. Moreover may select a-F double-phase steel, it the counter stress corrodes in Cl- and in the OH- medium not sensitively. After the initial tiny crack meets the ferrite, no longer continues to expand, the ferrite content should about 6%.
3. Austria make the stainless steel the deformation strengthening
The single-phase austenite stainless steel has the good cold-work performance, may the cold draw become the very thin steel wire, the cold rolling becomes the very thin steel belt or the steel pipe. After massive distortions, the steel intensity enhances vigorously, particularly when warm area rolling below zero, the effect is more remarkable. The tensile strength may reach above 2000 MPa. This is because besides hammer-harden effect, but also superimposed the deformation to induce the M transformation.
Austria make the stainless steel strengthens after the deformation to be possible to use for to make the stainless spring, the spring for watch, in the aviation structure steel wire and so on. After the deformation, if must weld, then can only use the spot welding craft, the deformation to make the stress corrosion tendentiousness to increase. And has ferromagnetism because of the part γ->M transformation, when use (for example in measuring appliance components) should give to consider.
The recrystallization temperature changes along with the deformation quantity, when the deformation quantity is 60%, its recrystallization temperature reduces to 650℃ the cold-work austenite stainless steel recrystallization annealing temperature for 850~1050℃,850℃ must keep warm 3h,1050℃ when to pass burns then, then water cooling.
4. Austria make the stainless steel the heat treatment
The austenite stainless steel commonly used heat treatment craft includes: Solid solution treatment, stabilizing processing and destressing processing and so on.
(1) solid solution treatment. Heats up the steel after 1050~1150℃ the water quench, the main purpose is causes the carbide to dissolve in the austenite, and retains this condition the room temperature, such steel's corrosion resistance will have the very big improvement. As stated above, to prevent the crystal to ask the corrosion, usually uses solutionizing processing, causes Cr23C6 to dissolve in the austenite, then rapid chilling. May use the air cooled regarding the thin wall, the ordinary circumstances use the water cooling.
(2) stabilizing processing. Generally is after the solid solution treatment carries on, in contains Ti, the Nb 18-8 steel commonly used, after solid processing, heats up the steel after 850~880℃ the heat preservation the air cooled, this time the Cr carbide completely dissolves, escapes, but the titanium carbide dissolves not completely, and in undercooling full separation, causes the carbon not to be impossible to form the chromium again the carbide, thus eliminated the intercrystalline corrosion effectively.
(3) destressing processing. Destressing processing is eliminates the steel heats up generally after the cold finishing or the welding residual stress heat treatment craft to 300~350℃ the tempering. Regarding including stabilizing element Ti, the Nb steel, the heating temperature does not surpass 450℃, in order to avoid the separation chromium's carbide causes the intercrystalline corrosion. Regarding the ultra low-carbon and including Ti, the Nb stainless steel's cold working work piece and the weldment, must in 500~950℃, the heating, then slow cold, the stress-relieving (eliminates welding stress to take upper limit temperature), may reduce the intercrystalline corrosion to favor and to enhance the steel the stress corrosion resisting force.
Fourth, austenite - ferrite double-phase stainless steel
In the Austria stainless steel's foundation, increases the Cr content suitably and reduces the Ni content, and with returns to dissolve processing to coordinate, may obtain has the austenite and the ferrite double-phase organization (including 40~60% Delta - ferrite) the stainless steel, the typical steel grade has 0Cr21Ni5Ti, 1Cr21Ni5Ti, OCr21Ni6Mo2Ti and so on. The double-phase stainless steel has the good weldability, after welding, does not need the heat treatment, moreover its intercrystalline corrosion, stress corrosion tendentiousness is also small. But because is high including the Cr quantity, easy to form Sigma, when use should perform to pay attention.
