To increase the crack growth resistance of retaining ring materials in hydrogen environments admixtures of La (up to 0.05 vol. %), Ce. (up to 0.1 vol.
HYDROGEN EMBRITTLEMENT AND CRACKING OF 18Mn-4Cr STEELS
A. Balitskii Karpenko Physico-Mechanical Institute of National Academy of Sciences of Ukraine, UA 79601
ABSTRACT The temperature dependence of the fracture toughness of 18Mn-4Cr-type steels is characterized by presence of the minimum o o at about 550 C. After heat treatment performed at 400 C, fracture toughness abruptly drops to its minimum value and then o somewhat increases but does not reach the original value. At temperatures higher than 600 C, fracture toughness begins to o decrease again. If tempering temperatures are not higher than 400 C, we observe plastic fracture. At higher tempering o temperatures (e.g. 600 C), we have a mixed type of fracture, i.e., a combination of plastic (intragranular) and brittle (intergranular) types of fracture. The time to failure of 18Mn-4Cr-type steels is the same in dry hydrogen and in dry air. Hydrogen makes heavier the elastic sliding in metal by increasing of strains of initial stage of elastic deformation. Atoms of hydrogen diffuse to active sliding planes (it is more advantageously thermodynamically), prevent from dislocation motion and lead to yield stress increasing. On macrogeometry of fracture surfaces of the specimens some steps typical for hydrogen embrittlement of materials with high elasticity reserve are obvious. Fatigue crack starts from superficial metal layers embrittled by hydrogen. Finale stage of o destruction (in the specimen core) happens under tangential stresses action at angle of 45 to normal stresses direction. To increase the crack growth resistance of retaining ring materials in hydrogen environments admixtures of La (up to 0.05 vol. %), Ce (up to 0.1 vol. %) in 18Mn-4Cr have been introduced during experimental melting. They have a positive influence on quantuity, geometry and distribution regularity of non-metallic inclusions, improve the metal quality, and change its dislocation structure. Applying Ca (up to 0.05 vol. %) for deaccidation of steels significantly influences the geometry and composition of nonmetallic inclusions, observed on fracture surfaces of specimens. Experimental steels with high content of Cu and other alloying components demonstrated increasing resistance to corrosion-mechanical fracture and long-term stength in hydrogen-containing environments.
Introduction Safety and reliability of power stations is of primary importance. While it is the type of power stations that is foremost in the news, there is also another aspect which is common to all types of power stations, nuclear, fossile-fired or hydro, advanced or less developed: This is the need to maintain the integrity of the generator rotors over the total life of the power stations, or extended life time.The number of generators retaining ring failures caused by the fracture of these details increases with the power of electric machines. A large number of modern turbogenerators (TG) are equipped by water - hydrogen cooling systems. Hydrogen is used as a generator rotor cooling medium since it has a high heat conductivity compared to that of many gases. Hydrogen is used as a cooling agent. Accidents with TG leads to fires, cause damage of generators, or even result in their complete failure. Statistical data of this sort are mainly accumulated for 18Mn-4Cr-type steels [1-8]. The most highly stressed part of the generator rotor is the retaining ring, which has the task to maintain the copper windings in position against the centrifugal forces. During last 50 years the electric machines output is sharply increased and in service now are the all spectrum of electrical machines output. Other tendency of this period is the number of accidents with rotor retaining rings, which decreased after new generation of ultrahigh strength steels introducing for the manufacture of retaining rings. These steels demonstrated the definite tendency of increasing of product of yield strength on fracture toughness, must also not be ferromagnetic, so as to keep eddy-current losses at a minimum. From all nonmagnetic materials they have the maximum product of yield strength on fracture toughness. Such retaining rings have a history of explosions in service in many countries of the world with resulting huge losses not only because of repair work but also due to down-time of the power stations. The overwhelming majority of the failures that have occured are due to stress corrosion cracking, that is the slow growth of cracks in retaining rings in service, i. e. in operating power stations.
Experimental and Results The quality of the steels used for retaining rings determines how fast the cracks grow and whether they can ultimately lead to an explosion of the power station. Presently, there is a world wide campaign to exchange all bigger retaining rings in large power stations and thus to switch over from an older material (8Mn-8Ni-4Cr) to the18Mn-4Cr or 18Mn-18Cr austenitic stainless steel. However, not enough information is available for a quantitative life time analysis to show whether the18Mn-4Cr and 18Cr-18Mn steels is sufficiently hydrogen resistant or whether possibly even more hydrogen resistant austenitic stainless steels will be needed to assure the reliable service of generator rotors for the generally planned life time of 40 years and may be for life time extension up to 70 years. The temperature dependence of the fracture toughness of 18Mn-4Cr-type steels is characterized by presence of the minimum o o at about 550 C (Fig.1). After heat treatment performed at 400 C, fracture toughness abruptly drops to its minimum value and o then somewhat increases but does not reach the original value. At temperatures higher than 600 C, fracture toughness o begins to decrease again. If tempering temperatures are not higher than 400 C, we observe plastic fracture. At higher o tempering temperatures (e.g. 600 C), we have a mixed type of fracture, i.e., a combination of plastic (intragranular) and brittle (intergranular) types of fracture. The time to failure of 18Mn-4Cr-type steels is the same in dry hydrogen and in dry air (Fig.2).
FIGURE 1.
FIGURE 2.
FIGURE 1. Temperature dependences of fracture toughness and sub critical crack growth rates in specimens from18Mn4Cr steel [9]. FIGURE 2. Long term strength of 18Mn-4Cr steel in dry hydrogen and in air (1), in distilled water without oxygen (2), in humid hydrogen (3), in water saturated with oxygen (4), and in a nitrate solution (5) [10].
Humid hydrogen is a much more aggressive medium than distilled water without oxygen. Saturation of water with oxygen and nitrated leads to a sharp decrease in the time of specimens fracture. The diagrams of structural strength of the materials of a rotor retaining ring (traditional and promising) presented in Fig.3 illustrated the advantages of 18Mn-4Cr and 18Mn-18Cr steels. For a long time, austenitic stainless steels had been considered one of the most stables against hydrogen embrittlement. Is noted that combined hydrogen and moisture action may involve hydrogen embrittlement and corrosion cracking of 8Mn-8Ni-4Cr steel. This fact has a great significance because generator rotor is cooled by hydrogen up to 20% relative humidity, which in more cases can raise up to higher values. Same ability to hydrogen and moisture caused cracking of 18Mn-4Cr steel. Hydrogen electrolytic saturation involves the significant acceleration in crack growth rate of retaining ring steels specimens under dynamic loading. In low amplitude region significant crack propagation rate increasing is observed compared with their air tested value. At that regime penetration probability of hydrogen atoms into crack tip during its opening depends on a time of exposition under cycle loading.
Analysis of the Results The fatigue strengths of 18Mn-18Cr and 18Mn-4Cr steels reduce accordingly to 168 and 135 MPa comparatively to the tested in air accordingly 436 and 350 MPa. The electrolytic hydrogenation exerts more influence on fatigue resistance of 18Mn-18Cr than 18Mn-4Cr steel. It is well known that hydrogen makes heavier the elastic sliding in metal by increasing of strains of initial stage of elastic deformation. Atoms of hydrogen diffuse to active sliding planes (it is more advantageously thermodynamically), prevent from dislocation motion and lead to yield stress increasing. Difficulties for elastic sliding in the materials due to hydrogen presence cause initiating critical dislocation density on more early stages of deformation than without hydrogen. It involves crack initiating at lower external stresses. Increasing chromiummanganese steels resistances to tangential stresses action under hydrogen influence prevent from dulling of crack tip during its growth. On macrogeometry of fracture surfaces of the specimens (Fig.4) some steps typical for hydrogen embrittlement of materials with high elasticity reserve are obvious. Fatigue crack starts from superficial metal layers embrittled by hydrogen. Finale stage o of destruction (in the specimen core) happens under tangential stresses action at angle of 45 to normal stresses direction. Electrolytic hydrogenation enhances the range of microbrittle fracture of chromium-manganese steels. High-nitrogen 18Mn18Cr steel shows transgranular fracture and 18Mn-4Cr steel shows a mixed fracture character with transgranular domination.
FIGURE 3. KIC – σT diagrams ({ – KJC, z – KIC) (а) (in air), Kfc – σT (b), Kth – σT (c) (electrolytical hydrogenation with current density 100 А/m2) of steels for rotor-retaining ring unit: 1 – 3,5Ni-CrMoV; 2 – 8Mn-8Ni-4Cr; 3 –18Mn-4Cr; 4 – 18Mn-18Cr. High strength steels with a content of nitrogen of about 1% are produced both under ordinary conditions (nitrogen is introduced with manganese and chromium ligature) and in furnaces with an elevated pressure of nitrogen. Steels produced by using special casting methods with a “counterpressure” of nitrogen at a very high content of nitrogen (up to 1,4%) with a subsequent cold working have strength properties which are unique for hardened austenitic materials ( σT can reaches 1000 MPa). However, today, these high technologies are seldom used and the content of nitrogen is usually restricted to 0,5%. The potential of steel alloyed with nitrogen has not been fully realized up to the present due to mainly, to metallurgical reasons. At the same time, in the last years, nitrogen has occupied an important place as an effective alloying component for producing steels with record strength characteristics. Electroslag remelting (ESR) of these steels in stationary electrolyser (diameter is equal 1100 mm) has been carried out with a o rate of melting 900-950 kg/hour. Flux of CaF2 -SiO2-MgO system, melting point of which is about 1100 C, has been used as working slag. Cast or forged electrodes (diameter 850 mm) have been used, beginning the process with a liquid start when cast consumable electrodes are being remelted, the flux deoxidation by cerium admixture is performed in the course of the electroslag remelting.
The amount of the cerium admixture is 1,5 kg per steel ton. The use of this working flux and tasking electrical parameter of remelting provide the optimal temperature-rate regimes of steel melting with saving of initial nitrogen containing and its distribution on height and diameter of ESR ingots.
a
b
FIGURE 4. Macrogeometry of fracture surface of chromium-manganese steels after fatigue tests under electrolytic hydrogenation: a – 18Mn-18Cr; b – 18Mn-4Cr. For certain melts, the concentration of nitrogen exceeds 1,2% and can reach an extraordinarily high level of 1,5%. The process of nitration in the course of electroslag remelting proceeds in the following way: nitrogen-containing solid additions (for melting, several hundreds of kilogram are spent) are continuously carried on the slag from three powerful weighing machines. Since nitrated ferroalloys contain a great amount of chromium, manganese, and vanadium, preference is given to ceramic nitrogen-containing materials, in particular, to silicon nitride (Si3N4 ). The main difficulties caused by the process of chamber electroslag remelting are problems connected with the performance of the slag bath at liquid start and the impossibility of observing the melting process and behaviour of the slag bath. The complexity caused by the maintenance of the coaxiality of electrode and ingot has already been overcome. However, such a serious disadvantage as the chemical inhomogenity of the ingot, which is typical of any procedure for remelting of an expendable electrode with additional filling of the melt with solid particles, remains. One way of overcoming it (despite very expensive repeated remelting) is electromagnetic mixing of the melt inside the crystalliser. Another way would be a method for remelting a metallic expendable electrode with an electric arc which burns between the surface of the liquid slag bath and the end of the expendable electrode, i.e., arc slag remelting. Chromium significantly influences stress corrosion cracking (SCC) of cold worked alloys, relaxation and corrosion resistance. Starting from 6% Cr an increase in its content considerably raises the durability of water quenched specimens. In more complex alloys there are beneficial additions of Ni, Mo, Nb(Cb), Ce, Ca, Cu. Chromium has an important effect on bend ductility . This property is related to the ability of the alloy to withstand the severe cold expansion used to attain the desired yield strength. Mn contributes to the stability of austenite, improves strength, work hardening characteristic and crevice corrosion resistance, but excessive additions might impair workability. At a total content of (Mn+Cr) more than 30% the SCC resistance is excellent over the whole composition range. Again, the high Mn-low Cr alloys have poor general corrosion resistance and a low rate of work hardening. The concentration of Mn in the steel must be up to 18...22%. Mn contents of more than 30% decrease the resistance to SCC as a result of intermetallics precipitation on grain boundaries. The microstructure of Mn-Cr steel shows globular grains with numerous annealing twins, which indicate small defects of the austenite structure. In Mn-Cr steel after a high level of cold work, stripes of sliding are formed. This steel is more resistant to hydrogen embrittlement than traditional steels (due to the higher purity concerning the content of sulphur and phosphorus). To increase the crack growth resistance of retaining ring materials in hydrogen environments admixtures of La (up to 0.05 vol. %), Ce (up to 0.1 vol. %) in 18Mn-4Cr have been introduced during experimental melting. They have a positive influence on quantuity, geometry and distribution regularity of non-metallic inclusions, improve the metal quality, and change its dislocation structure. Applying Ca (up to 0.05 vol. %) for deaccidation of steels significantly influences the geometry and composition of nonmetallic inclusions, observed on fracture surfaces of specimens. Experimental steels with high content of Cu and other alloying components demonstrated increasing resistance to corrosion-mechanical fracture and long-term stength in hydrogen-containing environments. The shot-peening treatment unambiously improves the characteristics of all investigated steels after their hydrogen saturation. Shot peening treatment is effective in retarding and eventually preventing SCC in water and hydrogen environment by increasing the surface hardening through cold work of the metal and providing residual compressive stresses at the surface.
In air a chromium-manganese steel fractures along the grain body due to the initiation and coalescence of micropores, which leads to the dimple appearence. At the same time, after electrolityc hydrogenation of this steel, smooth shear regions are clearly observed. In addition to alloying with nitrogen, which is added to chromium-manganese steels for stabilization of the austenite structure and for increasing of the yield strength after cold deformation, in our opinion, the addition of rare-earth elements to steels is important for improving their service characteristics. Due to the high activity of rare-earth elements (in particular, lanthanum and cerium) with respect to elements such as oxygen and sulphur, the refining action of rare-earth elements manifests itself in their ability to form oxides Ce2O3, La2O3, CeO2, sulphides La2S3, Ce2S3, Ce3S4, CeS and hexagonal oxysulphide Ce2O2S with a high melting point, in order to absorb a great amount of hydrogen, to form face-centred hydrides LaH2, CeH2 , and to purify a metal from harmful impurities and gases when conditions for removal of products of the interaction of rare-earth elements are formed. The addition of the rare-earth elements leads to the modification of the structure of a cast metal, a decrease in the grain size, an increase in the dispersivity of crystallized phases, which has a positive effect on the number, form, and uniform distribution of nonmetallic inclusions, structure and an increase in the mobility of dislocations. Cerium and lanthanum can harden a substitutional solid solution, form new phases with elements of the base (La,Ce)(Fe,Cr,Mn)2, (La, Ce)6(Fe, Cr, Mn)23, (La,Ce)(Fe, Cr, Mn)12, (La, Ce)(Fe, Cr, Mn)3, (La, Ce)2(F, Cr, Mn)17, and qualitatively influence the structural state of a metal. Cold deformation of Mn-Cr steels is accompanied by both microtwinning and ordinary dislocation sliding by formation of parallel stripes in one or several directions. The main danger in the case of disturbance of the balance of nitrogen in the formation of brittle intermetallic phases for the σ-type ((Cr, Mo)x (Fe, Ni, Mn)y) at concentrations of nitrogen smaller than 0,51 vol. % or ε-nitride Cr2N (at concentrations of nitrogen higher than 0,713 vol. %). Copper in 18Mn-18Cr and 18Mn-4Cr steels stabilizes the austenite and the magnetic permeability (at a level 1,003 – 1,01) for considerable cold deformation and also increases the resistance to hydrogen embrittlement, general corrosion (its rate is at most 0,001 mm per year), and SCC. Specimens subjected to electrolytic hydrogenation have stable high values of cyclic fracture toughness (Kfc) and a high yield strength. The use of calcium (up to 0,05 vol.%) for deoxidation of this steel has a considerable effect on the composition of nonmetallic inclusions (the local concentration of calcium can reach several vol.%). Among all the known methods for the improvement of crack resistance of retaining ring steels, the optimization of the chemical composition and the structural hardening by mechanical and thermal treatment are the most efficient. We note that the use of electrochemical inhibitive protection, surface plastic deformation, and control over operational modes of loading as well as protective coatings also give a positive effect. o All investigated steels have a high corrosion resistance to pitting and local corrosion in chloride solutions at temperatures 20 C owing to the formation of a stable passive layer. Steel 18Mn-18Cr has the highest pitting resistance of about 580 mV. An increase of the chloride concentration insignificantly narrows the passive region. The influence on the electrochemical behaviour of the steels investigated is very slight. However, it is necessary to note that the repassivation of pits is difficult and results in low potentials. An increase of the solution temperature significantly changes the metal’s surface behavior. All investigated steels at this temperature are susceptible to form pits. The process of forming pits and the growth is particularly dangerous in the steel 18Mn-4Cr because it is characterised by an abrupt increase of the current density and by forming of deep corrosion ulcers on O the metal surface, and repassivation is very difficult. An increase of the chloride concentration in the solution at 65 C leads to a decrease in corrosion resistance of all investigated steels. The phenomenon of a distinct decrease of the corrosion resistance of chrome-manganese steels in the presence of chloride species at high temperatures may be the main cause of stress corrosion cracking of such materials due to a mechanism which is related to the selective dissolution of the areas with high anodic activity. O The temperature 65 C is the main service temperature of retaining rings and a joint action of a specific potent chloride environment and the temperature may lead to cracking of retaining rings and thus is very dangerous because stress corrosion cracking of austenitic high strength steels is characterised by a low rate of crack initiation and a high rate of crack growth. The study of the ionisation process of the investigated steels in 3%NaCl+1N HCl solution shows the following: The ionisation of Fe is more intensive than for Cr. Mn transfers to the solution with the highest rate. Mechanical stresses intensify more the Mn dissolution and less the Cr dissolution. This fact leads to the decrease of ZCr to 0.10-0.26 and the increase of ZMn to 1.452.0. O High temperature water and 22% NaCl solutions also of temperatures below 100 C leads to transgranular stress corrosion cracking –11 with propagation rate of 0.5...40⋅10 m/s. The most aggressive environment for SCC of retaining ring steels is 22% CuCl2 solution. Contact at room temperature with this solution leads to intensive pitting formations, and even to catastrophic intergranular fracture. Thus, preventing the corrosion of the rotor windings made of copper strips and copper slip rings of turbogenerators have great significance. The energy losses for quasi-static crack propagation for TR-orientated 18Mn-18Cr specimens (especially in the case with crack initiation from the inner ring surface) are the highest. This has possible consequences as TR represents the orientation where the retaining ring would have the tendency to split. Cracks of this orientation are subject to the largest stresses. This results was confirmed due to series of SCC testing. It was found that high-nitrogen steel Cr-Mn steels are more resistant to corrosion than the 8-8-4 steel. The latter steel is attacked by pitting in NaCl solutions at the C Cl-- > 0.3 g-ion/l and T > 303 K; its corrosion depth index is as high as 1.48 mm/year in 22% NaCl (348 K). All the steels studied are resistant to pitting in NaCl solutions at C Cl-- < 0.1 g-ion/l and T = 348 K even upon anodic polarization to 1.5 V.
In hot concentrated NaCl solutions, the low pitting resistance of the Cr-Mn steels is due to deteriorated protective properties of their passive films. The electric resistance of these films in 22% NaCl at 348 K is three to six times lower and their capacitance is six to eight times higher than those in 3% NaCl at 293 K. This results permit us to show the common picture of various factors of new generation of high strength steel resistance to fatigue, corrosion fatigue, SCC and CSSCC and to propose the some ways of increasing of exploitation characteristic of Cr-Mn steels. References Balitskii, A.I., Modern Materials for Powerful Turbogenerators, National Academy of Sciences of Ukraine, Lviv, 1999. Balitski A., Krohmalny O., Ripey I., Intern. J. of Hydr. Energy, vol.25, №2, 167-171, 2000. Balyts’kyi O.I., Mater. Sci. , vol.33, №4, 539-552, 1997. Balitskii A.I., Mater. Sci. , vol.34, №4, 113-120, 1998. Balitskii A.I., Mater. Sci. , vol.35, №4, 485-490, 1999. Balitskii A.I., Mater. Sci. , vol.36, №4, 541-545, 2000. Balitskii A.I., Makarenko V.G., Shved M.M., Shokov N.A. In Abstr. of IV Sem. on Hydrogen in Metals, vol.2, Moscow, 1984, 137. nd 8. Balitskii A.I., Shokov N.A., In Abstr. of 2 Symp. on Fracture Mechanics, vol.II, Zhytomir, 1985, 61. 9. Scarlin R. B., Albrecht J., Speidel M. O., In Proc. 8th Int. Brown Boweri Symp., Plenum Press, New-York, London, 1984, 453–461. 10. Speidel M. O., VGB Kraftwerktechnik, vol.61, № 5, 1981, 417–427. 11. Lukas P., Kunz L., Bartos J., Mat.Sci.Eng.,vol.56, 1982, 11-18. 12. Shuju H., Xiaofang L., Shufeng Z., Yuanchun H., Haicheng G., Corrosion, vol. 55, № 12, 1999, 1182 – 1190. 1. 2. 3. 4. 5. 6. 7.
