FR2475577A1 - FORGED PART ALLOY FOR HIGH PRESSURE TURBINE ROTOR - Google Patents

Abstract

A steel alloy for forged rotors of high-pressure turbines is proposed which results in better characteristics in the region of the central part of the rotors. The alloy contains, in % by weight, from 0.25 to 0.35 % of C, from 0.15 to 0.35 % of Si, up to 1.00 % of Mn, from 0.90 to 1.50 % of Cr, from 1.00 to 1.50 % of Mo, from 0.20 to 0.30 % of V, up to 0.75 % of Ni, up to 0.015 % of P, no more than up to 0.002 % of S, the remainder essentially being iron.

Description

 The present invention relates to an alloy and more particularly to an alloy improvement for a high pressure turbine rotor forging such as is used to produce energy from a fossil fuel.

 To date alloys for forgings for high-pressure steam turbine rotors have been used, alloys having the chemical compositions shown in Table 1 or similar chemical compositions and in practice all these alloys contain 0.004 to 0.009% by weight of sulfur.

TABLE 1
Example of the chemical composition of a forged steel for high pressure turbine rotor (% by weight)

<tb> C <SEP> If <SEP> Mn <SEP><SEP><SEP> Cr <SEP> V <SEP> N <SEP> P <SEP> S
<tb> 0.23 <SEP> 0.15 <SEP> to <SEP> 0.90 <SEP> 1.00 <SEP> 0.20 <SEP> to <SEP> to <SEP><SEP> jus- <SEP>
<tb> a <SEP> to <SEP> than <SEP> | <SEP> until <SEP> only <SEP> than <SEP>
<tb><SEP> 1.00 <SEP> a <SEP>, 75 <SEP><SEP> 0.015 <SEP> 0.018
<tb> 0.35 <SEP> 0.35 <SEP> 1.50 <SEP><SEP> 1.50 <SEP> 0.30 <SEP>
<Tb>
Note t according to the "ASTM A470 class 8" standard.

Recently, however, the power generation industry has presented the following requirements for high pressure rotor rotors used to date and having the above-mentioned chemical compositions.
(1) Inspections of power generating plants used for a prolonged period of time have shown that cracks have occurred in the adjacent part of the central hollow of the high pressure turbine rotors. It has therefore been established that the properties of the part close to the central hollow of the high-pressure turbine rotors are very important and one must take special measures concerning them; and
(2) As in Japan in particular the regulation of the intensity of the electric current requires cyclic work, even in large capacity power plants, the forged part of the rotor must lend itself to these conditions of use.

 To meet these requirements, the turbine rotor manufacturing industry has considered various measures such as improvements in comparative tests used in the technology of casting rotor forgings as is the case in the United States, but To date, no definitive measure has been announced or is in sight.

The subject of the invention is
a forged alloy for a high-pressure turbine rotor satisfying the aforementioned requirements of the electric power generation industry, having a remarkably increased metallurgical and physical uniformity, toughness, which is closely related to the mechanical properties of breakage, remarkably improved so that the forging can withstand any severe operating conditions and suitable not only for forgings for high-pressure turbine rotors but also for low-pressure turbine forgings, rotor forgings generators, etc.

 According to the invention, a forging alloy for a high-pressure turbine rotor is characterized in that it consists essentially, in percentages by weight, of 0.23 to 0.33% of carbon, from 0.15 to 0s35 * of silicon , not more than 1.00 * manganese, 0.90 to 1.30 * chromium, 1.00 to 1.50 * molybdenum, 0.20 to 0.30 * vanadium, at most 0.75 * of nickel, not more than 0.015 s of phosphorus and not more than 0.018 * of sulfur, the balance being practically iron, the content of sulfur determined by casting analysis being not more than 0.002 *.

Other features and advantages of the invention will become apparent from the following description, made with reference to the accompanying drawings in which
FIG. 1 is a histogram of the relationship between the sulfur content (value determined by casting analysis) and the number of defects (detected by the magnetic particle test per 100 cm 2) in the central part of the rotor forgings. high pressure turbines.

 The figs. 2a and 2b show histograms expressing on the ordinate the frequency (*) of the upper energy plateau (in the radial direction) vEmax (J) at the surface and in the central part of the forged pieces for twists.

 The figs. 3a and 3b show hiatograms expressing the frequency (*) of the energy absorbed at the ordinary temperature (in the radial direction) GREEN (J) of the rotor forgings.

 The figs. 4a and 4b show histograms expressing the frequency (*) of FATT 50% (C) (in the radial direction) of rotor forgings.

 FIG. 3 represents a diagram expressing the relationship between the upper energy plateau in the longitudinal direction (J) as the abscissa and the upper energy plateau in the radial direction (J) in terms of the rotor.

 FIG. 6 illustrates a diagram expressing the relationship between the decrease in the area (, ') in the longitudinal direction in the abscissa and the decrease in the area (,') in the radial direction in the ordinate of the forged rotor parts.

 FIG. 7 represents a histogram expressing the frequency (*) of the difference # # between the values of the analysis at casting and the counter analysis values of forged parts for rotors.

FIG. 8 illustrates a diagram expressing the relationship between the sulfur content according to the casting analysis and the impact resistance in the radial direction (energy absorbed at the ordinary temperature expressed in
J) of the central part of the forgings for rotors.

Fig. 9 is a diagram similarly illustrating the relationship between the melt analysis sulfur content and the upper energy plateau (J); and
Figure 10 shows a diagram expressing a similar relationship between the sulfur content according to casting analysis and FATT 30 * (OC),
In order to achieve the objectives of the invention, the properties of forgings have been studied in detail. for conventional turbine rotors and turbine rotors manufactured by a process developed to reduce the sulfur content to as low as 0.001% by weight or less. It has been confirmed that if the sulfur content is reduced below 0.002% by weight (casting analysis value), the properties of the central part of turbine rotor forgings can be remarkably improved. According to the invention, in order to impart to the central part of a rotor forgings the various characteristics which are lacking and for which to confer uniform and improved metallurgical and physical characteristics, an alloy having a sulfur content of 0.002% by weight is prepared. or less (value of the analysis at casting).

 It is generally known that sulfur has an undesirable effect on the properties of steel. However, the increase of the accuracy of the criteria of the tests with magnetic particles has recently revealed, towards the central part of a piece forged for rotor, defects whose existence was not previously known, which shows that we had partially the influence of sulfur on a piece forged rotor and, in particular, the influence of the sulfur content. Consequently, in addition to the need to establish the quantitative relationship between the sulfur content and the properties of the part In the center of a forged part for burting, a technique has been developed to reduce the sulfur content of a rotor forging to an extremely low value and has carried out various researches and studies on forged parts for rotors made using this newly developed technique. on point. The invention follows from the results thus obtained.

 The invention will now be described in detail with reference to the accompanying drawings and photographs.

 First of all, the results of a study on classical alloys for parts will be exposed.

forged for high pressure turbine rotors.

 A. In a conventional alloy (sulfur content greater than 0.004% by weight), as there is a segregation concentration of sulfur in the upper central portion of the ingot, when forging the ingot into a rotor, a defect (non-inclusion) metal) can be detected in the central part of the forged part by a test with magnetic particles (see figure 1 and photograph no 1), a defect in the central part being considered as one of the causes of the cracks of the central part of the forged part. Figure 1 is a histogram showing the relationship between the sulfur content (value of the casting analysis) and the number of defects (non-metallic sulfide inclusions) of the central part of the part. forged rotor, the sulfur content (* by weight) being represented on the abscissa and the number of defects (per 100 cm) detected according to the test with magnetic particles being represented in order and photograph 1 is an example of a defect (considered to be a sulfide-type non-metallic inclusion) detected with magnetic particles.

 B. In conventional alloys, the differences in impact resistance properties between the center portion and the surface of a rotor forging are important. See FIGS. 2 to 4. In particular FIG. 2 is a histogram where the upper energy plateau (radial direction) yEmax (J) is represented on the abscissa and the frequency (%) is represented on the ordinate, which shows that the plateau higher energy is 102 J on average for the central part while the average on the surface is 122 J. As shown in Figure 3 which is a histogram where the energy absorbed at ordinary temperature (radial direction) GREEN (J) is plotted on the abscissa and the frequency (*) is shown on the ordinate, the energy absorbed at the ordinary temperature is 9.5 J on average for the central part while on the surface it is 14.2 J on average.

Thus Figures 2 and 3 show that the central part has considerably less energy than the surface in both cases. Moreover, as shown in FIG. 4, which is a histogram where FATT 50 * (radial direction) (OC) is represented on the abscissa and the frequency (*) is present on the ordinate, the FATT 50 ffi for the central portion is + 105C on average and +87 C on the surface, the value of the central part being considerably lower than that of the surface.

 Since these properties are closely related to fracture toughness, in the case of crack formation at the central recess of a rotor forging, it can be inferred that they are related to the bursting of the rotor. rotor of a turbine.

 C. In the central portion of a rotor forging, the difference in surface reduction and impact resistance properties between the radial direction and the longitudinal direction is important.

See FIGS. 5 and 6. As shown in FIG. 5 which is a histogram in which the upper energy plateau (longitudinal direction) (J) and the upper energy plateau (radial direction) (J) are respectively represented. on the abscissa and on the ordinate, the upper energy plateau in the radial direction is about 95 J while in the longitudinal direction it is about 122 J. Moreover, as shown in FIG. 6 which is a diagram of the the relationship between the radial directional area decrease and the longitudinal directional area decrease, where these percentage area decreases are respectively plotted on the abscissa and the ordinate, the radial directional area decrease is, for example, about 50% in the radial direction; % while the area decrease in the longitudinal direction is about 58 *.

As shown in Figures 5 and 6, the values in the radial direction are significantly lower than the values in the longitudinal direction. From this result, it is considered that for a crack in the central recess of a rotor forged part, the stress resistance force in the main direction (radial direction) which is associated with crack formation and their propagation is weak and the low-cycle fatigue resistance described below is impaired.

 D. With regard to the second requirement above, low cycle fatigue resistance has been considered.

où AE t gamme des tensions;
N t nombre des cycles à la rupture;
t t diminution de la surface;
e-w : résistance à la traction/2; et
E t module d'Young. Since the range of low-cycle fatigue strength and the number of cycles at break are influenced by the decrease of the surface, an improvement in the reduction of the surface is essential, but as already shown in Figure 6 , the decrease in the area in the central part of a rotor forging has a lower value in the radial direction, but also this value of the decrease in the area is small, so that it is considered that the decrease in the surface of the central part of a conventional rotor forging is unfavorable with regard to the low-cycle fatigue strength. In this respect, the following formula is known

where AE t range of voltages;
N t number of cycles at break;
tt decrease in area;
ew: tensile strength / 2; and
Young's modulus.

 It is apparent from the preceding discussion that a conventional alloy for forgings for high-pressure turbine rotors poses numerous problems relating to the properties of its central part.

Consequently, according to the invention, the sulfur content of the alloy is of interest and the alloy of the invention is characterized in that its sulfur content is less than 0.002% by weight according to the invention. casting analysis. The fact that all the problems typical of conventional alloys can be entirely solved by virtue of a sulfur content according to the invention will be explained in relation to embodiments of the invention and to experimental results.

 First of all, alloys for forgings for rotors of high pressure turbines having chemical compositions in accordance with the invention were melted in an electric furnace and after having desulfurized in a ladle, were cast under vacuum into ingots.

 The ingots were subjected to a conventional forging and heat treating operation and forgings were made for high pressure turbine rotors which were subjected to various tests. By way of example, the results of the casting analysis of an alloy of the invention and a conventional alloy are shown in Tables 2 and 3.

TABLE 2 Example of analysis result at casting
, (% by weight)

<tb><SEP> G <SEP> If <SEP> Mn <SEP> Cr <SEP> No <SEP> V <SEP> Ni <SEP> P
<tb> Alloy <SEP> of <SEP>
<tb> The invention <SEP> 0.31 <SEP> 0.230.78 <SEP> 1.13 <SEP> 1.15 <SEP> 0.24 <SEP> 0.33 <SEP> 0.007 <SEP>
<tb> classical <SEP> 0.30 <SEP> 0.28 <SEP> 0.75 <SEP> 1.15 <SEP><SEP> 1.10 <SEP> 0.24 <SE> 0.33 <SEP> 0.005 <SEP>
<Tb>
TABLE 3 Sulfur Content (% by Weight)

<tb><SEP> Scan <SEP> to <SEP><SEP> Scan <SEP> of <SEP> Control
<tb><SEP> casting <SEP> to <SEP> the <SEP> part <SEP> supe
<tb><SEP> upper <SEP> of the <SEP> ingot
<tb> Alloy <SEP><SEP> t <SEP> from
<tb> the invention <SEP> 0.002 <SEP> 0.002 <SEP>
<tb> Alloy
<tb> classical <SEP> 0.006 <SEP> 0.008
<Tb>
Note: In general the results of the analyzes present
slight differences depending on the method of analysis.

 As shown in Table 3, with respect to a conventional alloy, in the alloy of the invention, the sulfur content (% by weight) corresponding to the casting analysis has a very low value of 0.002 * and the homogeneity is also very good, the analytical values showing practically no discernible dispersion.

This fact also emerges from the examination of FIG. 7 which represents a histogram showing the relationship between the importance of the differences between the values of the analysis at casting and the values of the control analysis and the corresponding frequency. This fact also arises from the examination of the photographs Nos. 2 to 3, which are sulfur inserts respectively of a longitudinal section of a forged piece for a conventional rotor (the sulfur content of which is 0.006% by weight according to the casting analysis), a longitudinal section of the ingot according to the invention (whose sulfur content is 0.002% by weight according to the casting analysis), a cross-section of the central portion of a conventional rotor forging and a cross section of the central portion of the rotor forging according to the invention.

Examination of Tables 2 and 3 and Photographs 2 to 5 shows that in the alloy according to the invention, there is no segregation of sulfur which suggests that in this alloy the defects of the central portion that the In a conventional rotor forging are removed, that is, crack formation at the central recess of the rotor forging is made difficult.

 Although only one example of experimental results corresponding to an ingot of the alloy according to the invention with respect to an ingot of the conventional alloy has previously been described, the alloy according to the invention and the conventional alloy between which There are important differences in the sulfur content and segregation of sulfur as previously indicated, were respectively used to manufacture rotor forging according to a forging process and a conventional thermal treatment and then the forgings were subjected to different tests. The results of the tests are illustrated in FIGS. 2 to 6 which make it possible to compare them with the tests obtained with a conventional alloy as previously indicated. These results will now be explained.

 A. Figure 2 shows that the upper energy plateau in the case of an alloy according to the invention is 136 J on the average surface and 126.8 J average for the central part. Therefore, when comparing with the corresponding values of the conventional alloy, it appears that in the alloy of the invention the difference between the two values is reduced and that the level of the energy is higher.

 Similarly, as shown in FIG. 3, the energy absorbed at the ordinary temperature in the radial direction is 14.1 J on average at the surface and 11.4 J on average for the central portion. Thus with respect to the conventional alloy, it is evident that in the steel of the invention, the difference between the average values of the surface and the central portion as well as the level of these values are improved.

Moreover, as shown in Figure 4 which represents
FATT 50 * in the radial direction, it is apparent that the value of the central part is improved with an elevation reaching 97,4eu and that of the surface reaches 84,5 C, these values being better than those of the conventional alloy .

 Thus, the fact that the impact strength properties of the central part of a forged part for burping are remarkably improved and at the same time the level of impact resistance properties of the forgings for burping is increased as a whole reveals that even if a crack is formed in the central recess of the rotor forging, the risk of rupture is low.

 B. Also as the upper energy plateaus of the central part of a rotor forging piece in the longitudinal direction and in the radial direction are respectively as shown in Figure 5 of 122 J and 118 J, we see that the The improvement in the radial direction is remarkable and the energy level is improved as a whole. This means that the breaking strength of the rotor is increased.

 C. Finally, as shown in FIG. 6, the decreases in the area of the central portion of a rotor forgings in the radial direction and in the longitudinal direction are respectively 38 * and 37%, which shows that Increasing in the radial direction is remarkable and the level of values in both directions is greatly increased so that low frequency fatigue resistance is increased over the conventional alloy. It is also appropriate here to refer to the equation concerning A t previously indicated.

 Figures 9 and 10 are diagrams showing the relationship between the impact properties and the sulfur content resulting from the various results previously discussed. These diagrams show that if the sulfur content (casting value) falls below 0.002% by weight, the impact properties are considerably increased. In the invntion the reasons for which the sulfur content determined by the analysis to the casting is limited to a maximum of 0.002 * by weight derive from the above results, because, so far, for a piece forged for rotor, the relationship between this extremely low sulfur content and the properties of impact resistance had never been considered quantitatively, and the invention shows for the first time that this limitation of the sulfur content is an essential condition to remove in a rotor defects such as those observed at the central recess of the rotor forging, for example non-metallic inclusions and anisotsopie the decrease in the surface and significantly improve the values of various tests.

Claims (1)

 CLAIM  Forging alloy for a high-pressure turbine rotor, characterized in that it consists, in percentages by weight, of 0.25 to 0.35% of carbon, of 0.15 to 0.35% of silicon, not more than 1.00% manganese, 0.90 to 1.50% chromium, 1.00 to 1.50% molybdenum, 0.20 to 0.30% vanadium, not more than 0.75% of nickel, of not more than 0.015% of phosphorus and not more than 0.018% of sulfur, the balance being essentially of iron, the sulfur content of this alloy, determined by the casting analysis, being at most 0.002%.