EP0892078A1 - Grating for a recuperator - Google Patents

Abstract

A grating for a blast reheater consists of a cast iron material which exhibits linear expansion properties and no significant structure changes in the 20-600 degrees C range. The iron has a strength (DIN 1691) of more than 240 MPa at 600 degrees C and exhibits toughness with a strain rate of more than 10%. The cast iron may have (i) a bainite-free structure with vermicular graphite; or (ii) a pearlitic-ledeburitic structure with lamellar graphite and containing 0.6-1.0 wt.% Cr, 0.8-1.5 wt.% Ni and 0.4-0.7 wt.% Mo, the total content of Cr, Ni and Mo being 1.8-3.2 wt.%.

Description

The invention relates to a grate for a hot water heater, which consists of a easily machinable cast material, which is a long-term use with increased Exhaust gas temperatures on the grating up to 600 ° C allowed without permanent operation There is a risk of material failure over a period of 20 years.

Gratings made of cast materials are used in hot air heaters in the grate shafts as Support structure for the trimmings, i.e. for those that use regenerative heat storage inserted storage stones in systems with internal or external Burning shaft.

It is known that in modern systems with gas and air preheating the gratings are exposed to temperatures up to 450 ° C. Even higher application temperatures of up to 600 ° C would make it possible to reduce the stocking volume, to make better use of the preheating systems and thus to increase the overall efficiency of a hot water system while reducing the size by up to 20%. However, application temperatures of up to 600 ° C are dependent on the casting materials of the basic composition of a GG25, i.e. a gray cast iron with lamellar graphite, which were previously used for the production of gratings
and a chemical composition of about 3.3 weight percent carbon, about 0.6 weight percent manganese, 1.8 to 2 weight percent silicon, and more
a chromium content of about 0.7 percent by weight in long-term use.

The task therefore was to close a grate for a hot water heater develop, which consists of an inexpensive to produce casting material, the allows long-term use at elevated exhaust gas temperatures of up to 600 ° C, without fear of material failure.

• ein lineares Ausdehnungsverhalten und keine nennenswerten Gefügeveränderungen im Bereich zwischen 20 und 600°C zeigt;
• eine Festigkeit von mehr als 240 MPa (gemessen nach DIN 1691) bei einer Temperatur von 600°C und
• eine Zähigkeit mit einer Dehnrate von mehr als 10% aufweist.

This task is solved by a grate for a wind heater, which consists of a cast material, the

• shows a linear expansion behavior and no significant structural changes in the range between 20 and 600 ° C;
• a strength of more than 240 MPa (measured according to DIN 1691) at a temperature of 600 ° C and
• has a toughness with an elongation rate of more than 10%.

A casting material that meets these requirements is one of bainitic ones Structure components of free material with the basic composition of a GGG50 ferritic-pearlitic matrix and vermicular graphite, i.e. one Transitional form between lamellar and spheroidal graphite. A cast material with Vermicular graphite combines the good strength properties of the lamellar Graphite cast iron with the better toughness properties and the higher Temperature resistance of a GGG material.

• 0,6 bis 1,0 Gewichtsprozent Chrom,
• 0,8 bis 1,5 Gewichtsprozent Nickel und
• 0,4 bis 0,7 Gewichtsprozent Molybdän,

wobei die Gesamtmenge von Chrom, Nickel und Molybdän zwischen 1,8 und 3,2 Gewichtsprozent des Gußwerkstoffes beträgt.Another casting material that can be used for a grate in a hot air heater at temperatures up to 600 ° C consists of a pearlitic-ledeburitic casting material with a lamellar graphite structure and a content of

• 0.6 to 1.0 weight percent chromium,
• 0.8 to 1.5 weight percent nickel and
• 0.4 to 0.7 percent by weight molybdenum,

the total amount of chromium, nickel and molybdenum is between 1.8 and 3.2 percent by weight of the cast material.

The casting materials mentioned above were on the one hand with a conventional, chrome-alloyed GG25, showing a lamellar graphite structure, a standard material for gratings (V1), but also with Trial bars of gratings from the blast furnace systems of blast furnace 6 Thyssen Stahl AG (V5) and with samples from the grate of the hot water heater A1 des Blast furnace A of Preussag Stahl AG (V6) compared. The grating of the Blast heater system of blast furnace 6 of Thyssen Stahl AG has been around for about 30 years in operation. Blast furnace A of Preussag Stahl AG was built in 1981, whereby the hot air heater was first put into operation without air preheating and since 1986 after installing a heat recovery system Exhaust gas temperatures around 400 ° C was operated. With the grate from the Blast heater system of blast furnace 6 of Thyssen Stahl AG (V5) an unalloyed gray cast iron that corresponds to the composition of a GG25. Of the Grating from the blast heater system of blast furnace A from Preussag Stahl AG (V6) is in its chemical composition with the alloyed GG25 (V1) comparable.

The casting materials mentioned above were also included a material of the basic composition of a GGG60 with pearlitic-bainitic Basic matrix (V3) compared.

The chemical compositions of the grating materials examined are shown in Table 1 below:

The properties of the cast materials examined are shown in the diagrams 1 to 3 enclosed. Fig. 1 shows a schematic representation of the temperature profiles of the gratings of the

Winder heater A1 of the blast furnace A of Preussag Stahl AG and the winder heater of blast furnace 6 of Thyssen Stahl AG. The cyclical change of the is shown different maximum exhaust gas temperature.

2 shows the course of the hot tensile strength (R _m ) as a function of the temperature for the cast materials V1, V2, V3 and V4.

3 shows the temperature-dependent change in length for the materials V1, V2, V3 and V4 for a near-equilibrium heating up to 3 ° k / min Temperature of about 650 ° C.

The temperature profile shown in Fig. 1 shows the constant extreme Temperature changes, which those made from the cast materials V5 and V6 Gratings have been exposed for many years. To the thermal load enormous static, mechanical and oxidative stresses come when one assumes 8 to 10 temperature cycles per day, about 3,000 Temperature cycles per year are the same as those required by a grating Lifespan of 20 to 30 years, a low-frequency continuous load of far result in over 60,000 temperature cycles.

Fig. 2 shows the course of the hot tensile strength (R _m ) as a function of the test temperature. The casting materials V2 and V4 according to the invention show higher hot tensile strengths at all temperatures than the casting material V1 (GG25) corresponding to the prior art. However, the particularly high hot tensile strength of the cast material V3 cannot be used for a certain reason, which can be clearly seen from the diagram in FIG. 3.

The temperature-dependent change in length shown in Fig. 3 shows for the Material V3 in the range between 400 and 500 ° C an inconsistency caused by a Bainite transformation is to be explained. The caused by the bainite transformation Discontinuity in the change in length has the consequence that cast materials with bainitic structure parts not used for the production of gratings because this could lead to tension in the hot air heater would question its stability. Therefore, for the production of Gratings for hot water systems used only cast materials that have a linear Show expansion behavior, i.e. a linear expansion coefficient and have no significant structural changes in the range between 20 and show 600 ° C. The linear expansion behavior required for the Cast materials V1, V2 and V4 available.

However, the conventional casting material V1 is inferior to the casting materials V2 and V4 according to the invention, because - as FIG. 2 shows - its hot tensile strength, measured according to DIN 1691, is significantly below the strength values of V2 and V4. It is noteworthy that a significant decrease in strength occurs at 500 ° C for material V1, while a comparable decrease in strength can only be observed with material V2 from 600 ° C. The tensile strengths determined for the materials examined are summarized in Table 2 below:

The conversion behavior that is important for the mechanical properties of the gray cast iron materials is represented by the dilatometer curves of FIG. 3. The transition temperatures and thermal expansion coefficients found for the alloys examined are summarized in Table 3:

An expansion coefficient of 10.8 µm / mK was determined for materials V1 and V2 with lamellar graphite formation. Since the structure has a significant influence on the expansion coefficient, the measured values of the pearlitic-bainitic V3 and also of the material V4 with vermicular graphite deviate significantly from this value. It is important that the transition temperatures A _c1 and A _{c3 of} the examined casting materials V1, V2 and V4 are sufficiently far away from the required operating temperature of 600 ° C so that a structural transformation is not to be expected. In contrast, the different behavior of the material V3 can be clearly seen.

The examined casting materials show in metallographic examinations The following properties: While the structure of materials V1 and V2 by the usual lamellar graphite arrangement and the pearlitic-ledeburitic Structure is determined, the material V4 is a vermicular Formation of graphite before. The mechanical properties of materials this microstructure formation must be located between gray cast iron and nodular cast iron. The Examination of the cast material V4 has a 30% spherulitic formation surrender. Its matrix is ferritic-pearlitic and contains no ledeburitic Shares more. In contrast, spheroidal graphite cast iron GGG60 (V3) has a bainitic-pearlitic Structure on, for the reasons already mentioned to be less favorable mechanical properties. In addition, the high strength values at a pearlitic-bainitic structure to problems in machining lead, so that this material for the production of gratings in hot air heaters must also be considered unsuitable for this reason.

Crucial for the possible uses of the new cast materials in gratings their long-term behavior is for gas heaters. An investigation of the in the hot air heater Cast materials V5 and V6 that have been used for many years have shown that after 30 years Operating time in the exhaust gas temperature curves shown in Fig. 1 unalloyed material V5 in the examined temperature range up to 400 ° C Strength values between 140 MPa and 135 MPa. It drops to 550 ° C Strength then, however, to a value of around 70 MPa.

The chrome-alloyed material V6 obtained from Preussag Stahl AG shows up to the maximum operating temperature of 420 ° C strength values between 175 MPa and 145 MPa. For both materials V5 and V6 it could be demonstrated that none despite decades of use in the temperature range under consideration significant losses in strength. This can be explained by that these cast materials do not exist in the temperature range examined Experienced structural changes and a linear expansion coefficient demonstrate.

Since these properties also the cast materials V2 and V4 can be predicted that the new invention Cast materials even after an operating time of 20 years in the hot air heater Temperatures up to 600 ° C will still show strength values that 240 MPa will not fall below. That means the required security criteria too after an operating time of 20 years.

• Bei einem Gitterrost aus dem Gußwerkstoff V2 ist eine 5%-ige Gewichtseinsparung gegenüber dem Gußwerkstoff V1 zu erzielen;
• Bei einem Gitterrost aus dem Gußwerkstoff V4 ist eine 20%-ige Gewichtseinsparung gegenüber dem Gußwerkstoff V1 zu erzielen.

Gratings are not subjected to tensile stress in practice, but to bending and pressure. Nevertheless, a high level of security against tensile stress must be required in order not to endanger the long-term stability of the Winderhitzer building. The measured results for the materials V1, V2 and V4 give certainty that the following weight savings compared to the previously used casting material GG25 (V1) are achieved at the current maximum operating temperature of 400 ° C and with a safety factor of 8 when designing a grating:

• With a grating made of cast material V2, a 5% weight saving compared to cast material V1 can be achieved;
• With a grating made of the cast material V4, a 20% weight reduction compared to the cast material V1 can be achieved.

If you compare the permissible stresses of the materials examined, the following permissible stresses result with a safety factor of 8: Permissible stresses of the examined casting materials at 400 ° C Cast material Allowable voltage V1 30 MPa V2 34 MPa V4 53 MPa

Relative to a specific hot water heater with a lattice shaft diameter of 8.5 m, 1,900 t stocking weight and an operating temperature of 400 ° C would when using material V4 compared to material V1 Reduce the grating weight from 89 t to 71 t. The weight calculations are the established allowable stresses. This means that every component of the The grate is dimensioned so that a tension of 34 MPa (V2) or 53 MPa (V4) is not exceeded.

For the design criteria, one for long-term operation of up to 600 ° C developed grating, the following values result:

Both the cast material V2 and the cast material V4 show strength values at temperatures of 600 ° C, which are comparable to the strength value of the cast material V1 at 400 ° C. Based on the permissible voltages with a safety factor of around 8, the following permissible voltages result at an operating temperature of 600 ° C:

Since these stresses are in the range of the previously used 30 MPa, it can be safely concluded that gratings are made of these materials will have approximately the same weights at elevated operating temperatures as the previous gratings made of the chrome alloy cast material V1.

The specified allowable stresses of 30 MPa from the conventional Cast iron grating V1 represent a safety value that has proven itself in recent decades. If one also takes into account that the Design against pressure and bending takes place with the values of the tensile tests, see above there is an additional safety factor. This is a factor of pressure 4 and when bent by a factor of 2 above that of the tensile strength. With these Design values ensure that even in the event of malfunctions caused heating of the grate to temperatures around 650 ° C none there is an immediate risk of mechanical failure.

The same safety criteria for the cast materials V2 and V4 applied would mean that in the event of a fault at temperatures of up to 750 ° C is still sufficient mechanical strength.

The materials according to the invention thus allow, in the conventional Temperature range up to 400 ° C weight savings of the gratings. she Above all, they also enable hot air heaters in a temperature range up to 600 ° C to operate, reducing the size by saving energy achieve a higher efficiency and thus a higher one Offer environmental sustainability. This is associated with material savings of up to 30% without compromising the high standard of security criteria.

Claims (3)

Grating for a hot water heater,
characterized by
that it consists of a cast material that shows a linear expansion behavior and no significant structural changes in the range between 20 and 600 ° C; a strength of more than 240 MPa (measured according to DIN 1691) at a temperature of 600 ° C and has a toughness with an elongation rate of more than 10%. Grid according to claim 1,
characterized by
that the basic structure of the cast material is free of bainitic structure components and at the same time has a vermicular graphite structure. Grid according to claim 1,
characterized by
that the basic structure of the cast material is pearlitic-ledeburitic, at the same time has a lamellar graphite structure and 0.6 to 1.0 weight percent chromium, 0.8 to 1.5 weight percent nickel and Contains 0.4 to 0.7 percent by weight of molybdenum, the total amount of chromium, nickel and molybdenum is between 1.8 and 3.2 percent by weight of the cast material.

Images