DE2347728A1 - COATING DIMENSIONS FOR DELAYING SCALE FORMATION AT TEMPERATURES ABOVE 1177 DEGREES C. - Google Patents

Description

Dip! .- ι

<- ■: - .. un. 'ihjnstt. 43

gh se- i ^r ;? i

P 7040

USS EIiGIHEEES AlID CONSULTANTS, IKC.

600 Grant Street Pittsburgh, Pennsylvania

United States

Coating compound for the retardation of the scale formation

O.

Temperatures above 1177 C

The invention relates to a coating composition for the effective retardation of scale formation at temperatures above about 1177 G (2150 F) as well as a method for retarding scale formation in the heat treatment of ferrous Objects at temperatures above about 1177 G (2150 F) at which one touches at least one surface of the object applies a heat-resistant (weldable: meltable) protective coating. The invention relates in particular to a coating composition for reducing oxidation and scale formation in ferrous objects during heat treatment at high temperatures. -

During the heat treatment of ferrous objects shows the oxidation and the formation of scale on its surface, as well as the subsequent removal of it

4098U / 0960

hot rolling results in a loss of metal which can be quite considerable. This economic loss is even more significant when the article contains substantial amounts of expensive alloying additives. Additional problems arise with certain types of alloys. For example, on alloys containing Mickel, scale forms that is unusually tough and extremely difficult to remove. If this scale s is rolled during the T-Jarmwalz traversing s, recesses (Karben) remain after removal of the eggs resulting inclusions, which must then be removed again by grinding. This leads to an even higher loss of material and to higher operational processing costs.

So far, various methods have been used for. Reducing this Scale formation applied. Although the use of inert or reducing atmospheres in many cases Proven to be effective, it is used for large slabs or ingots because of the substantial increase in operating costs and the considerable capital investments required are not used on a large scale. Often it is also required (e.g. during hot rolling), the slab out of the furnace to take out with the controlled atmosphere while it is still at elevated temperature. At eii.am Such proceedings then inevitably become excessive Oxidation occurs when the slab is exposed to the atmosphere. To the known disadvantages of controlled atmospheres to overcome, a number of ceramic transfers have already been made

* 4098U / 0960

ORIGINAL BATHROOM

trains proposed. Such coatings usually consist of heat-resistant or hard-to-melt oxides such as aluminum oxide, silicon dioxide, magnesium oxide, and fluxes, e.g., silicates, borates, phosphates, in combination with a wide variety of other additives. Protective plates made of heat-resistant materials such as asbestos have also been used to protect against during the Heating to delay the oxidation. These. measures have proven successful in many cases. It was found, however, that under certain conditions these heat-resistant coatings themselves can contribute to the formation of scale depressions / it has in fact has shown that in many cases, for example when the temperature is above about 117 / G (2150 F), this Depressions (carbene) in the coating by implementing the Iron oxide is formed with the silicates. Besides this formation of pits occurs in the presence of silicates general and accelerated attack occurred after about 3 hours at temperatures above 1260 C (2300 F).

Because of these difficulties which occur with conventional, silicate-containing coatings, a new coating has now been developed which contains practically no silicon-containing materials. The basic coating, which contains 75 to 95% by weight of MgO, 2 to 10% by weight of B ₂ O and 0 to 15% by weight of carbon, not only gives better resistance to oxidation at elevated temperatures, but also makes it easier the formation of a scale that can be removed more easily. The corrosion protection can be improved even further, especially in the case of short-term heat treatments (for example

409814/0 960

wisely less than 3 hours) if about 11 to about 17 wt.% TiO _{9 are used} instead of an approximately equivalent amount of MgO.

The invention accordingly provides a coating composition for the effective retardation of scale formation at temperatures above about H77 ° C (215O ° F), which is characterized by a solids fraction in a suitable carrier which makes up about 10 to about 80 % of the total weight of the coating composition and im essentially from 75 to 95% MgO, 2 to 10% ^Β ₂ ^Ο ₃ ? ^{up to 15} % ^{C and} less than 2% SiO ₉ , preferably consisting essentially of 65 to 80 % MgO, 2 to 10% B ₂ O ₃ , 11 to 17% TiO ₂ , 6
up to 12% C and less than 2% SiO ₂ .

The invention is also a method for
Delaying the formation of scale in the heat treatment of iron-containing objects at temperatures above about 1177 C (2150 F), in which a heat-resistant or difficult to melt protective coating is applied to at least one surface of the object, which is characterized in that a coating mass which is in consists essentially of 40 to 70 % solids in a suitable carrier, the solids in turn consisting essentially of 75 to 95 ⁰ J ₀ MgO, 2 to 10 % B ₂ O ₃ , up to 15% C and less than 2% SiO ₉ or preferably consisting essentially of 65 to 80% MgO, 2 to 10 % B ₂ O, 11 to 17% TiO ₉ , 6 to 12% G and less than 2 % SiO ₉ , sprayed on in such an amount that a protective

* 409814/0960

coating from about 0.04 to about 0.25 g / cm thick Surface of the object is obtained.

The invention is described below with reference to attached drawing, which is a diagram showing the protection number (protection value) of a Steel article with a coating of the invention Coating mass compared with that of a bare steel and that covered with asbestos Stahls is specified.

The coating composition according to the invention is coated with a suitable one Carrier mixed and applied to the substrate in the usual way. It can be in the form of a slurry mixed or suspended using a suitable dispersant. In the following The examples given are a series of particularly effective carriers.

To evaluate the experimental data obtained on each coating tested and to compare this data with those involved in the oxidation of bare and asbestos-covered 3 Ni steel, which lasts for 1, 2, 4 and 6 hours had been heated to 704.C (1300 F) in the air, obtained VTUrden, a computer program was set up. Around To arrive at a standardized protection number, the weight loss (used as the standard) was the asbestos covered samples by the weight loss in the respective Test divided and multiplied by 100. This resulted in an empirical rating system for the protection numbers, in

4098U / 0960

change the asbestos the VJerl :. 100 and the bare steel the value 67 (see the accompanying drawing). To make the comparison To simplify, the Sehufczzabien for the 1-, 2-, Heat each coating for 4 and 6 seconds and add it through Divided 4 to get an average guard number.

The protection for a number of B ^ O ^ periclase formulations, those with a carrier-binder system, including from 30 ml shellac in 180 ml / 'ithanol, made vmrden, are given in Table I below.

409814/0960 BATH ORIGINAL

Table I.

Over- To G amines set- S chut ζ number (asbestos = 100) passage of the festival r. - ~~~ - ~ * ~~ - ~ _: - ~~~~~ -, »~~ _τ - ~ -» .. ™ - ^ ™ - _τ - ~ - ™ - ~ - ~ schnltt- ,, °, rr-. c, 1 hr. 2 sid. 4 hrs. B hrs. Through,. ,
Kr. Disturbed in%,. ,, borrowed

cut " , . ,

O7 "~ mSÖ ~~ coating

2 3 °. conditional

weight
(g / cm)

1 50 50 52 43 ^{+) +)} - - 0.0930

2 25 75 67 67 .81 86 75 0.1067

3 11 89 80 81 87 76 81 0.0789

4 9 91 91 84 114 103 98 0.2090

5 7 93 87 86 123 119 104 0.2171

6 6 94 100 95 113 126 108 02018

7 5 95 112 116 127 153 127 0.1549

Oie sample vmrde completely ox} ^'r diert.

The above data clearly shows the superiority of the B "0" periclass coatings, in particular with heating periods of more than about 2 hours. It has been found, however, that further improvements can be achieved in that.
my up to about 15 % finely divided carbon adds. Of the
The slower-burning carbon apparently provides additional protection during the breakdown of the organic
Binder. The dinrch addition of finely divided carbon
(in this case coke breeze) achieved improved protection

40 9 8U / 0 960

from the following Table II, in which the same ethanol / shellac carrier / binder system was used.

Table II

Over- composition chutzzahl S (asbestos = 100) passage of the solids for 1 hr. 2 hr. 4 hr. 6 hrs. Average no. in % . cut borrowed

Loading

₀ O ₀ MgO coke -,

2 3 scinch-

ttmgs weight,

8 4.6 88.3- 7.1 170

9 4.7 85, A 10.3 144 10 5 85 10 118

185 148 161 kV
( g / cm ") 141 160 149 146 0 , 1465 130 153 130 126 0 , 1650 103 0 , 1553

It is assumed that the superiority of the coatings according to the invention is due to the formation of solid magnesia-wustite solutions which have better protective properties than normal scale. Studies have also been conducted to determine whether the introduction of a third refractory (difficult to melt) oxide will further limit the solubility of FeO in MgO. Of a number of such oxides that were examined, only TiO _{9 gave} an improvement in the scale retardation of the basic (MgO-B ₂ O ₃ -C) -E

4098 14/0 960

In the experiments described in Table III below, 200 g of a solid mixture were suspended in an aqueous carrier containing 100 ml of water, 1 g of Ilydroxyäthylcellulose, 13 ml of concentrated ΝΗ, ΟΗ and 10 ml of a 25% ethylene / acrylic acid copolymer. The results reported for "Control Sample ¹ " are the average of 5 separate tests. Certain other samples listed below accordingly represent the average of more than one test.

Addition of amines
Solids in% coke MgO Table III 1 H. Guard number 4 hours 6 hours By
cut By
cut
liches Above
train
No. ^B 2 ° 3 7.1 87.9 the
(D 113 2 hours. 134 129 128 Be
schich
tion s-
ge
wichfc_
(g / cm) 5 Il 83.5 TiO ₂ 148 125 127 133 133 0.1021 Con
troll
sample If ti 79.1 129 124 144 86 116 0.1028 11 Il ti 74.7 4.4 160 137 - 205 175 0.1152 12th ti ti 73.4 8.8 164 162 121 104 134 0.1326 13th η Il 73.0 13.2 180 146 110 111 141 0.1149 14th Il It 72.5 14.5 218 163 207 149 192 0.1025 15th Il dd 72.1 14.9 177 196 119 132 135 0.1089 16 Il It 71.6 15.4 181 113 113 116 141 0.1103 17th It Il 70.3 15.8 117 153 118 77 101 0.1146 18th Il 16.3 92 0.1402 19th 17.6

The solids content in all cases was 200 g, ^{1 409814/0960}

The effectiveness of TiO ^ additives within the range from about 11 to about 17 % TiO 2 was particularly pronounced for short annealing times (for example of less than 3 hours). With annealing times of 4 hours and more, the _{coatings containing TiO 9} corresponded in terms of protection number approximately to those in which no TiO _{9 was} used.

The coating compositions according to the invention can be formulated as follows will:

When TiO _{9 is used} within the preferred range of 11 to about 17 percent, the MgO content of the solids fraction can be within the range of about 65 to about 80 percent. Considerably lower values of TiO ₇ , for example 1 to 5%, can be disadvantageous for the basic (MgO-B ₉ O ^ -C) coating. If TiO _{9 is} not used within the preferred range, the MgO content should be the solid

"ι

fraction always be greater than about 75 % and preferably greater than about 82% (which depends somewhat on the concentration of the other components).

Various forms of MgO, such as magnesite and precipitated magnesium oxide, provide some protection. However, it has been found that the more densely packed (more densely grained) periclase provides much better protection. Because of the adverse effect of SiO ₉ mentioned above, the periclase should be as pure as possible. In all cases the SiO ₉ content should be below 2%, preferably below 1%. The particle size should expediently be small

409814/0960

must be less than 0.25 mm (60 mesh), especially if the cover is sprayed on (instead of painted on). For a previously unknown reason, the protective effect becomes clear improved when the periclase ^ particle size is within the very narrow range from 0.125 to 0.104 mm (120 to 140 mesh).

When the borate content in the solid fraction is less than. is about 2%, there is a loss of adhesion of the coating at high temperatures and the protection achieved is less than that of asbestos. However, no more than about 10% borate should be used, since this additive increases the rate of scaling, similar to SiO ". A particularly advantageous range is from about 5.0 to about 7.5 % B "0". Since the addition of foreign cations is preferably avoided, the borate should either be in the form of (anhydrous) B ^ O ,. or added in the form of boric acid.

While C in any amount up to about .15 % provides improved protection, it is preferably used in an amount greater than about 6%. Any known source of finely divided carbon, for example pine black, coke breeze, can be used. During the initial stage of the annealing treatment, the carbon burns out of the coating. Therefore, the use of C in an amount greater than about 12 % (based on the solids fraction) or in a particle size greater than 0.25 mm (60 mesh) should preferably be avoided in order to prevent excessive porosity. Maximum benefits

* 409 ^ 14/09

are achieved if the particle size is below 0.074 mm (200 mesh).

Depending on the method of application and the parameters of the heat treatment, a wide variety of carrier-Birideinmittel formulations can be used can be used. The carrier itself can either be organic or based on water. For safety reasons a water-based carrier is preferred. Although the solids content can vary from 10 to 80%, is a more limited range of e.g. 40 to 70% preferred, especially when the coating is sprayed on. A solids content below about 40% would be excessive Heating required to evaporate the support, while difficulties with solids content greater than 70% could occur when spraying. The Suspenslcra the solids in water makes the use of a thickener to increase the viscosity and a dispersant to keep the particles in suspension. To achieve "green strength" (low temperature cohesion) a binding agent is required for the coating. However, if the total amount of organic material (Thickener + binder) about 2% of the "total weight derι exceeds solids, the coating dissolves in some cases from the substrate even if the binder provides excellent cohesion of the particles.

A specific example is given below in which a preferred water-based formulation (as described in attached drawing) was used. That

4098 147 0960

Carrier-binder system had the following composition ;

1 to 100 ml H "0

2 to 1 g of carboxyethyl cellulose

2 to 13 ml of NH. OH
4th

4 to 10 ml of a 24% ethylene / acrylic acid mixture * Polymer

The ingredients were added and mixed together in the order listed to maximize the effectiveness of the Maintain mixed polymer. The solids,. which actually form the coating have been separated into mixed in the following proportions:

178 g periclase, particle size 0.25 ram (60 mesh) 10 g BO ■ ■

12 g fine coke breeze (petroleum)

The solid fraction was then added to the carrier with stirring on a water basis and added to the steel slab to be protected sprayed on.

Regardless of the carrier system used and the method of application used, the coating should be within the range of at least about 0.04 to no thickness

2
greater than about 0.25 g / cm can be applied. Below

2
0.04 g / cm, the protection is completely inadequate. To provide adequate protection over a longer period of time than about

4098U / 0960

3 hours, vox * is supplied with a cover is used that is at least about 0.08 g / cm thick. the

?
the upper limit of 0.25 g / cni results from the inclination

thicker coatings, flaking off in case of thermal shock. Around these. Keeping inclination to a minimum is one thing in particular

2 upper limit of the thickness of about 0.15 g / cm 'complied with.

The invention has been described above with reference to preferred embodiments explained in more detail, but it is clear to those skilled in the art that these in many respects can be changed and modified without thereby departing from the scope of the present invention

409814/0960

Claims (20)

P ate η t an s ρ r ü c he (1. Coating compound for the effective retardation of scale formation at temperatures above about 1177 C (2150 F), characterized by a solids fraction in a suitable carrier, which makes up about 10 to about 80 % of the total weight of the coating compound and essentially 75 to 95 % MgO, 2 to 10% B ₂ O ₃ , up to 15% C and less than 2% SiO ". 2. Coating composition according to claim 1, characterized in that it contains periclase as MgO and that practically all of it MgO and all of C have a particle size less than 0.25 mm (60 mesh). 3. Coating composition according to claim 1 and / or 2, characterized in that the solids fraction makes up about 40 to about 70% of the total weight of the coating composition and that the SiO »content is less than about 1 % . 4. Coating composition according to claim 2 and / or 3, characterized in that the periclase content is more than about 82%, the B ₉ 0 "content is about 5 to about 7.5% and the C content is about 6 to about 12% %. 5. Coating composition according to at least one of claims 2 to 4, characterized in that practically the entire periclase A098U / 0960 particle size within the range 0.125-0.104mm (120-140 mesh) and practically all of the C content have a particle size of less than 0.074 mm (200 mesh). 6. Coating composition according to at least one of claims 1 to 5, thereby characterized in that the carrier consists primarily of water and a minor amount of an organic dispersant and an organic binder, the total content of organic material being less than is about 2%. 7. Coating compound for effectively retarding the formation of scale at temperatures above about 1177 G (2150 F), characterized by a solids fraction in a suitable carrier which makes up about 10 to about 80 % of the total weight of the coating compound and consists essentially of 65 to 80% MgO , 2 to 10 % B ₂ O ₃ , 11 to 17 % TiO ₂ , 6 to 12% C and less than 2% SiO ~. 8. coating composition according to claim 7, characterized in that the solids fraction makes up about 40 to about 70% of the total weight of the coating composition and that its Si0 _o content is less than about Ϊ % . " 9. Coating composition according to claim 7 and / or 8, characterized in that that their B "0" content is about 5 to about 7.5%. 409814/0960 10. coating composition according to at least one of claims 7 to 9, characterized in that the carrier mainly consists of Consists of water and contains a small amount of an organic dispersant and an organic binder, wherein the total organic material content is less than about 2%. 11. Process for retarding the formation of creases in the heat treatment of ferrous objects at temperatures above about 1177 C (2150 F) at which a heat resistant Applies (difficult to melt) protective coating, characterized in that a coating compound 'which essentially from 40 to 70% solids in a suitable carrier . stands, the solids in turn consisting essentially of 75 to 95% MgO, 2 to 10% B ₉ O., up to 15% C and less than 2% SiO ₉ , sprayed on in an amount such that a protective coating a Thickness from about 0.04 to about 2
0.25 g / cm surface of the object is obtained. 12. The method according to claim 11, characterized in that the heat treatment is carried out over a period of more than 1 hour and used as the MgO periclase, practically all of the periclase and all of the carbon being one Have particle size less than 0.25 mm (6.0 mesh). 13. The method according to claim 12, characterized in that one Periclase in an amount greater than about 82%, B ₉ O ₁ - in an amount from about 5 to about 7.5%, C in an amount within 409814/0960 of the range from about 6 to about 12% and SiO "in one Amount used less than 1%. 14. The method according to at least one of claims 11 to 13, characterized in that the coating in a thickness of about 0.08 to
Ranks. 2
from about 0.08 to about -0.15 g / cm surface of the counter 15. The method according to at least one of claims 11 to 14, characterized in that a steel containing Ni is used as the iron-containing object and the heat treatment is applied
Temperatures above about 1260 C (2300 F) over a
Performs for a period of more than about 3 hours. 16. Method of retarding scale formation in heat treatment of ferrous objects at temperatures above about 1177 C (2150 F), which is at least a surface of the object applies a heat-resistant (difficult to melt) protective coating, characterized in that that a coating composition consisting essentially of about 40 to about 70% solids in a suitable Carrier consists, with the solids in turn consist essentially of 65 to 80 % MgO, 2 to 10 % B ₃ O ₃ , 11 to 17% TiO ₂ , 6 to 12% C and less than 2% SiO ₂ , sprayed on in such an amount that a protective Coating a thickness of. about 0.04 to about 0.25 g / cm Surface of the object is obtained. 409814/0960 "'- 19 - ■ 7347728 17. The method according to claim 16, characterized in that there is used as MgO periclase and that practically the entire Periclase and all carbon have a particle size of less than 0.25 mm (60 mesh). 18. The method according to claim 16 and / or 17, characterized in that B "0" is used within the range of about 5 to about 7.5% and SiO "in" an amount of less than 1 % . 19. The method according to at least one of claims 16 to 18, characterized in that the coating is in a thickness from about 0.08 to ι Ranks. from about 0.08 to about 0.15 g / cm surface of the counter 20. The method according to at least one of claims 16 to 19, characterized in that the ferrous object uses a steel containing Ni as an appropriate alloying additive and the heat treatment Temperatures above about 1260 ° C (2300 ° F) over a Period of less than about 3 hours. 4098U / 0960 Blank page