PROCEDURE FOR THE PRODUCTION OF A PROTECTIVE LAYER ON WITH HOISTING GASES, IN PARTICULAR FLUE GASES SUBJECTED METALLIC WALLS

Procedure 'for producing a protective coating on metallic wails exposed to hot gases, especiaiiy flue gases The invention refers to a process for producing a protective .;oating on walls in oarticul.r -vails of combustion plants c hrat oxch inge made of meraliu; base matona' -nd exposed to hot gases, especially flue gases 'n which a powder of metallic, curtide oxide coramic or silicide materials or any mixtures thereof, is applied onto the previously cleaned metallic walls by means of the plasma jet technique to form the said prolective \ojtinq Such protective coatings are to be applied, for instance, onto the cooling walls -J heat recuperators tn steel converter plants These cooling walls are subject to extremely high stresses because the flue gases flowing along one side of the same have a lerrperaturc of approx 1400 to 1800 "C and arc loaded with ashes and slag particles while the saturated steam pressures reigning on their other side are approx 20 to 80 bar whereby 'he steam cooled tubular walls have internal pressure gradients of up to 2 bar/mm DE 23 55 532 C2 has already disclosed a process for deposition welding of metal and alloy powders on a sand blasted and preheated metal surface in which the said metal surface has previously been heated to a temperature of 100 'C minimum to approx 650 C Both in deposition welding oy means of rod oiectrooes and n powaer deposition w.jidir j or name spraying with subsequent melting down, the base material is heated up to a very high temperature during application of the protective coating which results in an undesired microstructural change Especially in name spraying, the melting-down temperature is between 980 and 1060 "C depending on the spraying powder used. Besides, the high heat input causes distortion of the walls to be coated which may make installation of these walls problematical and result in additional costs due to the dimensional inaccuracy If the protective coatings are applied afterwards by these state-of-the-art techniques the stresses caused by the high temperature cannot react in the form of distortion but entail cracte in the surface of the installed walls, especially in weld regions. Thickness of the protective coating is approx 8 to 10 mm in deposition welding and 1 to 2mm in flame spraying. Furthermore, DE-AS 26 30 507 has disclosed a process for producing protective coatings against hot gas corrosion and/or mechanical wear and tear on work pieces in which a coating powder consisting of different alloys is applied onto the work piece by plasma spraying in vacuo. For carrying out the coating, this vacuum spraying process requires a treatment chamber not accessible from the outside in which a vacuum has to be generated with considerable expenditure. The process is therefore not applicable to large walls such as installed, for instance, in heat recuperators. It is the aim of the present invention to ameliorate at least one of the problems of the prior art. It is an advantage that a process according to one aspect of the invention may reduce distortion of the work pieces and cracking stresses in the base material. In a process embodied by the invention, not only the surface of the walls is roughened before the powder is being applied by the atmospheric plasma jet technique, but also the base material of the walls is activated in such a way that disturbances in the metallic lattice are produced which increase the adhesive forces. Immediately following such activation, that is to say, before the said disturbances in the metallic lattice have been neutralised again, the powder is applied onto the walls, under atmospheric conditions, by the plasma jet technique, the surface of the walls meanwhile keeping approximately room temperature. The present invention provides a process for producing a protective coating on walls made of metallic base material and exposed to hot gases, in which a powder of metallic, carbide, oxide ceramic or silicide materials, or any mixture thereof, is applied onto the previously cleaned metallic walls by means of the plasma S:23610A/700 0« I * « - 2a - jet technique to form the protective coating, characterised in that: a) the surface of the walls is roughened and activated with high purity special fused alumina and b) immediately afterwards, the powder is applied by the plasma jet technique at ambient temperature and under atmospheric conditions, c) the composition of the powder having previously been selected so that the stress as a function of the temperature in the unstressed state (at room temperature) determined, by means of the thermal. expansion coefficients of the base material and test pieces made of various powders, for the transition zone between base material and coating applied gives tensile stresses of between 50 and 800 N/mm2 which is reduced to zero or exhibits slight compressive stresses in the specified temperature range of 300 to 18000C. The composition of the powder is selected in accordance with the respective base material and the future service conditions, in particular the specified temperature ranges. The transition zone between base material and coating applied, may have, in unstrained condition, that is to say, at ambient temperature, tensile stresses between 50 and 800 N/mm2, preferably between 500 and 800 N/mm2, which in the specified temperature range have been reduced to zero or show minor compressive stresses. These vV,'* S:23©10A/700 states of stress (see enclosed figure) are calculated by means of the thermal expansion coefficients of the base material, on the one side, and of test pieces made of different powders, on the other. This calculation can then be checked pursuant to DIN 50121. A process embodied by the invention allows a protective coating against hot gas corrosion and/or mechanical wear and tear, which is insensitive to thermal shock and easily reparable, to be produced, for instance, on plane or arched walls of combustion plants and heat exchangers, especially heat recuperators in steel converter plants «• • t i.*lt«Fias turned out that a final coating thickness of 0.1 to 0.5 mm, preferably 0.15 to 0 *. fuCn, is already sufficient for preventing appreciable wear and tear for a much longer period •Mtyan it has so far been possible.We would like to point out in this connection that an 80 KW ]$>15srna jet installation with internal powder feeding has proved to be particularly suited for applying such a protective coating, the powder used having a gram size of less than 75 (jm, preferably 20 to 40 pm. The said powder particularly allows a very thin coating to be applied which meets the requirements of insensitivity to thermal shock and resistance to hot '•* gas corrosion and avoids high internal stress owing to its process-inherent laminar '••'stJucture. It is most advantageous to produce the total protective coating in at least two , passes. m « 4 Before plasma spraying, the wall surfaces to be coated can be roughened and activated .'."Wiih special fused alumina, preferably with high-pur*/white special fused alumina It has further turned out to be advantageous that in the process according to the invention the surface be only heated to approx. 40 0C, maximum 60 "C, by the plasma jet and the powder particles contained in the same. Thus distortion of the walls Can in particular be precluded. It is advisable to use a nickel alloy containing powder. It has turned out that atmospheric plasma spraying should be earned out not later than minutes, preferably not later than 30 minutes, after activation of the wall surface Finally, the stress temperature of the walls provided with the said protective coating may range between 300 and '1800 C, preferably between 600 and 1000 "C The stress/temperature diagram of the enclosed figure shows exemplanly >tie stress behaviour in the transition zone between base material and proteotivp coating m the temperature range between 0 'C and approximately 1200 'C The diagram is based on the measured average linear thermal expansion coelTioients of the two materials in unstressed condition the coated wall of a converter heat reoi iperator shows tensile slresses of more than f)00 N/mm2 in the transition zone between the base material and the coating material While fhe recuperator is in operation the protective coating on the wall is suddenly exposed to the high temperatures of the steel melt and the slag spurting Irom the conver+er The diagram illustrates this process on the basis of the stress development The neutral range Is at about 700 "C while, above 700 'C compressive stresses oreventing breaking away of or cracking in the protective coating are building up in the transition one Owing 'o the usually water-cooled tubes of the recuperator walls, the state of tensile stress is Ihen slowly restored, that is to say that, in the diagram, the plotted line representing the stress development is passed through in opposite direction The figure shows only an exemplary stress development as a function of the temperature For other stress ranges ihe so-called zero state may certainly appear at 400 'C or at 800 C instead of at 700 C