Title of Invention

A PROCESS FOR FORMING METAL BORIDE COATING ON SURFACE OF STEEL AND ALLOYED STEEL COMPONENTS

Abstract

This invention relates to a process for forming metal boride coating on surface of steel and alloyed steel components subjected to high stress conditions comprising the steps of: a) preparing a packing mixture essentially consisting of boron (10%) boron carbide (20%), activator (Aluminium bi-flouride 2%) and the balance being silicon carbide; b) packing said alloyed steel component with said packing mixture covering a thickness of 10 mm; c) disposing said packed component in a suitable steel container, placing said container in a furnace having provision for pressure rise upto 30 kg/mm2; d) allowing a heating of said container under said pressure in said furnace in an inert atmosphere of Argon to a temperature of 735oC to 780oC; e) removing the heated container from said furnace and cooling the said container by compressed air; to obtain required thickness and the hardness of formed boride metal layer on said component.

Full Text

FIELD OF INVENTION This invention relates to a process for forming a metal boride coating on alloyed steel components subjected to high stress conditions. BACKGROUND OF INVENTION Many steels are known which possess the strength necessary for use in service under conditions where the steel is subjected to considerable stress. However, these alloys are often defi-cient in resistance to the erosion encountered in their intended service. One of the objectives of the present invention is to provide improved wear properties for components used in Hydrotur-bines, which undergo silt erosion. The present invention pro-vides erosion protection for the components of said turbine materials, with composition of about 13% Cr & 4% Ni, without sacrificing other properties. It is well known that steels can be boronized at temperatures more than 940o C using pack boroni- zation to obtain outer surface layers, which contain iron-boron compounds, possessing good erosion resistance. However the mechanical properties of 13Cr 4Ni material, after boronization at 940o C, gets adversely affected due to surpassing of the trans-formation temperature and also may result in distortions in the components. Document US4979998 relates to the boronization of the material by forming a metal boride coating on a carbonaceous substrate, however the process requires a temperature of at least 1200o C. Document US 5143559 relates to controlling the microstruc ture of the boronized material through cooling rates. Also the hardness achieved is of the level of only 350Hv. It is therefore an object of the invention to propose a process for forming a metal boride coating on alloyed steel components subjected to high stress conditions for improving the wear properties of the components. Another object of the invention is to propose a process for farming a metal boride coating on alloyed steel components sub-jected to high stress conditions in which mechanical proper-ties of the components are less deteriorated and risk of distorsion of the components being substantially reduced. Yet another abject of the invention is to propose a process for farming a metal boride coating on allayed steel components used in hydroturbines whereby a substantially higher hardness of the coating can be obtained. A further object of the invention is to propose a process for forming a metal boride coating on alloyed steel components subjected to high stress conditions wherein the process requires a substantially lower temperature. At the outset of the description which follows, it is to be understood that the ensuing description only illustrates a par— ticular form of this invention. However, such a particular form is only an exemplary embodiment and the teachings of the inven-tion is not intended to be taken restrictively. SUMMARY OF THE INVENTION A process has been developed in accordance with the present invention to form a uniform metal boride coating on alloyed steel components subjected to high stress conditions independent of the size or configuration of the components. The process of forming a metal boride coating on an alloyed steel component in accordance with the present invention comprises the steps of: preparing a packing mixture essentially consisting of boron carbide, silicon carbide, and an activator; packing said alloyed steel component with said packing mixture covering a thickness of 10 mm; disposing said packed component in a suitable steel container; placing said container in a furnace having provision for pressure rise upto 30 kg/mm2 ; allowing a heating of said container under said pressure in said furnace in an inert atmosphere to a temperature of 735o C to 780o C; removing the heated container from said furnace for allowing a cooling by compressed air; and measuring the thickness and the hardness of formed boride metal layer on said component. DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION The subject invention may be used for coating any steel substrate with a metal boride, but it is particularly advantageous for coating alloy steel components with a composition of 12 to 14% cr and 3 to 5% Ni being generally subjected to high stress condition such as the components used in hydroturbines undergoing silt erosion. In accordance with the process of the invention the surface of the components is reacted with a boron containing compound under specific conditions to cause a reaction between the metal of the component and the boron in the boron compound to produce a boride coating. The boron containing compound may be, for example boron carbide, amorphous boron and the like. The metal carbide is converted to a metal boride under reaction-conditions above a threshold temperature of at o least 735 C in a non reactive or essentially inert atmosphere of e.g. argon. The following is an example of the process of the present invent ion : The specimen is packed in a container with a mixture essen-tially consisting of boron carbide, silicon carbide and aluminium oxide etc . The container is placed in a furnace having provi- sion of uniform heating with pressure ranging upto 30Kg/mm2 . o o The container is heated to a temperature of 735 C to 780 C in an inert atmosphere under pressure. The specimen is taken out and hardness of about 1100 Hv is obtained. The sample is cut for examination under optical microscope. A minimum thickness of 40 microns of boride layer is obtained. Several trails were undertaken for carrying out experiments for boronisation using different composition of pack mixture and also varying the quantity of the activator used. The composition that gave the desired results is given below. Boron 10% Boron Carbide 20 % Aluminium bi-fluoride 2% Activator Calcium Chloride 2% Silicon Carbide Remainder Inert Filling Media Heating time in the furnace 6 to 8 hours followed by Air Cooling Example A specimen of dimensions 75X25X8 mm having composition of 13%Cr and 4% Ni was prepared. Packing mixture-containing Boron in the form of amorphous Boron & Boron Carbide was taken. Activator in the form of Ammonium bi fluoride to the tune of 2% was added along with 2% of Calcium Chloride. The remainder was Silicon Carbide as inert filling media. The container was kept in the furnace with a heating rate of about 10 degrees per minute. The pack mixture was prepared with 10mm minimum thickness around the sample and was kept in the furnace for about 6 hours at 735° C. Thereafter the specimen was taken out of the furnace and allowed to cool in air. The specimen was examined under optical microscope and the exami-nation revealed layer formation of more than 40 microns having hardness in excess of 100 Hv. The specimen was also examined under Scanning Electron Microscope and EDX analysis confirmed the presence of Boride Layer. WE CLAIM: 1. A process for forming a metal boride coating on alloyed steel components subjected to high stress conditions comprising the steps of: — preparing a packing mixture essentially consisting of boron carbide, silicon carbide, and an activator; — packing said alloyed steel component with said packing mixture covering a thickness of 10 mm; - disposing said packed component in a suitable steel container; placing said container in a furnace having provision for pressure rise upto 30 kg/mm2 ; - allowing a heating of said container under said pressure in said furnace in an inert atmosphere to a temperature of 735o C to 780o C; — removing the heated container from said furnace for allow- ing a cooling by compressed air; — and measuring the thickness and the hardness of formed boride metal layer on said component. 2. A process as claimed in claim 1 wherein said boron contain ing compound is selected from the group consisting of boron carbide, amorphous boron. 3. A process as claimed in claim 2 wherein said inert atmos phere is argon. 4. A process as claimed in claim 3 wherein said alloy steel component subjected to high stress conditions has a composition of 12 to 14% cr and 3 to 5% Ni.

Documents:

115-del-2004-abstract.pdf

115-del-2004-claims.pdf

115-del-2004-correspondence-others.pdf

115-del-2004-correspondence-po.pdf

115-del-2004-description (complete).pdf

115-del-2004-form-1.pdf

115-del-2004-form-19.pdf

115-del-2004-form-2.pdf

115-del-2004-form-3.pdf

115-del-2004-gpa.pdf