|Title of Invention||
"AN IMPROVED BORONISING MIXTURE AND PROCESS FOR PREPARATION THEREOF"
|Abstract||This invention relates to a process for the preparation of boronising composite mixture for enhancing the hardness, wear resistance, resistance to corrosion and oxidation of ferrous materials, certain non-ferrous materials and nickel alloys characterized by the step of preparing a charge of 2 to 4% by wt. of boron carbide, 2 to 4% by wt. of an activator potassium fluoborate and 90 to 95% by wt. of silicon carbide as diluent; composite prepared in (a) is charged into vibratory mill in layers; hardened steel balls are put in the boronising composite; comminution for 30 to 120 minutes.|
|Full Text||FIELD OF INVENTION
This invention relates to an improved boronising mixture and a process for preparation thereof, for boronising ferrous materials and certain non-ferrous materials like titanium, tantalum, tungsten and nickel-base alloys to enhance their hardness, wear resistance and resistance to corrosion and oxidation.
Boronising also referred in art as bonding, is a thermochemical process wherein a boron rich layer is formed on a metallic surface by treatment of such metallic surfaces with boron containing materials. When a metallic component is treated with a boronising mixture and adequate thermal energy is applied, boron of the boronising mixture, diffuses into the substrate resulting into the formation of the borides of substrate material. When steel components are boronised, it results into the formation of Fe2B with or without the presence of an overlay of FeB layer. The composition and morphology of the boride layer, depends upon the composition of the steel; the activity of boron in the boronising medium and the boronising temperature and time. All ferrous materials and a few non-ferrous materials like titanium, tantalum, tungsten and nickel-base alloys are amenable for boronising treatment. The boride layers are characterised by high hardness, excellent wear resistance and high resistance to corrosion and oxidation.
Various methods are known in the art for boronising metallic components viz; pack boronising, liquid boronising, gas boronising and plasma boronising. While plasma and gas boronising techniques are in experimental stage, liquid and pack boronising techniques are extensively used.
The liquid boronising techniques known in the art involves treatment of metallic surface with boron-liquid media such as borax melts. The process is carried out by immersion at temperature above 900 degree Celsius, in salt mixtures containing borax
with additions like boron carbide, boron chloride, etc. The process is also carried out by electrolysis in molten salts containing borax, metaboric acid, sodium fluoride, etc with the component to be boronised as cathode and graphite as anode, at bath temperature of over 900 degree Celsius.
The disadvantage of the above process is that handling of molten salts at high temperature is dangerous and messy due to spillage and exposure to moist environment.
Another disadvantage of the above techniques is that these are not environment-friendly as these involve generation of toxic and repugnant fumes which cause irritation to the operators and damage to the environment.
Still another disadvantage of the above process is that the process is not economical as maintenance cost for the fused salt vessel is high.
Further disadvantage of the above process is that it is time-consuming to remove excess salt from the component on which boride coating is applied.
The pack boronising technique known in the art involves use of a boronising mixture consisting of (i) boron source or boron donor (ii) an activator and (iii) a diluent. The boron source used is boron carbide or ferro-boron or amorphons boron and the concentration varies from 1 to 10%. The activator generally used is either potassium fluoborate or ammonium chloride and its concentration also varies between 1 to 10%. The diluent used is either silicon carbide or alumina which is the major constituent of the boronising mixture.
The drawback of the known pack boronising mixture is that such boronising mixtures which are based upon amorphous boron and ferro-boron are expensive.
The drawback of the known pack boronising mixture based upon Boron carbide (B4C) is that these mixtures vary in boron potential to such an extent that their commercial usage involves extensive prior process optimisation trials.
Another drawback of the known pack boronising mixture is that the particle size of the mixture vary significantly which adversely affects the sinterability of the pack to the component.
The primary object of the present invention is to propose an improved boronising mixture and a process for preparation thereof, for boronising of ferrous materials, some non-ferrous materials and nickel base alloys, for increasing their hardness, wear resistance and resistance to corrosion and oxidation..
Another object of the present invention is to propose an improved boronising mixture and a process for preparation thereof, wherein the process provides homogenous boronising mixture with particle size of 150 to 200µm.
Still another object of the present invention is to propose an improved boronising mixture and a process for preparation thereof, wherein the boronising mixture can provide coating with higher hardness in the range of 1800-2200 HV.,
Further object of present invention is to propose a process for preparation of an improved boronising mixture which is energy-efficient and time-efficient as effective boride coating thickness can be developed at lower temperature of 900°C within time duration of 2-4 hours or at higher temperature of 1000°C in 1-2 hours.
Still further object of the present invention is to propose an improved boronising mixture and a process for preparation thereof wherein the boronising mixture when applied to the component, provides coating free from brittle FeB layer and contains only the desired Fe2B layer.
STATEMENT OF INVENTION
The present invention provides an improved boronising mixture and a process for preparation thereof, wherein the boronising mixture is used for surface treatment to ferrous materials, some non-ferrous materials like titanium, tantalum, tungsten and nickel base alloys to provide coating to enhance their hardness, wear resistance and resistance to corrosion and oxidation. In the process of the present invention, the different constituents of the boronising mixture have been carefully balanced. It is known that too much of boron donor would result in the formation of undesirable brittle boride layer and too little of boron donor would not permit development of adequate boride layer thickness economically. The concentration of boron donor further dictates the concentration of activator. In the present invention, based upon the consideration of scientific principles and empirical results, the ingredients of boronising mixture, which yield optimum results in hardness, wear resistance, etc have been identified, selected and fixed and the range of constituents has also been taken carefully. The boron donor taken is boron carbide as compared to amorphous boron or ferro-boron preferred in the known mixtures, which are relatively expensive. The activator taken is potassium fluoborate while diluent taken is silicon carbide. The boron carbide is taken in the relatively lower range of 2-4% as compared to 5-50% used in known processes. Similarly, activator potassium fluoborate is taken in lower proportion of 2 to 4% as compared to 5-15% generally taken in the known process. While 90-95% of the weight of boronising mixture is silicon carbide as diluent. As content of the relatively costlier constituent boron carbide is optimised without compromise on the quality and depth of the coating obtained, the cost of the boronising mixture prepared by the process of the present invention is lower. This composition of the boronising mixture leads to coating of higher hardness and higher wear resistance. The hardness achieved with the boronising mixture
of the present invention is in the range of 1800-2200 HV which is higher than the hardness achieved with the known boronising mixtures.The coating obtained is free from the undesirable brittle FeB arid contains only the desirable Fe2B. The process is energy efficient as it enables development of effective coating thickness at 900°C in just 2-4 hours or at 1000°C in just 1-2 hours.
The major problem experienced in the known process for preparation of boron mixture, is lack of uniform distribution of small quantities of solid boron carbide and solid potassium fluoborate in the large quantity of diluent. In the process of the present invention, this problem of mixing of ingredients to ensure uniform and stable distribution, has been resolved. Taking into account the hardness and particle size differences between Boron carbide (B4C) and silicon carbide (SiC), large number of experiments were carried out to identify the correct method of mixing and to optimise the process related parameters such as initial particle size of constituents. In the present invention, the mixing was carried out by pounding the coarser SiC particles in the presence of finer B4C particles, resulting in fragmentation of SiC and impregnation of B4C on freshly formed facets of fragmenting SiC particles This was best achieved by using vibratory ball mill. The starting particle sizes of B4C was taken in the range of 200-500 um and starting particle size of SiC was taken in the range of 5-25µm. The time of comminution and the charge to ball ratio were critically controlled to obtain a homogenous boronising mixture, with particle size between 150 to 200µm. The charge to ball ratio was taken in the range of 2:1 to 1:1, ball sizes (single or mixed) are taken in the range of 6mm to 25 mm while the time of comminution was taken in the range of 30-120 minutes.
DESCRIPTION OF FIGURES
Fig 1: shows scanning electron photomicrograph of boronising mixture.
Fig 2: is the optical photomicrograph showing the boride layer on Ni-Si-Cr steel.
DESCRIPTION OF THE PROCESS
According to this invention there is provided a process for the preparation of boronising composite mixture for enhancing the hardness, wear resistance, resistance to corrosion and oxidation of ferrous materials, certain non-ferrous materials and nickel alloys characterized by the step of:
a) preparing a charge of 2 to 4% by wt. of boron carbide, 2 to
4% by wt. of an activator potassium fluoborate and 90 to
95% by wt. of silicon carbide as diluent;
b) composite prepared in (a) is charged into vibratory mill in
c) hardened steel balls are put in the boronising composite
wherein the ratio of boronising mixture to ball is taken in
the range of 2:1 to 1:1;
d) comminution for 30 to 120 minutes.
The different ingredients have synergetic effect on each other as the mixture is capable of producing uniform boride coating on ferrous and certain non-ferrous materials, non of individual ingredients is capable to doing so.
EXAMPLE OF PROCESS
In order to prepare 1kg of boronising mixture, 920 gm of silicon carbide of 300 micron particle size, 40 gm of boron carbide of 5 micron particle size of 40 gm of potassium fluoborate are mixed. The mixture is charged into vibratory mill in layers. To this mixture, 1 kg of hardened steel balls are charged and communication was done for 70 minutes.
METHOD OF USE OF BORONISING MIXTURE
The work piece to be boronised is packed in a container in such a way that the work piece is covered by 10mm thick mixture on all faces to be boronised. A layer of about 25 mm thick silicon carbide is maintained over the pack and finally covered with a thin layer of soda glass granules. Then the pack is heated to the desired temperature ensuring that the residence time between 700-800° C is as low as possible. The container with the pack is held at the selected temperature (say 900°C for boronosing AISI 4140 steel) for specified time (say 2 hours for obtaining 60um coating thickness on AISI 4140 steel). The container is then cooled in air. If the components is to be quenched and tempered, quenching can be integrated with the boronising step and quenched directly into the specified quenchant instead of air cooling. It is thus possible to eliminate an additional heat treatment step and boronising temperature can be matched with hardening temperature.
The Fig 1 shows the scanning electron micrograph of the boronising mixture. In the figure, the large particles are of SiC and the small dark particles embedded on SiC facets are of B4C:
The Fig 2 is a typical photomicrograph showing the boride layer obtained on Ni-Si-Cr steel with boronising mixture of the present invention. The absence of an outer layer of FeB and the presence of the characteristic saw-tooth profiled interface can be observed.
Typical hardness profile obtained on the boronised and quenched Ni-Si-Cr steel is given in Table-I below. The microstructure and hardness data clearly indicate that the boride layer obtained with the boronising mixture of the present invention nas properties similar to those reported for boride coatings on steel. Thus it is clear that the boronising mixture of the present invention, is uniform, hard and adherent.
Table-I Hardness Profile of the BoronisinR Mixture of the present invention
In order to verify the efficacy and consistency of the boronising mixture in developing metallurgically sound coating, isothermal and ischronial studies were carried out on a Ni-Si-Cr steel. The comparative data on the variation of the depth of coating with increasing temperature, at constant time and with increasing time at constant temperature, obtained with the boronising mixture of the present invention as compared with a boronising mixture known in the art, are given in Table-II:
Comparative data of coating thickness obtained with the present boronising mixture as compared to an imported boronising mixture
The results clearly indicate that the boronising mixture of the present invention is efficient in developing sound coatings of expected thickness at a given time and temperature of boronising. At 1000° C in just one hour, the mixture of the present invention is capable of providing effective coaling thickness of 70um as compared to 60um coating thickness obtained with known boronising mixtures.
It is to be understood that the process of the present invention is susceptible to changes, adaptations, modifications by those skilled in the art. Such adaptations, changes, modifications are intended to be within the scope of the present invention, which is further set forth with following claims:-
1. A process for the preparation of boronising composite mixture for enhancing the hardness, wear resistance, resistance to corrosion and oxidation of ferrous materials, certain non-ferrous materials and nickel alloys characterized by the step of :
a) preparing a charge of 2 to 4% by wt. of boron carbide, 2 to 4% by wt.
of an activator potassium fluoborate and 90 to 95% by wt. of silicon
carbide as diluent;
b) composite prepared in (a) is charged into vibratory mill in layers;
c) hardened steel balls are put in the boronising composite wherein the
ratio of boronising mixture to ball is taken in the range of 2:1 to
d) comminution for 30 to 120 minutes.
2. A process as claimed in claim 1 wherein said boron carbide particle
size is 5-25 µm.
3. A process as claimed in claim 1 wherein said silicon carbide has a
particle size is 200-500 µm.
4. The process for preparation of boronising composite substantially as herein described herein and illustrated.
|Indian Patent Application Number||1369/DEL/1999|
|PG Journal Number||09/2008|
|Date of Filing||13-Oct-1999|
|Name of Patentee||CHIEF CONTROLLER, RESEARCH AND DEVELOPMENT|
|Applicant Address||MINISTRY OF DEFENCE GOVT OF INDIA B - 341, SENA BHAWAN , DHQ P.O. NEW DELHI - 110011|
|PCT International Classification Number||C23C 008/68|
|PCT International Application Number||N/A|
|PCT International Filing date|