Title of Invention

AN IMPROVED BORONIZING COMPOSITION

Abstract

Boronizing, which involves diffusion of boron atoms into steel substrate to form iron borides, is a well-known diffusion coating process and numerous studies have demonstrated the outstanding tribological properties of boronized steel vis-a-vis carbonized or nitrided steels. A number of boronizing compositions are commercially available, among them "Kkabor" is the most popular. However, most of the available compositions suffer from the limitation that a two phase layer of iron borides (FeB & Fe2B) is formed upon boronizing of a steel substrate. Due to thermal expansion mismatch between these two phases, the coating is prone to crack under mechanical stress. Investigations have revealed that a single-phase boride layer, i.e. Fe2B is desirable for preventing cracks in the boride coating. ARCI has jointly worked NFTDC & DMRL to develop a novel composition of boronizing mixture in order to match the above specification of the boride coating. Accordingly, a new boronizing composition has been developed with posses low boron activity in the donor component. The added advantage of the new composition is that it is comparatively cheaper than the commercial mixture due to low boron activity. Boronizing of the different steels at different temperatures and time was carried out using the commercial mixture as well as the new composition. Characterization of the coating with the different combinations was carried out to analyze the phase composition and coating thickness. It was found that the newly developed composition successfully provides a single phase Fe2B layer upon boronizing of the different steels.

Full Text

This invention relates to an improved BORONIZING composition. The composition is useful for boronizing of various steels. Boronizing, or boriding, is a thermo-chemical surface hardening process that can be applied to a wide variety of ferrous, nonferrous, and cermet materials for improving their tribological properties. Boronizing provides higher hardness & high wear resistance when compared with carburizing & nitriding processes. Boronizing considerably enhances the corrosion-erosion resistance of ferrous materials in non-oxidizing dilute acids & alkali medium. Also, borided parts have an increased fatigue life and service performance under oxidizing and corrosive environments. Presently several boronizing compositions are commercially available for boronizing of steels. Some of the compositions of these mixtures are mentioned below: 5% B4C, 90% SiC, 5% KBF4, • 50% B4C, 45% SiC, 5% KBF4, • 85% B4C, 15%Na2C03, 95%B4C,5%Na2B407, • 84%B4C, 16%Na2B407, • Amorphous boron (containing 95 to 97% B), • 95% amorphous boron, 5% KBF4 It has further been observed that the most available boronizing mixtures suffer from the drawback that a two phase layer of iron borides is formed after boronizing which is prone to crack under stress due to a thermal expansion mismatch between the two layers. If this formation of a two-phase layer is avoided, a crack free layer could be obtained. Literatures also provided that single-phase iron sub-boride layer provides better performance. This prompted experimental studies to achieve a single-phase boride layer. Organizations in Germany have carried out pioneering work in the area of boronizing and different grades of boronizing mixtures have been developed & patented ("Pulverformiges Borierungsmittel", German Patent Nos. DE 22 08 734 & DE 22 08 737, by "Elektroschmelzwerk Kempten GmbH, 8000 Munchen, 1979 & 1980). The study of the patents DE 22 08 734 and DE 22 08 737 revealed that work has been carried out on development of composition of boronizing mix for production of an impervious, stress free, single phase boron compound layer on metals & alloys. The boronizing mixture consists of boron carbide, amorphous bdip"^n or ferro boron as boron donor substance individually or in combination with bor,^ or floro borate as activator and silicon carbide in quantities of 45 to 99.5%. as diluent. It is described in the patents that the borftx content varies from 4 to 0%, with regard to the total weight and can preferably be reduced to 0%i. Several compositions are described in the said patents where low quantity (upto 4.5%)) of the donor {B4C) is used to obtain a single phase iron sub-boride layer. One of the composition is 4.75% B4C, 90.25% SiC and 5%> KBF4- The commercially available boronizing composition contains 5% B4C, 5% KBF4 and 90% Sic. The results employing such mixture have shown that there are fair chances of formation of a two phase (FeB-Fe2B) structure due to higher boron activity in the composition (refer Table 1). Boronizing can be carried out in solid, liquid or gaseous medium, however, pack boriding is the most commonly used technique due to simplicity and non-toxicity of the process. In pack boriding, the components (textile machinery components, stamping and die casting dies, material handling equipment components, valves & cylinders used to extrude glass-filled plastics, 4140 steel screws etc.) are degreased, cleaned & packed in a boronizing mixture contained in a 3 to 5 mm thick, heat-resistant steel box so that the surfaces to be bonded are covered with a sufficient thickness (10-20 mm) of the powder. Several boronizing mixtures are commercially available as mentioned earlier, which commonly include three components e.g. a boron donor, an activator and a diluent. The typical boron donors are boron carbide (B4C), ferroboron, and amorphous boron; the last two have greater boron potential and provide a thicker layer. However, these compounds are very expensive when compared to B4C (N. Komustu, M. Oboyashi, and J. Endo, J. Jpn. Inst. Met., Vol. 38, 1974, pp 481-4862). Silicon carbide (SiC) is the most commonly used diluent, which doesn"t take part in the reaction. However, it controls the amount of boron and prevents caking of the boronizing agent. The commonly used boriding activators are NaBF4, KBF4, (NH4)BF4, NH4CI, NajCOa, BaF2, and Na2B407. In case of donor-activator-diluent mixture, reaction takes place at the boriding temperature and atomic boron releases as per the following reaction: BF3 B4C + 3SiC + 302 —^ 4B + 2Si + Si02 + 4CO (1) SiF4 The atomic boron thus released reacts with the substrate and forms iron borides. It has been found that with the commercially available mixtures, a two-phase boride layer (FeB/Fe2B) is obtained in most of the steels due to high boron activity in the mixtures. Generally, the formation of a single phase iron sub-boride (Fe2B) layer with sawtooth morphology is more desirable than a double phase layer consisting of FeB & Fe2B (R. Chatteijee-Fisher, Chapter 8, Surface Modification Technologies, T. S. Sundarshan, Ed. Marcel Dekker, Inc. 1989, pp 567-609). The boron rich FeB phase (containing approx. 16.23 wt% B) is not desirable because FeB is more brittle than the iron sub-boride, Fe2B (containing approx. 8.83 wt% B). Also, because of CTE mismatch between these two phases (ctFeB= 23x10"^ /°C, aFe2B= 7.85x10"^ /°C), crack formation is often observed at the FeB/Fe2B interface of a double phase layer. These cracks may lead to flaking & spalling when a mechanical load is applied (A. Galibois, O. Boutenko and B. Voyzelle, Acta Metall., Vol.28, 1980, pp 1753-1763, 1765-1771). Subrahamanyam and Gopinath (J. Subrahamanyam and K. Gopinath, "Wear studies on boronized mild steel". Wear, Vol. 95, 1984, pp 287-292) have shown that single-phase borides provide better performance than a two-phase boride layer on the surface. They conducted dry sliding wear tests using a pin-on-disc machine on boronized mild steel pins containing FeB, Fe2B, and both phases in the surface layers. The results indicated that single phase boride layers exhibited better wear resistance than two-phase boride layers. Below a critical PV value of 17 Mpa-m-min"", all the borides show negligibly small wear. Above the critical PV value dual-phase borides show high wear. The surface layer of two-phase borides consists of discrete FeB precipitates in the Fe2B phase (J. Subrahmanyam, Mater. Lett., Vol. 1, 1982, p. 100). This results in an inhomogeneous microstructure which may be responsible for the high wear. Habig and Chatterjee Fisher expressed a similar opinion for the wear of two-phase borides (K. H. Habig and R. Chatterjee Fisher, Triboi. Int., Vol. 14,1981, p. 209). The relative proportions of the outer layer of FeB and iimer layer of Fe2B depends on the availability of boron while boronizing; the lesser the boron, the greater the amount of Fe2B likely to be formed. The commercially available boronizing mixture contains 5% B4C with KBFi and SiC making rest of the part. Our study has shown that there are fair chances of formation of a two-phase layer when this mixture is used for boronizing (Table 1). Also, the cost of the mixture is too high due to its costly component, B4C. TTierefore, the available composition needs to be modified suitably in order to reduce the cost of the process and also to get a single phase Fe2B layer which is completely free from FeB. Boronizing has been found to enhance the surface hardness of steels to 1600-2000 HV as compared to the value of 650-950 HV for carburized steels. Boronized steel with single-phase boride (Fe2B) layer has been found to outperform nitrided steel as far as tribological behavior (abrasive & erosive wear) is concemed (G. Sundararajan, B. Venkataraman, K. Laxhminarayana and G. B. K. Harish, "A comparison of Tribological and Bulk Mechanical Properties of Boronized and Nitrided Steel", Surface Modification Technologies, Vol. X, Edited by T. S. Sundarshan, K. A. Khor and M. Jeandin, 1997, pp 852-862). Also, boronized steel has shown excellent resistance to sliding wear over a broad range of sliding speeds (B. Venkataraman and G. Sundararajan, "The High Speed Sliding Wear Behaviour of Boronized Medium Carbon Steel", Surface & Coating Technology, Vol. 73, 1995, pp 177-284). Inspite of the above advantages of the boronizing process over commercial carburizing & nitriding process, it is not commercially viable due to higher cost of the donor component (B4C) of the boronizing mixture as mentioned earlier. Also, as our investigations have shown, there are considerable chances of formation of two-phase layer (which is detrimental to the life of the component) when using the commercially available boronizing mixture. These facts made us to conduct studies on modification of the boronizing composition in order to reduce the cost of the powder and also to completely avoid formation of two-phase layer after boronizing. The main objective of the present invention is to provide an improved boomizing composition, which favors formation of the single-phase iron sub-boride (Fe2B) layer that is free from FeB. The formation of such structure avoids cracking of the boride layer. Another objective of the present invention is to provide an improved boronizing composition which is cheaper consequently making the boronizing process employing the new composition commercially viable. We have, accordingly developed a new boronizing mixture possessing low boron activity. The advantage of the new composition lies in better control for formation of a single phase FezB layer. Also, due to low concentration of the boron donor component in the said mixture, it is comparatively cheaper than the commercially available mixture, thus making the process less costly. In the boronizing composition of the present invention, boron activity is kept lower by keeping lower quantity of the donor component (B4C). Accordingly, the present invention provides an improved boronizing composition which comprises 1±0.1 wt% boron carbide (B4C), 9+0.5 wtVo potassium boro-fluoride (KBF4) & 90±2 wt% silicon carbide (SiC). Several compositions with different proportions of boron donor (B4C), activator (KBF4) and diluent (SiC) were prepared by weighing the appropriate quantity of the different compounds, blending the mixture and ball milling to obtain a homogenous mixture. Samples of approximately 25x25x5 mm size with different steel compositions were ground and ultrasonically cleaned. The samples were then packed in the different bonding mixtures in a 5 mm thick, heat-resistant steel box with around 10-15 mm thickness of the boronizing mixture around the specimens followed by heating the assembly at different temperatures for different times. Similar experiments were carried out with different steels by using the commercial boronizing mixture (5% B4C, 5% KBF4 & 90% SiC) also. Characterization of the boronized samples with different mixtures was carried out by XRD, SEM & image analysis. Scanning electron microscopy was taken in the back-scattered (BSE) mode to confirm whether FeB is present in the boride layer. FeB was found to be present for all the compositions either as a continuous layer or as serrations except in the case of the composition of the present invention. The results have been presented in Table 1 for the boronizing composition of the present invention (1±0.1 wt% B4C, 9±0.5 wt% KBF4 & 90±2 wt% SiC). A comparison of the results is shown with that of commercially available boronizing mixture containing 5% B4C.

Documents:

0289-mas-2001 abstract.pdf

0289-mas-2001 claims.pdf

0289-mas-2001 correspondence-others.pdf

0289-mas-2001 correspondence-po.pdf

0289-mas-2001 descritpion (complete).pdf

0289-mas-2001 form-1.pdf

0289-mas-2001 form-19.pdf