Sooner or later, certain treatment needs to be applied to any metal to convert it into some kind of product. Some metals almost do not resist such treatment, while others, e.g. tool steels, strongly oppose this process. Over 200 million carats of industri

DIAMONDS GET DESTROYED FOR THE METAL AND WIN

Anatoly SHEPELEV, deputy director, head of laboratory, candidate of sciences;

Valery LAVRINENKO, leading research fellow, Dr, Institute for Super-hard Materials under the National Academy of Sciences of Ukraine

Sooner or later, certain treatment needs to be applied to any metal to convert it into some kind of product. Some metals almost do not resist such treatment, while others, e.g. tool steels, strongly oppose this process. Over 200 million carats of industrial diamonds are annually produced worldwide to deal with this problem. Most of these diamonds are applied to work hard metals and materials. Meanwhile, the global industry requires more and more diamonds and diamond tools to replace the worn-out ones. That is precisely the reason why forty years ago Kiev became the first place in the world to house the Institute for Super-hard Materials, a specialised scientific centre that develops tool materials capable of treating the toughest metals. These forty years have seen creation of a range of tools made of tool materials (synthetic diamonds and cubic boron nitride) that are successfully made use of in working of modern tool and composite materials.

Most frequently, cutting tools are applied to make various products of metals and alloys. The process of metal treatment by cutting is somewhat of a drama, a duel between the metal being treated and the materials it is treated with. Destruction of the tool material is highly unwanted because it means destruction of the part itself. There is a growing demand for stronger and more durable parts; therefore, tool materials need to become more reliable. In turn, the efficiency of machining materials depends not only on their structure and chemical composition, but also on the quality of the very cutting tools.

Most tool materials can be split into two equal groups, namely tool steels, including high-speed steels, and hard alloys that cover almost 95% of the tool manufacture (see table 1).

According to modern classification of tool steels, the most intractable high-speed steels are the refractory steels that gain high strength, hardness and durability owing to double hardening, namely, the martensic one upon quenching and precipitation hardening accompanied with rather high tempering and release of hardening phases. Depending on the quantity of alloying elements added, steels are broken down into 3 groups, namely, moderate heat-resistant steels, tungsten (P18) and tungsten-molybdenum ones (P6M6); high heat-resistant steels, vanadium (Р6М5Ф3), cobalt (Р6М5К5) and high-carbon ones (А11Р3М3Ф2); and tungsten-free steels (ЭК41, ЭК42).

At the moment, Р6М5 steels account for 65 to 75% of high-speed steels applied in industry, Р6М5Ф3 contributes 10%, Р6М5К5 7 to 10% and composite and economical alloy steels make up the remaining 10 to 15%. Wider application of these steels is held back by their worse abrasion qualities compared to the most widespread P18 grade of steel. Meanwhile, abrasion is a significant technological property of tool steels that determines the quality of final tools.

It is the steels with high contents of vanadium carbide that show bad abrasion when being wrought with corundum grinding wheels, e.g., Р6М5Ф3 steels (abrasion coefficient of 0.4 compared to P18 steel) or А11Р3М3Ф2 (abrasion coefficient of 0.3). This happens because vanadium carbides VC and V4C3 have a hardness of HV ? 28 GPa; i.e. they are stronger than grains of white corundum, 20 to 22 GPa. Correspondingly, efficiency of the grinding process goes down, grinding wheels become worn out fast and require frequent stropping.

Consequently, qualitative and efficient treatment of high-speed steel should make use of high-strength abrasives (HV?100 GPa), like diamonds and cubic boron nitride. Application of diamonds has led to higher efficiency and better quality of treatment; however, rate of consumption of diamonds remains rather high. It has been determined that iron reacts with diamonds, surface of diamonds graphitises and carbon from graphite is subsequently diffused into iron. Besides, the true carbides that comprise high-speed steel are 15% low on carbon. Owing to lack of carbon atoms, carbides intensively appear on the surface of high-speed steels being exposed to diamond grinding, which subsequently leads to intensive deterioration of the diamond grinding wheels.

Therefore, formation of carbides is the key to selection of tools that can work high-speed steels. There is a hypothesis that carbon deficit of carbides in high-speed steels determines their capacity of being wrought. Iron also forms carbides, though not as strongly as alloying elements do.

During diamond grinding, diamond grains become worn out due to exposure to mechanical abrasion and chemical reactions, although the latter are rather weak should the steel contain much carbon. For instance, Р2М5 and А11Р3М3Ф2 steels are rather rich in carbon and they do not cause serious chemical action on diamond grains. Such steels can be treated with diamond grinding wheels. To the contrary, steels that have shortage of carbon cannot be treated with these wheels.

That is why the Institute has created a new super-hard material (that has no natural materials with similar qualities), namely, cubic boron nitride (C-BN) more commonly known under its trademark of cubonite. Application of grinding wheels made of C-BN makes it possible to improve the quality of high-speed steel tools being treated and reduce deterioration of wheels. The tool has become rather widely made use of in the Ukrainian industry, e.g. in ZaporozhInstrument, Vinnitsa Tool Factory and local sugar refineries.

Hard alloys are the second most widespread tool materials to tool steel. The USA and Western Europe prefer these alloys that account for 45 to 57% of total outputs there. Hard alloys produced in CIS and beyond are broken down into 4 groups, namely, tungsten-bearing ones (ВК), titanium-tungsten (ТК), titanium-tantalum-tungsten (ТТК) and tungsten-free ones (БВТС). The latter run into the biggest problems when being ground.

The existing grades of carbide-based tungsten hard alloys do not always meet modern demands, especially during treatment of intractable materials, mainly due to low scale resistance and chemical reactions with these materials. Therefore, it is a normal practice around the world to make use of tungsten-free hard alloys, e.g., in Europe, CIS and Asia (like China and Japan). These alloys have lower hardness than standard ones, though feature lower steel friction coefficient and lower susceptibility to adhesion. Simultaneously, tungsten-free alloys become ground much later than the standard hard alloys and exert several times the cutting forces that tungsten and titanium-tungsten hard alloys do (because titanium carbide has 1.5 times and titanium carbonitride has twice the hardness). Among modern composite hard alloys, there are several alloys that are difficult to treat, e.g., tantalum and vanadium carbide alloys, such as the TTK group (ТТ20К9, ТТ8К6, ТТ10К8Б and others) and OM group (ВК6 ОМ, ВК10 ОМ, ВК10 ХОМ). To great degree, this happens because additional alloying with vanadium and titanium carbides facilitates appearance of fine-grained hard alloys, less than 1 micrometer in grain size. Simultaneously, resistance to plastic strain becomes better, while abrasion properties deteriorate.

There are different recommendations regarding the ways to accurate and effective grinding technology for modern composite alloys.

Practical experience proves how important the tools made of synthetic diamonds and cubic boron nitride are for contemporary machine-building, metalworking and other industries. Grinding and subsequent treatment of tools made of hard alloys and high-speed steels improve quality, extend their useful lives 1.5 to 2 times, as well as boost productivity.

Besides to the Institute for Super-hard Materials, Ukraine also has Poltava and Lvov Diamond Factories that manufacture diamond tools. The two factories make standard tools, while the Institute produces both standard-sized and special grinding tools. What’s more, the Institute for Super-hard Materials is the only one to make cubonite and tools made of it. Pricing policy of Ukrainian companies remains favourable both for the domestic market and for export business. World prices for diamonds applied in grinding wheels fluctuate around US$1 to 2 per carat, while prices for cubic boron nitride are around $2 to 4 per carat. Meanwhile, diamond grinding tools made in Ukraine are offered at $0.3 to 1 per carat. When it comes to small batch orders, additional costs incurred can drive the price for diamonds in special grinding wheel to $3 per carat. This is a general practice around the world and export opportunities of the Institute for Super-hard Materials prove this policy right. The Institute makes standard and special tools upon orders coming from Japan, Yugoslavia, the Czech Republic, Hungary, Belarus, Latvia, Russia and other countries. For instance, in 2000 Russia-based clients of the Institute were Salda Iron and Steel Works, BMZ (Bryansk), Dyatkovsk Crystal, etc.

Today, this market needs economical tools made of synthetic diamonds and cubonite; the tools that feature high productivity and low wear. Analysis of relevant developments shows that the year 2000 saw a trend of durability and efficiency of grinding wheels reaching their upper possible limit (see figure 1). Further breakthroughs in this direction can only be achieved via development and implementation of new grinding technologies or usage of unique qualities of super-hard materials through novel design of grinding wheels. Notably enough, the Institute already has its own developments as regards the first and the second point, e.g., application of the new process of vibration grinding (see figure 1).

(rate of consumption of diamonds in grinding wheels (mg/g); grinding productivity (mm3/min); vibration grinding)

Figure 1. Main trends of changes in grinding efficiency and diamond consumption upon progress of metalworking processes.

There is one paramount trend typical both for the whole industry and specifically for metalworking, namely, lower energy consumption in treatment processes. Application of more and more energy-consuming processes to boost treatment efficiency was a typical feature of the 1970s (see figure 2). However, novel technologies pay greater attention to energy saving at the same rates of efficiency. We have shifted from the slogan of "Add more power to the process" to "Use the energy of the tool itself". Consequently, the process of vibration grinding was developed. The process does not need energy from external sources as the tool itself generates the energy in the grinding process. This is an extraordinarily environment-friendly way because the process needs no additional power sources.

(energy consumption of diamond grinding processes; diamond grinding without additional power; electrolytic; electrochemical; electro-erosion; electro-grinding; vibration grinding)

Figure 2. Main trends of energy consumption in metal grinding processes upon progress of metalworking.

The Institute for Super-hard Materials has developed new principles of activation of the grinding wheels’ cutting surface and new designs of the wheels themselves. The experience of application of such tools shows that grinding wheels can be split into several main designs (depending on their functional properties) that can:

- treat groups of tools;

- stabilise certain treatment parameters, such as temperature, productivity, wear, etc.;

- combine functional properties of several grinding wheels in a single tool.

There have been created effective technologies of efficient treatment of cutting tools made of tungsten and tungsten-free hard alloys, tool steels and other tool materials for deep, vibration and electrochemical grinding.

These technologies make it possible to:

- perform treatment regardless of the structural and phase condition of tool materials, i.e. hard and soft materials can be treated with the same efficiency and effectiveness;

- eliminate thermal influence on the structure of tool materials via application of non-calorific processes at the temperature below 6000С;

- increase productivity two to threefold;

- more than double the durability of tools.

Implementation of these technologies requires application of both standard and special grinding wheels made of diamonds and cubonite composites. For the first time ever, new grinding tools made of tool materials have been designed, namely:

- large-sized shapes 1А1 and 6А2, 400 to 900 mm in diameter;

- small-sized shapes 1А1, 1V1 and 12R4, 30 to 100 mm in diameter;

- tools with intermittent and alternate working surfaces.

The Institute for Super-hard Materials has always worked to support the high standard of domestic market for abrasive tools made of super-hard materials. The International expo Intertool-2000 held in Kiev in December 2000 once again proved this statement correct.

Table 1. Performance attributes of tool materials