Abrasives - Grinding Wheels - Steel

?Introduction

First, this chapter deals with the most essential abrasive for general precision grinding: high-grade aluminum oxide (Al2O3), its manufacture, and application. Second, as aluminum oxide is mainly used for the grinding of steel, the chapter also deals with the nomenclature of steel and its alloys.

Aluminum oxide was first produced in 1900 by fusing bauxite. Before that time, abrasives were naturally occurring minerals such as emery, consisting of about 50% aluminum oxide. In Europe, emery was found on the Greek island of Naxos. Hence, several grinding wheel companies contain the word "Naxos," such as Naxos Union or Slip Naxos, for example.

Bauxite produces normal and semi-friable aluminum oxide, also called carborundum. Pure aluminum oxide (pure white alumina) is extracted from bauxite in the form of Bayer alumina, which is produced by treating bauxite with hot caustic to produce relatively pure aluminum hydroxide. Calcinating aluminum hydroxide produces pure?α?alumina powder, which is melted electrothermally in electric arc furnaces at a reaction temperature of over 2000°C.?

Illustration 1: Fusing of pure white alumina oxide

Illustration 1 schematically shows the production of aluminum oxide abrasives. First, pure white alumina is melted in an electric arc furnace at over 2000° C. Second, the alumina is placed in a cooling crucible and cooled for several days. Third, the cooled mass is broken up with large pneumatic hammers. In the fourth step, the large fragments are put into ball mills to be crushed into abrasives, which are sieved into various sizes in the fifth step, as shown in Illustration 2.

Illustration 2: Manufacturing process chain of aluminum oxide

Since normal and semi-precious corundum are not widely used in precision grinding, this chapter concentrates on pure white aluminum. The fracture and self-sharpening properties can be influenced by adding specific alloying elements such as chromium or titanium. Adding about 0.25% chromium oxide produces pink aluminum oxide. If the concentration of the fusible alloy is increased to 2.5% chromium oxide, the result is red aluminum oxide. The basic colors of the aluminum oxide have a great influence on the final color of the grinding wheel. However, one should not be deceived because white aluminum oxide wheels become brown, blue, or green by adding iron, cobalt, and chromium oxide dyes. Silicon carbide (SiC) exists as green or black SiC. Black SiC is only used for rough grinding operations, while green SiC is used for high-precision grinding when high surface finishes are required or steel > 64 HRc.

Illustration 3: Range of abrasives before crushing

Grinding wheels

Traditionally, vitrified wheel bonds consist mainly of naturally occurring raw materials like kaolin, feldspar, and glass frits. Being "natural" means that they are subject to variations in quality. Today, however, bonds are mostly synthetic recrystallized glasses consistent in quality. Crystallized glass bonds have higher inherent strength than bonds of naturally occurring raw materials and can be used in smaller quantities. The smaller amounts allow a bond reduction in the overall grinding wheel composition without losing wheel strength. The following Illustrations 4 & 5 show the three elements that make up a grinding wheel.

Illustration 4: Elements of a grinding wheel

Illustration 5: Elements of a grinding wheel

Production of grinding wheels

The manufacturing process of vitrified bonded grinding wheels is shown schematically in Illustrations 6 to 9.

Illustration 6: Mixing of grinding wheel components

Abrasive grains, binding agents, temporary binders, and pore-forming agents are mixed in countercurrent mixing units (Illustration 6) for a long and precisely defined period to achieve a homogeneous mixture.

Illustration 7: Pressing of grinding wheels

After mixing, the loose mass is dosed into pressing molds for individual pressing processes. The quantity of mixed mass, the length of pressing distance, and the pressing pressure result in the desired structure of the grinding wheels. For grinding wheels for tap grinding, pressing forces of up to 2000 tons can be used to obtain the required dense structure, making it possible to achieve the necessary edge retention on the grinding wheels. Lower pressures are often used for highly porous grinding wheels for creep-feed grinding or grinding rubber rolls.

Since abrasives create high internal friction in the mold, ideally, pressing is done from above and below at the same time. This dual motion prevents the grinding wheel from having a steep hardness gradient over the entire width.

After pressing, the grinding wheels do not yet have great strength. We speak of "green" grinding wheels, which are still very fragile. It is not until the firing process, Fig. 8, at about 1000° C and the subsequent cooling, during which the bonding material, which can be compared to glass, is melted, and coats the abrasive grains and bonds them to each other during cooling, that the grinding wheel becomes a solid body. A firing process takes at least four days. This process is determined by a firing curve that must be adhered to, in which the heating, holding and cooling times are precisely defined and must be adhered to.

Illustration 8: Firing of grinding wheels in a kiln

After firing, the now hard grinding wheels have a firing skin all around, which is now turned to the finished size (picture 9). This is done on special lathes with PCD turning tools.

Illustration 9: Machining of grinding wheels

Once the grinding wheels have been machined, they are tested for hardness. This is done by measuring the E-modulus. The grinding wheel is placed on three rubber elements and struck with a hammer to create a vibration (picture 10). This vibration is measured and compared with a reference vibration. This ensures that all production batches remain within a defined hardness tolerance band.

Illustration 10: Checking wheel hardness via the modulus of elasticity

In the final step, each grinding wheel is measured using a test speed (Fig. 11), generally 1.5 times the operating speed. The factor 1.5 corresponds to the American safety standard (ANSI), stricter than the European standard. This allows the grinding wheel manufacturer to cover many countries. Asian countries such as China, Taiwan, Japan, and Korea have even higher safety factors, and the grinding wheel supplier must take special tests when delivering to these countries. In the case of a grinding wheel with an operating speed of 50 m/s, this means a testing speed of 75 m/s. In addition, grinding worms are systematically blasted at the factory at a prescribed frequency. This ensures that all production batches remain within a narrow tolerance band.?

Illustration 11: Speed testing of grinding wheels

Alloy components of steel and the selection of corresponding abrasives

To correctly select an abrasive wheel specification, the material to be ground must be known. The alloying elements used to give the steel improved properties, such as increased tensile strength, high-temperature strength, abrasion resistance, etc., generally reduce the grindability. Alloying elements such as chromium, vanadium, and tungsten, for example, form carbides above a steel hardness of approximately 64 HRc, the hardness of which is higher than the hardness of the abrasives, as can be seen in the table of Illustration 12.

Illustration 12: Hardness of abrasives and alloying elements

Suppose one compares the grindability of three different types of steel, low-alloy steel, tool steel, and powder metallurgical steel. In that case, there is a factor of 100 in the grindability between low-alloy and powder metallurgical steel. For powder metallurgical steel, only the choice of CBN abrasive makes sense. The differences in grindability have to do with the proportion and type of alloying elements. Powder-metallurgical tool steels allow the highest alloy contents of molybdenum, cobalt, tungsten, vanadium, and chromium. Powder metallurgical steels are characterized by high toughness, maximum wear resistance, high working hardness, and long tool life, whereby these properties markedly reduce grindability.

Illustration 13: Types of steel and the influence on grindability

Reading steel specifications

Reading steel specifications is not easy at first sight. As an example, consider the specification 32CrMoV5-3. The first number refers to the carbon content C in percent. The effect of carbon is of very high importance for materials technology. On the one hand, carbon as an alloying element in iron lowers the melting point while it increases hardness and tensile strength through Fe3C formation. An iron alloy is also called steel if the carbon content is between 0.002 % and 2.06 %. However, steel can only be hardened from a carbon content of 0.3%.

Illustration 14: Composition 32CrMoV5-3 low alloyed steel

The number 32 (Fig. 3) refers to the carbon content (C), whereby the alloying elements have different factors. For carbon (C), this factor is 100, so the number 32 from the specification 32CrMoV5-3 results in 0.32% carbon after division.

Alloying elements all have a different division factor

Cr, Co, Mn, Ni, Si, W?(4)

Al, Cu, Mo, V, Pb, Nb, Ti, Ta, Zr, Be?(10)

C, Ce, N, P, S?(100)

While this definition seems strange, one must accept it and move on and look at further examples. Here: Low alloyed steel 18CrNiMo7-6, shown in detail in Illustration 15.

Illustration 15: Low alloyed steel 18CrNiMo7-6

The next example is high-alloyed tool steel, which has a percentage of alloys > 5%. It is recognizable by the initial letter “X.” In this case, the percentage of alloying elements is given without any factor, except for carbon, which carries the multiplying factor of 100.

Illustration 16: High-alloyed tool steel

High-alloyed tool steel starts with the letter HS, standing for high-speed. The alloying elements appear in a given sequence: W, Mo, V, Co.

Illustration 17: High-alloyed tool steel

The alloying constituents, Illustration 18, have a great influence on the grindability of steel. On the one hand, they can promote carbide formation in the steel structure, especially from hardness 63 HRc. They also influence chip formation. Chromium, for example, leads to long chips in unhardened steels, which tend to clog the grinding wheel and require longer dressing intervals.

Illustration 18: Influence of alloys on grindability

Illustration 19: Alloying elements and their influence of grindability

Illustration 20 shows how the hardness of the abrasive grit relates to the tendency to chip. White corundum is not quite as hard as pink corundum or red corundum, which are tougher. However, white corundum has a higher tendency to splinter, which has a positive effect on slender workpieces that bend under high grinding pressure. Sintered corundum is harder than white aluminum oxide but requires high grinding pressure to achieve self-sharpening.

Illustration 20: Hardness and friability of abrasives

The following tables, Illustrations 21 to 25, show the properties and applications of the different abrasives.?

Illustration 21: White aluminum oxide

Illustration 22: Pink aluminum oxide

Illustration 23: Ruby colored aluminum oxide

Illustration 24: Microcrystalline ceramic abrasives

Illustration 25: Silicon carbide

Walter Graf, The Philosopher’s Grindstones, Copyright?April 2022