High Speed Steel

 

High speed steels are the most common in cutting tools. They combine high heat resistance (600-650 ° C, depending on the composition and processing) with high hardness (up to HRC 68-70), wear resistance at elevated temperatures and increased resistance to plastic deformation. High-speed steels allow you to increase the cutting speed 2-4 times compared to the speeds used when machining with tools from carbon and alloy tool steels.

High-speed steels are widely used for cutting tools operating under conditions of significant loading and heating of the working edges. High-speed steel tools have a fairly high stability of properties, which is especially important in a flexible automated production.

The performance of tools of a simple shape with a massive cutting edge during continuous turning is limited by secondary hardness, heat resistance and wear resistance. For tools of complex shape, thin-blade, as well as for tools used for interrupted turning, the strength and toughness of high-speed steel are of greater importance. An increase in one property or another, achieved as a result of a change in the chemical composition of steel, as well as the modes of quenching and tempering, is often accompanied by a decrease in other indicators. For example, with an increase in secondary hardness and heat resistance, a decrease in the strength and toughness of steel is usually observed.

High cutting properties of high-speed steels are provided by alloying with strong carbide-forming elements (tungsten, molybdenum, vanadium), elements that increase the temperature (a -> y) - transformation (cobalt, aluminum), and the use of special heat treatment, which consists in quenching at high temperatures (1200 -1300 ° C) and tempering causing precipitation hardening.

For high-speed steels, M6C carbide is the main one.

To obtain high heat resistance and hardness, a sufficiently large proportion of decomposing carbide must be transferred during quenching into a solid solution (austenite, martensite), which saturates it with carbon, tungsten, molybdenum, vanadium, and chromium.

Subsequent tempering at temperatures of 550-560 ° C increases the hardness to maximum values ​​due to the precipitation of dispersed carbides and the decomposition of retained austenite.

Depending on the chemical composition and, consequently, the level of basic properties, high-speed steels are subdivided into steels of normal and increased heat resistance (productivity). If the vanadium content does not exceed 2%, they are classified as high-speed steels of normal heat resistance (productivity). These are steel R18, R9, R6M5.

High-speed steels with a higher vanadium content, as well as additionally alloyed with cobalt, are classified as steels with increased heat resistance (R12FZ, R6M5FZ, R18K5F2, R9K5, R6M5K5, R9M4K8, etc.).

Compared with steels of normal productivity, high-vanadium steels with increased productivity have mainly increased wear resistance due to the presence of high-hard carbide of the MC type, and cobalt-containing steels have higher secondary hardness, heat resistance and thermal conductivity.

The group of high-speed steels with increased productivity should also include high-speed precipitation-hardening alloys with intermetallic hardening. Their high heat resistance and cutting properties are provided by high temperatures a -> y transformation and hardening due to the release of intermetallic compounds during tempering, which have a higher resistance to coagulation upon heating than carbides. The most widely used alloy is V11M7K23 (EP831).

The main properties of high speed steels as delivered are shown in the table below. The final heat treatment modes and properties of high-speed steels of normal and increased productivity are shown in the table below.

A group of low-alloy high-speed steels with a total content of tungsten and molybdenum not exceeding 5-6% is intensively developing.

Tools made of high-speed steels of this group are intended mainly for processing non-hardened steels and cast irons, as well as non-ferrous metals and alloys. The durability of tools made of these steels when processing the above-mentioned groups of materials is close to the durability of tools made of steel R6M5.

The highest properties in this group of steels are possessed by P2M5 and 11M5F steels. They are significantly superior to 11R3AM3F2 and 9Kh4M3F2AGST steels in terms of both basic properties and grindability.

The use of low-alloy tungsten-free steel 11M5F is especially promising. Steel 11M5FYUS with 1% Al has higher heat resistance and cutting properties than high-speed steel R6M5..