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..