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Metal Cutting of Ck 45 Steel - Cutting Process in the Microstructure; Influence of Heat Treatment


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Chip formation is the fundamental process of metal cutting and is influenced by several material properties. These properties
of cut material and with them the machinability of steel are determined not only by its alloying constituents but also by the type and size of its structural constituents.
Four different textures of a carbon steel, Ck 45 , have been induced by heat treatment. Heat treatment processes differ in temperature, thermal retardation and rate of cooling.
Normalising induces a texture of light ferrite and dark perlite, which is lamellar cementite in a ferritic matrix. By soft-annealing, lamellar cementite of pearlitic constituency is converted into spheroidal cementite. The tempered texture appears as martensite annealed at high temperature within precipitated spheroidal cementite. The coarse-grained forgetype perlite is induced by
controlled cooling down from forging
temperature. Coarser pearlitic constituents are incorporated in a ferritic network. The different texture types determine the mechanical properties of this Ck 45 steel and with
them chip formation.
At the extremely low cutting speed of 0.01 m per min flow chips are being formed by continuous shearing of normalized structure. Due to a number of small shearing fissures the upper chip surface appears rough. The varying depth of these shearing fissures signifies differential deformability of the surface layer.
The spherodized structure of the Ck 45 steel tends to adhere. When being formed, chip material sticks to the rake face and is upset until the retained material starts to glide on the rake face and shear off, starting from the cutting edge. In this manner separate or cohesive shear chip elements are formed.
Good deformability and high elastic limit of tempered texture induce formation of shear chips. Material is adhering and being increasingly upset; then detaching from the rake face and shearing off as a completely formed shear chip element.
Continuous shearing of a forgetype pearlitic structure creates flow chips. Shearing angle, chip relation and uncut chip thickness vary in relation to the coarse crystallises. The upper chip surface is developing wavily and appears to be rough.
While a sharp cutting edge favors the process of cutting, a rounded edge, as here with a radius of about 30 micrometer, induces a zone of high material deformation which causes strainhardening and, with that, formation of built-up edges. When cutting the normalized structure of the Ck 45 steel, flow chips are formed at changing built-up edges in conjunction with the positive rake angle of about 20Grad.
High plastic deformation of the spherodized structure in the zone in front of the rounded cutting edge induces formation of built-up edges. Changing material deformation at a stationary nucleus of the built-up edge causes flake-shaped material overlapping on the cut surface and the lower chip surface.
Distinctly outlined built-up edges are being shaped out of the tempered steel structure. Instabilities are causing an alternate settling and gliding-off of the budt-up edges. The separating particles of the built-up edge are pressed into the cut surface layer which exhibits flakey roughness. Flow chip formation proceeds consistently with varying chip thickness. When cutting
the forgetype texture, a small and stable built-up edge has settled at the rounded cutting edge and flow chips are being formed.The built-up edge has filled up the zone in front of the rounded nose and has thus shaped a sharp cutting edge that causes material separation without considerable plastic deformation of this structuretype in the zones of contact.
With a cutting speed of 1 m per min flow chips are formed in the normalised structure. A small, stable built-up edge favours chip's running off and causes roughness of the cut surface by quickly varying plastic deformation on the apex of the built-up edge.
When cutting the spherodized structure of this carbon steel Ck 45 with 1 m per min a discontinuous flow chip is running off from a small, stable, high rising built-up edge.
At a big built-up edge, discontinuous flow chips are being formed out of the surface influenced structure of the tempered steel. The cut surface of the workpiece has been determined by the preceding cutting process and shows flakes. The shape of the built-up edge is varying by changing plastic deformation at the apex and shearing deformations of the nucleus.
When cutting forgetype structure, a distinctly outlined built-up edge has been formed and is determining the formation of continuous flow chips. The shape and size of the built-up edge are varying through momentary deformations, accumulation, and slipping away of small particles. Flake-shaped overlappings of material thereby appear at the lower chip surface and the cut surface of the workpiece.
At a cutting speed of about 10 m per min a stable built-up edge has been shaped out of the normalized texture. Material deposits are adhering and accumulating contrary to the chip running direction on the rake face, momentarily linking up with the built-up edge. These momentarily changing processes of shaping the built-up edge influence flow chip formation.
When cutting the spherodized texture of Ck 45 steel a small, stable built-up edge has accumulated on the cutting wedge of the tool, and a continuous chip is running off. Friction in the zone of contact between rake face and chip causes shearing in the lower surface, and a flow layer appears as a bright ribbon.
The martensite annealed at high temperature has induced a big built-up edge on the rake face. Fluctuating plastic deformation is intensely varying the shape of the built-up edge and causing discontinuous flow chips with initiation of shear chip formation. These also cause the rough cut surface with flaky inclusions of the strongly deformed and strainhardened particles of the built-up edge.
When cutting the forgetype structure at 10 m per min: cutting speed, and at first small, but increasingly built-up edge is being shaped in connection with continuously alternating deposition and shearing-off. Its shape and size varies irregularly, momentarily jutting out the cutting edge of the tool so that a gap occurs between cut surface and clearance face and correspondingly the uncut chip thickness increases.
When cutting the normalized structure of the Ck 45 carbon steel at a cutting speed of about 30 m per min, a continuously running flow chip is being formed at a large, high angle towered built-up edge. Temporarily, the flow chip seems to glide without any contact and friction between the apex of the built-up edge and the rake face of the tool. Because the stable built-up edge juts out, a clearance gap has occurred.
A stable built-up edge, formed out of the spherodized steel-structure, appears more strongly rounded with intensive material deformation and is more extended with a near-horizontal transition to the rake face. Temporarily, free gliding of the chip over built-up edge of the rake face, a clearance gap, and flaky shearing off, characterise processing under these conditions.
When cutting at 30 m per min, the tempered-type material flows nearly rectangularly around a stable built-up edge nucleus. The lower surface of the chip is developing flaky roughness because of variation in the flowing layer. Compared with that, the cut surface of the workpiece appears smooth because the material glides on a short section of the built-up edge with a slightly negative clearance angle.
When cutting the forgetype structure, material in the lower chip surface and in the cut surface runs continuously around a stable, well marked built-up edge. At the apex only temporary small deformations and momentary depositing and shearing-off of small particles occur.
At a cutting speed of about 60 m per min., increased affects of higher cutting temperature influence built-up edge processing of normalised type Ck 45 steel. Shape and size of built-up edges, chip formation, and cut surface vary momentarily through material deformation, depositing and shearing-off of larger sections of the built-up edge.
The spherodized type of the steel Ck 45 produces a stable built-up edge, whose apex appears rounded because of intense material flow. Again and again material wedges are piled up on the rake face contrary to the chip running direction. Friction between lower chip surface and rake face, shearing, and strain-hardening cause these material deposits as well as built-up edge formation.
Because of the higher mechanical properties of the tempered texture at a cutting speed of about 60 m per min, the cutting temperature is so high that strain-hardening no longer more occurs when material flows around the cutting wedge of the tool, and therefore a built-up edge no longer occurs. Material is flowing continuously with the chip on the rake face as with the workpiece around the cutting edge.
When cutting Ck 45 forgetype steel, built-up edge formation changes over a wide range through intensive deformation, momentary accumulation and shearing-off of sections of built-up edge. Chip and cut-surface shaping are correspondingly irregular in appearance.
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Formal Metadata

Title Metal Cutting of Ck 45 Steel - Cutting Process in the Microstructure; Influence of Heat Treatment
Alternative Title Zerspanen von Stahl Ck 45 - Schnittvorgang im Feingefüge; Einfluß der Wärmebehandlung
Author Warnecke, Günter
License No Open Access License:
German copyright law applies. This film may be used for your own use but it may not be distributed via the internet or passed on to external parties.
DOI 10.3203/IWF/E-2949eng
IWF Signature E 2949
Publisher IWF (Göttingen)
Release Date 1987
Language English
Producer IWF
Production Year 1984

Technical Metadata

IWF Technical Data Film, 16 mm, LT, 178 m ; F, 16 1/2 min

Content Metadata

Subject Area Engineering
Abstract With the aid of a special experimental method it is possible to make the process of chip formation accessible for direct observation. Microcinematographical shots show penetration of the cutting blade into the skin of the polished and etched work piece structure. Hereby, due to the interaction of various types of behaviour from aspects of deformation and resistance of cut material on the one hand, and material stress on the other, caused by the cutting too, depending on cutting conditions, the formation of several chip shapes and built-up edges can be observed. The Ck 45 tempering steel, which was transformed by heat treatment into four different structural states with different mechanical properties, was cut at a speed of between 0.01 and 60 m/min. A wide spectrum of types of chips and built-up edge shapes can be seen. This is to be attributed to the interrelationship of influence on the work material, strain causes by the cutting wedge and cutting conditions. Resistance, stretch limit and stretching, combined with the cutting temperature caused by the cutting speed, go to create a complex mechanism of different influence.
Keywords metal cutting / steel
machining / steel
steel / metal cutting
steel / machining
heat treatment / steel machining

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