ISSN 1068-798X, Russian Engineering Research, 2009, Vol. 29, No. 1, pp. 93–96. © Allerton Press, Inc., 2009.Original Russian Text © V.I. Kotel’nikov, O.A. Krasnov, 2008, published in STIN, 2008, No. 10, pp. 23–27.

93

Experiments are conducted on the cutting of surfacedcylindrical blanks, with additional heating. Before sur-facing, the surface of the cylindrical blank is machined.Both ends of the blank are trimmed (so that the distancebetween them is 155 mm). At one end, a central hole(diameter 5 mm) is produced; at the other, a hole (diam-eter 28 mm, depth 60 mm) is made for the attachment ofthe blank to a conical mandrel (with end clamping of thepart by the rear center), with measurement of the cuttingforce by a dynamometric cutting head mounted on a16K20 machine tool. The outer diameter of all the blanksis 45 mm. A metal layer thicker than 5 mm is applied byelectric-arc welding with a metal electrode [1]. Threebands (width 20–35 mm) are applied on each blank, sothat the ridges of applied metal are, respectively, perpen-dicular to, aligned with, and tangential to the axis of theblank (Fig. 1).

During cutting of the surfaced bands, the cuttingforce and temperature are measured. A cylindrical blankafter cold cutting of the surfaced band is shown in Fig. 2,where the cut band is clearly visible. The cold-cuttingconditions are varied as follows: cutting depth up to0.8 mm (in cold cutting) and 0.8–5.5 mm (in cutting withadditional heating); supply 0.05–0.15 mm/turn (in coldcutting) and 0.25–0.52 mm/turn (in hot cutting); spindlespeed 200–1800 rpm. The blank is made of steel 45(State Standard GOST 1052–80). Hot cutting (with heat-ing by a gas-burner flame) of the surfaced band on acylindrical blank is shown in Fig. 3. As we see, the sur-face of the band is heated to a pink coloration, corre-sponding to around 800

°

C. The chip leaves the cutter at900–1000

°

C.

Cutting with large margins permits the removal ofthe whole surfaced layer on the cylindrical blank in onepass. The surface of the hot-machined part has a char-acteristic blue coloration (Fig. 4). Measurements of thecutting temperature by the natural-thermocouplemethod show that the temperature in the cutting zone is860–950

°

C (in hot cutting) and 910–1020

°

C (in coldcutting of the surfaced layer).

Experience with cold metal cutting shows that themachining of surfaced surfaces and weld seams entailspreliminary annealing of the surfaced part so as toweaken the finely crystalline crust on the surfacedlayer, which is saturated with welding-slag inclusions.Such heat treatment is required to preserve the cuttingedge of the tool.

In addition, in cutting a surfaced layer with a highcontent of nonmetallic inclusions formed in the solidi-fication of metal surfaced at the surface, considerabletemperatures arise in the cutting zone, leading ulti-mately to heating of the tool’s cutting edge, whichsheds crumbs of metal.

As a result of accelerated tool wear, the machiningprecision declines sharply, and the loss of metal parti-cles from the cutting edge increases the roughness ofthe machined surface. As shown by experiments on thecutting of unannealed surfaced zones, the resistance ofthe metal is so great that cutting is not possible beyonda depth of 0.8 mm and a supply of 0.15 mm/turn. Coldcutting of bands of unannealed metal blank with a sur-faced layer is shown in Fig. 5.

It is evident from Fig. 5 that the cutting depth is verysmall and the supply is small. In cold cutting of the sur-face crust, the chip leaves the cutter in the form of smallburned crumbs. Note that the tool wear after each passin a single blank is so great that the cutter must bereground. T5K10 hard-alloy cutting plates areemployed. In hot cutting, the cutter is reground onlyonce (after machining 20 blanks).

This sharp difference in tool life in cold and hot cut-ting is explained by the change in chip formation on cut-ting. In cold cutting, chip formation is accompanied byplastic deformation of the cut metal. The size of the plas-tic-shear zone of the crystals is chip formation is clearlyseen on the microsection in Fig. 6, where the plastic-shear zone of the crystals may be determined from char-acteristic features of the texture lines in the steel.

The initial boundary of the chip-formation zone isthe line dividing the region into metal grains that areand are not deformed in plastic shear; the final bound-ary is the line parallel to the initial boundary at the pointcorresponding to maximum height of the zone of con-tact plastic deformation. The crack along which the

Hot Cutting of Blanks after Surfacing

V. I. Kotel’nikov and O. A. Krasnov

DOI:

10.3103/S1068798X09010237

Fig. 1.

Blank with surfaced metal bands.

94

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2009

KOTEL’NIKOV, KRASNOV

block of crystals moves in further chip motion along thecutter’s front surface may be seen in Fig. 6.

The cutting force consumed in chip formationdepends on the strength of the metal, the cut cross sec-tion, and the angles of cutting, friction, and shear of themetal in plastic deformation [2]. In hot cutting, theadditional energy introduced in the system sharplychanges the plastic deformation of the metal in the chip.Increasing the temperature of the metal surface softensit and reduces its resistance to cutting. Heating disruptsthe strong intercrystallite bonds of the metal grains ondeformation, under the pressure of the cutting edge.

As a result, in hot cutting, chip formation ischanged. The temperature propagation in a cylindricalbody on heating by an external source is shown sche-matically in Fig. 7 (by means of a grid).

In hot cutting, the softened metal in the chip slipsalong the shear plane. However, separation of the crys-tals along plastic-deformation lines or along cleavagecracks, with loss of the strong intercrystalline bonds, isnot observed. As a result, the chip leaves the cutter inthe form of a straight intact strip. The chip thickness isclose to the linear dimension of supply, and its width isequal to the length of the tool’s cutting edge in contactwith the hot metal on cutting.

On account of the softening of the hot metal, hotcutting may be conducted at high speed and with con-siderable cutting depth up to 500–850

°

C.

As shown by experiments, the cutting of a softenedlayer of surfaced metal is possible at any speed and withany supply rate. It is found that the cutting depth of themetal depends on the degree of heating of the part andis not limited by the power of the machine tool’s pri-mary motor, the chemical composition of the surfacedmetal, or its initial strength.

In practice, hot cutting of surfaced metal may occurover the whole cutting edge of the tool. With decreasein the resistance of the hot metal to cutting, tool lifeincreases. For example, the wear area on the rear cuttersurface is only pronounced after turning five blanks tothe maximum depth (hot cutting of 15 bands, at fivespindle speeds: 200, 350, 500, 800, and 1600 rpm; sup-ply 0.52 mm/turn; cutting depth 4 mm). Intensificationof the cutting conditions increases the productivity by afactor of 2–4 [3].

The increase in tool life may be explained in thatheating sharply reduces the pressure of the rear cuttersurface on the blank. In hot cutting of a surfaced sur-face, the hardness ratio of the cutter and the blank issharply changed. Experiments show that increase inthis ratio reduces the error due to hardness fluctuationsin turning.

In addition, with the adopted experimental con-straints, the fluctuations in the margin removed arereduced on account of the reduced vibration of themachine-tool system in hot cutting relative to cold turn-ing. Accordingly, we may assume zero error due to

Fig. 2.

Cylindrical blank after cold cutting.

Fig. 3.

Hot cutting of a band of surfaced metal.

Fig. 4.

Hot cutting of part with surfaced bands.

Fig. 5.

Cold cutting of bands of unannealed metal blankwith a surfaced layer.

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HOT CUTTING OF BLANKS AFTER SURFACING 95

elastic deformation and zero error due to tool wear, inthe first approximation. Then the sum of the dynamicerrors may be expressed as

(1)

where

∆

mt

is the error due to geometric imprecision ofthe machine tool;

∆

t

is the error due to temperaturedeformation of the part in turning.

In hot turning of shafts with a diameter of 30, 40,and 50 mm, the errors are determined at constant cut-ting speed, supply, and cutting depth.

For comparison, the total errors in cold turning aredetermined. The following conditions are specified: 1)steel 45 shaft blanks with a surfaced surface are turnedby a cutter with a T15K6 hard-alloy plate (

ϕ

= 45

°

,

ϕ

=10

°

); 2) the cutting conditions are as follows: supply

s

=0.15 mm/turn; cutting speed

v

= 130 m/min; cuttingdepth

t

= 0.5 mm.

The total diametric error in machining is

(2)

where

∆

el

is the error due to elastic deformation of thesystem;

∆

s

is the error in the machine tool’s settings;

∆

w

is the error due to tool wear;

∆

mt

is the error due to geo-metric imprecision of the machine tool;

∆

t

is the errordue to temperature deformation of the system.

After turning, the diameter is measured in threecross sections (the beginning, middle, and end of theshaft). For example, in cold cutting, the tool settings are

∆i

i 1=

n

∑ f ∆mt ∆t+( ),=

∆i

i 1=

n

∑

= 2 ∆el2 ∆s

2 1.73∆mt( )2 1.73∆w( )2 1.73Σ∆t( )2+ + + + ,

intended to produce a shaft diameter of 32 mm, but itsactual dimensions are as follows:

D

1

= 31.97 mm

∆

1

= 32 – 31.97 = 0.03 mm

D

2

= 31.99 mm

∆

2

= 32 – 31.99 = 0.01 mm

D

3

= 31.97 mm

∆

3

= 32 – 31.97 = 0.03 mm

∆

y

= 0.03

+

0.04 = 0.07 mm = 70

µ

m.

The machining error in cold cutting is 70

µ

m. Afterhot cutting with the same tool settings, the results are asfollows

D

1

= 31.97 mm

∆

1

= 32 – 31.97 = 0.03 mm

D

2

= 31.99 mm

∆

2

= 32 – 31.99 = 0.01 mm

D

3

= 31.97 mm

∆

3

= 32 – 31.97 = 0.03 mm

∆

y

= 0.03 – 0.01 = 0.02 mm = 20

µ

m.

The machining error in hot cutting is 20

µ

m, whichrepresents a 3.5-fold decrease.

After hot cutting (

T

= 450–600

°

C), there is no coldhardening at the surface [4]. This may be explained inthat the theoretical recrystallization temperature for themetal employed is 450

°

C. At 450

°

C, the lattice distor-tion in the surface layer due to the stress in cutting iseliminated. In recrystallization, the hardness andstrength are reduced by 20–25% relative to the initialvalues, while the plasticity increases. There is littlechange in metal structure.

In hot cutting, the surfaced layer is heated to thesoftening temperature within a short period, in whichthere are no marked structural changes. However, thestress is removed, and the lattice distortion is reduced.As a result, the cold hardening disappears.

The metal structure and grain size affect the suscep-tibility of the surfaced layer to cutting. The large-grainstructure of the surfaced steel flux, with reduced ductil-ity, is better for machining. In hot cutting (

T

= 450–600

°

C), on account of softening of the metal, there isno difference in the machining properties of the sur-faced layer and the basic metal in 30, 45, and 40Khsteel parts [5].

Fig. 6.

Microsection of chip formation in cutting quenchedsteel by a tool with T15K6 hard-alloy cutting plate.

1000

°

C 500

°

C

1000

°

C

20

°

C

1 2

3

Fig. 7.

Heat propagation in the cross section of a cylindricalblank on hot cutting:

1)

burner;

2)

part;

3)

cutter.

2000–1500

°

C

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The restoration of worn surfaces (for example, cat-erpillar tracks) by surfacing and subsequent hot cutting,to ensure the required strength and hardness, ends withsurface quenching by high-frequency currents. By con-trast, in ordinary repair by surfacing, the whole part issubjected to tempering, followed by cold machining,and final hardening by quenching. Experiments showthat, in hot cutting of the surfaced layer, there is no needfor tempering of the surfaced metal. After hot cutting,the part may immediately undergo quenching by high-frequency currents.

The structure of surfaced metal samples (30, 44, and40Kh steel) is investigated metallographically. The dia-metric cross section of the surfaced part is ground pol-ished, and etched by appropriate reagents. The microstruc-ture is analyzed on a MIM-7 microscope (360-fold mag-nification).

Photographs of the microstructure in the region ofthe surfaced metal and the material of the cylindricalsections show that, in hot cutting of surfaced parts,heating of the surfaced layer does not change the grainsize or the structure of the metal and does not impair theproperties of the surfaced layer on the metal part.

Parts restored by surfacing and subsequent cuttingare subjected to mechanical tests by the method of [6].The results indicate that hot cutting of surfaced parts(shafts) during repair considerably extends their work-ing life. The number of load cycles to failure for allparts repaired by surfacing and subsequent hot cuttingis comparable with that for parts made entirely of themetal used for plating. In strength tests of surfaced

parts, no peeling or disintegration of the surfaced layeris observed.

REFERENCES

1. Tolstov, I.V. and Korotkov, V.A.,

Spravochnik ponaplavke

(Surfacing Handbook), Chelyabinsk: Metal-lurgiya, 1990.

2. Plotnikov, A.L. and Cheremushnikov, N.P., ComplexInfluence of Steel Strength on the Components of theCutting Force,

Stanki Instrum.

, 2006, no. 10, pp. 27–33.3. Sorokin, V.M., Gavrilov, G.N., and Kotel’nikov, V.I.,

Restoring Worn Parts by Cutting with Heating of the Tar-get Layer,

Progressivnye tekhnologii v mashino- i pri-borostroenii: Mezhvuz. sb. statei po materialam

VNTK

(Progressive Technologies in Machine and InstrumentManufacture: Papers Based on Conference Proceed-ings), Nizhni Novgorod–Arzamas: NGTU, 2004,pp. 78–81.

4. Kotel’nikov, V.I., Abdullaev, Sh.R., Milovanov, V.A., etal., Influence of the Lack of Cold Hardening after HotMetal Cutting on the Operational Properties ofMachines,

Progressivnye tekhnologii v mashino- iproborostroenii: Materialy Vseros. nauch.-tekhn. konf.

(Progressive Technologies in Machine and InstrumentManufacture: Conference Proceedings), Arzamas:NGTU, 2005, pp. 130–132.

5. Bogodukhov, S.I., Grebenyuk, V.F., and Sinyukhov,A.V.,

Kurs materialovedeniya v voprosakh i otvetakh:Ucheb. posobie

(Course in Materials Science: A Text-book Based on Questions and Answers), Moscow:Mashinostroenie, 2005.

6. Grib, V.V. and Lazarev, G.E.,

Laboratornye ispytaniyamaterialov na trenie i iznos

(Laboratory Frictional andWear Tests of Materials), Moscow: Nauka, 1968.