Surface plastic deformation of plasma-arc hardened layersof steel class X12 (D2, D3 AISI)

Ivan IvanovTechnical University – Varna, Bulgaria

Abstract: The paper is presented results from microstructure,

hardness, electron microscopy and roughness analysis of plasma-arc

hardened and surface plastic deformed sidelong specimen of tool

steels class X12.

Key words: plasma-arc hardening, surface plastic deformation,

residual austenite

Introduction:The working capability of cutting and deforming tools depends

on the quality of their heat and mechanic treatment. Poor

implementation of one of the technologies worsens the working

capability and durability of the tool, and, respectively, the

surfaces it is used to treat.

In recent years surface heat treatment with concentrated

energy flows and finishing surface plastic deformation become

widely applied. The usage of CEF as a heat source allows local

hardening at places with highest tool wearing. In addition, we

reach hardness surpassing the hardness after the usual heat

treatment, and for the unhardened parts the initial properties

remain [1,2,4-8].

The application of surface plastic deformation secures the

decrease of the surface roughness, increment of surface hardness,

generation of residual pressure stresses [9,11], and structural

changes in the surface layer. This predisposes the durability of

the details [3,9].

Combining surface quenching and CEF with surface plastic

deformation of the hardened layer leads to additional increase in

the hardness while relatively keeping the phase structure

[1,2,4,5]. There are two major schemes for combined treatment

implementation – plastic deformation before CEF treatment [5,10]

and after CEF treatment [1,2,4,5].

The aim of current paper is to examine the combined plasma-

arc and subsequent surface deformation treatment of steel class

Х12.

Methodology:

We undertake a plasma-arc treatment of specimens of steels Х12,

Х12М, Х12МФ with sizes 20х20х60 mm. The advance heat treatment is

presented in Table 1.

The surface plasma-arc treatment of the specimens of steel

class Х12 was implemented this appliance РМ6601П, which guarantees

linear movement of the plasmotrone. Plasma-generating and

protecting gas is argon.

The plasma-arc quenching regime is presented in Table 2.

The surface elastic-plastic deformation is implemented with a

universal milling machine at rotational movement of the

instrument/tool and linear step-by-step shifting of the sample.

The tool is eccentrically adhered so that we have a width of the

deformed layer of 15 mm. The deforming treatment is implemented

with a sphere with a diameter of d=10,5 mm, and the treatment

power is F=650N. The deforming treatment on the surface of the

specimens is done with a rotation speed of 250 min-1, circle

diameter D=15 mm and linear speed of 12,5 mm/min. The number of

transitions N of the deforming instrument is determined by the

following law (1):

N=2i, i=1÷5

(1)

Scheme of linear surface plastic deformation is presented on

Figure 1.

The hardness analysis is conducted with 0,2,4,8,16 and 32

deformation transitions of the instrument. In order to achieve

better results in the specimens research in the micro hardness

changes we created oblique metallographic specimens. The specimes

are grinded in angle 0,014о in depth 0,3 mm, so that we achieve

large enough surface for layer-by-layer deformation analysis.

The structure is developed with 3%-solution of HNO3 in C2H5OH.

The microhardnesses are measured using the Vikers method with

microhardness measurer ПМТ3 and pressure 100g and microhardness

measurer Heckert and pressure 5kg. The microstructures are

photographed with an optical microscope NEOPHOT 32, and the

electron microscopy ones on an electronic microscope JOEL–JXA-50A

with zoom of 4000 times.

The roughness is measured crosswise the grinding direction. We

have used Taylor-Hobson Surtronic 3 instrument/tool, with standard

width of 4,5 мм and base length of 4 мм. The length of the piece

is 0,8 мм, and the translation speed is 0,25мм/s.

Results and analysis:

Regardless of the advance heat treatment after plasma-arc

treatment a white melted zone appears in the surface layer, with a

characteristic dendrite morphology consisting of more than 90%

residual austenite [1,4]. During the crystallization in the

melting zone a disperse quasi-eutectic is formed around the axes

of the dendrites. The hardness in this zone reaches 500 – 600 HV,

surpassing that of the ordinary austenite. We suppose this is a

consequence of the full solution of the carbides in this zone and

the saturation of the austenite with carbon and all additives.

From the standpoint of classic volume heat treatment the achieved

hardness in the melting zone is not enough for this class of

steels, working at high pressures and relatively high

temperatures.

A large quantity of the residual austenite and its eventual

thermo-deformational transformation during the usage of the

instrument leads to size instability of the instrument as well. On

the other side, the destruction of the carbide texture and its

complete solution in the metal matrix leads to homogenization of

surface layer properties. However, the achieved relatively low

hardness in the melted zone does not contribute to the increase of

the exploitation characteristics of instruments. It is different

with the quenching zone from solid state. The hardness in this

zone reaches 750 – 820 HV and is comparable and even higher than

that achieved in volume quenching. The forming of instruments’

plasma-arc treated surfaces of zones with different phase and

structural content and properties, is not always beneficial for

their exploitation characteristics. The presence of large

quantities of residual austenite in the melted zone, deteriorating

the durometric properties, can be removed in two main ways –

thermal destabilization and plastic deformation. From the view

point of its saturation with carbon and additives, the thermal

destabilization is difficult to achieve in temperatures below

400оС, the residual austenite keeps its hardness, and the further

increase of temperature is not desirable, because of the decrease

of the hardness in the volume quenched part. The deformational

martensite produced as a result of the transformation of the

austenite during exploitation of the instrument is inadmissible.

Te application of surface plastic deformation on plasma-

hardened layers is one of the options to increase the surface

hardness. In current research we observe an increase in every

zones, achieved in plasma-arc quenching regardless of the

structural condition. In greatest extent increase in hardness is

observed in the melted zone – up to 30%, which is logical

considering the quantity of austenite inclined to hammer

hardening. In lower extent, hardening is observed in the quenched

from hard state layers – 10-15% (fig.2). The expectations for γ→α

transformation in zone with 90% Аres. under multiple surface

deformation are not confirmed by the X-ray structural analysis of

specimens of steel Х12 [2]. Nevertheless, the increased hardness

after surface plastic deformation, especially in the melted zone,

and the residual pressure stresses suppose an increase in the

exploitation durability of the instrument, and from an a priori

information is well known that in surface plastic deformation we

achieve favorable pressure stresses. In addition, we observe

decrease in roughness. Largest decrease of Ra is observed after

the second transition of the deforming element. Subsequent

deformation does not contribute to significant changes in

roughness (fig.3).

Micro structurally, we note in the plasma hardened layer a

deformed texture, which is best expressed in the melted layer,

because the austenite possesses more gliding surfaces in its

crystal net than the martensite, and therefore, greater depth of

the deformed layer (fig.4). In the periphery of the zone quenched

from solid state there are formed parallel deformation lines with

distance between them of 1 – 2 μm, and ordered location in the

direction of the lines of dispersed carbides with sizes from 0,5

to 2 μm (fig.4e). We observe crashes of big primary carbides as well

(fig.4 e,f).

Conclusion:

Combining surface plasma-arc and deformation treatment of

steels class Х12 leads to an increase in the hardness in the

melted zone with around 30% and in the zone quenched from solid

state with 10÷15% and hardening depth of 0,2 [mm]. Dispersed

carbides are formed after plasma-arc quenching, and their ordering

in the direction of the deformational lines after the deformation

treatment. The surface plastic deformation decreases the roughness

to Ra 0,63.

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Heattreatment Т q.оС Т оС Т an.оС τ, min HRC

Cooling

mediumQuenching 1050 200 - 120 60-62 Oil

Annealing - - 1050 720 25 withoven

Table 1 Advance heat treatment regimes

Speed oftranslation, V

mm/s

Power density,Ns W/cm2

Expense ofplasma-

generating gas,Q, l/min

Distance betweenthe plasmotroneand the surface,

H, mm4÷10 1,5÷1,7*104 4÷6 2

Table 2. Plasma-arc treatment regime parametres

F=650N

n=250 min-1

12,5mm/min

Ø 10,5

Ø15

Figure 1. Scheme of linear surface plastic deformation

0

100

200

300

400

500

600

700

800

900

0 2 4 8 16 32

Number of transitions

HV5

melted zone tempering zone hardened zone

Figure 2. Influence of stage of plastic deformation on microhardness hardened and

annealing specimens

0 0.5 1

1.5 2

2.5

0 2 4 16 Number of translations of the deforming instrument

Ra

Figure 3. Changes in Ra depending on the number of

translations

30 μm30 μm

a b

c d

e f

Figure 4. Optical and scanning electron micrograph of zones of the combined plasma-arc treatment and surface plastic deformation