金属热处理15

dc=0.2√t (9.4)

where t is the heating time (seconds).

In induction hardening, the component is heated usually for a few seconds only. Immediately after heating, the surface is quenched by a jet of cold water. Due to quenching, a martensitic structure is formed, which makes the outer surface hard and wear resistant. Figure 9.2 shows the operation of induction hardening.

In hardening, temperature for plain carbon steel is about 760℃. For alloy steels, higher hardening temperatures are required. For example, for Cr-Mo steels the hardening temperature is about 800℃.

A striking feature of induction hardening (which is true of other surface hardening processes also) is that in this process the original toughness and ductility remain unaffected even after heat treatment.

Table 9.1 gives the process conditions for induction hardening of steels.

Coolant Entry

Inductor High Frequency Capacitor Generator

Job

Magnetite Field

Sprayer

Fig.9.2 Diagrammatic representation of induction hardening process.

Table 9.1 Process Conditions for Induction Heating of Steels Range of desired depth of frequency required (Hz)

hardening (mm)

0.5-1.1 450 1.1-2.3 450 1.5-2.3 10 2.3-3.0 10 3.0-4.0 10 3.0-4.0 3 4.0-5.0 3

Range of power input

required (kW)

15-19 8-12 15-25 15-23 15-22 22-25 15-22

9.3 ELECTRON BEAM HARDENING

This process is used for hardening those components which cannot be induction hardened because of associated distortion. Automatic transmission clutch cams (SAE 5060 steel) are hardened by this process. The work-piece is kept in vacuum at 0.06m bar pressure. Electron beam is focused on the

work-piece to heat the surface. In the beginning, energy input is kept high. Whit time, power input is reduced as the component gets heated up. This is done to avoid melting. Normally, case depth up to 0.75mm can be achieved by this method. A mini-computer is used to control voltage, current, beam, dwell time and focus.

9.4 LASER HARDENING

Laser beams are also used for surface hardening treatment. Since these have very high intensity, they may melt the work-piece when they are Width

Surface

used at such high intensity. Therefore,

Depth a lens is used to reduce the intensity by

producing a defocused spot or scans Hardened Area from 1-25mm wide. A laser beam of 1 Over-tempered Zone kW produces a circular spot whose diameter may vary from 0.50 mm to Surface 0.25 mm. Industrial lasers up to 20 kW are now available. Case depth of about 0.75 mm is obtained by self quenching.

Pass Pass The depth of hardening is governed by

No.1 No.2

both time and energy density. In laser hardening process, less time is required than in induction and flame Width

Surface

hardening processes, and the effect of

Depth

heat on the surrounding surface is less, thus leading to less distortion. Some

Hardened Zone Over-tempered Zone heat patterns are shown in Fig. 9.3. No

Fig. 9.3 Heat effects of three different beam separate quenching media are required

optics, viz. defocused beam, single pass; defocused

since quench is effected by the mass of beam, overlapping passes; and oscillating beam,

single pass employed in laser heat treatment. the surrounding unheated portion. The

microstructure of laser-heated steel

consists of bainite + ferrite at the surface of the heated spot and pearlite and ferrite in the interior.

The relationship between depth of hardening and power is as follows:

P (9.5) ? 3 . 02 Y??0.11(DbV)12where

Y ＝case depth (mm) P ＝laser power (W)

Db＝incident beam diameter (mm) V ＝traverse speed (mm/s)

However, experimental data show scattering. At a constant value of P/(DbV)1/2 case depth may vary by a factor of 2.

In many countries, the industrial scale application of laser. Heat treatment

has already commenced. Rapid progress in this area has been achieved because of the availability of high power CO2 lasers and advanced cost effective laser production techniques.

The main advantages of laser heat treatment are as follows:

(i) It is possible to achieve high production rates since light has no inertia and, consequently, it is possible to obtain high processing speeds with rapid stopping and starting.

(ii) Input distortion is quite low because specific energy is very low. (iii) It is possible to give localized treatment with this process.

(iv) No external quenching is needed. At times external quenching may be adopted for such small parts which have insufficient mass for self quenching.

(v) There is hardly any contamination during surface hardening treatment. (vi) It is possible to control the process with the help of a computer.

(vii) Those areas which are difficult to be treated by conventional methods can be easily treated with this technique.

(viii) It is not necessary to carry out any final machining operation subsequent to hardening.

Laser heat treatment is best suited for steel and cast irons. During laser heating, heat transfer takes place by inverse Bremsstrahlung effect, i.e. by interaction of laser beam with the free electrons of the substrate. As a result, the energy state of the electrons of the conduction band is raised.

For successful laser heat treatment, it is necessary that the temperature of the zone which is being hardened must reach closer to the austenitizing range. Further, between the heating and cooling cycles, the substrate must be maintained at the austenitizing temperature for sufficiently long time to ensure adequate diffusion of carbon. Also, there should be enough mass so that the cooling rate achieved by self quenching is greater than the critical cooling rate required for martensite transformation.

While considering laser heat treatment, it is necessary to apply the same metallurgical concepts as in the case of other conventional heat treating processes. However, there are some basic differences between the laser heat treatment process and other conventional processes. Some of these are as follows:

(i) It is possible to harden low carbon steel with relative ease due to extremely rapid heating and cooling rates associated with laser heating. There is hardly any effect due to differences in hardenability between plain carbon steels and alloy steels since the cooling rates normally achieved during laser heat treatment are much higher than the critical cooling rate required for martensitic transformation.

(ii) It has generally been observed that the level of hardness achieved by laser hardening is higher than that obtained by conventional hardening.

(iii) Laser heat treatment is not well suited for alloys requiring rather long soaking time such as steels containing spheroidal carbides or cast irons rich in graphite instead of pearlite. The large soaking time required for the diffusion of

carbon would restrict the operating parameters associated with the laser. Consequently, the process would lose its inherent advantage of rapid heating and cooling rate.

Cast irons with a combination of pearlite and graphite, on the other hand, can be heat treated successfully with lasers. When the pearlite is being dissolved to be converted into austenite and subsequently to martensite, some carbon diffusion takes place out of graphite flakes which will produce martensite around the original graphite flakes. However, the predominant hardening mechanism will be based on austenite formed by dissolution of pearlite.

Some of the major independent process variables connected with laser heat treatment are incident laser beam power. diameter of incident laser beam, absorptivity of laser beam by the coating, and the substrate and transverse speed across the substrate surface. Another important factor in this context is the thermophysical properties of the substrate.

The depth of hardness, geometry of the heat-affected zone, and micro-structure and metallurgical properties of the laser heat treated material are the dependent variables.

For efficient laser heat treatment, it is necessary that proper absorption of light energy by the work-piece takes place. All heat transfer calculations for laser processing are based on this absorbed energy.

Melting and key hole formation should be strictly avoided during laser heat treatment. Hence, some absorbent coatings are invariably used during laser heat treatment. Colloidal graphite, manganese phosphate, zinc phosphate and black paint are some of the commonly used absorbent coatings. High absorptivity can also be achieved with the help of a mixer of sodium and potassium silicate. Absorptivity depends on coating thickness, coarseness and adherence to the substrate. Heat transfer between the coating and the substrate also plays an important role in this context.

With a given beam diameter and traverse speed, the depth of hardening by laser heat treatment is proportional to the laser power.

The surface heat source is defined by the diameter of the laser beam and distribution intensity.

The power density as also the coverage rate depends on the diameter of the laser beam. For laser heat treatment, a wide beam with uniform intensity distribution is preferable. This in turn ensures uniform case depth.

Different methods of beam manipulation can be adopted to obtain a broad beam with uniform intensity distribution. Different techniques used for this purpose are shown in Fig. 9.4.

The interaction time depends on traverse speed. The depth of hardening is inversely proportional to traverse speed. For proper hardening, it is necessary to ensure a minimum interaction time of the order of 10-2s with a power density exceeding 103 W/cm2.