PULSE-PLASMA HARDENING OF TOOLS
Treatment of surfaces by concentrated energy sources (laser, electron beam, plasma, etc.) is extensively used in machine building for hardening of tools. It provides unique mechanical and physical properties of surfaces of parts (high hardness, wear resistance, fatigue strength, corrosion resistance, etc.)
Concentrated energy sources cause rapid heating (time of heating - 10-3…10-6 s) of the surface layer of metal followed by its intensive cooling through removal of heat both into the bulk of metal and into the environment. High rates of heating and cooling of the surface layer of metal (104…108K/s) lead to formation of a dispersed crystalline structure, high density of dislocations and variations in the carbon and nitrogen concentrations.
Thermal effect exerted by the concentrated energy sources is combined with alloying processes. Surfaces of parts are alloyed by melting of the preliminarily heated coatings or by adding gaseous alloying elements, such as nitrogen, hydrocarbon gases, cyanides, into the working medium (plasma). As shown by investigations, the pulsed action on the workpiece surface is most efficient. This is attributable to increased heating and cooling rates, elasto-plastic deformation
of the surface and, thus, intensification of the diffusion mechanisms.
The E.O.Paton Electric Welding Institute developed the technology for pulse-plasma treatment of materials, which allows different methods of affecting the workpiece surface, such as elasto-plastic deformation, effect by sound and pulsed magnetic field, heat and electric-pulse treatment and deformation of metals and alloys during the process of reversible (α—γ) transformations, to be simultaneously implemented.
At initiation of the detonation the ionized combustion products are fed from detonation chamber 1 to inter-electrode gap, thus completing the electric circuit. This leads to formation of conducting layer of the combustion products, which is accelerated under the effect of gas-dynamic and electromagnetic forces. Consumable metal rod is fixed in a position along the axis of central electrode. The end of the rod is evaporated during heating and thus provides introduction of alloying elements into the plasma jet.
Being ejected from the plasmatron, plasma jet completes the circuit between the anode electrode and the workpiece surface. As a result of the current flowing through the jet the plasma is heated due to Joulean heat.
The time-averaged temperature of the plasma jet was determined by the results of spectral analysis of the jet. The temperature of the plasma was determined by a relative intensity of iron lines. Plasma radiation spectra integrated by time showed that temperature of the plasma at the plasmatron exit was 15000-20000 K. The time of interaction of the plasma pulse is 0.2…0.6 ms
Density of the electric current in the plasma jet, j, is (1-7)·10 A/cm2. The heat flow into a workpiece depends upon the current density and varies within a range of q= (0.1-5.0)·106 W/cm2.
The time of interaction of the plasma pulse and its energy parameters were controlled by varying capacitance of the capacitors, voltage charge at plates of the capacitor bank, inductance in the discharge circuit, distance to the workpiece surface and variation in size of the active spot of interaction of the plasma jet with the workpiece surface.
As indicated by the experiments and shown by the calculation data, treatment of the workpiece surface by the pulsed plasma containing alloying elements is accompanied by a complex (thermal, electromagnetic and deformation) pulsed effect. This provides alloying of the workpiece surface with components of the plasma and hardening of this surface. Alloying elements are added into the plasma in the form of metal electrode (rod) erosion products and gas (propane, nitrogen).
Pulse-plasma treatment of a workpiece of iron-based alloy results in the formation of a microcrystalline alloyed layer. Structure of the layer depends upon the plasma composition and quantity of the treatment pulses.
X-ray phase analysis of the pulse-plasma hardened layers on samples of carbon steels detects widening of the α-Fe lines and emergence of the Fe residual austenite lines. An increase in the number of pulses favours further widening of the α-Fe lines with a decrease in their intensity, as well as an increase in the relative intensity of the γ-Fe lines. Comparison of the intensities of the residual austenite and ferrite lines shows that the amount of austenite under the same treatment conditions is maximum in the case of using a tungsten electrode. X-ray spectrometry indicated that the consumable electrode material penetrated into the hardened layer on a workpiece. For example, in the case of using a titanium consumable electrode titanium was detected at a depth of down to 20 μm in the hardened layer.
Experience of application of the technology.
Maximum microhardness of the surface layer was obtained in treatment by the plasma containing tungsten or molybdenum vapours and in excessive nitrogen and carbon content of the plasma. In addition to the above process parameters, the level of microhardness is affected also by the quantity of the plasma pulses. An increase in their number leads to an increase in thickness of the hardened layer and its uniformity.
High values of service properties of alloys used for the manufacture of tools are provided by alloying them with tungsten, molybdenum and vanadium. The chromium content of alloys is 3.0-4.5 %.
The technology and technological equipment for pulse-plasma hardening of tools are employed under conditions of metalware production and mass metallurgical production at the OpenStock-Holding Company "Cherepovets Steel-Rolling Plant". Metal cutting tools, dies and punches for hot and cold deformation of metal were subjected to hardening. Prior to hardening, the parts were subjected to standard heat treatment and machining. Pulse-plasma treatment was used as a final operation. Hardening was done only to surfaces of cutting edges of the tools (Figure 11). Productivity of hardening was up to 0.5 m2/h, which in re-calculation to actual tools amounts to up to 100 punches per hour. The tool surface requires no cleaning or any other preparatory operation for hardening.
The experience of commercial application showed that performance of the tools increased 2-6 times.
1. The offered pulse-plasma technology is a resource-saving one, which is provided by a low consumption of alloying elements and electric power in combination with high productivity (up to
2. The technology makes it possible to treat (heat) only working (cutting) surfaces, which solves the problem of increasing wear resistance without any change in structural state of the entire workpiece.
3. Treatment of surfaces of tools by the pulsed plasma containing alloying elements is accompanied by electromagnetic (up to H = 1.6·105 A/m) and thermal (up to q = 5·106 W/cm2 )
effects. This accelerates mass transfer of elements from the plasma deep into the surface being hardened.
4. Commercial application of the pulse-plasma technology proves its efficiency.