SURFACE PLASTIC DEFORMATION OF PLASMA-ARC HARDENED LAYERS OF STEEL CLASS X12 (D2, D3 AISI)
Technical University - Varna, Bulgaria
In the paper are presented the results from microstructure, hardness, electron microscopy and roughness analysis of plasma-arc hardened and surface plastic deformed sidelong specimen of tool steels class X12.
Keywords: plasma-arc hardening, surface plastic deformation, residual austenite.
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.
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.
Table 1. Advance heat treatment regimes
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.
Table 2. Plasma-arc treatment regime parametres
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=650 N. 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.
Fig.1. Scheme of linear rotational surface plastic deformation
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 0,014° 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 100 g and microhardness measurer Heckert and pressure 5 kg. 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 0,25 мм/s.
3. 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 400°С, 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 ga transformation in zone with 90% Ares. under multiple surface deformation are not confirmed by the X-ray structural analysis of specimens of steel X12 . 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).
Fig.2. Influence of stage of plastic deformation on Fig.3. Changes in Ra depending on the number of
microhardness of hardened and annealing specimens translations
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 mm, and ordered location in the direction of the lines of dispersed carbides with sizes from 0,5 to 2 mm (fig.4e). We observe crashes of big primary carbides as well (fig.4 e,f).
Fig.4. Optical and scanning electron micrograph of zones of the combined plasma-arc treatment and surface plastic deformation
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.