METALLOGRAPHIC AND ELECTRON MICROSCOPY RESEARCHES OF STEEL H12MF (D2 AISI)
AFTER COMBINED PLASMA ARC AND SURFACE DEFORMATIONAL TREATMENT *

Sergey Kirov
Ivan Ivanov

Technical University - Varna, Bulgaria


Abstract

In the paper are presented results from metallographic and electromicroscopic researches of tool steel H12MF after combination of massive and concentrated thermal and additional plastic deformation. The conducted hardness measurements and structural examinations are in the basis of the comparative analysis for interconnection between the changes of the surface characteristics and the implemented machining. It is stated that there is significant increase in the microhardness of the surface layers, especially in the local micro volumes of the residual austenite that are susceptible to deformational strengthening.

Keywords: plasma-arc treatment, surface plastic deformation, steel H12MF.



1. Introduction

Modification of the surface characteristics is done not only under technological influences, but also in the process of detail and tool exploitation. The formation of contact or friction interaction creates tensely condition, under which the surface plastic deformation occurs. This leads to the origin of nanocrystal layers with size of the crystallite of 5-100 nm. Their formation occurs over the friction surfaces of not only plastic and relatively soft materials (native metals, austenite steels), but also of highly robust and hardly-deformational alloys including high-carbon and high-speed steels [10]. The applying of surface thermal treatment via concentrated energy fluxes, especially on high-carbon and alloyed steel, forms structures susceptible to future deformational strengthening. From the point of view of stabilizing the size and improving the mechanical and other characteristics, is necessary that this process be realized before exploitation of the details and the tools.

The purpose of the following work is that by using Vickers microhardness test, microstructure and electro microscopic (scanning electronic microscope) researches to be determined the character of the strengthening processes during combined plasma-arc and surface deformational impact of steel H12MF.


2. Methodic

Developed methods for surface plastic deformation are being used [6], by utilization of spinning tool with elastically connected ball deformational element, achieving width of the deformational layer of 15 mm. The samples with dimensions of 20x20x60 mm are processed in advance under massive thermal treatment and then put under combined plasma-arc surface deformational impact (Table 1).

                      Table 1. Thermal treatment conditions
                     

The surface plastic deformation is implemented through rotational movement of the indenter (hardened-steel sphere) and linear step-by-step shifting of the sample (fig.1),

           
                                                       Fig.1. Scheme of linear rotational surface plastic deformation

where the count of the transitions N is defined by the following law /1/:

N = 2i,    i = 1÷5                 /1/

The hardness analysis is conducted with 2, 4, 8, 16 and 32 deformational transitions and jam force of the indenter on the sample of
650 N (Table 2).

                Table 2. Parameters of surface plastic deformation's conditions
               

For more precise results, the research of the microhardness and depth-structure was implemented on oblique metallographic specimen (fig.2). The metallographic examinations were conducted on a optical microscope Neophot 32, and the electron microscopy ones on electronic microscope JOEL-JXA-50A with zoom of 4000 times. For the hardness analysis is used hardness measurer PMT3 with loading of 0.1 kg.

                                       
                Fig.2. Microstructures in the typical zones of the combined plasma-arc treatment and surface plastic deformation


3. Results and discussion

By combining plasma-arc and surface deformational treatment are formed several typical zones with different structural and mechanical characteristics. In most conditions, a zone with full melting forms on the surface, transforming smoothly into one with partial melting, and respectively into a zone of hardening from hard state (fig.2). Under high speed of cooling, crystallization in the melting zone occurs with formation of thin dendrite structure from austenite, satiated with chrome and carbon (over 80%), which maintains its stability until reaching room temperature. The end of the crystallization goes in the interdendritic spaces with formation of two-phased carbide-austenite (ledeburite) structures, or after prolonged cooling, of carbide-martensite ones.

The formed structure and the microhardness in this zone practically don't depend on the massive thermal treatment conducted in advance. Right after the plasma-arc treatment, the hardness is within 550-600 HV and after surface deformational impact it reaches 780-820 HV. The increase of the achieved deformational strengthening is over 30% and the speed of the increment is highest until the eighth transition, after which it fades (fig.2). The depth, until which this treatment is significant, is 0.05 - 0.07 mm, which is determined by measuring the microhardness of an oblique specimen. The effectiveness of the process of strengthening in this zone, after plasma-arc and especially deformational treatment is related to the possibility of realizing increased dislocated density in the austenite. Especially effective obstacles against the movement of dislocations are the border surfaces of the austenite dendrites, which contact with relatively harder and non-deformational carbide-martensite mixture [8]. The examined intensive strengthening after surface deformation is also due to activation of processes of dynamic deformational aging, which is characterized with segregation of alloyed atoms over the numerous dislocations [7,8].

After conducting X-ray structural researches on samples of similar steel X12, which passed the same treatment [6], it wasn't experienced any deformational activation of the process of transforming the satiated with chrome and carbon austenite into martensite.

In the rest of the zones of hardening from hard state - surface plasma-arc and massive one, is observed, even though to less extent, increasing of the microhardness after deformational treatment. This degradation of deformational strengthening is based on the less ability of the more fragile martensite and non-dissolved primary carbides to be plastically deformed. The increase of the hardness in this case is no more than 130-150 HV, which is slightly more than 20%, compared to the 30% of the melted zone, which includes big amounts of residual austenite. No matter the structural non-homogeneity after the eighth deformational transition, equalization of the values of the microhardness is observed (fig.3,4).

                       
                              Fig.3. Microhardness in width in hardened                 Fig.4. Influence of stage of plastic deformation
                                            and annealing specimens                         on microhardness hardened and annealing specimens

In the microvolumes, where the energy-time parameters of the process of plasma-arc impact are optimum according temperature and respectively the solubility of the carbides, is observed thin acicular and needleshaped martensite with pin's length of just few micrometers (fig.5). The circumscription of their growth is favored by the non-dissolved dispersed carbides. In the adjacent areas, where the temperature is higher are observed thin light layers on the substructural borders (fig.5), where probably takes place an initial stage of melting, which is fixated during the next crystallization.

There are observed interesting structures with clearly marked deformational texture in the relatively more plastic zones of the non-hardened samples on the periphery of the plasma-arc impact. Parallel deformational lines are being formed, with distance between them of 1-2 mm, as well as arranged situation on the direction of the lines of the dispersed carbides with dimensions of 0,5 - 2 mm (fig.5).

                                         
                Fig.5. Scanning electron micrograph of zones of the combined plasma-arc treatment and surface plastic deformation


4. Conclusion

The applying of surface deformational impact in the zones of plasma-arc hardening increases the hardness of the surface layers with
20 - 30%. The most significant is the strengthening of the structures with prevailing austenite phase. After eight deformational transitions occurs equalization of the microhardness of the two typical structures - the austenite-carbide and martensite-carbide.


References

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* Paper of the 4th International Conference ArtCast 2008: Casting, from Rigor of Technique to Art, May 2008
   University Dunarea de Jos of Galati - Faculty of Metallurgy and Materials Science, Galati, Romania

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