The tool steel surface layer strengthening by shot peening.

Bilik, Jozef ; Ridzon, Martin ; Suba, Roland 等

1. INTRODUCTION

The lifetime of dynamically stressed forming tools and other parts depends in especially on surface layer. Surface layer is critical for wear and fatigue fracture. So the paper is focused on surface layers strengthening based on surface plastic deformation by the method of shot peening.

2. THE EFFECTS OF DYNAMICAL IMPACT DURING SURFACE LAYER STRENGTHENING BY DYNAMICAL SHOT PEENING

The high speed impact of bodies causes the physical mechanical processes of elastic--plastic waves and it affects the properties of metals during and after stress (Baca, 2005), (Baca, 1995). The accompanying effects of impact are thermal effect, localization of plastic deformations in surface layers, effects in structure (Baca, 2005), (Polak & Hobemagi 1988). The important demonstrations of surface layers strengthening at dynamical shot are the density and configuration of thermally stable dislocations, dislocations nets, dislocation barriers, density of slip lines, slips and twins, fragmentations of carbides or impurities joined with equiaxed grains properties homogenization levelling, very fine austenitic and martensitic structures with two or even three times higher wear resistence (Baca, 1995), the creation of beneficient pressure stresses system in surface layer, increasing of hardness and strength in surface layer, the creation of texture in strengthened surface layer, the change of surface quality.

3. CHOSEN RESULTS OF EXPERIMENTS FOCUSED ON FORGING DIE LIFETIME INCREASING BY APPLICATION OF DYNAMICAL SHOT PEENING

The experiments of surface layers strenghtening by dynamical shot peening included: a) the testing of samples cyclic stress resistance (fig.1). Samples were quenched and tempered on required hardness and so called strengthening effectivnes was measured by number of cycles to failure at impact energy 160 J, b) the measurement of hardness distribution from surface by gradual grinding of thin layers and measurement of microhardness distribution of grinded and polished cross section, c) the measurement of surface roughness and its character after shot peening, d) the measurement of samples size change after shot peening, e) the estimation of residual austenite content before and after strenghtening by xray difraction analysis, f) the estimation of character and morphology of fracture surfaces after cyclic stress resistance test. (Pfeiffer, 2005), (Kirk, 2005)

[FIGURE 1 OMITTED]

There would be mentioned only some of obtained results, due to restricted number of papers page, for Cr-Mo-Si-V steel STN 419552, surface layers strenghtening. The chemical composition of this tool steel is 0,32/0,42 % C, 0,2/0,5 % Mn, 0,8/1,2 % Si, 4,5/5,5 % Cr, 1,1/1,6 % Mo, 0,35/0,6 % V, max. 0,03 % P, max. 0,03 % S. For experiments single purpose machine with one shooting wheel with tip speed approximately 32 ms-1 was used. The steel balls with diameter 1 up to 1,5 mm and weight 85 [+ or -] 10 kg.min-1 were used for shot peening.

4. THE MEASUREMENT OF RESIDUAL AUSTENITE CONTENT BY X-RAY DIFFRACTION ANALYSIS

The residual austenite content was measured by y-ray diffraction analysis in surface layers of shot peened specimens andspecimens without treatment from STN 419552 steel. The analysis was made on PHILIPS PW 1710 x-ray diffractometer. There was established that the content of residual austenite decreased by 4 to 5 % after shot peening due to plastic deformation and residual pressure stresses in surface layers. It was transformed to martenzite.

5. THE MEASUREMENT HARDNESS DISTRIBUTION AFTER SHOT PEENING

The measurement of hardness at gradual grinding of layers with 0,05 mm thickness showed that all measured specimens with initial hardness from 48 to 50 HRC and shot peening time 4, 6, 8, 10 minutes has depth of strengthened surface layer from 0,35 to 0,4 mm. The beneficial distribution of hardness is proved also by mocrohardness measurement on specimens from STN 419552 steel with initial hardness before stzrengthening 40 up to 48 HRC. The measured values of microhardness are in table 1.

6. THE OBSERVATION OF INITIAL HARDNESS INFLUENCE ON SURFACE ROUGHNESS AFTER SHOT PEENING

The dependence of roughness after shot peening on initial hardness before shot peening for STN 419552 steel is on Fig.2. The surface roughness before shot peening was Ra = 0,46 um.,

[FIGURE 2 OMITTED]

7. THE FRACTURE SURFACES OBSERVATION AFTER STRENGTHENING EFECTIVNESS TEST

The character of fracture surfaces changed at specimens with strengthened surface layers and crack initiation and their development is suppressed. It is visible in different morphologies of fracture surface near notch and in the middle of specimen. The morphology of fracture surface of specimen from STN 419552 steel shot peened 8 minutes on the surface near notch (Fig.3a) and in the middle of specimen is shown (Fig.3b).

[FIGURE 3 OMITTED]

It is visible from Fig.3a and 3b that fracture surface of strengthened specimen have different morphologies on surface near notch and in the middle. It is connected with plastic deformation of surface layer during shot peening. The split facettes are finer in strengthened surface layer and thus the energy required for their creation and development is higher than at non-strengthened specimen.

8. CONCLUSION

The experimental measurements proved beneficial effects of surface layer strengthening of STN 419552 tool steel, used for forging die and forging die inserts production, by dynamical shot peening. The analysis of deformation size and measurements of hardness and microhardness allowed to establish the depth of plastic deformation penetration, i.e. the thickness of surface layer, which in dependence on initial hardness was from 0,3 to 0,5 mm. The observed specimens proved the minimal increase of surface layer hardness from 2 to 3 HRC at initial hardness 40 up to 50 HRC. The observation of surface roughness after shot peening of heat treated and grinded surface established that its roughness was increased in dependence of initial state of specimens. The specimens with hardness 40 up to 48 HRC, which corresponds to usual forging die hardness, has surface roughness from Ra = 3 to 8 um. But the shot peening significantly changed the roughness profile character in comparison with grinded surface. The observation of fracture surfaces on strengthened and non-strengthened specimens showed different character of fracture surfaces in their surface layer. The mechanical strengthening of surface layers is one of very effective fatigue strength increasing methods for dynamically stressed tools and parts. This is evident from international conferences focused on this problem.

9. ACKNOWLEDGEMENTS

The paper was realised with VEGA 1/3192/06, VEGA 1/0060/08 support.

10. REFERENCES

Baca, J. (2005). Volumen forming--Drop forging Bratislava, STU

Baca, J. (1995). Contribution to problematics of cavity tools production by forming (collection of papers). Habilitation thesis. Trnava, MTF STU

Polak, K & Hobemagi, A.(1988). Production of cavity tools by forming. Bratislava, Alfa

Pfeiffer, W. (2005). Characterization of peened components by x-ray diffraction. ICSP-9, Paris

Kirk, D. (2005). Evolution of shot peening experimental technigues. ICSP-9, Paris

Tab. 1 Measured values of microhardness on various distances
from surface for STN 419552 steel (shot peening time 8
minutes)

Hardness Distance of Measurement Measurement Average
 of indention No 1 No 2 hardness
specimen from Hardness Hardness HV 0,1
 before surface HV HV
 shot [mm]
peening

40 HRC 0,04 478 478 478
 0,06 478 464 471
 0,15 444 396 420
 0,24 409 422 415
 0,36 411 394 402
 0,45 398 398 398
 0,55 404 394 399

48 HRC 0,04 530 540 535
 0,08 527 511 519
 0,14 499 527 513
 0,23 508 496 502
 0,32 502 493 498
 0,44 476 511 494