EXPERIMENTAL ANALYSIS OF SHOT PEENING INDUCED RESIDUAL STRESSES IN STEEL SAMPLES

P. Sedlák^a,1, N. Ganev^a,2, L. Berka^b,3

^a Faculty of Nuclear Sciences and Physical Engineering, CTU in Prague

^b Faculty of Civil Engineering, CTU in Prague

¹ sedlak@troja.fjfi.cvut.cz, ²ganev@troja.fjfi.cvut.cz, ³berka@fsv.cvut.cz

The aim of this contribution is to present a comparative analysis of macroscopic residual stress distribution in the surface layers of shot-peened steel samples by means of X-ray diffraction and semi-destructive ring cutting technique.

Shot peening process consists of the controlled bombardment of the metal surface by spherical shot including steel shot, steel and stainless steel pieces of wire, ceramic or glass beads. The shots may be driven by a high velocity stream of air or liquid or by mechanical device in which the shots are fed into a rotating wheel and thrown at the desired velocity. The treatment causes plastic flow of the surface layers, thereby inducing surface compressive stresses, change of microstructure and may cause phase transformation in the surface layers.

However, there is not a simple relationship between the parameters of shot peening and the course of residual stresses induced in the surface layers of treated materials [1,2]. Final results of shot peening depends on its conditions and on properties of treated material including microstructure and crystallographic structure. Therefore reliable methods of experimental stress analysis have to be applied for control of the resultant state of residual stresses.

X-ray diffraction is the most accurate and best developed method of quantifying the residual stresses produced by surface treatments such as shot peening, and the method is widely used in automotive and aerospace applications. X-ray diffraction is applicable to most polycrystalline materials, metallic or ceramic, and is non-destructive at the sample surface.

Classical X-ray methods are limited to relatively fine-grained materials; the most experimental errors are caused by extreme preferred orientation and near-surface stress gradients.

A common mechanical methods for stresses measuring involves the removal of stressed materials and measurement of the strain relaxation in the adjacent material [3]. These methods are widely used in industry for many years, mostly the hole drilling method with electrical resistance strain gages. However, they are not straightforwardly applicable for the residual stress determination of shot peened samples because of the steep near-surface stress gradients. The removal area has form of ring in the ring cutting technique and the strain relaxation is measured in the centre of the ring. The use of finite element calculation is always needed for the interpretation of results.

Shot peened plates were prepared from steels Cz grade 12050 (sample A), Cz grade 14220 (sample B), Cz grade 19312 (sample C) and PN 17145 (sample D).

A ω-goniometer was used to measure the diffraction line α-Fe {211}. In order to determine the stress gradient beneath the samples surface, the layers were gradually removed by electrolytic polishing.

Distributions of macroscopic residual stresses obtained are plotted in Fig.1. Surface residual stresses (MPa) obtained in two directions φ = 0º and φ = 90º perpendicular each to another are summarized in the Tab.1.

Tab.1.Surface residual stresses [MPa]obtained by sin²ψ method

Sample	Orientation j = 0°	Orientation j = 90°
A	-496 ± 42	-485 ± 33
B	-358 ± 15	-363 ± 9
C	-339 ± 12	-319 ± 18
D	-628 ± 36	-676 ± 72

Tab.2. Comparison of stresses [MPa] evaluated by ring cutting method and calculated by FEM from depth profiles

Sample	Ring cutting method	Calculated by FEM
A	-696 ± 66	-600
B	-444 ± 72	-407
C	-241 ± 54	-381
D	-708 ± 105	-674

The stereo-image technique for strain measurement was used at the ring cutting method. The maps of strains were obtained by the stereo-comparison of the images taken before and after the ring cutting process. Stress relaxation was measured in the inner part of the ring. The ring diameters were: inner radius 1.07 mm; depth 3.50 mm; width 1.70 mm. The ring was removed by electric-corrosive process in petroleum bath. The values of stresses were calculated from the maps of shifts using Hook’s law.

The results of both methods were compared using the finite element analysis. Tab. 2 shows the comparison of stresses calculated by finite element model with input from X-ray determined stress profiles with stresses evaluated by the ring cutting method.

X-ray diffraction was found as the most accurate method for quantifying the residual stresses but extremely time-consuming (one depth distribution - about 10 hours) that can be a big disadvantage in the process of quality control testing.

Ring cutting measurement is not so lengthy but, on the other side, does not give any information about depth profile. The values of surface stresses for both methods are in a good agreement despite near-surface stress gradients in samples. Finite element analysis showed us that more detailed analysis of measured shifts in the ring cutting method can give additional information about depth of stressed layer. However, more accurate system for shift measurement is needed.

Fig.1. Depth profiles of residual stresses obtained by means of sin²y method in the surface layers of shot-peened samples

ACKNOWLEDGEMENTS

The research has been supported by the project Grant Agency of the Czech Republic №106/03/1039.

[1] Kraus,I., Ganev,N.: in Industrial Application of X-Ray Diffraction. New York, Marcel Dekker 1999.

[2] Noyan I. C., Cohen J. B.: Residual Stresses, Springer - Verlag, New York 1987

[3] Berka,L., Sova,M., Růžek,M., Fischer,G. Nové možnosti mechanických metod analýzy zbytkových napětí. Sborník přednášek z 34. konference EAN Plzeň 1996.