Introduction

In practice engineers and technicians often have to deal with residual stresses occurring after different machining and heat-treatment processes [1-4]. It is interesting that design engineers and technologists have already introduced specifying of the magnitude and variation of the residual stresses in a thin surface layer of individual machine parts, which are usually the most exacting in dynamically loaded parts. It is also interesting that various work hardening processes are still predominantly applied in industry for surface hardening.
Residual compressive stresses are commonly introduced into ductile metals mechanically by localized plastic deformation within the outer surface region. Commercially, this is often accomplished by a process termed shot peening. Small, hard particles (shots) having diameters within the range of 0,1 to 1,0 mm are projected at high velocities onto the surface to be treated. The resulting deformation induces residual compressive stresses to a depth of between one quarter and half of the shot diameter [5, 6].
 
Residual Stresses after Strain Hardening of Machine Parts

Shot peening is a process of mechanical hardening of the surface layers of machine parts applied in order to ensure compressive residual stresses in the surface layer to improve: fatigue strength of a material, resistance to fretting, and stress-corrosion resistance.
In the assessment of the quality of the shot-peened layer, the highest value of the compressive stress at the surface or just below it and the gradient of compressive stress are of major importance. Suitably selected machining conditions may ensure ideal variations of residual stresses in the shot-peened or laser processed surface layer of a machine part so that the required life may be achieved [7-9].
In practice the quality of surface layers is mostly determined by various destructive testing methods applied to specimens specially prepared for a micro-structural analysis, hardness measurements, and measurements of residual stresses. In case of larger series of machine parts, measurements of a sufficiently large number of machine parts are needed to be accomplished, and the adequacy of machining to be confirmed by statistical processing of the results obtained.
Fig. 1 shows through-depth residual-stress profiles for quenched-and-tempered normalized steel 42CrMo4. The flat specimen is made of quenched-and-tempered steel and shows, therefore, an increased strength in the entire volume. The flat specimens made of normalized steel showed compression residual stresses amounting to 200 MPa whereas the same specimens in the quenched-and-tempered condition showed compression residual stresses amounting to 400 MPa. If a still higher hardness is to be obtained at the surface, i.e., in the surface layer, various hardening processes using cold deformation may be employed [5].
In hardening S110 shot was used, a nozzle diameter being 15 mm and a nozzle-to-surface distance 100 mm.
For a comparison of results two shot velocities, i.e. v1 = 25 m/s and v2 = 40 m/s, and two mass flow rates, i.e. 1 = 200 g/min and 2 = 600 g/min, were chosen.
Residual stresses were measured always in the centre of the specimens using standard X-ray diffraction methods. -radiation was used to measure lattice strain distributions at {211}-lattice planes of ferrite or martensite respectively. The area irradiated by the X-ray beam was collimated to a diameter of 1,0 mm. For the calculation of residual stress values from the measured lattice strain distributions, the elastic constants E = 210.000 MPa and Poisson’s ratio  = 0.285 were used. Depth distributions of residual stresses were determined by successive electrochemical layer removal.
Characteristic data on the residual stress variations permit very important findings, i.e., after hardening by shot peening relatively high compression stresses ranging between 600 and 900 MPa can be achieved and the process provides favourable stress gradients in the greater depths of the subsurface [5].
Fig. 2 shows residual-stress profiles for various hardening processes employed with surface layers. The investigation conducted was based on turned specimens made of alloyed, hardening-and-tempering 42CrMo4 steel. The residual stresses were measured using the incremental hole drilling method. Thus by drilling a hole the material relaxes and the strain occurred is measured by resistance rosette. From the size of the measured gradual strain generated by the relaxation during drilling, the magnitude of the residual stress in the individual depths can be calculated. Thus a description of the residual-stress profile through the case depth is obtained when the measurements are finished [8]. The turned specimens showed very low compression residual stress. At the surface it did not exceed 100 MPa. Thus a real description of the cold-hardening process can be made.
In the diagram two residual-stress profiles after laser shock hardening are distinguished. One is obtained in moderate laser shock processing (LSP) and the other in heavy laser shock processing (HLSP). A comparison of the two residual-stress profiles shows that the values obtained for residual stresses at the surface are very similar whereas those obtained for the variation through the depth of the hardened layer differ considerably. The moderate LSP provides a very high stress gradient, which is very favourable for the operation of a machine part under heavy dynamic loads since higher values for fatigue strength are provided too [8].
With heavy laser shock processing (HLSP) at the surface of the hardened layer a somewhat lower residual stress is obtained. To a depth of 0.2 mm it remains almost constant whereas it changes to a greater depth with a low stress gradient. The results obtained show that the types of strong treatments such as heavy shot peening (HSP) and roller burnishing show very similar the residual-stress profiles through the case depth. An exception is low laser shock processing (LSP) which provides an extremely high value of the residual stress at the surface, which then slopes steeply, i.e., with a high stress gradient, through the depth. The case depth thus obtained is very small and the variation of the residual stresses in the hardened layer is unfavourable, primarily because of the steep stress gradient.

Fatigue of Strain Hardened Machine Parts

In many types of service applications metal parts subjected to repetitive or cyclic stresses will fail at a much lower stress than that which the part can withstand under the application of a single static stress. These failures which occur under repeated or cyclic stressing are called fatigue failures.
One of the most effective methods of increasing fatigue performance is by imposing residual compressive stresses within a thin outer surface layer. Thus, a surface tensile stress of external origin will be partially nullified and reduced in magnitude by the residual compressive stress. The net effect is that the likelihood of crack formation and therefore fatigue failure is reduced [7-9].
Fig. 3 shows S-N curves and fatigue strength with different specimens made of the same steel, i.e., 42CrMo4, after different hardening processes in rotational loading.  It turns out that the lowest fatigue strength in heavy shot peening amounts to 630 MPa but after surface treatment by heavy laser shock processing it increases to 740 MPa, which means an almost 20 % improvement of the fatigue strength [8].
Also the cycles to failure depend strongly on the type of surface treatment applied so that in shot peening treatment they range between 105 and 106, more precisely between 200.000 and 800.000 cycles to failure. With different surface hardening processes the amplitude stress of the fatigue strength is relatively high, i.e., even exceeding 0.5 Rp0.2. This means that with the given quenched-and-tempered 42CrMo4 steel showing a tensile strength of around 1300 MPa, the fatigue strength can exceed 600 MPa. For practical applications it would be more suitable to choose the torsion-bend amplitude stress of fatigue strength with which, in exceptional cases, only 0.33 Rp0.2, i.e. one third of the yield stress obtained in  a static test of the material given, is obtained.
 
Reference

1. J. Grum: Induction hardening. In Handbook of residual stress and deformation of steel, Eds. G.F. Totten, M.A.H. Howes and T. Inoue. ASM Int., Materials Park, Ohio, 2002, 220-247.
2. J. Grum: Induction hardening, materials science and technology series, A comprehensive treatment, Series Editor: Janez Grum, Volume 1, Univ. of Ljubljana, 2003.
3. J. Grum: A review of the influence of grinding conditions on resulting residual stresses after induction surface hardening and grinding, J. Mater. Processing Technol., 114(3), 2001, 212-226.
4. J. Grum: Laser surface hardening. Eds.: Totten G. E., Funatani K., Xie L. Handbook of metallurgical process design, (Materials engineering, 24). New York; Basel: M. Dekker, 2004, 641-731.
5. W. Zinn, J. Schultz, R. Kopp, B. Scholtes: The Influence of the velocity of a peening medium on the almen intensities and residual stress states of shot peened specimens, Ed.: L. Wagner, DGM, Wiley-VCH, Weinheim, Proc. of the 8th Int. Con. on Shot Peening, ICSP-8 in Garmisch-Partenkirchen, Germany, 2002, 161-166.
6. V. Schulze, O. Vöhringer, E. Macherauch: Thermal relaxation of shot peening induced residual stresses in a quenched and tempered steel 42CrMo4, Ed. J. Champagne, 1996 Conf. Proc., The Sixth Int. Conf. on Shot Peening ICSP6, San Francisco 265-274;
7. P. Peyre, P. Merrien, H.P. Lieurade, R. Fabbro, A. Bignonnet: Optimization of the residual stresses induced by laser shock treatment and fatigue life improvement of 2 cast Aluminium alloys, Ed. D. Kirk, Coventry Univ., The fifth Int. Conf. on Shot Peening, ICSP5, Proc., Oxford, UK,301-310.
8. D. Wierzchowski, A. Ostertag, L. Wagner: Fatigue performance of the mechanically surface treated steels 42CrMo4 and 54SiCr6: shot peening vs. roller-burnishing, Ed.: L. Wagner, DGM, Wiley-VCH, Weinheim, Proc. of the 8th Int. Conf. on Shot Peening, ICSP-8 in Garmisch-Partenkirchen, Germany, 2002, 468-473.
9. A. Sollich, H. Wohlfahrt: Optimization of the fatigue strength of heat treated steels as a consequence of an optimum state of the surface and of subsurface layers after shot peening, Ed.: J. Champagne, 1996 Conf. Proc., The Sixth Int. Conf. on Shot Peening ICSP6, San Francisco 251-262.

Author: Janez Grum, Prof. Ph. D.
Dr. Janez Grum is a professor for the field of Materials Science and Technology of Materials at the Faculty of Mechanical Engineering, University of Ljubljana, Slovenia. He graduated with Ph. D. in technical cybernetics, manufacturing systems and computer technology at Faculty of Mechanical Engineering, University of Ljubljana in 1977. He is currently Head of Department for Technology of Materials, Head of the Laboratory for Heat Treatment and Laboratory for Materials Testing and President of Slovenian Society for Non-destructive Testing.
He is also editor of the Journal News of Society for Nondestructive Testing, Member of the Editorial Board, Acta Stereologica, Journal for Stereology and Quantitative Image Analysis – Int. Society for Stereology; Member of the Editorial Board, CIRP – Journal of Manufacturing Systems.
He is Member of the Editorial Board of the ASNT Journal "Research in Non-destructive Evaluation" and "Int. Journal Materials and Product Technology – IJMPT". He is founder and Editor in Chief of a new journal "Int. Journal of Microstructure and Materials Properties – IJMMP". He is editor of the seven NDT Conf. Proceedings, two ASM and Marcel Dekker book Chapters, and four books with several reprints.

For Information:
Faculty of Mechanical Engineering
University of Ljubljana
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Tel. +386.1.4771 203
Fax +386.1.4771 225
E-mail: janez.grum@fs.uni-lj.si