VOL. 8 March ISSUE YEAR 2007
in Vol. 8 - March Issue - Year 2007
Improvement Of Maraging Steel Tools With Roto Peening
Table 1: The parameters of roto peening of the surface and a reference depth of influence
Fig. 1: Residual-stress variation immediately after grinding of individual specimens.
Fig. 2: Residual-stress profiles obtained after roto peening using different parameters.
Surface Integrity and Surface Engineering
In practice, engineers and technicians often have to deal with residual stresses occurring after different machining and heat-treatment processes. Design engineers and technologists have already introduced specifications for the magnitude and variation of the residual stresses in a thin surface layer of individual machine parts, which are usually the most exacting and dynamically- loaded parts.
Internal residual stresses are those existing in a material or product when no external load is acting on the material or product. The residual stresses in metal products became interesting to technicians and engineers only after the quality of manufacturing processes exceeded the magnitude of strain, i.e. product distortion due to residual stresses. The scientific discipline dealing with influences of manufacturing processes on the surface of a machine part is called surface integrity [1,2].
After cutting-off, i.e. after mechanical treatment, machine parts are subjected to various processes of heat treatment or thermo-chemical treatment. Additional heat treatment makes it possible to enhance strength properties of a machine part, which can, consequently, withstand higher external loads. The additional thermal or thermo-chemical treatment can increase surface hardness and thus improve wear resistance of the machine part concerned and also improve resistance to chemical influences and corrosion. Thus after 1980 a scientific discipline called "surface engineering" appeared. It treats processes and procedures to improve surface integrity of machine parts [3,4].
Roto Peening of Maraging Steel
Die-casting tools are subjected to strong thermo-mechanical loads. In every casting cycle the surface layer of such a tool is subjected to influences of high mechanical loads due to melt pressure, closing forces of the tool, and of high thermal loads due to increased temperatures and high thermal differences. Some common hot-working tool steels and maraging steels, showing high fatigue performance, are considered particularly suitable materials for the manufacture of tool engravings.
The group of maraging steels is based on various iron-based alloy systems. It is characteristic of these steels that hardening involves precipitated intermetallic phases of the order of magnitude of several tens up to several hundreds and more nanometers. The precipitated phases are formed due to a diffusion mechanism in a certain temperature interval with reference to a local composition, i.e. portion of individual alloying elements. Their formation partly deforms the crystal lattice of the matrix, which results in plastic strain and higher dislocation density. The formation of precipitated phases changes the mechanical properties of steel, i.e. the steel becomes stronger and harder. The best hardening effects can be obtained with a uniform distribution of coherent precipitated phases.
The processes enhancing fatigue performance of the steel concerned also include strain-hardening techniques for the surface layer such as shot peening, ultrasonic peening, roto peening, roller burnishing and deep rolling. Some of the techniques, e.g. laser shock processing, are mainly performed in laboratories whereas practical applications have been known only in recent years [5,6]. Increase in fatigue performance of materials has been the subject of numerous studies.
In order to determine the influence of roto peening on the final properties of maraging steel, several specimens were produced. They were all prepared in the same way and showed supposedly the same initial microstructure condition, mechanical properties, and residual stresses. The material used to produce the specimens was 12% Ni maraging steel with a Co-Mo-Ti system for precipitation hardening. The maraging steel chosen was in the as-delivered state, i.e. solution-annealed and homogenized by forging . The specimens were cut from a forged rod of 80 mm in diameter; the rod was cut into discs, which were then ground to their final size. On one side the specimens were then subjected to rotary peening. The processing parameters used are given in Table 1. The flap rotation speed was variable, the flap standoff distance and operator techniques were the same in each case. The processing intensity was determined by the flap rotation speed and processing time . The study included several specimens in the soft condition and in the precipitation-hardened state with subsequent hardening by rotary peening.
Fig.1 shows residual stress profiles of the maraging steel specimens after grinding and prior to processing by rotary peening. The residual stress profiles vary in the range of -40 to +180 MPa. In the surface layer the stresses of tensile character prevail, which is a result of the grinding process. The respective directions of the principal residual stresses in the surface layer are parallel and perpendicular to the direction of the grinder. This was inferred from the angle measured of the principal residual stresses to the location of a measuring rosette at the specimen surface. Residual stresses were measured with a hole-drilling method according to ASTM E837 and Vishay’s Technical Note TN-503. The grinding processing and the resulting residual stresses constituted a starting point for subsequent roto peening.
The depth of influence of cutting-off and grinding zig can be determined in various ways: the most easily-surveyable way is by measuring the gradient or magnitude of residual stresses, i.e. the variation of a magnitude with a chosen threshold of ±25 % difference with reference to the residual stresses in underlying layers. With the conditions used the depth of influence was less than 20 ?m. In a depth zig > 0.25 mm the influence of the preliminary processing is insignificant.
Residual Stresses after Roto Peening
Fig. 2 shows residual stress profiles to a depth of 1.2 mm after different roto peening processes. It can be observed that the residual stress profile has improved significantly, since a compressive zone and a high stress gradient to a depth of 400 ?m were obtained. A less-important influence of compression stresses reaches, however, also to a greater depth. The angle of the principal residual stresses can indicate the changes occurring during processing of the surface layer as well. The angles of residual stresses will change significantly – similarly as for stress profiles – after roto peening of the surface layer with reference to the stress angle in the specimen core.
Fig. 2A shows the two profiles of the principal residual stresses after roto peening with a rotation speed of 2000 min-1. The two principal stresses are relatively close together. This indicates the absence of higher shear stresses in the surface layer, which is favourable from the viewpoint of fatigue performance.
Fig. 2B shows the two residual stress profiles obtained after processing with a speed of 3000 min-1. In this case the principal stresses are higher and the difference between them has significantly increased, particularly at a lower depth, i.e. up to 300 ?m. Fatigue performance of the material concerned will be presumably higher than in the previous case although there are major undesired shear stresses in the surface layer.
The condition obtained after roto peening at n = 4000 min-1 is shown in Fig. 2C. In this case the high compressive stresses in the surface layer at a depth of 650 ?m, which is deeper than in the previous two cases, are even more conspicuous. The absolute difference between the maximum and minimum stresses is higher just below the surface whereas the relative difference is smaller than in processing at n = 3000 min-1. The absolute values of residual stresses are extremely high due to high yield stress of the chosen maraging steel, which can exceed 850 MPa already in its soft state. The yield stress will additionally increase due to surface hardening. Please note that the stresses in the diagram exceeding 70% of the yield stress in the soft state are only for information, since the stress calculation cannot accurately simulate the conditions close to the actual yield stress.
Roto peening of the chosen maraging steel was carried out in the soft state. It was followed by precipitation annealing at a temperature of 450 °C for 2 hours to obtain the final mechanical properties. As anticipated, the stress state changed as well, but in the surface layer within the depth of influence the compressive stresses up to 700 MPa persisted.
The research results show that high-strength maraging steel can be efficiently processed in the soft state by using rotary peening. Roto peening resulted in strong improvement as regards the residual stresses in the maraging steel. The stresses in the surface layer have a compressive character, which usually means a considerable increase in dynamic strength, i.e. resistance to thermo-mechanical loads.
In precipitation annealing the residual stresses due to the preceding grinding process will be, to a large extent, preserved. Consequently, the profiles of the compressive residual stresses after maraging steel hardening in the soft state are very favourable for further heat treatment. Precipitation annealing, which proceeds, with the maraging steel given, below the temperature of recrystallisation annealing, can provide increases in hardness and strength, but does not, however, annul the favourable residual-stress variation in the surface and the thin surface layer. The preserved residual stress variation after hardening by rotary peening will result in relative surface improvement in die-casting tool parts.
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3. Bell T., Bloye A., Langan J.: Surface Engineering of Light Metals, Heat Treatment and Surface Engineering: New Technology and Practical Applications, Proc. of the 6th Int. Conf. on Heat Treatment of Metals, Ed.: Krauss G., Chicago, Illinois, ASM Int., 1988, 1-7.
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5. Grum J., Zupan?i? M., Ocana J.L.: Laser Shock Processing as a Method of Decreasing Fatigue of a Die-Casting Die Made of a Maraging Steel; Ed.: Rosso M., Actis G.M., Ugues D., Tooling Materials and Their Applications from Research to Market: Proceedings of 7th International Tooling Conference : Politechnico di Torino, 2-5 May 2006, Vol. 2, 487-496.
6. Zupan?i? M.: Precipitation Hardening and Properties of a Maraging Steel, Ph.D. Dissertation, Faculty of Mechanical Engineering, Ljubljana, 2005.
7. Grum J., Zupan?i? M.: Suitability Assessment of Replacement of Conventional Hot-working Steels with Maraging Steel, Part 2, Microstructure of maraging steel after precipitation hardening treatment, Z. Met.kd., 2002, Bd. 93, Hf. 2, 171-176.
8. Bailey P.G.: Manual Peening With the Rotary Flap Process, 7th Int. Conf. on Shot Peening, Ed.: Nakonieczny A., Warsaw, Poland, 1999, 405-414.
Prof. Janez Grum (1)
Mr. M. Zupancic (1)
Mr. R. Kikelj (2)
Faculty of Mechanical Engineering,
University of Ljubljana, Slovenia (1)
Adria Airways, Ljubljana, Slovenia (2)