effect of shot peening on fatigue properties of steel...

G. Rosenberg: Effect of shot peening on fatigue properties of steel in different structural states

Materials engineering - Materiálové inžinierstvo 18(2011) 68-72

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EFFECT OF SHOT PEENING ON FATIGUE

PROPERTIES OF STEEL IN DIFFERENT

STRUCTURAL STATES Gejza Rosenberg1,*

1Institute of materials Research, SAS Košice, Watsonova 47, 043 53 Košice *Corresponding author: Tel.: +421 55 7922443, Fax.: +421 55 792408, e-mail: [email protected]

Resume

The aim of this paper is to evaluate the effect of shot peening on fatigue properties of dual-phase steels DP 600 in received condition, as well as in condition after different regimes of heat treatment. In the work, there are presented results of tests on smooth samples as well as on samples with stress concentrator (a drilling hole, size d = 4mm). The expectations, that shot peening of flat steel samples improves resistance to fatigue failure, were approved by all tested structural states. The tests on samples with the drilled hole showed, that effect of shot peening on fatigue properties may be negative, if the hole was drilled after shot peening of surface of samples.

Available online: http://fstroj.uniza.sk/PDF/2011/12-2011.pdf

Article info

Article history:

Received 12 July 2011 Accepted 28 July 2011 Online 29 July 2011

Keywords:

Dual phase steel Fatigue strength Heat treatment Microstructure Shot peening

1. Introduction

Dual phase steels (DP steels) are the most used steels, from the group of Advanced High Strength Steel, in the automotive industry. It is expected, that percentage of DP steels used in automobile body will be continuously increasing, because these steels are relatively cheaper and have higher ductility and better processing properties in comparison to conventional steels with the same strength [1].

One of the main disadvantages of DP steels is their relatively low stability of mechanical properties at increased temperatures [2], as an area of local softening occurs in by heat affected zone of welded joint of these steels [3-5]. The authors of the work [6] showed, that it is possible to eliminate the area of local softening by shot peening of welded steel sheet. In Fig.1, there is a profile of hardness values in perpendicular direction to the welding path before and after shot peening of DP steel sheet that were measured in the work [6]. Fig.1 shows that besides significant hardening of the location

of previous softening, shot peening caused hardening of weld metal, as well as base material. Taking into consideration results from Fig.1 and well-known dependence between hardness and fatigue strength we expect that shot peening may cause even higher fatigue resistance of welded parts of DP steels, than of sheets in received condition.

Fig. 1 Surface hardness profile in perpendicular

direction to the welding path [6].

This expectation is supported by results [7], where it was discovered, that DP 600 steel



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in received condition has in comparison with HSLA steel slightly lower fatigue resistance, while in pre-deformed condition the resistance is significantly higher.

Shot peening causes hardening of surface layers, changes in surface microgeometry, formation and redistribution of already existing residual stress, as well as changes in physical, mechanical and technological properties of steel sheets [8, 9]. Some contradictory results of shot peening on fatigue properties of materials usually refer to concurrent positive effect of deformation strengthening and presence of compressive stress and negative effect of irregularity of surface and disadvantageous distribution of residual stress in surface layers [10, 11].

The results of analysis of many works (cited in [12]) show: a) the effect of shot peening is mostly stronger in area of lower amplitude of loading; b) the lower is the fatigue ratio of tested material before shot peening, the stronger is positive effect of shot peening on fatigue behaviour of steels; c) in contrast to increase by 10-20% of fatigue limit of flat steel samples, it is possible to obtain increase by more than 100% at samples with stress concentrator or it is possible to eliminate the effect of the concentrator absolutely.

The measurement of positive effect of shot peening on fatigue behaviour of steels may be influenced by redistribution of residual stress [13]. In the work [14], results of several authors show that if the hole is drilled in flat samples after shot peening, fatigue resistance of these samples is at the same level as of samples without shot peening. Similar experiments with Al alloys show that fatigue resistance of such samples may be even lower than of samples without shot peening.

The aim of this work is to evaluate the effect of shot peening on fatigue properties of DP 600 steel in dependence on its microstructural condition. Another aim is to verify above-mentioned results of work [15].

2. Experimental methods and materials

Dual phase steel DP 600, commercially produced by U.S. Steel Košice, was used in the experiments. Chemical content and mechanical properties of the steel in received condition are listed in Table 1 and Table 2.

Table 1

Chemical composition (in wt. %) of steels

Steel C Mn Si P S Cr

DP 0.060 1.13 0.02 0.046 0,04 0,56

Table 2

Mechanical properties of steels

Steel

Rp0.2

MPa

Rm

MPa

Rp0.2/Rm

MPa

Ag

%

A5

%

Z

%

DP 405 625 0.65 14.4 28.5 54.4

The effect of shot peening was examined on DP 600 steel samples in received condition and heat treated samples, which were heated at temperature of 670 - 860 °C, holding time 10 min., followed by water or air cooling.

Heat treated samples with dimensions 2,5x20x90 mm were after HT worked from steel sheet, initial thickness 3,6±0,5 mm. The surface of samples was shot peened on pneumatic device in free space. Demetalized steel slag was used as shot peening device. For the evaluation of effect of stress concentrator on fatigue behaviour of DP 600 steel, there were also used the samples with drilled hole. Because the main aim of these experiments was verification of results of the work [15], the hole was drilled before (1st set) and after (2nd set) surface shot peening. After drilling of the hole, d = 4mm, both sets of samples were exposed to fatigue tests. The samples were exposed to fatigue loading, frequency f = 25 Hz and stress ratio R = σmin/σmax = -1.

3. Results and discussion

Effect of particular heat treatment (HT) regimes on change of hardness of DP 600 steel is shown in Fig. 2



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Fig. 2 The effect of annealing temperature on change

in hardness (HV30) of steel DP 600

The type of regime HT significantly influenced the size of irregularity of shot peened surfaces, as shown in Fig. 3. The values Rz were measured metallographically (Rz - maximum height of profile) and in dependence on heat treatment of samples were in range Rz = 76,8 µm (860 °C/10 min→ air) to Rz = 43,7 µm (860 °C/10 min→ water).

Fig. 3 Surfaces of shot peened samples:

a) 740 °C/10 min→ air; b) 860 °C/10 min→ H2O

Observed surfaces were significantly morphologically different, shown in Fig. 4, the surfaces of shot peened samples intercritically quenched at temperature tIQ = 860 °C showed many facettes of cleavage failure, as a result of brittle failure of martensite particles.

The results of the experiments aimed at evaluation of effect of shot peening on change of fatigue resistance of steel DP 600 are owing to better visualization shown only in form of Wohler curves in Fig. 5.

Fig. 4 Morphology of shot peened surface of sample

in SEM (86 °C/10min → H2O)

Fig.5 Influence of shot peening and heat treatment on

fatigue resistance of steel DP600

Shot peening caused increase of fatigue resistance in all investigated structural conditions. Fatigue resistance of samples in received state and in state after heat treatment by regime 740 °C/10 min→H2O was the same before and after shot peening and after shot peening both sets of samples reached the results of samples quenched from temperature tIQ = 860 °C (curve 4 in Fig. 5).

Similar effects of shot peening on change of fatigue properties was observed at samples treated by regime 750 °C/10 min→ air. For all three above-mentioned conditions we can say, that in the area of samples loading that causes their failure in range Nf = 5.104 - 106 cycles, there was more-or-less the same shift of Wohler curves to higher level of loading as a result of shot peening (at same value Nf roughly 10 – 15 % higher values σa).



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In contradiction with literary knowledge, the effect of shot peening on increase of fatigue resistance was higher with samples with lower strength (compare the curves 2 – 5 and 4 - 6 in Fig. 5). However, in contrast to other works, where the shot peening device of spherical shape had been used for this kind of experiments, in this work sharp-edged shot peening device was used (steel slag). From that reason, the above-mentioned discrepancy between results of these work and data from literature can relate to relatively higher level of irregularities in surface of samples with higher hardness. For the verification of these assumption, there were made other experiments on samples quenched from temperature tIQ = 850 °C. It was shown, that after metallographical grinding of surface of these samples, fatigue resistance was increased to the level of HSLA steels, hardness Rm > 900 MPa (curve 7 in Fig. 5).

The results of experiments aimed at evaluation of effect of drilled hole on fatigue properties of samples in received condition, as well as after heat treatment are listed in Fig. 6. As usual for fatigue tests, all experiments confirmed, that regimes heat treatment, that increased hardness of DP steel, also increased their resistance to cyclic loading.

The highest and lowest resistance showed samples annealed at temperature 860°C, which were after 10 min cooled in water, or air (Fig. 6). The results measured for all heat treated samples cooled in air showed, that in contrast to samples in received condition, decrease in fatigue strength (loading amplitude corresponding failure of samples in range Nf = 5.104 - 106 cycles ) was adequate to change in hardness caused by particular annealing temperatures. It was found, that this was not valid for samples that were intercritically quenched (samples cooled in water from temperatures between AC1 - AC3) and failure of these samples (Nf ) occurred at lower amplitude of loading, as it would be expected from the ratio of hardness of intercritically quenched

samples to hardness of samples in received condition.

Fig.6 Fatigue resistance of samples with drilled hole

(full symbols – holes drilling after shot peening)

Especially markedly it turned out on samples quenched at temperature tIA = 820 °C which had higher hardness by 30% in comparison to received condition, but fatigue hardness, or shift of Wohler curves to higher amplitudes of loading was only about 5% (Fig.1 and Fig.2). Fig.6 shows, that in case of samples quenched from temperature tIQ = 860 °C, disproportion between increase of hardness and fatigue resistance is increasing as the amplitude of loading is decreasing. These results show, that hardening of steels by increasing of volume fraction of martensite in dual phase steels do not have to cause adequate increase of fatigue resistance.

Measured data on samples in received condition, as well as all Heat treated samples cooled in air show that fatigue resistance of samples with drilled hole do not depend on whether the surface was or was not shot peened. In contrast, intercritically hardened samples confirmed knowledge acquired on Al alloys in work [15]. However, negative effect of shot peening of sample surface before hole drilling on fatigue properties was remarkable only at samples hardened from temperature tIQ = 860 °C.

4. Conclusion

Experimental results and presented discussion in the work lead to this conclusion:



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• effect of shot peening of steel samples, in received condition (steel DP600) as well as in heat treated condition was always positive in term of resistance to fatigue loading

• measure of effect of shot peening on increase of fatigue resistance of steels is dependent on regime of heat processing, as well as size of surface irregularities initiated by process of shot peening

• intercritically quenched samples, with hole drilled after grinding and shot peening show slightly lower fatigue resistance than samples with only grinded surface.

Acknowledgements

This research was supported by the grant No.

2/0195/09 of the Grant Agency of Ministry of

Education and Slovak Academy of Sciences

(VEGA).

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