Elsevier

Surface and Coatings Technology

Volume 296, 25 June 2016, Pages 117-123
Surface and Coatings Technology

Influence of laser processing of the low alloy medium carbon structural steel on the development of the fatigue crack

https://doi.org/10.1016/j.surfcoat.2016.04.032Get rights and content

Highlights

  • Round Compact Tensile specimen was melted on multiple paths using a CO2 laser beam.

  • Laser treated sample withstand 7.2 mln and no treated sample only 0.27 mln cycles.

  • Laser treatment induced residual compressive stress in native material.

  • Residual compressive stress was 1.165 GPa in the vicinity of the crack tip.

  • Sulphides of manganese significantly reduce the stress before the tip of the crack.

Abstract

The paper contains the results of the structural analysis, hardness tests and fatigue tests conducted for the medium carbon structural steel with low content of Cr and Ni after its processing with CO2 laser beam. Pre-cracks were made in the round compact tension (RCT) specimen used for fatigue test. Next, four paths, parallel to each other, were melted on both sides of the samples using a laser beam. The paths were perpendicular to the direction of the axis of the cut notch. The first melted path ran at a distance of about 2.5 mm from the pre crack tip. Fatigue test results were compared with the sample which was not subjected to laser treatment. The fatigue tests showed that the sample with no laser treatment fractured after 270,000 cycles and the laser treated sample was able to withstand 7.2 million cycles for the same load and during this time the crack length increased only 0.4 mm. Hardness tests to estimate the residual internal stresses in the melted zone, in the heat affected zone (HAZ) and in the native material were carried out using nanoindenter. It was shown that compressive residual stress, in native material, close to HAZ in half-length of the melted path, just before the front of the crack, was 1165 MPa. These residual stresses contribute to the stopping of the development of the fatigue crack. It was also shown that the longitudinal manganese sulfide inclusions reduce crack development rate probably by blunting the crack blade.

Introduction

Lasers have been used in many fields since their invention. Lasers have also found applications in surface engineering. Cases of various materials can be modified by using a laser beam to give them other chemical, physical and mechanical properties. Laser treatment can improve, for example, the tribological properties [1], [2], [3], [4], [5], corrosion resistance of the surface layers [6], [7], [8], [9], [10] or significantly increase the surface hardness [11], [12], [13], [14]. Using a laser beam, thermal stresses are generated in the surface layer during its heating [15]. For some materials, heating may also cause phase changes in the solid state which can further generate structural stress. Then the residual stresses are the sum of thermal and structural stresses. Residual stresses can significantly affect the fatigue resistance of the processed material. If residual stresses are tensile, the fatigue strength decreases significantly. In the case of compressive stress, fatigue strength increases significantly since then it reduces the tensile stresses before the face of the fatigue crack. For example Wei and Ling [16] presented a three dimensional model to predict the development, magnitude and distribution of residual stress field induced by laser shock processing (LSP). They reached the conclusion that the overlap between two laser surface locations improves the magnitude and the affected depth of the residual stress. Morales et al also reported [17] that from the practical point of view, the LSP technology allows the effective induction of residual stresses fields in metallic materials. Cvetkovski et al showed in [18] that the development of residual stresses in medium carbon steels during their laser treatment depends on heating rate, temperature and mainly two material properties, thermal expansion/contraction and yield strength. In turn, Zhang et al studied the effect of two-sided laser heating of the aluminum 7050-T6 alloy on its fatigue resistance [19]. According to them, the laser treatment is very beneficial because crack initiation is delayed greatly, which plays a leading role in prolonging the fatigue life of specimen. Additionally crack propagation rate slows down which is attributed to superficial compressive residual stress induced by laser. Altus and Konstantino, in their work [20], enhanced fatigue resistance of Titanium 6Al–4 V alloy using 1.8 kW continuous wave (CW) — CO2 Laser. They attempted to find the optimal conditions of laser treatment which will improve the material resistance to fatigue failure, and explore the mechanisms involved. They identified two basic mechanisms. One is related to healing mechanism (HM), and the other is connected to microstructure mechanism (MM). Healing was found to be effective for surface temperatures above 400 °C. They reached a conclusion that the changes of microstructure adversely affect fatigue resistance except for temperatures lower than 600 °C and specific laser conditions. They also found a positive correlation between hardness and fatigue life. Bień also reported [21] the beneficial effects of selective laser re-melting of the steel on fatigue crack propagation. According to her research, the modification of the steel microstructure due to the laser beam re-melting increases fatigue strength of the processed materials.

This paper presents the possibility of stopping a fatigue crack by melting the material before its front, using a laser beam. Matching parameters of the laser beam and the appropriate arrangement of the re-melted paths, large compressive stresses able to stop the fatigue crack can occur before the face of the crack.

The aim of this paper is to check whether using laser beam is capable of generating high enough compressive residual stresses in the areas before the face of the crack (near the heat affected zone but still outside of the zone) able to stop the fatigue cracks in the treated steel.

Section snippets

Investigated material

Low-alloy steel with a content of 0.30% C was used for research. Chemical composition of testing steel is presented in Table 1. This steel is designed for quenching and tempering and it is designated for the most loaded parts of machines and engines, which require a very good plastic properties. This grade of steel is used, inter alia, in the aircraft industry. Round compact tensile (RCT) specimen for fatigue testing was made from steel after softening. Next six samples were hardened by

Results and discussion

Fig. 3 shows the changes of crack length versus the number of cycles. Both graphs are presented separately and in different scales in order to better illustrate the change in the rate of crack propagation. The sample with no laser treatment fractured after 270,000 of cycles at the crack length of 31.35 mm. The laser treated sample was able to withstand 7.2 million cycles at the same load and during this time the crack length increased only 0.4 mm. Because after the 7.2 million cycles the laser

Conclusion

Round Compact Tensile specimen was melted on multiple paths using a CO2 laser beam and then subjected to fatigue test. The paths were perpendicular to the direction of the axis of the cut notch. Fatigue test results were compared with the sample which was not subjected to laser treatment. In order to determine the level of the residual stress in melted paths, in HAZ and in the native material caused by a laser treatment, hardness tests were made by using nanoindenter. Also, structural

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