Microstructure and mechanical properties of laser surface treated 44MnSiVS6 microalloyed steel
Introduction
Microalloyed steels have good machinability and hardenability properties that can reduce production costs related to heat treatments [1]. Grain refinement and precipitation hardening due to alloying elements improve the strength of microalloyed steels, which makes these steels suitable for automotive application [2]. The first automotive application of microalloyed medium carbon steel was in the manufacturing of crankshafts [3], where fatigue properties are a very important consideration. Long-term fatigue life improvements can be achieved by modifying the materials composition and improving manufacturing processes. High strength materials can sustain heavy load and thus offer longer fatigue life for automotive application, such as 38MnSiVS microalloyed steel that has been commonly used in the automotive industry [4], [5], [6], [7]. Meanwhile, 44MnSiVS6 microalloyed steel offers higher strength properties than 38MnSiVS due to its higher carbon content [8]. Therefore, the properties after heat treatment of 44MnSiVS6 microalloyed steel are of interest.
Surface hardening is one processing strategy to improve fatigue properties by inducing martensitic transformation through heat treatment. The martensitic structure increases the surface hardness and thus induces the residual stress characteristics that eventually hamper the fatigue crack propagation [9]. Common steel hardening methods, such as induction hardening and deep rolling, have several limitations [10]. For instance, the depth of hardened layers can be inhomogeneous and the processes are potentially less energy efficient than advanced hardening methods such as laser surface hardening [11], [12]. The laser beam acts as a thermal energy source in the material to induce martensitic transformation in the steels [13]. An advantage of using this hardening method is that the characteristics of the laser beam allow flexibility in controlling the heat-treated zone for processing and the gradient of thermal energy input. In this way, the manufacturing process and final products can be more energy efficient and improved. Beam shaping has been reported to deliver different spatial thermal energy inputs during laser processing [14], [15], [16], [17]. This gives a chance to create a specific thermal energy input that is the most efficient for desired processes and products.
Consequently, understanding the martensitic transformation of 44MnSiVS6 microalloyed steel during laser surface treatment and observation of the impact of laser beam shaping are important. Therefore, the present work was conducted in order to gain knowledge about laser heat treating 44MnSiVS6 microalloyed steels and the influence of laser beam shaping techniques.
Section snippets
Methodology
Fig. 1 illustrates the experimental set-up for this study. Beam shaping optics, namely a kaleidoscope and a Diffractive Optical Element (DOE), were mounted on a Yb:fibre laser system from IPG photonics with a wavelength of 1070 nm to produce single, straight, hardened tracks. The laser output power was 3000 W and the process speed was 100 mm/s for all experiments shown in Table 2. Illustrations of the optical set-up and beam shapes are shown in Fig. 2.
The specimens were 44MnSiVS6 microalloyed
Temperature measurements
Fig. 4 shows the thermal cycles of the pixel outlined in red in Fig. 1 when heating with a Gaussian beam and a rectangular top-hat beam (kaleidoscope). The emission intensity from the hardening process with the DOE was too low for recording the temperature during the process.
The thermal cycle of the rectangular top-hat beam shows different characteristics compared to the Gaussian beam. The rectangular top-hat beam process shows an abrupt increase and drop of temperature in the beginning and the
Microstructures created by short thermal cycles
According to the microstructural results in Fig. 5, Fig. 6, the laser-hardened area can be schematically divided into the three main areas highlighted in Fig. 8.
Temperatures measured on the surface were at values above the known austenization temperature, see Fig. 4. Appropriate heating and cooling rates combined with sufficient time above the A3 temperature were available in the region close to the surface. Therefore, the creation of fully martensitic structures in the ‘A’ area is a reasonable
Conclusions
Laser surface treatments of 44MnSiVS6 microalloyed steel using different laser beam shapes have been carried out. Three processes involving different laser beam distributions were characterised by different thermal cycles and power losses, affecting microstructural development and thus hardness values and the residual stress fields. The results using different spatial laser intensity distributions indicate that the laser power and power density mainly defines the hardened zone dimensions. Three
CRediT authorship contribution statement
Handika Sandra Dewi: Investigation, Writing - original draft. Andreas Fischer: Investigation, Writing - review & editing. Joerg Volpp: Writing - review & editing, Methodology, Supervision. Thomas Niendorf: Supervision. Alexander F.H. Kaplan: Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors gratefully acknowledge funding from the EC Research Fund for Coal and Steel, RFCS, for the project Stiffcrank, no. 754155 and OptoSteel, no. 709954.
References (24)
- et al.
Fracture toughness behavior of 38MnSiVS5 microalloyed steel after isothermal transformation and thermomechanical processing
Mater. Today:. Proc.
(2017) - et al.
Distortion and residual stress measurements of induction hardened AISI 4340 discs
Mater. Chem. Phys.
(2013) - et al.
Variable beam intensity profile shaping for layer uniformity control in laser hardening applications
Int. J. Heat Mass Transf.
(2014) Mechanisms of bainite transformation in steels
- et al.
The effect of heating rate on reverse transformations in steels and Fe-Ni-based alloys
- et al.
Influence of surface pre-treatments on the high-cycle fatigue behavior of Ti-6Al-4V – From anodizing to laser-assisted techniques
Int. J. Fatigue
(2016) - et al.
On strength of microalloyed steels: an interpretive review
Mater. Sci. Technol.
(2013) Microalloyed steels
Ironmak. Ironmaking & Steelmaking
(2016)- et al.
Microalloyed steels through history until 2018: review of chemical composition, processing and hydrogen service
Metals
(2018) - et al.
The influence of reheating temperature and direct-cooling rate after forging on microstructure and mechanical properties of V-microalloyed steel 38MnSiVS5
ISIJ Int.
(2006)
Ti / AI interaction and austenite grain control in crankshaft manufacturing with 38MnSiVS5 steel
Steels Automot. Apl.
A full factorial design-based desirability function approach for optimization of hot forged vanadium micro-alloyed steel
Metallogr. Microstruct. Anal.
Cited by (14)
A novel electropulsing treatment to improve the surface strength and repair the pore of additively manufactured Ti-6Al-4V alloy
2023, Surface and Coatings TechnologyInfluence of secondary-pass laser treatment on retained ferrite and martensite in 44MnSiVS6 microalloyed steel
2022, Materials Today CommunicationsCitation Excerpt :In the third step, the regions that have more than 0.05% of carbon will quench to martensite and the rest will revert to ferrite, leaving retained ferrite embedded with martensite as the final microstructure [25]. Homogenization of carbon in the austenite has a major impact on the appearance of the retained ferrite in the resulting microstructure after laser heat treatment [21,26–28]. Previous investigations on adjacent tracks of laser surface hardening found that tempering effects occurred in the overlapping region [12,14,29].
A pragmatic approach for assessment of laser-induced compressive residual stress profiles
2021, Journal of Manufacturing ProcessesCharacterizing corrosion properties of carbon steel affected by high-power laser cleaning
2021, Construction and Building MaterialsShort thermal cycle treatment with laser of vanadium microalloyed steels
2020, Journal of Manufacturing ProcessesCitation Excerpt :These regions of austenite that have more than a certain amount of carbon (0.05 %) will quench into martensite and the rest will revert into ferrite, leaving retained ferrite along with martensite in the final microstructure [42]. Later works also highlighted that homogenisation of carbon in austenite has a major impact on the appearance of retained ferrite [43–47]. Since fast thermal cycles were found to result in different microstructure and mechanical characteristics compared to longer thermal cycles processes, one may question how this knowledge applies for 38MnSiVS5 and 44MnSiVS6 microalloyed steels.
Evaluation of extremely steep residual stress gradients based on a combined approach using laboratory-scale equipment
2021, Journal of Applied Crystallography