Microstructure and mechanical properties of laser surface treated 44MnSiVS6 microalloyed steel

doi:10.1016/j.optlastec.2020.106139

Optics & Laser Technology

Volume 127, July 2020, 106139

https://doi.org/10.1016/j.optlastec.2020.106139 Get rights and content

Highlights

•
Temperature cycles that exceed A3 temperature are likely leading into ferrite structures.
•
The dwell time during the austenitic phase is the main influencing factor for the homogeneity in the heat-affected zone.
•
Existence of the ferrite grains reduce the hardness values of the heat-affected zone.

Abstract

Fatigue property improvement for automotive components such as crankshafts can be achieved through material selection and tailored surface design. Microalloyed steels are of high interest for automotive applications due to their balanced properties, excellent hardenability and good machinability. Lasers facilitate efficient and precise surface processing and understanding the laser-material-property interrelationships is the key to process optimisation. This work examines microstructural development during laser surface treatment of 44MnSiVS6 microalloyed steel and the resulting mechanical properties. Laser beam shaping techniques are employed to evaluate the impact of beam shaping on the process. It revealed that ferrite structures remain in the treated area surrounded by martensite due to insufficient heating and dwell time of carbon diffusion.

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)

P. Singh et al.
Fracture toughness behavior of 38MnSiVS5 microalloyed steel after isothermal transformation and thermomechanical processing
Mater. Today:. Proc.
(2017)
J. Yi et al.
Distortion and residual stress measurements of induction hardened AISI 4340 discs
Mater. Chem. Phys.
(2013)
D. Wellburn et al.
Variable beam intensity profile shaping for layer uniformity control in laser hardening applications
Int. J. Heat Mass Transf.
(2014)
S.B. Singh
Mechanisms of bainite transformation in steels
Y.Y. Meshkov et al.
The effect of heating rate on reverse transformations in steels and Fe-Ni-based alloys
A. Taube 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)
A.J. Deardo et al.
On strength of microalloyed steels: an interpretive review
Mater. Sci. Technol.
(2013)
T.N. Baker
Microalloyed steels
Ironmak. Ironmaking & Steelmaking
(2016)
J.C. Villalobos et al.
Microalloyed steels through history until 2018: review of chemical composition, processing and hydrogen service
Metals
(2018)
G. Dini 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)

S.F. Medina et al.

Ti / AI interaction and austenite grain control in crankshaft manufacturing with 38MnSiVS5 steel

Steels Automot. Apl.

(2006)

I. Equbal et al.

A full factorial design-based desirability function approach for optimization of hot forged vanadium micro-alloyed steel

Metallogr. Microstruct. Anal.

(2018)

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 Technology
The surface strength and pore content in electron beam melted (EBM) Ti-6Al-4V alloy play a key role in its tribological performance and fatigue life. In this study, a novel surface electropulsing treatment was applied to generate local phase transformation strengthening on the surface of EBM Ti-6Al-4V while simultaneously repairing the pores in the hardened zone. The surface microhardness was enhanced by 36 % from ∼260 HV_0.2 to ∼355 HV_0.2 when electropulsing treated for 120 ms, and keep the hardness with the prolongation of time to 240 ms. The thickness of the hardened surface increased from 1.3 mm to 1.7 mm with the increase in processing time. In addition, the hardened zone showed much fewer pores than the matrix. Mathematical simulation indicated that the high cooling rate in the treated zone was sufficient to result in a martensite transformation, and the compressive stress induced by thermal stress and electrode loading could cause deformation and furtherly improve the strength. More importantly, the combination of the large-scale compressive stress with the current crowding effect induced compression in the hardened zone is a benefit for repairing the pore. Predictably, this novel electropulsing treatment can be extended to other AM α + β Ti alloy systems and AM martensite steel systems for strengthening and pore repair.
Influence of secondary-pass laser treatment on retained ferrite and martensite in 44MnSiVS6 microalloyed steel
2022, Materials Today Communications
Citation 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].
Overlapping regions of laser surface treatment are necessary features when processing large surface areas or cylindrical specimens. However, complex microstructural changes that appear in the regions with multiple heat treatment can affect their mechanical properties. Therefore, this study focuses on examining thermal cycle characteristics and resulting microstructures, particularly martensite and retained ferrite structures, to better understand the correlation between experienced thermal cycles and resulting microstructures. Laser surface hardening experiments on 44MnSiVS6 microalloyed steels together with thermal diffusion simulations were conducted to relate microstructures after the secondary pass of the laser treatment to the local thermal cycles experienced during the process. The amount of retained ferrite was calculated and compared to the respective thermal cycle characteristics. Regions which experienced thermal cycles below Ac₃ temperature showed microstructures similar to those after tempering. The sizes of retained ferrite structures were found to decrease as the total holding time increases regardless of how the holding time is distributed in multiple laser treatments. However, the size of retained ferrite structures were constant in the region where tempering effect occurred. This shows that the amount of retained ferrite can be tailored by modifying the experienced total holding time and a reduction of retained ferrite structure happens only if the secondary thermal cycle is above Ac₃ temperature.
A pragmatic approach for assessment of laser-induced compressive residual stress profiles
2021, Journal of Manufacturing Processes
Laser hardening is a very efficient technique for local surface treatment, however, in case of complex and large components robust processing is highly challenging due to limitations in terms of the absolute size of the overall heat-affected zone. As is shown in the present work, an increased in-depth effect can be achieved by tailoring the laser parameters without melting the surface layer. Optimization of process parameters leads to an elaborate test design demanding numerous verification measurements to determine essential material properties. In this context, the evaluation of compressive residual stress values in the surface layer is very important, e.g. in case of fatigue loaded components. However, residual stress profile measurements obtained by X-ray diffraction are very time-consuming and, thus, can significantly impair the laser parameter development cycle. For this reason, the present study introduces a novel pragmatic approach allowing for qualitative evaluation of laser-induced compressive residual stress states, in particular for multiple laser pass processes based on a Gaussian-like intensity profile. Based on straightforward analytical evaluation, several characteristic features of the affected surface layer, e.g. the position of the residual stress transition zone, can be correlated to a change of the local energy input. A novel parameter referred to as modified area energy is established in present work for this purpose. This novel energy approach provides for an essential contribution to the field of laser hardening to considerably shorten the experimental effort within the laser parameter search.
Characterizing corrosion properties of carbon steel affected by high-power laser cleaning
2021, Construction and Building Materials
A high-power CW laser equipment was used to irradiate bare carbon steel plates to evaluate the effects of laser surface cleaning on the corrosion properties of the base metal. Different laser processing parameters were adopted, representing lower to higher laser ablation effects. The surface oxide content was analyzed using XRD and SEM-EDX, the microstructure was observed under optical microscopy, and the corrosion properties of the specimens were evaluated using electrochemical tests. The results demonstrated that the CW laser-treated steel plates possessed better corrosion resistance than the unirradiated steel plates. Higher laser power density and interaction time result in denser iron oxide formed on the steel surface, which is the main reason for the improved corrosion resistance. Layers with modified microstructures are present in all laser-treated steel plates. The laser ablation effects and gradient microstructures can be well described by the theoretical peak temperature distribution. The laser surface cleaning technique for steel structures is thereby confirmed to have a positive impact on the corrosion properties of the base metal.
Short thermal cycle treatment with laser of vanadium microalloyed steels
2020, Journal of Manufacturing Processes
Citation 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.
Improvement of crankshaft fatigue properties can be approached by altering its mechanical properties in the surface, such as laser surface treatment. Laser beam treatment offers efficient and precise surface hardening processing with possibility of reducing the production cost compared to the conventional hardening techniques. However, its characteristic of having short thermal cycle can be a challenge for the development of laser surface hardening techniques, such as inadequacy of literatures in phase transformation and resulting mechanical properties under rapid heating and cooling rate. Therefore, this work investigated the impact of short thermal cycles induced by the laser beam on the resulting microstructure and hardness properties in the surface of 38MnSiVS5 and 44MnSiVS6 microalloyed steels. Temperature cycles during the process were recorded and examined with the resulting microstructure along with microhardness values. 44MnSiVS6 microalloyed steel, which contains ca. double the amount of vanadium compared to 38MnSiVS5 steel, produces finer ferrite grains in the treated area for all investigated short thermal cycles. This fine-grained microstructure leads to steady hardness distributions in the treated area. The short thermal cycle was assumed to be unable to dissolve the vanadium precipitates that reside in the ferrite grains, which then initiate precipitation hardening.
Evaluation of extremely steep residual stress gradients based on a combined approach using laboratory-scale equipment
2021, Journal of Applied Crystallography

View all citing articles on Scopus

View full text

Microstructure and mechanical properties of laser surface treated 44MnSiVS6 microalloyed steel

Highlights

Abstract

Introduction

Section snippets

Methodology

Temperature measurements

Microstructures created by short thermal cycles

Conclusions

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgements

Mater. Today:. Proc.

Mater. Chem. Phys.

Int. J. Heat Mass Transf.

Int. J. Fatigue

On strength of microalloyed steels: an interpretive review

Mater. Sci. Technol.

Microalloyed steels

Ironmak. Ironmaking & Steelmaking

Microalloyed steels through history until 2018: review of chemical composition, processing and hydrogen service

Metals

The influence of reheating temperature and direct-cooling rate after forging on microstructure and mechanical properties of V-microalloyed steel 38MnSiVS5

ISIJ Int.

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.