Microstructure and properties of 316L cladding layer on Hydraulic support cylinder

Scanning speed refers to the motion of the laser beam on the surface of the substrate. The transformation of heat affected zone and microstructure of the matrix were observed by increasing the motion speed and martensite content. The effect of scanning speed on the microstructure and properties of the coating was studied. In addition, the electrochemical properties and hardness of the workpiece are also improved.


Introduction
27SiMn steel is widely used to support key components of equipment.The hydraulic cylinder operates under high-pressure emulsion conditions, which makes it highly susceptible to corrosion, wear and fatigue damage due to the presence of impurities in the emulsion [1] .
After a long time of service, corrosion and wear are common failure problems of hydraulic cylinders.Therefore, strengthening, protecting and repairing the inner wall of the cylinder has attracted wide attention [2] .By applying a laser cladding layer to the hydraulic cylinder, the substrate undergoes localized heat treatment at different temperatures and depths, resulting in significant changes in its mechanical properties [3][4] .The best scanning speed ensures that there are no cracks and pores in the cladding layer, and it has a good metallurgical bond [5][6] .In conclusion, taking into consideration the cylinder's service environment, for the laser cladding strengthening technology of the hydraulic cylinder surface, the coating preparation process is very important [7] .
In this study, the coatings were prepared on the same substrate with different scanning speeds.The effect of preparation technology on coating properties was further studied.

Materials and experiments
27SiMn is used as the base material, specific composition is shown in Table 1.316L austenitic stainlesssteel powder (later called 316L powder) from Xi'an BiSheng Laser Technology Co., Ltd is taken as the cladding powder.The specific composition is shown in Table 2.The particle size ranges from 50 to 180 μm.The spherical powder has the advantages of good fluidity and good process formability.In the preparation process, the process parameters are adjusted to ensure that there are no inclusions, voids and cracks in the cladding layer.  1 shows the results of XRD analysis of three laser-cladded coatings that were primarily composed of austenite with no significant variation in phase composition observed with changing scanning speed.However, as the scanning speed increased, there was a transition in the grain orientation of the austenite phase.Specifically, there was an increase in the relative intensity of the diffraction peak at around 2θ=44°.Additionally, with higher scanning speeds, the peak width at 2θ = 51° in the laser-cladded coatings increased.The SEM micrograph of the coating is shown in Figure 2. By carefully observing the local magnification of different areas of the coating, the top area of the coating can be observed.The rest is mainly composed of columnar crystals and equiaxed crystals [8] .In the initial stage of molten pool solidification, the supercooling zone of liquid phase metal is not obvious, resulting in grain growth on the plane (Figure 2 a1, b1, c1).When solute atoms gather in front of the solid-liquid boundary, a narrow supercooling zone exists, leading to the evolution of cell particles (Figure 2 a2, b2, c2).With further increase in undercooling, the cellular grains transition into columnar dendrites (Figure 2 a3, b3, c3).In the significant subcooling zone on the surface of the coating, free-growing equiaxed dendrites are formed in the subcooling solution, which hinders the unidirectional extension of the columnar dendrites.This can be attributed to the reduction of heat and the increase of temperature difference during laser cladding, which leads to grain refinement.Combined with Table 3, it can be seen that the inner region of the coating is composed of Fe, Cr and Ni elements while the intermediate region is rich in Cr and Mo elements (points 1, 3 and 6).The generated Fe and Ni in the interdendrimer region are low (point 2, point 4, point 5).In addition, the Mo content in this area is reduced.As a result, the temperature difference is reduced, the transport capacity of Mo element in the molten pool is weakened and the component segregation in the dendrite region is reduced.3 illustrates the distribution of cross-section hardness in cladding samples at various laser scanning speeds.The hardness of the heat-affected zone demonstrates a significant increase compared to the cladding layer and matrix, averaging over 600 HV 0.5 .This also shows that the length of the heat-affected area depicted in the figure accurately represents its actual size.Furthermore, with the acceleration of laser travel speed, the heat-affected zone decreased significantly, which was about 2.0 mm, 1.0 mm and 0.6 mm in sequence.

Corrosion performance.
In Figure 4 (a), each of the three polarization curves shows three distinct regions: activation, passivation and hyper passivation.This film effectively prevents the sample from reacting with the corrosive solution, resulting in an almost constant corrosion current density as the potential increases [9] .However, when the passivation film breaks, pitting occurs on the sample surface, leading to a substantial increase in current density.This phenomenon indicates that the sample has been corroded and a violent electrochemical reaction has taken place.Figure 4 (b) shows the impedance spectrum.With the change of laser speed, the reactance half-arc radius of the sample also increases, indicating that the corrosion resistance is improved.The Bode plot in Figure 4 (c) shows that all three samples present a single crest.The high phase angle value proves that a stable passivation film is formed on its surface.Table 4 shows the corresponding polarization curve and impedance spectrum fitting results.The results of impedance spectrum fitting parameters show that the Rs (solution resistance) falls within the range of 5-8 Ωꞏcm 2 , indicating minimal deviation in solution resistance and ensuring the stability of the electrochemical testing system for the three types of austenitic stainless steel cladding layers.Under dry friction conditions, the average friction coefficient is 0.520, 0.417 and 0.365 respectively.In the working state, the average friction coefficient of the coating does not change significantly which is between 0.1 and 0.2.The coating loss rate of 3 mm/s sample is 9.61×10 -4 mm 3 ꞏN -1 ꞏm -1 .The loss rates of 5 mm/s and 7 mm/s coatings were 50.26% and 57.02% respectively lower than those of 3 mm/s coatings.When the travel speed is 7mm/s, the coating wear rate is as low as 1.30× 10 -4 mm 3 ꞏN -1 ꞏm -1 , which is 60.60% lower than that at 3mm/s.As shown in Figure 6, under dry friction conditions, the width of wear marks gradually decreases.When the travel speed is 6 mm/s, there are fewer spalling pits on the ground surface.There are tears and micro-grooves on the surface of the coating.Small particles are attached to it.In the medium, the width of the wear marks also decreases gradually.There are many micro-grooves on the surface of the coating.The tearing edges are small, so the mechanism is judged to be abrasive wear.Combined with the EDS results in Figure 7, under dry friction conditions, the oxygen content on the surface of wear marks increases, indicating that the surface of wear marks oxidizes.In the medium, the oxygen content of the wear surface has no obvious change, which indicates that the oxidation degree of the coating surface decreases.

Conclusion
The microstructure of the laser-clad layer predominantly consists of austenite.With the increase of laser travel speed, the grain size of the coating decreases.The composite structure of martensite, bainite, ferrite and pearlite are presented in the HAZ.The HAZ was shallower.The proportion of martensitic tissue increased.After subjecting the cladding layer to electrochemical corrosion in a 3.5% NaCl solution, uniformly distributed pitting pits with a honeycomb-like microscopic morphology are observed.Interdimeric corrosion serves as the primary mechanism of corrosion for the austenitic stainless steel cladding layer.

Figure 1 .
Figure 1.XRD patterns of cladding layers of austenitic stainless steel under different scanning speed.

Figure 3 .
Figure 3. Microhardness distribution of cladding samples cross-section under different scanning speeds.

Table 4 . 3
Electrochemical fitting parameters of cladding layers at three scanning speeds.Friction and wear.The wear rate and friction coefficient of the two media are shown in Figure5.

Figure 5 .
Figure 5. Wear properties of austenitic stainless steel cladding layers in different media with different scanning speeds: (a) Average coefficient of friction; (b) Wear rate chart.

Figure 6 .
Figure 6.Wear marks morphology of austenitic stainless steel cladding layers in different media.

Figure 7 .
Figure 7.Chemical composition in the middle of the abraded surface of the three cladding layers.(a) Dry friction; (b) Emulsion lubrication.

Table 3 .
Chemical composition of micro-area of austenitic stainless steel cladding layers under different scanning speeds (wt.%).
3.2 Influence of scanning speed on performance 3.2.1 Hardness.Figure