METHOD FOR LASER SURFACE TREATMENT OF FURNACE FURNITURE

Abstract

A method for a laser surface treatment of furnace furniture of a heating furnace, which furnace furniture is used for the support of metal products in the heating furnace, the method including the steps of: setting a laser device to generate a laser beam of a pre-defined power, guiding the laser beam over the surface of the furnace furniture with a pre-defined velocity, such that the surface of the furnace furniture is heated locally to above its melting temperature.

Claims

1. A method for a laser surface treatment of furnace furniture of a heating furnace, which furnace furniture is used for the support of metal products in the heating furnace, wherein the furnace furniture is made of a creep resistant Ni—Cr alloy, the method comprising the steps of: setting a laser device to generate a laser beam of a pre-defined power in a range sufficient to deliver a fluence in a range of 1.3×10.sup.6-1.3×10.sup.8 J/m.sup.2, guiding the laser beam over the surface of the furnace furniture with a pre-defined velocity, such that the surface of the furnace furniture is heated locally to above its melting temperature.

2. The method according to claim 1, wherein the laser device and/or the laser beam and the furnace furniture are moved with respect to each other.

3. The method according to claim 1, wherein one or more laser devices are used to generate multiple laser beams.

4. The method according to claim 1, wherein the laser beam or multiple laser beams are each guided in a single track or in multiple tracks over the surface of the furnace furniture.

5. The method according to claim 4, wherein the tracks of the laser beam or laser beams run parallel to each other.

6. The method according to claim 4, wherein adjacent tracks of the laser beam or laser beams overlap.

7. The method according to claim 4, wherein the laser device or the laser devices are controlled such that the generated laser beam or laser beams result in tracks with a width in a range of 1-6 mm.

8. The method according to claim 4, wherein the overlap of adjacent tracks of the laser beam or laser beams is in the range of 20-80%.

9. The method according to claim 1, wherein continuous or pulsed laser beam or laser beams are used.

10. The method according to claim 1, wherein the velocity of the movement of the laser beam or the laser beams and the furnace furniture with respect to each other is in a range of 5-100 mm/s.

11. The method according to claim 1, wherein the velocity of the movement of the laser beam or the laser beams and the furnace furniture with respect to each other is in the range of 5-50 mm/s.

12. The method according to claim 1, wherein the laser surface treatment is carried out under a protective atmosphere.

13. The method according to claim 10, wherein the protective atmosphere is an argon atmosphere.

14. The method according to claim 1, wherein the furnace furniture is made from Ni—Cr alloys wherein the Ni+Cr content is in a range of 50-90%.

15. The method according to claim 1, wherein the laser surface treatment is followed by a mechanical surface treatment.

16. The method according to any of claim 4, wherein the laser device or the laser devices are controlled such that the generated laser beam or laser beams result in tracks with a width in a range of 2-5 mm.

17. The method according to any of claim 4, wherein overlap of adjacent tracks of the laser beam or laser beams is between 40-80%.

18. The method according to any of claim 4, wherein overlap of adjacent tracks of the laser beam or laser beams is between 60-70%.

19. The method according to claim 1, wherein the furnace furniture is made from Ni—Cr alloys wherein the Ni+Cr content is in a range of 75-85%.

20. The method according to claim 5, wherein adjacent tracks of the laser beam or laser beams overlap.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The invention will be further explained by means of the example shown in the drawing, in which:

[0035] FIG. 1 shows a graph of the depth of re-melting at different surface speeds as a function of energy delivered to the surface,

[0036] FIG. 2 shows a BSE image of an interface between a re-melted zone (top part) and the bulk material,

[0037] FIG. 3 shows a graph microstructure sizes in the re-melted zone for different processing speeds,

[0038] FIG. 4 shows a representation of a re-melted zone with processing parameters,

[0039] FIG. 5 shows a BSE image of a transition from a thermally affected porous region to the bulk material that has not been re-melted, and

[0040] FIG. 6 shows a BSE image of a transition from a thermally affected region in a re-melted zone to a non-re-melted zone and a thermally affected porous region within the re-melted zone.

EXAMPLE AND DETAILED DESCRIPTION OF THE DRAWINGS

[0041] A particular alloy with the composition given in Table 1 has been tested extensively in static and dynamic high temperature oxidation and wear tests, and compared to the standard material/surface.

TABLE-US-00001 TABLE 1 Example composition of material tested. Element Ni Cr W Fe C Si Mn wt. % 45~50 30~35 10~20 1.0~2.0 0.35~0.45 0.25~0.35 0.05~0.15

[0042] A series of laser processing parameters has been followed utilising a 3 kW continuous wave solid-state fibre laser by IPG Photonics with a wavelength of 1.07 μm, 4 axis CNC table and delivery of a shielding gas (argon). Defocusing of the laser beam was tuned to obtain single laser track width of slightly more than 3 mm.

TABLE-US-00002 TABLE 2 Typical laser processing parameters during trials Surface Speed (mm/s) Laser Power (W) 5  300-1100 20 1000-1800 100 2000-3000

[0043] The combination of laser parameters as given in Table 2 can be used to determine the fluence or the energy delivered to the surface of the part being processed in J/m2 which provides an energy density for the process, see Table 3. The test has been carried out on a round bar wherein the round bar is rotated resulting in the given surface speed.

TABLE-US-00003 TABLE 3 processing parameters, power density and fluence Power Speed Beam radius Power density Processing Fluence [W] [mm/s] [mm] W/m{circumflex over ( )}2 time [s] (J/m{circumflex over ( )}2) 300 5 1.5 4.24E+07 0.6 2.55E+07 1100 5 1.5 1.56E+08 0.6 9.34E+07 1000 20 1.5 1.41E+08 0.15 2.12E+07 1800 20 1.5 2.55E+08 0.15 3.82E+07 2000 100 1.5 2.83E+08 0.03 8.49E+06 3000 100 1.5 4.24E+08 0.03 1.27E+07

[0044] FIG. 1 shows the influence of the changing process parameters, that is the energy delivered on the re-melted depth of the layer formed.

[0045] The series of process parameters reveal a good surface quality, without any porosity or cracking and a well-defined interface between re-melted regions and the bulk material. The exception were tracks produced at highest scanning speed 100 mm/s which all had severe cracking.

[0046] The given composition is characterised by a cast microstructure having a dendritic microstructure with eutectic solidified between the dendrites. Laser re-melting results in a considerable decrease in the size of the dendrites for all scanning speeds and laser powers.

[0047] FIG. 2 shows an energy dispersive x-ray spectroscopy (EDS) image of an interface between a re-melted zone and the bulk material. It shows that additionally to a dendritic microstructure, a columnar microstructure is found in some regions after re-melting, especially close to an interface between the re-melted layer and the original bulk material.

[0048] FIG. 3 shows a graph of the refinement of the microstructure characterised by the mean dendrite size. It shows that in general the microstructure has become finer by about an order of magnitude and this does not change much for the different processing parameters. Further it can be seen from that for each set of speeds there is a small decrease in the size of the microstructure with increased laser power.

[0049] By overlapping the tracks of individual re-melted tracks re-melted layers can be formed of any length and width. The depth of such layers is determined as the minimum amount of re-melted depth corresponding to overlapped regions. FIG. 4 reveals one such layer produced with a power P=1600 W, overlap OR=66% and velocity S=20 mm/s resulting in a re-melting depth of 0.62 mm. Experiments show that the depth can be increased by increased laser power and/or by increasing the overlap.

[0050] A combination of EDS mapping and electron back-scattered diffraction (EBSD) has been used to identify individual phases present on a micro-scale. Ni and Fe predominantly forms the dendrites and Cr, W and Mn have a higher concentration in the eutectic where this phase alternates with the Ni-rich phase. Further, it has been shown that carbon is more abundant in the chromium and tungsten rich phases, forming carbides. The main phases are Ni solid solution, Cr—C phase(s) and W—C phase(s) and W dissolved in Cr23C6 phases.

[0051] Hardness tests have shown a 15% increase in hardness after laser re-melting but no noticeable variation with laser power (after re-melting hardness=390-425 HV2.5; cast material=300-360 HV2.5.

[0052] Isothermal testing was conducted using a simple heat induction furnace at ambient air and pressure at a temperature of 1200° C. for up to 8 days. During this thermal testing, an upper part of the re-melted layer developed porosity with the remainder of the layer unaffected. FIGS. 5 and 6 shown a comparison of porosities formed on re-melted and untreated surfaces.