An electron beam melting method for removing inclusions from hafnium-containing superalloy return materials

Abstract

This invention discloses an electron beam melting method for removing inclusions from hafnium-containing superalloy return materials. The invention employs electron beam droplet melting technology, utilizing the unique temperature and flow fields combined with interfacial tension generated during the electron beam droplet melting process to enrich high-density inclusions that are difficult to float to the surface of the molten pool. This solves the problem of poor hafnium inclusion removal efficiency in traditional melting methods such as vacuum induction remelting, enabling large-scale industrial-scale ultra-pure melting, and achieving an inclusion content of ≤0.3 (cm³) in the obtained electron beam ingots. 2 / kg), and achieves a deep removal effect of hafnium inclusions in the recycled master alloy ingot, which helps to realize the industrial-scale same-level recycling of a large amount of hafnium-containing casting high-temperature alloy recycled materials, and reduces the waste of my country's strategic metal resources.

Description

TECHNICAL FIELD

[0001] The embodiment of the present application relates to the alloy smelting technical field, and in particular to an electron beam melting method for removing inclusions in hafnium-containing high-temperature alloy return material. BACKGROUND

[0002] Cast high-temperature alloy has become an irreplaceable key material for hot end components of aerospace gas turbine engines due to its excellent high-temperature strength and good fatigue resistance and oxidation corrosion resistance. Cast high-temperature alloy usually contains strategic metals such as Ni, Co, Cr, W, Mo, Ta, and Hf, but the material utilization rate of cast high-temperature alloy parts is usually only 10-20%, and the cast high-temperature alloy return material produced in China each year is as high as several thousand tons. Because the cast high-temperature alloy parts undergo a high-temperature remelting casting process during production, the alloy melt reacts with the crucible, the mold shell, and the like, resulting in a substantial increase in the content of inclusions in the return material, which significantly reduces the high-temperature plasticity and fatigue service life of the alloy, and the alloy must be treated by a strict purification process before it can be considered for recycling and reuse.

[0003] Cast high-temperature alloy containing hafnium, such as DZ125 and Mar-M247, has strong interaction with impurity elements during smelting and casting due to the active element hafnium, so the return material mainly contains large specific gravity refractory hafnium oxide, which cannot be effectively removed by traditional vacuum induction remelting and electroslag remelting smelting processes. The electron beam melting technology uses high-energy electron beams generated by high-voltage electric fields as a heat source to bombard raw materials to make them melt. The high vacuum degree and high energy density of the electron beam make the purification rate of the electron beam higher than that of other vacuum melting equipment. This technology is mainly applied in the smelting of refractory metals and alloys, the preparation of high-purity titanium alloy, and other fields. Currently, the patent technology for purifying high-temperature alloy by electron beam melting is mainly small-scale cold bed furnace refining at the laboratory level, which cannot realize large-scale continuous melting of return material. In addition, there is a problem of main element loss in the electron beam refining of alloys. Unconditionally increasing the refining steps and refining time and pursuing high-energy beam spot bombardment and decomposition of inclusions will cause severe volatilization of main elements in the return material, significantly increasing the cost of recycling the return material. For the deep removal of high-density inclusions in high-temperature alloy, the patent CN112813282B proposes to use large-size inclusions to settle and capture the mechanism through the condensate, which requires two sets of water-cooled copper crucibles and electron gun devices, and the process is complex. The settling distance of inclusions increases with the extension of the refining time, and the refining power and crucible size must be certain to ensure sufficient inclusions settlement, which is difficult to realize large-scale industrialization. Therefore, how to effectively realize the ultra-low content control of refractory inclusions in large-scale industrialized purified cast high-temperature alloy containing hafnium in the return material is still a challenge for the recycling of hafnium-containing return material. SUMMARY

[0004] To solve the problem of the large proportion of refractory inclusions in the return material of the hafnium-containing cast high-temperature alloy mentioned above, the traditional vacuum induction remelting and electroslag remelting smelting processes cannot make it float and effectively remove it, and the electron beam cold bed refining technology process is complex and high in cost, and the existing technology cannot realize large-scale industrialization of the return material purification, and the technical problems such as ultra-low content control of refractory inclusions are faced, and the present application provides an electron beam melting method for removing inclusions in hafnium-containing high-temperature alloy return material. The present application generates molten droplets by high-energy electron beam bombardment, and fully carries out volatilization and decomposition reaction in the furnace environment, and utilizes the unique temperature field and flow field generated during the electron beam droplet melting process combined with the interfacial tension to make the difficult-to-float large proportion inclusions enriched to the surface of the molten pool, thereby solving the problem of poor hafnium-containing inclusion removal effect by using traditional melting methods such as vacuum induction remelting. The electron beam melting method for removing inclusions in hafnium-containing high-temperature alloy return material provided by the present application has the advantages of being able to realize large-scale industrialization of ultra-pure smelting, obtaining an electron beam ingot with a content of inclusions ≤0.3 (cm 2 / kg), and achieving the deep removal effect of hafnium-containing inclusions in the return material master alloy ingot, which helps to realize the industrialized same-level recycling of a large amount of hafnium-containing cast high-temperature alloy return material, and reduces the waste of strategic metal resources in China.

[0005] In order to achieve the above-mentioned purpose, the embodiments of the present application provide the following technical solutions:

[0006] An electron beam melting method for removing inclusions in hafnium-containing high-temperature alloy return material, which utilizes electron beam droplet melting technology to realize the migration and enrichment of large proportion refractory inclusions in hafnium-containing cast high-temperature alloy return material, achieves the deep removal effect of the difficult-to-float hafnium-containing inclusions in the return material, and obtains a return material ingot with a content of inclusions ≤0.3 (cm 2 / kg), the method comprising the following steps:

[0007] S1, vacuum induction remelting the hafnium-containing high-temperature alloy return material to obtain a raw material rod with a suitable size for electron beam melting;

[0008] S2. Place the raw material rod for electron beam melting into the feeding system of the electron beam melting equipment. Fix the bottom ingot to the ingot lifting system and adjust it to the bottom of the water-cooled copper crucible. Turn on the electron beam melting equipment for preheating and vacuuming. After the vacuum degree of the melting chamber and electron gun chamber reaches the required level and remains stable, turn on the electron gun to start melting. Adjust the electron beam spot focus to the center of the crucible and slowly feed the raw material into the electron beam bombardment zone. After the raw material melts, the molten liquid drips into the water-cooled copper crucible. Operate the ingot lifting system to descend at a certain speed and adjust the electron beam melting power and feeding speed to keep the molten pool in the crucible at a certain height. Gradually pull the ingot, which has been rapidly cooled and solidified by the water-cooled copper crucible, out of the bottom of the molten pool from the bottom of the crucible. Maintain this state and continue melting until the raw material rod is completely melted. Gradually reduce the melting power to zero. After the furnace body cools down, take out the ingot and cut off the top inclusion-rich area to obtain a high-purity hafnium-containing return material electron beam ingot.

[0009] Further, in step S2, the inner diameter D of the water-cooled copper crucible is 90 ~ 200 mm, and the descent speed V of the ingot lifting system satisfies the following relationship with the inner diameter D of the water-cooled copper crucible: V = 5 × 10 4 / D 2 ~ 1×10 5 / D 2 mm / min.

[0010] Furthermore, in step S2, the melting power P of the electron beam melting furnace during the melting process is 35 ~ 60 kW, the height H of the molten pool in the crucible is 25 ~ 50 mm, and P, V, and H satisfy the following relationship: .

[0011] The parameters of the melting power P are directly related to the molten pool height H and the ingot descent speed V. Different molten pool heights alter the temperature distribution during melting, while different ingot descent speeds change the cooling loss during the melting process. To ensure that the hafnium-containing recycled material achieves hafnium oxide enrichment and flotation during electron beam melting, these three parameters are defined by formulas. When the three parameters are substituted into the formula... When the temperature gradient is less than 12, the temperature gradient within the molten pool is small, and high-density hafnium oxide is not easily floated by stirring. Therefore, when the three parameters are substituted into the formula... When the value is greater than 20, although hafnium oxide can be removed through decomposition reaction, the main element is severely burned off, and the smelting cost increases significantly.

[0012] Furthermore, in step S1, the diameter of the raw material rod used for electron beam melting is in the range of 80 to 150 mm.

[0013] Furthermore, the inner diameter of the water-cooled copper crucible is not less than the diameter of the raw material rod used for electron beam melting.

[0014] Further, in step S2, the electron beam melting equipment is turned on for preheating and vacuuming. After the vacuum levels in the melting chamber and electron gun chamber reach the required level and remain stable, the electron gun is turned on to begin melting. The vacuum level in the melting chamber reaches less than 2 × 10⁻⁶. -2 The requirement is that the vacuum degree of the electron gun chamber should be less than 8 × 10⁻⁶ Pa. -3 Pa's requirements.

[0015] Furthermore, in step S2, the bottom ingot fixed to the ingot lifting system is obtained by machining a hafnium-containing high-temperature alloy of the same grade.

[0016] Furthermore, in step S2, the thickness of the inclusion-rich region at the top of the electron beam melting ingot is 10-30 mm.

[0017] The embodiments of the present invention have the following advantages:

[0018] 1. The present invention provides an electron beam melting method for removing inclusions from hafnium-containing superalloy return materials. This method innovatively applies electron beam droplet melting technology to the ultrapure melting of return materials such as hafnium-containing directional superalloy runners, risers, and scrap parts, achieving an inclusion content ≤0.3 (cm³). 2 Electron beam casting with a yield of / kg enables ultra-low control of refractory inclusions, bringing the purity of hafnium-containing recycled materials to the level of virgin materials, promoting the same-level recycling of recycled materials, and reducing the waste of my country's strategic metal resources.

[0019] 2. This invention employs electron beam melting technology, which bombards the surface of raw materials with a high-energy electron beam to completely melt them. The molten droplets undergo degassing and impurity volatilization / decomposition reactions in a high-vacuum, high-temperature environment within the melting furnace, effectively reducing the content of impurities and inclusions. By utilizing the unique temperature and flow fields combined with interfacial tension generated during electron beam droplet melting, high-density inclusions that are difficult to float are enriched to the surface of the molten pool, solving the problem of poor hafnium inclusion removal efficiency in traditional melting methods such as vacuum induction remelting. Compared with electron beam cold bed refining technology that utilizes the settling of high-density inclusions and capture through a solidified shell, the high-density refractory inclusion removal mechanism utilized in this invention is fundamentally different. It does not require additional refining equipment, has low process complexity, and can achieve large-scale industrial ultrapure melting, obtaining ultrapure hafnium-containing return ingots in the hundreds of kilograms or more, exhibiting significant advantages of high efficiency and low cost. Attached Figure Description

[0020] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely exemplary, and those skilled in the art can derive other embodiments based on the provided drawings without creative effort.

[0021] Figure 1 This is a scanning electron microscope image of the region rich in high-density refractory inclusions removed by electron beam melting, provided in Embodiment 1 of the present invention.

[0022] Figure 2 A scanning electron microscope image of an ultrapure ingot obtained by electron beam melting according to Embodiment 1 of the present invention;

[0023] Figure 3 The image provided is a scanning electron microscope (SEM) image of the interior of a DZ125 ingot obtained by electron beam melting, which is provided as Comparative Example 1 of this invention. Detailed Implementation

[0024] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] Example 1

[0026] This embodiment provides an electron beam melting method for removing inclusions from hafnium-containing superalloy return materials:

[0027] (1) Vacuum induction remelting was performed on the return material of hafnium-containing oriented high-temperature alloy DZ125 to obtain a raw material bar with a diameter of 80 (mm) for electron beam melting;

[0028] (2) Place the 80 mm diameter electron beam melting raw material bar into the feeding system of the electron beam melting equipment. Melting is carried out using a water-cooled copper crucible with an inner diameter of 100 mm. The DZ125 alloy bottom ingot is processed by induction melting. The vacuum degree of the melting chamber reaches 8.5 × 10⁻⁶. -3 (Pa), the vacuum level of the electron gun chamber reaches 5.1×10⁻⁶. -3After (Pa), electron beam melting begins. The ingot lifting system is lowered at a speed of 10 (mm / min). The electron beam melting power is 42 (kW). The height of the molten pool in the crucible is maintained at 30 (mm). Melting continues until all the raw material rods are melted. The melting power is gradually reduced to zero. After the furnace body cools, the ingot is removed and the top 15 (mm) thick inclusion-rich area is cut off to obtain a high-purity DZ125 return material electron beam ingot.

[0029] The hafnium-containing high-temperature alloy return material electron beam casting ingot provided in Embodiment 1 of the present invention has a slag content of 0.22 (cm³). 2 / kg). The scanning electron microscope (SEM) images of the high-density refractory inclusion-rich regions removed by electron beam melting in Example 1 of this invention are shown below. Figure 1 As shown, the scanning electron microscope image of the ultrapure ingot obtained by electron beam melting is as follows: Figure 2 As shown, comparing the scanned images of the removed inclusion-rich area and the ultrapure ingot obtained from melting reveals that a large amount of heavy refractory inclusions such as hafnium oxide are concentrated in the removed top inclusion-rich area, while no refractory inclusions such as hafnium oxide are found in the electron beam ingot. This demonstrates that the electron beam melting technology described in this invention can utilize the unique temperature field and flow field combined with interfacial tension generated during electron beam dripping to enrich heavy inclusions that are difficult to float to the surface of the molten pool, achieving the effect of deep removal of inclusions in the return material of hafnium-containing casting high-temperature alloys.

[0030] Example 2

[0031] This embodiment provides an electron beam melting method for removing inclusions from hafnium-containing superalloy return materials:

[0032] (1) Vacuum induction remelting was performed on the return material of hafnium-containing oriented high-temperature alloy DZ125 to obtain a raw material bar with a diameter of 100 (mm) for electron beam melting;

[0033] (2) Place the 100 mm diameter electron beam melting raw material bar into the feeding system of the electron beam melting equipment. Melting is carried out using a water-cooled copper crucible with an inner diameter of 120 mm. The DZ125 alloy bottom ingot is processed by induction melting. During the electron beam melting process, the vacuum degree of the melting chamber reaches 9.5 × 10⁻⁶. -3 (Pa) below, electron gun chamber vacuum reaches 7.0×10 -3 (Pa) or below, the ingot lifting system is operated to descend at a speed of 5 (mm / min), the electron beam melting power is 55 (kW), the height of the molten pool in the crucible is maintained at 45 (mm), and continuous melting is carried out until all the raw material rods are melted. The melting power is gradually reduced to zero. After the furnace body is cooled, the ingot is taken out and the inclusion-rich area with a thickness of 25 (mm) at the top is cut off to obtain a high-purity DZ125 return material electron beam ingot.

[0034] The hafnium-containing high-temperature alloy return material electron beam casting ingot provided in Embodiment 2 of the present invention has a slag content of 0.18 (cm³). 2 The oxygen content decreased to 4 ppm, achieving a deep removal effect of high-density oxides in the return material of hafnium-containing casting high-temperature alloys. The electron beam melting ingot provided in Example 2 of this invention has a Cr content of 8.69 wt% and a burn-off rate of less than 1.2%, meeting the Cr element chemical composition range requirements of DZ125 alloy.

[0035] Example 3

[0036] This embodiment provides an electron beam melting method for removing inclusions from hafnium-containing superalloy return materials:

[0037] (1) Vacuum induction remelting was performed on the Mar-M247 high-temperature alloy return material to obtain two raw material rods with a diameter of 150 (mm) and a length of 1 (m) for electron beam melting;

[0038] (2) Place the 150 mm diameter electron beam melting raw material bar into the feeding system of the electron beam melting equipment. Melting is carried out using a water-cooled copper crucible with an inner diameter of 200 mm. The bottom ingot is processed by induction melting of Mar-M247 master alloy. During the electron beam melting process, the vacuum degree of the melting chamber reaches 1.8 × 10⁻⁶. -2 (Pa), the vacuum degree of the electron gun chamber reaches 6.5×10. -3 (Pa), the ingot lifting system is operated to descend at a speed of 2.5 (mm / min), the electron beam melting power is 35 (kW), the height of the molten pool in the crucible is maintained at 25 (mm), and the melting is continuously carried out until all the raw material rods are melted. The melting power is gradually reduced to zero. After the furnace body is cooled, the ingot is taken out and the inclusion-rich area with a thickness of 20 (mm) on the top is cut off to obtain a high-purity Mar-M247 return material electron beam ingot.

[0039] The Mar-M247 high-temperature alloy return material electron beam casting ingot provided in Example 3 of this invention has a slag content of 0.27 (cm³). 2 / kg), the total weight of the ingot is 290 (kg), and the actual burning loss rate of Cr element in the ingot is 0.8%. This achieves ultra-low content control of inclusions in the return material of hafnium-containing high-temperature alloy castings of more than 100 kg, while effectively controlling the burning loss of active elements.

[0040] Comparative Example 1

[0041] This comparative example provides an electron beam melting method for removing inclusions from hafnium-containing superalloy return materials:

[0042] (1) Vacuum induction remelting was performed on the return material of hafnium-containing oriented high-temperature alloy DZ125 to obtain a raw material bar with a diameter of 90 (mm) for electron beam melting;

[0043] (2) Place the 90 mm diameter electron beam melting raw material bar into the feeding system of the electron beam melting equipment. Melting is carried out using a water-cooled copper crucible with an inner diameter of 100 mm. Induction melting of DZ125 alloy processing ingot is used. The vacuum degree of the melting chamber reaches 1.2 × 10⁻⁶. -2 (Pa), electron gun chamber vacuum level is less than 8.0×10 -3 After (Pa), electron beam melting begins. The ingot lifting system is operated to descend at a speed of 20 (mm / min). The electron beam melting power is 30 (kW), and the height of the molten pool in the crucible is maintained at 20 (mm). Melting continues until all the raw material rods are melted. The melting power is gradually reduced to zero. After the furnace body cools, the ingot is removed and the top 10 (mm) thick inclusion-rich area is cut off to obtain DZ125 return material electron beam ingot.

[0044] The hafnium-containing high-temperature alloy return material electron beam casting ingot provided in Comparative Example 1 of this invention has a slag content of 0.65 (cm³). 2 / kg). The scanning electron microscope image of the interior of the DZ125 ingot obtained by electron beam melting in Comparative Example 1 of this invention is shown below. Figure 3 As shown in the comparison image of the ultrapure ingot obtained by electron beam melting in Example 1, it can be seen that using the melting parameters provided in Example 1, the ingot still contains large-density refractory inclusions such as hafnium oxide with a size greater than 200 μm. However, no refractory inclusions such as hafnium oxide were found in the electron beam ingot melted using the parameters provided in Example 1. This is because the melting power is less than 35 kW and includes the relationship between the three parameters of melting power, molten pool height, and ingot descent speed. This results in a small temperature gradient within the molten pool, making it difficult for high-density hafnium oxide to float to the surface through stirring, thus failing to achieve the desired deep removal of inclusions from the return material of hafnium-containing high-temperature alloy castings.

[0045] Comparative Example 2

[0046] This comparative example provides an electron beam melting method for removing inclusions from hafnium-containing superalloy return materials:

[0047] (1) Vacuum induction remelting was performed on the return material of hafnium-containing oriented high-temperature alloy DZ125 to obtain a raw material bar for electron beam melting with a diameter of 120 (mm);

[0048] (2) Place the 120 mm diameter electron beam melting raw material bar into the feeding system of the electron beam melting equipment. Melting is carried out using a water-cooled copper crucible with an inner diameter of 150 mm. The DZ125 alloy bottom ingot is processed by induction melting. During the electron beam melting process, the vacuum degree of the melting chamber is kept below 1.0 × 10⁻⁶. -2 (Pa), electron gun chamber vacuum level is less than 8.0×10 -3 (Pa), the ingot lifting system is operated to descend at a speed of 4 (mm / min), the electron beam melting power is 65 (kW), the height of the molten pool in the crucible is maintained at 50 (mm), and the melting is continuously carried out until all the raw material rods are melted. The melting power is gradually reduced to zero. After the furnace body is cooled, the ingot is taken out and the inclusion-rich area with a thickness of 30 (mm) on the top is cut off to obtain DZ125 return material electron beam ingot.

[0049] The hafnium-containing high-temperature alloy return material electron beam casting ingot provided in Comparative Example 2 of this invention has a slag content of 0.16 (cm³). 2 / kg), but the content of the active main element Cr in the electron beam casting is 8.45wt%, and the burn-off rate reaches 3.8%, which is lower than the chemical composition range requirement of Cr element in DZ125 alloy. This is because the melting power is higher than 60kW, and it contains the relationship between the three parameters of melting power, molten pool height and ingot descent speed. This results in severe burning loss of the main elements and a significant increase in smelting costs.

[0050] Although the present invention has been described in detail above with general descriptions and specific embodiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. An electron beam melting method for removing inclusions from a hafnium-containing superalloy return material, characterized by, The migration and enrichment of large specific gravity refractory inclusions in the return material of cast high-temperature alloy containing hafnium are realized by using electron beam droplet melting technology, so as to achieve the effect of deep removal of the return material internal difficult-to-float hafnium-containing inclusions, and obtain the return material ingot with the inclusion content ≤0.3 (cm 2 / kg), and the method comprises the following steps: S1, the hafnium-containing high-temperature alloy return material is vacuum induction remelted to obtain a raw material rod with a proper size for electron beam melting; S2, the raw material rod for electron beam melting is placed in a feeding system of an electron beam melting device, a bottom ingot is fixed to an ingot lifting system and adjusted to the bottom of a water-cooled copper crucible, the electron beam melting device is started to preheat and vacuumize, the melting chamber and the electron gun chamber are vacuumized to the required degree and kept stable, then the electron gun is opened to start melting, the electron beam spot focus is adjusted to be aligned with the center of the crucible, the raw material is slowly fed into the electron beam bombardment area, the raw material is melted to form molten droplets which are dropped into the water-cooled copper crucible, the ingot lifting system is operated to be lowered at a certain speed, the electron beam melting power and the feeding speed are adjusted, the molten pool in the crucible is kept at a certain height, the bottom of the molten pool is rapidly cooled and solidified by the water-cooled copper crucible, the ingot is gradually pulled out from the bottom of the crucible, the state is kept to continuously melt until the raw material rod is completely melted, the melting power is gradually reduced to zero, the ingot is taken out after the furnace body is cooled, and the top inclusion-rich area is cut off to obtain a high-purity hafnium-containing return material electron beam ingot; In step S2, the water-cooled copper crucible has an inner diameter D = 90 ~ 200 mm, and the casting ingot lifting system has a lowering speed V and the water-cooled copper crucible inner diameter D satisfy the following relationship: V = 5 × 10 4 / D 2 ~ 1 × 10 5 / D 2 mm / min, the melting power P of the electron beam melting furnace during the melting process is 35 ~ 60 kW, the molten pool height H in the crucible is 25 ~ 50 mm, and P, V and H satisfy the following relationship: .

2. The electron beam melting method for removing inclusions in a hafnium-containing superalloy return material according to claim 1, characterized by, In step S1, the diameter of the raw material rod for electron beam melting is in the range of 80-150 mm.

3. The electron beam melting method for removing inclusions in a hafnium-containing superalloy return material according to claim 1, characterized by, The inner diameter of the water-cooled copper crucible is not less than the diameter of the raw material rod for electron beam melting.

4. The electron beam melting method for removing inclusions in a hafnium-containing superalloy return material according to claim 1, characterized by, In step S2, the electron beam melting device is started for preheating and vacuumizing, and the vacuum degree of the melting chamber and the electron gun chamber reaches the requirement and is kept stable, then the electron gun is opened to start melting, wherein the vacuum degree of the melting chamber reaches less than 2×10 -2 Pa, and the vacuum degree of the electron gun chamber reaches less than 8×10 -3 Pa.

5. The electron beam melting method for removing inclusions in hafnium-containing superalloy return material according to claim 1, characterized by, In step S2, the bottom ingot fixed to the ingot lifting system is obtained by machining a hafnium-containing cast high-temperature alloy of the same grade.

6. The electron beam melting method for removing inclusions in hafnium-containing superalloy return material according to claim 1, characterized by, In step S2, the thickness of the top inclusion-rich area of the electron beam melting ingot which is cut off is 10-30 mm.

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