A resistance spot welding process for dissimilar steels based on differentiated electrode parameter optimization

Abstract

A dissimilar steel resistance spot welding process based on differentiated electrode parameter optimization belongs to the field of welding technology. Carbon steel and stainless steel plates are assembled and fixed on a fixture in an overlapping manner as workpieces. Chromium-zirconium-copper electrodes are selected as welding electrodes. The electrode end face diameter is 5mm to 9mm, and the electrode end face diameter on the carbon steel plate side is smaller than the electrode end face diameter on the stainless steel plate side. The electrode end face is fully contacted with the workpiece surface and pre-pressed. Welding is performed using resistance spot welding. During welding, the electrode pressure is 7kN to 10kN, the welding current is 7kA to 10kA, and the welding time is 0.2s to 0.4s. After the power is turned on, the current is removed, and the electrode pressure is maintained to allow natural cooling. At this point, the dissimilar steel resistance spot welding of carbon steel and stainless steel plates is completed. By using electrodes of different sizes on the stainless steel and low-carbon steel sides, the present invention effectively controls the nugget offset and nugget size, thereby improving the quality and stability of spot welding.

Description

Technical Field

[0001] The present invention belongs to the field of welding technology, and in particular relates to a dissimilar steel resistance spot welding process based on differentiated electrode parameter optimization. Background Art

[0002] 301L austenitic stainless steel cold-rolled series plates are the main material for manufacturing rail vehicles. In addition to the welding of 301L stainless steel, there are also many dissimilar steel welding structures of 301L stainless steel and low carbon steel. Currently, the dissimilar steel welding of 301L stainless steel and low carbon steel still has the following problems:

[0003] 1. The arc welding and plug welding currently used not only have low production efficiency and high cost, but also need to be improved in terms of strength, stiffness and fatigue reliability.

[0004] 2. Traditional spot welding processes usually weld two plates directly and cannot be adjusted according to changes in steel properties.

[0005] 3. Due to the differences in resistivity, thermal conductivity and expansion coefficient between 301L stainless steel and low carbon steel, the weld nugget will shift, affecting the weld strength.

[0006] 4. Since 301L stainless steel and low carbon steel dissimilar steel welding is widely used in automobile manufacturing, the following problems may arise during welding:

[0007] (1) The diameter and depth of the weld nugget on the stainless steel side are greater than those on the mild steel side.

[0008] (2) The high temperature zone of the temperature field shifts toward the 301L stainless steel side.

[0009] 5. The patent document "A three-layer steel plate resistance spot welding process and the electrodes used therein" (publication number: CN116586732A, publication date: 2023.08.15) uses upper and lower differentiated electrodes to solve the problem that the outer low-resistance steel plate of the three-layer steel plates of unequal thickness cannot form a qualified welding nugget with the high-resistance steel plate. However, this solution is applicable to multiple layers of unequal thickness steel plates. It uses spherical electrodes and is only suitable for resistance spot welding of mild steel and high-strength steel. It optimizes weldability, but is not suitable for resistance spot welding of thick steel plates such as stainless steel / low-carbon steel. There is no clear data on the improvement of weldability and the problem of nugget offset in resistance spot welding of dissimilar steels is not solved.

[0010] The above methods are of great reference significance to the present invention. For resistance spot welding of dissimilar steels, not only the defects caused by differences in resistivity, thermal conductivity and expansion coefficient should be considered, but also the problem of nugget shift should be considered.

[0011] Therefore, the existing technology urgently needs a new technical solution to make full use of the welding characteristics of stainless steel and low carbon steel, and to cleverly use electrodes with different sizes to optimize the problems existing in welding between stainless steel and low carbon steel. Summary of the Invention

[0012] The purpose of the present invention is to provide a resistance spot welding process for dissimilar steels based on differentiated electrode parameter optimization. By adopting electrodes with different sizes on both sides of stainless steel and low carbon steel, effective control of the weld nugget offset and weld nugget size is achieved, thereby improving the quality and stability of spot welding.

[0013] The technical solution adopted by the present invention to achieve the above-mentioned purpose is: the present invention proposes a dissimilar steel resistance spot welding process based on differentiated electrode parameter optimization, comprising:

[0014] Step 1: Clean and polish a carbon steel plate with a thickness of 1.5 mm to 3 mm and set aside;

[0015] Step 2: Clean and polish the stainless steel plate with a thickness of 2mm to 3.5mm and set aside;

[0016] Step 3: The carbon steel plate and the stainless steel plate are assembled and fixed on the fixture in an overlapping manner as workpieces. A chromium-zirconium-copper electrode is selected as the welding electrode. The electrode end face diameter is 5mm to 9mm, and the electrode end face diameter on the carbon steel plate side is smaller than the electrode end face diameter on the stainless steel plate side.

[0017] Step 4: Make the electrode end face completely contact with the workpiece surface, set the electrode pressure, and perform pre-pressing treatment;

[0018] Step 5: Welding is performed using resistance spot welding, with an electrode pressure of 7kN to 10kN, a welding current of 7kA to 10kA, and a welding time of 0.2s to 0.4s.

[0019] Step 6: After the power is turned on, remove the current, continue to maintain the electrode pressure, and allow it to cool naturally;

[0020] At this point, the resistance spot welding of carbon steel plates and stainless steel plates is completed.

[0021] According to a specific embodiment of the present invention, the carbon content of the carbon steel plate is less than 0.25.

[0022] According to a specific embodiment of the present invention, the stainless steel plate is composed of an alloy having Fe as a main component and containing 10.5% by mass or more of Cr.

[0023] Among them, when the thicknesses of the carbon steel plate and the stainless steel plate are 1.5 mm and 2 mm respectively, the diameter of the electrode end surface on the carbon steel plate side is 5 mm; the diameter of the electrode end surface on the stainless steel plate side is 7 mm.

[0024] Among them, when the thicknesses of the carbon steel plate and the stainless steel plate are 2 mm and 2 mm respectively, the diameter of the electrode end surface on the carbon steel plate side is 6 mm; the diameter of the electrode end surface on the stainless steel plate side is 7 mm.

[0025] Among them, when the thicknesses of the carbon steel plate and the stainless steel plate are 2 mm and 2.5 mm respectively, the diameter of the electrode end surface on the carbon steel plate side is 6 mm; the diameter of the electrode end surface on the stainless steel plate side is 8 mm.

[0026] Among them, when the thickness of the carbon steel plate and the stainless steel plate are 2 mm and 3 mm respectively, the diameter of the electrode end surface on the carbon steel plate side is 6 mm; the diameter of the electrode end surface on the stainless steel plate side is 8 mm.

[0027] Among them, when the thicknesses of the carbon steel plate and the stainless steel plate are 3 mm and 3 mm respectively, the diameter of the electrode end surface on the carbon steel plate side is 6 mm; the diameter of the electrode end surface on the stainless steel plate side is 8.5 mm.

[0028] Among them, when the thicknesses of the carbon steel plate and the stainless steel plate are 3 mm and 3.5 mm respectively, the diameter of the electrode end surface on the carbon steel plate side is 7 mm; the diameter of the electrode end surface on the stainless steel plate side is 9 mm.

[0029] Wherein, in step 4, the electrode pressure is set to 7 kN to 10 kN.

[0030] The above-mentioned design scheme can bring the following beneficial effects: The present invention proposes a dissimilar steel resistance spot welding process based on differentiated electrode parameter optimization. By using electrodes of different sizes on both the stainless steel and mild steel sides, the process effectively controls the nugget offset and size, improving the quality and stability of the spot weld. Differentiated electrodes are used to optimize current distribution, adjust pressure distribution, control the heat-affected zone, and adapt to differences in sheet material properties, effectively improving the weld structure and balancing thermal stresses during welding. The electrode size can be flexibly adjusted to accommodate workpieces of varying materials and thicknesses.

[0031] Furthermore, the process is simple to operate and can be widely applied to the spot welding production of stainless steel and mild steel. By using the process of the present invention, product quality and production efficiency can be improved, production costs can be reduced, and strong support can be provided for the sustainable development of enterprises. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The drawings described herein are used to provide a further understanding of the present invention and constitute a part of the present invention. The exemplary embodiments of the present invention and their descriptions are used to understand the present invention and do not constitute improper limitations of the present invention. In the drawings:

[0033] Figure 1 This is the process flow chart of resistance spot welding of dissimilar steels based on differentiated electrode parameter optimization.

[0034] Figure 2Schematic diagram of the position of the weld nugget using the same electrode according to one embodiment of the present invention.

[0035] Figure 3 Schematic diagram of the position of the weld nugget using different electrodes according to one embodiment of the present invention. DETAILED DESCRIPTION

[0036] The present invention provides a resistance spot welding process for dissimilar steels based on differentiated electrode parameter optimization. By adjusting the electrode size, the size-differentiated electrodes can alter the current distribution in the weld area. When a large electrode contacts one type of plate and a small electrode contacts another, the current flows more concentratedly toward the thicker plate or the plate with lower resistivity. The plate corresponding to the large electrode receives a relatively uniform and moderate current, preventing local overheating, while the plate corresponding to the larger electrode receives a higher current density. This targeted current distribution helps improve weld quality and reduce weld defects. Electrodes of different sizes enable different pressure distribution. In resistance welding, the pressure applied by the electrodes is crucial to weld quality. Large electrodes provide more stable and uniform pressure, while small electrodes can apply higher pressure in localized areas. For welding stainless steel to carbon steel, this differential pressure can better adapt to the different characteristics of the two plates. For example, stainless steel may require less local pressure to avoid excessive deformation, while carbon steel can withstand greater overall pressure to ensure good contact. By properly adjusting the electrode size, optimal pressure distribution can be achieved, improving the strength and sealing of the weld joint. Electrodes with differentiated sizes help control the size and properties of the heat-affected zone (HAZ). During welding, the HAZ is the area around the weld joint that undergoes structural and property changes due to welding heat. By selecting the appropriate electrode size, heat input and heat dissipation can be adjusted during welding. Large electrodes dissipate heat more quickly, reducing the extent of the HAZ; small electrodes concentrate heat in a localized area, enabling precise welding. This reduces the adverse effects of the HAZ on the properties of the parent material and improves the overall performance of the welded joint. Stainless steel and carbon steel differ in their physical properties and welding characteristics. Stainless steel typically has higher resistivity and thermal conductivity, while carbon steel has lower resistivity and thermal conductivity than stainless steel. Electrodes with differentiated sizes allow for optimized design based on the characteristics of the two materials. For example, for stainless steel with high resistivity, a small electrode can be used to increase current density and welding temperature; for carbon steel with high thermal conductivity, a large electrode can be used to increase heat dissipation and prevent overheating. This targeted design better adapts to the different characteristics of the two materials, improving welding efficiency and quality. Effective control of nugget offset and nugget size improves the quality and stability of spot welding. By rationally designing the electrode size ratio and position, welding parameters can be flexibly adjusted to suit workpieces of different materials and thicknesses.

[0037] The present invention proposes a dissimilar steel resistance spot welding process based on differentiated electrode parameter optimization. According to different plate thickness combinations of stainless steel and carbon steel plates, electrodes of appropriate sizes are selected to improve the dissimilar steel resistance spot welding process. Due to the differences in resistivity, thermal conductivity and expansion coefficient between stainless steel and carbon steel, the welding nugget is offset, which affects the welding strength.

[0038] like Figure 1 As shown, a dissimilar steel resistance spot welding process based on differentiated electrode parameter optimization includes:

[0039] Selection of carbon steel plate and stainless steel plate:

[0040] The composition of the carbon steel plate is not particularly limited. Considering the practicality of rail vehicles, a carbon content of less than 0.25 is sufficient. The thickness should also be selected based on practicality, so 1.5 mm to 3 mm is selected.

[0041] The composition of the spot-welded stainless steel plate is not particularly limited. For example, austenitic stainless steel, ferritic stainless steel, duplex stainless steel and martensitic stainless steel are used, the main component of which is Fe and is composed of an alloy containing 10.5% or more Cr by mass. According to practicality, the thickness of the stainless steel plate is 2 mm to 3.5 mm.

[0042] Electrode selection:

[0043] Considering the welding performance of carbon steel and stainless steel, chromium-zirconium-copper electrodes should be selected as welding electrodes. They offer higher strength and hardness, as well as good electrical and thermal conductivity. In spot welding dissimilar steels, they can withstand higher currents and pressures, are less susceptible to deformation and wear, and effectively conduct current, generating sufficient heat to weld the plates. Based on the plate thickness, the electrode tip diameter should be between 5mm and 9mm.

[0044] Matching of base material and electrode:

[0045] Stainless steel has a resistivity five times that of carbon steel and a thermal conductivity one-third that of carbon steel. Therefore, smaller electrodes should be used on the carbon steel side, and larger electrodes on the stainless steel side. Common plate thickness combinations in rail vehicles include: 1.5mm carbon steel and 2mm stainless steel, 2mm carbon steel and 2mm stainless steel, 2mm carbon steel and 2.5mm stainless steel, 2mm carbon steel and 3mm stainless steel, 3mm carbon steel and 3mm stainless steel, and 3mm carbon steel and 3.5mm stainless steel. For these plate thickness combinations, the electrode end face diameters on the carbon steel side are 5mm, 6mm, 6mm, 6mm, 6mm, and 7mm, respectively, and on the stainless steel side are 7mm, 7mm, 8mm, 8mm, 8.5mm, and 9mm. That is, when the thicknesses of the carbon steel plate and stainless steel plate are 1.5mm and 2mm, respectively, the diameter of the electrode end face on the carbon steel plate side is 5mm, and the diameter of the electrode end face on the stainless steel plate side is 7mm. When the thicknesses of the carbon steel plate and stainless steel plate are 2mm and 2mm, respectively, the diameter of the electrode end face on the carbon steel plate side is 6mm, and the diameter of the electrode end face on the stainless steel plate side is 7mm. When the thicknesses of the carbon steel plate and stainless steel plate are 2mm and 2.5mm, respectively, the diameter of the electrode end face on the carbon steel plate side is 6mm, and the diameter of the electrode end face on the stainless steel plate side is 8mm. When the thicknesses of the carbon steel plate and stainless steel plate are 2mm and 3mm, respectively, the diameter of the electrode end face on the carbon steel plate side is 6mm, and the diameter of the electrode end face on the stainless steel plate side is 8mm. When the thicknesses of the carbon steel plate and stainless steel plate are 3mm and 3mm, respectively, the diameter of the electrode end face on the carbon steel plate side is 6mm, and the diameter of the electrode end face on the stainless steel plate side is 8.5mm. When the thicknesses of the carbon steel plate and the stainless steel plate are 3 mm and 3.5 mm respectively, the diameter of the electrode end face on the carbon steel plate side is 7 mm; the diameter of the electrode end face on the stainless steel plate side is 9 mm.

[0046] Resistance spot welding:

[0047] For carbon steel plates, a soft welding method with a long welding time and low current should be used, while for stainless steel plates, a hard welding method with a short welding time and high current should be used. Taking into account the weldability of carbon steel and stainless steel plates, a hard welding method with a high current and long welding time should be used. Based on the plate thickness in the above steps, the electrode pressure is 7kN to 10kN, the welding current is 7kA to 10kA, and the welding time is between 0.2s and 0.4s. In spot welding, increasing the electrode tip diameter, applying pressure, and increasing the current or current duration can increase the diameter of the weld nugget and the weld depth. Therefore, the electrode tip diameter, pressure, current duration, and current should be adjusted according to the plate thickness and composition.

[0048] At this point, the process steps for optimizing differentiated electrode parameters for resistance spot welding of dissimilar steels are completed.

[0049] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention. In the present invention, unless otherwise specified, all equipment and raw materials can be purchased from the market or are commonly used in the industry. The methods in the following embodiments are all conventional methods in the art unless otherwise specified.

[0050] Example 1

[0051] Select 2mm thick 301L stainless steel and 2mm thick Q235 low carbon steel materials and process them into a size of 100mm×25mm. The specific steps are as follows:

[0052] Step 1: Immerse 2mm thick 301L stainless steel in an ultrasonic cleaning machine and use sodium hydroxide solution for ultrasonic cleaning. Then immerse the stainless steel in a pickling tank and use a mixed solution of HCL, H2SO4, and HNO3 as an HCL acid deoxidizer for pickling. Finally, place the stainless steel in a polishing machine.

[0053] Step 2: Immerse the 2mm thick Q235 mild steel in a solvent cleaning tank and clean it with petroleum ether. Then immerse the mild steel in a pickling tank and use a mixed solution of HCL and hexamethylenetetramine as an acidic deoxidizer for pickling. Finally, place the mild steel in a polishing machine.

[0054] Step 3: Stainless steel and mild steel are assembled and fixed on the fixture in an overlapping manner. A conical electrode with an end face diameter of 6 mm is selected as the small-size electrode, and a conical electrode with an end face diameter of 7 mm is selected as the large-size electrode. The large-size electrode corresponds to the stainless steel side, and the small-size electrode corresponds to the mild steel side.

[0055] Step 4: Make effective contact between the electrode end face and the workpiece surface, and set the electrode pressure to 8kN for pre-pressing treatment;

[0056] Step 5: Perform spot welding, set the welding current to 9.5kA, the electrode pressure to 9kN, and the welding time to 0.3s;

[0057] Step 6: After the power is turned on, the current is removed and the electrode pressure of 9kN is maintained. It is allowed to cool naturally and the shear strength test of the weld joint is performed. The shear strength of the weld joint reaches 1850N.

[0058] Example 2

[0059] Select Q215 low carbon steel with a thickness of 1.5mm and 304 stainless steel with a thickness of 2mm and process them into a size of 100mm×25mm. The specific steps are as follows:

[0060] Step 1: Immerse 2mm thick 304 stainless steel in an ultrasonic cleaning machine and use sodium hydroxide solution for ultrasonic cleaning. Then immerse the stainless steel in a pickling tank and use a mixed solution of HCL, H2SO4, and HNO3 as an HCL acid deoxidizer for pickling. Finally, put the stainless steel into a polishing machine;

[0061] Step 2: Immerse the 1.5mm thick Q215 mild steel in a cleaning tank and clean it with petroleum ether. Then, immerse the mild steel in a pickling tank and use a mixed solution of HCL and hexamethylenetetramine as an acidic deoxidizer for pickling. Finally, place the mild steel in a polishing machine.

[0062] Step 3: Fix the mild steel and stainless steel on the fixture. The stainless steel and mild steel are assembled and fixed on the fixture in an overlapping manner. Select a conical electrode with an end face diameter of 5mm as the small-size electrode and a conical electrode with an end face diameter of 7mm as the large-size electrode. The large-size electrode corresponds to the stainless steel side, and the small-size electrode corresponds to the mild steel side.

[0063] Step 4: Make effective contact between the electrode and the workpiece surface, and set the electrode pressure to 7kN for pre-pressing treatment;

[0064] Step 5: Perform spot welding, set the welding current to 8kA, the electrode pressure to 8kN, and the welding time to 0.2s;

[0065] Step 6: After the power is turned on, remove the current and continue to maintain the 8kN electrode pressure. Allow it to cool naturally and perform a shear strength test on the joint. The shear strength of the joint reaches 1633N.

[0066] According to the above embodiments, Figure 2 and attached Figure 3As can be seen, during resistance spot welding of dissimilar steels, differences in resistivity, thermal conductivity, and expansion coefficient between stainless steel and mild steel can cause the weld nugget to shift, affecting weld strength. Using differentiated electrodes on either side of the dissimilar steel plates for resistance spot welding can alter the current distribution within the weld area. When a larger electrode contacts the stainless steel plate, and a smaller electrode contacts the mild steel plate, the current flows more concentratedly to specific areas. Large electrodes deliver a relatively uniform and moderate current, preventing localized overheating, while smaller electrodes achieve higher current density at specific locations, promoting fusion within the weld. This targeted current distribution improves weld quality and reduces defects. Different electrode sizes enable different pressure distribution. During resistance welding, electrode pressure is crucial to weld quality. Large electrodes generally provide more stable and uniform pressure, while smaller electrodes apply higher pressure in localized areas. This differential pressure distribution in stainless steel to carbon steel welding better accommodates the differing properties of the two plates. Stainless steel requires less local pressure to avoid excessive deformation, while carbon steel plates can withstand greater overall pressure to ensure good contact. By properly adjusting the electrode size, optimal pressure distribution can be achieved, improving the strength and sealing of the weld joint. Electrodes with different sizes help control the size and performance of the heat-affected zone. This can effectively improve the problem of nugget offset and improve welding quality. Figure 2 and Figure 3 By comparison, it can be clearly seen that the nugget offset is reduced and the nugget size is more reasonable. Figure 2 and Figure 3 In the diagram, A represents stainless steel, B represents mild steel, and C represents the nugget.

[0067] In summary, the present invention has conducted a large number of experiments and summarized the matching relationship between differentiated electrodes and plate thickness based on the experimental results to solve the above-mentioned difficulties in rail vehicle spot welding.

Claims

1. A resistance spot welding process for dissimilar steels based on differentiated electrode parameter optimization, characterized in that: include: Step 1: Clean and polish a carbon steel plate with a thickness of 1.5 mm to 3 mm and set aside; Step 2: Clean and polish the stainless steel plate with a thickness of 2mm to 3.5mm and set aside; Step 3: The carbon steel plate and the stainless steel plate are assembled and fixed on the fixture in an overlapping manner as workpieces. A chromium-zirconium-copper electrode is selected as the welding electrode. The electrode end face diameter is 5mm to 9mm, and the electrode end face diameter on the carbon steel plate side is smaller than the electrode end face diameter on the stainless steel plate side. Step 4: Make the electrode end face completely contact with the workpiece surface, set the electrode pressure to 7kN~8kN, and Pre-pressing treatment; Step 5: resistance spot welding is used for welding, with an electrode pressure of 7kN to 9kN, a welding current of 7kA to 9.5kA, and a welding time of 0.2s to 0.3s; Step 6: After the power is turned on, remove the current, continue to maintain the electrode pressure, and allow it to cool naturally; At this point, the resistance spot welding of carbon steel plates and stainless steel plates is completed.

2. The dissimilar steel resistance spot welding process based on differentiated electrode parameter optimization according to claim 1, characterized in that: The carbon content of the carbon steel plate is less than 0.

25.

3. The dissimilar steel resistance spot welding process based on differentiated electrode parameter optimization according to claim 1, characterized in that: The stainless steel plate is composed of an alloy having Fe as a main component and containing 10.5% by mass or more of Cr.

4. The dissimilar steel resistance spot welding process based on differentiated electrode parameter optimization according to claim 1, characterized in that: When the thicknesses of the carbon steel plate and the stainless steel plate are 1.5 mm and 2 mm respectively, the diameter of the electrode end face on the carbon steel plate side is 5 mm; the diameter of the electrode end face on the stainless steel plate side is 7 mm.

5. The dissimilar steel resistance spot welding process based on differentiated electrode parameter optimization according to claim 1, characterized in that: When the thicknesses of the carbon steel plate and the stainless steel plate are 2 mm and 2 mm respectively, the diameter of the electrode end face on the carbon steel plate side is 6 mm; the diameter of the electrode end face on the stainless steel plate side is 7 mm.

6. The dissimilar steel resistance spot welding process based on differentiated electrode parameter optimization according to claim 1, characterized in that: When the thicknesses of the carbon steel plate and the stainless steel plate are 2 mm and 2.5 mm respectively, the diameter of the electrode end face on the carbon steel plate side is 6 mm; the diameter of the electrode end face on the stainless steel plate side is 8 mm.

7. The dissimilar steel resistance spot welding process based on differentiated electrode parameter optimization according to claim 1, characterized in that: When the thicknesses of the carbon steel plate and the stainless steel plate are 2 mm and 3 mm respectively, the diameter of the electrode end face on the carbon steel plate side is 6 mm; the diameter of the electrode end face on the stainless steel plate side is 8 mm.

8. The process for optimizing differential electrode parameters for resistance spot welding of dissimilar steels according to claim 1, characterized in that: When the thicknesses of the carbon steel plate and the stainless steel plate are 3 mm and 3 mm respectively, the diameter of the electrode end face on the carbon steel plate side is 6 mm; the diameter of the electrode end face on the stainless steel plate side is 8.5 mm.

9. The process for optimizing differential electrode parameters for resistance spot welding of dissimilar steels according to claim 1, characterized in that: When the thicknesses of the carbon steel plate and the stainless steel plate are 3 mm and 3.5 mm respectively, the diameter of the electrode end face on the carbon steel plate side is 7 mm; the diameter of the electrode end face on the stainless steel plate side is 9 mm.

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