In the machinery manufacturing industry, the production of welded structural parts accounts for a large proportion. The quality of structural parts directly affects product quality, especially appearance quality. The first step to improve the quality of welded structural parts is to improve the cutting quality of plates. In plate cutting, flame gas cutting is one of the main cutting methods widely used by enterprises. How to improve the quality of CNC flame gas cutting surface and avoid the occurrence of cutting defects is the most concerning issue of each enterprise. The following is a preliminary discussion and analysis of the causes and solutions of various cutting surface quality defects based on the cutting situation of our company over the years.
Quality defects and cause analysis of the cutting surface
In the actual production process of CNC flame cutting, various quality problems often occur, which can be summarized as the following types of quality defects: edge defects, cutting section defects, slagging, cracks, etc. There are many reasons for these quality defects. If the oxygen purity is guaranteed to be normal and the equipment is operating normally, the reasons for the quality defects of CNC flame cutting are mainly manifested in the following aspects: cutting torch, cutting nozzle, steel quality, steel plate material, etc. The following is a specific analysis based on the specific situation.
The upper edge cutting quality defect
This is a quality defect caused by melting.
(1) The upper edge collapses: the edge melts too fast, causing the rounded corners to collapse. Causes: the cutting speed is too slow, the preheating flame is too strong; the height between the cutting nozzle and the workpiece is too high or too low; the nozzle size used is too large, and the oxygen in the flame is excessive.
(2) A string of drop-shaped molten beans (see Figure 1), which is represented by a string of drop-shaped molten beans formed on the upper edge of the cut. Causes: the surface of the steel plate is corroded or has oxide scale; the height between the cutting nozzle and the steel plate is too small, the preheating flame is too strong; the height between the cutting nozzle and the steel plate is too large.
(3) The upper edge collapses and presents the shape of an eave (see Figure 2), which is shown as an eave-like protruding edge on the upper edge of the cut. Causes: The preheating flame is too strong; the height between the cutting nozzle and the steel plate is too low; the cutting speed is too slow, the height between the cutting nozzle and the workpiece is too large, the nozzle number used is too large, and the oxygen in the preheating flame is excessive.
(4) There is dross on the upper edge of the cut section: it is manifested as depression and dross on the upper edge of the cut. Causes: The height between the cutting nozzle and the workpiece is too large, the cutting oxygen pressure is too high; the preheating flame is too strong.
The cutting section is uneven. Poor flatness
(1) There is a concave defect under the edge of the cutting section: it is manifested as a depression at the edge of the cutting section, and at the same time, the upper edge has different degrees of melting and collapse. Causes: The cutting oxygen pressure is too high; the height between the cutting nozzle and the workpiece is too large; the cutting nozzle is blocked by debris, which causes the wind line to be disturbed and deformed.
(2) The slit shrinks from top to bottom: the slit is wide at the top and narrows at the bottom. Reason: The cutting speed is too fast; the height between the cutting nozzle and the workpiece is too large, and the cutting nozzle is blocked by debris, which causes the wind line to be disturbed and deformed.
(3) The slit is narrow at the top and wide at the bottom: the slit is narrow at the top and wide at the bottom, forming a trumpet shape. Causes: The cutting speed is too fast and the cutting oxygen pressure is too high; the cutting nozzle number is too large, which makes the cutting oxygen flow too large; the height between the cutting nozzle and the workpiece is too large;
(4) Concavity in the cutting section: It is manifested as a depression in the entire cutting section, especially in the middle part. Causes: cutting speed is too fast; the cutting nozzle used is too small, the cutting pressure is too low, the cutting nozzle is blocked or damaged; the cutting oxygen pressure is too high, and the wind line is blocked and deteriorated.
(5) The cut section shows a large wave shape: the cut section is uneven, showing a larger wave shape. Cause: The cutting speed is too fast; the cutting oxygen pressure is too low, the cutting nozzle is blocked or damaged, and the wind line is bad; the cutting nozzle number used is too large.
(6) Angle deviation in the vertical direction of the incision: the incision is not vertical and there is a bevel. Cause: The cutting torch is not perpendicular to the workpiece surface; the wind line is not right.
(7) The lower edge of the incision is rounded: the lower edge of the incision is melted to varying degrees and becomes rounded. Causes: The cutting nozzle is blocked or damaged, which makes the wind line bad; the cutting speed is too fast, and the cutting oxygen pressure is too high.
(8) The lower part of the incision is recessed and the lower edge is rounded: it appears as a recess near the lower edge and the lower edge melts into a rounded corner. Cause: The cutting speed is too fast; the cutting nozzle is blocked or damaged, and the wind line is blocked and deteriorated.
Roughness defects of cutting section
The roughness of the cut section directly affects the processing quality of the subsequent process, and the roughness of the cut section is related to the advanced amount and depth of the cut pattern.
(1) Excessive drag after cutting section: It is manifested as the backward deviation of the cutting section cut pattern, and at the same time, different degrees of depression appear with the size of the deviation. Causes: The cutting speed is too fast; the cutting nozzle used is too small, the cutting oxygen flow is too small, and the cutting oxygen pressure is too low; the height between the cutting nozzle and the workpiece is too large.
(2) In the half part of the cutting section, there is a cutting pattern advance: a kind of performance is that a certain degree of cutting pattern advance is formed near the upper edge (see Figure 3(a)) o Cause: cutting torch and cutting The direction is not vertical, the cutting nozzle is blocked or damaged; the wind line is blocked and becomes bad. Another manifestation is that the cutting edge is too large near the lower edge of the cutting section (see Figure 3(b)). Cause: The cutting nozzle is blocked or damaged, and the wind line is blocked and deteriorated; the cutting torch is not vertical or there is a problem with the cutting nozzle, which makes the wind line not straight or inclined.
Dregs
The main types of dross that are difficult to remove on the cutting section or lower edge are:
(1) Dross on the lower edge (see Figure 4): It is shown as continuous dross on the lower edge of the cut section. Hiraoka: The cutting speed is too fast or too slow, the nozzle number used is too small, and the cutting oxygen pressure is too low; there is excess gas in the preheating flame, and the surface of the steel plate is corroded or dirty; the gap between the cutting nozzle and the workpiece The height is too large and the preheating flame is too strong.
(2) Dross generated on the cut section: it is manifested as dregs on the cut section, especially in the lower half. Cause: The alloy content is too high.
Crack
It is manifested as visible cracks on the cut section, or pulsating cracks in the interior near the cut section, or only cracks visible on the cross-section. Cause: The carbon content or alloy content is too high. When the preheating cutting method is used, the preheating temperature of the workpiece is not enough, the cooling time of the workpiece is too fast, and the material is hardened by cold work.
Solutions for cutting surface quality defects
Based on the above analysis, it can be seen that in actual production, the cutting nozzle number, cutting oxygen pressure, acetylene or propane pressure, cutting speed and other cutting parameters should be correctly selected according to the plate thickness. Please refer to Table 1 for specific parameter selection; to adjust the cutting nozzle correctly The distance to the surface of the workpiece and the inclination angle of the cutting direction; to keep the cutting torch clean and smooth, and there should be no splashes of iron oxide slag sticking to the nozzle head, to effectively improve the quality of the gas cutting surface.
Oxygen
| Table 1 GK3 cutting nozzle cutting performance and basic parameters table | |||||||
|---|---|---|---|---|---|---|---|
| Cut nozzle number | Diameter of cutting mouth throat (mm) |
Cutting thickness (mm) |
Cutting speed (mm/min) |
Propane pressure (Mpa) |
Cutting oxygen pressure (Mpa) |
Preheating oxygen pressure (Mpa) |
Cutting oxygen consumption (m³/h) |
| 1 | 0.6 | 5-10 | 700-500 | ＞0.03 | 0.4~0.5 | 0.39 | 1.25 |
| 2 | 0.8 | 10-20 | 600-380 | ＞0.03 | 0.5~0.6 | 0.47 | 2.23 |
| 3 | 1.0 | 20-40 | 500-350 | ＞0.03 | 0.5~0.6 | 0.57 | 3.48 |
| 4 | 1.25 | 40-60 | 420-300 | ＞0.03 | 0.6~0.7 | 0.68 | 5.44 |
| 5 | 1.5 | 60-100 | 320-200 | ＞0.03 | 0.7~0.8 | 0.97 | 7.84 |
| 6 | 1.75 | 100-150 | 260-140 | ＞0.04 | 0.8~1.0 | 1.36 | 10.68 |
| 7 | 2.0 | 150-180 | 180-130 | ＞0.04 | 1.0~1.2 | 1.60 | 13.69 |
The oxygen used for cutting steel must have higher purity, generally requiring a purity of 99.5% or more. The oxygen cutting pressure and preheating pressure are determined by the type of cutting nozzle used and the thickness of the steel plate to be cut (see Table 1). When oxy propane cutting, the oxidizing flame is used at the beginning of the preheating flame to shorten the preheating time and cut normally. When using a neutral flame, the oxy propane cutting speed should be slightly lower. When cutting thick plates, the cutting speed is similar to that of oxyacetylene cutting. When cutting straight, choose an appropriate back angle of the cutting nozzle to improve the cutting speed and cutting quality. Also, the energy of the preheating flame is closely related to the cutting speed and cutting quality. As the thickness of the workpiece to be cut increases and the cutting speed increases, the energy of the flame should also increase, but not too strong. Especially when cutting thick plates, the reaction heat generated by metal combustion increases, which strengthens the cutting The preheating ability of the pointed front, at this time, the too strong preheating flame will seriously melt the upper edge of the incision. If the preheating flame is too weak, the steel plate will not get enough energy, which will reduce the cutting speed and even interrupt the cutting process. Therefore, the relationship between the strength of the preheating flame and the cutting speed is mutually restricted.
Cutting speed
The cutting speed directly affects the stability of the cutting process and the quality of the cutting section. In actual production, the cutting speed should be adjusted according to the performance parameters of the cutting nozzle used, gas type and purity, steel plate material, and thickness (see Table 1). When the fire bouquet is parallel to the cutting oxygen flow, the cutting speed is considered constant. Generally, it is judged by observing the characteristics of the slag sprayed from the incision and listening to the sound produced during cutting to adjust the appropriate cutting speed.
Cutting nozzle
For steel plates of different thicknesses, use cutting nozzles with different parameters and adjust the corresponding height. To ensure a high-quality cut, the height of the cutting nozzle to the surface of the workpiece must be kept basically the same throughout the cutting process. Under normal circumstances, pay attention to the following points: ①Determined according to the length of the preheating flame and the thickness of the cut piece. Generally, the end of the flame core is 3~5mm away from the cut piece. Too close will cause the carburized to melt the edge of the cut; ②The cut piece is thicker, The spacing is appropriately reduced to prevent the edge of the cut from melting; ③The thickness of the cut piece is adjusted by the angle. The spacing is appropriately increased to reduce the thickness of the hardened layer.
The size of the inclination angle of the cutting nozzle causes the difference in the trailing amount, which affects the cutting quality. Generally, the cutting nozzle should be perpendicular to the surface of the workpiece. When cutting in a straight line, when the thickness of the workpiece is less than 30mm, the cutting nozzle can tilt backward in the cutting direction (back tilt angle) 20°~30°, and it can be increased to 45° below 18mm. , To reduce the trailing drag and increase the cutting speed; when the thickness of the workpiece is greater than 30mm, the inclination angle before cutting is 5°~10°, and it is perpendicular to the surface of the workpiece after cutting through; when the curve is cut mechanically and manually, the cutting nozzle is generally Pieces are vertical.
Other
Remove the oxide scale produced on the steel plate, avoid increasing the preheating time and reducing the cutting speed; keep the cutting torch clean and the inner hole of the cutting nozzle smooth, and there should be no splashes of iron oxide slag sticking to the nozzle head; the carbon content and alloy content of the use are qualified of plates.
In actual production, many reasons affect the quality of the cutting surface. It is necessary to conduct a comprehensive analysis from the above factors, and correctly select cutting parameters such as the cutting nozzle number, cutting oxygen pressure, acetylene or propane pressure, and cutting speed according to the thickness of the plate to avoid cutting. The occurrence of quality defects effectively improves the quality of the gas cutting surface. If you want to have a more comprehensive understanding of the factors that affect the quality of flame cutting, you can refer to the article "What are the factors that affect the quality of CNC flame cutting?".
