With the large-scale equipment and the widespread use of steel instead of casting, ultra-thick plates are more and more widely used in equipment manufacturing.
Flame cutting is the first step in the production and processing of structural parts. Coupled with the irreversibility of flame cutting, the CNC flame cutting process for ultra-thick plates is an essential technology for large equipment manufacturers.
1. Characteristics of flame cutting of ultra-thick plate
>> Oxygen and acetylene
Because ultra-thick plate parts are usually larger, the total amount of oxygen and acetylene consumed is much greater.
For CNC cutting of ultra-thick plates, it is necessary to ensure the continuous and stable supply of sufficient oxygen and acetylene to cut a single part at a time.
>>The size and weight of the thick plate are large
For example, 220mm × 2200mm × 8000mm thick plate, the weight is about 30t. The weight of individual parts is sometimes very large. The upper connecting rod number 9 steel plate in Figure 1 can reach more than 4t.
>> Easy to produce cutting defects
In addition to common cutting defects, ultra-thick plates can also produce defects such as impervious cutting.
>> Large material scrap loss
Due to the large loss of edging of the ultra-thick board, it is difficult to reuse the edging.
>> Large cutting distortion
The heat of the steel plate is large, which leads to the deformation of the material when the steel plate is cut, and the deviation of the zero size.
Under the tremendous stress, if the steel plate suddenly bounces off, it may also cause a safety accident.
Therefore, it is necessary to prevent the quality and safety problems caused by cutting deformation when formulating the cutting process.
2. Ultra-thick plate cutting section is prone to quality defects
(1) Upper edge cutting defects
The upper edge collapses or drops of molten bean string, and the upper edge of the slit melts too quickly, causing the rounded corners to collapse.
First, the steel surface has thick refractory scale;
Second, the cutting speed is too slow and the preheating flame is too strong;
Third is that the height between the cutting nozzle and the work piece is too high or too low, the size of the cutting nozzle used is too large, and the oxygen in the flame is excessive. As shown in picture 2.
(2) Poor flatness of the cutting surface
① There is a concave defect under the edge of the cutting section (see in Figure 3).
At the same time, the upper edge has different degrees of melting collapse.
This is because the cutting oxygen pressure is too high, or the height between the cutting nozzle and the work piece is too large, the cutting nozzle is clogged with debris, causing the wind line to be interfered.
②The roughness of the cutting section is too large. This is because the cutting speed is dragged too fast or there are impurities in the steel plate that affect the forming. As shown in Figure 3.
(3) Poor verticality
①The cutting seam is wide at the top and narrow at the bottom or narrow at the top and wide at the bottom, because the cutting speed is too fast or too slow.
The cutting nozzle is blocked by debris, which interferes with the wind line.
Cutting oxygen is too small or too large, resulting in insufficient and excessive metal burning.
②The oblique angle is caused by the cutting torch is not perpendicular to the work piece surface or the wind line is not correct.
(4) Defects in lower edge cutting
① It is depressed near the lower edge and the lower edge melts into rounded corners.
The reason is that the cutting speed is too fast, the cutting nozzle is blocked or damaged, and the air line is blocked and deteriorated.
② The slag is difficult to remove on the cut surface or the lower edge.
The reasons are:
the cutting speed is too fast or too slow, the cutting 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 height between the cutting nozzle and the work piece is too large, the preheating flame is too strong;
if the content of the alloy is too high, slag will be caused on the cross section and bottom edge (see Figure 4).
Micro-cracks appeared in the cutting section or heat affected zone.
The reason is that the carbon equivalent of the steel plate is too high, and the crack sensitivity is high, but proper preheating and slow cooling measures are not adopted.
Due to the local heating of the steel plate during cutting, the material shifting deformation occurs during the cutting of the steel plate, thereby causing dimensional deviation of the cut parts and affecting the quality of the product. As shown in Figure 5.
3. Cutting process
(1) Cutting gas supply system
In order to ensure the continuous and stable supply of oxygen and acetylene gas, multiple bottles of gas can be used in parallel to ensure the stable and continuous supply of acetylene gas.
Make a parallel gas supply package (see in Figure 6). A φ100mm steel pipe is used as the air bag, and both ends are welded with steel plates.
Six intake pipes and one air outlet pipe are made on the steel pipe (pay attention to the welding quality and ensure that the air bag does not leak).
Add a gas-tight ball valve and a connecting device to each air inlet and outlet.
(2) Cutting support frame
Due to the large size and quality of the thick plate, the maximum weight is 30t, and the size and weight of the individual parts are large, the maximum weight is 4t, so the original CNC cutting machine support frame cannot meet the cutting requirements because of the insufficient support of the slats (see Figure 7a).
In addition, in order to ensure the stable support of the support frame, the support frame needs to be modified.
After analysis, research and discussion, it was decided to use waste H-shaped steel as the thick steel plate support frame.
(A) Support frame before transformation
(b) Support frame after transformation
(3) Cutting program optimization
First, lead-out point processing is introduced.
The most difficult thing to ensure when cutting an ultra-thick board (up to 220mm thick) is the quality of the cut, especially the position of the lead-in and lead-out points of the part, which often results in cutting defects.
As shown in Figures 8a and 8b. Thick plate cutting points are often not vertical.
When the cutting point coincides with the introduction point, if the cutting line turns at this time, the root will not be cut.
Defects are caused by fracture caused by the gravity of the parts.
In order to prevent the occurrence of similar defects, it can be avoided by optimizing the lead-in and lead-out in the cutting procedure.
Secondly, if you do not pay attention to the direction during the cutting process, it will cause the parts deforming.
That is, the part is squeezed away by the expansion force, resulting in the inconsistency of the part size and the program size.
In order to solve this problem, according to our analysis: When the steel plate is cut, the light weight is due to the small pressure, which causes the friction with the support frame to be small and is squeezed away by the expansion force.
The mass weight is due to the high pressure, which causes the friction force with the support frame to be large and not squeezed away by the expansion force.
You must pay attention to this when writing the program. When cutting, the part should be connected to a heavy piece as much as possible.
According to this principle, the discharge, cutting sequence and cutting direction of the upper connecting rod are shown in Figure 9.
Finally, the layout optimizes the size, which can save greater costs.
When designing the program, more time can be considered in the optimization of the layout.
It is best to use the remaining leftover material, and two or more people will interactively check the size of the part and generate the program after confirming that it is correct.
The thicker the part to be cut, the torch model, cutting nozzle number, and oxygen pressure should be increased.
Oxygen pressure and cutting piece thickness, cutting torch model, selection according to field equipment and cutting experience, ultra-thick plate cutting parameters are shown in the attached table.
Ultra-thick plate oxyacetylene flame cutting parameters
|Thickness||Cutting nozzle diameter||Oxygen pressure||Acetylene pressure||Preheat time||Cutting speed||Gas flow|
(4) Cutting performing
Once the steel plate is cut, it can only succeed once.
Try cutting at the discarded corners of the steel plate, adjust the cutting air line, and ensure that the cutting section does not have the above-mentioned defects.
You need to follow up on the spot during the cutting process and find the problem to deal with in time.
Through sufficient preparation and strictly formulated cutting process, the ultra-thick board cutting has achieved one-time success, the product cut quality and appearance have reached the process requirements, and qualified product parts are cut (see in Figure 10).
The production process of flame-cutting ultra-thick steel plates under the existing equipment has been opened, which laid a technical support for the production of similar products.
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