Define high-strength steel from the perspective of plasma cutting, and elaborate on the application characteristics of plasma cutting technology on high-strength steel. The plasma cutting process is compared with three other widely used cutting processes, and the influence of the next welding process in these cutting processes is compared. Analyze the relationship between the air plasma cutting process HAZ effect, cutting quality and cutting cost, and describe the equipment requirements for plasma cutting of high-strength steel plasma cutting power, height adjustment and CNC numerical control system.
The concept of alloy high-strength steel
Low-alloy high-strength steel (HSLA) or microalloyed steel is specially designed to provide better mechanical properties and/or greater environmental corrosion resistance than conventional carbon steel under normal conditions because it is designed to meet specific requirements. Mechanical properties require only non-chemical composition. The carbon content of HSLA is relatively low, W(C)=0.05%~0.25%. The purpose is to produce sufficient forge ability and weldability. The manganese content is at most 2.0%. There is also a small amount of chromium, nickel, molybdenum, copper, nitrogen, vanadium, niobium, titanium and zirconium components. HSLA materials are often used in cars, trucks, cranes, ships, bridges, roller coasters, and other structures that specialize in handling a large amount of stress or require a better strength-to-gravity ratio. Under the same strength, HSLA steel is usually 20% to 30% lighter than carbon steel.
HSLA can be divided into the following categories:
(1) Weathering steel. It is designated to exhibit excellent environmental corrosion resistance.
(2) Controlled rolling steel. The hot rolling process is carried out according to the predetermined rolling plan, and the purpose is to develop a high formability austenite structure so that it can be transformed into an extremely fine equiaxed ferrite structure during cooling.
(3) The pearlite reduction in steel. The hardness is enhanced by the fine-grained ferrite and the precipitation hardening phase, but because the carbon content is very low, there is almost or no pearlite in the microstructure.
(4) Microalloyed steel. By adding a small amount of niobium, vanadium and (or) titanium, the particle size and (or) precipitation hardening are improved.
(5) Acicular ferritic steel. The very low carbon content has sufficient hardenability and can be transformed into a very fine high-strength acicular ferrite structure during the cooling process instead of the conventional polygonal ferrite structure.
(6) Dual-phase steel. After being processed into a ferrite microstructure containing high-carbon martensite uniformly distributed in small areas, the product has both low yield strength and high work hardening rate, thereby providing high-strength steel with excellent formability.
How to cut HSLA steel plate?
There are four practical processes for cutting HSLA steel plates.
(1) Plasma cutting
The plasma cutting process can cut all conductive materials, such as steel plates, stainless steel plates, aluminium plates, etc. The thickness of carbon steel is 0.5-50mm, and the thickness of stainless steel can be up to 160 mm. In the plasma cutting process, air, O2, N2, Ar/H2 or other mixed gases can be used. The gas is blown from the cutting nozzle at high speed, and an arc is formed on the surface of the workpiece through the cutting nozzle electrode, and part of the gas is converted into plasma. The plasma gas at a temperature of up to 30 000°C melts the cut metal and moves quickly to blow the molten metal water away from the cutting opening.
(2) Flame cutting
In the flame cutting process, depending on the gas, the cutting torch can release up to 3500℃ to heat the low carbon steel (other metals cannot be cut) to the melting point temperature, and then a stream of oxygen is aimed at the metal to burn the metal into oxidation The material flows out from the incision in the form of slag.
(3) Laser cutting
Laser cutting uses light energy with a wavelength of 488-10 600 nm and different types of gases (such as N2, O2, air) to cut all kinds of metal materials and non-metal materials such as wood, glass, and fiber. Such as carbon dioxide laser, solid-state (YAG) laser, fiber laser and semiconductor laser.
The power consumption of laser cutting is about twice that of plasma cutting, and high-pressure and high-capacity N2 is required to cut non-ferrous metals (gas usage is about 35 times that of plasma cutting), and the cost of cutting medium and thick plates is high. The speed of laser cutting thin plates is very high, but when the thickness of the plate is greater than 10 mm, the cutting speed is greatly reduced, and the perforation time is prolonged.
(4) Waterjet cutting
Waterjet is an industrial cutting tool that can cut a variety of materials. It uses extremely high-pressure water to cut softer materials, such as wood or rubber, or uses a mixture of water and abrasives to cut harder materials, such as metal or granite stone. The waterjet cutting process is usually used in the manufacture of mechanical parts. When the material to be cut is very sensitive to the high temperatures generated in other cutting processes, water jets are the best cutting method. Waterjet cutting is widely used in cutting, forming and reaming operations in various industries, such as mining and aerospace.
The disadvantage of the waterjet cutting process is that the cutting speed of the metal is extremely slow, and the abrasive material needs to be continuously added to the high-pressure jet, so the output is low, the abrasive cost is higher, and the hourly cost is very high. Plasma cutting can obtain cutting quality similar to waterjet cutting, but the cutting speed is faster, and the production cost is extremely low. Therefore, it is an excellent alternative to water jet cutting in many cases.
Table 1 shows a summary of the above four cutting processes.
| Method | Cutting quality | Advantages and disadvantages | Thickness/mm |
|---|---|---|---|
| Waterjet | High cutting quality High precision Medium repetitive processing ability |
Slow Medium equipment investment cost High operating cost |
1~50 |
| Oxy-fuel | Low cutting quality Low accuracy High repetitive processing ability |
Slow Low equipment investment cost High operating cost |
30~300 |
| Plasma | Medium cut quality Medium precision Low repetitive processing ability |
high speed Low equipment investment cost Low operating cost |
5~50 |
| Laser | High cutting quality High precision High repetitive processing ability |
high speed Low equipment investment cost High operating cost |
1~12 |
Table 2 is a summary ranking of the four cutting process performance, one is the best, and four is the worst. The heat-affected zone (HAZ) is a key indicator in the mass application of HSLA because HAZ must be eliminated in many cases to achieve the best solderability. Although the waterjet cutting process can provide the best performance in the heat-affected zone (no heat-affected zone), its ultra-slow cutting speed for thick plates undoubtedly limits the practical use of this process. Laser cutting is second only to waterjet cutting and is the preferred process for thin sheet cutting. However, due to its high initial investment, many potential users will not choose laser cutting. Therefore, the plasma cutting process has become the first choice for HSLA cutting due to its good performance and flexibility.
| Project | Plasma | Oxy-fuel | Laser | Waterjet |
|---|---|---|---|---|
| Heat Affected Zone (HAZ) | 3 | 4 | 2 | 1 |
| Cutting speed (plates within 6mm thickness) | 2 | 3 | 1 | 4 |
| Cutting speed (board thickness above 6mm) | 1 | 3 | 2 | 4 |
| Initial investment cost | 2 | 1 | 4 | 3 |
| Cutting cost | 1 | 2 | 3 | 4 |
| Precision | 3 | 4 | 1 | 2 |
| Material thickness | 2 | 1 | 4 | 3 |
System requirements for plasma cutting HSLA
The plasma cutting system consists of a power supply, gas control box, cutting torch and CNC numerical control system.
1. Plasma power system
The plasma power supply is designed according to the principle of plasma arc generation. The plasma power supply uses compressed air as the working gas and high-temperature and high-speed plasma arc as the heat source to partially melt the metal to be cut, and at the same time blow away the molten metal with a high-speed airflow to form a narrow slit to achieve the purpose of cutting. The plasma power supply design uses innovative inverter technology, serial input/output signal connection technology to achieve unlimited communication between the plasma power supply, CNC control system and gas control, and the overall efficiency of the system is greatly improved. Microprocessor controlled equipment can provide detailed operating information in real-time. In a highly integrated plasma cutting system, this information can be directly displayed in the CNC control system to facilitate users to accurately understand the current system operating conditions in real-time.
2. Cutting torch and wearing parts
The cutting torch is the key part for generating plasma arc and cutting. The torch electrode generally adopts an indirect water-cooled tungsten electrode. The working gas can be O2, N2, air or Ar/H2 mixture, and the protective gas can be O2, N2, Ar or water. Modern cutting torches have stronger piercing capabilities, and the high-density arc produced greatly improves the cutting quality, achieving the effects of small slits, flat cuts, and small material deformation. The new fast cardholder technology makes the replacement of wearing parts convenient and achieves the shortest time in history, greatly reducing the downtime caused by replacing worn parts.
3. Height adjustment control
The height adjustment control is to use the basic constant current characteristics of the plasma power supply to measure the torch height change in the plasma cutting process by detecting the change of the plasma arc voltage and to realize the height control of the cutting torch. The basic functions usually have initial automatic positioning, start-up piercing and arc breaking lifting functions, cutting torch anti-collision, given and actual arc voltage display monitoring, manual and automatic operation, etc.
4. CNC control system
In order to achieve high-speed, high-precision surface contour finishing, it is necessary to improve the interpretation and processing capabilities of the tiny contour line segments and the performance of the servo drive. The CNC system must have a sufficiently high data processing speed to accurately control the walking trajectory of the cutting table to ensure high precision, repeatability, speed and acceleration, so as to achieve the best HSLA steel plate cutting quality.
In the plasma cutting process, O2 or air can be used as plasma gas for cutting carbon steel. Although air is cheaper than O2, it has obvious disadvantages compared to using O2. Using air as the plasma gas, the concentration of nitrate on the cutting surface is very high, so secondary treatment before welding is required, which increases the workload and cost. If O2 is used, in most cases, no secondary treatment is required. When cutting HSLA sheets, the heat-affected zone needs to be removed in most cases.
Another advantage of a high-precision plasma cutting system is that the highly constrained plasma arc results in a smaller slit width and a smaller slope on the cutting surface, which will result in a smaller heat-affected zone. Equipped with a professional height controller and CNC numerical control system can achieve the best cutting effect. Especially in high wear-resistant steel and ballistic steel cutting applications, height control is a key element to avoid damaging the consumable parts of the plasma cutting torch during arc starting. The height control can realize functions such as perforation, retraction, and lifting delay to optimize perforation performance. If the CNC numerical control system is seamlessly integrated into the plasma cutting system, the cutting performance can be further improved by improving the in and out of the lead and optimizing the quality of hole cutting.
In general, an integrated plasma system is the best choice for cutting HSLA (low alloy high strength steel) in terms of performance, cutting cost, and user-friendliness. Although other cutting processes can provide better cutting performance, they have different disadvantages, such as low flexibility, slow cutting speed, or high initial investment cost.
