According to the sequence of spraying and remelting, the spray welding technology of oxygen acetylene flame can be divided into "one step" and "two steps" ".
(1) "one step" spray welding process
The so-called "one step" spray welding, spraying and remelting using the same spray gun, in a process to complete, that is, the edge of the spray edge melt.
1) process
The working process of the "one step" spray welding is generally carried out according to the following procedures: Workpiece surface preparation - pre - spray protection - pre - spray protection layer from the edge of the local heating - edge spray edge melt (by a certain line of the edge of the spray melt forward, until complete) - slow cooling of the workpiece.
Pre spray protection layer refers to the whole surface of the workpiece is prepared on the surface of a layer of mm 0.1~0.2 alloy powder coating, to protect the workpiece, to avoid the surface of the workpiece during the spray welding oxidation.
2) process control points
The key of process control is to adjust the flame and to control the heating temperature of the workpiece. Its operating points are as follows:
Preheat temperature. Flame spray welding preheating flame should adopt neutral flame or micro carbide flame, the preheating temperature is often higher than the spray preheating temperature, generally 250~350 degrees Celsius, depending on the specific material.
Control of flame and temperature at the beginning of local heating. Should adopt soft neutral flame, the distance from the gun down to the mm 20~30, the angle of 60~80 degrees, the workpiece surface heated to 500~600 degrees Celsius (dark red), or the beginning of the protective layer of wetting, can start edge spray edge melt. In the process of spray welding, the distance between the spray gun and the workpiece surface is different because of the different models of the spray gun. Changes in general use SPH-2/h spray welding distance of 20~50 between mm, heating and melting at about 20 mm, when the powder is about 50 mm.
Temperature and speed control of edge spray melting. The use of gap type spraying and heating, the local area to achieve the required thickness and melting, showing "mirror", and then moving forward evenly and slowly moving the spray gun to the next local area of the spray welding. The whole process of spray welding is sprayed and fused alternately, periodically, muzzle velocity and forward along the surface of spray welding powder feeding rate, should be based on the powder melting and spraying layer thickness, ensure uniform, spray penetration.
The cooling. Because of self fluxing alloy linear expansion coefficient, poor plasticity, thus it is easy to crack during cooling. Especially when the spray welding layer is thicker, or the part is larger, the matrix is better in the crack when the matrix is hardened. Therefore, the parts should be evenly cooled after spray welding, if necessary, it should be treated by isothermal annealing.
Spray welding parts can be spray welding layer of the post processing.
Characteristics of "one step" spray welding process:
The spraying and remelting are alternately or simultaneously, the powder is sprayed directly into the molten pool, so the powder size small, scattered.
The heat input of the workpiece is low, the effect on the microstructure of the base metal is small, and the deformation of the workpiece is small.
The thickness of spray welding layer can be changed in a larger range, generally mm 0.8~1.2, rarely more than 2 mm. The process is simple, flexible operation.
Its disadvantages are generally used manual operation, poor working conditions, low productivity, the uniformity and quality of spray welding layer with the operator proficiency and different, not easy to stable. The forming of spray welding layer with mechanical operation is better, and the quality is stable.
Application scope of "one step method". Protection of small parts of the surface protection and repair; medium or large parts of the surface protection of the surface protection and treatment; some parts damaged or even broken, can be partially repaired and connected.
(2) "two step" spray welding process
The so-called "two steps", is the coating and remelting of two processes completed, the first coating coating for remelting. 1) process
"Two - step" spray welding process generally follow the following procedures:
Preparation of workpiece surface -- preheating of workpiece -- spraying protection layer -- spraying -- workpiece temperature rising -- remelting -- workpiece temperature rising or cooling -- two times of spraying -- two remelting -- slow cooling of workpiece.
"Two - step" spray welding is the key process of spraying and remelting, and the control of the temperature of the workpiece in the whole process.
2) spraying
It is suitable to use high power dual purpose gun (SPH-E type gun or SPHT-6/h, SPHT-8/h) or high power spraying gun (SPH-E2000 type spray gun) for spraying, which makes the high efficiency of spraying.
To use neutral flame spraying, spraying distance is 100~200 mm. Spray gun model is different, spraying distance is also different. The spraying distance of the SPH-4/h spray gun is about mm 100~150, and the spraying distance of the SPH-6/h spray gun is 200 mm. In order to ensure the coating thickness uniformity and good bonding strength, the thickness of each powder should be mm 0.1~0.2. To obtain thicker coating, can be used repeatedly for dusting. The thickness of the general spray welding layer is not more than 1.5 mm. Tungsten carbide containing alloy powder, due to the remelting is more difficult, the coating should not be too thick, generally not more than 0.5 mm. The wear depth of more than 1.5 mm can be achieved through several parts, dusting and remelting, the spray welding process, the workpiece temperature should be maintained at about 350. If the temperature drops rapidly, the coating is easy to crack and spalling. The workpiece is larger, the spray welding time is longer, can use other methods to assist heating. In the spraying process of spray gun is matched with the spraying parameters to move uniformly, so as to ensure the whole thickness of the coating was uniform in the plane parts of spraying nozzle and the spray angle, the best plane between 60 DEG ~80 deg.
3) remelting
In the "two step" spray welding process, the remelting is a key process. The operation is the remelting coating is heated to the temperature range between the solid and liquid phases, the original relatively porous coating into the cladding layer continuous and compact, complete diffusion bonding or micro metallurgical bonding with the substrate.
In the temperature of the workpiece after remelting powder has not been reduced immediately, so that one can reduce the heat loss, on the other hand can be reduced due to repeated heating and cooling to produce a coating shell. According to the heating method, it has many kinds of forms, such as induction remelting, remelting in the furnace and flame remelting. Laser remelting technology has been developed in recent years.
The heating mode of high frequency induction heating remelting is similar to that of high frequency quenching. It has the characteristics of fast speed and little influence on the thermal effect of the workpiece.
In the furnace, it is suitable for the large workpiece which is difficult to be re melted by other methods. Remelting should strictly control the furnace temperature control in general alloy solidus above 10 DEG C, the maximum not more than 30 degrees Celsius above the solidus alloy. In a Ni based self fluxing alloy powder for example, the solid point of 1035 DEG C, the liquid phase point is 1280 DEG C, then the temperature should be controlled at 1045~1065 DEG C. In this temperature range, the alloy is about 60%~80% liquid phase and 20%~40% solid phase, at this time there is enough viscosity, not to flow.
In the furnace, the workpiece is heated slowly. In general, remelting is done in a protective atmosphere, which can protect the surface of the workpiece to avoid high temperature oxidation.
Flame remelting: simple equipment, flexible use, the most widely used. In the flame remelting, the general use of remelting gun, the operation should pay attention to the following points:
Remelting gun should have enough flame energy, one is not enough, the need to increase one or two auxiliary gun. Flame remelting, due to the protection of flame gas is not good, so we should pay special attention to the 700 degrees from degrees to remelting time should not exceed 20. If you stay too long in the above 700 degrees, due to high temperature oxidation coating, more than self fluxing alloy deoxidation ability, the powder will be "slag" self fluxing bad, even remelting cannot continue;
Remelting flame should adopt neutral flame, the nozzle is not too close to the workpiece, generally about 40 mm, thin pieces should be a little far, thick parts can be a little closer;
Remelting can not be at the end of the workpiece or the edge of the start, from the end of the end or edge of about 30 mm at the beginning;
The remelting process, the temperature control is very important, the remelting temperature and the powder melting point is generally 1000 degrees Celsius, with experience in control, when the welding surface 'mirror' reflection, found that the powder layer has been melting, remelting temperature has been reached. If the remelting temperature is too low, the slag is not easy to surface, to achieve a good combination of spraying layer and substrate is not easy to crack, cooling, uneven hardness and low. The temperature is too high, the melting layer overflow, appear wave shape, affect the surface level. Laser remelting, laser energy density, can be used as the heat source of remelting, laser remelting of the workpiece matrix heat affected small, deformation of the workpiece is small, can obtain excellent performance of the coating. But there is a high power laser.
After the remelting process for dusting. Remelted spraying layer thickness will shrink, the shrinkage of powder with different shrinkage, generally is 25%, for example, found after remelting allowance not required to fill the spraying, should pay attention to the preheating temperature. There are on the surface of the nickel oxide instead of iron oxide, if the preheating temperature is 250 DEG C or slightly higher, the temperature of boron, silicon powder in this can not make the nickel oxide reduction, to preheat temperature increased to 670~700 DEG C, then spray, to be successful.
Cooling and spray welding layer after the processing: after the end of the remelting, should allow the workpiece to slow cooling. Easy to quench the matrix, in the cooling process, should be prevented due to the organization phase transformation and cracking. For this kind of material, it is generally used in the isothermal annealing process. If the coefficient of linear expansion coefficient of the alloy is larger than that of the matrix, it should take the slow cooling measures, such as in the asbestos powder, or the cooling in the furnace.
Thin piece in the spray welding process is easy to produce deformation, it can take advantage of the workpiece has not been completely cooling of the thermal state of plastic correction. If the cooling is plastic, the spray welding layer is easy to crack.
4) workpiece cooling
The coating after remelting temperature is higher, due to the brittleness of the alloys, the cooling process will produce large contraction, resulting in spray welding layer crack. In particular to the special material, complex shape of the workpiece and the thickness of the spray welding layer, should take measures to slow cooling, to avoid crack.
Low carbon steel, medium carbon steel work piece, natural air cooling can. Rigid, workpiece with complicated shape, and high carbon steel, high manganese steel, hot forging die steel workpiece after remelting, available asbestos cloth wrapped, or covered with dry heating to 100 degrees above the slow cooling of asbestos powder. Can also be put into the furnace cooling. This type of work should take corresponding measures for insulation and slow cooling in the remelting process. Slow cooling at 430~480 DEG 18-8 type austenitic stainless steel in the intergranular precipitation of chromium carbide, the alloy resistance to intergranular corrosion ability, which should be placed on the workpiece after remelting 860~900 degrees around the furnace or protective atmosphere furnace for stabilizing treatment. For the air-cooled hardened tool steel or martensitic stainless steel workpiece, change from cooling of austenite into martensite, will produce the phase expansion, but also easy to produce cracks in slow cooling, so the need for the isothermal tempering in the holding furnace, to control the change of organizational structure, the furnace temperature is 700~800 DEG C, holding 8 hours then, slow cooling.
