Spraying Welding Basics You Should Know

What is spray welding?

Spray welding is to reheat the preheated self soluble alloy powder coating to 1000~1300 ℃ to melt the particles, float the slagging to the coating surface, and disperse the boride and silicide in the coating, so as to achieve a good combination between particles and the substrate surface.

The final deposit is a dense metal crystal structure and forms a metallurgical bonding layer of about 0.05~0.1mm with the matrix, with a bonding strength of about 400MPa, good impact resistance, wear resistance and corrosion resistance, and a mirror like appearance.

Spraying Welding Basics You Should Know 1

Advantages of spray welding

Compared with spray coating, spray welding coating has significant advantages.

However, due to the local temperature of the matrix after being heated up to 900 ℃ during the remelting process, greater thermal deformation will occur.

Therefore, the application scope of spray welding has certain limitations.

Parts and materials suitable for spray welding are generally:

① Vulnerable parts with high surface hardness and good wear resistance under impact load, such as sander blade, crusher tooth plate, excavator bucket tooth, etc;

② Large vulnerable parts with simple geometry, such as shafts, plungers, sliders, hydraulic cylinders, chute plates, etc;

③ Low carbon steel, medium carbon steel (less than 0.4% carbon), structural steel containing less than 3% manganese, molybdenum and vanadium, nickel chromium stainless steel, cast iron and other materials.

Self fluxing alloy powder for spray welding

Self fluxing alloy powder is an alloy with nickel, cobalt and iron as the base materials, in which appropriate boron and silicon elements are added to play the role of deoxidizing slag forming welding flux and reduce the alloy melting point, which is suitable for remelting the coating with acetylene oxygen flame.

There are many kinds of domestic self fluxing alloy powders, and nickel base alloy powders have strong corrosion resistance, oxidation resistance up to 650 ° C, and strong wear resistance;

Cobalt based alloy powder is characterized by good red hardness, which can maintain good wear resistance and corrosion resistance at 700 ℃;

The abrasive wear resistance of iron base alloy powder is superior to the other two types.

Spray welding process

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The process procedure of spray welding is basically the same as that of spraying, and the remelting procedure is added in the powder spraying procedure for the difference.

Spray welding includes one-step spray welding and two-step spray welding.

Before construction, attention shall be paid to:

① The carburizing layer or nitriding layer on the workpiece surface must be removed during pretreatment;

② The preheating temperature of the workpiece is 200~300 ℃ for ordinary carbon steel and 350~400 ℃ for heat-resistant austenitic steel.

Use neutral or weak carbon flame for preheating flame.

In addition, the thickness of the spraying layer will be reduced by about 25% after remelting, which should be taken into account when measuring in hot state after spraying.

One step spray welding.

One step method is to melt one section after spraying, spraying and melting are conducted alternately, and the same spray gun is used.

Medium and small spray welding guns can be selected.

After the workpiece is preheated, the 0.2mm protective layer shall be sprayed first, and the surface shall be sealed tightly to prevent oxidation.

The spraying and melting shall start from one end with a spraying distance of 10-30mm.

The protective layer shall be partially heated in sequence until the melting starts to wet (no flowing), and then the powder shall be sprayed again and again with the melting until the predetermined thickness is reached, the surface shows “mirror” reflection, and then expands forward until the surface is completely covered with the spraying and welding layer.

If the thickness is insufficient at one time, it can be thickened repeatedly.

One step method is applicable to small parts or small area spray welding.

Two step spray welding method.

The two-step method is to finish the spraying layer first and then remelt it.

High power spray guns are used for spraying and remelting, such as SpH-E spray and welding guns, to fully melt the alloy powder in the flame and produce a plastic deformation deposit layer on the surface of the workpiece.

Weak carbon flame shall be used for spraying iron based powder, and neutral or weak carbon flame shall be used for spraying nickel based and cobalt based powder.

The thickness of each layer of powder spraying is less than 0.2mm, and the repeated spraying reaches the remelting thickness, which can be remelted at 0.5~0.6mm generally.

If the spray welding layer is required to be thick and one remelting fails to meet the requirements, it can be sprayed and remelted several times.

Remelting is the key process of two-step method, which is carried out immediately after spraying.

The high-power soft flame of neutral flame or weak carbonization flame is used.

The spraying distance is about 20~30mm, and the angle between the flame and the surface is 60 °~75 °.

Starting from the position about 30mm away from the coating, the remelting speed is properly controlled, and the coating is heated until the “mirror” reflection of the coating appears, and then the remelting of the next part is carried out.

During remelting, it is required to prevent over melting (i.e. mirror surface cracking), coating metal flowing, or surface oxidation due to excessive local heating time.

During multi-layer remelting, the previous layer shall be cooled to about 700 ℃, and the surface slag shall be removed before secondary spraying.

The remelting should not exceed 3 times.

Cooling of the workpiece.

The workpieces of medium and low carbon steel and low alloy steel, thin welding layer and iron castings with simple shape are naturally cooled in air.

For iron castings with thick welding layer and complex shape, alloy steel castings with large content of manganese, copper and vanadium, and parts with high hardness, they should be buried in lime pits for slow cooling.

Process difference between spraying and spray welding

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The combination of spraying layer and spray welding layer with the base metal is different.

When nickel clad aluminum is heated by spraying flame beam, exothermic chemical reaction occurs, and the rust removal by sand blasting reaches Sa3, RZ>50 μm carbon steel surface forms a mechanical bonding coating with a micro metallurgical bonding bottom layer and a working layer, which also produces an “anchor hook” effect, while the bonding between the spray welding layer and the substrate is purely a metallurgical bonding coating.

Different spraying materials require the use of self fusible alloy powder for spray welding, while spraying requires low self fusibility of the powder, which is not necessarily self fusible alloy powder.

Various self fusible alloy powders can be used for both spray welding and spraying, but the spraying powder does not have self fusibility, which can only be used for spraying and cannot be used for spray welding process.

The workpiece is heated differently.

During spraying and spray welding, the preheating temperature before spraying is different, the workpiece is heated differently, and the microstructure and properties of the workpiece after spraying are also different.

The density of the coating is different, the spray welding layer is dense, and there are a few pores in the spray coating.

The ability to bear load is different.

Generally, the spray coating can withstand large area contact, and it is mostly used on lubricated working surfaces, mating surfaces and other working conditions with less stress.

However, the spray welding coating can withstand large impact force, extrusion stress or contact stress.

Application of spray welding

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(1) The workpiece surface load of various carbon steel and low alloy steel is large, especially under the impact load.

For the workpiece with the bonding strength between the coating and the substrate of 350-450N/mm2, the spray welding hardness HRC150 ≤ 65, the coating thickness from 0.3 to several millimeters, and the surface roughness of the spray welding layer after grinding can reach Ra0.4-0.1 μ M above.

(2) When used in corrosive medium, the coating shall be compact and free of pores.

(3) The original design of the workpiece surface adopts quenching, carburizing, nitriding, hard chromium plating and other processes, and the surface is required to have high hardness.

(4) The working environment of the workpiece is poor, such as strong abrasive wear, erosion wear, cavitation, etc.

(5) The oxyacetylene flame alloy powder spray welding process is suitable for surface strengthening or repair of various carbon steel and low-alloy steel parts, but some characteristics of the part materials should be noted.

When the linear expansion coefficient of the base material is greatly different from that of the alloy spray welding layer, it is less than 12 × 10-6/℃ greater than 12 × 10-6/℃, this process should be used with caution to avoid cracks.

If the content of elements with high affinity to oxygen in the base metal is more than 3%, such as tungsten and molybdenum, the total content of aluminum, magnesium, cobalt, titanium, molybdenum and other elements is more than 0.5%, or the sulfur content in steel is more, it will also bring difficulties to spray welding.

This is because these materials are easy to generate a dense and stable oxide film under the action of oxygen, which blocks the wetting effect of molten alloy on the base.

During remelting, the liquid alloy will roll down like beads like “sweat beads”.

Therefore, when using spray welding process, attention should be paid to the adaptability of this process to the sprayed base material.

(6) Metal materials that can be spray welded without special treatment:

① Carbon structural steel with carbon content ≤ 0.25%.

② Alloy structural steel with total content of Mh, Mo, V, Cr and Ni<3%.

③ 18-8 Stainless steel, nickel stainless steel, grey cast steel, malleable iron, ductile iron, low-carbon pure iron, red copper.

(7) The metal materials to be slowly cooled after spray welding at 250-375 ℃ shall be preheated.

① Carbon structural steel with carbon content>0.4%;

② Manganese, molybdenum, vanadium Alloy structural steel with total nickel content>3%;

③ Alloy structural steel with chromium content ≤ 2%;

(8) Metal materials requiring isothermal annealing after spray welding:

① Martensitic stainless steel with chromium content ≥ 13%;

② Nickel molybdenum alloy structural steel with carbon content ≥ 0.4%.

After the spray welding process is determined, the one-step or two-step spray welding process shall be selected according to the following conditions:

(1) The workpiece needs to be repaired locally, and large heat input is not allowed at the spray welding position.

For example, the one-step spray welding process should be used to repair local scars on various machine tool guides;

(2) If the workpiece surface is complex or irregular, such as sprocket, gear tooth surface, screw feeder, etc., one-step spray welding process should be used;

(3) If it is difficult to heat large workpieces as a whole, such as locomotive and tramcar wheels, one-step spray welding process should be used;

(4) Two step spray welding process should be used for general shaft parts that can rotate on the machine tool;

(5) The hardness of the obtained coating shall be as close as possible to the surface hardness of the original design.

For example, if the original design uses quenching or chemical treatment process to make the surface hardness reach HRC ≥ 55, the so-called “hard coating” powder shall be used, such as Ni15, Ni60, Fe65 or Wc composite powder;

(6) Highly worn non mating surfaces, such as impeller, shell of mud pump, shovel teeth of rock loader, spiral surface of spiral feeder, etc., shall be made of high hardness composite powder such as Ni15, Ni60, Fe65 or Wc;

(7) For workpieces that need to be processed but cannot be used on lathes and grinders, they can only be processed by hand with files and other tools, such as the repair of local scars on the guide rail surface of the machine tool, only low hardness spray welding powder such as SH, F103, Ni15, etc. can be used;

(8) Differences between spray welding process and arc surfacing:

The bonding between the spray welding layer and the matrix is solution diffusion metallurgical bonding, while the surfacing is melting metallurgical bonding.

The matrix is not melted during the spray welding process, but there is a dissolution effect between the spray welding layer and the matrix, and there is a diffusion mutual solution zone between the two.

Because the matrix is insoluble, the spray welding layer will not be diluted by the base material.

Therefore, the dilution rate is extremely low, which can ensure the good performance of the spray welding layer.

The overlay welding matrix is melted, and the dilution rate of the overlay welding layer is high.

The performance of the welding layer can only be guaranteed if the overlay welding is very thick, and it is difficult to guarantee the edges and corners of the part contour.

Common undercuts and corners collapse, while spray welding will not have such defects.

Spray welding characteristics of cast iron parts

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(1) Cast iron is an iron carbon alloy with a carbon content of more than 2%.

It is commonly used in industry, with a carbon content of 2.5~4%, a silicon content of 1~3%, and a small amount of manganese, sulfur, phosphorus, etc., of which gray cast iron is the most widely used.

Due to the high content of cast iron, low strength, sensitivity to temperature changes, and local heating during welding repair, large temperature difference and fast cooling speed, it is difficult to weld the cast iron.

The weldability of cast iron is poor, and the following problems are likely to occur during welding repair:

(2) The white microstructure is easy to appear at the welding repair part, which is hard and brittle, difficult to machine after welding, and easy to cause cracks;

(3) It is easy to produce cracks, and thermal stress cracks are easy to occur due to uneven heating and fast cooling speed during welding repair;

In addition, many impurities such as sulfur and phosphorus in the cast iron are easy to crack at the weld repair;

(4) Porosity and slag inclusion.

Because of the high carbon content and more impurities in the cast iron, and because of the fast cooling speed in the welding repair process, the gas and some oxides can not precipitate and float in time, they will form porosity or slag inclusion in the weld zone.

The one-step spray welding with oxygen acetylene flame alloy powder can satisfactorily solve the above problems;

(5) The melting point of alloy powder used in spray welding is lower than the melting point of the matrix.

During remelting, the cast iron matrix does not melt, there is no dilution problem of the spray welding layer, and there is no semi melting zone.

Therefore, correct spray welding will not produce white microstructure in the weld repair area, which is convenient for processing.

Moreover, because the matrix does not melt, it naturally controls the sulfur, phosphorus and other impurities in the matrix to melt into the spray welding layer, which is conducive to preventing cracks;

(6) The one-step spray welding has less heat input to the substrate and less heat effect on the substrate, which is conducive to reducing the thermal stress and effectively controlling the thermal stress cracks;

The heat input is small, which is unique for local spray welding repair of parts with high dimensional accuracy.

At the same time, oxygen acetylene flame heating is used, which is slower than electric welding cooling, which is also beneficial to prevent cracks and deformation;

(7) The alloy powder contains strong deoxidizing elements boron and silicon, which not only protects other elements in the powder from oxidative burning, but also reduces the oxides on the surface of the matrix by boron and silicon, preventing pores and slag inclusion;

(8) The spray welding layer is compact and flat in structure, well formed, without undercut, and can be used only with a small amount of processing, saving materials and high efficiency;

Spray welded cast iron parts are commonly used as SH · F103, nickel base alloy powder Ni15, etc.

Iron base alloy powder has high melting point, high brittleness, great influence on the matrix, and poor effect.

(9) During spray welding, the main purpose of workpiece preheating is to remove the moisture on the workpiece surface and generate certain thermal expansion to reduce the temperature difference, thus reducing the thermal stress is conducive to improving the coating bonding strength and ensuring the quality of spray welding layer.

Generally, 250-300 ℃ is used for steel, 450-500 ℃ is used for austenitic stainless steel, 350-400 ℃ is used for nickel chromium stainless steel, 250-300 ℃ is used for low alloy steel and cast iron, and the preheating temperature of general small workpieces and easily oxidized steel is lower;

The thickness of the spray welding layer is based on the thermal expansion and cold contraction characteristics of the workpiece after spraying, and the shrinkage after remelting is about 25-30%.

Therefore, when determining the thickness of the spray coating layer, in addition to considering the machining allowance and the diameter of the workpiece before spraying, the shrinkage must be taken into account.

The thickness of the spray coating is calculated.

The coating thickness before remelting=(thickness of the spray welding layer+machining allowance) ÷ (1-0.3).

Related reading: How to Weld Cast Iron?

Circumstances where spray welding process cannot be used.

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(1) Materials lower than alloy melting point, such as aluminum and its alloys, magnesium and its alloys, brass and bronze;

(2) The workpiece is a slender shaft or a very thin plate without deformation;

(3) The original design requirements of the workpiece are very high, and the metallographic structure is not allowed to change;

(4) Nickel chromium molybdenum alloy steel with high hardenability;

(5) Martensitic high chromium steel with chromium content>18%.

(PS: High precision spray welding can be carried out for long shafts with an outer diameter of more than 28mm and a length of less than 8m)

Key points of spray welding process for iron castings

Spraying Welding Basics You Should Know 7

① There are quite a few parts in vehicles and mechanical equipment that are made of cast iron, and various problems are inevitable in the process of manufacturing or use.

The oxygen acetylene flame spray welding process is not only an effective method to strengthen the cast iron, but also an ideal means to repair various defects and damages (such as sand holes, air holes, wear in use and other damages) of the castings;

② Spray welding is mostly used to repair local defects of iron castings, and the defects are different in size and depth.

It is suitable for one-step spray welding.

According to the parts of the workpiece and welding repair, small power spray guns should be used as far as possible to reduce the heat input to the substrate.

Generally, QH-1/h, QH-2/h, QH-4/h, etc. are selected;

③ During spray welding, due to the poor weldability of cast iron parts, nickel base alloy powder (Ni-B-Si series) should be preferred.

The melting point of nickel base alloy powder is generally 950~1050 ℃.

The matrix will not be melted during remelting.

At the same time, the nickel base powder spray welding layer with low carbon content has low hardness and good plasticity.

Relaxation of spray welding stress is helpful to prevent cracks, which is extremely important for unskilled operators;

④ When selecting spray welding specifications, factors such as cast iron material, size of defect parts, working condition requirements, etc. shall be considered.

On the premise of ensuring the necessary flame energy, the heat input to the substrate shall be reduced as much as possible.

The lower limit of oxygen and acetylene gas pressure is appropriate.

During powder spraying and remelting, the spraying and melting distance shall be properly adjusted to control the heat input;

⑤ Local small defects, such as air holes and sand holes, can be repaired by welding without preheating before spray welding, and the heating area around the spray welding layer shall be minimized to make the high-temperature area as small as possible.

⑥ For the repair of local small and deep defects, spray welding is suitable for continuous spray welding.

This method has high powder deposition rate, rapid thickness growth and high efficiency, but requires skilled operation technology to coordinate the powder feeding amount with spraying and melting speed, so as to achieve uniform spraying and penetration;

⑦ For large and deep defects, in order to prevent the thermal stress from increasing due to excessive heating of the substrate, intermittent spray welding can be used.

If necessary, electric welding and spray welding can also be used.

The bottom and upper parts can be filled with welding rods for spray welding.

If the wall thickness of the casting is large, pre spray wire planting can be used to not only increase the bonding strength, prevent the spray welding layer from peeling off from the base metal, but also eliminate some of the thermal stress of spray welding;

⑧ The thermal stress of large complex castings for spray welding is large, so measures shall be taken to reduce the stress accumulation during spray welding, such as heating stress reduction method and subsection symmetry method;

Preheating before welding and slow cooling after welding can achieve good results;

⑨ Control the remelting temperature.

If the remelting temperature is too high, not only the alloy elements will be burned, the matrix will be overheated, and even the matrix will be melted, which will lead to the carbon in the cast iron entering the spray welding layer, which will increase the carbon content in the spray welding layer, increase the hardness, and reduce the plasticity.

However, the matrix is easy to appear white because of the carbon content, and the high temperature will also cause greater thermal stress, which will lead to the generation of cracks.

The remelting temperature cannot be too low, otherwise, it is easy to form ash inclusion or impenetrability.

It affects the bonding strength.

For the control of the remelting temperature, the operator should pay attention to the change of the coating surface state during remelting.

With the help of the “mirror reflection” during remelting, the “mirror reflection” is displayed, indicating that the powder has been melted and the slag has floated.

At this time, the remelting gun should be removed immediately, and the remelted parts should not be reheated.

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