5 Questions to Ask When Choosing a Thermal Spray Vendor

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In recent years the internet has greatly changed the buying process, certainly in the business to consumer market but also in the business-to-business market. You may not even realize that your buying habits have changed. With the answers to most of your questions just an internet search away, businesses are relying more on internet research to identify their problems and determine the ideal solutions rather than on the friendly, neighborhood salesperson.

According to CSO Insights, '70% of buyers fully define their needs on their own before engaging with a sales representative, and 44% identify specific solutions before reaching out to a seller.' While this ability is great, you may miss out on some key first impressions of a vendor. Being able to ask the right questions once you do speak to someone has become more important than ever.

After you've done your research and know that thermal spray is the solution you want to look into, what kind of questions should you be asking to determine which vendor is the right one for you? We've compiled five questions we think are key in helping you determine which companies will provide the quality thermal spray solution you should expect. From standard procedures to EPA disposal, how should a thermal spray company conduct themselves to earn your business?

Do they have written standard procedures? Do they allow you to view them?

You are trusting this company with your equipment. You should get to know what will happen to it. Also, if a company has a written set of procedures that means they've sat down and thought about the best way to do things, which typically means your part will be better cared for. If a company knows what they're doing they should have these at the ready for potential customer review and shouldn't fear a quality audit or even a shop visit. If you sense tension on the line when you ask these questions, take heed. You should also ensure their standard procedures include steps for quality and coating checks.

Do they perform regular quality checks or use lab testing to determine coating effectiveness?

Sometimes you won't have to ask this question if you are already aware that they are ISO certified, but it still wouldn't hurt to know if they have the capabilities to perform lab testing. You can ask how these companies measure the quality of their services. Do they use micrometers or thickness gages, such as an Elcometer, to check coating thickness at regular intervals along the part and throughout the spray process? Do they have a coordinate-measuring machine, or CMM, to measure spherical or complicated surfaces such as valve balls? These tools and procedures will ensure that your part will return to you at your desired dimensional specifications.

Coating quality is another topic to ask about; do they measure their coating quality using industry standard lab tests? Do they outsource their lab testing or do they have an in-house lab for faster result turnaround? The most common coating lab tests are bond strength, hardness, porosity and basic coating structure. If you need certain coating properties, such as friction mitigation or anti-slip coatings, you will want to ask about surface roughness testing as well.

While you may not need these tests every time you have a part sprayed, it is good to know whether a company has a plan in place to evaluate their processes and coatings and if they have those services available to you, should you desire them. It is also important to know whether they keep up with regular calibration of their lab and thermal spray equipment; as well as their measuring devices. You can even ask to them to provide certifications of calibration for their equipment, if you desire.

How often do they calibrate and maintain their equipment and machinery?

After asking about lab testing, it's a good idea to ensure the quality of the lab tests and equipment. The results you request will mean nothing if the vendor is not doing regular machine maintenance and monitoring. This also applies to all of their service equipment and measuring devices.

This starts in their prep processes such grinding or machining. Do they regularly check their grinding wheels to ensure the grit effectiveness? Do they have a preventative maintenance plan in place to mitigate errors from machine breakdowns?

This gets even more crucial when it comes to thermal spray equipment. Have their powder feeder pressures been checked? Do they regularly check their gun pressures and temperatures? Do they obtain material certifications for their wires and powders? These things will not only affect the quality of your part's coating but also affect their process times and materials, potentially costing you extra time and money. An easy way to ensure consistent, quality thermal spray coating is the use of robotic technology.

Are their thermal spray operations robotically controlled?

Robotically controlled thermal spray is the most consistent way to apply coatings. Asking if they spray via programmable robots or similar mechanical devices, such as bug tracks, is a good indicator of the quality of coating you will receive. Mechanically controlled thermal spray guns apply more even coatings and allow for exact control over spray parameters which highly affect the hardness and bond strength of the coating. Some parts and processes will require hand spray and can't be fully automated, but knowing a company uses robotically controlled spray guns will show you how seriously they take their spray parameters and coating quality. Another thing a thermal spray vendor should take seriously is what they do with the byproducts of their processes.

Do they follow EPA guidelines for material and waste disposal?

Some of the materials used in thermal spray operations such as chrome carbide powder and aluminum oxide grit are considered hazardous substances and require special steps for handling, storage, and disposal. While this may not have a direct impact on the quality of your coating, it says a lot about the character of the people you're working with. Proper material handling displays a sense of responsibility for the work they are doing. If they aren't taking care to dispose of their materials properly, they might not be taking the utmost care for your parts either. If you sense forethought and care in the way they answer this and other questions, you are on the right track to finding the thermal spray company for you.

Be Informed, Get Quality Results

In the business-to-business market there's a lot of competition out there and it can be hard to weed out who to trust with your machine components. While a cheaper price might be tempting, you may be sacrificing on quality. Not getting the answers you want on one of the above questions might not be a deal breaker but working with a company who is able to answer these questions with poise and planning, says a lot of about the quality of work you will receive.

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The meaning and characteristics of thermal spraying
Thermal spraying technology refers to a series of processes, in these processes, fine and dispersed metal or non-metallic coating materials, in a melting or semi-melting state, deposited on the surface of the prepared matrix, forming a spray coating layer. Is the use of a certain heat source (such as arc, plasma spraying, etc.) to powder or filament-shaped metal or nonmetallic materials heated to molten or semi-molten state, and then with the help of the flame itself or compressed air at a certain speed to spray to the surface of the pretreated matrix, deposition and formation of a surface coating with various functions of a technology.

The characteristics of thermal spraying :

(1)The substrate is not limited, can be sprayed on a variety of substrates.

(2) Spraying coating materials are very wide, can be used to spray almost all solid engineering materials.

(3) The temperature rise of the substrate material is small in the spraying process, and no stress and deformation are generated.

(4) Flexible and convenient operation process, not limited by the shape of the workpiece, convenient construction.

 (5) coating thickness can be from 0.01 to a few millimeters.

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(6) coating performance is diverse, can form wear resistance, corrosion resistance, heat insulation, oxidation resistance, insulation, electrical conductivity, radiation and other coating with a variety of special functions.

 (7) Strong adaptability and good economic benefits.

Thermal spraying methods include detonation spraying. Wire arc spraying. Flame spraying. High velocity oxy-fuel coating spraying (HVOF).

Due to the advantages of convenient operation, strong adaptability and good economic effect, thermal spraying is widely used in chemical industry (all kinds of containers, reactors, pipelines, pumps, valves and sealing parts), transportation (such as gear shaft, mud pump water seal neck, mud door, mud impeller, sand scraper, etc.). Electric power industry (ball mill, turbine rotor journal and generator rotor journal, cylinder joint surface), metallurgy (blast furnace tuyere, guide roller and delivery roller), etc.

Thermal spraying process:
Thermal spraying workpiece: workpiece surface pretreatment - workpiece preheating - spraying - coating post treatment

Workpiece surface pretreatment: in order to make the coating and the substrate material is well combined, the substrate surface must be clean and rough, there are many ways to purify and coarsely surface, the selection of methods should be based on the suitable requirements of the coating and the substrate material, shape, thickness, surface original condition and construction conditions and other factors.
The purpose of purification treatment is to remove all the dirt on the surface of the workpiece, such as oxide skin, oil stains, etc. The purpose of coarsening treatment is to increase the contact surface between the coating and the matrix material and increase the mechanical biting force between the coating and the matrix material. For some coating materials which are not well bonded with the substrate, a transition layer of coating materials which are bonded with the substrate material should be sprayed, called the bottom layer of the bond, and the thickness is generally 0.8-0.18μm.
Preheating: The purpose is to eliminate moisture and moisture on the surface of the workpiece, improve the interface temperature when spraying particles contact with the workpiece, and improve the binding strength of the coating and the collective.
Spraying

Which spraying method is used for spraying mainly depends on the selected spraying material, the working condition of the workpiece and the requirements for coating quality. The pretreated workpiece should be sprayed in the shortest possible time, and the spraying parameters should be determined according to the coating material, the performance of the spray gun and the specific situation of the workpiece. The optimized spraying conditions can improve the spraying efficiency, and obtain the high-quality coating with high density and high binding strength.
After coating processing: thermal spraying coating sometimes cannot be used directly, mustturningry out a series of post-processing, such as for under impact load or high stress abrasion artifacts, in order to improve the bonding strength of coating, to remelting the coating layer, make the combination of porous and substrate with only mechanical coating into and the density of the metallurgical bonding coating to the substrate.
There are dimensional accuracy requirements, to the coating for mechanical processing. Because the spray coating has different characteristics with the general metal and ceramic materials. The strength of the coating itself is low, can not withstand a large cutting force; In the coating.

Mechanical processing of workpiece after thermal spraying
The above mentioned dimension accuracy requirements of the workpiece, the need for mechanical processing, to ensure the size and accuracy requirements. However, the workpiece hardness after thermal spraying is generally above HRC50, and part of the thermal spraying coating is above HRC65, and there are hard points in the coating, which requires the tool material not only to have high hardness, but also to have good wear resistance and impact resistance.
According to the coating thickness and processing allowance of thermal spraying materials and the substrate material to choose the turning or grinding processing; Due to the low grinding efficiency, the thermal spraying workpiece with small machining allowance and spray thickness is generally selected. On the other hand, when the allowance is large, it is a more economical choice to replace the grinding with turning.

Thermal spraying are widely used in metal surface strengthening, workpiece after thermal spraying, the surface hardness can reach 50 to 55 HRC, general high up to 70 HRC, and most of the coating on the surface of the demand is higher, the surface after spray to cold working, some large machining allowance or restricted to large cylindrical grinding machine equipment, often used in car generation grinding machining method.

1. The type of thermal spraying materials

(1) Types of thermal spraying materials:

Thermal spraying commonly used spraying powder including nickel base, iron base and cobalt base alloy powder, according to different coating hardness, respectively applied to repair and protection of mechanical parts; Spray thickness can be from 0.01 to a few millimeters; The hardness is generally above 50HRC.

General machining allowance and spraying thickness can be used for grinding; On the contrary, instead of grinding car is an economic choice.

2. Cutting tool selection for turning thermal sprayed workpieces

(1) When the hardness of the thermal spray coating is less than HRC45, the carbide tool material can be selected. The commonly used grades are YS30, YG3X, etc., and the line speed can be selected as νc=40 m/min.

(2) When the hardness of the thermal spray coating is greater than HRC45, ceramic or CBN cutters can be used; ceramic cutters and welded composite CBN cutters are often used in finishing due to their poor impact resistance (the cutting depth is preferably within 0.3mm).

(3) When the hardness of the thermal spray coating is greater than HRC55, CBN tools can have better tool life and processing efficiency.

3.  Cutting parameters of the tool

(1) When using carbide cutting tools, the cutting depth is usually aO=0.15~0.6mm. The higher the hardness of the surface layer, the smaller the aP should be. aP=0.05'0.1mm during fine turning. The feed rate is generally taken as f=0.1'1mm/r, and f should be smaller during fine turning. The first speed is generally selected according to the hardness of νc=40 m/min; when the hardness of thermal spraying is HRC45-65,choose the carbide tool and the linear speed is better at νc=10 m/min.

(2) When using ceramic cutting tools, the cutting depth is generally controlled within 0.2mm, and the feed rate is about 0.1mm/r; however, the linear speed can be increased to about 3 times that of the original cemented carbide.

--EDITOR:Doris Hu/Anna Wang

---POST: Doris Hu

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