Longitudinal welds in stainless steel bars are electrochemically deburred to ensure proper passivation. Image courtesy of Walter Surface Technologies
Imagine that a manufacturer enters into a contract to manufacture a key stainless steel product. Sheet metal and pipe sections are cut, bent and welded before being sent to the finishing station. The part consists of plates welded vertically to the pipe. The welds look good, but it’s not the ideal price a buyer is looking for. As a result, the grinder spends time removing more weld metal than usual. Then, alas, a distinct blue appeared on the surface – a clear sign of too much heat input. In this case, this means that the part will not meet the requirements of the customer.
Often done by hand, sanding and finishing require dexterity and craftsmanship. Mistakes in finishing can be very costly considering all the value that has been placed on the workpiece. Adding expensive heat-sensitive materials such as stainless steel, rework and scrap installation costs can be higher. Combined with complications such as contamination and passivation failures, a once profitable stainless steel operation can become unprofitable or even damaging to reputation.
How do manufacturers prevent all this? They can start by expanding their knowledge of grinding and finishing, understanding the roles they play and how they affect stainless steel workpieces.
These are not synonyms. In fact, everyone has fundamentally different goals. Grinding removes materials such as burrs and excess weld metal, while finishing provides a fine finish to the metal surface. The confusion is understandable, given that those who grind with large grinding wheels remove a lot of metal very quickly, and very deep scratches can be left in the process. But when grinding, scratches are only a consequence, the goal is to quickly remove material, especially when working with heat-sensitive metals such as stainless steel.
Finishing is done in stages as the operator starts with a coarser grit and progresses to finer grinding wheels, non-woven abrasives and possibly felt cloth and polishing paste to achieve a mirror finish. The aim is to achieve a certain final finish (scratch pattern). Each step (finer grit) removes the deeper scratches from the previous step and replaces them with smaller scratches.
Since grinding and finishing have different purposes, they often do not complement each other and can play against each other if the wrong consumables strategy is used. To remove excess weld metal, the operator makes very deep scratches with a grinding wheel, and then passes the part to the dresser, who now has to spend a lot of time removing these deep scratches. This sequence from grinding to finishing can still be the most efficient way to meet customer finishing requirements. But again, these are not additional processes.
Workpiece surfaces designed for workability generally do not require grinding or finishing. Parts that are sanded only do so because sanding is the quickest way to remove welds or other material, and the deep scratches left by the grinding wheel are exactly what the customer wanted. Parts that require only finishing are manufactured in such a way that excessive material removal is not required. A typical example is a stainless steel part with a beautiful weld protected by a tungsten electrode that simply needs to be blended and matched to the finish pattern of the substrate.
Grinding machines with low material removal discs can pose serious problems when working with stainless steel. Likewise, overheating can cause bluing and change in material properties. The goal is to keep the stainless steel as cold as possible throughout the process.
To this end, it helps to select the grinding wheel with the fastest removal rate for the application and budget. Zirconium wheels grind faster than alumina, but ceramic wheels work best in most cases.
The extremely strong and sharp ceramic particles are worn in a unique way. As they gradually disintegrate, they do not become flat, but retain a sharp edge. This means that they can remove material very quickly, often several times faster than other grinding wheels. Generally, this makes ceramic grinding wheels worth the money. They are ideal for machining stainless steel, as they quickly remove large chips and generate less heat and deformation.
Regardless of which grinding wheel a manufacturer chooses, potential contamination must be kept in mind. Most manufacturers know that they cannot use the same grinding wheel for both carbon steel and stainless steel. Many people physically separate carbon and stainless steel grinding operations. Even tiny sparks of carbon steel falling on stainless steel parts can cause contamination problems. Many industries, such as the pharmaceutical and nuclear industries, require consumables to be rated as non-polluting. This means that stainless steel grinding wheels must be practically free (less than 0.1%) of iron, sulfur and chlorine.
Grinding wheels don’t grind themselves, they need a power tool. Anyone can advertise the benefits of grinding wheels or power tools, but the reality is that power tools and their grinding wheels work as a system. Ceramic grinding wheels are designed for angle grinders with a certain power and torque. While some pneumatic grinders have the required specifications, in most cases the grinding of ceramic wheels is done with power tools.
Grinders with insufficient power and torque can cause serious problems with even the most modern abrasives. Lack of power and torque can cause the tool to slow down significantly under pressure, essentially preventing the ceramic particles on the grinding wheel from doing what they are designed to do: quickly remove large chunks of metal, thereby reducing the amount of thermal material entering the grinding wheel. grinding wheel.
This exacerbates the vicious cycle: sanders see that no material is being removed, so they instinctively press harder, which in turn creates excess heat and bluing. They end up pushing so hard that they glaze the wheels, which forces them to work harder and generate more heat before they realize they need to change the wheels. If you work this way with thin tubes or sheets, they end up going right through the material.
Of course, if operators are not properly trained, even with the best tools, this vicious cycle can occur, especially when it comes to the pressure they put on the workpiece. Best practice is to get as close as possible to the rated current of the grinder. If the operator is using a 10 amp grinder, he must press so hard that the grinder draws about 10 amps.
The use of an ammeter can help standardize grinding operations if a manufacturer processes a large amount of expensive stainless steel. Of course, few operations actually use an ammeter on a regular basis, so it’s best to listen carefully. If the operator hears and feels the RPM drop rapidly, he may be pushing too hard.
Listening to touches that are too light (i.e., too little pressure) can be difficult, so attention to spark flow can help in this case. Sanding stainless steel produces darker sparks than carbon steel, but they should still be visible and protrude evenly from the work area. If the operator suddenly sees fewer sparks, it may be due to not applying enough force or not glazing the wheel.
Operators must also maintain a constant working angle. If they approach the workpiece at nearly a right angle (nearly parallel to the workpiece), they can cause significant overheating; if they approach at too great an angle (nearly vertical), they run the risk of slamming the edge of the wheel into the metal. If they use a type 27 wheel, they should approach work at an angle of 20 to 30 degrees. If they have type 29 wheels, their working angle should be around 10 degrees.
Type 28 (tapered) grinding wheels are typically used for grinding flat surfaces to remove material on wider grinding paths. These tapered wheels also work best at lower grinding angles (around 5 degrees) so they help reduce operator fatigue.
This introduces another important factor: choosing the right type of grinding wheel. Type 27 wheel has a metal surface contact point, type 28 wheel has a contact line due to its conical shape, type 29 wheel has a contact surface.
Today’s most common type 27 wheels can do the job in many areas, but their shape makes it difficult to work with deep profiled parts and curves, such as welded stainless steel tube assemblies. The profile shape of the Type 29 wheel facilitates the work of operators who need to grind combined curved and flat surfaces. The Type 29 wheel does this by increasing the surface contact area, which means the operator does not have to spend a lot of time grinding at each location – a good strategy to reduce heat buildup.
Actually, this applies to any grinding wheel. When grinding, the operator should not stay in the same place for a long time. Suppose an operator is removing metal from a fillet several feet long. It can drive the wheel in short up and down motions, but this can cause the workpiece to overheat as it keeps the wheel in a small area for a long period of time. To reduce heat input, the operator can run the entire weld in one direction at one nose, then raise the tool (allowing the workpiece to cool) and pass the workpiece in the same direction at the other nose. Other methods work, but they all have one thing in common: they avoid overheating by keeping the grinding wheel in motion.
This is also helped by widely used methods of “combing”. Suppose the operator is grinding a butt weld in a flat position. To reduce thermal stress and excessive digging, he avoided pushing the grinder along the joint. Instead, he starts at the end and runs the grinder along the joint. This also prevents the wheel from sinking too far into the material.
Of course, any technique can overheat the metal if the operator works too slowly. Work too slowly and the operator will overheat the workpiece; if you move too fast, sanding can take a long time. Finding the sweet spot for feed speed usually takes experience. But if the operator is not familiar with the job, he can grind the scrap to “feel” the appropriate feed rate for the workpiece.
The finishing strategy depends on the surface condition of the material as it enters and leaves the finishing department. Determine a start point (obtained surface condition) and an end point (finish required), and then make a plan to find the best path between those two points.
Often the best path does not start with a highly aggressive abrasive. This may seem counterintuitive. After all, why not start with coarse sand to get a rough surface and then move on to finer sand? Wouldn’t it be very inefficient to start with a finer grain?
Not necessarily, this again has to do with the nature of the comparison. As finer grit is achieved in each step, the conditioner replaces deeper scratches with finer, finer ones. If they start with 40 grit sandpaper or a flip pan, they will leave deep scratches on the metal. It would be great if these scratches would bring the surface closer to the desired finish, which is why there are 40 grit finish materials available. However, if a customer requests a #4 finish (directional sanding), the deep scratches left by #40 grit take a long time to remove. Craftsmen either go to multiple grit sizes or spend a lot of time using fine grit abrasives to remove those big scratches and replace them with smaller ones. All this is not only inefficient, but also heats the workpiece too much.
Of course, using fine grit abrasives on rough surfaces can be slow and, combined with poor technique, results in too much heat. Two-in-one or staggered discs can help with this. These discs include abrasive cloths combined with surface treatment materials. They effectively allow the craftsman to use abrasives to remove material while leaving a smoother finish.
The next step in finishing can include the use of non-woven fabrics, which illustrates another unique finishing feature: the process works best with variable speed power tools. An angle grinder running at 10,000 rpm can handle some abrasive materials, but it will completely melt some non-woven materials. For this reason, finishers slow down to 3,000-6,000 rpm before finishing nonwovens. Of course, the exact speed depends on the application and consumables. For example, nonwoven drums typically rotate at 3,000 to 4,000 rpm, while surface treatment discs typically rotate at 4,000 to 6,000 rpm.
Having the right tools (variable speed grinders, various finishing materials) and determining the optimal number of steps basically provides a map that shows the best path between incoming and finished material. The exact path depends on the application, but experienced trimmers follow this path using similar trimming methods.
Non-woven rolls complete the stainless steel surface. For efficient finishing and optimum consumable life, different finishing materials run at different rotational speeds.
First, they take time. If they see that a thin piece of stainless steel is heating up, they stop finishing in one place and start in another. Or they might be working on two different artifacts at the same time. Work a little on one and then on the other, giving the other piece time to cool.
When polishing to a mirror finish, the polisher can cross-polish with the polishing drum or polishing disc in the direction perpendicular to the previous step. Cross sanding highlights areas that should merge with the previous scratch pattern, but still does not bring the surface to a #8 mirror finish. Once all scratches have been removed, a felt cloth and buffing pad will be needed to create the desired glossy finish.
To get the right finish, manufacturers must provide finishers with the right tools, including real tools and materials, as well as communication tools, such as creating standard samples to determine how a certain finish should look. These samples (posted next to the finishing department, in training papers, and in sales literature) help keep everyone on the same wavelength.
As far as actual tooling (including power tools and abrasives) is concerned, the geometry of some parts can be challenging even for the most experienced finishing team. This will help professional tools.
Suppose an operator needs to assemble a thin-walled stainless steel pipe. Using flap discs or even drums can lead to problems, overheating, and sometimes even a flat spot on the tube itself. This is where belt grinders designed for pipes can help. The conveyor belt covers most of the pipe diameter, distributing contact points, increasing efficiency and reducing heat input. However, as with everything else, the craftsman still needs to move the belt sander to a different location to reduce excess heat buildup and avoid bluing.
The same applies to other professional finishing tools. Consider a belt sander designed for hard-to-reach places. A finisher can use it to make a fillet weld between two boards at a sharp angle. Instead of moving the finger belt sander vertically (kind of like brushing your teeth), the technician moves it horizontally along the top edge of the fillet weld and then along the bottom, making sure the finger sander doesn’t stay in one place too much. for a long time. long .
Welding, grinding and finishing stainless steel comes with another challenge: ensuring proper passivation. After all these disturbances, did any contamination remain on the surface of the material that would prevent the natural formation of a stainless steel chromium layer over the entire surface? The last thing a manufacturer needs is an angry customer complaining about rusty or dirty parts. This is where proper cleaning and traceability come into play.
Electrochemical cleaning can help remove contaminants to ensure proper passivation, but when should this cleaning be done? It depends on the application. If manufacturers clean stainless steel to ensure complete passivation, they usually do so immediately after welding. Failure to do so means that the finishing medium may absorb surface contaminants from the workpiece and distribute them to other locations. However, for some critical applications, manufacturers may add additional cleaning steps—perhaps even testing for proper passivation before the stainless steel leaves the factory floor.
Suppose a manufacturer is welding an important stainless steel component for the nuclear industry. A professional tungsten arc welder creates a smooth seam that looks perfect. But again, this is a critical application. A member of the finishing department uses a brush connected to an electrochemical cleaning system to clean the surface of a weld. He then sanded down the weld with a non-woven abrasive and a wiping cloth and finished everything to a smooth surface. Then comes the last brush with an electrochemical cleaning system. After a day or two of downtime, use a portable tester to check the part for proper passivation. The results, recorded and saved with the job, showed that the part was fully passivated before leaving the factory.
In most manufacturing plants, grinding, finishing, and cleaning the passivation of stainless steel typically occur in subsequent steps. In fact, they are usually performed shortly before the job is submitted.
Improperly machined parts create some of the most expensive scrap and rework, so it makes sense for manufacturers to take another look at their sanding and finishing departments. Improvements in grinding and finishing help eliminate key bottlenecks, improve quality, eliminate headaches and, most importantly, increase customer satisfaction.
FABRICATOR is North America’s leading steel fabrication and forming magazine. The magazine publishes news, technical articles and success stories that enable manufacturers to do their job more efficiently. FABRICATOR has been in the industry since 1970.
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