To ensure proper passivation, technicians electrochemically clean the longitudinal welds of the rolled sections of stainless steel.Image courtesy of Walter Surface Technologies
Imagine a manufacturer enters into a contract involving key stainless steel fabrication.Sheet metal and tube sections are cut, bent and welded before landing at a finishing station.The part consists of plates welded vertically to the tube.The welds look good, but it’s not the perfect dime the customer is looking for.As a result, the grinder spends time removing more weld metal than usual.Then, alas, some distinct blues appeared on the surface – a clear sign of too much heat input.In this case, it means that the part will not meet customer requirements.
Often performed manually, grinding and finishing require dexterity and skill.Errors in finishing can be very expensive, given all the value that has been given to 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 lucrative stainless steel job can turn into a money-losing or even a reputation-damaging mishap.
How do manufacturers prevent all this?They can start by developing their knowledge of grinding and finishing, understanding the roles they each play and how they affect stainless steel workpieces.
They are not synonyms.In fact, everyone has a fundamentally different goal.Grinding removes materials such as burrs and excess weld metal, while finishing provides a finish on the metal surface.The confusion is understandable, considering that those who grind with large grinding wheels remove a lot of metal very quickly, and doing so can leave very deep scratches.But in grinding, scratches are just an after-effect; the goal is to remove material quickly, especially when working with heat-sensitive metals such as stainless steel.
Finishing is done in steps, as the operator starts with a larger grit and progresses to finer grinding wheels, nonwoven abrasives, and perhaps felt cloth and polishing paste to achieve a mirror finish.The goal is to achieve a certain final finish (scratch pattern).Each step (the finer grit) removes the deeper scratches from the previous step and replaces them with smaller scratches.
Because grinding and finishing have different goals, they often don’t complement each other and can actually play against each other if the wrong consumable strategy is used.To remove excess weld metal, operators use grinding wheels to make very deep scratches, then hand the part over to a dresser, who now has to spend a lot of time removing these deep scratches.This grinding-to-finishing sequence may still be the most efficient way to meet customer finishing requirements.But again, they are not complementary processes.
Workpiece surfaces designed for manufacturability generally do not require grinding and finishing.Parts that are ground only do this because grinding is the fastest way to remove welds or other material and the deep scratches left by the grinding wheel are exactly what the customer wants.Parts that only require finishing are manufactured in a way that does not require excessive material removal.A typical example is a stainless steel part with a beautiful gas tungsten shielded weld that just needs to be blended and matched to the finish pattern of the substrate.
Grinders with low-removal wheels can present significant challenges when working with stainless steel.Likewise, overheating can cause bluing and change material properties.The goal is to keep the stainless steel as cool 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.Zirconia wheels grind faster than alumina, but in most cases, ceramic wheels work best.
Extremely tough and sharp ceramic particles wear in a unique way.As they gradually disintegrate, they do not grind flat, but maintain a sharp edge.This means they can remove material very quickly, often in a fraction of the time of other grinding wheels.This generally makes ceramic grinding wheels worth the money.They are ideal for stainless steel applications because they remove large chips quickly and generate less heat and distortion.
No matter which grinding wheel a manufacturer chooses, potential contamination needs to be kept in mind.Most manufacturers know that they cannot use the same grinding wheel on carbon steel and stainless steel.Many people physically separate their carbon and stainless steel grinding operations.Even tiny sparks of carbon steel falling on stainless steel workpieces can cause contamination problems.Many industries, such as the pharmaceutical and nuclear industries, require consumables to be rated as pollution-free.This means that grinding wheels for stainless steel must be almost free (less than 0.1%) of iron, sulfur and chlorine.
Grinding wheels can’t grind themselves; they need a power tool.Anyone can tout 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 amount of power and torque.While some air grinders have the necessary specifications, most ceramic wheel grinding is done with power tools.
Grinders with insufficient power and torque can cause serious problems, even with the most advanced abrasives.The 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 were designed to do: quickly remove large pieces of metal, thereby reducing the amount of thermal material entering the grinding wheel.
This exacerbates a vicious cycle: Grinding operators see material not being removed, so they instinctively push harder, which in turn creates excess heat and bluing.They end up pushing so hard that they glaze the wheels, which makes them work harder and generate more heat before they realize they need to replace the wheels.If you work this way on thin tubes or sheets, they end up going straight through the material.
Of course, if operators are not properly trained, even with the best tools, this vicious cycle can happen, especially when it comes to the pressure they put on the workpiece.Best practice is to get as close as possible to the nominal current rating of the grinder.If the operator is using a 10 amp grinder, they should press so hard that the grinder draws about 10 amps.
Using an ammeter can help standardize grinding operations if the manufacturer processes large quantities of expensive stainless steel.Of course, few operations actually use an ammeter on a regular basis, so your best bet is to listen carefully.If the operator hears and feels the RPM drop rapidly, they may be pushing too hard.
Listening to touches that are too light (i.e. too little pressure) can be difficult, so in this case, paying attention to spark flow can help.Grinding stainless steel will produce darker sparks than carbon steel, but they should still be visible and protrude from the work area in a consistent manner.If the operator suddenly sees fewer sparks, it may be because they are not applying enough pressure or glazing the wheel.
Operators also need to maintain a consistent working angle.If they approach the workpiece at a near-flat angle (nearly parallel to the workpiece), they can cause extensive overheating; if they approach at an angle that is too high (nearly vertical), they risk digging the edge of the wheel into the metal.If they are using a Type 27 wheel, they should approach the 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 on flat surfaces to remove material on wider grinding paths.These tapered wheels also work best at lower grinding angles (about 5 degrees), so they help reduce operator fatigue.
This introduces another critical factor: choosing the right type of grinding wheel.The Type 27 wheel has a contact point on the metal surface; the Type 28 wheel has a contact line because of its conical shape; the Type 29 wheel has a contact surface.
By far the most common Type 27 wheels can get the job done in many applications, but their shape makes it difficult to handle parts with deep profiles and curves, such as welded assemblies of stainless steel tubes.The profile shape of the Type 29 wheel makes it easier for operators who need to grind a combination of curved and flat surfaces.The Type 29 wheel does this by increasing the surface contact area, which means the operator doesn’t have to spend a lot of time grinding in each location – a good strategy for reducing heat build-up.
In fact, this applies to any grinding wheel.When grinding, the operator must not stay in the same place for a long time.Suppose an operator is removing metal from a fillet several feet long.He can steer the wheel in short up and down motions, but doing so may overheat the workpiece because he keeps the wheel in a small area for long periods of time.To reduce heat input, the operator can traverse the entire weld in one direction near one toe, then lift the tool (giving the workpiece time to cool) and traverse the workpiece in the same direction near the other toe.Other techniques work, but they all have one feature in common: they avoid overheating by keeping the grinding wheel moving.
Commonly used “carding” techniques also help to achieve this.Suppose the operator is grinding a butt weld in a flat position.To reduce thermal stress and over-digging, he avoided pushing the grinder along the joint.Instead, he starts at the end and pulls the grinder along the joint.This also prevents the wheel from digging too much into the material.
Of course, any technique can overheat the metal if the operator goes too slowly.Go too slowly and the operator will overheat the workpiece; go too fast and grinding can take a long time.Finding the feedrate sweet spot usually requires experience.But if the operator is unfamiliar with the job, they can grind the scrap to get the “feel” of the appropriate feed rate for the workpiece at hand.
The finishing strategy revolves around the surface condition of the material as it arrives and leaves the finishing department.Identify the starting point (surface condition received) and the ending point (finish required), 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 sound counterintuitive.After all, why not start with coarse sand to get a rough surface and then move to finer sand?Wouldn’t it be very inefficient to start with a finer grit?
Not necessarily, this again has to do with the nature of collation.As each step reaches a smaller grit, the conditioner replaces the deeper scratches with shallower, finer scratches.If they start with 40-grit sandpaper or a flip disk, they’ll leave deep scratches on the metal.It would be great if those scratches brought the surface close to the desired finish; that’s why those 40 grit finishing supplies exist.However, if the customer requests a No. 4 finish (directional brushed finish), deep scratches created by a No. 40 abrasive will take a long time to remove.Dressers either step down through multiple grit sizes, or spend a long time using fine-grained abrasives to remove those large scratches and replace them with smaller scratches.Not only is this all inefficient, but it also introduces too much heat into the workpiece.
Of course, using fine grit abrasives on rough surfaces can be slow and, combined with poor technique, introduce too much heat.This is where a two-in-one or staggered flap disc can help.These discs include abrasive cloths combined with surface treatment materials.They effectively allow the dresser to use abrasives to remove material while also leaving a smoother finish.
The next step in final finishing may involve the use of nonwovens, which illustrates another unique feature of finishing: the process works best with variable-speed power tools.A right angle grinder running at 10,000 RPM may work with some grinding media, but it will melt some nonwovens thoroughly.For this reason, finishers reduce the speed to between 3,000 and 6,000 RPM before starting the finishing step with nonwovens.Of course, the exact speed depends on the application and consumables.For example, nonwoven drums typically spin between 3,000 and 4,000 RPM, while surface treatment disks typically spin between 4,000 and 6,000 RPM.
Having the right tools (variable speed grinders, different finishing media) and determining the optimal number of steps basically provides a map that reveals the best path between incoming and finished material.The exact path varies by application, but experienced trimmers follow this path using similar trimming techniques.
Non-woven rollers complete the stainless steel surface.For efficient finishing and optimum consumable life, different finishing media run at different RPMs.
First, they take their time.If they see a thin stainless steel workpiece getting hot, they stop finishing in one area and start in another.Or they might be working on two different artifacts at the same time.They work a little on one and then the other, giving the other workpiece time to cool.
When polishing to a mirror finish, the polisher may cross-polish with a polishing drum or polishing disc, in a direction perpendicular to the previous step.Cross sanding highlights areas that need to blend in the previous scratch pattern, but still won’t get the surface to a mirror finish of No. 8.Once all scratches have been removed, a felt cloth and buffing wheel are required to create the desired glossy finish.
To achieve the right finish, manufacturers need to provide finishers with the right tools, including actual tools and media, as well as communication tools, such as establishing standard samples to determine what a certain finish should look like.These samples (posted near the finishing department, in training documents, and in sales literature) help get everyone on the same page.
With regard to actual tooling (including power tools and abrasive media), the geometry of certain parts can present challenges even for the most experienced employees in the finishing department.This is where professional tools can help.
Suppose an operator needs to complete a stainless steel thin-walled tubular assembly.Using flap discs or even drums can cause problems, cause overheating, and sometimes even create a flat spot on the tube itself.Here, belt sanders designed for tubing can help.The conveyor belt wraps around most of the pipe diameter, spreading out the points of contact, increasing efficiency and reducing heat input.That said, as with anything else, the dresser still needs to move the belt sander to a different area to mitigate excess heat build-up and avoid bluing.
The same applies to other professional finishing tools.Consider a finger belt sander designed for tight spaces.A finisher might use it to follow a fillet weld between two boards at an acute angle.Instead of moving the finger belt sander vertically (kind of like brushing your teeth), the dresser moves it horizontally along the upper toe of the fillet weld, then the bottom toe, while making sure the finger sander doesn’t stay in one for too long .
Welding, grinding and finishing of stainless steel introduces another complication: ensuring proper passivation.After all these disturbances to the surface of the material, are there any remaining contaminants that would prevent the stainless steel’s chromium layer from naturally forming over the entire surface?The last thing a manufacturer wants is an angry customer complaining about rusted or contaminated 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 performed?It depends on the application.If manufacturers do clean stainless steel to promote full passivation, they usually do so immediately after welding.Failure to do so means that the finishing medium may pick up surface contaminants from the workpiece and spread them elsewhere.However, for some critical applications, manufacturers may choose to insert additional cleaning steps—perhaps even testing for proper passivation before the stainless leaves the factory floor.
Suppose a manufacturer welds a critical stainless steel component for the nuclear industry.A professional gas tungsten arc welder lays a dime seam that looks perfect.But again, this is a critical application.An employee in the finishing department uses a brush connected to an electrochemical cleaning system to clean the surface of a weld.He then feathered the weld toe using a non-woven abrasive and dressing cloth and got everything to an even brushed finish.Then comes the final brush with an electrochemical cleaning system.After sitting for a day or two, use a handheld test device to test the part for proper passivation.The results, recorded and kept with the job, showed that the part was fully passivated before it left the factory.
In most manufacturing plants, the grinding, finishing and cleaning of stainless steel passivation typically occurs downstream.In fact, they are usually executed shortly before the job is shipped.
Incorrectly finished parts generate some of the most expensive scrap and rework, so it makes sense for manufacturers to take another look at their grinding and finishing departments.Improvements in grinding and finishing help alleviate major bottlenecks, improve quality, eliminate headaches, and most importantly, increase customer satisfaction.
FABRICATOR is North America’s leading metal forming and fabrication industry magazine.The magazine provides news, technical articles and case histories that enable manufacturers to do their jobs more efficiently.FABRICATOR has been serving the industry since 1970.
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