Lehigh Valley Abrasives January 2010 Newsletter

Welding and the Fabricator Magazines Feature Lehigh Valley Abrasives Article on Stainless Steel Finishing Techniques.

 

 

By Christopher Stone | Dec. 1, 2009 12:00 PM

As the demand for fabricating higher-value materials increases, a three-step process will guide operators through the fastest, most cost-effective way to achieving customers' requirements.

In North America, the use of stainless steel continues to grow in popularity, due to its many advantages for consumer and industrial products. Architects, engineers and contractors increasingly specify stainless steel for use in construction, appliances, food processing equipment and the medical field. As stainless steel usage grows, proficiency in finishing stainless will take on increased importance.

 

STEP 1: Define the finish
 
The first step for fabricators is to work with each customer to define the finish required. The finish for the base material and treatment of the welds should be discussed and agreed upon. Generally, the material finish is specified in grit (the particle size for the last step inthe abrasive finishing process). Stainless steel material specifications are defined below.
 
Mechanical Finish Designations for SteelMaterials
 
#2D Finish - a uniform, dull silver gray mill finish that is applied to thinner stainless steel coils, the thickness of which has been reduced by cold rolling.

#2B Finish - A bright cold rolled mill finish commonly produced in the same manner as No. 2D, except that the final light cold rolling passis done using polished rolls.

#3 Finish - A semi-polished surface achieved by finishing with the equivalent of a 80 to 120-grit abrasive. This finish has a pronounced grit line.

 #4 Finish - Also called brushed, directional or satin finish A number 4 finish, characterized by fine polishing grit lines that are uniform and directional in appearance. The final abrasive used in the process is 150 to 220 grit.

 #6 Finish - Polished finish achieved with the equivalent of a 240-grit abrasive. Finer grit lines and higher reflectivity than  No. 4 finish.

 #7 Finish - Highly reflective surface obtained with the equivalent of a 320-grit abrasive. Minimal grit lines.

#8 Finish or Mirror Finish - produced by polishing with at leasta 320 grit belt or wheel finish. The part is then sisal and color buffed to achieve a mirror-like finish.
 
Weld treatment
 
The treatment of the welds should also be defined with the customer. Weld treatment options are as follows;

1. Remove weld spatter and discoloration only.
2. Remove weld spatter, discoloration and weld ripple.
3. Remove weld spatter, discoloration and weld ripple, plus grind weld smooth with base material.
4. Remove weld spatter, discoloration and weld ripple, plus grind weld smooth with base material, and producing all welds pit and crevice free.
 
Once the customer and fabricator have defined the material finish and weld treatment, a sample swatch can be created to use as reference. The sample swatch can be as simple as two pieces of stainless steel square tubing welded together and finished per the aforementioned specifications.

 

 

STEP 2: Choosing Abrasives and Power Tools
 
Choosing Abrasives
 
Discs and belts are the most popular abrasive mediums for mechanical finishing of stainless steel. Most often abrasive discs are used for weld treatment, and belts are used for material surface treatment. The one notable exception is the use of Dynafile small portable belts for treating welds in hard to reach locations.

 

The following illustrates the typical mechanical finishing steps for weld treatments along with the appropriate abrasive.
 
For removal of excess weld, spatter and discoloration, use the following abrasives:
   - Grinding wheel
   - Resin fiber disc
   - Abrasive flap disc
   - Dynafilecoated abrasive belt
 
For creating a uniform scratch pattern in weld, use the following abrasives:
   - Weld disc
   - Surface conditioning disc
   - Unitized disc
   - Dynafile coated surface conditioning belt
 
The advent of the flap disc has significantly reduced the steps required for stainless steel finishing. In the past, operators used a two step process for weld treatment consisting of a grinding wheel for the first step (material removal) and then a resin fiber disc for the second step(uniform scratch pattern).
 
Increasingly, the flap disc isreplacing the older two-step process with a single step, requiring only one abrasive product. Layers of coated abrasive are arranged on a backing padin a fan like arrangement. In addition to reduced processing times, flap discs are less prone to gouging and scratching the work piece (very common with grinding wheels). Gouging and scratching can significantly increase downstream polishing times.
 
For finer weld finishing, a bevy of new products are now available that improve surface finishes and reduce polishing times. The newer products include surface conditioning flap discs, combination flap discs, and unitized disc. Surface conditioning material is a nylon web impregnated with abrasive grit. The material is available incoarse (50-80 grit), medium (100-150 grit), fine (180-220 grit), and very fine (240 -360grit) and produces a matte like finish when applied to stainless steel. Combination discs combine coated abrasive and surface conditioning on the same backing pad in an interleaf pattern, thus removing the weld and polishing at the same time. Unitized discs consist of layers of non-woven material impregnated with abrasive grit. They are available in coarse through fine grits and excel at producing fine finishes on stainless steel welds.
 
Abrasive Belts
As previously discussed, belts are generally used to process larger material surface areas.The two main categories of abrasive belts used in stainless steel finishing are coated and surface conditioning.
 
Traditionally, zirconia-coated abrasives belts were used for stainless steel finishing. However, the introduction of newer ceramic grain belts caused a rise in popularity of stainless steel finishing. The newer ceramics offered by leading coated abrasive manufacturers, such as VSM Abrasives and Norton Abrasives, require less pressure, last longer than other coated abrasives, and cut cooler-particularly important for stainless steel finishing as heat generation is an obstacle to effective finishing.
 
Belts made out of surface conditioning are generally applied after the coated abrasives do their work. The surface conditioning belts texture more than remove material. The belts generate the appearance of a linear brushed finish on the material.
 
When sequencing abrasives, fabricators should work from coarse to fine. Skipping more than one grade level when moving finer in grit tends to give the material an uneven choppy look. When blending welds and large surfaces, using longer strokes will result in a more even appearance.
 
Power Tools
 
While the angle grinder is still the workhorse for stainless steel weld finishing, linear belt finishing tools are required to achieve the higher level stainless steel finishes that are more frequently being specified.

 

 
Power tools are available as pneumatic or electric. Pneumatic tools offer the advantage of being lighter, and therefore, easier for the operatorto hold and handle. On the other hand, electric power tools offer the advantage of more power and variable speed control.
 
The ability to control the speed of the tool is essential for stainless steel finishing. As the surface finish becomes finer, the speed of the tool must be reduced to achieve a uniform appearance in the surface finish. Therefore, fabricators attempting #4 finishes and finer, need to have variable speed angle grinders and linear belt finishers in their workshops. By reducing finishing speeds as the surface finish becomes finer, abrasive consumption is also reduced resulting in the need for fewer abrasives to complete the project.

 

New grinders, such as the WE14-125VS from Metabo, offer variable speed, vibration reduction, quick wheel change and a powerful 12 amp motor. The yellow dial at the bottom controls the speed. This 6" unitized disc must be run at 3000 rpm (a normal grinder runs at 11000 rpm) and will grind and polish the corner weld in a single step.
 
For linear finishing andblending welds with adjacent pre-polished material, variable speed burnishing machines such as the flex machine below, allow operators to more easilyachieve desired finishes in stainless steel. It is important to note that a linear scratch pattern (frequently specified for stainless steel finishes) cannot be achieved with an angle grinder. The angle grinder will leave swirl marks in the material. Only a burnisher, like the unit below, can achieve the desired finish.
 
A relatively new class of power tool has been gaining popularity for polishing pipe and hand rails. The pipe sander wraps 270 degrees around the work piece and quickly accomleft: plishes finishing tasks with ease. The pre-tensioned arms apply even pressure to the workpiece producing a uniform finish.
 
The Dynafile is another tool required for most stainless steel finishing jobs. The tool accepts small portable belts and excels at accessing hard to reach areas. Dynafiles are typically used where access to use the grinder or polisher cannotbe gained.

 

 

The final step is to create the mechanical stainless steel finishing plan and execute completion of the project. This involves pre-planning the actual work steps involved in order to complete the required finish. Each work step should list the power tool, abrasive type and grit which will be used.
 
Nowhere is preparing more important than in the area of mechanical polishing. By carefully planning the project, hours of needless re-work can be avoided. If fabricators are attempting a finish they have not achieved in the past, they may want to consult with their abrasives and powertool distributor for guidance.
 
The following points should be considered during the planning process:
   1.  Ordering pre-polished material when possible.
   2.  Protecting the material with a plastic film coating.
  3.  When possible using TIG welding.
   4.  Minimize weld diameter and spatter.
   5.  Pre-polishing parts that have difficult access points.
   6.  Educate employees concerning care in material handling (stainless steel is easily    
        scratched)
Below is an example of a polishing plan created for a stainless steel housing. The housing walls are 11 gauge 304ss, the flanges and end plates are =BC" thick 304ss. The requirement is a #4 finish with all welds ground smooth and flush with the base material.
 
 
Finishing Plan
Order all 11 gauge 304 ss and 1/4" thick 304ss material, pre-polished to a #4 finish.
Keep protective plastic on pre-polished material.

 

 
 
Task - Tool -  Abrasive
Weld treatment -Metabo 4-1/2" variable speed grinder, Flap disc ceramic grain 80 grit, then 120 grit Unitized disc 6A medium
Hard to reach areas - Dynafile Belt ceramic grain 80 grit, then 120 grit, then med. surf conditioning.
Blending -weld areas withbase pre-pol­ished material Flex burnisher Medium Scotch-Brite belt. 
 
 
Summary 
 
In summary, the three steps critical to successful stainless steel finishing include defining the customers, requirements, planning the sequence, which includes choosing the abrasives and tools that will be used, and executing the plan. By adopting this approach, fabricators can fulfill their customers' requirements in the least time and withthe least cost.

LVA