Three Contributors to Plating Problems on Steel Parts

December 4, 2013

The causes of plating difficulties  on parts manufactured from cold drawn steel bars are neither mysterious nor beamed on to the parts from the outer reaches of the galaxy. Plating difficulties invariably are related to three potential contributors: Inadequate cleaning, Insufficient stock removal, and Features of the part being plated.

Your plating problems are not beamed down from here...

Your plating problems are not beamed down from here…

The location of the plating problems on your parts  gives you a key to determining the mechanism of failure

  • If the plating problems are occurring on both original bar surfaces as well as on as machined surfaces, Inadequate Cleaning is likely the cause.
  • If the plating difficulties are only on the portion of your parts that are original bar stock surface, Insufficient Stock Removal is the most likely cause of the problem.
  • If the plating is fine everywhere else on the part except near a particular feature, Retention of contaminating fluid by a feature of the part is the likely cause.

Inadequate Cleaning

I used the term inadequate  to describe the situation where despite efforts to clean, some soil or contaminant remains, interfering with the plating. The cleaning method employed could just be insufficient to the task of cleaning- not enough time, agitation, etc.

Or it could be the incorrect cleaning process being used. Acidic cleaners do not remove oils or greases.  In fact, we oil our metallic products like tools, firearms, and fishing reels to prevent them from being attacked by the acids in our skin and the environment. Alkaline cleaners are needed to remove oils and greasy residues from steel parts. Solvents can be used to remove the bulk of oily residues as well.  If an insufficient or improper preclean is performed prior to plating,  oils or oily residues can remain on the surfaces of the parts and mask or obstruct the deposition  of the metallic plate.

If the plating problems are occurring on both original bar surfaces as well as on as machined surfaces, this is likely the cause.

Insufficient Stock Removal

When I first started out in the industry, most steel bars were acid pickled prior to cold drawing. In pickling, acid “wets” the entire bar surface,  is able to penetrate through the surface scale to react with the acid soluble iron oxide known as Wustite, (FeO) on the innermost bar surface, and thus assures the nearly complete  removal of all scale from the bar surface.  (See scale note  below)

Acid disposal became a significant operational challenge, and the industry moved to  the use of mechanical descaling (shotblasters) to abrade away the hard iron oxide scale from the surface of the bars.

Unlike pickling, shotblasting does not fully remove every bit of scale- the shot stream abrades off most, but not every single bit of scale on a bar’s surface. If the bar surface has many fine depressions or pits, the abrasive shot may not be able to contact the scale at the bottom of these depressions. The presence of this scale could interfere with the subsequent plating of parts by the following mechanisms:

  • It can retain metalworking fluids or cleaner from the precleaning step  and then release these during plating causing localized reactions and staining;
  • Because scale is an insulator,  it will prevent electrical current flow at its location and thus mask  or prevent the deposition and adhesion of the plate;
  • It can create an air bubble by geometry as well as perhaps a hydrogen bubble if the bath is acidic. this bubble could form a mechanical barrier masking its location and preventing deposition/ adhesion of the plate.

If the plating difficulties are only on the portion of your parts that are original bar stock surface, this is the most likely cause of the problem.

Part Geometry Features and Location

Many times the design of the part can be the cause of the plating difficulties.

Features like small diameter holes, blind holes and recesses and grooves which can retain fluids, create bubbles or support a meniscus can result in localized contamination, create staining,  and interference with deposition by providing a fluid or bubble barrier.

If the plating is fine everywhere else on the part except near a particular feature, retention of fluid by a feature of the part is the likely cause. Adding a wetting agent to reduce surface tension in cleaner or rinse can eliminate the problem.

There are other problems that can arise during plating that can be attributed to the plating process itself, but  it has been my experience that these 3 categories  will cover most of the problems  encountered when both the machine shop and the plater claim that “there must be something wrong with the steel.”

If the machined surfaces plate fine- but not the original bar surface nor the inside of a hole- it isn’t the steel. It’s one of the above.

_____

Scale Note: There are two additional iron oxides that could be present- Hematite (Fe2O3) and Magnetite (Fe3O4) – both of these are acid insoluble, but for this discussion, it is sufficient to say that they are removed  by the removal of the underlying Wustite scale during pickling.

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How Much Stock Is Required?

December 18, 2012

If you think that just multiplying the part length by the number of parts needed is the answer you will be quite disappointed…

The number of these needed is more than just the number of parts times the part length in inches...

The number of these needed is more than just the number of parts times the part length in inches…

The part length usually accounts for the vast majority of stock required.

But the amount of material lost by cutoff tooling(kerf loss), the first piece and remnant in the machine “steal” parts makeable from the material you purchase.

Parts can also be lost from production by failure to conform to requirements for dimension or during an extended campaign to get the set-up dialed- in.

Quick rules of thumb to allow for bar end and scrap loss based on the length of the finished part (plus cutoff loss)  include:

  • For short parts under 2 inches in length, allow for 5% extra material needed to make the desired quantity;
  • For parts between 2 to 3 inches inclusive allow for 6.5% extra material needed to make the desired quantity;
  • For parts between 3 to 4 inches inclusive allow 8.5% extra material needed to make the desired quantity;
  • For parts 4 inches and over allow 10.0% extra material to make the desired quantity.

Your mileage may vary. If you use narrower than usual cutoff tools, this may be reduced a bit.

If you use cutoff saws, you may achieve a significant savings.

But if your team can’t get the setup and dimensional control right, these numbers are downright optimistic.

Getting the setup correct the first time is key to effective use of material too.

Getting the setup correct the first time is key to effective use of material too.

Note, do not confuse this for for scrap loss by weight. Heavy stock removal parts may have up to 90% of total material (by weight) removed to create the desired geometry during machining. These guidelines are just an estimating tool to give you a minimum order quantity to assure you can deliver the required number of parts ordered.

Curly adjusts the machine.


5 Ideas To Thrive Despite Market Uncertainty

January 5, 2012

Volatility. Uncertainty. Complexity. Ambiguity. Welcome, to the year 2012!

The headlines are filled with factors that make our planning for the year ahead futile at best. Eurozone Sovereign Debt. China Currency. U.S. Economic Problems. High Unemployment.

With all of these issues potentially ready to emerge at any second, what can we do to intelligently manage our risk?

Here are 5 ideas to thrive in the face of today’s uncertain market environment:

1) Edit your customer list. In the old days, adding customers was the thing to do. But noone ever made up for low margins with volume.Edit your customer list to those customers that provide you with a return at or above your cost of capital. Just like we add value by taking stock removal from barstock, removing the customers who cost you more than they contribute will free up limited resources so that you can better serve the customers who do cover your costs. (Note: this will really pay dividends for you with your banker, who is no longer lending on B.S.)

2) Work your supply chain- in both directions. Communication and cooperation are critical to success when facing uncertainty. You just might find that your communications is what is keeping your part of the turned products market thriving. Communicate, communicate, communicate.

3) Manage your inventory and dollar cost average in high variability times.There is no doubt that the year 2012 will be one of high variability. Rather than making a few outsize buys of raw materials, placing smaller regular purchases will enable you to dollar cost average as the market does its roller coaster thing based on whatever news story is hot that day. You can’t sell parts this week if raw material is 4 month lead time. You know what you typically use. Intelligently manage risk, have material on hand to make parts, and then you won’t have to listen so carefully to the news.

4) Producing low value added in a high cost location is a losing business strategy. Your peers have been leaning, innovating, and reducing their costs to remain competitive. In addition to editing your customers, you need to edit those low value added jobs out of your business plan. Editing adds value.

5) Improve cross functional communications between your team and your customers’. This is a great year to take your people to meet your  their customers and counterparts and to exchange ideas on ways to improve performance and reduce costs and waste all the way around. While this might be seen as a corollary of idea number 2, the fact is that taking your people to customers is a great idea for continuous improvement anytime.

We have no idea when any of the crises we know of will blow up. We have no idea when the sword might fall. But inspite of increased volatility, uncertainty, complexity and ambiguity, we can improve our chances for success by taking the steps outlined above.

What do you think is the most likely issue to emerge in 2012 that we have not listed above which will affect your business?

Sword of Damocles


5 Not So Obvious Advantages of Centerless Grinding

September 29, 2011

It is well known that centerless grinding processes allow parts to be held to tighter dimensional tolerances, achieve smoother surface finishes, and hold high degrees of straightness. 

Cincinnati is the legacy technology.

Beyond these obvious advantages, all centerless grinding processes offer the following 5 Not So Obvious Advantages:

  1. The grinding process is essentially continuous, because the loading time, when compared to grinding between centers, is exceedingly small.
  2. The work is rigidly supported directly under the grinding cut as well as for the full length of the cut. This means  that no deflection takes place during the grinding operation, permitting heavier passes than grinding between centers.
  3. No axial thrust is imposed on the work while grinding. The absence of end pressure makes it possible to grind long pieces of brittle materials and to grind easily distorted parts.
  4. Because the error of centering is eliminated, a true floating condition exists during the grinding process. This results in less stock needed and longer wheel  life / yield.
  5. Large quantities of smaller size work can be automatically ground by means of a magazine, gravity chute, or hopper feeder attachments, depending on the shape of the workpiece.

A few final thoughts: The degree of error in the centerless grinding process (setup or compensating for wheel wear) is reduced by half, since stock removal is  measured on the diameter rather than the radius. Centerless grinding is a mature process, with few wear surfaces in the machine, and automatic lubrication, making maintenance a small part of the total cost of this process.


7 Causes For Quench Cracking Of Steel

August 3, 2010

Failures of steel parts in service or production occur very infrequently. However, when steel parts fail, the consequences are dire.

Quench crack- this is not good!

Here are 7 ways that steel can fail as a result of Quench Cracking from heat treatment.

  1. Overheating during the austenitizing portion of the heat treatment cycle can coarsen normally fine grained steels. coarse grained steels increase hardening depth and are more prone to quench cracking than fine grain steels. Avoid over heating and overly long dwell times while austenitizing.
  2. Improper quenchant. Yes, water, brine, or caustic will get the steel “harder.” If the steel is an oil hardening steel, the use of these overly aggressive quenchants will lead to cracking.
  3. Improper selection of steel for the process.
  4. Too much time between the quenching and the tempering of the heat treated parts.  A common misconception is that quench cracks can occur only while the piece is being quenched. This is not true. If the work is not tempered right away, quench cracks can (and will) occur.
  5. Improper design– Sharp changes of section, lack of radii, holes, sharp keyways, unbalanced sectional mass, and other stress risers.
  6. Improper entry of the part/ delivery of the quenchant to the part. Differences in cooling rates can be created, for example, if parts are massed together in a basket resulting in  the parts along the edges cooling faster than those in the mass  in the center. Part geometry can also interfere with quenchant delivery and effectiveness, especially on induction lines.
  7. Failure to take sufficient stock removal from the original part during machining. This can leave remnants of seams or other surface imperfections which can act as a nucleation site for a quench crack.

Finally, materials that are heat treated to very high strength levels, even though they did not quench crack, may contain localized concentrations of high residual stresses. If these stresses are acting in the same direction as the load applied in service, an instantaneous failure can occur. This will be virtually indistinguishable from a quench crack during an examination, due to its brittle failure mode, lack of decarburization on surface of the fracture, or other forensic evidence of a process failure.

When looking at quench cracking failures under the microscope, cracks and crack tributaries that follow the prior austenitic grain boundaries are a pretty good clue that grain coarsening and or its causes-  overheating or too long time at temperature- occurred. Temper scale on the fracture surface helps the metallurgist know that the crack was present before tempering. Decarburization may show that the crack was open prior to quenching.

Photo1 Thanks to WIP SAMI over at British Blades for the photo.

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3 Actions To Minimize Cracks In Our Shops

July 29, 2009

Paying attention to draft, chemistry, and steel melt source processes can help you minimize the potential for cracks at your customer after cold work operations.

After a crimping, staking or swaging operation, cracks can develop. This is because the cold work needed to swage,  stake, crimp, etc. was greater than the material’s available elasticity. This is the case in the part photographed here.

Cracks can develop after cold work is performed on machined parts.

Cracks can develop after cold work is performed on machined parts.

In order to minimize cracking during or after crimping, or thread rolling, or other substantial cold work, take the following steps:

  1. Specify non-renitrogenized material;
  2. Inform your supplier of your cold work application. They can consider reducing cold draft, or changing suppliers of the hot roll to get basic oxygen process, low residual, low nitrogen steel;
  3. Ask the customer to consider changing the grade. Resulfurized steels are capable of being somewhat cold worked, but their high volume fraction and weight percent of nonmetallic inclusions (What makes them cut so well!) is also what works against successful cold work.

To minimize the occurrence of cracks  that are not a result of cold work, try this:

  • Assure that adequate stock removal is taken in machining;
  • Buying from reputable sources whose quality systems employ rototesting and eddy current testing;

When cracks are discovered in your shop, what actions do you take?

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