Thank You to My Readers

April 12, 2017

Actually, a million thank yous! Thanks to readers like you, SPEAKINGOFPRECISIONBLOG has over 1,000,000 views.

1,000,000 views!

Our first post  was from June 30, 2009 and remains as relevant to day as it did then: 5 Reasons to choose a Career in Precision Machining

We’ve come quite a way since that very first post!

What do you like? 

Our post on The Difference Between Accuracy and Precision Measurement in Your Machine Shop is our most popular with 44,445 views.

Hardness vs. Hardenability- There is a Difference is our second most popular post at 35,780 views.

5 Facts About Manganese in Steel, 7 Causes for Quench Cracking of Steel, and Why Manufacturing is the Right Career Choice- DATA! all came in between 21,000 and 30,000 views each.

Average number of times a post gets seen on PMPASPEAKINGOFPRECISIONBLOG: 1048 times.

1,000,000 is an aspirational number. How many of us get to measure anything that we do in quantities of millions?  (Actually, all of our shops do, they call it “normal production!”)

But as an individual, as someone sharing knowledge and experience- 1,000,000 views;  1,000,000 shares of information; 1,000,000 human to human connections- that is an unexpected and very satisfying validation.

Thank you for spending a small part of your day with me here at my blog.

Together, we’ll continue to make sense of the the issues that make a difference to all of us in North American Manufacturing.

Especially Precision Machining.

1,000,000 views!

Do I believe in the power of social media to help us connect and share?

You bet I do! 1,000,000 times YES!

Tooting Own Horn  photo credit.


Decarburization and Your Precision Machine Shop

March 17, 2017

Decarburization on surface layers can affect heat treatment and hardness attained on parts. Decarburization also provides evidence of where in a process a defect or imperfection occurred.

Most defects  in steel workpieces encountered in our precision machine shops are longitudinal in nature. While their presence alone is enough to concern us, for the purposes of corrective action, it becomes important to identify where in the process the longitudinal imperfection first occurred. Visual examination alone is not enough to confirm the source. Did it occur prior to rolling? During rolling? After rolling? Understanding decarburization and how it presents in a sample can help us to identify where and when  in the process the imperfection first occurred.

The question that we want to answer as part of our investigation is usually “When in the process did the defect first occur?” Looking at decarburization and any subscale present can help us answer that question with authority.

What is Decarburization?

The light area (ferrite) surrounding the dark intrusion is decarburization. note the lack of pearlite in this decarburized (lighter) zone. There is no evidence of scale, indicating that this defect was created during, rather than prior to rolling.

“Decarburization is the loss of carbon from a surface layer of a carbon containing alloy  due to reaction with one or more chemical substances in a medium that contacts the surface.”Metals Handbook Desk Edition

The carbon and alloy steels that we machine contain carbon. In the photo above, the carbon is contained in the pearlite (darker) grains. The white grains are ferrite. In an etched sample, decarburization surrounding a defect is identified as a layer of ferrite with very little, or none of the darker  pearlitic structure typically seen in the balance of the material. The black intrusion in the photo above is the mount material that has filled in the crevice of the seam defect.

What is Subscale?

The grey material adjacent to defect within the white decarburized area is subscale. This subscale is evidence that the crack was present on the bloom prior to reheat for rolling

Subscale is a reaction product of Oxygen from the atmosphere with various alloying elements as a result of time at high temperatures. The presence or absence of the subscale is the indicator that helps us to pinpoint the origin of the defect. For a subscale to be present,  the time at temperature must be sufficient for oxygen to diffuse  and react with the material within the defect. According to Felice and Repp, 2250 degrees F and fifteen minutes  is necessary to develop an identifiable subscale. Lower temperatures would require longer times. Typically rolling mill reheat cycles offer plenty of time to develop a subscale in a prior existing defect. However, for defects that are created during rolling, the limited time at temperature and the decreasing temperatures on cooling make formation of subscales unlikely.

Reading Decarb and Subscale to Understand the Defect

Decarburization is time and temperature dependent. This means that its relative depth and severity are clues as to time at temperature, though interpretation requires experience and understanding of the differences in appearance from grade to grade based on Carbon content.

Symmetrical Decarburization

If the decarburization is symmetrical this is an indication that the defect was present in billet or bloom prior to reheat and rolling. oxygen in the high temperature atmosphere of the reheat furnace depletes the carbon equally from both sides of the pre-existing defect.

Asymmetrical Decarburization

Decarburization that is obviously asymmetrical indicates that the defect is mechanical in nature and was induced some time  during the hot rolling process.

Ferrite Fingers

Unetched specimen of seam (top). Etched specimen showing “ferrite finger.” (Bottom)

Ferrite fingers are a surface quality problem that is associated with longitudinal bar defects. During reheat, a defect in the bllom or billet is exposed to high temperature atmosphere, forming decarburization and subscale  around the defect. Rolling partially closes or “welds shut” the crack. However, a trail of of subscale is entrained in a  formation of almost pure ferrite which has been depleted of pearlite, carbon and alloy by the reaction at elevated temperature.   This trapped scale remains a potential oxygen source, driving further internal oxidation and decarburization if temperatures remain high.

Continuous improvement requires  taking root cause corrective action. Obviously identifying the root cause is critical. When we encounter longitudinal linear defects in our steel products, using a micro to characterize the nature of the decarburization and presence or absence of sub scale or ferrite fingers are important evidence as to when, where, and how in the process the defect originated.

 

 

 


Stress Cracks in Steel Bar Products.

March 14, 2017

“Stress cracks are defined as transverse or near transverse open crevices created when concentration of residual stresses exceed the local yield strength at the temperature of crack formation. These stresses can be mechanically induced or can be attributable to extreme temperature differences and /or phase transformations. They can originate at almost any point in the manufacture of the steel.”AISI Manual Detection, Classification, and Elimination of Rod and Bar Surface Defects

Stress cracks are often found visually  at locations that experience bending or straightening. They are also referred to as “Cross Cracks” or “Transverse Cracks.” Originally they were identified in mill billet and bloom products, prior to rolling.

Micro examination can help determine crack origin by noting:

  • Orientation
  • Intergranular nature
  • Presence of scale
  • Presence of subscale

Additional microstructural characteristics can reveal the thermal history of heating and cooling at the crack location.

This photo shows stress cracks on a conditioned billet.

Causes and Corrective Action

  • Excessive load during straightening can exceed the local yield strength of the material causing it to crack; reduce load applied by machine, or consider tempering or stress relieving material prior to straightening or further cold work.
  • Cooling too quickly can also induce stress cracks. Critical cooling rates are highly dependent on steel chemistry. Crack sensitive chemistries (Medium carbon and high carbon steels; also medium and high carbon steels with straight chromium or straight manganese additions.) These steels should be slowly cooled through transformation temperatures to minimize the occurrence.
  • Design faults such as
    • Heavy sections adjacent to light sections and sharp corners 
    • Failure to fillet sharp corners
    • Use of fillets rather than tapers
    • Undercuts
    • Overloading the material during fabrication, processing, or application.

Detection of stress cracks  is problematic as their transverse orientation makes them difficult to detect on equipment set up to detect longitudinal defects.

Final caveat: The term stress crack is arbitrarily defined based on industrial usage in the market. It does not necessarily imply anything about the specific metallurgical nature of the crack, I know that a number of people use the term “stress crack” to describe longitudinal cracks on steel bar products as well, which the AISI calls “Strain Cracks.”

 

 


EU Commission- RoHS Exemptions Delayed Again

January 24, 2017

The current exemptions for Lead in work piece materials for our shops will remain in effect and will not expire until the EU Commission completes the current ongoing review of the applications.

They can fly the flags, but getting regulatory decisions on time is not something the EU commission does very well.

They can run the flags up the flagpoles on schedule, but getting regulatory decisions on time is not something the EU Commission does very well.

According to Electronics Industry Portal I-Connect007,  the EU is unlikely to publish  the RoHS exemptions any time soon. Apparently the EU Commission only got around to reviewing the requests last month.  December 15, 2016. December 2016!

And they are only now in the process of preparing draft legislation to be sent to the member states for consideration later this year.

Later this year!

Bottom Line for your precision machining shop and your customers:

  • A decision on the RoHS exemptions of interest to the precision machining industry is unlikely to be made until Fall of 2017.
  • The current exemptions for Lead as an alloying element in steel, Lead as an alloying element in aluminum, Lead as an alloying element in copper will remain in effect and will not expire until the EU Commission completes the current ongoing review of the applications.

Here’s the full statement from I-Connect007

“The EU Commission and Member States continue to meet to discuss the disposition of RoHS exemption renewal requests submitted by industry in January 2015.  During their December 15, 2016 meeting in Brussels, the experts reviewed requests on: Lead as an alloying element in steel (Annex III exemption 6a); Lead as an alloying element in aluminum (Annex III exemption 6b); Lead as an alloying element in copper (Annex III exemption 6c); Lead in high melting temperature type solders (Annex III exemption 7a); Lead in a glass or ceramic other than dielectric ceramic in capacitors (Annex III exemption 7c-I), jointly with exemption request 2015-1; Lead as activator in the fluorescent powder (Annex III exemption 18b), jointly with exemption request 2015-3; Lead in solders for the soldering to machined through hole discoidal and planar array ceramic multilayer capacitors (Annex III exemption 24); and Lead in cermet-based trimmer potentiometer elements (Annex III exemption 34).

“The Commission is currently working on preparing the draft legislative proposals for these and other exemptions) which will be sent to Member States for written consultation.  The drafts will likely be published in the spring; final legislative acts could be published in the Official Journal of the EU (OJEU) in the fall at the earliest. Under the EU RoHS2, all existing exemptions were set to expire by July 21, 2016. However, all exemptions for which industry submitted a renewal application will not expire until the EU Commission completes the current ongoing review of the applications.”

I-Connect007

(PS. Interesting footnote, I searched for an image of the European Union Commission and found the one above-on a blog about the EU Commissions delay to decide on a VISA issue– last April. Not an isolated case of Bureaucratic delay!)

Photo credit: Pulse Blog


RoHS Exemptions for Machining Industry- PMPA Members Know

January 12, 2017

The EU decision on the  RoHS 2011 recast appeals for exemptions for leaded materials for machining is almost a year late- the decisions were due  from the commission January 21, 2016.

January 21, 2016!

rohs-word-cloud

PMPA has reviewed the findings of the study group assigned to review and report on the appeals regarding Lead for machining purposes in Steels, Aluminum, and Copper alloys.

If you are a PMPA member, you can get our summary  providing the latest status of the RoHS exemptions that affect our shops. The EU is almost a year late with their decision on the exemption appeals for leaded materials for machining under the latest (2011) RoHS recast.

PMPA has posted a summary of the consultants’ findings  and their potential impacts on our machining companies for our members information on the PMPA website here.

We have really been challenged by the EU to stay up to date on these exemptions, being almost a year late with the decision certainly keeps the uncertainty high for manufacturers.

I wonder if the European Commission is as  lenient with the deadlines on the regulated communities as they are for their own?

If you are a PMPA member, you can get the latest Status of the RoHS exemptions that affect our shops here : January 2017 RoHS Exemption Summary Report

If you are not a PMPA member, where do you go to stay up to date on regulatory issues that affect your business?


December ISM PMI Positive Bellwether for 2017

January 5, 2017

The Institute for Supply Management announced on Tuesday that its Purchasing Manager’s Index (PMI) for Manufacturing index rose 1.5 percentage points to 54.7 in December, its highest level in two years and up from 53.2 in November.

This is great news- let’s look at some details to find out why:

  • New Orders component– new orders rose 7.2 percentage points to 60.2 – their highest level since November 2014
  • Strength in Employment component– employment rose 0.8 percentage point to 53.1 – the highest since June 2015
  • And strength in the Production component-production improved to 60.3- very unusual outcomes for Manufacturing in December.

This is an unexpectedly solid report showing Manufacturing industry performance stronger in December than in November.

Highest in 2 years and up 1.5 points over November 2016

Highest in 2 years and up 1.5 points over November 2016

Here is why we see this as a bellwether for a great 2017 for our precision machining companies.

  • U.S. Light Vehicle (Auto) Sales set annual sales record according to the Wall Street Journal and reports we heard from online videos from AUTO NEWS. “A total of 17.55 million vehicles were sold in 2016, roughly 60% of which were classified as light trucks” according to the Wall Street Journal.
  • 2016 sales volume was up ~ 700,000 light vehicles, according to reports from Auto News
  • The average age of the U.S. Light Vehicle Fleet in 2016 was a record 11.6 years, According to Statista online, 

There is still plenty of reason to expect demand for light cars and trucks to be sustained based on the average age of the U.S. Fleet and the current low unemployment rate reported by the Federal government. Knowing that Automotive is the precision machining industry’s most heavily served market convinces us that these numbers reported by ISM, WSJ, Auto News and Statista bode well for our precision machining shops in 2017. I hope that you are preparing for success, not for hunkering down in 2017.

Happy New Year, indeed!

Auspicious!

Auspicious!

 

Link to ISM December 2016 report

Statista Age of Light Vehicles in U.S.

Calculated Risk December ISM Post and Chart


Thread Rolling Thin Walls- CJ Winters

November 3, 2016

Guest post by Lib Pietrantoni, CJWinter

Flaking threads and thread damage can be avoided when thread rolling thin walled parts.

Distortion during the thread rolling process can cause

  • Flaking,
  • Non-uniform thread geometry
  • Tearing
  • Collapse of threaded portion of part

These are particularly troublesome issues on thin walled parts.

These can be avoided if you assure that a minimum wall thickness is maintained for the process.

 

Minimum Wall thickness is determined by Nominal Thread diameter and thread pitch

Minimum Wall Thickness is determined by Nominal Thread Diameter and Thread Pitch

Larger nominal thread diameters require thicker minimum wall thickness; so do coarser thread pitches.

The way that you roll the thread can also be a factor.

According to Lib Pietrantoni at CJWinter, specialized pneumatic radial-pinch-type thread rolling machine attachments can apply equalized rolling pressure across the workpiece, ensuring thread concentricity, eliminating side pressure on both the parts and the machine, and allowing precise control of the penetration rate — especially important for thin-walled parts.

You can download the Thread Rolling Reference Chart at CJWinter’s website: reference chart

As a steel mill Quality Metallurgist, I saw my share of complaints that “the steel was flaking- it must be the steel.”

But the lab results never found the flaking anywhere except where the thread had been rolled – it was never on the bars as shipped.

Pay attention to minimum wall thickness when thread rolling!

And  don’t forget to pass this handy chart along to the engineer at your customer that is designing the parts that you make.

Thanks to Lib Pietrantoni at PMPA member CJWinter for providing this reference information.