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Speaking of Precision

Speaking of Precision is a knowledge preservation and thought leadership blog covering the precision machining industry, its materials and services. With over 36 years of hands on experience in steelmaking, manufacturing, quality, and management, Miles Free (Milo) Director of Industry Research and Technology at PMPA helps answer "How?" "With what?" and occasionally "Really?"

OSHA Mandatory Electronic Injury and Illness Reporting Delayed

Posted June 19, 2017 12:00 PM by Milo
Pathfinder Tags: injury osha reporting

No Formal Announcement. Why?

No Entry in the Federal Register. Why?

No Official Statement. Why?

Just a tiny little update buried in the middle of the OSHA Injury and Illness reporting Web page. We discovered it while doing some fact checking to make sure that we gave our members advance notice of the new reporting requirements which would have been July 1, 2017!

No Federal Register Entry, No Official Statement, just a sneaky little snippet buried without fanfare on a web page…

Since OSHA hasn’t even given us the courtesy of a formal statement, we have no idea how long the delay will last.

But here are some questions we’d ask the agency that threatens our shops with fines and penalties for non-compliance with their dictates:

  1. Have you set up that electronic infrastructure (secure computer system) for our reporting to be entered that you assured us at the hearing would be available?
  2. Have you hired contractors to create that secure computer system for our reporting to be entered?
  3. Have you tested the security protocols on this secure electronic system so that employers and their employees can be assured of the security of our data?

Note to shops- despite the Agency’s inability to hit its own deadlines, you can be assured that the retaliation provisions of this rule and their new take on “employee right to report” will remain enforceable.

Our original post on this rule

Link to OSHA Injury and Illness Recordkeeping Page


Editor's note: CR4 would like to thank Milo for sharing his blog, which can also be read here.

3 comments; last comment on 06/20/2017
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Decarburization and Your Precision Machine Shop

Posted June 12, 2017 9:00 AM by Milo
Pathfinder Tags: decarburization Hardness subscale

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.


Editor's note: CR4 would like to thank Milo for sharing his blog, which can also be read here.


Stress Cracks in Steel Bar Products.

Posted June 05, 2017 10:00 AM by Milo
Pathfinder Tags: cracks machining steel stress

“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.”


Editor's note: CR4 would like to thank Milo for sharing his blog, which can also be read here.


PMPA Selected as National Partner to Grow Apprenticeships in Manufacturing

Posted December 21, 2016 10:00 AM by Milo

PMPA is proud to be partnering with NIMS, to help companies find new ways to help students and workers gain skills for success.

Apprenticeships to build a pipeline of skilled professionals for a great manufacturing career.

The National Institute for Metalworking Skills (NIMS) has been selected by the U.S.Department of Labor as an industry intermediary to support the expansion of registered apprenticeships within MANUFACTURING. The Precision Machined Products Association (PMPA) a founding stakeholder member of NIMS, will work with NIMS to increase access to apprenticeships and assist employers in developing new programs that reach diverse talent pools among our membership. As part of this initiative, $500,000 is available to support companies in establishing a registered apprenticeship program with the Department of Labor.

“For over two decades, NIMS has worked with companies, workforce development groups and community colleges to stand-up high-caliber apprenticeship programs across the country,” said Jim Wall, Executive Director, NIMS. “This contract gives us the unique opportunity to create more impact in our industry by expanding apprenticeships to underrepresented populations and to new companies looking to establish a sustainable talent pipeline.”

What’s in it for your company? NIMS will focus on providing companies with tools and resources to develop customized registered apprenticeship programs. These programs combine on-the-job training with job-related classroom instruction and meet national standards for registration with the Department of Labor or State Apprenticeship Agencies. PMPA is working with NIMS to help facilitate the creation of registered apprenticeships for our member companies.

If you are interested in enhancing your talent pipeline through apprenticeships, this program may be for you.

Companies that are interested in building an apprenticeship program or organizations that are interested in partnering with NIMS should contact Sterling Gill sgill@pmpa.org; for more information go to www.mfgapprenticeship.com or email the NIMS ApprenticeshipUSA team at apprenticeship@nims-skills.org.

1 comments; last comment on 12/21/2016
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Thread Rolling Thin Walls--CJ Winters

Posted December 07, 2016 10:00 AM by Milo

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

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.


Editor's note: CR4 would like to thank Milo for sharing his blog, which can also be read here.

1 comments; last comment on 12/09/2016
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