As Director of Technology and Industry Research for PMPA, Miles brings 38 years of hands-on experience in areas of manufacturing, quality and steelmaking. He helps answer "HOW?","WITH WHAT?" and "REALLY?"
Vacuum treated (vacuum degassed) steel is used for critical applications that require steel with an exceptionally high degree of structural uniformity, internal soundness, and other characteristics that may be impaired by the effects of uncontrolled amounts of dissolved gases. Vacuum degassing treatments, along with various deoxidation practices, are specified to control the amounts of dissolved gases in the steel. The benefits of vacuum treatment include:
Reduced hydrogen content, which reduces steel’s tendency to “flake” or become “embrittled.”
Reduced oxygen content, which makes it easier for the steel to conform to restrictive microcleanliness requirements.
Improved recovery and uniformity of alloying elements and other additive distribution.
More controlled steel composition.
Higher and more uniform transverse ductility, improved fatigue resistance, and improved high temperature performance.
Can be used to achieve exceptionally low carbon content that are otherwise unobtainable by conventional means.
What are some situations where vacuum treatment is employed?
Large forgings and large cross sections where hydrogen would otherwise remain and contribute to flaking and embrittlement.
Bearings where uniformity throughout the section is important for critical performance.
Inverted delta, human critical safety applications where steel toughness and performance place high demands on the steel’s properties in all directions.
The removal of oxygen by degassing is a challenge for the steelmaker, because this element is extremely reactive: It can exist in the steel in many forms, such as free oxygen; it can dissolve in the melt as a soluble nonmetallic oxide; it can combine with carbon to form gaseous oxides; and it can exist as complex oxides in the accompanying slags and refractories in the process.
While it is best to measure using the parameter specified in the print, there are rules of thumb available that can help clear up the confusion and convert Ra to Rz or Rz to Ra.
The methodology of measurement and what is measured are quite different. This is critical to understand if you will not be paid for your parts because the Ra you measured is not in fact the Rz surface profile that customer specified.
According to an article in Modern Machine Shop written by George Schuetz, director of precision gages at Mahr Federal, “Ra is calculated by an algorithm that measures the average length between the peaks and valleys and the deviation from the mean line on the entire surface within the sampling length. Ra averages all peaks and valleys of the roughness profile and then neutralizes the few outlying points so that the extreme points have no significant impact on the final results.
“Rz is calculated by measuring the vertical distance from the highest peak to the lowest valley within five sampling lengths, then averaging these distances. Rz averages only the five highest peaks and the five deepest valleys—therefore, extremes have a much greater influence on the final value.”
According to doctor blades manufacturer Swedev’s website, “Ra is the arithmetical average value of all absolute distances of the roughness profile from the center line within the measuring length. Rz is the average maximum peak to valley of five consecutive sampling lengths within the measuring length. Ra averages all measurements and does not have any discriminating value in separating rejects from acceptable cylinders.”
And by the way, the definition of Rz has also changed over the years. Which definition of Rz exactly is your customer using? How do you know?
You will find “Conversion Ratios” on the internet provided by well-meaning people. But how useful can these be when the range said to be equivalent goes from 4:1 to 7:1 to 2-:1? 4:1 is equivalent to 20:1? Really? Not in my math class.
Smart shops will avoid using these “approximations in name only” and communicate with their customers to determine the customer’s true need. Gambling on conversion factors that you found on the internet is not professional. It is an example of poor engineering practice, and it fails to serve and protect your customer.
Surface finish measurement procedures, general terminology, definitions of most parameters and filtering information can be found in American Standard ASME B46.1 – 2009, Surface Texture, and in International Standards, ISO 4287 and ISO 4288.
The National Technical Conference (NTC) is one of PMPA’s most valued deliverables. Produced by members for members, this conference shares how-tos across the range of our industry’s challenges—operations, management and quality. Presenters are people that can (and do) do the work. Presenters at the conference include:
Building an Effective Training Program being presented by Shingo Silver Award winning shop experts Dan Vermeesch of Micron Manufacturing Company and Dave Masereau of Boston Centerless.
Gary Griffith (the highest ranked presenter) is back with a great workshop on GD&T.
Diane Thielfoldt will talk more about our millennial workforce.
There are also sessions on troubleshooting, ISO 9001:2015, rapid improvement events, finish issues, shopfloor math, innovating with CAM and CNC, print and part review, just to name a few. This conference is truly packed with a host of opportunities for your team to bring back new ideas and new capabilities to your shop.
The NTC runs from April 19-21 in Columbus, Ohio, with the Precision Machining Technology Show (PMTS) immediately following the conference. (Your registration to the NTC will automatically register you for PMTS.)
Sign up now for the National Technical Conference.
Want more info on programs offered? Click this link to review more than 30 sessions that are packed within these 2 ½ days of training.
Don’t miss your chance to upgrade the skills of your team and the capabilities of your shop.
The 0.15- 0.35 weight percent of lead contained in these bars helps them machine 25 percent faster with less power required.
Leaded steel bars historically have been a mainstay raw material in the screw machining industry. As more applications and newer technology move toward non-leaded steel applications, I thought that a brief refresher about lead and its role in shops might be timely.
Leaded steel bars are standard steels and widely available. In the U.S. 12L14 is the predominant grade. 11SMnPb30, 11SMnPb28, 9SMnpb28, and 9SMnPb36 are German designations nominally equivalent to 12L14. The Chinese version of 12L14 is Y15Pb; Japanese nominal equivalents include SUM22L, SUM23L and SUM24L.
Leaded steels are selected for use for the savings achieved in producing parts by machining.
Leaded steels are not appropriate for all parts, and parts with low amounts of stock removal may not create any noticeable savings.
Today’s leaded steels are more consistent, more uniform, than they were when produced by the ingot process.
The decision to use leaded steels for a specific part must be based on the economics for that part—volume, stock removal, part complexity, tolerances required, surface finish needed are all factors that contribute to that economic calculation.
There is no sacrifice in mechanical properties when adding lead to steel. Neither longitudinal nor transvers mechanical properties are affected by the addition of lead to steel.
Leaded steels are currently permitted under European Union regulations covering End of Life Vehicles, RoHS.
The reduction in energy required and time needed (about 25 percent!) to machine a part make leaded steels environmentally friendly by reducing carbon dioxide emissions to create parts compared with using unleaded materials.
In order to be dangerous to humans, lead must be in a soluble form. The lead in steel bars is a separate solid phase. IARC lists lead under its Group 2B category, “possibly carcinogenic to humans.”
Lead, as well as chromium, copper, manganese, nickel, and phosphorous is required to be reported under Sara 313 (40 CFR 372.65) when they are above established thresholds.
Manganese and sulfur have a powerful effect in reducing flank wear on HSS tools.
Manganese sulfides are a separate internal phase.
Flank wear is the “normal” failure mode for tools when machining steels. The volume fraction of manganese sulfides is a determinant of the tool’s wear rate. “The wear rate of high speed steel tools decreases rapidly up to about 1 percent volume fraction of MnS and then levels off to a constant wear rate as the volume fraction is increased.“-Roger Joseph and V.A. Tipnis, “The Influence of Non-Metallic Inclusions on the Machinability of Free-Machining Steels.”
As sulfur rises beyond 1% volume fraction, surface finish improves, chips formed are smaller with less radius of curvature, and the friction force between cutting tool and chip decreases because of lower contact area.
How does manganese sulfide improve the machinability? The MnS inclusions act as “stress raisers” in the shear zone to initiate microcracks that subsequently lead to fracture of the chip. MnS inclusions also deposit on the wear surfaces of the cutting tool as “built-up edge (BUE).”
BUE reduces friction between the tool and the material being machined. This contributes to lower cutting temperatures. BUE mechanically separates or insulates the tool edge from contact with work material and resulting heat transfer. This is why resulfurized steels in the 11XX and 12XX series can be cut at much higher surface footage than steels with lower manganese and sulfur contents.