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Qualitatively predicting Charpy impact toughness

  • 1.  Qualitatively predicting Charpy impact toughness

    Posted 03-03-2022 12:10
    I wanted to reach out to the community for input on whether anyone knows of a reliable means for predicting Charpy impact toughness as a function of composition, thermal processing, etc. in steels. For comparison, things like hardenability and hardness are very well-established in the literature; there are formulas for DI as a function of composition, TTT diagrams, charts for hardness vs. tempering temperature, etc. However, Charpy's seem to be a little less predictable. Some grades are just known to have good or poor toughness (think F22 vs. 1095) and on some level this is directly attributable to obvious variables like carbon content or extent of martensitic transformation but sometimes it gets more nuanced. For example, 4130 is not typically thought of as an optimal material for Charpy's due to the carbon content but we produce QT 4130 parts for an application with 237 HBW max hardness, which necessitates tempering very hot, and as such we can achieve acceptable Charpy's even at -75F. However, there is an inflection point where if you temper colder to achieve a slightly higher hardness, you will fail Charpy's in the single digits. Of course, you could further complicate the equation by factoring in thick section sizes, modified chemistries (a small vanadium addition, for example, which is quite common), etc.

    Usually when people come to me asking about whether a given material can meet a certain toughness requirement, unless it's an obvious yes or no based on the grade, my default answer is "we'll just have to test it out and see what we get" but I wish I had something better to offer since that can be costly in both money and time and it poses a catch 22 if the question comes up in something like a quote review where we'd need to commit to buying a heat of material, tooling, etc.

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    Sean Piper
    Product / Process Metallurgist
    Ellwood Texas Forge Houston
    Houston TX
    (713) 434-5138
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  • 2.  RE: Qualitatively predicting Charpy impact toughness

    Posted 03-04-2022 06:37
    Too many parameters have influence on steel Charpy absorbed energy to have a fully  comprehensive formula.

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    Donato FIRRAO FASM
    Professor
    Politecnico di Torino
    TORINO
    +393351494032
    Italy
    ------------------------------

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  • 3.  RE: Qualitatively predicting Charpy impact toughness

    Posted 03-04-2022 13:47
    The impact test is ment to verify the soundness of the material for a given set of processing. However unlike other mechanical tests , it doesn't generate any concrete design data. Its very difficult to quantify or have a formula. In fact that is not the aim either. You can predict tensile, fatigue etc but impact has to be verified.

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    Rahul Gupta
    Managing Director
    N D Gupta Enterprises
    Pune
    +919423009829
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  • 4.  RE: Qualitatively predicting Charpy impact toughness

    Posted 03-08-2022 09:24
    Processing (Tempering) and presence of residual elements, such as Tin, Arsenic has significant impact on testing results. It segregates around grain boundaries upon cooling and ends up in low testing results.

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    Sanjay Kulkarni
    Materials Engineer
    MSSC
    Troy, MI
    248-840-1056
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  • 5.  RE: Qualitatively predicting Charpy impact toughness

    Posted 03-07-2022 09:06
    When I was first starting out at Westinghouse Nuclear, we would periodically test Charpy monitoring specimens that were placed in the reactor vessels when the units were built. We used the change in impact energies over time as a crucial part of our model to assess irradiation effects on reactor vessel integrity during emergency conditions requiring rapid cooling. The assessment of course included remaining life assessment that was based on modeling the reduction of impact properties over time.

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    Aaron Tanzer
    Senior Metallurgical Engineer
    Metallurgical & Materials Technologies
    Baton Rouge LA
    (407) 247-9557
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  • 6.  RE: Qualitatively predicting Charpy impact toughness

    Posted 03-07-2022 14:15
    Sean,
    There are many publications on the subject of Charpy impact properties (shelf energy and transition temperature) versus chemical composition, microstructure, and processing. One source you might examine is the Book "Steels - Processing, Structure, and performance by George Krauss that is published by ASM International. Clearly, carbon and grain size have a major influence for all sorts of thermal processes. In general, there is a big difference between as-rolled or as forged properties versus quenched and tempered properties. Within the quenched and tempered group, the phosphorous content can have a substantial influence on temper embrittlement as a result of segregation at prior austenite grain boundaries. Sulfur can form Manganese sulfides that elongate during hot working and can be very detrimental for fracture along the direction of major elongation. As you note, heavy sections that cool more slowly can aggravate the effects of phosphorous, etc. related to grain boundary embrittlement.

    You do pose an interesting question as to whether a predictive model could be developed to estimate the impact properties. I think the answer is yes - provided that the models are segregated by the type of thermal processing and  the data is available to cover all the known effects. In the current state, I think folks try to do what they know will work and adjust if the first attempt falls short - as you say, make a heat and see what happens (costly if it doesn't work).

    Bob cryderman

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    Robert Cryderman FASM
    Research Associate Professor
    Colorado School Of Mines
    [Golden] [Colorado]
    (734) 735-3093
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  • 7.  RE: Qualitatively predicting Charpy impact toughness

    Posted 03-08-2022 12:31
    I have recently completed an intensive study of the effects of composition on the transition temperature and upper shelf energy for hot rolled steels and developed two equations which predict both these quantities.  They have surprising accuracy when compared to actual published CVN energy vs. temperature curves for a variety of structural alloys.  It will be published shortly after review is complete.  Also, I have also been very interested in why the ductile-to-brittle transition occurs in bcc steels, but this report now explains why various elements, physical properties of iron, and strain rate influence the formation and shape of the curve.  I will also have to check out the work of Prof. Krauss that you have mentioned for further insights.  An interesting report by Iskander & Stoller documents the effects of radiation on CVN properties for a structural steel.

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    Christopher Hahin
    Engineer of Structural Materials & Bridge Investigations
    Illinois Department of Transportation
    Springfield IL
    (217) 522-4023
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  • 8.  RE: Qualitatively predicting Charpy impact toughness

    Posted 03-09-2022 20:22
    Sean, 
    I hate to be a wet blanket but I spent 20+ years of my early career in the Materials Laboratory at Babcock & Wilcox testing thousands of Charpy specimens from many heats of carbon and low alloy (Mn-Mo-Ni) steels, both rolled plates and forgings.  Part of my job was to pick tempering temperatures for components in the shop based on temper studies we performed on each test section removed from the components after austenitizing.  The one thing I learned quickly was that there was no magic formula, every heat was different and these individual studies were the only way to guarantee the impact strengths of every part that would meet our customers' specifications.  I believe the bottom line is there are just too many variables involved.

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    [Gene] [Homer] [P.E.]
    [Metallurgical Engineer]
    [Babcock & Wilcox - Retired]
    [Barberton] [OH]
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  • 9.  RE: Qualitatively predicting Charpy impact toughness

    Posted 03-10-2022 01:06

    Hi, Sean and Gene,
    I agree with Gene that there is variability between heats, and there is great complexity to the prediction of toughness, but I do not believe that makes the situation hopeless. A very important variable (the main variable in high-strength steels), I believe is the inclusion content, which varies between heats of steel, but can now be quantified if this is built into the grade specification. The ductility and toughness of high-strength steels depends greatly on the number of inclusions larger than a critical size, (in addition to their composition, shape, and distribution), so at least the inclusion size distribution should be measured and provided as part of the grade specification. This depends mainly on the casting process, but of course can be altered by subsequent forming operations (eg rolling), heat treatments, and other down-stream processes. The importance of inclusion size distribution is well known in the bearing industry, when companies such as Timken will have special specifications regarding "cleanliness" (inclusion content) with individual customers which go far beyond the classic grade specifications which involve composition and heat treatment. New automated measurement methods for inclusions in steel are faster and more accurate than ever before. I believe that the ASTM standard grade specifications for all high-strength steels should be changed to include some measure of the inclusion content (in addition to composition and thermal-mechanical processing history). This would enable an accurate, quantitative answer to your very reasonable question,
    --Brian



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    Brian Thomas
    Professor of Mechanical Engineering
    Colorado School of Mines,Cleveland State University
    Golden CO
    (217) 493-9633
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  • 10.  RE: Qualitatively predicting Charpy impact toughness

    Posted 03-11-2022 10:06

    Sean, Brian and all:

     

    Prediction of the impact toughness of steels in general based on the Charpy test is a challenging proposition and depends very much on what you are looking for, determining the transition temperature or the upper shelf energy?  Going back to Brian's comments, inclusions will affect the upper shelf energy but not the transition temperature of the steel, which is determined by the microstructure and texture of the material.  So, if you have to meet a certain spec on "impact energy at a certain test temperature" then I agree that inclusions will have a role to play.  Someone else mentioned in this string of comments that there are too many variables to develop a predictive equation and I generally agree with that statement.  On the other hand, I am old enough to know that as soon as someone proclaims something to be impossible, someone else smarter goes ahead and does it just to prove you wrong!  My advice to Sean is to define a limited range of steel product that you are interested in (HSLA, Q&T, etc.) and limit the chemistry range to that of interest and then try to develop a correlation between what you might consider the most important parameters and impact energy and see if that works.  You can then make adjustments to your equation, as necessary, to improve its accuracy.  Hope these comments help.

     

    Nassos Lazaridis




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  • 11.  RE: Qualitatively predicting Charpy impact toughness

    Posted 03-11-2022 19:25

    What I have recommended in my most current analysis of CVN requirements is to first determine what are the operating conditions for your application in terms of temperature variations over an entire year.  We have records going back to the 1880s to determine average low temperatures in the 3 coldest months.  We have fracture-critical machinery that operates in temperatures as low as -15F, so transition temperature is important.  With that information, a realistic CVN impact toughness can be specified, with knowledge of maximum and cyclic stresses and any crack tolerance.  Since the CVN can be correlated with fracture toughness, this will determine what tolerable flaws or stress concentrations are permissible.  All products, whether they be castings, rolled plates, bars, shapes, weldments, etc. have some inherent flaws which may or may not be significant to operation of the product or structure.  In any event, I would always have a full CVN energy curve test run of the selected material before committing to its final specification.

     

    Christopher Hahin, MetE/MechE, CorrE, PE

    States of Illinois & California

    Illinois DOT

    Springfield IL 62704

     




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  • 12.  RE: Qualitatively predicting Charpy impact toughness

    Posted 03-18-2022 20:25
    About 30 years ago I developed formulas to predict tensile and yield strength and ductility for plain carbon and carbon manganese steels from our own Foundry because all we really needed to know was that the steel exceeded the minimum requirement as used by the design engineers. As long as the predicted strength was 10% over the minimum the product was accepted. Less then that and an actual test was carried out to be certain. This was still a considerable cost saving in testing. Provided the microstructure of the steels was fine grained after normalising, the charpy tests were always OK in that we only needed to know that the transition temperature was below 0 degrees C. These were relatively simple steels in non critical applications. Being in the rail industry, I always considered the safety aspect. Hence the degree of testing undertaken depended on the consequence of failure of the item being manufactured. I wouldn't trust my own predictive formulas if the item was safety critical. Hence while predictive formulas are handy, are you going to trust a predictive formula in a safety critical application? Are you prepared to stand up in a court of enquiry and say no we didn't test because this formula said it would be OK. Sorry if I'm putting a dampener on this enquiry, but I think even from the comments above, there are so many factors to consider when you consider alloy steels from various sources etc. the base analysis, the tramp elements, the microstructure, the inclusion content, shape and location etc, I suspect the cost to determine the variables to fit any really reliable formula would exceed the cost of Charpy testing.

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    David Schonfeld
    Graceville QLD
    61407894727
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  • 13.  RE: Qualitatively predicting Charpy impact toughness

    Posted 03-19-2022 04:46
    Mr. Schonfeld, when you say the CVN values were "ok", what specific value was required?  If you fully read my post, it stated that a full set of CVN values at the range of temperatures for the product material in question to be encountered should be completed.  We have some critical machinery running on our movable bridges that operate at -15F, so 0F might not be sufficient.  The traditional 15 ft-lbs is derived from Liberty ship steels studied in the 1950s.  The value of estimating impact energy from composition just gives an indication of whether testing is needed for the application, along with what are the operating stresses associated with the application.  The tolerable flaw size is then determined from the correlation of CVN impact energy absorption to fracture toughness.  For a pre-cracked steel under dynamic loading, KId = sq rt [4 x CVN x E].  Flaw size is K = 1.1sq rt[[pi x a], where a is the critical crack size.

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    Christopher Hahin
    Engineer of Structural Materials & Bridge Investigations
    Illinois Department of Transportation
    Springfield IL
    (217) 522-4023
    ------------------------------

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  • 14.  RE: Qualitatively predicting Charpy impact toughness

    Posted 03-20-2022 03:38
    Most everyone who has worked extensively with Charpy impact testing generally agree it primary use is in finding the DBTT the values themselves have never been used in design and although can mostly be roughly correlated with Kic and specified on prints as minimum at a certain temperature based on past experience they are of little use.  The question is rather is there a way to predict DBTT based on easy to determined material properties from a dreaded "tensile test". Unfortunately, these are not state properties and as past correlations between chemistry and processing to predict the dreaded tensile test results have been poor.

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    Edward Vojcak
    Senior Metallurgical Engineer
    SGS North America
    Blue Island IL
    (708) 595-8734
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  • 15.  RE: Qualitatively predicting Charpy impact toughness

    Posted 03-21-2022 00:24

    This conversation is certainly interesting.  Unfortunately, the thread reveals a lack of appreciation that virtually all of the commercial steel presently being utilized in North America is steel that was solidified in a continuous casting machine (caster).  In modern steelmaking (since about 1970), continuous casting has replaced virtually 99% of ingot casting, the previous technology. 

    Now, continuously cast steel has several characteristics that differ from ingot cast steel.  Many good, some bad.  One of the negative characteristics of continuously cast steel, that can be particularly devastating to Charpy-test performance, is "center segregation", which actually has several manifestations, one of which is "banding".

    An important example concerns the manufacture of steel for "Arctic-Grade" linepipe, as is in the Trans-Alaska pipeline system.  Because of the severe cold, the Charpy test was an important aspect of evaluating steel applicability.  One of the requirements was to locate Charpy specimens so that it included the center of the plate, with the notch perpendicular to the plate surface, and pointing in the final-rolling direction.  If you think this is tough, it got even tougher, because if the Charpy specimen fracture-face showed any sign of "splitting", the test was disqualified, irrespective of the ft-lb value.  Metallurgists familiar with appropriate metallographic techniques (rarely taught in NA universities) proved the splitting to be related to aspects of center segregation.

    So much for all the data you guys have in your files.



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    Edward Szekeres
    Consultant
    Utica NY
    (585) 766-3536
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  • 16.  RE: Qualitatively predicting Charpy impact toughness

    Posted 03-21-2022 09:26
    Chris,

    That's interesting that you brought up the Liberty Ships as a genesis for the 15 ft-lb requirement; this is something else I've always wondered about. Would you happen to have that paper? I'd be interested to get deeper into that history.

    To Ed V's point, it's my experience as well that Charpy's are usually not used directly as a design parameter in that way that, say, YS/UTS are, but rather they are codified in industry standards which blanketly say "if you meet this impact at this temperature, you're good" and it's fairly black and white. I've always assumed that this relies on some behind the scenes fracture mechanics similar to the equation you shared where it's figured that, for a typical application and product covered by that standard, flaws will not exceed a certain size (maybe that's the UT FBH requirement) and temperature/load will not exceed certain worst cases and, within those boundaries, 15 ft-lb (or 20 or whatever the case may be) is adequate to ensure that equipment won't catastrophically fail in service.

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    Sean Piper
    Product / Process Metallurgist
    Ellwood Texas Forge Houston
    Houston TX
    (713) 434-5138
    ------------------------------

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  • 17.  RE: Qualitatively predicting Charpy impact toughness

    Posted 03-22-2022 04:43
    I thought I had replied to this question but for some reason it didn't register.  Actually CVN requirements are used in a wide variety of design applications, and widely specified in pressure vessel steels, bridges, machinery, oil field and aircraft parts.  For example, we have bascule, vertical lift and rolling lift bridges where there machinery operates in very cold weather (-10 F in winter is typical in the Chicago area).  We had to switch to 8620 normalized because I had specified 20 ft-lbs at -10 F; 4130 was not adequate.

    Fracture toughness is directly related to impact toughness; correlations can be taken from the transition zone or the upper shelf.  I prefer the Roberts-Newton equation because it is not strain-rate related.  I don't know when you received your metallurgy degree, but fracture mechanics has only been emphasized for only the last 25 years, and it is making its way into many specifications.  Impact toughness is very important in transportation because many of our structures are subject to impact and the steels and components used in bridges have sustained fatigue cracks, particularly since they are primarily welded together and then the major sections are bolted in the field for final assembly.

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    Christopher Hahin
    Engineer of Structural Materials & Bridge Investigations
    Illinois Department of Transportation
    Springfield IL
    (217) 522-4023
    ------------------------------

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  • 18.  RE: Qualitatively predicting Charpy impact toughness

    Posted 03-22-2022 23:20
    Never ran across "Roberts-Newton" ​know of Barsom-Rolfe, Rolfe-Novak -please enlighten us.

    note also:


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    Edward Vojcak
    Senior Metallurgical Engineer
    SGS North America
    Blue Island IL
    (708) 595-8734
    ------------------------------

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  • 19.  RE: Qualitatively predicting Charpy impact toughness

    Posted 03-21-2022 09:39
    Hi Edward,
    The point you made about " banding" is true. Currently I am facing the challenge in our  incoming steel.



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    Sanjay Kulkarni
    Materials Engineer
    MSSC
    Troy, MI
    248-840-1056
    ------------------------------

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  • 20.  RE: Qualitatively predicting Charpy impact toughness

    Posted 03-22-2022 10:07
    I think your refereeing to Mr Szekeves correct point above about segregation and banding in concast steels.  I was worked in the cold finished bar industry back in the mid 1980's when the transition between ingot and continuous casting was occurring - in general top, middle and bottom of an ingot differed enough that one generally experienced poor, good and excellent performance for Charpy respectively -  were as concast was either uniformly good or poor - rarely excellent.

    Another point to all the "correlators" out there - the mathematical act of correlation is an enumerative process applicable to the sampled "frame" of data and ignores many contributing factors  that may be "constant for the frame".  In the case of Charpy impact values the oxygen and nitrogen contents for the steels is very important - these are generally constant for a given steelmaker (common cause) due to their specific steel making practice but these factors may become "special causes" when other steelmakers material is included and Charpy test found not to follow the "model" of the correlation.

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    Edward Vojcak
    Senior Metallurgical Engineer
    SGS North America
    Blue Island IL
    (708) 595-8734
    ------------------------------

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  • 21.  RE: Qualitatively predicting Charpy impact toughness

    Posted 03-22-2022 11:11

    Ed,

    Thanks for pointing out that ingots had tremendous segregation from top to bottom and surface to core. Many of the ingots were top poured which created very poor surface. During cooling after stripping from the molds, ingots were prone to form deep longitudinal cracks from thermal stresses. I had a lot of experience in steelmaking quality with top poured ingots from 1978 to 1995 and continuous casting from 1978 to 2011. There is no comparison – I don't ever want to see another top poured ingot!!!

     

    Yes, there is consistent center segregation and periodic centerline voids in continuous cast billets and blooms. The degree of the segregation can be reduced by controlling the casting superheat, applying magnetic stirring, or using soft reduction to compact the centers. Larger cast cross sections can also help. Conditions during roughing in the rolling mill can also affect the final bar center quality.

     

    Transverse impact test results are highly affected by elongated inclusions such as MnS – especially when elevated S levels are required for machinability. In addition, heat treatment can magnify the effects of inclusions on impact shelf energy.

     

    See the following reference:

    Effects of Sulfur Level and Anisotropy of Sulfide Inclusions on Tensile, Impact, and Fatigue Properties of SAE 4140 Steel

    N Cyril, A Fatemi, B Cryderman

    SAE Journal of Material Manufacturing 1 (1), 218-227

     

    Bob Cryderman

     

     

    Sent from Mail for Windows

     




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  • 22.  RE: Qualitatively predicting Charpy impact toughness

    Posted 03-28-2022 22:10
    This discussion has been really good because of the different dimensions and factors that continue to be expressed and described by the contributors.  Thanks to all who have responded!
    Within really limited ranges of products, suppliers have managed to sell adequately performing materials by merely looking at the product specification and adjusting their processes to fit.  Some of the correlations and rules described here have been through that sort of adaptation.  And the reasons that was a successful approach was that the groups who wrote the specifications in the first place, or revised them over the years, were composed of people with enough industry experience with what worked and what didn't.  They got together, drew a boundary around a set of rules that mostly included success and mostly excluded failure.  Then designers used materials from that pool of success to deliver the final products.

    A problem as I see it with the drive to produced engineered data sets, such as described in the upcoming webinar promoted by ASM, is that sheer multitude of dimensions of compositions, production processes, and even weather (for instance affecting cooling rates) which are not captured.  For instance, elemental compositions which are not specified are not analyzed for.  Many of the reasons that the specification Committees wrote the rules the way that they did were never captured or preserved.  Therefore when industrial practice changes outside of the original process boundaries or just pushes against the edge of the spec in too many ways, the new user does not know the "why" of failures.  We had an expression at work about "dead guy's rules" for when a process or activity had been defined by an expert or group who was no longer available and who had not recorded the reasons for why something was done that way. 

    After several generations of materials specifiers may have been using a call-out of properties without a full understanding of why, without a full understanding of the statistical variation of properties even within a fairly tight spec, it becomes more and more likely that minor or unforeseen changes will result in inadequate service.  And that is not even getting into the use of somewhat hazy correlations such as Charpy values to any of the explicitly defined methods of fracture toughness/crack growth resistance.  Or the use of a powder-derived base stock for a previous wrought product, or using wrought product defined chemical specification and properties but producing an item by casting.  The more approximations, the less precise the final answer.

    Engineering is often a compilation of what has been good enough in the past, with the unknown or poorly defined covered by factors of safety.  But this has led to people who do not know how much they do not know, pulling values out of a handbook or old purchase order, and semi-blindly charging forward.  This works only as long as the tolerances of what is required and factors of safety in design (room for ignorance of the unknown) are adequate to cover the conditions present in manufacturing and service.  So Sean's response of "we'll just have to test it out and see what we get" is totally valid without specific experience to back up a guess, and the hard part is quantifying the cost to a manager who asks why not just accept it.  What is the cost if you decide to go forward on insufficient data and then production fails to meet cost/schedule/quality or, usually worse, the delivered product fails catastrophically?

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    Paul Tibbals
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  • 23.  RE: Qualitatively predicting Charpy impact toughness

    Posted 03-28-2022 22:53
    The cost is the sum of appraisal plus prevention exceeding failure cost (internal and external) by just about 50% to bring "Total quality cost" to a minium - See: Principles of Quality Cost -J. Campanella ASQC

    But getting mangement to pay that when they can hide the true cost is very tough, in fact impossible since the extra effort removes most of managemental control - and that's politics -the old enemy of the systems approach (Churchman).

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    Edward Vojcak
    Senior Metallurgical Engineer
    SGS North America
    Blue Island IL
    (708) 595-8734
    ------------------------------

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  • 24.  RE: Qualitatively predicting Charpy impact toughness

    Posted 03-29-2022 06:35
    Very good comment, Paul

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    Donato FIRRAO FASM
    Professor
    Politecnico di Torino
    TORINO
    +393351494032
    Italy
    ------------------------------

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