Gallery of Dramas. Broken Enfield Parts!

[breakeyp]

I had the unplesant opportunity to investigate and resolve? heat treat issues in the automotive business. Given the volume of fasterners Ford uses every day, it is not surprising that problems happen. Chassis Division had 7 metallurgists/material engineers alone.

Problems can be due to:
1. Improper heat treat
2. Incorrect materials or materials out of spec
3. Batch heat treat may mean parts on the out side of the heat source don't see proper temperatures or parts too close to heat may be over heated. A hardened part may not be drawn down to the final temperature. I have seen a hardend part drop on the floor and break.
4. Slag/impurities imbedded in part leads to weak cross section
5. During the milling process at the steel mill--one billet is mechanically welded to the next and the seam becomes a srtess point subject to failure in a later manufactured part.
6. Part notched or dented during manufacturing process prior to heat treat.

A coarse cross section at a break indicates an immediate failure. If the break surface is smooth with a series of curved swirls--the failure was ductile and happened over time.

I have a three inch thick textbook from a class on the Theory of Failures that attempts to cover the subject. If someone is really desperate for reading material, I can furnish the title and particulars.

Information

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[jmoore]

Thanks for the "Reader's Digest" version, breakeyp!

I'm thinking there's out of spec. materials dramas here, as well as heat treating issues. The cocking pieces above seem to have failed in two cases via slow crack propagation ("fatigue"), whilst the other went all at once. As it occurred in an area w/ rapid section change, it's possible heat treatment was a contributing factor, but I suspect a bit of "notch sensitivity" as well. Fairly roughly machined, most are. The Enfield example (Early type cocking piece) shows the classic crack propagation features the best (I think...they're going to be re-photoed in a few days, so maybe we'll have a better illustration.)

I have a three inch thick textbook from a class on the Theory of Failures that attempts to cover the subject

Yah, I get that! I still don't know of one "bible" for this subject. Most of my best references seem to be small papers and studies. ASM and ASTM affiliated, the majority; old AWS magazines as well.

[breakeyp]

Thanks for the "Reader's Digest" version, breakeyp!

I'm thinking there's out of spec. materials dramas here, as well as heat treating issues. The cocking pieces above seem to have failed in two cases via slow crack propagation ("fatigue"), whilst the other went all at once. As it occurred in an area w/ rapid section change, it's possible heat treatment was a contributing factor, but I suspect a bit of "notch sensitivity" as well. Fairly roughly machined, most are. The Enfield example (Early type cocking piece) shows the classic crack propagation features the best (I think...they're going to be re-photoed in a few days, so maybe we'll have a better illustration.).

I am sure Peter can say more as he is there and knows the current British system of material callouts. The WWII period part drawings I have had access to show material callouts that allow a great variance in material components/processing. We (the US) use SAE (Society of Automotive Engineers) material descriptions. This method has become world wide much like the Doctors using Latin to provide a common language for body parts. If you call out SAE 1020 steel--it means it is made to specific ingredients and there are specific heat treat procedures to obtain a desired surface hardness and it can be expected to have specific material properties. I have seen British drawing that call out material to be a stock number from a specific supplier. If he goes out of business---you have to start over. If he makes a bad batch---how to you prove it. If he takes shortcuts due to material component unavailability, how do you prove it.

Mr. Moore I can tell you have been down the road. I had some oldtimers that helped me through the maze explaining the differences between ductile and catastrophic failures. One thing they were absolutely crazy about was if a failure was attributed to material crystalization--you got a rough lecture as all metal is crystaline in nature. Steel has infinite life, unlike Aluminum, and does not change properties over time. Steel must be worked or have a weak crossection vs. load for a failure to happen. Unlike steel, rubber continues to cure, age, over time to the point cracks/failures happen. For example look at antique car steering wheels--guaranteed cracks. Re-reading this I find that I would have been in trouble at work if I had written this. Due to the lawyers, we couldn't write using the word failure. We had to say the part was unable to react to the present load.
Yah, I get that! I still don't know of one "bible" for this subject. Most of my best references seem to be small papers and studies. ASM and ASTM affiliated, the majority; old AWS magazines as well

[jmoore]

Steel has infinite life, unlike Aluminum, and does not change properties over time.

Just a clarification on breakeyp's excellent post (He's gone to an extreme "shorthand" to keep from filling pages!):

Steel parts will theoretically survive an infinite number of stress repititions or cycles IF the applied stress is below a certain level. Above that point- failure will
happen. How soon can be predicted with increasing certainty as the variables are controlled (or are known), reducing the "factor of stupid". (A phrase I picked upfrom a favourite pre-PC professor)

Some parts can be designed to last millions of cycles, others don't need to last ten! It's maddenly hard to control ALL variables, though!