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I have an ASTM A311-04 1.25" diameter round 1050 steel bar that exhibits a tensile strength reading of 155,227 PSI. Normally, we receive material in the 13-133,000 PSI range. The bar is machined to a 5/8-18 UNF thread.

We have experienced instances where the threads of these machined parts get damaged when installing a prevailing torque nut. The nut is hand tightened and then driven tight with a pneumatic gun. Threads are being cracked off from the shaft during the installation of the prevailing torque nuts. This is a new phenomenon and seems to be centered around threads machined in steel that has this "high" tensile strength (155,227 PSI). The nuts and assembly process have been ruled out as root cause.

Could the material tensile strength create a situation where the threads become susceptible to damage? In tests, it appears as though the threads are being pulled off the shaft and getting embedded in the threads of the nut. The hardness is 33 HRC if that is of interest.

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    $\begingroup$ Is that 13 to 133 ksi? Or should it have been 130 to 133? $\endgroup$ – Air Mar 25 '15 at 21:43
  • $\begingroup$ In general, higher strength and hardness come at the cost of less ductility in steel. Seeing as threads are basically one giant stress riser, it wouldn't be too shocking if higher strength steels are more susceptible to brittle failure. Are the bolts being coated in anything? Hydrogen embrittlement from galvanizing is a common concern. $\endgroup$ – Ethan48 Mar 25 '15 at 21:53
  • $\begingroup$ It isn't clear exactly when the threads fail. Are they failing when the nut is threaded but the bar is not under tension? Is it only when a tension test is performed? Is it while the nut is being turned to pretension the rod? $\endgroup$ – hazzey Mar 25 '15 at 22:47
  • $\begingroup$ Thank you both for your comments. To clarify - The bar stock is being machined into a shaft that contains 5/8-18 UNF threads. The threads are failing in a downstream assembly operation when a prevailing torque nut is installed. It is hand tightened and then driven tight with a pneumatic gun. the threads. We have eliminated the nut and process and root cause. Seeking to understand the role of high tensile in the material. $\endgroup$ – Kevin14009 Mar 26 '15 at 12:25
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First, I want to say that I'm suspect of that material actually being ASTM A311 (though I don't have a copy of that standard available to verify.) Looking on Matweb, this material is the closest I found in the 1050 steels. This search turns up quite a few results, but from what I can tell of A311, it's for cold-drawn and stress relieved bars, but doesn't include quenched and tempered bars. ASTM's page seems to suggest A434 is for Q&T bars. So my first thought is that your supplier is doing something weird.

Independent of whether or not you've got the right material, as was noted in the comments, increasing strength generally decreases ductility. The material may yield at a higher strength, but after it yields, there's less elongation before the material fractures. This can be more prevalent in situations where there are high stress concentraters, such as screw threads. Fracture mechanics is a big messy area, but the key here is that a more brittle (less ductile) material is less resistant to crack growth because it can't dissipate enough energy during its deformation.

In the extreme, think of a piece of glass and a rubber band. Glass won't deform as easily, but once you start to move it, it breaks fairly easily. A rubber band stretches with no problem, but you have to stretch it a lot farther before it breaks. I think that's what you're seeing with your threads. There may technically be more strength there, but any defects in the material will weaken it more than a less strong, more ductile material.

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The issue described here sounds like galling. This is where the mating threads adhere to each other and tear chunks out of the surface which then wedge into the thread locking it completely.

The symptoms are that the nut will go on fine for a number of turns and very suddenly seize in both directions, at this point its usually impossible to remove without destroying the threads and you often see quite long sections of one thread embedded in the other.

Some steels are more susceptible than others, with stainless being a particular offender and the finish of the thread is also a factor is is the use of impact wrenches etc. So a change in steel specification could indicate that the new steel is more susceptible to galling or that it has different machining properties which affect the surface finish.

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