1144 Stressproof 6

Stressproof is a tradename of Niagara LaSalle for a modified SAE 1144 steel, alloyed with nitrogen. SAE 1144 is a heavily resulfurized steel, which substantially improves machinability. The Stressproof grade is processed by cold drawing a hot-rolled steel bar, and subsequently stress-relieving it. LaSalle uses a "heavy draft", more than the typical 10% draft (reduction in cross-section), which increases strength and hardness due to strain hardening, also called cold working.

This heavy draft increases the strength, and many times allows Stressproof 1144 to be used in applications without further heat treatment, such as Quenching/Tempering. However, cold drawing affects the surface more than the core, resulting in lower core hardness. Quenching & Tempering produces a more uniform surface, but introduces another processing step, potential for distotion, etc.

One suggestion is to not change the steel grade, but just require a minimum core hardness. Is the steel currently making acceptable threads? If not, then you may want to consider other free-machining grades from LaSalle like Fatigueproof or ETD 150. You can read more about these grades at the following website/.pdf file:


LaSalle does not have their own website-- they use space allotted to them at whose server is misbehaving today, otherwise I would add the link to this reply. Try accessing the homepage, and then find LaSalle Steel on it.

If you feel that the strength of the part is an issue, using something like 1137/1144/4142/4150 but with subsequent heat treating (Q&T) will provide increased strength, but with increased cost, process time, and potential distortion.

Is it possible to eliminate the machining? You may be able to forge the part (cold/warm/hot?) using a carbon steel like 1038 or 4140, then roll the threads instead of machining them. I'm guessing it was deemed cheaper to machine the entire spindle, instead of this, but you may want to investigate. Best of luck, and if you have any additional questions, just ask.

I was doing some research on the Stress Proof from lasalle and was reading from this website:


It may not be the official web site or anything but it may help you with more information about the ETD and the Stress Fatigue options that Lasalle has. Good Luck.

What other machining is being done other than threading?
If that is the bulk of it then don't bother with a free machining grade and roll the threads. They will be smoother and stronger.
The StressProof line has a good reputation.

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Corrosion never sleeps, but it can be managed.

One problem with using free machining steels in products that experience cyclical loading is that they have very poor fatigue properties. The sulphur or lead or selenium inclusions that make machining properties so good make for stress risers and crack initiation points. IMHO, if there is any possibility that failure of the part could result in injury to persons near it, free machining grades should not be used. You have to ask yourself, what would happen if the blade were to come off because a stress riser in the thread root on the shaft propagated a crack through the shaft?

There was a fairly hairy lawsuit that happened here in Houston a few years back where an eye bolt used on a window washing scaffold failed and the window washer fell from near the top of a sky scraper. The eye bolt was made from a free machining alloy and was manufactured in China if memory serves. Free machining materials have thier place, but you have to be careful how you apply them.

I have to concur with rorschach based on our experience and especially when you mention spindles. We had a lot of machines that used what might be called a spindle and a lot of short heavily machined shafts. StreeProof was an ideal choice but it didn't take long to find out under our loads it was the ideal we had originally envisioned. We switched to E.T.D. but still had some problems with the more highly loaded components. Reevaluation of the design and the imposed loading we decided to give FatigueProof a try. We made a complete swap to FatigueProof to ensure the material wouldn't be comingled or misused. This material evaluation was due to the loss of material and the company who made it, we had previously used.
If there is any impact or alternating loads on the spindle I would seriously consider FatigueProof.