Are all Coil springs work hardened so that annealing will reduce their stiffness? 4

[vonsteimel]

I was just wondering if coil springs relied partially on work-hardened wire to give them their "spring" properties or is it simply the property of spring steel?

so that if one were to heat/anneal a coil spring, would it lose much of its stiffness/spring properties?

What about machined coil springs? Are they machined out of work-hardened round stock (CRS maybe?) or again, is it simply a property of the spring steel so that annealing will not affect its spring/stiffness?

Just wondering this as I was doing some work with them.

VS

 

[drawoh]

vonsteimel,

The stiffness of your coil springs comes from the elastic modulus, which is not affected by heat treatment or work hardening.

Annealing a heat treated or work hardened coil spring will affect the yield and ultimate strengths of your coil spring. Since springs generally are highly stressed, annealing means your spring will not spring back after loading.

The only issue I can see with a machined coil spring, all else being equal, is that the surface finish might not be as good. This will increase your metal fatigue. Again, your springs probably are highly stressed. If your loads are not constant, you will have fatigue problems.

--
JHG

 

[moon161]

Machined springs nicely summarized.

http://www.taylordevices.com/machined-springs.html

 

[MikeHalloran]

It's possible to design perfectly satisfactory coil springs that don't reach the yield point of annealed steel under a specified load. ... but they will be much bigger and heavier than springs made from hardened steel with a higher yield point.

FYI, work hardening is not the only way to increase a steel's yield point. A quench and temper can do it if the steel has enough carbon to start with. If starting with a low-carbon steel, one can precede the q&t with carburizing, by any of several means. Small springs that you can hold in your hand are commonly made of 'music wire', pretty much the same stuff used for guitar springs, which has a fair amount of carbon and is supplied in a mostly hardened condition, with enough ductility remaining to allow forming the wire into a helix. Automobile size coil springs are commonly formed hot, then quenched to develop hardness.

Mike Halloran
Pembroke Pines, FL, USA

 

[vonsteimel]

"springs generally are highly stressed" -- from....? Work hardening/heat-treatment?

"annealing means your spring will not spring back after loading" -- this mean you have effectively destroyed the spring for all practical purposes...

So as it sounds, the answer is yes... Coil springs rely much on being "pre-stressed" to give them their stiffness/spring properties? This all traces back to an example a professor performed for us in school; He took about 4" of your normal MIG weld wire (which is stressed/workhardened) and wound it around a pencil to form a spring. He passed it around to demonstrate its springy properties... Then heated it red-hot and let it cool. Then squished it and it did not spring back... (because he essentially annealed it, causing it to lose its work-hardness).

Since MIG wire is not a purpose made "spring steel", I was just wondering if most coil springs relied on the pre-stress/work-hardening or if it was a property of the steel itself to give it its propensity to return to its normal shape, so much so that annealing would not affect its usefulness as a coil spring.

Thanks for the help

VS

 

[drawoh]

vonsteimel,

Springs are highly stressed when you use them. The manufacturing process is irrelevant, as long as the spring can take the stress.

--
JHG

 

[mfgenggear]

Drawoh

You mean well. but I too was confused by your statement. please clarify.

The stiffness of your coil springs comes from the elastic modulus, which is not affected by heat treatment or work hardening.

the way Mike described it, is my understanding but I am not a spring specialist.
Hot formed then quench, the steel is essentially formed in the stress flow condition. then harden.
to retain it's form under load.

Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[btrueblood]

They are both right. Mike is a bit more specific in stating that an annealed wire spring would have to be made from thicker wire than work-, age- or quench-hardened wire, in order to support a given load without yielding, all else being equal. But the stiffness (K, in lbs/inch) of the two springs would be equivalent, for similar geometry springs, i.e. the elastic modulus of steel doesn't change. The annealed one just yields and (usually) becomes useless at a lower load value.

 

[stanweld]

vonsteimel,
Your professor's example only showed that the allowable stress range was lowered after annealing/stress relieving. If he had not overloaded the heated spring beyond its yield point, it would have exhibited the same spring constant.

 

[drawoh]

mfgenggear,

The spring rate of the spring is a function of its geometry and its elastic modulus. The ability of the spring to bend elastically is a function of its maximum yield stress. Elastic modulus is not affected by work hardening or heat treatment. Yield stress is affected by work hardening and heat treatment.

In most practical cases, if you are designing a spring, you need a low elastic modulus and/or a high yield stress, to make the thing work. Hence MikeHalloran's comments about work hardening and quenching and tempering.

Making the thing thicker, as noted above, is a good strategy for designing structures out of softer, cheaper materials. Springs are way more complicated.

--
JHG

 

[mfgenggear]

von

Sorry for hijacking your thread.

drawoh

thank you.

Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[vonsteimel]

Its all good stuff here mfgenggear... This is what I was hoping for.

So then, what are the properties that describe spring steel? From what I remember, Nickel is a common alloy used in it. Since a spring is designed to under millions of cycles I would assume they are trying to achieve a very high fatigue limit in spring steel? Or simply a high yield point?

 

[MikeHalloran]

Nickel is an element.
Steel is an alloy of iron and carbon and sometimes other stuff.
VonSteimel, it's time to review your old machine design textbooks.



Mike Halloran
Pembroke Pines, FL, USA

 

[mfgenggear]

http://www.pss-corp.com/pdf/coiling-terminology_1.pdf

I found this about springs

Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[vonsteimel]

I meant Nickel is a common alloying element used in it... And I'm pretty sure steel is on the periodic table

 

[GregLocock]

I'm pretty sure it isn't.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Walterke]

http://en.wikipedia.org/wiki/Steel

start here.

NX 7.5.5.4 with Teamcenter 8 on win7 64
Intel Xeon @3.2GHz
8GB RAM
Nvidia Quadro 2000

 

[btrueblood]

No nickel in music wire, or many other common spring steels - just carbon and manganese, with some Cr, Va, Si sometimes thrown in. Funny, the wikipedia article for "spring steel" says exactly what vonsteimel said...but then lists several spring steel alloys with no nickel in them. They quote tensile strengths more appropriate for bars also, when drawn and tempered spring wire can reach 200 ksi. Ah well, that's the internet for ya. A good spring design book, like those by Wahl, will show that tensile strength for drawn/quenched and tempered spring wire becomes a function of wire diameter for wires under about 1/2 inch - smaller diameters achieve much higher tensile strengths. You see similar numbers when looking at fastener breaking strengths in typical ASTM/ASME/SAE specifications. This phenomenon may be due to equal parts that defects are less likely to pass crude inspection in smaller diameters (i.e. defects of a certain size will cause the wire to break during drawing), and possibly because heat treatments are more likely to achieve full through-hardening for smaller wire diameters. Ain't metallurgy fun?

 

[vonsteimel]

So... What properties are they trying to obtain in spring steel that differ from mild steel or tool steel.etc What makes a good spring steel?

 

[drawoh]

vonsteimel,

For spring steel? A high yield stress, obviously.

--
JHG