44 replies to this topic

[Greg Locock]

http://www.asmintern...5e-a7caa5e22eba

 

You'll be hearing a bit of chatter about this. While i don't doubt it will find its uses, in general, in chassis parts, we struggle to use a material with a  ductility of less than 12%, and for steel on the body side, 20% is more typical.

 

Now, if we were to redesign how we design cars, so that they can be built from brittle, strong materials, instead of ductile, weak ones, we'd be laughing. Also bear in mind that many parts of a car are designed to stiffness targets, which this wonder material does not address.

 

 

[gruntguru]

Low ductility normally means reduced toughness and compromised impact energy absorption.

 

OTOH with those amazing tensile strength numbers (1.9 GPa) combined with 10% elongation, the toughness is probably better than mild steel.

[Greg Locock]

I just found that Unbrako bolts, and TiAl6V4 T6, are roughly the same ductility and strength as this stuff.

[desmo]

Will this be significantly cheaper than whatever steel they use for Unbrako bolts and 6-4 Ti?

[MatsNorway]

12.9 is often 42CrMo4

 

12.9 is actually according to a standard and can be a range of alloys i believe. At least fairly sure it is for 8.8 etc.

 

i use 42CrMo4 on occation for disassembly tools, bushingpresses etc. If your in a sticky there is allways the tiny bit harder and toughter: 34CrNiMo6 (+QT)
 

http://www.steelnumb...php?name_id=335

http://www.steelnumb...php?name_id=196

 

I have yet to need the real exotic stuff.

 

I don't understand too much after a quick glance but if it has the same numbers it must be after welding or something. That chances everything.  You might just compare it do Weldox 1100 ++ I think they even go higher.

 

http://www2.ssab.com...ts/Weldox-1300/

Edited by MatsNorway, 14 December 2015 - 19:19.

[Lee Nicolle]

As a matter of interest I am led to believe that various steels are used for different components. eg subframes and main structures use a different grade to pressed bolt on panels. Bumper frames are different again.

I know these are different guages, and materials that have to welded together are compatible.

Modern cars, even with all their crash numbers still scare me.Sure I know the materials are far better but the lengths they go to to achieve lightness  eg the interior door trim is part of the doors structure and in some cases supports the window reg etc..

The front of the car once the plastic is removed is bare back to the engine, these days it seems that is part of the crash structure.

Yes I know the plastic itself does a surprising good job in a crash, better than many steel bumpers of the past. But only by the bulk of plastic and its support structure. But a lot more complex and in many cases heavier than the old simple steel structures. And a damn sight more expensive too.

Plastic structures too could be a good deal stronger by the simple addition of proper fasteners. All the plastic fsteners do is fail at the first bump. Brush a gate, sometimes just scrape the low hanging assembly on a driveway or culvert and there is bumpers and undertrays flapping every where. Hardly ideal. On some models I have actually bolted them back together using 6mm bolts with mudguard washers. Problem solved

[Greg Locock]

Desmo - most things are cheaper than Ti, in a previous job our order for tons of the stuff per year got us a lot of attention. Here's a more amusing article on that steel http://www.gizmag.co...-testing/40774/

 

I'm still a bit puzzled. Most of the car's body is stiffness limited, using higher strength material serves no purpose, increases your stamping costs, and makes forming good parts more difficult - you have to hit them harder and the low ductility makes them more likely to tear.. 

Edited by Greg Locock, 14 December 2015 - 22:45.

[Fat Boy]

I'm still a bit puzzled. Most of the car's body is stiffness limited, using higher strength material serves no purpose, increases your stamping costs, and makes forming good parts more difficult - you have to hit them harder and the low ductility makes them more likely to tear.. 

 

What the hell do I know about OEM car building, but this is what my initial thoughts revolve around. If you can make the non-critical areas of the chassis super thin, then you could probably make some progress in the weight department. At that point, though, it might make more sense to chop out the non-critical areas of the floorpan and bond in carbon or AL panels.

 

It looks like what they're most interested in is crash structures like door reinforcment or bumpers. Grab the low-hanging fruit first and all that.

[Magoo]

I know about these guys. A few decent pages at this site. 

 

http://www.flashbainite.com/

[Greg Locock]

Here's a more complex example. http://articles.sae.org/8512/

 

I vaguely remember that at least in Oz the steel mill charges proportional to the yield strength, so the economic argument is weaker over here.

[Canuck]

Bainite Bicycles. Because we need a new thing to buy.

[Greg Locock]

The (rigid) forks on racing bikes would be a good use for it, more compliance for a given strength.

[bigleagueslider]

Forming of sheet metal requires applying enough force to yield the material. Otherwise the material will simply spring back to its previous shape. The amount of force required to plastically deform a piece of sheet metal is greater with higher tensile strength materials.

[Canuck]

[gruntguru]

The (rigid) forks on racing bikes would be a good use for it, more compliance for a given strength.

Probably still not as good as carbon fibre on a compliance/strength/mass basis?

[bigleagueslider]

If my comment was so obvious to you, please provide your expert opinion of the manufacturing issues involved with using this high tensile steel material in place of a lower tensile steel material. What die forming tool modifications do you recommend when switching to the high tensile material? How much adjustment do you feel is needed with this high tensile steel vs a lower tensile steel for design factors like thinning, springback, min corner radii, etc? What are the apropriate analysis knock down factors one should apply at the weld HAZ with this material's metallurgy?

 

The main advantage of this material processing technique is that it allows a lower cost steel raw material to be used in certain applications. As GregLocock noted above, with an auto unibody structure there are a few places where the material could replace HSS to reduce cost a bit. But for most production autos, the chassis and bodywork will still use lots of mild steel for cost reasons.

[Canuck]

Awwww, did I make you sad face? Your comment was and remains, laughably obvious. You might have said "welding requires heating the parts to be joined until they reach a temperature at which they puddle. Otherwise they will not join. The amount of energy input required is higher in materials with higher heat transfer rates". You feel my mockery requires an expert opinion on the subject do you? I think not. I freely admit I'm not a sheet metal stamping expert. Doesn't change my amusement with your desire to hear (or see) your own voice.

The big takeaway that I left with was, they're achieving the same performance with a material that not only starts with a cheaper base metal, but uses less energy input to achieve its final strength so, as you pointed out, a cheaper material overall. They're also able to achieve the same levels of performance with thinner sections, so less weight and again less cost. That has to be the single biggest attraction in a world dominated by ever-ballooning size and weight competing wth ever increasing fuel efficiency requirements.

[gruntguru]

. . .  They're also able to achieve the same levels of performance with thinner sections, so less weight and again less cost.

. . . . and perhaps similar forces during the stamping process! 

[bigleagueslider]

Don't worry Canuck, I was not bothered by your post. However, since this is suppose to be a technical forum, it would be nice if you made an effort to refrain from that type of response in the future. How was my comment any more "laughably obvious" than the previous comment from GregLocock stating, "........ more compliance for a given strength."  This post was a similar statement of an obvious fact, and one might even argue this statement is not technically correct. Compliance could mean elastic modulus, while the increased ductility (or elongation) of this material means that there is a greater difference between the yield and ultimate tensile properties. Yet for some reason you did not criticize his post in the same way you did mine.

 

It is also unfortunate that you chose to intentionally misstate what I said about welding this material and the effects at the weld HAZ. Steel unibody structures primarily use resistance welding. Resistance welding is not a type of fusion process that you implied, where the metal at the joint undergoes a phase change from solid to liquid and back to solid. Resistance welding uses a combination of heat and pressure to produce a diffusion bond between the part surfaces. The materials never undergo a phase change from solid to liquid. Instead they are only heated to a plastic state, and thus a resistance weld is a solid state joining process and not a fusion process.

 

The heating and uncontrolled air quench conditions at the HAZ around a resistance weld will alter the metallurgy of the flash bainite steel material. This means the weld HAZ will have significantly reduce mechanical properties. So when analyzing a resistance welded structure made from the material in question, the appropriate knock-down factor must be applied at the weld HAZ. Here's a good technical paper somewhat relevant to the subject if you wish to learn more.  http://www.maneyonli...812Y.0000000100

[Canuck]

Forming of sheet metal requires applying enough force to yield the material.

I think the obviousness of this statement is obvious. Needs no explanation unless you don't understand the vocabulary but you wrote it so I assume you do.

Otherwise the material will simply spring back to its previous shape.

As above. Unless you don't know what yield is in the context of material strength, this statement too is obvious.

The amount of force required to plastically deform a piece of sheet metal is greater with higher tensile strength materials.

Really? You don't see the obviousness of the statement?
If one understands the topic-specific terms used - yield, plastic deformation, tensile strength - then everything in your statement is blatantly obvious. If you don't understand the vocabulary, the paragraph is meaningless.

My welding example was poor it seems because I failed to convey (to you at least) the intent. Nowhere did I link my comment to the material, unibody construction methods or armour plating. I just picked an easy analogue. Another: incandescent light bulbs generate luminous output relative to the input voltage. Insufficient input will not result in full light output. The amount of input voltage required to obtain full output is higher in bulbs with higher levels of luminous output.

Still obvious.

[Fat Boy]

The (rigid) forks on racing bikes would be a good use for it, more compliance for a given strength.

 

The steel that Reynolds has for bike tubing right now is about the same yield strength (250ksi, +/- 20k). It's an amazing material, but it can't really compete with carbon.

[Fat Boy]

Don't worry Canuck, I was not bothered by your post. However, since this is suppose to be a technical forum, it would be nice if you made an effort to refrain from that type of response in the future. How was my comment any more "laughably obvious" than the previous comment from GregLocock stating, "........ more compliance for a given strength."  This post was a similar statement of an obvious fact, and one might even argue this statement is not technically correct. Compliance could mean elastic modulus, while the increased ductility (or elongation) of this material means that there is a greater difference between the yield and ultimate tensile properties. Yet for some reason you did not criticize his post in the same way you did mine.

 

It is also unfortunate that you chose to intentionally misstate what I said about welding this material and the effects at the weld HAZ. Steel unibody structures primarily use resistance welding. Resistance welding is not a type of fusion process that you implied, where the metal at the joint undergoes a phase change from solid to liquid and back to solid. Resistance welding uses a combination of heat and pressure to produce a diffusion bond between the part surfaces. The materials never undergo a phase change from solid to liquid. Instead they are only heated to a plastic state, and thus a resistance weld is a solid state joining process and not a fusion process.

 

The heating and uncontrolled air quench conditions at the HAZ around a resistance weld will alter the metallurgy of the flash bainite steel material. This means the weld HAZ will have significantly reduce mechanical properties. So when analyzing a resistance welded structure made from the material in question, the appropriate knock-down factor must be applied at the weld HAZ. Here's a good technical paper somewhat relevant to the subject if you wish to learn more.  http://www.maneyonli...812Y.0000000100

 

Honestly, BLS, you're getting your ass chapped a little too easy.

 

When Greg wrote what he did, this is what he meant. The modulus of the material is not affected much by the alloying elements. Steel is pretty much steel in terms of modulus. Having said that, when you increase the yield strength, then you are giving yourself the opportunity to reduce the cross-section of a given structure. Let's say you were using a tube wall of 3mm. Maybe now you have the opportunity to change the wall to 2mm because the greater strength. That reduction in wall thickness will allow more compliance even though the actual modulus of the material has not changed.

[Kelpiecross]

If I remember correctly - in the early 1980s there was quite a lot of publicity (in Oz)  about the BHP producing a new type of steel for car bodies that was higher tensile and thus could be made  thinner and lighter.  Were car bodies ever made from this steel? What type of steel is used currently in car bodies?    

[Greg Locock]

That'd be Extraform 500, yes often used where strength is a priority over stiffness. 

[Kelpiecross]

What is the distinction between "strength"  and "stiffness"  - I would have thought they would both amount to much the same thing? 

[Greg Locock]

No, especially with steels. Strength is how much force a given structure can take before taking a permanent set, (yield) or breaking apart (Ultimate Tensile Strength). Stiffness, or its inverse, compliance, is how much it deflects for a given load. Ductility is crudely how much it'll stretch before it breaks. Stiffness, or elastic modulus, doesn't vary much between steels, whereas ductility and yield stress and UTS vary a great deal. The other variable property of great interest to car designers is toughness, which could be badly described as the ability of the metal to ignore small defects in shape, such as notches. Mild steel is very good at this (yes I know it sounds like I'm a fan of mild steel, I don't apologise for that). 

[bigleagueslider]

Honestly, BLS, you're getting your ass chapped a little too easy.

 

When Greg wrote what he did, this is what he meant. The modulus of the material is not affected much by the alloying elements. Steel is pretty much steel in terms of modulus. Having said that, when you increase the yield strength, then you are giving yourself the opportunity to reduce the cross-section of a given structure. Let's say you were using a tube wall of 3mm. Maybe now you have the opportunity to change the wall to 2mm because the greater strength. That reduction in wall thickness will allow more compliance even though the actual modulus of the material has not changed.

Here's exactly what GregLocock wrote, "The (rigid) forks on racing bikes would be a good use for it, more compliance for a given strength."

 

Note his use of the phrase, "..for a given strength."  You propose to explain to me what GregLocock meant in his post, yet you apparently did not bother to actually read it. You wrote, "..when you increase the yield strength..." and "...Maybe now you have the opportunity to change the wall to 2mm because the greater strength", while GregLocock's post clearly described similar strength. Cost is usually no object with "racing bikes", so the best quality materials available are typically used. So how would replacing a high-performance steel alloy having a 250ksi UTS used for the forks of a racing bike, with this flash bainite treated steel having a 250ksi UTS, alter the strength or compliance of the forks? Same UTS and same elastic modulus should give similar results, right?

 

The improved elongation characteristics of the flash bainite treated material might allow a more aggressive forming operation to be used for the fork profile without inducing cracks in the material. But you made no mention of this. A fork tube is structure loaded mostly in compression and bending, so the improved elongation characteristics might be beneficial in the event of a frontal impact. The front fork structures must also consider fatigue life. Do you have any understanding of this material's fatigue properties?

 

Ultimately, use of this flash bainite steel for high-end racing bicycle forks could provide some savings in raw material cost. But saving $20 in raw material cost of the forks is not significant compared to the $1500+ sales price of a typical racing bike.

[Canuck]

I think you forgot a zero on your pricing there. I'm not sure that would cover a set of wheels on an actual race bike these days. Fork assembly loading will see an increase in torsion loads now with the adoption of single-side disc brakes.

[desmo]

Does this new stuff actually have significantly superior properties to ancient air hardened steel like Reynolds 753 or what used to be called maraging steel? Maybe it's a lot cheaper.

I'm not really seeing the point of disc brakes on a road bike, rim brakes act near outside of the wheel where there is far better leverage and the braking load paths are much nicer.

[Canuck]

Well...if they don't come up with new products to sell us, they'll go broke . I bought my plastic bike a couple of years ago, complete with disc brakes and heard no end of malarkey from the rim brake enthusiasts. Here's what I've found: despite the reduced lever of the smaller diameter, the discs are far easier to modulate, more predictable on initial application, vastly superior when riding in the rain and snow and don't wear out the rim. For sure they introduce all sorts of new load paths for braking forces but it's neither insurmountable nor new (hello motorcycles). It's also not my domain of concern as a rider - that's the fork assembly engineer's concern.  If rim brakes were more capable, they'd be on the downhill bikes where brakes play a much bigger role.

 

That said, I'd still happily ride road bikes with rim brakes.

[desmo]

I've got a road bike with ancient Campagnolo Record sidepull brakes and once they are set up properly* I can modulate them to the point of lockup on either wheel. There's really not much upside I can see remaining once you are to that point with a(ny) braking system.

*meaning well trued rims with the pads set fag paper close, a nice clean non-anodized braking track and Kool Stop salmon pads.

[gruntguru]

Well...if they don't come up with new products to sell us, they'll go broke . I bought my plastic bike a couple of years ago, complete with disc brakes and heard no end of malarkey from the rim brake enthusiasts. Here's what I've found: despite the reduced lever of the smaller diameter, the discs are far easier to modulate, more predictable on initial application, vastly superior when riding in the rain and snow and don't wear out the rim. For sure they introduce all sorts of new load paths for braking forces but it's neither insurmountable nor new (hello motorcycles). It's also not my domain of concern as a rider - that's the fork assembly engineer's concern.  If rim brakes were more capable, they'd be on the downhill bikes where brakes play a much bigger role.

 

That said, I'd still happily ride road bikes with rim brakes.

 

I wondered about this myself when discs started appearing.

 

Another benefit that comes to mind is - the rim profile can be better optimised for aero and strength/weight (weight at the periphery counting double for straight line inertia). This is particularly relevant for carbon rims which could be significantly improved once the need for a friction surface and the ability to withstand the brake clamping forces are removed.

 

Desmo. I haven't tried the Salmon pads but my own experiences with rim brakes has always been a nasty delay in the wet. In a panic stop, you have to grip the levers much harder until the water is wiped off the rim, then be ready to avoid lock-up once that is done.

[Canuck]

Being able to lock up the brake doesn't (necessarily) mean an equal amount of modulation. Think of it as 256 steps of brake position (rim) vs 2048 (for disc...obviously).

[Greg Locock]

BLS - Fatboy's interpretation of my post is what I meant.

[JacnGille]

I'll take discs on my mt bike all day long. My road bike...not so much.

[Kelpiecross]

GL - thanks for the definitions of "strength" , "stiffness" etc.   I have to admit that I was pretty much ignorant of all this.   Presumably "stiffness"  being much the same between steels is why there is no real advantage in making a spaceframe out of anything but mild steel? 

  (I also have to admit that I find it slightly puzzling just why "stiffness" is much the same between steels). 

[Greg Locock]

Bear in mind many of the better steels soften considerably when welded.

 

 

The quick explanation for all steels being roughly equally stiff is that the vast majority of the bonds are between iron atoms, which all have the same stiffness. Ths same happens with all metal alloys to some extent. Here's a handy plot  http://www.lehigh.ed...rces/chart1.jpg

 

Edited by Greg Locock, 24 December 2015 - 07:18.

[Kelpiecross]

Does hardening/tempering etc. (as with a car's leaf spring) have much effect on the stiffness, strength, hardness etc.?  Where would the leaf spring material be on the plot? 

[desmo]

In the tiny circle labelled "steels". Obviously.

[Canuck]

[Greg Locock]

Hardening and tempering affect the strength (and toughness) but not the stiffness of the steel. Hardness is yet another word for yield strength incidentally. If you are interested in this stuff in a general way then I strongly recommend the book "Engineering materials" by Ashby and Jones, which is based on their lecture handouts. youtubes of their lectures would be better, Jones is a funny and articulate presenter. There are pdfs around. Most materials books are dry as dust, in fact for steel/carbon phase diagrams and heat treatment and concrete I found I needed a dry as dust book as well, but for explaining why all steels are roughly the same stiffness and density, that book is terrific. It also explains where those dashed lines on the plot above come from.

[Kelpiecross]

GL - thanks for that - I will chase up some of these sources you mentioned.  I have been using these different steel types for years without (apparently) knowing much about them. I usually rely on standard previous practice or what my machinists recommend.  For instance - they like to use aluminium if possible (because it doesn't wear their tools etc.)  - instead of mild steel they use something slightly higher carbon  (as it machines better they say) -  for things that need to be very strong we normally use only 4140  (also as it also can be nitrided).  For really strong things we have used EN40 which they apparently regard as the ultimate in steels (and it's expensive and hard to get). 

 

 D and C  - Duhhh  -  obviously I meant the stiffness/strength etc.  after it had been heat treated.         

.  

Edited by Kelpiecross, 29 December 2015 - 05:06.

[Canuck]

Okay, but you did ask "does hardening <snip> have much effect on hardness...".

[Kelpiecross]

I wish I was clever. 

[Fat Boy]

Here's exactly what GregLocock wrote, "The (rigid) forks on racing bikes would be a good use for it, more compliance for a given strength."

 

Note his use of the phrase, "..for a given strength."  You propose to explain to me what GregLocock meant in his post, yet you apparently did not bother to actually read it. You wrote, "..when you increase the yield strength..." and "...Maybe now you have the opportunity to change the wall to 2mm because the greater strength", while GregLocock's post clearly described similar strength. Cost is usually no object with "racing bikes", so the best quality materials available are typically used. So how would replacing a high-performance steel alloy having a 250ksi UTS used for the forks of a racing bike, with this flash bainite treated steel having a 250ksi UTS, alter the strength or compliance of the forks? Same UTS and same elastic modulus should give similar results, right?

 

The improved elongation characteristics of the flash bainite treated material might allow a more aggressive forming operation to be used for the fork profile without inducing cracks in the material. But you made no mention of this. A fork tube is structure loaded mostly in compression and bending, so the improved elongation characteristics might be beneficial in the event of a frontal impact. The front fork structures must also consider fatigue life. Do you have any understanding of this material's fatigue properties?

 

Ultimately, use of this flash bainite steel for high-end racing bicycle forks could provide some savings in raw material cost. But saving $20 in raw material cost of the forks is not significant compared to the $1500+ sales price of a typical racing bike.

 

F-me, BLS, do you _really_ want to attack me? I think not.

 

I know WTF Greg was saying because it's what a good engineer should have said. If you missed, then do your homework and figure it out.