Recommendation? Steel to use in a security product?

K

KozzyK

Joined
Oct 25, 2018
Messages
9
Hi,

Let me start with apologizing for that I'm probably not writing this in the best place on this forum.

I have designed and developed a ( patent pending ) a new locking device for motorcycles.
The plan was to order a metal that has a really good wear resistance but its not brittle.
So it was first ordered in as 20CrMnTi metal case hardened with boron.

Well, the factory for some reason said that this metal cannot reach 60HRC so they replaced it with SDK11 / D2 metal.

I have been to a workshop who has said the D2 would be really good, hardened and tempered, as this will give maximum surface hardness but still ductile in the middle.

Now i have seen that a lot of people make knives of this D2 and I wanted to ask someone's opinion who is knowledgeable in metals and their properties.

So the metal we had on testing was really good and strong on the surface but, when the tester froze the metal with a can of air and inserted a chisel and hit it twice with a hammer this happened.


https://prnt.sc/lai3la
http://prntscr.com/lai3y1

I know i need to work on the design as the piece was thin on the stop but this item had HRC60 in the middle after being sliced in half and tested. I was told it was to brittle.

Thank you very much for your help and again sorry i don't know where to go anymore to ask.
 
You are more likely to find the metallurgists over in the knifemakers subforum. We do have some actual experts around that could give you some help, as it seems like both your factory and whoever is doing your testing is really happy to spend your money.

The biggest question I have is why the need to have your steel get to hrc60, are you looking at that as a way to keep it resistant to cutting by sawblades?

In any case, I'd recommend starting a thread asking for help in the knifemakers subforum for advice on picking a steel that will get your requirements without costing an arm and a leg.
 
Hi, thank you for your response gadgetgeek.

The reason of me posting here was that its not about knives and i didnt want to get banned or to upset anyone.

Regarding the issues.

As this is a security product i really wanted a hard material to overcome cutting tools ( or just put up a good fight).
This is the reason i wanted a really hard surface.
To be honest my knowledge of HRC is not that good so i could have aimed for a lower number.

Im stuck now.

Maybe an admin could move this post to somewhere more relevant ?
 
KozzyK said:
Regarding the issues.

As this is a security product i really wanted a hard material to overcome cutting tools ( or just put up a good fight).
This is the reason i wanted a really hard surface.
To be honest my knowledge of HRC is not that good so i could have aimed for a lower number.

Im stuck now.

Maybe an admin could move this post to somewhere more relevant ?
Click to expand...
Hard chrome on surface of steel will resist to any hacksaw blade. . . . and is extremely rust resistant .
 
for this specific application the hardness of the steel is important but the fundamental characteristic is the abrasion resistance. Today thieves have available battery angle grinders with which they can virtually cut any steel, so the real need translates into a steel that extends these times as much as possible. The crucible produces excellent steel with excellent resistance to wear, but they are also very expensive, this is an example: https://www.crucible.com/PDFs//DataSheets2010/Datasheet CPM S110Vv12010.pdf
If the resistance to corrosion is not a feature that requires the audience of candidates it widens a lot!
 
Carbide would be virtually impossible to cut.

Stellite 6K (or 6B) would be an excellent choice.

I would think any alloy steel with cobalt would be a good choice. Other alloying ingredients that will make a strong and tough lock would be tungsten and vanadium.

A hard chrome plating will take care of rust issues.
 
Last edited:
Hi,

This D2/SDK11 had HRC60 in the middle after being sliced in half and tested. I was told it was to brittle.
To be honest we are happy with the results of wear resistance and strength is just the toughness is what let it down.
Also the design changed from:
lar9pd
http://prntscr.com/lar9pd
To:
lar8z8
http://prntscr.com/lar8z8

I have marked the area where it has broken.

What is the recommendation to keep this D2 ductile ? Or this is how the steel is ?
The factory mentioned that it was through hardened.
 
Also forgot to say that the item are Teflon coated to overcome rust issues.

And this pin is enclosed in to the device it self like this (-) ( - being this pin ) holding two parts together, you cannot really get a disc in there.
The test facility tried with a 45 degree hacksaw with probably 1mm in 1 min 30 sec.
Then they frozen it with a can and inserted a chisel (′-) then hit the chisel with a hammer 2/3 times and it was off.
 
Yes i have, but the prices would be over the roof as the whole device is made of 3 parts all full CNC machined.
We might release a special version but thats in the future :)
 
KozzyK said:
Also forgot to say that the item are Teflon coated to overcome rust issues.

And this pin is enclosed in to the device it self like this (-) ( - being this pin ) holding two parts together, you cannot really get a disc in there.
The test facility tried with a 45 degree hacksaw with probably 1mm in 1 min 30 sec.
Then they frozen it with a can and inserted a chisel (′-) then hit the chisel with a hammer 2/3 times and it was off.
Click to expand...
Well if angle grinder cannot be used ........you can use any steel .Hand hacksaw will hardly cut 55 HRC steel , considering it would be difficult for someone to keep it stable that device and cut in same time .Remain that cold can spray ... the one I use in my service can get to -50 C degree .So you should look for steel resistant to some degree in minus temp ....
http://www.spartaengineering.com/effects-of-low-temperature-on-performance-of-steel-equipment/
https://www.bladeforums.com/threads...ll-work-best-in-extreme-cold-weather.1237086/
 
as someone in the (Security) field for 30 plus years
my advise is regardless of cost to use a mix of a magnesium/hard steel mix
this has already been done by Medeco/Abus /other brands of locks .
i have cut many locks in half using a battery powered abrasive saw.
Stainless mix also works pretty well but has to be a mix of "Ordinary" and hard steel
the business side of things being hardened...
 
Well, the design of the lock is made so it protects the shackle of the padlock and the main pin that holds it together, so there is not much you can do with a grinder.

Would a D2 be good if hardened and tempered after?
Would it resist shock or freezing?

Thank you
 
You need a material that has good low temp properties for that particular test. Something like a 17-4 PH stainless. It handles low charpy impact testing down to -75F pretty easily. If age hardened properly it can be hard and still somewhat ductile. Down know how it will perform at a 60 RC hardness. But if you stayed around 40 it would do really well. Like someone said above research materials that contain chrome and or nickel.
 
we should discuss this in private message.
i will say i have cleanly cut through guarded,(shrouded) padlocks in very short time using battery powered tools.
 
With today's tools, I'd guess it's pretty much impossible to make an "indestructable" lock. You can make them "cut resistant" with the harder steels, though the typical trade off is that they become brittle, as you've seen.

With battery powered grinders, drills, and saws, hydraulic bolt cutters, and even .22 caliber ramset guns, most locks can be bypassed in pretty short order, if not through the shackle itself, than through the key cylinder, and/or the housing. I'd encourage you to check out videos by "the lock picking lawyer" for some good examples of lock bypass and destruction tests.

The more "beefed up" you start to make the locks, or the more advanced alloys you start trying to implement, typically the more cost prohibitive things tend to start getting.

Now please don't get me wrong, I'm not trying to discourage you from pursuing your lock design. I'm sure you already know, however, that locks are really only good for keeping the honest people away. What I'm saying is, don't get hung up on over engineering any one part of the lock, if another part of the lock is easily bypassed, or if that engineering choice is going to make the lock too difficult or too expensive to manufacture. You've got to keep up with your nearest competition, after all.
 
Thank you guys for the replies.
I only whish i could share the design with you, maybe soon as the patent lawyers say so : ).

What i am trying to achieve and did so with the design of the lock to make it be as hard as possible to cut it off.
This is a motorcycle brake disc lock and the goal is to keep them there for ages and/or delay the process as much as possible.

There are a lot of locks out there for a lot of money and all have a single point of failure, usually, 10mm shackle on the show, or u shape locks on the market that pry open like a flower.

I have shown to a few select people that i trust and they were really amazed, including some police officers and the testing facility officials.

Now my question is that what sort of hardness you would think I need to overcome hacksaw, but still be hard and ductile for impact?
If we are going down the route of D2 what can i tell the factory? Harden and then temper? Any process you guys would recommend degrees and other ?

We were even thinking of going down the route of case hardened 20CrMnTi as i read on some online books.
https://www.researchgate.net/profil...ion-Resistance-of-Cement-Based-Composites.pdf

"The matrix of normalized chromium-manganese carburizing steel grade 20MnCrTi is ferritic-pearlitic. After quenching the microstructure of this steel is martensitic"
steel Heat Treatment Hardness (HV) Abrasion resistance
20MnCrTi A 8700C/air 214 1.28
Q 8500C/oil 446 1.45

and
Steel grade Heat treatment Hardness (HV) Abrasion resistance a
20MnCrTi C 940°C + Q 8200C/oil + T 1500C/1h/air 754 2.38
Screenshots from a document, maybe someone could read it with better understanding than me?
http://prntscr.com/lbcuux
http://prntscr.com/lbcv2u
 
Did some work back in the day with another bike lock company. Look at S5. 60-62 HRC and very shock resistant. Was found to be the best overall combination of both hardness and toughness. Only down side is the oil quench but on the plus side most newer machines can work with this hardened.
 
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