Which grade of steel do I need?

I want to make what is essentially an 8mm diameter cold chisel about 250mm long. It needs to be hard but also resist bending when struck with a hammer. It needs to be circular in cross section so using something like and old drill bit is not an option.

I have tried silver steel tempered at a range of different temperatures but it either breaks because it is too hard or bends or both.

Is there a better grade of steel for an application like this? And ideally available in 8mm dia stock, I don't have turning facilities and the diameter needs to be very close to 8mm.

p.s. I know this post will raise questions about why and what for but I really don't want to get into that as it will just raise other concerns. I am certain that this is what I need there is no other option but a circular 8mm hard resilient rod

Silver steel works fine for me when making cold chisels. So I suspect your hardening and/or tempering is off. Are you using an electric furnace or the heat 'n' hope method?

An 8mm diameter 250mm long rod is very slender. So irrespective of what steel you use it will be prone to buckling under load; basically a design flaw.

Andrew

Does the chisel have to be 8mm for the whole length ? Engravers tools and old fashioned stone masons hand chisels are in many cases much smaller at the business end than the handle/shaft.

John

I'm using heat and hope I don't do a lot of heat treatment so alternatives aren't really cost effective. And yes, getting consistent temperature over 250mm with a blow torch is challenging. Suggestions for low cost better ways of consistent heating most welcome. I use the domestic oven for tempering.

The 8mm over 250mm is what it is, it screws into a larger diameter holder.

I wonder if a length of en19T or en24T used without any extra heat treatment would work?

You don't really need to heat treat the whole length, just the business end will do. Good chisels will be some form of alloyed steel probably with vandium, chromium, etc in it but chances of being able to buy a short length will be slim.

Also the angle of your cutting edge should be right for what you are intending to cut so least likely to be damaged Also do your shaping of the end before heat treat to reduce risk of grinding overheating things. An 8mm chisel will only suit light cutting so trying to take of a big chip will more likely result in a bent shank which is on a looser anyway due to dia/length ratio.

The end that you strike should be soft and the business end should be hardened and tempered. A straw colour for the tempering may chip more easily than tempering down to blue. If you are hoping to heat treat the long shaft to be more resistant to bending then you have a problem, if you leave it at straw it is likely to snap but blue could be less resistant to bending and less likely to snap. Commercial cold chisels seem to often be made from a chrome vanadium steel but I think you will struggle to find that in the size and quantity you require and heat treatment is likely to be more fussy than silver steel. Cold chisels that are very long always tend to bend which may be why commercial products don’t come in long lengths. I have an odd special that was made for a particular one off job that wants to bend when you hit it. The bending tendency is always a problem with long rods, over long punches always bend as it is impossible to hit them so accurately that all the force arrives at the tip, some of the blow always tries to bend the long shaft. The optimum material with perfect heat treatment will not make a satisfactory tool with the length diameter proportion you require. You say you want round material but if hexagonal could be acceptable then buy an 8mm T style long Allen key which will be as tough as possible but may not hold an edge particularly well, cut the T handle part off.

Mike

Edited By Mike Poole on 23/07/2022 08:05:28

If you're going to be stuck with 8mm which REALLY sucks then I would say you need one of those pile driver type hammers to give you accurate maximum force blows every time

The hammer weight has a hole in the middle and it travels up and down the chisel shaft until it hits the dead stop, you can make one with a cheap welding kit

Dont kno what they call them

aha! A sliding hammer!

crowbars are made of very tough steel but I can't say I've ever seen one in 8mm

Edited By Ady1 on 23/07/2022 09:36:46

Hi Mike Poole, I don't know what length you consider as long commercial cold chisels are, but I have this 18" x 1" octagonal cast steel one manufactured to BS3066 and has had plenty of heavy thrashing with a 4 lb lump hammer and has shown no sign on bending. This is the second one of these that I've had and the first one had loads more thrashing without bending, but that one got nicked when the firm I was working for had a break in. However I do agree that a long slender one like Chris wants won't stand up to a lot of hammering and only the cutting end should need heat treating.

Regards Nick.

Edited By Nicholas Farr on 23/07/2022 09:25:00

Hi Nick, you could probably pound that all day with a 14lb sledgehammer without a problem, the chisel I had in mind was about 2ft long and 1/2 diameter, it was my grandfather’s who was a plumber by trade. Faced with some concrete chiselling for some new fence posts I bought a Screwfix cheapy SDS drill/ breaker, it did the job in style and if it never comes out of it’s box again it was worth every penny. Cast steel used to be the material of choice for cold chisels and was what my apprentice made chisel was made from, CrV seems to have become popular nowadays.

Mike

I was about to suggest repurposing an SDS chisel... cheap ones are cheap enough and the shaft size can't be far off the OP's need. If you can't find one with the correct tip then it'd be a bit of time on a grinder..

pgk

If I wanted something stiff at that length, I would be re-engineering a 3/8’ socket drive extension bar. Not sure how good a chisel tip they might make, but likely better than the OP has managed.

Again, with cheap welding kit you could weld a hardened tip piece onto a good bar

There are better grades of steel for making cold-chisels, but they're harder to heat treat than silver-steel, and I suspect Chris' problem is caused by faulty heat treatment. The metal needs to be heated quickly and evenly to the right temperature and then smartly quenched. Like telling jokes, timing is critical!

• Important to prevent heat loss, I always build a nest around the item out of soft white insulating firebrick.
• My small 1kW torch is unable to heat a long length of steel evenly. I have to use two, which are cheap but hard to manage. Big torches deliver a lot of heat quickly over a broad area and are much easier to get right. Sadly, a big torch and gas cylinder are expensive for odd jobs.
• The nature of the requirement makes the heat treatment more critical than anything I've done. I usually only need a hard tip at one end and leave the rest softish. Chris seems to want a chisel hard at the tip, which only gradually softens towards the hammer end to resist bending. Unfortunately, the ratio between length and diameter of his chisel is over 30:1 making that difficult! Less than 20:1 is more normal and cold chisels are usually even stubbier; my slenderest is 15:1. To get close I think the heat-treatment needs to be near perfect, which is difficult with ordinary equipment.

What might help short of making a perfect chisel is arranging for the blow to the hammer end to accurately directed. When hit with an ordinary hammer swung in an arc, the force applied to the shaft will be slightly off-axis causing it to flex. Once a bend develops, lever action multiplies the bending force and the shaft kinks and probably fails catastrophically. A tool that delivers a hammer blow exactly inline with the axis allows much more force to be applied without bending the shaft. Perhaps a slide hammer?

Commerical cold-chisels are made of drop-forged hardened and tempered steel, usually one of the extra tough alloys. Much better than anything that can made in a home workshop or even by a well-equipped blacksmith. Buying one and grinding it down to shape would be the best answer, but it's a lot of work.

Dave

Thanks for the replies, I had considered something like a screwdriver repurposed, what I hadn't though of was welding/brazing different materials together. As I briefly mentioned in my second post the slender rod screws into a holder which is the part that gets hit with a hammer, so may be a screw driver of the right diameter with a threaded rod welded on one end and a hardened piece on the other is the way to go.

I suppose the question there is around whether it is possible to weld a tool steel tip on to the screwdriver and then harden it without affecting the strength of the screwdriver shaft too much. I'm pretty busy for the next few weeks but I'll do some tests when I get chance. It may be that a chrome vanadium screwdriver would be hard enough anyway for the intended application.

Just to add that I have got a decent propane torch and what I have been doing is setting up a firebrick nest on top of the gas BBQ and letting everything get up to about 150-200c then bringing it up to the curie point with the propane torch and quenching. After quenching I test with a file which usually just skates over so I am fairly convinced I am getting the rod pretty hard. As I said I am tempering in a domestic oven so the whole rod is tempered, obviously the oven temperature setting might be off but I do have a fairly new, decent quality toaster oven with a digital temperature setting. I know this might not be accurate but its probably as good as anything else without going to much expense.

And just out of interest here is one that failed, as you can hopefully see a piece has sheered off the business end and the other end is bent

Edited By Chris Hunter on 23/07/2022 13:10:18

Given that good screw drives and socket extensions will be chrome vandium steel then little need to add a tip, just grind the end to the desired shape.

Tool steel shatters under hammer blows when you shock load a point, it lacks elasticity/bending resistance

Would be fine if the load was applied via increasing pressure like with a nut splitter

Welding it is quite difficult, you get stress cracks on the mild steel side because of cooling differences

You need some sort of chisel steel that is hard but you can belt the snot out of it

Lidl had boxes of SDS chisels recently for about 15 quid

Edited By Ady1 on 23/07/2022 13:56:29

Rebar is quite good for chisels