Simple table of potential hardness of metals

Hi, what would be very useful is for me to be pointed in the direction of a table showing the potential hardness that can be achieved with the various metals we would commonly use in home machining. I believe this is called the Rockwell index but stand to be corrected.

Regards

Chris

Harness of metals, usually steel, can be stated in different units, depending upon the machine and method used.

it can be Firth, or Vickers, Rockwell, Brinell, depending on whether the indentor is a diamond or ball. And the pressure used can vary. Hence Rockwell B or Rockwell C. Zeus tables quote the loads and indentors used.

So, the same metal hardness may be given different values, depending which method is used. 

Howard

Edited By Howard Lewis on 22/08/2019 12:29:43

The question I would like answering is exactly what is the use for me as a home machinist in a hardness scale (lets assume we are referring to the Rockwell Scale). For example if a metal can be heat treated to a higher HRC is this of value in relation to another metal yielding less. At first this question seems simplistic but many factors could arise so its a question I would be interested in anyone answering as there's always something you don't know.

Regards

Chris

Ifs and buts off the scale on this question because there are so many variables. Roughly though, and with many overlaps, softest first:

Lead
Tin
Copper
Aluminium
Magnesium
Dead mild steel (Car Bodies)
Black Mild Steel (Holding up buildings)
Cast Iron (varies)
Soft Brass (plumbing)
Hard Brass (cartridges)
Medium Carbon Steels (stronger than mild-steel, not so easy to work)
High Carbon Steels (Files and Razors, heat treatment essential)
Ultra-high Carbon Steels (punches)
Alloy steels, HSS, & camshafts etc
Tungsten

Hardness is a bit like Easter though - it moves about! Try stretching a length of solid copper core from a mains cable. Starts soft, then goes hard. Many materials can be softened by heat - annealing. Some, especially the high-carbon steels, can be hardened by heating and rapidly cooling - quenching. Metals like bronze and stainless steel work harden. Some types of stainless are vicious, going from usable to glass hard the instant a cutting tool is allowed to rub.

Hardness isn't the whole story. Though soft, pure aluminium is nasty to work with because it's sticky and highly likely to weld itself to a tool's cutting edge. Quenching steel for maximum hardness is rarely done because the process leaves the steel brittle to the point it can shatter like glass. Balancing Heat treatment for hardness versus toughness can be rather complicated. O1 (aka Gauge Plate) and W1 are favoured in workshops because they are easy to heat treat. O1 is designed to be quenched in oil, a slow process that strikes a good balance between hardness and toughness, as is needed in knife blades. W1 (aka silver steel) is designed to be quenched in water without cracking. The result is a bit harder for making drills, punches, and screw-taps, and it often benefits from being tempered (cooked in an oven) to reduce brittleness. The two alloys are similar.

When buying metal look for mention of machinability in the description. Many alloys are tuned to make them more suitable for turning and drilling etc. and life is made easier by buying them. Ordinary mild-steel is neither awful nor nice to machine but it's well worth spending a little extra on EN1APb, aka EN1A Leaded, or 230M07PB. Most Brasses machine well, but the Bronzes can be a pig, especially Phosphor Bronze.

Dave

Edited By SillyOldDuffer on 22/08/2019 14:31:02

Chris,

Your question is a little too complex for a simple answer because once a tool has been quenched to give its ultimate hardness it is usually tempered to give a compromise between hardness and toughness (resistance to fracture) suitable for the job in hand.

If you are interested in this subject then I strongly recommend Tubal Cain's "Hardening, Tempering and Heat Treatment", No.1 in the Workshop Practice series.

Just as a taster, TC quotes the "as quenched" hardness of silver steel at c.67 Rockwell C and High Speed steel as 65 HRc. Both are pretty brittle in this state. If you really get in to it then you can make Dave Lammas' Hardness tester, castings available from Blackgates!

HTH,

Rod

Some metals have a range from fully annealed to (usually) so hard that it is too brittle for most uses. The only importance to the home machinist is (usually) the final hardness after tempering. That will always be a compromise between hardness and toughness with springiness somewhere in between for some items. One can get the final hardness on many metals but that does not mean they will be suitable for any particular job.

Then there is the difference between surface- and through-hardness. The hobbyist needs to consider what heat treatment is appropriate - a very hard skin (case hardening) with a soft centre or hardness across the whole section. Rates of cooling (oil or water quenching will change that, too, as well as the cross section of the piece).

Sooo, a simple table of hardness is likely not too appropriate for any or many jobs dependent on the metallurgy, the method used and the end properties required. Hobbyists generally keep it simple - industrial hardening, tempering, soaking at particular temperature(s), heating and cooling gradients, etc is a work of technical artistry!

Your question should be, I think, more about which metal composition to choose before hardening and tempering, to a particular standard, and that may depend on your individual requirements for the end product.

Tungsten is very different from tungsten carbide. I have a small supply of tungsten alloy called Densimet. The 1/2" bars make very good boring bars for TC inserts and can be turned, milled, drilled and tapped. The relative density of the alloys runs about 18, and they have a higher Youngs modulus than steel. I also have a couple of plates of the stuff, 5/8" thick and about 10" by 4", which I have not thought of a use for yet, other than an excellent high inertia backing plate.

The exact answer is very little. All you really need to know is that some steels with a high carbon content (silver steel and gauge plate) can be made harder by heating, quenching and tempering. What the exact hardness number is on the Rockwell scale isn't critical. In a home workshop you're unlikely to be able to accurately measure it unless you're very well equipped. Even in the professional world it's not easy to get consistent results. And of course the scales are arbitrary, based upon the indentation of a specified anvil shape under a given load.

We now wait for Rod to show us his hardness tester.

Andrew

I have been looking for a way to describe hardness to young engineers , we have Vickers, Rockwell, firth and brinell, I add mohs and shore. "The nice thing about standards is there are so many to chose from".

Mhos is more useful as a geological scale, but is similar to hardness gauging using sets of standard files, I suppose.

But still just a comparative scale - what will, or won’t, scratch what - likely more useful for choosing a surface preparation material within a polishing system.

But who, in hobby model engineering, seeks out grinding paste or polishing compounds by using that scale? It is left to the suppliers to provide the correct grinding material and the correct sizes of particles (within each grade of paste) and carrier, for users to decide on how much they wish to spend - mostly dependent on speed of production.

Hi Guys thanks for all the replies. My thoughts are ; I have only come across references to the Rockwell scale in terms of how hard it may be to machine or how hard it may become after heat treatment. Therefore it can be used as an indicator of hardness which does not mean in many cases the best suited material.

Regards

Chris

Slightly off topic but does anyone else experience dipping a lead bell in liquid nitrogen then having it ring a clear resonant note?

Along with other hardness scales the Rockwell scale simply gives an indication of hardness. It says nothing about machinability. With the right tooling it is easy to mill high speed steel toolbits (hard) but milling copper or pure aluminium (soft) is 'orrible.

There are many different Rockwell scales. The most common is Rockwell C as it covers many steels and cutting tool materials. Brass and very soft steels might use the Rockwell B scale. The scale can be used on material in any state, soft, annealed, hardened, tempered and so on.

Andrew

Itmay help to add a few basic facts:

All common metals can exist in harder and softer forms, but with many of them the difference is not great. Changing from hard to soft can usually be done with heat - not enough to melt, and then allowed to cool fairly slowly. This is called 'annealing'. Lead is an odd one as its annealing temperature is lower than normal room temperature so it is always in the soft condition, unless, as pgk pgk has discovered, further cooling is taken seriously.

Another exception is steel, in which the difference between hard and soft in the same alloy is greater than other metals, and in steel including a small % of carbon the range can be extreme. It all depends on the exact alloy, the possible addition of other metals to the alloy, and the heat treatment.

So, your 'hardness table' is going to be complicated by each metal having a range of hardnesses, some with a much wider range of hardness than others. This probably explains why such tables are not readily found.

I have found the book 'Machinery's Handbook' very useful in this regard. My edition (No 21, 1980) has 16 index references to Hardness, and 51 about Heat treatment of Steel. Don't look at the most recent versions, which will be expensive - the book has many editions back to about 1910. Hardness and what to do about it have always interested engineers.

Hope this helps

Tim

Edited By Tim Stevens on 24/08/2019 11:51:11

...and then allowed to cool slowly...

Just watch out for the common exceptions. Copper is one - it matters not how - quickly or slowly - it is cooled. Hardening of copper is not really possible with heat - but it does work harden quite easily.

Hi Chris, There are not many choices for cutting tools in the home workshop, Tungsten Carbide is probably the hardest but tends to chip easily, it is very resistant to high temperatures. HSS is readily available as square or round blanks to grind a tool of your choice, again it is supplied hardened and will still hold its edge at high temperature. High carbon steel can be supplied in a soft state like silver steel and gauge plate and fashioned into a tool and then hardened and tempered to your requirements and finish ground, excellent tools can be made but overheating will very quickly destroy the edge. Tungsten Carbide can be as simple as a brazed tip ground to a basic tool or as a preformed tip with a wide variety of shapes and characteristics, HSS has a number of different flavours to choose from. High carbon steel is available as mentioned and also various other hardenable tool steel alloys are available. The absolute hardness is only part of the story for making tools, toughness of a tool can be desirable and with High carbon steel the tempering process trades some hardness for increased toughness. A Tungsten Carbide tool will machine a HSS tool even in its hard state. Unfortunately Tungsten Carbide needs special grinding wheels as it is harder than Aluminium Oxide and silicon carbide, you will need a green grit wheel or diamond wheel which is probably one of the reasons to use preformed tipped tooling. The absolute value of the materials hardness is interesting but as long as the tool is harder than the material you wish to cut then all should be well.

Mike