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New Rockwell Hardness Tester testing results


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I just picked up a new to me Rockwell hardness tester today because I am planning on doing more of my own heat treating.  The things I most want to bring in house require hardness testing results submitted to the customer with the parts..

After checking the calibration with the test blocks,   I was eager to try the tester out on something so I tested a couple of things around the shop.    I was surprised how soft a number of things were.

A piece of mild steel did not even register on the RC scale I would have had to use the B Brailer and weights.  I then tested a couple of drill bits.  Hardware store drill bits were only 38RC and some good quality were about 42.  I wondered if it was the fact I was measuring the shank rather than the tip so I measured on the web near the tip and I got the same results.  I checked a Sorby chisel that I bought when I started my patternmaking apprenticeship it was only 29RC!!  It is about 2" shorter than when I bought it due to sharpening so it may have only been fully sharpened at the end.  I think I will be re hardening it.  

I have an old Marples gouge that is a favorite of mine it tested at 62RC and a solid carbide end mill was 75RC so not everything I tested was softer than I expected. 

The results were very interesting and I can see how having a better idea of the actual hardness of things will be useful going forward.

 

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I use an old Wilson table for cylindrical correction values. The information in the table comes from ASTM, but that spec costs money for access. I assume that you already have the spec if you are promising certified hardness values. The Wilson chart is freely supplied. From searching for the official website, I think that Buehler may own Wilson now. 

http://www.buehler.com/hardness-testing.php

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Oh, for anyone not already familiar with hardness testing: grind or file parallel flats if you need accurate tests on something with too tight of a radius. If you can't grind a flat large enough for the anvil on the hardness tester, make a small flat on the test side and use a V block anvil. The flat needs to be at least large enough for 5 indentation diameters (2.5 diameters from the center of the indentation to the nearest edge in any direction). 

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Thanks for the information on the adjustment tables for round items.  I couldn't find one at the link but I found one elsewhere.  The drills were 5/16 and 11/32 so i had to add between 3.5 and  2.5 to my results, still softer than I would expect but better.  I will print off the chart and keep it with the tester. 

I have a set of the hardness files shown but they are a little more subjective in use and give a range not a specific number for my customer  As well they are starting to wear out. 

The parts I will be testing have a tapered side and a parallel  side so I will be testing on the parallel side.  If I use a tapered block between the anvil and the tapered side of what I am testing so the top of what I am testing is parallel with the top of the anvil will that give me a reliable result on the tapered side? 

I am going to have to make a stand for the end of the piece I am testing to ensure it is flat on the anvil.

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It's worth mentioning that the surface condition shape and thickness all make a difference to the readings.  If I take a sample piece and test it with a 40 grit grinder finish it can read up to 5 points softer than the same piece taken to a 600 grit finish. The piece meeds to be parallel as a tapering piece often reads softer as the indenter slips. Thin (like 1mm) pieces read much harder than the same steel at 6mm thick, which it could be but I suspect is that the dial is reading some resistance due to the table instead of just the sample. If you have forgescale or coarse grounds surface on the underside it will affect the result too

When I play with heat treating I run some test pieces through the same HT regime as the tools in that batch, normally 5mm thick, and then use those as references as to how hard the tools are likely to be. That way I don't have to worry about the above problems ;)   I also have a set of those files and are OK for a VERY rough idea, but the fresher areas will still mark harder than read steel I've found

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The charts are under "hardness testing resources, " and the direct link is http://www.buehler.com/Posters/Hardness%20Conversion-ASTM.pdf

This table also includes minimum acceptable stock thickness based on hardness, which will give you a good idea of when you would be getting a false measurement. 

Like Dave Budd said, a witness coupon made from the same stock as the part (but with flat, parallel faces) will help when you have something with a difficult geometry and can't put a flat on.  Make sure that the witness coupons have the same thickness as the part and are heated and quenched at the same time as the part so that you don't have any variance from quench rate differences or a maximum through hardenability. 

The angled shim idea can be made to work in extreme cases, but is not good practice. If you have guaranteed the test to ASTM E18, or the heat treatment to some other spec (which should specify inspection criteria), avoid doing anything "creative." Like Dave already said, the shim and part will tend to slip relative to each other. 

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I will just use the parallel side. and may may use test coupons as well.  I have sent test coupons to heat treaters in the past.

I will not be guaranteeing these to any spec as my customer's requirements  do not mention any inspection criteria.  The certs I currently get from 2 of  the 2 heat treaters I use do not reference any specs I assume because I have not asked for then to be. 

I have had other more critical parts done to DIN specs in the past but at this point I would still send out these more critical type parts.  I have other parts that I have to continue sending out as they are 4' and 6' long and have to be oil quenched and I have no way of reliably heating bars that long all at once.  I actually had a hard time finding a heat treater who could oil quench the 6' bars.  I had to call over a dozen different heat treaters to find one that could do it. 

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I bought a second hand electric furnace from the place where I served my apprenticeship (they were closing down), it allows me to heat bars up to 83 inches, (2.100 metres).  I've always got work for it, I have figured out that there are not many HT places that can do up to 83 inches long and have an oil quench bath.  Seems that you have confirmed that this is not an Australia only problem.

Yes I know that places like Scot Forge can handle this sort of stuff, but places like that are not in every town.

Phil

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There are a couple of places around here that have huge furnaces but they do water or polymer quenching.  The parts I am going to continue to send out are only 1" dia so the one place that will heat treat up to 82' long x 21' wide (they have smaller furnaces as well)  felt they could not get them from the furnace to their quench tank fast enough. 

I would love to bring them in house as well and if the right furnace came up for the right price I might consider it as the place I have to send them has a terrible turnaround and has a high minimum charge. 

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