During our recent trip through the mid-west I had an opportunity to stop in and chat with John D.,a metallurgist, and Joe P.,a heat treating professional. I ended up taking a number of pictures and John was nice enough to do a write up for me concerning what we looked at, what it does [...]]]>  

During our recent trip through the mid-west I had an opportunity to stop in and chat with John D.,a metallurgist, and Joe P.,a heat treating professional. I ended up taking a number of pictures and John was nice enough to do a write up for me concerning what we looked at, what it does and how it works.

Nikon inverted stage ‘Optiphot’ optical metallurgical microscope with a CCD camera and flat screen display for group viewing..  This type of reflected light microscope is usually referred to as a ‘metallograph’.  Metallographs provide useful magnifications from 10x to 1,000x plus and are distinguished from most scientific microscopes by their use of reflected light, rather than transmitted light.  The illumination light is introduced into the image axis via a half silvered mirror and broadcast upon the sample surface by the objective lens, the same objective lens which generates the reflected image..

Image on the flat screen is a polished and etched, through hardened, medium carbon, manganese-chromium alloy steel transverse section (cut perpendicular to steel rolling direction).  Area being viewed is deep in the sample, away from its quenched surface. This steel sample image reveals decided chemical segregation remaining from its original solidification at the steel mill.  The light yellowish blotches are alloy rich ‘strands’ in the steel which fully hardened to martensite at this depth from the quenched surface.  The darker greenish envelopment is the much softer, alloy depleted matrix which did not fully transform to martensite (the strong, hard constituent of hardened steel) at this depth.

This type of alloy segregation is termed ‘banding’ by metallurgists and can severely degrade mechanical properties in the directions perpendicular to steel rolling.  Banding in alloy steels can be diminished by greater reductions in steel mill hot rolling (requiring a larger initial size cast form), or adding alloying elements like nickel which promote more chemically uniform solidification.  Banding is a major issue in the AISI/SAE 4140 series (chromium-molybdenum) steels popular in American firearms construction.  It is also a major issue in the manganese and manganese-chromium steels popular for firearms construction in Europe.

Fully automated Newage ‘MT-90′ microhardness test system used to measure the hardnesses of very small volumes in metals.  Microhardness is usually measured on the Vickers or Knoop scales using single indentation loads ranging from 100 grams to 2.5 kilograms.  Microhardnesses are determined by measuring the width of impressions made by known dimension diamond penetrators.  Microhardness testing – due to its inherent sensitivity – can only produce accurate hardness measurements on highly prepared (polished) surfaces.

The more commonly used Rockwell scale differential-depth (dual load to eliminate instrument lash) hardness values are determined by directly measuring the depth of penetration, using indentation loads ranging from 60 kg to 150 kg.  Microhardness measurements can be converted to the more commonly understood Rockwell values by using conversion tables, but the microhardness measurements only read the hardness to a depth of about 0.1mm, while Rockwell measurements routinely read the hardness to a depth of 1mm.  This is an important distinction when evaluating metals with hardness gradients.  Side note: the indispensable Rockwell hardness test is 100 years old this year, having been invented in 1914.

At the top of the lower right quadrant in this screenshot from the Newage MT-90 automated microhardness tester, you can see what metallurgists call a ‘microhardness traverse’.  This shows the gradient in hardnesses from near the surface, towards the interior, in a carburized steel section.  The surface of the steel sample is directly under the center cross of the quadrants.  The material imaged to the left of this point is a phenol formaldehyde plastic mounting compound (‘bakelite’) used to preserve flatness of the metallurgical specimen during polishing.  The material imaged to the right of this point is the actual steel metallographic specimen.

The light area at the surface is extremely hard, high carbon martensite which has fully transformed.  As the steel darkens away from its surface, its carbon content is lower and hardness is declining due to reduced hardenability and less transformation to martensite. [Hardenability is a direct function of carbon content in this case].  The individual HV(1kg) impressions are increasing in size further from the carburized surface, reflecting declining hardness.  This particular traverse has been performed to determine ‘effective case depth’, the depth below a carburized surface where hardness falls below 50 Rockwell ‘C’ scale equivalent [the term equivalent is used to indicate that the original measurements were not performed using the Rockwell ‘C’ scale].  Effective case depth is the most important property of precision carburized steel parts.  Core hardness and percentage of retained austenite (a soft constituent) in the case are the other two commonly specified properties which influence part performance.

Metallographic image of an etched section through fully hardened and then tempered medium carbon steel specimen without a carbon gradient.  Bakelite on the left, steel on the right hand side.  A good metallographic structure not evincing much evidence of banding, such as might be obtained from AISI/SAE 8645 or 4340 nickel containing steels.  The faint striations visible are polishing artifacts which do not reflect upon the actual sample structure.

Another view of the Nikon ‘Optiphot’ inverted stage metallurgical microscope.  The sample being viewed is in the exact center of the square stage, in front of the binocular eyepieces, on the round white metal stage insert.  The sample is fully encapsulated in bakelite, which is also seen on the left side of the flat screen screenshot.

Two Struers ‘Tegramin’ automatic metallographic polishing machines.  Six bakelite mounted steel specimens are locked in the holder at the bottom of the vertical drive column and then spun on abrasives affixed to the spinning platen below.  Steel metallographic specimens are first leveled and polished on 120 grit to 600 grit silicon carbide abrasive papers in progression, then 6 micron diamond particles in oil on cloth, and finally 0.05 micron alumina particles suspended in water on cloth.  Specimens are ultrasonically cleaned and the platens changed at every progressive polishing step. The resulting mirror finished surface is then etched in weak acid solutions to reveal the steel microstructure.  Some steel microconstituents like hard martensite are highly corrosion resistant, others like iron carbide easily corrode in weak acids.  This corrosion differential provides the structural images seen in a metallograph.

John Dingell, III, Charles Kramer, and Joseph Pieprzak, Jr. discuss the metallurgy of several firearms parts and assemblies.  Note the VG.1-5 rear assembly to the left of the reading glasses; can anyone recognize the shiny bolt to the right of the reading glasses?  Hint: it is from a firearm that could be politely described as Chuck’s obsession!

A few other pictures testing the shiny bolt thing.

This is a real close up of the part being tested.

This is the rest of the shiny part being tested.

Thanks to John for adding the technical aspect for this write up.

You will get extra brownie points for guessing the shiny thing.

I had the opportunity to chat with a great design team that has been working on some new magazine and weapon designs that unfortunately at this point I can not go into. However, what I [...]]]> I am not going to do a library post today. Instead I am going to post about my day yesterday.

I had the opportunity to chat with a great design team that has been working on some new magazine and weapon designs that unfortunately at this point I can not go into. However, what I can talk about is what I learn in the few hours that I was able to spend with them. I realize that I have so much more to learn that I can not possible get to where I want to be in my life time. Just the few minutes we spent with them talking about magazine design open my eyes to what I have been doing wrong in some of my own designs. There help way beyond measure and with the insight provided I can now continue with my 7.62x54r Bren gun magazine design.

They were able to help me find other contractors that could help with wire EDM and deep hole drilling problems that we have been having. Finding good sub contractors is really a task. We have been looking for a good deep hole drilling company for quite some time. With there knowledge I will be able to take the next step on the FG-42 first model. The conversion  about dwell time open my eyes to a way to correct a design problem that we have been having with a new rifle project that we are working on.

The wire EDM guy they are using understands the accuracy needs that we are facing on one of our projects as well as the grinding and heat treating requirements that we need. The conversation concerning steels and heat treating was just great.

I was able to test fire there newest design which is better then anything else that I have shot recently. It is truly years ahead of the rest of the weapons designs out there.

One of the services they offered and I will take them up on is their use of their high speed camera. With this service I can now take detail pictures of the weapons we make making. This will answer a number of questions for us concerning the design and operation of our guns.

I just hope that I did not embarrass myself to much as to not be allowed to come back and talk with them again. I really enjoy the time I spent with people that know there stuff. I always feel a little less then adequate after a day like today, not because of anything they said or did but because of what they open my eyes to.

I hope in the future I can mention the design team by name. These are the type of people that truly make the world turn on its axis. Great men just designing and building with no fan fare.I hope that in some small measure we are able to help them continue with there project.

And as if this day could get no better I received a call from one of the members of this site and we went into the some of the problems that we have been dealing with concerning our stampings. The knowledge that he shared with me dealing with sheet metal and stamping has truly helped in the design of the next set of dies we make.

This day was just GREAT. What I hope to do with this site is to present you with the kind of information I obtained today. I know that my post today does not do the day justice. However, I have written down two pages of notes of information that I have learned today and in the future will be able to incorporate that knowledge into the designs that we build here.

Thanks again to everyone for making this a great day.

So this first post is [...]]]> First off you can see that we changed the title for the Thursday post from book review to library. This was done because I wanted to add more then just book reviews. I wanted to add information to the Gun Lab library that I thought would be useful to you.

So this first post is from a friend of mine that wants to remain anonymous. So for this post we just call him Joe. Joe had the opportunity to have every piece of brass test that he had that was attached to a weapon. This included Maxim guns, Nordenfelt weapons, Gardner guns and any thing else he could find. This is that report. I found it interesting and very useful. I hope you do to.

Bronze Composition Analysis document