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HEAT TREATING

 

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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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DSC_1729sThis is a real close up of the part being tested.

DSC_1728sThis is the rest of the shiny part being tested.

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

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

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