Metals. It’s easy to take them for granted. They’re all around us, in our cars, computers, knives and forks, pens. It’s pretty "common" stuff. But when you think about it, metals are pretty amazing materials. Some are light and relatively weak (magnesium), while others are heavy and strong (tungsten). But the opposite is also true. Some are light and strong (titanium), while others are heavy and relatively weak (lead).

Some metals can be either weak (soft) or strong (hard) depending on what we do to them. A piece of steel can be soft and flexible. Throw it in a furnace for an hour or two then drop it in a bucket of water and it will become hard (almost as hard as a diamond) and brittle. Throw it back in the furnace— but not too long—then air cool it and it has some of both characteristics, hardness and flexibility. Combine nitrogen, which has no strength (it’s a gas after all), with a piece of steel, and you’ve made the steel’s surface harder and stronger (nitriding). Add a lot of carbon to iron and you’ve created a metal-cast iron containing graphite—that is somewhat soft and self lubricating. Add a little carbon to iron and you’ve created a metal—steel—that is hard and abrasive. Bending a hard piece of steel makes it softer. Bending a soft piece of steel makes it harder.

These are just a few of the almost countless characteristics that metals can exhibit. Many of these characteristics are the result of a wide range of processes which have been developed over the past five thousand years including forging, alloying, heat treating, and others. All of these processes have one thing in common; they manipulate or alter the microstructure of metals. The microstructure of a metal is what determines its properties. So, what exactly is microstructure?

Like all matter, metals are composed of atoms. These atoms combine in small clusters which are called crystals. Groups of crystals combine to form grains. Since I’ve just condensed several thousand books into three sentences, you can assume I’ve left a few things out, but it will suffice for the purposes of our discussion. It is the grains that are, for the most part, the objects of our analyses and examinations. Basically, the grains size, shape, orientation, and combination with other constituents gives us information about a metals properties, how a component composed of that metal was made, and allows us to predict the performance of the component in service.

How the microstructure of a metal is revealed is another subject about which many books have been written. In the simplest terms, a piece is cut from the component to be analyzed. The piece is molded into a thermosetting plastic mount, called a micro, which looks something like a small hockey puck. The plastic mount allows us to hold the piece at a consistent angle through the next step, polishing through progressively finer abrasives until the encapsulated metal exhibits a mirror-like finish. The metal is then etched with an appropriate acid, revealing the grain structure.

This gallery of photos, taken here at Metallurgical Associates, shows a variety of microstructures and describes their implications to the performance and physical properties of the components from which they came.

"Super Iron" Gray iron, the most common form of cast iron, is relatively inexpensive. It’s also reasonably strong and hard. But it’s also very brittle. If stretched or bent, it breaks. This lack of "give", defined as low ductility, once required the substitution of more expensive steel in components subjected to bending or stretching (tensile) loads. The microstructure of gray iron, shown below, consists of laminations of carbide and iron called Pearlite (A), relatively pure iron called Ferrite (B), and a high level of carbon in the form of graphite flakes (C). In the late 1940’s, a new form of cast iron was developed called ductile iron. Magnesium was added to the molten iron just before it was poured. This resulted in the microstructure shown below, which also contains Pearlite (D) and Ferrite (E), but with the graphite converted to spheres (F). This change dramatically improved the irons ability to accept bending and tensile loads, or its ductility. While gray iron will “stretch”, or elongate, only 0.06% before breaking, ductile iron will elongate 18%.

Manganese Steel Gear Teeth Manganese steel (10-14% manganese) is an extremely tough, wear resistant material. So tough, in fact, that carbide tipped saw blades are required to cut it. This makes it an ideal material for high wear applications. The image below shows a polished cross section from a gear used to articulate machinery in a mine, an extreme wear environment. The gear was cast, machined, and heat treated. Defects generated during casting and heat treating caused it to fail in service. Massive chromium carbides (A) formed during casting when the molten metal solidified too slowly resulting from too high of a pouring temperature. Carbide films (B) were produced during heat treating when the gear was not quenched quickly enough after coming out of the furnace. Both of these constituents were generated at the grain boundaries, weakening the grain-to-grain bond strength. Titanium carbonitrides (gold colored particles at C) were intentionally formed to combine with gases in the metal which might otherwise embrittle it.

Electric power companies schedule regular analyses of critical components in their generators to predict service life. This enables them to replace these key components before they fail, preventing power outages. The image shows a section from a medium carbon steel steam turbine generator shaft. The specified microstructure for this shaft is Martensite, a combination of iron and carbon atoms obtained by quench and temper heat treating. Martensite normally has the appearance of interlaced needles. Our analysis of the microstructure of this shaft, however, showed that the Martensite had become rounded, or, sheroidized (the small black “dots”) due to many years of exposure to 1000° F steam. This microstructure indicates that continued exposure to these temperatures, along with rotational stresses, will eventually produce small internal voids which will grow, link together, and eventually result in a fracture, a failure mode known as creep. The rate at which this process occurs can be predicted and the shaft replaced before failure.

Micro-Processor Solder Joint

Consumer electronics; computers, digital cameras, TV’s, are abundant and cheap in today’s marketplace. One of the many reasons for this is the speed with which they can be manufactured. Micro-processors and other components must be soldered to circuit boards. The best solder joints are made by hand, by skilled technicians. But this process is time consuming and is used only in very demanding applications like satellites and smart weapons. To make consumer electronics affordable, furnace soldering is used. In this process, a pre-determined amount of solder is applied to all the solder joints on a circuit board and the assembly is heated in a furnace to a temperature which melts and flows the solder, forming all the joints at once. This requires precise quality control. To maintain this control, solder joints are routinely cross sectioned and examined. The image below shows a polished cross section of a furnace solder joint which exhibits good flow and wetting (contact) of the solder (A) with the copper alloy lead (B) and circuit board pad (C). 
 

Chipped Cutting Tool

A chipped drill bit is a nuisance on a home project. It can be a disaster in industry. Thousands of drill bits are used, for example, in the manufacture of a single commercial airliner. Rough, jagged drilled holes can act as initiation sites for fatigue cracks. Bits that chip in use result in lost time and increased costs while drilled holes are re-worked, and when one drill chips, the entire lot from which it came are suspect and may have to be replaced. Industrial drills are made from very hard tool steels. Their cutting edge is formed by grinding. If this grinding process is done at too fast a feed rate or with insufficient coolant, a condition known as grinder burn occurs. This results in the formation of hard but brittle untempered Martensite at the cutting edge, which is susceptible to chipping. The image shows this condition (brown area) on the cross section of a drill.

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