Hardening and Softening of Metals

All along the walk on the Greenway trail couldn't wait to come back to write about it. Though it is taught only in colleges that have something to do with manufacturing, including machine tools manufacturing, i think it should be part of mainstream general minimal knowledge.

Theoretically it should have been a nice walk but it was yet another one from hell. Just before i left i checked under the hood. About six months ago i closed with the soldering iron a crack in the nylon part of the radiator and a few days ago i smelled again the smell of coolant (That truck did that for most of its life, once i was fixing something something else was popping). The crack opened again and there was coolant by it. At the trail itself there was some sort of fireplace smoke in the air that also had a chemical component. For that to happen somebody must burn a piece of plastic in a cold fireplace. And before checking under the hood when i got outside i smelled some solvent. Here at this complex like in the other before and the other before there was always some major remodeling/fixing work to be done at one apartment nearby and today was no exception. Also. All morning since woke up like always there was a continuous roar of modified exhaust cars taking turns in vibrating my whole universe and spine to the point i got sick to the stomach. It has always been like that but i started to become more aware after i got rid of the last colita at the door and had not seen yet another after a week or so. Among others a guy dressed in neon green and blue, reminding me of the failure with the (winter, cold) blue-green tablet of denture cleaning detergent in the blue denture box.

So i opened Wikipedia for this one. Wow. Incogniscible. Far away from the mainstream eye this definition at the beginning of the article transcends my power of understanding though i studied it a lot in college and i got it re-freshenning after the previous post about WTC disaster.

But couldn't i just find something on the web close to my my current perception so i don't have to re-invent the wheel write right now in this afternoon?

No luck there. I wanted to find something that puts together both hardening through cold laminating and thermal treatment. Ok let's bring on the definition of metal, metal structure and start with the basics.

All people know in nature we have crystals. They find them in mines. Diamonds, rubies, quartz, amber. Just kidding. Amber is not a crystal. Amber is raisin from a tree that have fossilized. Polymeric. Amorphous. The opposite of crystal.

A crystal is a symmetric, repetitive arrangement of atoms in a pure substance upon cooling and solidifying. In a way is like a bigger 3d molecule because nearby crystals stick together by electric forces as well. Crystals come in a variety of shapes but most common and most interesting crystallizing systems are the cubic ones. It's the system in which diamond, ruby and iron crystallize. Didn't know metals are crystalline substances? They are, in a way. In another they are not due to fact that continuous crystals in metals are very small, confined to micron size grain-like crystals. Metals are thus hybrid crystalline-amorphous substances. Luckily there is a site on the web that allows us to see interactive 3d intuitive images of some crystallizing systems. Mono crystals or single crystals including two purely theoretic fcc (face cubic center) and bcc (body cubic center) are present in this website's archive. (Drag with the mouse to rotate).



Best way to figure crystal grains in metals is to look at a fracture. Here is a fracture of a piece of of steel increased 1000 times under microscope.

But what in the world makes metal crystallize in small grains and not in big crystals like other substances, like by example silicon dioxide which is grown in single crystals the size of an elephant's leg in wafer manufacturing facilities?

In my opinion there are two causes. One is thermal conductivity of metals. Unlike silicon dioxide, when cooling a metal there is enough time for the temperature change to diffuse in the whole or large contiguous volumes of metal for crystals to start growing simultaneously in all volume. Thus crystallization starts at many points at once and crystals stop growing when they bump into each other. This is being aided by the impurities. Real metal contains a number of impurities that may solidify first that act like seeds for growing the small grain.

Hardness in any substance is given by the symmetry of arrangement of atoms. The more symmetric, or the the closer to an average distances between atoms are the stronger that material or harder to find a weak point or area with larger distances that may initiate a fracture.

Intracrystalline hardness or hardness of crystals themselves is very easy to explain and it pertains to the crystallizing system. Both fcc and bcc give strong materials because of symmetry of arrangements but bcc is stronger because is more symmetric. Steel by example which is actually an alloy of iron and cementite (a chemical combination of iron and carbon) depending upon temperature crystallizes in both forms, being bcc under some 912 Celsius and fcc above, where it is heated to be easily forged or rolled because is softer but with some volume differences due to rearrangement in different size and shapes of crystals upon cooling (not talking about thermal deformation or dilation or contraction within the same crystallizing systems which is a different process of its own and it actually adds up to a total difference of size upon cooling), whic means if we process iron steel above that temperature in fcc state the cooled resulting shape and size in bcc state will be slightly smaller by two different causes, which needs to be taken both into account by manufacturers.

Intercrystalline hardness of metals is the total hardness or what we perceive as hardness of a metal and usually is less than that of crystals themselves. Metallic bonds, electric forces that hold together the atoms in mono-crystals and crystals or grains are the same forces responsible for holding the grains together though arrangement of atoms at the border of the grains is less than the ideal one inside crystals, or with different distribution of distances, other than close to average inside crystals.

Elasticity is the property of any crystal, metallic or ionic, to give in or to change shape within certain limits when a force is applied and atoms pulled apart or pushed into each other are still left in the same arrangement and then fully recover, that is the deformation is completely reversed upon cessation of the force. This is characteristic to pure crystals like diamonds.

Any pure non metallic (ionic) crystal will deform after a force is being applied and get back in the exactly same unchanged initial shape with any change of properties if the force is being ceased before fracture and will break after a certain limit, without a permanent deformation left (the two fractured parts are still the exact sum of the initial one). We say crystals are brittle.

Metals because of metallic bonds and the grain like structure and defects within grains due to impurities will permanently deform before fracture and if the deforming force is ceased before fracture they might be left deformed and functional and they can become part or of a working structure.

Upon deformation the grains will start moving onto each other and inside along defects and rearranging allowing the metal to basically flow before fracture which is both inter and intracrystaline. However the rearrangement will not be as neat as the initial puzzle anymore. Grain deformation means sliding of crystal planes inside in areas with defects which makes them harder to further deform by exhausting the possibilities but also closer to fracture. We say metals are plastic.

Due to this property many manufacturers use processes of shaping the metal through plastic deformation. Metals is deformed to known tolerated limits sometimes in several steps while becoming harder which can make the parts smaller and lighter basically with the same material. One of the processes is called cold rolling and is used to produce beams including those used in building of skyscrapers. Cold deformation always introduces some stress in metal with some volumes applying permanent forces to others. This stress makes the final products somehow prone to breakage in certain directions more than others when applying a load.

When metal parts like I or H beams is produced through hot rolling, which is done at a temperature above re-crystallization where metal flows without breakages they can be hardened through a different process, called quenching, which is controlling crystal growth through controlling the speed of cooling.

Also. It is known from experience the smaller the grains in a metal, the harder and brittle the metal and the properties are closer to those of crystals the grains are made of. That is because the smaller the grains, the harder to move them against each other cause the surface between them or bonding surface is bigger. (surface/volume ratio increases with decreasing volume) and also harder to initiate intracrystaline deformation because smaller crystals have fewer initial defects.

Grains in metal can permanently change size and quality (reducing the number of intracrystaline defects, increasing size) through annealing and decreasing size through quenching, processes called by metallurgists thermal treatments.

Annealing is a thermal treatment that is used to decrease hardness and reduce stress in metals, sometimes after plastic deformation used in fabrication of different shapes of steel and has three stages. Annealing can be done at temperatures way below melting or fcc/bcc transition in steel.

"The temperature range for process annealing ranges from 260 °C (500 °F) to 760 °C (1400 °F), depending on the alloy in question."

"if annealing is allowed to continue once recrystallization has completed, then grain growth (the third stage) occurs. In grain growth, the microstructure starts to coarsen and may cause the metal to lose a substantial part of its original strength. This can however be regained with hardening."

https://en.wikipedia.org/wiki/Annealing_(metallurgy)#Stages

We also know from experience the faster we cool a metal the smaller the grains. Temperature dropping suddenly in a volume or at the surface only will initiate many new solidifying points or grain seeds reducing final grain size. Blacksmiths call this quenching and they do it by immersing a piece of steel in water for a certain amount of time. It is done after achieving the phase of re-growth of incomplete annealing. Blacksmiths in the past knew the temperature of the hot metal and when to do this bu the shade and intensity of the color of the hot red metal and other signs.

Quenching is a form of hardening the steel through thermal treatment rather than plastic deformation. But not any steel will quench. You need a certain amount of carbon in the form of cementite in the steel for this to happen through initiating more grains. Cementite has a slightly higher melting point then iron and solidifies first. Being dispersed throughout the hot alloy, it will initiate many new crystals.

Iron processing was known since at least 2500 years ago in Northern Europe. Scandinavians are among the people who have a long tradition. There are infinite possibilities and ways to produce high quality steel parts and tools and they are still best at. Blacksmiths in Medieval Japan needed about 3 days of continuous firing of the smelting furnace just to produce the steel, the starting point for making samurai swords. The whole process that included folding and welding of the final product in many strata took about 6 months and the price for a sword was the equivalent of 1 million dollars in today's money. In the Middle East they were making the famous Damascus swords, which actually had a texture inside the metal and it is said they could cut the barrel of a gun.

With all these being said we can figure by now. A cheap steel beam with a low carbon content that will not quench can be produced through several stages of cold rolling, also increasing its strength through introduction of intra and inter crystalline deformation. The final product will have a strength higher than initial steel after slow cooling from being produced in liquid form. However if the final product is submitted to an annealing process will become as soft as the initial cast steel ingot before cold rolling.

I am pretty much convinced all these things are thought in the US as well. But they are so basic are oftenly overlooked by engineers who nowadays specialize in very narrow fields of manufacturing and neglect the greater picture. The few who however figure it out don't have a voice or the courage or the motivation to say it out loud.