Practicing machinists often rely on experience to set up a particular metal for cutting or rely on the tribal knowledge of their shop and industry. While it is typically up to the design engineers or the customer to specify the materials needed for a part, sometimes even materials within specs seem just a little more difficult to machine. That will often depend on how a material was heat-treated, cold worked and what percentage of alloys was used, which can vary to a degree. The science of getting the properties of a metal just right is extensive, and beyond the needs of working machinists.
But there is benefit for shop specialists in knowing some of the fundamentals.
The key to fully understanding metals and how to strengthen them is knowing they are crystalline. Atoms of a crystal are arranged in a lattice, but deviations from the “perfect” lattice explain mechanical properties such as strength. Perfect lattices have no dislocations or defects—and would, in theory, be weak and ductile since there would be little reason to stop the slipping of adjacent planes of atoms in that crystal. But most metals are not perfect and instead have defects in the crystals (grains), and it is the energy surrounding these defects that makes for stronger, but less ductile, metals.
Metals are aggregates of grains and within each grain is this crystalline structure. You can see evidence of the crystals in images of microstructures for specially prepared samples of metal. Defects can be singular (point) defects within the lattice, or lines that can snake through a grain (dislocations). The grain boundaries themselves are also defects—classified as “planar defects.” But it is the movement of the dislocations that allow for the movement of metal, and some would say allow for “weaker” behavior. Strengthening, then, is based on stopping dislocation movement, since if the dislocation cannot move, the metal cannot “move.” Not being able to move when a stress is applied is “making the metal strong.”
One way to inhibit dislocations from moving is making metals with smaller grains, which makes the metal stronger. Controlling the cooling from a melt is one way to get smaller grain sizes. There are others.
A second common method for strengthening is cold work, also called work hardening or strain hardening (they all mean the same thing). When a metal piece is forged, rolled, extruded, or drawn at a relatively cool temperature, grains are shaped and squished. Dislocations are put into the metal. In fact, so many dislocations are added that none of them can easily move—they are nearly bumping into each other and become pinned. A piece that has been overly work hardened can be made softer by annealing above the recrystallization temperature, to form new, dislocation-free grains.
A third way is what material scientists call “solid-solution strengthening.” When a pure elemental metal like copper is heated to a liquid and another element, say zinc, is added in small amounts, a new lattice structure is formed that is interspersed with the added element. The small amount of the substituted second element can inhibit dislocation motion in the lattice structure itself, since there is “lattice strain energy” surrounding the individual wrong-size atom. The moving dislocation, itself being lattice strain energy, is inhibited from moving near the substituted element. If the dislocations cannot move, the metal cannot move. Metal that cannot move when a stress is applied is said to be “strong.” See the pattern?
There is another special case of adding another element to a host metal and obtaining a stronger alloy, but it does not rely on solid solution strengthening, nor even inhibiting the movement of dislocations. This special case is well known to forgers, welders, machinists and heat treaters—adding carbon to iron to make steel. In the case of steel, an exceptionally hard phase called martensite occurs when steel with just the right amount of carbon is heated to orange-hot and quickly quenched. Rapid cooling through quenching in water or oil is vital to forming martensite.
The fourth mechanism of strengthening is called “precipitation strengthening” or “precipitation hardening.” Elements can only accept so much of another element. When more than a certain solubility limit is exceeded, then a second phase—another crystal—precipitates from the solution during cooling. Iron can absorb only so much carbon in its crystal lattice. In iron, a microstructure called pearlite, made of ferrite (a form of pure iron) and cementite (a carbon-rich phase), relies on precipitation to obtain desired strength and ductility properties. Also, many tool steels have a carbide as a second phase. Think of this as adding a new grain of a different alloy into a lattice structure, which adds strength by further inhibiting movement of dislocations in the lattice.
There are other examples of precipitation strengthening, especially with aluminum alloys, which have a temper designation indicating that the alloy is relying on precipitation strengthening. For example, 6061-T6 has a temper designation (T6), so one can conclude that it relies on precipitation strengthening.
Steel is an alloy iron with a low amount of carbon added, usually 0.5 percent and almost always below 1 percent. The hardest form of steel, martensite, is created by rapidly quenching orange hot steel. Martensite or martensitic steels, while strong, are usually harder to machine and process because of the resulting low ductility. In fact, martensite is so strong that practically speaking it is usually heated again to form a tempered martensite. This contains fine, unresolvable particles of cementite in a ferrite matrix. While less hard, it offers improved ductility and machinability.
But steel with only carbon added is just the beginning. What about all those steel alloys that use chromium, molybdenum, vanadium or nickel? Adding small percentages of these elements, from 0.5 to 2.5 percent, can improve the properties of iron quite significantly. They make the steel easier to harden deeper into the metal (hardenability). These elements provide hardenability by making it easier to form martensite at slower cooling rates.
For reference, martensitic stainless alloys will contain 11–17 percent chromium with 0.15–0.63 carbon and are ideal for medical instruments, blades, and hand tools. Precipitation hardened stainless steels usually have a PH in their name, such as PH 13-8 Mo from Sandvik that includes 0.5 percent carbon, among other alloying elements. A common grade is PH 17-4.
For more information on Tooling U-SME and access to this webinar in particular, visit https://www.toolingu.com/resources/Watch-and-Listen
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