Case studies 467

19.5 Case studies 467

are anisotropic (i.e. different in different directions). This can be a problem if you want to make something smooth and circular out of a sheet, like a bever- age container.

Alloy steels and heat treatment: cranks, tools and gears. The moving and con- tacting parts of machinery (Figure 19.12(c)) are subjected to very demanding conditions—high bulk stresses to transmit loads in bending and torsion (as in a crank or a drive shaft), high contact stresses where they slide or roll over one another (as in gears), and often reciprocating loads promoting fatigue failure (as in connecting rods). Strength with good toughness is everything. Plain carbon steels extend up to 0.8% carbon—this introduces more iron carbide into the microstructure, giving greater precipitation hardening. They do a reasonable job, but the density of iron is a problem. Make the steel stronger, and use less of it, and we can save weight. This is particularly true for fast-moving parts in engines, since the support structure can also be made lighter if the inertial loading is reduced (so-called ‘secondary weight savings’). The trick is yet more alloying, and more processing.

A bewildering list of additions to carbon steel can be used to improve the strength—Mn, Ni, Cr, V, Mo and W are just the most important! Some contribute directly to the strength, giving a solid solution contribution (e.g. high-alloy tool steels, with up to 20% tungsten). More subtle though is the way quite modest additions (⬍5% in low-alloy steels) affect the alloy’s response to heat treatment. The key process is ‘quench and temper’ (Figure 19.7). The steel is heated to a temperature at which the carbon and the alloy additions dissolve (850–1000°C) and then quenched rapidly to room temperature, usually in oil or water. Tempering is reheating of the steel to an intermediate temperature to precipi- tate a fine, uniform dispersion of iron carbide in iron in every single grain. The quench is critical—slow cooling produces a softer mixture of grains, some being pure iron and some containing a coarse dispersion of iron carbide. A bit of additional precipitation strength comes from the formation of alloy carbides, but this is not the main reason for alloying. Plain carbon steels can also be quenched and tempered. The problem is the severity of the quench needed to avoid the formation of the much less effective microstructure associated with slow cooling. Alloying effectively shuts this down, enabling slower quench rates to achieve the target microstructure. In consequence, bigger components can be heat treated this way—the cooling rate at the center being limited by heat conduc- tion through the steel. The technical term for this is enhancing hardenability—it is another example of how material, process and design detail can interact in meeting the objectives of a design.

In passing, it is worthy of note that the intermediate microstructure (known as ‘martensite’) formed on quenching a hardenable steel is very hard, but also very brittle—like a ceramic. It is therefore useless as a bulk microstructure— tempering is essential to restore useful toughness. However, some surface treat- ments can produce a thin layer of martensite on a component—excellent for wear resistance on gears, bearings and so on. This is how laser hardening works

468 Chapter 19 Follow the recipe: processing and properties

(Figure 19.10). In other respects, martensite is a bit of a hazard. It can cause cracking problems on quenching, due to stresses induced by differential thermal contraction (Chapter 12). And even worse, welding causes a thermal cycle similar to the first stage of heat treatment, so higher carbon and alloy steels are suscep- tible to inadvertent embrittlement if the cooling rate is too quick (Figure 19.9). Oil rigs and bridges have collapsed without warning as a result of this behavior.

Stainless steel: cutlery. The city of Sheffield in England made its name on stainless steel cutlery (Figure 19.12(d)), and today manufactures more steel (of all types) than ever before. The addition of substantial amounts of chromium (up to 20% by weight) imparts excellent corrosion resistance to iron, avoiding one of its more obvious failings: rust. It works because the chromium reacts more strongly with the surrounding oxygen, protecting the iron from attack (Chapter 17). Nickel is also usually added for other reasons—one being the preservation of the material’s toughness at the very low cryogenic temperatures needed for the stainless steel pressure vessels used to store liquefied gases. Another is that both chromium and nickel provide solid solution hardening—this and work hardening (during rolling and forging) being the usual routes to strength in stainless steels, rather than heat treatment for precipitation strength.

Summary. In all of these applications, strength and toughness dominate the property profile. The manipulation of these properties was illustrated in Chapter 8 on a property chart for the main structural light alloys (aluminum). How do these varied ferrous alloys map out on this chart? Figure 19.13 shows a small selection of steels of the four types discussed above. This paints a remarkable picture. Starting with soft, tough pure iron (top left), we can manipulate it to produce more or less any combination of strength and toughness we like— increasing the strength by more than a factor of 20. Every final material (with the exception of as-quenched martensite) is comfortably above the typical fracture toughness threshold (⬇15 MPa.m 1/2 ) for structural or mechanical application. The normalized condition is standard for structural use. The chart shows how the quench-and-temper treatment enhances the strength without damaging the toughness. This figure explains why steels are so important—no other material is so versatile, with literally hundreds of different steels and ferrous alloys avail- able commercially.